RU2102547C1 - Method of preparing bleached pulp and a method for delignification and bleaching of lignocellulose material - Google Patents

Abstract

FIELD: pulp-and-paper industry. SUBSTANCE: two methods and their versions involving no gaseous chlorine are fulfilled through partial delignification of pulp until permanganate number about 10 or less and viscosity within the range from about 10 to about 13 centypoise are attained, after which partially delignified pulp is further delignified with a certain effective amount of ozone for a period of time sufficient to produce essentially delignified pump with permanganate number about 5 or less, viscosity about 5, and whiteness about 50%. The pulp thus obtained can be bleached with such bleaching agent as chlorine dioxide or peroxide to yield final product with whiteness about 65%, preferably 70-90%. Absence of chlorine allows regenerating filtrates from all stages except that where chlorine dioxide treatment is involved, when available. EFFECT: provided drainless process with considerably reduced environmental damage. 124 cl, 5 dwg, 30 tbl

Description

 The invention relates to a method for producing bleached pulp and to a new and environmentally acceptable method of delignification and bleaching of lignocellulosic pulp, which eliminates the need for chlorine gas and produces a pulp of acceptable strength. The application of this method also allows you to reduce the amount of pollutants.

 The wood consists of two main components, it is a fibrous hydrocarbon or cellulosic part and a non-fibrous component. The polymer chains forming the fibrous cellulose part of the wood are oriented identically and form strong associative bonds with neighboring chains. The non-fibrous part of the wood contains a three-dimensional polymer material, consisting mainly of phenylpropane units, also called lignin, some of the lignin is located between the cellulose fibers, making them strong bond, however, the main part of the lignin is distributed inside the fibers themselves.

 So that wood can be used in paper production, it should first be crushed to pulp. A similar mass is a fiber of wood that can be suspended and then deposited on a grid, resulting in a sheet of paper. To turn wood into pulp, it is subjected to physical or chemical treatment or their combined action in order to change the chemical form of wood and give the finished product the desired properties. So, there are two main methods for producing pulp from wood: mechanical and chemical methods. By machining the wood is divided into separate fibers. During chemical processing, wood chips are boiled with chemical solutions to dissolve part of the lignin and, as a result, remove it. The most widely used chemical processing methods can be classified as follows: (1) the soda method of cooking; (2) a sulfite method of cooking, and (3) a sulfate method or craft process; the latter method has found the widest application, its various modifications are known, which are described in detail below.

 The soda method of cooking is well known. It uses sodium hydroxide (NaOH), which acts as an active reagent that softens lignin and helps to remove it. Sulphite cooking is also well known (see, for example, “Pulp and Paper Process Manual”, Chapter 6 Sulphite Cooking (TAPPI, USA).

The main chemical process used in the manufacture of paper is the sulfite method of cooking, as well as its various variations. In accordance with the main sulfate cooking method described in the Handbook of pulp and paper technologist, chapter 7 "Kraft process" (TAPPI, USA), wood chips are cooked in an aqueous solution of sodium hydroxide (NaOH) and sodium sulfide (Na ₂ S). This method has proven to be highly effective when cooking southern coniferous species of wood, as well as when cooking easily processed types of wood, namely northern hardwood and coniferous species. The use of the kraft process, as a rule, allows to obtain a relatively high-strength material, since when it is used, the cellulosic component of wood undergoes less damage.

 Various modifications of the Kraft process can achieve even less degradation of the polymer structure of cellulose fibers, so that the strength of the resulting paper is higher than with a standard Kraft process. One of the modified sulphate cooking methods is called the “extended delignification” method; it covers many modified kraft processes, for example, the addition of chemicals in a strictly defined sequence or at different points in the digester or at different times, or the removal and subsequent supply of cooking solutions in a given sequence in order to provide more efficient lignin removal while mitigating the chemical effects of cooking liquors on cellulose fibers . Another modification of the kraft process is the process in which a small amount of anthraquinone is added to the sulphate cooking liquor in order to accelerate delignification and limit the effect on cellulose fibers.

 In addition to them, many other delignification methods are known, for example, the modified Kamyur continuous cooking (MSS) described by V. A. Kortelainen and E. A. Backlund in TAPPI, Volume 68 (II), 70 (1985); Belois's method of accelerated substitution upon heating (RDN), described by R. S. Grant in TAPPI, Volume 66 (3), 120 (1983); and Sands cold-blown cooking, described by B. Peterson and B. Ernerfeld in Pulp and Paper, Volume 59 (II), 90 (1985).

 As a result of cooking wood according to the main or modified kraft process, a suspension is formed that is painted in dark color and therefore is called an unbleached mass. The dark color of the unbleached mass is attributed to the fact that not all lignin is removed during cooking, it is chemically modified and forms chromophore groups. To lighten an unbleached mass, i.e. to make it suitable for use as printing or writing paper, it is necessary to continue to remove residual lignin by adding delignifying materials and by chemically converting residual lignin into colorless compounds using a process known as bleaching or lightening.

 However, usually at the end of the chemical treatment carried out during the cooking of the wood, and before the start of bleaching, the pulp is placed in a separate blow tank. Inside the blow tank, the pressure created during the initial chemical treatment of the lignocellulosic material is removed and the pulp is separated. The resulting pulp is subjected to several washes to remove a combination of residual chemicals and soluble materials (such as lignin) that are separated from the fibrous materials during cooking. Quite often, the pulp is subjected to several sortings aimed at separating the unmilled wood for further processing (digestion, mechanical grinding, etc.).

 After washing, a residue is obtained, usually called black liquor, it is collected, concentrated and then calcined in an environmentally friendly way in a recovery boiler. The technology for collecting, concentrating and burning black liquor is standard, it is well known.

 The delignification and bleaching processes are carried out on the washed pulp for several stages, using the selected combination of chemical reagents. Previously, a variety of combinations of chemical treatments were offered. In addition, individual processing steps can be rearranged, and the number of such combinations and permutations is almost unlimited. Therefore, to simplify with further explanations of the various processes and systems, letter codes are applicable, which we will use to describe the specific reagents used and the sequences of steps.

Further, where convenient, the following letter codes will be used:
C chlorination reaction with chlorine gas in an acidic environment;
E alkaline treatment dissolution of reaction products with NaOH;
E _o oxidation-alkaline treatment dissolution of reaction products with NaOH and oxygen;
D chlorine dioxide reaction with ClO ₂ in an acidic environment;
P peroxide reaction with peroxides in an alkaline environment;
O oxygen reaction with gaseous oxygen in an alkaline environment;
O _m modified oxygen uniform alkaline treatment of pulp of small or medium concentration, followed by the reaction of highly concentrated mass with oxygen;
Z ozone reaction with ozone;
Z _m - a homogeneous reaction with ozone;
C / D mixtures of chlorine and chlorine dioxide;
H hypochlorite reaction with hypochlorite in an alkaline solution.

O _m and Z _m are modified processes according to this invention, their description is given later in the text.

 Over the years, chlorine gas has been used to delignify and bleach wood pulp.

 Although chlorine gas is an effective bleaching agent, it is difficult to handle and is potentially hazardous to both equipment and personnel. For example, wastewater from chlorine bleaching processes contains large amounts of chlorides as by-products of these processes. Under the influence of chlorides, equipment quickly corrodes, which is why expensive materials have to be sent to the manufacture of such plants. At the same time, the presence of chlorides inside the unit prevents the filtrate from being recycled after the completion of the chlorination step in a closed-loop system, if regeneration systems with complex and therefore expensive modifications are not used. But the concept of the potential environmental impact of chlorinated organic compounds in wastewater and found to be toxic to humans and animals according to decisions of the US Environmental Protection Agency has undergone significant changes in the sense of stricter requirements and standards, the observance of which may not be possible using standard methods bleaching and pollution control.

 To avoid these inconveniences, they try to reduce or completely eliminate the use of gaseous chlorine and chlorine-containing compounds in multi-stage processes for bleaching lignocellulosic material. An additional factor complicating the attempts made is the need to maintain a high level of whiteness of the fibrous prefabricated product.

 Therefore, efforts were directed to the development of such a bleaching process, in which oxygen is used to bleach the mass instead of chlorine-containing substances. The use of oxygen allows the recycling of wastewater from this stage along with a significant reduction in the amount of chlorine gas used. Several processes have been proposed for bleaching and delignification of wood pulp using oxygen, for example, US patent N 2926114, CL. 162/16 for 1960 and N 3024158 according to cl. 162/17 for 1962, US patent N 3274049 for CL. 162/65 for 1966 US patent N 3384533 CL. 162/65 for 1968, US patent N 3423282 for CL. D 21 C 9/10 (162/65) for 1969 and other US patent N 3661699 for CL. D 21 C (162/65) for 1972, publication of P. Christensen, “Bleaching of Sulphate Masses Using Hydrogen Peroxide,” Norsk Skoqindustri, 268-271 (1973).

 In US patent N 4806203 according to CL. D 21 C 3/26 (162/19) for 1989 provides methods for pre-alkaline treatment of pulp prior to oxygen delignification.

 However, the use of oxygen cannot be considered a completely satisfactory solution to the problem associated with the use of gaseous chlorine. Oxygen is not as selective a delignifying agent as chlorine gas, which is why the use of standard methods of delignification with oxygen allows only to a limited extent to reduce the permanganate number of the pulp to a disproportionate start, i.e. unacceptable effects on cellulose fiber. In addition, upon completion of oxygen delignification, residual lignin was typically removed by chlorine bleaching methods to obtain a completely bleached mass; a much smaller amount of chlorine was used. However, even at such low concentrations of chlorine due to the closed working cycle, completely unacceptable levels of the concentration of corrosive chlorides were soon reached.

 To avoid the use of bleaching agents with a chlorine content, it was proposed to remove residual lignin using ozone. At first, ozone seemed to be an ideal material for bleaching lignocellulosic materials, however, the extremely high oxidizing properties of ozone and its relatively high cost until recently had severely limited the development of satisfactory processes for ozone bleaching of lignocellulosic materials, and especially southern conifers. Ozone easily reacts with lignin, effectively reducing the permanganate number, however, in many cases, it destroys the hydrocarbon contained in cellulose fibers and significantly reduces the strength of the resulting pulp. However, ozone is extremely sensitive to process parameters such as pH, which affect its oxidative and chemical stability, which is why such changes can significantly change the reactivity of ozone with respect to lignocellulosic materials.

 From the very beginning of our century, when the delignifying properties of ozone were first recognized, many researchers have carried out important and continuous work to create an industrially acceptable method for bleaching lignocellulosic materials using ozone. The result of this work was numerous articles and patents, which reported attempts to implement ozone bleaching on a non-industrial scale. For example, in US patent N 2466633, CL. 162/65 for 1949 describes a bleaching process in which ozone is passed through a pulp, where the absolute humidity is maintained in the range from 25 to 55% and the pH is from 4 to 7.

 Other chlorine-free bleaching proposals are described in S. Rotenberg, D. Robinson, and D. Jiosonbo's publication, “Ozone-Bleaching of Oxygen-Based Pulp Pulps,” TAPPI, 182-185 (1975) Z, ZeZ, Zp, and ZPa (Pa Peracetic Acid ); and R. Soseland’s publication, Oxygen and Ozone Whitening of Industrial Pulp, Canada Pulp and Paper Review. T153-58 (1974) OZEP, OP and ZP.

 In US patent N 4196043, D 21 C 9/10 (162 / 30K) for 1980 disclosed a multi-stage bleaching method, which also made an attempt to get rid of the use of chlorine compounds; it contains examples specifically related to hardwood. Specialists are well aware that hardwoods are easier to bleach than many conifers. The method proposed there is characterized by the use of one to three stages of obtaining, using the Kraft process, a mass of ozone bleaching and final treatment with alkaline hydrogen peroxide, with each stage being separated by alkaline extraction. A similar sequence can be abbreviated as ZEZEP. According to this method, wastewater from each step is collected and recycled for use in bleaching operations, preferably at an earlier stage. The patent also describes the so-called counterflow of wastewater.

 Nevertheless, despite all the studies carried out to date, it is not known about any industrially feasible method for the production of lignocellulosic masses through ozone bleaching, especially southern soft conifers; there are only reports of numerous failures.

 The problem to which this invention is directed, is to create new combinations of the stages of cooking and bleaching, eliminating the previously encountered difficulties listed above, with the exception of the release of chlorinated organic compounds, as well as minimizing the color and biological demand for oxygen for the production of high-grade bleached pulp on an industrial scale.

 SUMMARY OF THE INVENTION

 The technical result of the invention is the creation of a multi-stage process of delignification and bleaching of lignocellulosic pulp without the use of bleaching substances with a chlorine content to significantly reduce or eliminate the environment while optimizing the physical properties of pulp in a way that is effective in terms of energy and cost. This invention can be applied with almost all species of wood, including those with difficult to bleach southern soft coniferous species.

 The method according to this invention consists of three or more steps, while various variations are possible within and between the steps. All these steps can be described as follows.

 The first stage includes the delignification of wood chips into lignocellulosic pulp using one of the well-known chemical cooking processes, followed by removal by washing of dissolved organic substances and cooking chemicals for their recycling and regeneration. Then, the sorting of the mass is usually carried out in order to remove bundles of fibers that did not separate during cooking. The delignification step is carried out in such a way as to obtain a softwood pulp with a permanganate number from 20 to 24 (preferably 21), a viscosity in a solution of cupriethylenediamine ("SED") from about 21 to 28, and a whiteness from about 15 to 25. Of U.S. hardwood typically produces pulp with a permanganate number of 10 to 14 (preferably 12.5) and an SED viscosity of about 21-28.

Possible options for the implementation of the first stage include, but are not limited to:
a) sulfate cooking using a continuous or cyclic stage;
b) continuous sulphate cooking with expanded delignification during the phased addition of alkali and final countercurrent cooking;
c) cyclic sulfate cooking with extended delignification and using methods of rapid displacement of liquor and cold purging; or
g) sulfate cooking using anthraquinone to achieve enhanced delignification using a continuous or cyclic stage.

 The methods of extended delignification mentioned in paragraphs b) and c) above may contain, for example, Kamyur cooking techniques of the MCC, Belois RDN and Sands, discussed in the introductory part of this description. Depending on the particular type of lignocellulosic material used, the above soda and sulfite processes may also be used.

 At the second stage of the method, oxygen delignification is carried out to further remove lignin, which is not accompanied by a significant loss in strength of cellulose fibers. At this stage, removal can be carried out by washing the dissolved organic substances and alkalis for recycling and regeneration. Upon completion of the oxygen delignification, the pulp is also sorted.

At the stage of oxygen delignification, the permanent number of pulp with an increased concentration decreases by at least about 45% (for O) and by 60% (for O _m ) without noticeable damage to the cellulose component. In addition, the ratio of permanganate number to viscosity is usually reduced by at least 25%. When processing pulp, a permanganate number equal to or less than 10 is achieved. When processing pulp from softwood using O _{m, a} permanganate number from 7 to 10 is easily achievable and a viscosity of approximately equal to or greater than 13. When processing pulp from hardwood after the oxygen delignification step, a permanganate number of 5 to 8 and a viscosity of approximately equal to or greater than 13 are achieved.

Possible options for the implementation of the second stage include, but are not limited to:
a) typical oxygen delignification, which includes oxygen-alkaline treatment of the pulp at low, medium or high concentration of pulp (O); or
b) a preferred alkaline treatment at a low to medium mass concentration, i.e. less than about 10 weight. then for the mass with a high concentration (i.e. more than about 20 wt.) follows oxygen treatment (O _m ).

 As a pulp, the whiteness of which can not exceed 35% CEB (often called semi-whitened mass), you can use the mass processed up to 2 stages.

At the third stage of the method, acid bleaching with gaseous ozone (Z or Z _m ) is carried out subject to certain process parameters to achieve highly selective removal and bleaching of lignin for minimal destruction of cellulose. Process parameters include chelating agents for controlling metal ions, pH control, particle size control, mass consistency control, ozone concentration and contact between the gas and the pulp. Before treatment with ozone, chelating agents can be added to the pulp, such as oxalic acid, diethylene triamine pentaacetic acid (DTPA) or ethylenediaminetetraacetic acid ("EDTA") to bind to the metal ions contained in it. It is advisable to adjust the pH from 1 to 4, doing this before the start of the third stage. For this purpose, a sufficient amount of acidic material can be added to the pulp, it is also preferable that the consistency of the pulp is in the range from 35 to 45 wt. and fiber flakes, it is desirable to give a size of approximately 5 mm or less, carrying out this before the stage of ozone delignification. This also includes the stage of washing the dissolved organic substances for recycling and regeneration.

Throughout the ozone treatment step, it is desirable that the temperature of the mass be maintained at normal or at least below about 120 ^° F (48.9 ^° C). Ozone can come in the form of an ozone-containing gas, which, for example, can include oxygen or air. If a mixture of oxygen and ozone is used, it is desirable that the ozone concentration is from about 1 to about 8 vol. whereas in a mixture of ozone with air, a concentration of about 1 to 4 vol. The delignified mass is moved inside the reaction tank so that ozone has a uniform effect on all its particles.

 It was found that pulp with a permanganate number greater than about 10 after the second stage is not suitable for use in the described third stage, since a large amount of ozone is required to reduce the permanganate number to the desired level, which usually undesirably affects the properties of the cellulose fibers. If a pulp with a permanganate number of less than 10 is ozonized, then a lower concentration of ozone is used and the most insignificant destruction of cellulose is observed. The product obtained at the stage of ozonation from pulp from coniferous or deciduous species has a permanganate number of about 5 or less than 5 and usually ranges from 3 to 4 (preferably 3.5), the viscosity exceeds 10, and the CE brightness is, according to at least 50% (usually in coniferous wood about 54% and above, in deciduous 63% and above).

Possible options for implementing the third method include, but are not limited to:
a) processing the acidified mass in countercurrent contact with ozone in an oxygen-containing gas or air; or
b) processing the acidified mass upon contact with a parallel stream of ozone in an oxygen-containing gas or air.

Then, an additional bleaching step can be carried out to give the pulp mass the desired fully bleached state when the whiteness reaches approximately 70-95% due to the use of one of the well-established bleaching and alkalization processes. Possible implementation options include:
a) a typical alkaline stage with washing, followed by a peroxide stage with washing, i.e. EP;
b) typical stages of alkalization and washing, followed by a typical treatment with chlorine dioxide with washing, that is, ED;
c) typical stages of alkalization and washing, followed by a typical treatment with chlorine dioxide with washing, then re-alkalization and treatment with chlorine dioxide, i.e. EDED or
d) the alkalization stage, enhanced either by oxygen, or by oxygen and peroxide, followed by a typical treatment with chlorine dioxide, i.e. (E _o ) D or (E _op ) D.

 As an additional implementation option, at the stage of alkalization, a mainly delignified mass can be combined with a sufficient amount of alkaline material in an aqueous-alkaline solution for a certain time and at a certain temperature in accordance with the available amount of alkaline material to dissolve a significant fraction of the lignin remaining in the mass. Then part of the aqueous alkaline solution can be removed to remove almost all of the dissolved lignin.

 Upon completion of the alkalization step, the delignified pulp can be sent to an additional bleaching step to increase its whiteness by at least 70%. Preferred bleaches include chlorine dioxide or peroxide.

The greatest degree of whiteness is achieved in the implementation options (E _o ) D, (E _op ) D or EDED. In the ED embodiment, the filtrate obtained after treatment with chlorine dioxide cannot be recycled for chemical regeneration without pretreatment due to the presence of inorganic chlorides. However, such a filtrate is the only one leaving the process, and therefore a significant reduction in the volume of wastewater, the presence of chlorinated organic compounds, COD and BOD in it is achieved. It is entirely possible to achieve dye levels of less than 2 pounds per ton, BOD ₅ less than 2 pounds per ton and a total organic chloride content of less than 2 and preferably less than 0.8. In addition, the filtrate after treatment with chlorine dioxide can be sent to membrane filtration, which will complete the recycling. In the EP embodiment, at the bleaching stage, the formation of chlorinated materials does not occur and almost all liquid filtrates can be recycled and regenerated, which results in a process without wastewater.

 In FIG. 1 shows, in a block diagram, preferred methods according to the invention, the solid line depicting the flow of pulp and the dashed stream of wastewater; figure 2 preferred method according to this invention; figure 3 is a view of the device for ozonation of figure 2, in section, made along the line 3-3; figure 4 is a view of the preferred device for ozonation of figure 2 in section, made along the line ZZ; 5 is a comparison of recycling and waste streams in various pulp processing methods.

 The invention relates to new methods of delignification and bleaching of pulp, allowing to minimize the degree of destruction of the pulp fraction of wood and therefore to obtain a product with strength properties acceptable for the production of paper and various products from it. For convenience in understanding the improvements achieved by using the inventive method of delignification and bleaching, definitions of several parameters involved at various stages in all delignification and bleaching processes will be presented below.

 Throughout the description, the following definitions are used.

 By "consistency" is meant the amount of cellulosic fibers in suspension, expressed as a percentage of the total weight of absolutely dry fiber and water. It is sometimes called the concentration of pulp. The consistency of the mass depends on the operation of the dewatering equipment used and its type. The following definitions are based on the definitions given by Rüdholm in the publication "Cooking Processes" Interscience Publishers, 1965, p. 862-863, and monographs TAPPI N 27, "Pulp Whitening", publication of the Technical Association of the Pulp and Paper Industry, 1963, p.186-187.

 The concept of "small consistency" includes a range from 6%, usually from 3 to 5%. This suspension is pumped with a conventional centrifugal pump, it is obtained using thickeners and filters without squeezing shafts.

 The concept of "average consistency" includes a range of about 6 to 20%, a section point within this range is a value of 15%. You can get a consistency below 15% using filters. It is this consistency that the mass exiting the drum vacuum filter has in the washing system of the unbleached mass and in the bleaching system. The consistency of the suspension coming from the washer, whether it be a vacuum filter for washing sulfate pulp or a washing machine after bleaching, is from 9 to 15%. If the consistency is more than 15%, squeezing shafts are required for dewatering. Rüdholm believes that the usual range for the average consistency is 10–18%, while Rapson indicates 9–15%. The suspension can be pumped with special machines, although at high temperatures and under pressure it is a homogeneous liquid phase.

 The concept of "large consistency" includes a range of more than 20 and up to 50%. Rüdholm considers the range from 25 to 35% to be common, according to Rapson 20-35%. Such a consistency can be obtained only by using presses. Cellulose fibers have completely absorbed the liquid phase, and the pulp can be pumped only at very short distances.

 Further, in the present description, the term "cooking" is used in its usual sense, indicating the cooking of lignocellulosic material to produce unbleached pulp. The concept of cooking includes a kraft process, as well as a craft process using anthraquinone, as a result of cooking delignification is achieved.

 The concept of “modified kraft process” includes extended delignification and all other modified kraft processes with the exception of the kraft process using anthraquinone, since this process has acquired a special status and is known by its special name. The concept of extended delignification does not include the stage of oxygen delignification, following the completion of cooking; it is preferable to call the first stage of the delignification process for bleaching or clarification of the mass.

 Then, two main types of measurements used to determine the completion of the cooking or bleaching process are mentioned here, these are the “degree of delignification” and “whiteness” of the pulp. The concept of the degree of delignification is usually used in connection with the cooking process and the initial stages of bleaching. As the amount of lignin decreases, this indicator becomes less accurate, which is characteristic of the last stages of bleaching. The whiteness coefficient is usually used in connection with the bleaching process, since it becomes more accurate as the pulp becomes lightened when its reflectivity increases.

 There are several methods for measuring the degree of delignification, however, many of them are only variations of the permanganate test. In a normal permanganate test, a permanganate number (K) is obtained, i.e. the number of cubic centimeters of decinormal permanganate potassium solution absorbed by one gram of absolutely dry mass under certain conditions. The permanganate number is determined by the standard TAPPI T-214 test.

 There are also several methods for measuring the whiteness of pulp. This parameter usually represents a measure of reflectivity, and its value is expressed as a percentage on a particular scale. The standard method is CE whiteness, expressed as a percentage of the maximum CE whiteness determined by the standard TAPPI TRD-103 methodology.

 In addition, the letter code described above in the section "Background to the invention" will be used in appropriate places; throughout the detailed description, it will denote the various stages of pulp processing.

 The following values of permanganate number, viscosity and whiteness obtained using the proposed method of cooking, delignification and bleaching will show the ability of this method to improve the degree of removal of lignin from the pulp while minimizing the resulting destruction of cellulose.

 At the end of the oxygen delignification step and prior to bleaching, there is a partially delignified mass with a permanganate number of about 5 to 10, preferably 7 to 10 for softwood growing in the USA and 5 to 7 for hardwood. The viscosity of the partially delignified mass is approximately 10, usually more than 13 and preferably at least 14 (for coniferous mass) or 15 (for hardwood). The partially delignified material had good strength and the required viscosity and therefore could withstand the effects of ozone. The partially delignified mass was subjected to ozone to further delignify the mass, as a result of which the permanganate mass number was reduced to approximately 3-4 for both coniferous and deciduous species, while the whiteness of the mass increased to at least 50-70%. conifers usually reached whiteness of about 54% and higher, while hardwoods reached values of more than 63%. Further increase in pulp whiteness was achieved by alkalization and additional bleaching with Iew chlorine dioxide or peroxide.

For ease of understanding of the present invention, figure 1 shows in schematic form the various steps used in cooking, delignification and clarification of the mass according to this invention. As can be seen from figure 1, the present invention proposes a multi-stage process in which:
(a) cooking lignocellulosic material, and the chemicals used in the cooking can be regenerated and reused using known methods;
(b) the mass is washed to remove chemical waste from the cooking solution along with residual lignin, and, as a rule, the pulp mass is sorted to remove those fiber bundles that did not separate during cooking;
(c) carry out oxygen-alkaline delignification of the mass (i.e., O or O _m );
(g) washing the partially delignified mass obtained in step (c) to remove dissolved organic compounds;
if necessary, at this moment it is possible to sort and direct at least part of the wastewater from this stage to the previous stage for recycling;
(e) chelating and acidifying the pulp to bind metal ions and adjust the pH to a predetermined level;
(e) contacting the mass with ozone (i.e., Z or Z _m ) for further delignification and partial bleaching of the material;
(g) washing the ozonated mass by recycling at least a portion of the wastewater from this stage to the previous one;
(h) carry out alkaline treatment to remove residual lignin;
(i) washed the treated mass, recycle at least part of the wastewater to the previous step;
(k) adding a second whitening reagent (i.e., D or P to lighten and bleach the mass);
(l) washing the clarified mass to obtain a bleached product whose whiteness is approximately 70-90% and
(m) recycle at least part of the wastewater from the bleaching step P to the previous step; or drain the wastewater from the bleaching step D, or after appropriate treatment, recycle the wastewater to the previous step.

1. Cooking
The first step in the method of this invention, in which procedures can be used to remove lignin from lignocellulosic materials while minimizing cellulose degradation, is cooking. The particular cooking method used in this invention is highly dependent on the type of lignocellulosic material and more specifically on the type of wood used as the starting material. Figure 1 shows that the cooking solution used during cooking can be regenerated and reused by a known method. After this step, washing is usually followed to remove most of the dissolved organic compounds and cooking chemicals for recycling and regeneration, as well as a sorting step in which the pulp passes through a sorting device to remove bundles of fibers that did not separate during cooking.

 Almost all types of wood can use the Kraft process, as the resulting material has acceptable physical properties, although the unbleached mass is darker in color.

 Depending on the initial lignocellulosic material, the results obtained using a typical kraft process can be improved by applying the extended delignification method or the Kraft process with the addition of anthraquinone. This method is preferable when it is necessary to obtain a higher degree of reduction of the permanganate number of the pulp without undesirable deterioration of its strength and viscosity properties.

When using the Kraft process with the addition of anthraquinone, the specific amount of anthraquinone in the cooking solution should be at least about 0.01 wt. when calculating the absolutely dry weight of the wood being digested, the amounts from 0.02 to 0.1% are generally preferred. The inclusion of anthraquinone in the Kraft process significantly contributes to the removal of lignin, without adversely affecting the strength properties of residual cellulose. In addition, the additional costs of anthraquinone are partially offset by the savings in chemicals in the subsequent steps Z _m , E and D or P.

 As an option or addition to the craft process using anthraquinone, you can also use the extended delignification methods of Camur, Belois or Sands, proposed for batch boilers. These methods also allow you to remove more lignin from the pulp during cooking, without adversely affecting the desired strength properties of the residual cellulose.

2. Oxygen delignification
The next step in the method according to this invention is part of the bleaching process, which is primarily aimed at removing residual lignin from the processed unbleached mass. According to the method of the invention, an oxygen delignification step is carried out at this stage. At this stage, oxygenated materials are removed, they can, like a cooking solution, be collected, concentrated and then calcined in an environmentally friendly way in a typical regeneration boiler. As can be seen from figure 1, recycles at least part of the liquid phase.

 It was found that the stage of oxygen delignification can be carried out in such a way that helps to remove large amounts of residual lignin in the unbleached mass, without causing an undesirable deterioration in its viscosity. In general, the described method is implemented by processing an unbleached mass leaving the cooking stage with a consistency of small to medium, using the amount of alkali that is required for the oxygen delignification step to ensure a uniform alkali supply and then increase the consistency and delignification to large values. The most preferred high consistency delignification, however, low or medium oxygen delignification can also be used instead.

The highly consistent delignification step is preferably carried out in the presence of an aqueous alkaline solution with a consistency of mass of from about 25 to 35%, preferably about 27%. Such an improved method (O _m ) ensures the removal of at least 60% of the residual lignin from the unbleached mass compared to 45-50% removed at typical stages of oxygen delignification, while there is no previously inevitable deterioration in relative viscosity. Due to the nature of the modified process, it is the preferred oxygen process used in the method of this invention.

The processing step of the modified oxygen process (O _m ) involves essentially uniformly combining the wood pulp, preferably the unbleached pulp from the Kraft process, with an aqueous alkaline solution while maintaining a pulp consistency of at least about 10% and more preferably less than 5 wt. It is desirable that the aqueous alkaline solution is present in an amount sufficient to obtain from about 0.5 to 4 wt. active alkali after thickening when calculated on an absolutely dry unbleached mass, even more preferably a value of about 2.5% active alkali by weight after thickening when calculated on an absolutely dry unbleached mass.

 At this stage, the aqueous alkaline solution is evenly distributed throughout the low-consistency mass, while all fibers of the unbleached mass should experience uniformly the action of the alkaline solution. It was completely unexpectedly found that a similarly treated unbleached mass is generally not delignified at the processing stage, however, the delignification of an unbleached mass at the subsequent stage of highly consistent delignification is more effective than that of an unbleached mass processed at high consistency in alkaline solutions according to standard methods. In this case, local inhomogeneities in the alkali distribution of a typical highly consistent mass are eliminated, and the heterogeneous oxygen delignification accompanying them is excluded.

 Preferably, the homogeneous distribution step includes uniformly combining the mass with an aqueous alkaline solution for at least about 1 minute, and preferably not more than about 15 minutes. We can assume that the processing time of less than 1 min, as a rule, is not enough to achieve uniform distribution, while the processing time of more than 5 minutes will not give a significant gain.

The alkaline treatment of the mass according to this invention can be carried out in a wide temperature range. Practice has shown that processing can be carried out at a temperature of from about room temperature to about 150 ^° F. (65.6 ^° C.), with temperatures from 90 to 150 ^° F. (32.3 65.6 ^° C.) being most preferred. Both atmospheric and elevated pressure can be used. The processing step is completed when the aqueous alkaline solution is evenly distributed throughout the low-consistency mass. The amount of aqueous-alkaline solution present in the processing step can vary widely within the limits of the specific parameters of the delignification reaction process. In this case, an effective amount of an alkaline solution will depend primarily on the degree of delignification desired at the stage of oxygen bleaching, as well as on the strength of the solution used. It is desirable that the number of aqueous alkaline solutions include a solution of sodium hydroxide with a concentration of from about 20 to 120 g / L. Such a solution is mixed with a low consistency mass so that the concentration of alkaline material in the resulting mixture is between about 6.5 and 13.5 g / l, preferably about 9 g / l.

If the processing time is from 5 to 15 minutes with a consistency of the mass from 3 to 5% and a temperature of from 120 to 150 ^o F (48.9-65.6 ^o C) at the specified concentration of alkaline material, then it is uniformly distributed throughout the unbleached mass.

 In accordance with a preferred embodiment of the present invention, an aqueous-alkaline hydroxide solution is added to the low consistency mass in an amount that is sufficient to obtain from about 15 to 30 wt. sodium hydroxide when calculated on a completely dry mass. Other alkaline solutions with an equivalent content can also be used, for example, white liquor from a typical cycle of a regenerative kraft process.

 After the low consistency alkaline treatment step, the consistency of the treated mass increases to not less than 20% and preferably from 25 to 35%. To increase the consistency of the mass, known methods can be applied, for example, pressing wood pulp to remove liquid from it. The initially formed mass is an unbleached mass, and at least a portion of the liquid obtained from the alkaline solution at the stage of increasing the consistency of the mass is recycled to the alkaline processing stage.

Further, oxygen delignification is carried out on a highly consistent mass. In order to influence delignification, known methods for dissolving gaseous oxygen in the liquid phase of a highly consistent mass can be applied. Any of the known methods may be used in accordance with this invention. However, it is desirable that gaseous oxygen with an overpressure of about 80 to 100 psi be supplied during oxygen delignification of the present invention. inch into the liquid phase of the high consistency mass, while the temperature of the mass is maintained in the range of about 90 to 130 ^° C. The average contact time of the high consistency mass with gaseous oxygen is preferably kept at about 20 to 60 minutes.

 If you follow the preferred process according to this invention, it is possible after the stage of oxygen delignification to achieve a reduction in permanganate number by at least 60%, while the cellulose part of the mass will not be affected. As a comparison, we note that with typical oxygen delignification, a reduction in permanganate number of about 50% can be achieved before cellulose breakdown begins. Thus, the proposed method allows to increase delignification by at least 20% in comparison with typical delignification processes, i.e. from 50 to 60% reduction in permanganate number in the incoming mass. With minimal destruction of cellulose, a reduction of 70% or more can be achieved. The fact that the destruction of cellulose does not occur, is obvious from a minimal change in the viscosity of the mass processed according to this invention.

 At the beginning of the oxygen delignification stage, the permanganate mass number ranges from 10 to 26 depending on the type of wood (for example, when cooking according to Kraft, approximately 10-14, preferably 12.5 for hardwoods, for conifers from 20 to 24, preferably 21 ), whereas after oxygen delignification the permanganate number is usually in the range from 5 to 10.

 In FIG. 2 shows in schematic form the processing method of the present invention. Using the steps presented therein, a preferred scheme is depicted, allowing to extract the maximum benefits from this invention. Wood chips 2 enters the digester 4, where it is cooked in a mortar, for example, sodium hydroxide or sodium sulfide. From the brewing unit 4, an unbleached mass 8 and black liquor 6 are supplied, which contain the products of the reaction of dissolution of lignin. The unbleached mass is processed in washing plants, which preferably include a blow tank 10 and a washing apparatus 12, where the residual working solution contained in the mass is removed. A wide variety of washing methods for unbleached pulp are known, for example, diffusion washing, rotational washing under pressure, horizontal filtration and dilution and extraction. All of these methods are within the scope of this invention. In addition, it is very common to sort unbleached pulp both before and after the washing step to remove large pieces of ground wood for special processing.

 Next, the washed wood is fed to a processing unit 14, where it is treated with an alkaline solution, while its consistency is maintained at a level of less than about 10% and preferably less than 5%. The process according to this invention also includes means for supplying fresh caustic 16 to the processing stage to maintain the desired level . The treated mass 18 is fed to the thickening unit 20, where the consistency of the mass is increased, for example, by pressing, at least up to 20 wt. and preferably from 25 to 35%. Liquid 22 removed from the thickener 20 is desirably returned to the washing unit 12 for further use. The highly pressed “pressed” unbleached mass 24 obtained in the thickener 20 is sent to the oxygen delignification reactor 26, where it is in contact with oxygen gas 28. It is desirable to pass the delignified unbleached mass 30 through the blowing tank 32 and then direct it to the second washing unit 34, where the mass washed with water to remove dissolved organic compounds and obtain a high-quality slightly colored mass 36. It is advisable to return at least part of the effluent 38 from the washing step to washing installation 12 for use in it. In turn, the wastewater 13 from the washer 12 can be recycled individually or together with the effluent 38, or part thereof, to the blow tank 10 or the black liquor line 6. The partially delignified mass obtained during the oxygen delignification step can be sorted to remove beams from it fibers not previously separated for further processing, for example mechanical defibration. From there, the mass 36 can be sent to the subsequent stages of finishing to get a completely bleached product.

According to the most preferred method of this invention depicted in FIG. 2, for the most successful application of ozone bleaching, Kraft wood cooking can be carried out before the low consistency modified alkaline treatment and high consistency delignification (O _m ) procedure described above. As noted above, when using coniferous species, this combination leads to a mass with a permanganate number of approximately or less than 10, preferably 9, and a viscosity of more than 13. Alternatively, it is also possible to cook wood using a kraft process with the addition of anthraquinone, followed by the step typical oxygen delignification (i.e., when a highly consistent alkaline treatment is performed, followed by a highly consistent oxygen delignification), resulting in a mass with similar properties tvam. Instead of cooking using the Kraft process with the addition of anthraquinone, extended delignification processes can be used, followed by the standard oxygen delignification step to obtain a mass with the desired properties. Also very effective, although less preferable from the point of view of increased costs, is the combination of the Kraft process with advanced delignification technology, for example, the Camura MCC, Belois RDN and Sands processes described in the introductory part of this description, followed by a typical oxygen delignification.

 You can apply a variety of combinations of the stages of cooking and delignification, provided that they allow you to achieve the above permanganate number and viscosity before the ozone stage.

 It is generally undesirable to use Kraft type cooking in the framework of this invention with subsequent typical oxygen delignification, with the exception of some hardwoods such as aspen, which are relatively easy to delignify and bleach, because using the above typical technologies usually require more ozone at the ozonation stage, which is accompanied by a large cellulose breakdown.

Thanks to the application of this invention, it is possible to reduce ozone consumption by using several alternative methods, for example, a standard Kraft process followed by a modified oxygen delignification step (O _m ), or a modified Kraft process with extended delignification (for example, Camura processes MSS, Belois RDN and Sands), followed by the stage of a typical oxygen delignification (O), or a Kraft process with the addition of anthraquinone, followed by a stage of a typical oxygen delignification tion (O) discussed above.

An even greater reduction in ozone consumption can be achieved by using a modified Kraft process with extended delignification (Kamyur MSS, Belois RDN and Sands) followed by a stage of modified oxygen delignification (O _m ), or using a Kraft process with the addition of anthraquinone and with extended delignification (Kamyur MSS, Belois RDN or Sands) followed by a typical oxygen delignification (O) step. In the case of applying all of the listed technological solutions in one process, i.e. Kraft process with the addition of anthraquinone with extended delignification (Kamur MSS, Belois RDN or Sands), followed by a stage of modified oxygen delignification (stage О _m ), it is possible to further reduce ozone consumption. In turn, reducing the amount of ozone consumed allows maintaining the viscosity of the mass at an acceptable level.

The above advantages from the modified bleaching stage described above with highly consistent oxygen delignification (O _m ) can be visually shown by comparing the viscosity and permanganate numbers obtained on southern conifers, on related processes with identical conditions. So, the use of the Kraft process in its typical form with typical bleaching with highly consistent oxygen delignification allows one to obtain cellulosic pulp, the permanganate number of which lies in the range from 12 to 14, and a viscosity of approximately 15. Such a permanganate number is too large for subsequent delignification with the use of the ozone stage according to this invention. If you use a typical Kraft process with modified bleaching, then the resulting mass will have a permanganate number less than about 9, while the viscosity of the mass will exceed 12-14. This permanganate number allows you to use the mass at the stage of ozone bleaching according to this invention.

 According to the invention, the next step is ozone delignification and bleaching of an oxygen delignified unbleached mass. Such ozonation is carried out in an ozone reactor, described in detail below in FIGS. 2, 3 and 4. Before treatment of the mass with ozone begins, it should be conditioned in order to guarantee the most effective selective delignification of the mass and minimize the chemical effect of ozone on cellulose . The incoming mass 36 is sent to the mixing tank 40, where it is diluted to a small consistency. Acid 42 is added to the low-consistency mass, for example sulfuric acid, formic acid, acetic acid, etc., to reduce the pH of the mass in mixing tank 40 to about 1-4 and preferably to 2-3. Regulation of the pH value is carried out for the reason that the relative effectiveness of ozone mass bleaching is known to depend on the pH of the mixture. Lower pH values will not have any positive effect during subsequent processing of the mass, when as an increase in pH of more than 4-5 will lead to a decrease in viscosity and an increase in ozone consumption.

 The acidified mass is treated with chelating agent 44 to form complexes of any metals or their salts that may be present in the mass. Such a chelating agent is used to make metals less reactive or harmful in the ozone reactor so that they do not lead to the decomposition of ozone and, accordingly, to a decrease in lignin removal and a decrease in the viscosity of cellulose.

 Chelating agents themselves are known, for example, polycarboxylate and its derivatives, for example, di-, tri-, and tetracarboxylates, amides, etc. Preferred chelating agents for cost and effectiveness reasons are DTRA, EDTA and oxalic acid. The number of chelating agents is approximately in the range from 0.1 to 0.2 wt. by weight of absolutely dry weight, although at high concentrations of metal ions, additional quantities may be required.

 To monitor the effectiveness of the ozone bleaching process, a number of interrelated parameters are used, which include the pH level and the amount of metal salts in the mass. Another very important parameter is the consistency of the mass during the ozone bleaching process. The bleached mass should contain enough water, and water should exist as a continuous phase in all individual fibers, i.e. the fiber must be sufficiently saturated with water. The presence of water in the fibers ensures the transfer of ozone from the gaseous ozone atmosphere to the processing of the outer surface of the fibers, and more importantly, the aqueous phase ensures the transfer of ozone to the less accessible inner region of the individual fibers, thereby guaranteeing a more complete removal of lignin from the fibers.

 On the other hand, the consistency should not be so small that the ozone will begin to dissolve and its chemical decay will begin instead of bleaching the mass.

 It was found that the preferred range of consistency, especially for southern US conifers, is in the range of 25 to 50%, while optimal results are obtained in the range of 35 to 45%. Relative value of delignification indicates the achievement of the preferred results precisely at these limits. a small degree of degradation of cellulose and a noticeable increase in the brightness of the treated mass.

An important control factor is the reaction temperature at which ozone treatment is carried out. The ozone stage can be effectively carried out up to a certain critical temperature at which the reaction causes excessive cellulose degradation. The critical temperature to a large extent depends on the specific type of wood, as well as on the history of previous processing of the mass. Thus, the maximum temperature of the fibers at which the reaction should be carried out should not exceed the temperature at which excessive degradation of cellulose occurs, for southern US softwoods, the maximum is 120-150 ^o F (48.9-65.6 ^o C).

 Ozone gas used during bleaching can be used as a mixture of ozone with oxygen and / or an inert gas, or used as a mixture of ozone with air. The amount of ozone that can be included in the gas is limited by the stability of the ozone in the mixture. For use in this invention are suitable ozone-gas mixtures containing approximately 1-8 wt. ozone in a mixture of ozone with oxygen or about 1-4 wt. ozone in a mixture of ozone with air. In relatively small reactors and with a shorter reaction time, large concentrations of ozone in the mixture can be used to process equivalent amounts of mass, which can reduce the capital cost of equipment. However, ozone-gas mixtures with less ozone are not so expensive to produce and can reduce operating costs.

 Another control factor is the relative weight of the ozone used to bleach a given amount of mass. This amount is determined, at least in part, by the amount of lignin that should be removed during the ozone bleaching process with a balance of the relative amount of cellulose degradation allowed during ozone bleaching. In accordance with the preferred method of this invention, an amount of ozone is used that will react with approximately 50-70% of the lignin present in the mass. At the ozone bleaching stage, there is no removal of all lignin in the mass, which is confirmed by a permanganate number equal to 3-4 after this stage, since the absence of all lignin in the reaction zone can lead to an excessive reaction of ozone with cellulose, which will significantly reduce the degree of cellulose polymerization. According to a preferred method of the invention, the amount of ozone added, calculated on a dry weight basis, is from about 0.2 to 1% in order to achieve a lignin level of 3-4 permanganate numbers. Large quantities may be required if significant amounts of dissolved solids are present in the system.

 The reaction time required for ozone bleaching is determined by the desired rate of completion of the ozone bleaching reaction, as indicated by the complete or almost complete absorption of the ozone used. This time varies depending on the concentration of ozone in the ozone-gas mixture, with relatively more concentrated ozone mixtures reacting much faster, and on the relative amount of lignin that is desired to be removed.

 It is desirable that this time be less than two minutes, however, the procedure may be longer depending on other reaction parameters.

 An important feature of this invention is the uniform bleaching of the mass. This feature is partially achieved by grinding the mass into separate particles of flakes with a sufficiently small diameter and sufficiently low density, so that the ozone-gas mixture completely penetrates almost all fiber flakes, i.e. fiber agglomerate flakes. During grinding, it is not possible to completely separate the mass into individual fibers. As a rule, the particles obtained by grinding have a relatively compact core surrounded by a multitude of protruding fibers. In the present invention, the particle size is determined through the smallest diameter of the relatively rough core. Particles of said mass with increased consistency are crushed to a size that facilitates uniform contact with ozone in the absence of significant destruction of the cellulosic components of the mass.

 The homogeneity of bleaching largely depends on some other process parameters, however, it was found that if the particle size is limited to a maximum of 5 mm, and preferably even less, such as 3 mm, it is easy to achieve uniform processing of a significant number of particles, as evidenced by a small number dark unbleached flakes. If the particle size exceeds 5 mm, the bleaching is heterogeneous, as evidenced by a large number of dark unbleached centers. Hence, the importance of such grinding, when the size of the main share of flakes is on average 5 mm for their uniform treatment with ozone, is understood.

 Another important parameter of the process is related to the fact that during ozone bleaching, the particles should be affected by the ozone bleaching mixture with such a stirring that the ozone-gas mixture has access to all surfaces of the flakes along with the same access of the ozone-gas mixture to all flakes. Stirring the mass in the ozone-gas mixture allows one to obtain higher homogeneity results in comparison with the results obtained with the static layer, when some flakes are isolated from other gaseous ozone flakes and therefore are whitened to a lesser extent.

 Thanks to the movement of the flakes, their more uniform treatment with the ozone-gas mixture is achieved. As a result, the desired amount of lignin is removed from the mass, without over-breaking the cellulose in the fibers that make up the flakes. Studies have shown that control of the ozone treatment in accordance with this invention through the use of particles with a controlled size, as well as due to their turbulent movement during ozone treatment, provides the finished mass with such variations in whiteness, permanganate number and viscosity that do not exceed 5 % For comparison, we note that in the non-uniform treatment typical of reactors with a static layer (i.e., in reactors where particles are not mixed during ozone treatment), some sections of the layer are excessively sprinkled, while others almost do not react due to the heterogeneity of the flow of gaseous ozone in the reactor with a static layer.

In the case of ozone treatment of a highly consistent mass, special attention must be paid to grinding the fibers of the mass and to the correct contact between the individual fibers and the gas flow, otherwise inhomogeneous ozone bleaching of the fibers will inevitably occur. In this invention, such heterogeneous processing is indicated by the letter "Z". The use of the modified ozone treatment technology of the present invention, when the fibers are ground to a size of about 5 mm or less and come into regular and uniform contact with the gaseous ozone stream, is denoted by Z _m .

 The whiteness of the mass at the outlet of the ozone reactor is at least 50%, usually it is in the range from 50 to 70%, and the whiteness of hardwood usually exceeds 55%. The permanganate mass number of both coniferous and hardwood is in the range from 3 to 4 ( preferably 3.5), which is completely satisfactory for the mass at this stage of the invention.

 In FIG. 2, 3 and 4 depict the device most suited to ozone treatment according to this invention. As mentioned above, the washed mass 36 is sent to the mixer 40, where it is treated with acid 42 and a chelating agent 44. The acidified and chelated low-consistency mass 46 is fed to the thickening unit 48, where excess liquid 50 is removed from it, it can be a double roller press, where the consistency of the mass increases to a predetermined level. At least part of the excess liquid 50 can be recycled to the mixer 40, and the rest sent to the blow tank 32. Next, the obtained highly consistent mass 52 passes through a screw feeder 54, which acts as a gas seal for gaseous ozone, and through a grinding unit 56, for example a baking powder, where the mass is crushed to flakes 60 of a predetermined size, which should not exceed 5 mm, as mentioned above. The crushed particles are introduced further into the dynamic reaction chamber with ozone 58 using a conveyor 62 driven by an engine 64. The conveyor 62 is specially designed to transport and transport particles of mass 60 so that the entire surface of the particles is exposed to the action of the ozone-gas mixture 66 during movement. From FIG. 2 it can be seen that after processing the flakes 60 fall freely into the solution tank 68.

 In FIG. 3 is a cross-sectional view of the ozone reactor 58, from which the arrangement of the particles of mass 60 is seen moving through the reactor by conveyor 62. FIG. 4 shows a preferred cross-section of the conveyor, and a blade structure is used to move the crushed particles through the reaction chamber 58.

 Figure 2 shows that the ozone-treated mass moves in the same direction as the ozone-gas mixture. Alternatively, that part of the mass that needs to be whitened to a greater extent may first come into contact with the newly introduced ozone mixture containing the maximum amount of ozone due to the passage of the ozone-containing gas in countercurrent with mass 60. The mass entering the reactor has the highest lignin content and is initially in contact with outgoing and almost exhausted stream of ozone-gas mixture, which allows to absorb ozone almost entirely. This method is very effective for the selection of ozone from a mixture of ozone with oxygen or ozone with air.

 If ozone 66 is in contact with the mass in countercurrent, as shown in FIG. 2, the remaining gaseous ozone 70 can be regenerated from dilution tank 68. Water 72 is added to tank 68, it reduces the consistency of the mass to a lower level to facilitate the movement of bleached mass 74 by subsequent processing steps, and also acts as a sealant for gaseous ozone.

 From reservoir 68, spent gaseous ozone 70 is sent to a pre-treatment stage 76 with a carrier gas, where carrier gas 78 is added in the form of oxygen or air. The resulting mixture 80 is fed to an ozone generator 82, where the amount of ozone that is required to obtain the desired concentration is generated. Next, the necessary mixture of ozone with air 66 is sent to the ozone reactor 58 for delignification and bleaching of the mass.

 At the end of the ozone bleaching stage, the entire essentially delignified mass 74 is again washed in the washing unit 84, as shown in FIG. 2, whereby at least a portion of the recovered water 86 is recycled to the washing unit 34, thereby significantly improving environmental protection, since the presence of Wastewater.

After ozonation, the bleached low consistency mass 74 contains a reduced amount of lignin and therefore has a lower permanganate number and an acceptable viscosity. The specific permanganate number and viscosity values depend on the treatment to which the mass has been subjected. For example, the mass of conifers from the south of the USA processed according to the standard kraft method is first delignified by the method of modified highly consistent oxygen delignification (O _m ), and then further delignified by ozone, preferably using a modified uniform ozone treatment (Z _m ), this permanganate number is approximately 3-4, and a viscosity of approximately 10. Coniferous mass from the south of the USA, cooked by the Kraft process with the addition of anthraquinone and then modified with a high consistency oxygen bleaching (O _m ) and modified ozone treatment (Z _m ), as a rule, has a permanganate number of approximately 2 and a viscosity of more than 12.

The resulting mass of 74 is noticeably whiter than the initial mass. For example, the mass of southern conifers after cooking has a whiteness of about 15 to 25% after the oxygen bleaching process, the whiteness is about 25 to 45%, and after the ozone bleaching process, the whiteness is about 50 to 70%
Alkaline treatment
Then, the washed mass 88 was combined with a sufficient amount of alkaline material 90 in the reaction tank 92 for alkaline treatment. The mass 88 was subjected to the action of an aqueous alkaline solution for a certain time and at a certain temperature, consistent with the available amount of alkaline material to dissolve in the tank 92 a significant proportion of the lignin remaining in the mass. The alkalization process also increases the whiteness of the mass, usually by 2 points on the CE whiteness scale. Next, the treated mass 94 was sent to a washing unit 96, where an aqueous alkaline solution was washed off the mass in order to remove almost all of the dissolved lignin and, as a result, to obtain a mass practically free of lignin. This step is well known to those skilled in the art, and there is probably no need for any additional comments. An illustration of the preferred alkali parameters of this step can be found in the examples. At least a portion of the recovered alkaline solution is recycled in the washing unit 84. The fact that there is also no wastewater at this stage also contributes to a significant improvement in environmental protection.

In some cases, and especially when it is necessary to achieve the greatest whiteness, the alkaline treatment step can be enhanced by introducing oxygen treatment (E _о ) into it. Any comments on this stage, also well known to specialists, are not required.

 Additional bleaching stages.

 Quite often in the production of paper, the final whiteness, lying in the range from 50 to 65, is completely unsatisfactory. In order to further raise the whiteness to a more preferred range of from about 70 to 95%, the mass is subjected to brightening bleaching, which primarily ensures the transformation of the ligin chromophore groups into a colorless state.

 After alkalizing and washing the mass, it is possible to bleach the alkaline and ozone treated masses; for this purpose, a variety of materials can be used. Figure 2 shows that the washed mass 100 combines with the selected bleach 102 in the bleach tank 104. The preferred bleach is chlorine dioxide or peroxide. After bleaching, the mass 106 is washed with water 114 in the washing unit 108, and the effluent is either recirculated 110 or discharged 112. In the case of recirculation, at least part of the water stream 110 is sent to the washing unit 96. The obtained bleached mass 116 can be collected and used for a variety of purposes .

 One of the main materials that have been widely used in the past and are highly effective at the present time is chlorine dioxide (e) (see Fig. 1). In accordance with this invention, by using an appropriate amount of chlorine dioxide, a high-strength mass can be obtained whose whiteness exceeds approximately 80%. Since the lignin content in the mass entering the chlorine dioxide treatment step is relatively small, bleaching can be carried out in the presence of as little as 0 , 25 to 1% chlorine dioxide based on the weight of the completely dry mass.

 Preferably, chlorine dioxide used in the clarification process does not contain chlorine gas. A less desirable option is when chlorine dioxide is used, which contains insignificant amounts of chlorine gas, which nevertheless does not increase the discharge of harmful substances because a relatively small amount of lignin is present in the mass entering this stage after ozone bleaching. The effluent from the last bleaching step of the present invention, when chlorine dioxide is used, is extremely small and can be safely discharged, as shown in FIG. 2.

 If the discharge of wastewater from the last stage of bleaching with chlorine dioxide is completely unacceptable, then such a stream can be further purified by treatment in the process of membrane filtration as reverse osmosis. With this technology, a clean filtrate is obtained, which can be recycled back to the previous bleaching stages for further use. This reduces the need for clean water. In addition, the volume of concentrated chloride streams obtained after membrane filtration is relatively small.

A case may arise when it is necessary to obtain a mass with extremely high whiteness, for example, 92-95 CEV, then several additional bleaching stages can be applied. Most often, the choice is based on additional alkalization and treatment with chlorine dioxide, i.e. it turns out the bleaching sequence O _m Z _m EDED.

Instead of using chlorine dioxide for the final bleaching, hydrogen peroxide can be used in bleaching bleaching, which is also shown in FIG. In this case, a bleaching cycle is obtained in the complete absence of chlorine (for example, a sequence of the type О _m Z _m ЕР), while chlorinated materials are not formed during the bleaching process, and the leaching product can be recycled without resorting to complex filtering equipment. At the same time, when using peroxides as a bleach, it is necessary to reduce the permanganate mass number from both coniferous and deciduous species to about 6 prior to the ozonation stage so that after the peroxide bleaching stage as a final product a mass with acceptable whiteness is obtained, for example, more than 80% since peroxide is not as effective as chlorine dioxide. In cases where it is necessary to obtain a process in which chlorine or chlorine dioxide is not used at all, peroxide can give satisfactory results.

 The specific types of peroxide clarifiers and their use at this stage are well known, and therefore it will not be difficult for a specialist to select the necessary concentration, type and method of using peroxide. Most preferred is hydrogen peroxide.

 The whiteness of the washed and additionally clarified mass is from 70 to 95%, preferably from 80 to 95%. According to its physical properties, such a mass is close to the mass obtained by standard processes CEDED or OS / DED.

Sewage recycling
In any technological process with pulp, one of the significant factors affecting economic efficiency or the total cost of the process is the neutralization of the filtrate. There is also the problem of access to an appropriate source of water and the problem of treating wastewater before it is discharged.

 To reduce the need for water in this process, it is advisable to recycle as much wastewater as possible. This practice cannot be applied in processes where chlorine or chlorine dioxide is used, since the wastewater produced by these processes contains large amounts of chlorides from the by-products of these chemicals. Therefore, the recycling of such wastewater leads to the formation of chlorides, which, in turn, lead to corrosion of the equipment or to the use of expensive materials. In addition, such wastewater must undergo serious treatment until it is discharged, which requires additional costs associated with equipment and chemicals.

 In FIG. Figure 5 shows that the application of the typical CEDED process or the OS / DED method leads to serious difficulties in the discharge of wastewater obtained at the washing stages, due to the presence of high levels of chlorine-containing compounds in them. It has already been noted above that these flows cannot be recycled, it is advisable to process them before discharge into the environment. Wastewater recycling can be used to reduce the amount of water used. But then the equipment may suffer from accelerated corrosion caused by increased levels of chlorine in the recycled effluent.

In contrast, by using the O _m Z _m ED process of the present invention, the smallest amount of chlorinated material is formed in the wash water, and such water can be safely discharged without violating basic environmental standards. Alternatively, the wastewater can be treated with reverse osmosis to obtain an even cleaner filtrate, which can be recycled to the previous bleaching steps, as shown in the drawing, without an increase in chloride concentration. In the washing step of a substantially lignin-free mass, the mass is separated from the obtained washing water and at least a portion of said washing water is directed to the washing step of the substantially delignified mass. At the same time, at the stage of washing the essentially delignified mass, the mass is washed with water from the washing of the mass essentially free of lignin, the mass is separated from the obtained washing water, and at least a part of the said washing water is directed to the washing stage of the partially delignified mass.

 At the stage of washing the partially delignified mass, the said mass is washed with water from the washing of the essentially delignified mass, the mass is separated from the obtained washing water, and at least a part of the said washing water is directed to the mass washing step.

In addition, at the stage of washing the mass, the said mass is washed with water from washing the partially delignified mass, the mass is separated from the obtained washing water, the said washing water is collected and concentrated before calcination in the recovery boiler. If it is desired to carry out the bleaching stage D, then additional steps can be taken to reduce the need for chlorine dioxide. So, you can use stage E _about , contributing to the achievement of higher degrees of whiteness, although the use of sodium hydroxide and oxygen leads to increased costs. In addition, industrial technologies for producing chlorine dioxide are known in which residual chlorine levels are minimized (for example, process R8 with process R3). It is such chemicals with a reduced level of chlorine that are preferably used in step D to reduce the level of chlorides in wastewater.

In order to obtain additional significant advantages over the known technical solutions, it is possible to use the process O _m Z _m EP instead of the O _m Z _m ED process, and the amount of chlorinated compounds obtained is reduced. As a result, wastewater can be recycled without fear of the problem associated with an increase in chloride concentration in wastewater streams.

 So, the method according to this invention has significant advantages in terms of reducing the volume of wastewater, COD, BOD and chlorinated organic compounds, as well as in the sense of color. Since the wastewater from the washing stages contains significantly less chlorides in comparison with the known processes where chlorine is used, chlorinated organic compounds or gases that require treatment before discharge to the atmosphere will not enter the ventilation systems.

 For a more complete understanding of the principles of this invention, consider the following examples, presented solely for illustrative and not restrictive purpose. Unless otherwise indicated, everywhere in the examples the percentage of chemicals is calculated by weight of absolute fiber (OD). In addition, the specialist should be clear that the exact achievement of a given level of whiteness is difficult, since a deviation of whiteness within plus or minus 2% of a given value is considered acceptable. In all examples where Stage D is present, with the exception of Example 11, a chlorine dioxide solution with a ratio of dioxide to gaseous chlorine of 6: 1 was used.

Example 1 (comparative). Incense pine wood chips were periodically cooked under the conditions indicated in Table I to obtain a mass according to a typical Kraft process. The resulting mass had a permanganate number of 22.6 at a viscosity of 27.1 cP. The mass was then subjected to a typical oxygen treatment (Tables II and V), followed by bleaching to a final whiteness of 83 GEB using a typical OS / DED sequence (Table III) and an OZ _m ED bleaching sequence (Tables IV and V). The ozone bleaching stage was carried out with a consistency of 35% and an ozone intake of 0.61%
As can be seen from the following tables VI and VII, as a result of bleaching OZ _m ED under these conditions, a mass is obtained that has acceptable strength properties in comparison with a mass processed by OS / DED to a brightness of 83%. Under these conditions, the mass of OZ _m ED has ultimate viscosity 9.7 cP. Strength properties were measured on a mass of OZ _m ED, when the final supply in stage D was 2.5%. The desired whiteness was achieved only with an excess charge of chlorine dioxide. From the reaction of the mass OZ _m E to the treatment with chlorine dioxide, it can be seen that a large whiteness can only be achieved by increased ozone supply, which leads to a significant loss of viscosity and strength.

 Example 2. In a laboratory cooking apparatus of periodic action according to the Kraft method with the addition of anthraquinone, an unbleached mass was obtained from incense pine wood chips, see Table VIII. The permanganate number of the obtained unbleached mass was 18.3, and the viscosity was 20.6 cP. During cooking by the Kraft process with the addition of anthraquinone, a mass with a significantly lower lignin content was obtained than that of Example 1, as evidenced by the permanganate number, while there was no deterioration in the strength of the mass, as evidenced by its viscosity.

Then, the unbleached mass was bleached using the typical OS / DED sequence and the OZ _m ED sequence shown in Tables II, III, IV, and V to obtain the desired whiteness in 83% CEB. Due to the use of the Kraft process with the addition of anthraquinone, the required mass was obtained with a small permanganate number and the desired viscosity properties required for ozone bleaching. The ozone bleaching stage was carried out with a consistency of 35%, with an ozone input of 0.35% at the last D stage, 1.6% ClO ₂ was used to achieve the desired whiteness.

 As can be seen from the following Tables IX and X, there is an improvement in the optical properties measured by the bleaching reaction at the last stage of chlorine dioxide treatment, while the strength properties also remain satisfactory in comparison with the values obtained by the OS / DED method.

Example 3 (comparative). The unbleached mass obtained by the Kraft process and having a permanganate number of approximately 24 was pressed to a consistency of approximately 30-39 wt. to obtain a highly consistent mat. This mat was sprayed with a 10% sodium hydroxide solution in an amount sufficient to obtain approximately 2.5 wt. sodium hydroxide based on the weight of dry matter. To dilute, water was added in an amount sufficient to obtain a consistency of approximately 27%. Further, the mat from a highly consistent unbleached mass was oxygen delignified under the following conditions: temperature 110 ^° C, duration 30 minutes, overpressure oxygen 80 psi.

 Example 4. The unbleached mass from Example 3 was introduced into the treatment tank together with a sufficient volume of a 10% NaOH solution to carry out a 30% NaOH addition based on the dry weight. In order to achieve an unbleached mass of approximately 3 wt.% In the treatment tank. enough water was added there. The mass and aqueous sodium hydroxide solution were thoroughly mixed at room temperature with a tape-screw mixer for approximately 15 minutes. The treated unbleached mass was then pressed to a consistency of approximately 27 wt. After pressing, the amount of sodium hydroxide in the fibers was approximately 2.5% as in Example 3. Next, the treated unbleached pulp was delignified in accordance with the oxygen delignification procedure described in Example 3. Comparative data are presented in table XI.

From a comparison of the data obtained in examples 3 and 4, it can be noted that as a result of applying the preferred method according to this invention using a low-consistency alkaline additive, followed by a highly consistent oxygen treatment (O _m ), a bleached mass with a greater delignification (less permanganate) was obtained number) than by known methods, and no significant change in the strength properties was observed.

 Since the mass obtained by this process has a lower permanganate number, the subsequent bleaching steps can be adjusted to a mass with a lower lignin content and higher whiteness. Therefore, for the bleaching stages in this case, less bleaching agents are required, or a shorter bleaching time is compared with the mass not treated according to this invention.

Example 5. The mass obtained from pine by the process of O _m from example 4 of the present invention, compared with the mass obtained by the standard method (O), (ie, in the absence of a stage of low-alkaline treatment). The average caustic dosage for highly consistent oxygen delignification of the unbleached mass was 45 pounds per tonne dry matter (lb-t) or 2.3%. At this level, the average decrease in permanganate number after the oxygen delignification reactor was 10 units. At the same level of caustic supplied to the mass in accordance with the preferred processing step, the average decrease in permanganate number during delignification was 13 units, i.e. there is a 30% improvement over the standard method.

 The preference of the proposed delignification can also be shown by comparing the viscosity of the mass. The average permanganate number and viscosity of a typical mass is 12.1 and 14.4 cP, respectively. According to the preferred processing method of this invention, the average permanganate number at the same viscosity (14.0 cP) was 8.3.

 The delignification selectivity can also be expressed through a change in viscosity as a function of a change in the permanganate number between the unbleached mass and the correspondingly processed one. When the change in permanganate number begins to exceed 10 units, a very rapid drop in oxygen delignification selectivity occurs. A decrease in selectivity is observed with a rapid increase in viscosity changes for a given change in permanganate number. For example, when the change in permanganate number is 12 units, it can be expected that the corresponding change in viscosity will be from 12 to 13 cP. In contrast, with the same change in the permanganate number (12) obtained on the delignified mass treated according to this invention, the change in viscosity was approximately 6 cP. A change in viscosity with a change in permanganate number occurs almost constantly before the change reaches 16-17 units for the mass obtained by the preferred processing method. The results are presented in table XII.

Example 6. At a 600 t / day production facility for producing fake pine thin paper, a pulp was made using a modified oxygen delignification process (O _m ) under the conditions indicated in Table II in combination with the uniform alkaline treatment described in Examples 4 and 5 and under the conditions presented in table XIII below. The properties of the mass obtained at the stage of oxygen treatment according to the new method required the addition of a bleaching process using ozone described in the preferred embodiment of the invention. After the oxygen treatment step, the permanganate mass number was 7.9 (after the typical oxygen treatment step, the permanganate number is about 12). The viscosity of the delignified mass was approximately 15 cP, while its significant decrease was not observed after the delignification carried out by the method of the modified oxygen process. Next, the mass was subjected to further bleaching using ozone, using one of the implementation options in order to obtain a mass having the required final strength and optical properties.

 Mass bleaching using the C / DED method was completed in the laboratory, as shown in Table XIV and used to compare properties.

 Stage ozone bleaching was carried out on a pilot plant, depicted in figure 2. The operating conditions of the experimental reactor are presented in table XV.

Then, the bleached mass in the experimental setup was treated with alkali and chlorine dioxide in the laboratory conditions described in Table V, and as a result, the finished bleached mass with a given whiteness was obtained. Download at the last D stage was only 1.0% ClO ₂ .

 Tables XVI and XVII present data for comparing the strength and optical properties of the mass separated using ozone and the mass processed according to the standard OS / DED method.

 Example 7. To give an additional example of the usefulness and applicability of the process according to this invention, using ozone in the pilot plant described above in example 6, fiber bleaching from southern deciduous species, consisting mainly of eucalyptus and oak, was performed. The mass obtained at the working unit 600 turbojet engine, after the stage of a typical oxygen treatment was treated with ozone in the experimental reactor.

 The permanganate mass number after the oxygen stage was 5.7, and the viscosity was 14.1.

 Part of the mass from the oxygen treatment stage was bleached according to the typical C / DED sequence in laboratory conditions to obtain comparative data. The conditions in the C / DED sequence are given in Table XVIII.

The processing conditions in the ozone reactor are presented in table XIX. The mass obtained from the pilot plant after stage Z _m , was finally subjected to bleaching at typical stages E and D to the desired whiteness, the conditions at these stages are presented in table. XX. The ClO ₂ charge in step D was only 0.35% of the dry matter. Strength and bleaching properties are given in comparison in table XXI and XXII.

Example 8. Comparative tests similar to the tests of example 5 were carried out with a mixed mass of hardwood, which included mainly eucalyptus and oak. In this case, it was also found that a much larger decrease in the permanganate number in the oxygen delignification reactor using a modified oxygen process (O _m ) is achieved compared to a typical oxygen treatment (O). The average caustic dosage for hardwood was 27 lbs / ton, or 1.4%. The decrease in permanganate number at the oxygen treatment stage was 5 units. When using caustic at the same level in a modified oxygen treatment process, an average decrease in permanganate number by 7.3 units was observed, i.e. almost 50% increase
The same advantage in delignification selectivity can be shown by comparing the viscosity of the mass. The average permanganate number and viscosity were 7.6 and 16 cP, respectively. According to the present invention, a permanganate number of 6 was obtained at a viscosity of 17.7. In addition, it was found that the permanganate number at the same mass viscosity as that of the untreated material (16 cP) was 5.8.

 The delignification selectivity can also be expressed in terms of the change in viscosity as a function of the change in the permanganate number of the unbleached mass and the mass passed through the modified oxygen treatment. When comparing the mass after a typical treatment with the mass obtained according to this invention, a significant decrease in delignification selectivity was noted with an increased degree of delignification. When the permanganate number changed by 4 units, the average viscosity change was 4 cP for the mass produced by the standard process. In contrast, the change in permanganate number in the mass obtained by the modified oxygen method with the same change in viscosity was 7 units. Thus, the selectivity of the modified method was 1.8 units of the permanganate centipoise number, the selectivity of the typical process was 1 unit, the permanganate number was centipoise, i.e. an increase of 80%. The results are presented in table XXIII.

 Example 9. In the experimental setup, a series of experiments were carried out on the mass obtained from the installation of 600 tons / day for the production of thin paper, with a typical stage of oxygen delignification (O). The experiments should illustrate the effect of pH during ozone bleaching on southern hardwoods. The operating conditions of the reactor were maintained constant, they are listed in Table XIV, the only variable was only the pH at the ozone stage.

 From the table. XXV, one can see how great the influence of pH is during ozone bleaching, and low pH values improve the selectivity of the bleaching process.

Example 10. It is of interest to cite a number of properties illustrating the positive influence of the OZ _m ED process in the production of fully bleached pulp. Typical operating data and wastewater measurements were collected from various operating plants, where CEDED and OS / DED pulp bleaching methods were used. These values were compared with values belonging to the wastewater obtained by the method OZ ED _m using a weight ED O _m and wastewater obtained in Example 1. Data on typical procedure SEDED cm. Table. XXVI, data on the standard OS / DED methodology, see Tables II and III, data on the OZ _m ED methodology, see Tables IV and V.

Note that in the CEDED method, wastewater is a combination of wastewater from stages C, E ₁ , D ₁ , K ₂ and D ₂ . In the OS / DED method, wastewater is a combination of wastewater from stages C / D, E and D, and wastewater in the OZ _m ED method is only wastewater from stage D. As can be seen from the table. XXVII, the ozone bleaching method can significantly reduce the impact of wastewater from the bleaching stage on the environment. To determine the color, the EPA 110.2 method was used. From the obtained data it can be seen that this invention allows to obtain wastewater with a dye content of not more than 2 pounds per ton, the value of BOD ₅ does not exceed 2 pounds per ton and the amount of organic chlorides in the amount of not more than 2, and preferably less than about 0.8.

Example 11. The mass of pine obtained by the Kraft process, was bleached using three modifications of the main method OZED. In the first case (Z _m ED), the mass was bleached in accordance with the data in Tables IV and V during a typical oxygen treatment, a modified treatment with ozone, alkali and treatment with chlorine dioxide according to sequence R-3 with a ClO ₂ / Cl ₂ ratio of 6.1. In the second case, a modified oxygen process (O _m ) was used, with R-3 chromium dioxide being used at the last stage. In the third case, a modified oxygen process (O _m ) was also used, while in the last stage a solution of chlorine dioxide in a ratio of 95: 1 in R-8 was used. From Table XXVIII, a positive environmental effect is seen when using the modified oxygen process (O _m ). The positive effect of the R-8 bleaching solution is also visible.

Example 12. According to the Kraft process and the Kraft process with the addition of anthraquinone from incense pine, pulp was obtained in accordance with the conditions given in Tables I and VIII above. The mass was subjected to typical as well as modified oxygen delignification according to the conditions of examples 4 and 5 in order to show the effect of a combination of these processes (for extended delignification with minimal impact on the strength of the mass) during ozone bleaching. From the table. XXIX shows that these processes give an additive effect. It is possible to achieve very small permanganate numbers when acting according to the method of O _m Z _m E with little effect on the final viscosity. Conversely, it is possible to significantly reduce the amount of ozone required to achieve the permanganate number 3.5 specified in the O _m Z _m E method for the previously described bleaching processes. In addition, due to the additive effect, a mass is obtained which can be whitened completely using the O _m Z _m EP method, where a very small permanganate number is required at the peroxide stage.

Example 13. A mass of conifers, such as frankincense, was bleached to a given whiteness of 83 GEB using the standard CEDED method presented in Table. XXVI, using the standard OS / DEDE methodology presented in Table. II and III, and using the OZ _m ED method presented in Tables IV and V. In order to test the ability to remove contaminants using this method, compared with bleaching using the CEDED and OS / DED methods, the ground polluted material was added to the unbleached mass based on wood at a level of 0.75 wt. In all three methods, the properties for removing contaminants were equivalent when measured by the effective black area, bark and wood chips.

 Example 14. This example illustrates the applicability of the process according to this invention in ozone bleaching. Bleached mass can be obtained with a variety of whiteness values due to appropriate combinations of ozone and chlorine dioxide while minimizing environmental pollution and reducing operating costs. From the above table. XXX shows that with various combinations of ozone and chlorine dioxide, it is possible to obtain products whose whiteness exceeds 65% GEB, while maintaining the required strength properties.

 Although it can be seen from the foregoing that the invention achieves its goals, it is obvious to the skilled person that various modifications and variations are possible.

Claims (98)

 1. The method of obtaining bleached pulp, including chemical digestion of lignocellulosic material to obtain mass, oxygen delignification of the mass to remove a significant portion of lignin, a combination of the stages of chemical digestion and oxygen delignification, chosen so as to obtain an intermediate pulp containing a given amount of lignin and having a given amount viscosity, and ozone delignification of the intermediate mass, characterized in that the ozone delignification is carried out when regulated and the consistency and pH of the intermediate mass to certain values, crush the intermediate mass to separate particles of sufficiently small diameter and low enough density to facilitate the substantially complete penetration of ozone into most particles and bleach the particles with ozone in an amount sufficient to remove a significant portion, but not total lignin due to the close contact and mixing of individual particles with ozone over time and temperature sufficient to achieve essentially uniform delignification and bleaching most of the particles to form a bleached mass, wherein a certain amount of lignin in the intermediate mass is such that after ozone delignification, a certain whiteness is achieved in the bleached mass, while a certain viscosity of the intermediate mass is high enough to compensate for the decrease in viscosity during ozone delignification and produce bleached masses of a certain strength.  2. The method according to claim 1, characterized in that the particle size of the mass is less than approximately 5 mm  3. The method according to claim 1 or 2, characterized in that it contains chemical boiling of lignocellulosic material according to the Kraft process, the Kraft process with the addition of anthraquinone or extended delignification.  4. The method according to claim 1, characterized in that at the stage of oxygen delignification a mass is formed with a consistency of small to medium, the mass is treated with an aqueous solution of alkaline material with a consistency of small to medium for a certain time and at a certain temperature depending on the amount of alkaline material for essentially uniform distribution of alkaline material throughout the mass, increase the consistency of the mass to high and subject the resulting highly consistent mass of highly consistent oxygen deligne fication for the intermediate pulp.  5. The method according to claim 1, characterized in that the pulp is obtained, a certain whiteness of which is at least 50%, a certain viscosity at a certain strength exceeds 10 cPs, and a permanganate number equal to or less than 10 indicates a predetermined amount of lignin in the intermediate mass , and the set viscosity exceeds 13 cPs.  6. The method according to claim 1, characterized in that the lignocellulosic material is coniferous wood, a certain whiteness is at least 50%, a certain viscosity at a certain strength exceeds 10 cPs, a permanganate number of approximately 7 10 indicates a predetermined amount of lignin in the intermediate mass , and the set viscosity exceeds 13 cPs.  7. The method according to claim 1, characterized in that the lignocellulosic material is deciduous wood, a certain whiteness exceeds 55%, a certain viscosity at a certain strength exceeds 10 cPs, a permanganate number of approximately 5 8 indicates a predetermined amount of lignin at an intermediate mass, and a predetermined viscosity exceeds 13 cps.  8. The method according to PP.5, 6 or 7, characterized in that the amount of lignin contained in the mass after oxygen delignification is determined through the permanganate number of the bleached mass, equal to approximately 3 4.  9. The method according to claim 1, characterized in that it further comprises bleaching the mass after ozone delignification using a brightener to increase the whiteness of the bleached mass.  10. The method according to claim 9, characterized in that it further comprises the step of combining the bleached mass with a sufficient amount of alkaline material in an aqueous alkaline solution at a certain temperature, correlated with the available amount of alkaline material to dissolve a significant portion of the lignin remaining in bleached mass, and then part of the aqueous-alkaline solution is removed in order to essentially remove all dissolved lignin from there and to obtain a leached mass before bleaching with a clarifier.  11. The method according to claim 9, characterized in that the clarifier is chlorine dioxide or peroxide.  12. The method of delignification and bleaching of lignocellulosic material to a pulp having a certain whiteness and a certain strength, which is indicated by a certain viscosity, characterized in that the lignocellulosic material is digested to obtain a mass having a first permanganate number and a first viscosity, and the resulting mass is delignified with oxygen for to obtain a partially delignified mass having a second permanganate number less than the first permanganate number and sufficient for For further delignification by ozone of the partially delignified mass while maintaining the viscosity at a level at which there is no significant chemical destruction of the cellulose components of the said partially delignified mass due to oxygen delignification, the partially delignified mass of ozone delignification is subjected to supplying sufficient ozone for a sufficient time, linked to the amount of ozone supplied to said partially delignified mass for p radiation delignified pulp having a third K No. significantly less said second K No. of said partially delignified pulp and the brightness value exceeding the brightness value at the partially delignified pulp while maintaining viscosity at a certain level, prevents the destruction of the cellulose components of the mass to prevent substantial reduction of its strength.  13. The method according to p. 12, characterized in that the partially delignified mass contains such an amount of lignin that after ozone delignification allows a certain whiteness to be achieved, and also has a viscosity that is high enough to compensate for the viscosity drop during ozone delignification to achieve certain strength of the delignified mass.  14. The method according to p. 12, characterized in that it further comprises the step of combining the delignified mass with a sufficient amount of alkaline material in an aqueous alkaline solution at a certain temperature, linked to the amount of alkaline material to be connected, to dissolve a significant part of the lignin remaining in the delignified mass, and then part of the aqueous-alkaline solution is removed in order to remove almost all of the dissolved lignin and obtain a mass practically free of lignin.  15. The method according to 14, characterized in that it further comprises the step of bleaching virtually no lignin-containing mass to such a brightness value that is significantly higher than the brightness value of the delignified mass.  16. The method according to p. 12, characterized in that the viscosity of the said partially delignified mass support more than approximately 13 SP.  17. The method according to clause 16, characterized in that the decrease in viscosity of the said partially delignified mass is maintained at a level of 30% or less from the first value.  18. The method according to p. 12, characterized in that the said lignocellulosic material is deciduous wood.  19. The method according to p. 18, characterized in that the said first permanganate number is in the range of 10 14.  20. The method according to p. 18, characterized in that the said first viscosity value is in the range of 21 to 26 cps.  21. The method according to p. 18, characterized in that the said second permanganate number is in the range of 5 to 8.  22. The method according to item 21, wherein the said third permanganate number is less than 5.  23. The method according to p. 12, characterized in that said lignocellulosic material is coniferous wood.  24. The method according to p. 23, characterized in that the said first permanganate number is within 20 24.  25. The method according to item 23, wherein the said second permanganate number is in the range of 7 to 10.  26. The method according to p. 12, characterized in that the oxygen delignification treatment is carried out on a mass of medium consistency.  27. The method according to p. 12, characterized in that the partial delignification step further comprises the steps of treating said mass with a certain amount of alkaline material in an aqueous alkaline solution for a certain time and at a certain temperature, coupled to the amount of alkaline material, in order to achieve a uniform distribution of alkaline material throughout the mass, increase the consistency of the mass at the end of the processing step, subject the mass to a high consistency of highly oxygenated th delignification to obtain a partially delignified mass.  28. The method according to p. 12, characterized in that the viscosity of the specified delignified mass support more than approximately 10 SP.  29. The method according to p. 28, characterized in that the decrease in viscosity of the specified delignified mass is maintained at a level of 30% or less of the viscosity of the specified partially delignified mass.  30. The method according to p. 12, characterized in that the additional stage of delignification further comprises the steps of increasing the consistency of said partially delignified mass, crushing said mass of increased consistency to particles of a certain size, and uniformly contacting said crushed mass with said sufficient ozone at the passage of mass in this process.  31. The method according to p. 30, characterized in that the particles of said mass with increased consistency are crushed to a size that facilitates uniform contact with ozone in the absence of significant destruction of the cellulosic components of the mass.  32. The method according to p, characterized in that the mass is ground to a size of approximately 5 mm and moved through the process in such a way as to avoid uneven supply of ozone to the mass.  33. The method according to p, characterized in that the ground mass is advanced in the same direction as ozone.  34. The method according to p, characterized in that the ground mass is advanced countercurrently to ozone.  35. The method according to clause 15, characterized in that the mass, practically not containing lignin, is bleached with chlorine dioxide.  36. The method according to p. 15, characterized in that the mass, practically not containing lignin, is bleached with peroxide. 37. The method according to p. 35 or 36, characterized in that at the stage of bleaching the whiteness of the mass, practically not containing lignin, increases at least to about 70%
38. The method according to clause 35 or 36, characterized in that at the stage of bleaching the whiteness of the mass, practically not containing lignin, increases at least to about 80
39. The method according to p. 35 or 36, characterized in that at the stage of bleaching the whiteness of the mass, practically not containing lignin, increases at least to about 90%
40. A method for delignifying and bleaching lignocellulosic material, characterized in that the lignocellulosic material is partially delignified to obtain a mass having a permanganate number of about 10 or less and a viscosity of more than about 13 cPs, and then the mass is delignified with some sufficient ozone for a time sufficient to obtain a substantially delignified mass having a permanganate number of about 5 or less, a viscosity of more than about 10 cPs and a whiteness of at least approximately 50% flax
41. The method according to p. 40, characterized in that the lignocellulosic material is coniferous and partially delignified to a mass having a permanganate number of approximately 7 10 and a viscosity of more than 13 cPs before the subsequent ozone delignification.  42. The method according to paragraph 41, wherein the softwood mass has a permanganate number of about 3 4, a viscosity of more than 10 cPs and a whiteness of at least 50% after said additional ozone delignification.  43. The method according to p, characterized in that the lignocellulosic material is deciduous wood and partially delignified to a mass having a permanganate number of approximately 5 8 and a viscosity of more than 13 cPs to additional ozone delignification.  44. The method according to item 43, wherein the hardwood mass has a permanganate number of about 3 4, a viscosity of more than 10 centipoise and whiteness exceeds 55% after additional ozone delignification.  45. The method according to p. 40, characterized in that it further comprises the steps of connecting mainly delignified mass with some effective amount of alkaline material in an aqueous alkaline solution for a certain time and at a certain temperature, linked to the amount of alkaline material for dissolution of a significant part of the lignin remaining in the mass, a part of the aqueous-alkaline solution is removed to remove almost all of the dissolved lignin and obtain a mass practically free of lignin.  46. The method according to item 45, characterized in that at the said stage of extraction, the whiteness of the mass increases by about 2 points on a scale of GE whiteness. 47. The method according to p. 45, characterized in that it further comprises the step of bleaching a mass practically free of lignin with chlorine dioxide or peroxide to increase the brightness of at least up to about 70%
48. The method according to item 47, wherein the whiteness is increased at least to about 80%
49. The method according to item 47, wherein the whiteness is increased at least to about 90%
50. The method according to item 45, wherein the lignocellulosic material is partially delignified using oxygen delignification treatment.  51. The method according to p. 50, characterized in that the oxygen delignification treatment is carried out on a mass of medium consistency.  52. The method according to item 45, wherein the lignocellulosic material is partially delignified at the stages in which a mass of comparative low consistency is formed of approximately less than 10 weight. treat the low-consistency mass with a certain amount of alkaline material in an aqueous-alkaline solution for a certain time at a certain temperature, linked to the amount of alkaline material to evenly distribute the alkaline material throughout the mass, increase the consistency of the mass to at least about 20 weight. at the end of the processing step, the high consistency mixture containing the alkaline material is subjected to highly consistent oxygen delignification to obtain a partially delignified mass having a permanganate number of about 9 or less and a viscosity of about 13 cPs or more. 53. The method of delignification and bleaching of lignocellulosic material, characterized in that it comprises stages in which lignocellulosic material is partially delignified to obtain pulp, and the mass consistency is reduced to less than 10 weight. treat the low-consistency mass with a certain amount of alkaline material in an aqueous-alkaline solution for a certain time at a certain temperature, associated with the amount of alkaline material to achieve a substantially uniform distribution of alkaline material throughout the mass; increase the consistency of the mass to at least 20 weight. at the end of the processing stage, the mass of increased consistency, containing alkaline material, is subjected to highly consistent oxygen delignification to obtain a partially delignified mass with a permanganate number of approximately 9 or less and a viscosity of more than 13 cPs, then the said partially delignified mass is delignified with an effective amount of ozone over time, sufficient to obtain a substantially delignified mass having a permanganate number of about 5 or less, a viscosity more than 10 cPs and a whiteness of at least about 50% connect the practically delignified mass with some sufficient alkaline material in an aqueous alkaline solution for a certain time at a certain temperature, linked to the amount of alkaline material to dissolve a significant part of the lignin remaining in the mass a portion of the aqueous alkaline solution is recovered in order to essentially remove all dissolved lignin and obtain a mass substantially free of lignin, and practical bleach Ski containing no lignin mass to increase its whiteness of at least about 70%
54. The method according to item 53, wherein the whiteness is increased to at least about 80%
55. The method according to item 53, wherein the whiteness is increased to at least about 90%
56. The method according to item 53, wherein the lignocellulosic material is coniferous and delignified to a permanganate number of approximately 8 9 and a viscosity of more than 14 cPs before additional ozone delignification.  57. The method according to p. 56, characterized in that the mass of coniferous wood has a permanganate number in the range of about 3 4, a viscosity of more than 10 centipoise and a whiteness of at least about 54% upon completion of the aforementioned additional ozone delignification.  58. The method according to p. 53, characterized in that the lignocellulosic material is deciduous wood, and it is partially delignified to a permanganate number in the range of about 6 7 and a viscosity of more than 15 cPs before the said stage of additional ozone delignification.  59. The method according to § 58, wherein the hardwood mass has a permanganate number in the range of about 3 4, a viscosity of more than 10 cPs, and a whiteness of at least about 63% upon completion of said additional ozone delignification.  60. The method according to item 53, wherein the partially delignified lignocellulosic material is obtained using the Kraft process, a Kraft process with the addition of anthraquinone or extended delignification of the lignocellulosic material.  61. The method according to p, characterized in that they reduce the permanganate mass number of increased consistency by at least about 60% during the stage of oxygen delignification, without causing significant harm to the cellulosic components of the mass.  62. The method according to item 53, wherein the mass is subjected to highly consistent oxygen delignification without significant changes in the viscosity of the mass. 63. The method according to item 53, wherein the ratio of the permanganate number to the viscosity of the mass is reduced during the stage of oxygen delignification by at least 25%
64. The method according to item 53, wherein the consistency of the mass treated with an aqueous-alkaline solution prior to the stage of oxygen delignification, lies in the range of approximately 1 to 4.5%
65. The method according to item 53, wherein the consistency of the mass is increased in the range of approximately 25 to 35 weight. to the stage of oxygen delignification.  66. The method according to item 53, wherein the amount of alkaline material distributed throughout the low-mass mass to the stage of oxygen delignification, lies in the range of approximately 15 to 30 weight. when calculating the dry weight of the mass.  67. The method according to p, characterized in that the aqueous-alkaline solution has a concentration of alkaline material lying in the range of approximately 20 to 120 g / l so that the concentration of alkaline material in the low-consistency mass is in the range of approximately 6.5 to 13 g / l 68. The method according to item 53, wherein the step of alkaline treatment is carried out for 1 to 15 minutes at a temperature of 90 150 ^o F (32.3 - 65.6 ^o C).  69. The method according to item 53, wherein the initially formed mass is unbleached, and at least a portion of the liquid obtained from the alkaline solution at the stage of increasing the consistency of the mass is recycled to the alkaline processing stage. 70. The method of delignification and bleaching of lignocellulosic material, characterized in that it comprises the steps of obtaining an unbleached mass with a permanganate number of approximately 10 24 due to the implementation of the Kraft process, a Kraft process with the addition of anthraquinone or extended delignification of lignocellulosic material, reduce the consistency of said masses up to about 1 45 weight. treat the mass of reduced consistency with a certain amount of alkaline material in an aqueous alkaline solution, where the concentration of alkaline material is in the range of about 20 120 g / l over a period of 1 15 minutes and at a temperature of about 90 150 ^o F (32.3 65.6 ^o C ) so that during this stage the concentration of alkaline material in the mass of reduced consistency is in the range of 6.5 to 13 g / l to ensure a substantially uniform distribution of alkaline material throughout the mass, the consistency of the alkali-treated mass is increased blizitelno 35 to 25 wt. subjecting the mass to an increased consistency of highly consistent oxygen delignification without changing the viscosity of the mass to obtain a partially delignified mass with a permanganate number of about 10 or less and a viscosity of 13 cPs or less and a viscosity of 13 cPz or more, while the ratio of the permanganate number of said mass to its viscosity during oxygen delignification decreases by at least about 25%; then said oxygen-delignified mass with an effective amount of ozone is delignified and a time sufficient to obtain a substantially delignified mass with a permanganate number of about 5 or less, a viscosity of about 10 cPs or more and a whiteness of at least about 50% connect the practically delignified mass with a sufficient amount of alkaline material in an aqueous alkaline solution for a certain time and at a certain temperature, linked to a given amount of alkaline material, a part of the aqueous-alkaline solution is removed to dissolve a significant part of the lignin remaining in the mass. and in order to remove practically dissolved lignin from there and obtain a mass practically free of lignin, and essentially lignin-free mass is bleached by increasing whiteness to at least up to about 70%
71. The method according to item 70, wherein the whiteness is increased to at least about 80%
72. The method according to item 70, wherein the whiteness is increased to at least about 90%
73. The method according to p. 70, characterized in that during the stage of oxygen delignification, the permanganate mass number of increased consistency is reduced by at least 60% without significant damage to the cellulosic components of this mass.  74. The method according to item 70, wherein the practically lignin-free mass is bleached with chlorine dioxide or peroxide.  75. The method according to p. 74, wherein the peroxide is hydrogen peroxide.  76. The method according to p. 70, characterized in that it further comprises the step of adding a chelating agent to the mass before the start of ozone delignification in order to make metal ions practically non-reactive to ozone.  77. The method according to p, characterized in that the chelating agent is a DTPA, EDTA or oxalic acid.  78. The method according to p. 70, characterized in that it further comprises the step of adjusting the pH of the mass in the range of approximately 1 to 4 by adding to the mass of a sufficient amount of acidic material prior to the ozone delignification step.  79. The method according to p. 70, characterized in that it further comprises the step of increasing the consistency of the mass to approximately 25 to 50 weight. to the ozone delignification phase.  80. The method according to p, characterized in that the consistency of the mass is increased to approximately 35 to 45 weight. to the ozone delignification phase.  81. The method according to item 70, wherein the said mass is ground to a diameter of less than about 5 mm after oxygen delignification and before ozone delignification. 82. The method according to p. 70, characterized in that during the ozone delignification, the mass is maintained at a temperature of less than about 120 ^o F (48.9 ^o C).  83. The method according to item 70, wherein the ozone is supplied in a mixture containing ozone and oxygen.  84. The method according to p, characterized in that the concentration of ozone in the mixture is in the range of approximately 1 to 8 weight.  85. The method according to item 70, wherein the ozone enters a mixture of ozone with air.  86. The method according to p, characterized in that the ozone concentration is in the range of approximately 1 to 4 weight.  87. The method according to item 70, wherein the partially delignified mass at the stage of ozone delignification is moved in such a way that in fact the whole mass is exposed to ozone.  88. The method according to p, characterized in that the ozone is fed in countercurrent to the displaced mass.  89. The method according to p, characterized in that the ozone is fed in the same direction as the mass to be moved. 90. A method for delignifying and bleaching lignocellulosic material, characterized in that it comprises stages in which the lignocellulosic material is partially delignified by digesting to obtain pulp and the oxygen is delignified to obtain a partially delignified mass with a permanganate number of approximately 10 or less with a viscosity of approximately greater than 13 cPs, a chelating agent is added to said mass so as to render the metal ions contained therein essentially incapable of reactions with ozone, regulate the pH of the said mass within about 1 to 4 by adding a sufficient amount of acidic material, increase the consistency of the said mass to about 25 50% grind the said mass with increased consistency to particles with a diameter of less than about 5 mm, further delignify the said mass with increased consistency by means of a sufficient amount of ozone for a sufficient time, moving the crushed mass in such a way that the whole mass is tested the action of ozone to produce an essentially delignified mass having a permanganate number of about 5 or less, a viscosity of more than 10 cPs and a GE whiteness of at least about 50% combine the practically delignified mass with a sufficient amount of alkaline material in an aqueous alkaline solution for a certain time and at a certain temperature, coupled with the available amount of alkaline material to dissolve a significant part of the lignin remaining in the mass, a part of the aqueous-alkaline solution is removed in order to remove almost all of the dissolved lignin and obtain a practically lignin-free mass, and the practically lignin-free mass is bleached with chlorine dioxide to increase its whiteness by at least about 70%
91. The method according to p. 90, characterized in that the whiteness of the mass is increased at least to about 80%
92. The method according to p. 90, characterized in that the whiteness of the mass is increased to at least about 90%
93. The method according to p. 90, characterized in that the cooking process is carried out at the cooking stage, and at the stage of oxygen delignification the permanganate mass number is reduced by at least about 60% without significant destruction of the cellulosic components of the mass or without a significant change in its viscosity.  94. The method according to p. 93, characterized in that at the stage of cooking carry out a kraft process with the addition of anthraquinone.  95. The method according to p. 94, characterized in that at the stage of oxygen delignification, the permanganate mass number is reduced by at least about 60% without significant destruction of the cellulosic components of the mass or without a significant change in its viscosity.  96. The method according to p. 90, characterized in that at the stage of cooking the cooking is combined with the addition of anthraquinone and expanded delignification, and at the stage of oxygen delignification, the permanganate mass number is reduced by at least about 60% without significant destruction of the cellulose components or without a significant change in the viscosity of the mass. 97. The method according to p. 90, characterized in that at the stage of ozone delignification, the mass is moved so that its temperature is maintained below 120 ^o F (48.9 ^o C).  98. The method according to p. 90, characterized in that the chelating agent and acid are added to the mass in the mixing tank.  99. The method according to p. 98, characterized in that at least a portion of the liquid separated from the mass at the stage of increasing consistency is recycled to the mixing tank.  100. The method according to p, characterized in that the mass is mixed in one direction with ozone.  101. The method according to p, characterized in that the mass is mixed in a countercurrent to ozone. 102. A method for delignifying and bleaching lignocellulosic material, characterized in that it comprises stages in which the lignocellulosic material is partially delignified by cooking to obtain a pulp and oxygen is delignified to obtain a partially delignified mass having a permanganate number of approximately 10 or less and a viscosity of more than 13 centipoise, add a chelating agent to the mass to make the metal ions present in it incapable of reaction with ozone, regulating comfort pH of the said mass in the range of about 1 to 4, adding a sufficient amount of acidic material, increase the consistency of the said mass to about 25 50% grind the mass of high consistency to a diameter of less than about 5 mm, further delignify the mass of high consistency with a sufficient amount of ozone for a sufficient time to obtain essentially delignified mass having a permanganate number of about 5 or less, a viscosity of more than 10 cPs and white well, at least about 50% combines a practically delignified mass with a sufficient amount of alkaline material in an aqueous-alkaline solution for a certain time and at a certain temperature linked to a given amount of alkaline material to dissolve a significant part of the lignin remaining in the mass, a part of the aqueous-alkaline is recovered solution in order to remove essentially all of the dissolved lignin and obtain a mass practically free of lignin, and bleach a mass practically free of lignin with the help of peroxide to increase its whiteness of at least about 70%
103. The method according to p. 102, characterized in that the whiteness is increased at least to about 80%
104. The method according to p. 102, characterized in that the whiteness is increased at least to about 90%
105. The method according to p. 102, characterized in that the cooking of the material is carried out using the Kraft process, and at the stage of oxygen delignification the permanganate number is reduced by at least about 60% without significant damage to the pulp components of the mass or without a significant change in its viscosity.  106. The method according to p. 102, characterized in that the cooking of the material is carried out using a Kraft process with the addition of anthraquinone.  107. The method according to p, characterized in that at the stage of oxygen delignification, the permanganate mass number is reduced by at least about 60% without significant damage to the cellulosic components of the mass or without a significant change in its viscosity.  108. The method according to p. 102, characterized in that at the cooking stage a combination of the Kraft process with the addition of anthraquinone and enhanced delignification is carried out, and at the stage of oxygen delignification, the permanganate mass number is reduced by at least about 60% without significant damage to the cellulosic components of the mass or without a significant change in its viscosity. 109. The method according to p. 90 or 102, characterized in that at the stage of ozone delignification, the permanganate mass number is reduced by at least 50%
110. The method according to p. 90 or 102, characterized in that at the stage of bleaching, the whiteness of the mass is increased to at least 50%
111. The method according to p, characterized in that the whiteness of the mass is increased to at least 83%
112. The method of delignification and bleaching of lignocellulosic material, characterized in that it comprises stages in which the lignocellulosic material is partially delignified by cooking to obtain a mass, the specified mass is washed and the specified mass is delignified with oxygen to obtain a partially delignified mass having a permanganate number of about 10 or less and a viscosity of more than 13 centipoise, wash the partially delignified mass, additionally delignify the partially delignified mass with sufficient full-time amount of ozone for a time sufficient to obtain a substantially delignified mass having a permanganate number of about 5 or less, a viscosity of more than 10 cPs and a whiteness of at least about 50%, washed the practically delignified mass, combine the practically delignified mass with a sufficient amount of alkaline material in aqueous-alkaline solution for a certain time and at a certain temperature, linked to a given amount of alkaline material for dissolution a significant part of the lignin remaining in the mass, a part of the aqueous-alkaline solution is removed in order to remove almost all of the dissolved lignin and obtain a mass practically free of lignin, the mass which is practically free of lignin is washed, this mass is bleached with chlorine dioxide or peroxide to in order to increase the whiteness of the mass to at least about 70% and wash the bleached mass. 113. The method according to p. 112, characterized in that the whiteness is increased to at least about 80%
114. The method according to p. 112, characterized in that the whiteness is increased to at least about 90%
115. The method according to p. 112, characterized in that at the stage of washing the bleached mass, the said mass is washed with clean water and the mass is separated from the obtained wastewater.  116. The method according to p. 112, characterized in that at the stage of bleaching, chlorine dioxide is used, and waste water after bleaching is discharged.  117. The method according to p. 112, characterized in that chlorine dioxide is used at the bleaching stage, and the wastewater after bleaching is treated with reverse osmosis to obtain a treated filtrate, at least a portion of which is directed to the washing stage of a mass practically free of lignin.  118. The method according to p. 112, characterized in that at the stage of bleaching, peroxide is used and at least a portion of the bleached mass is recycled to the stage of washing the mass practically free of lignin.  119. The method according to p. 117 or 118, characterized in that at the stage of washing the mass, practically not containing lignin, said mass is washed with water from washing the bleached mass, the mass is separated from the obtained washing water and at least a portion of said washing water is directed to the step washing practically delignified mass.  120. The method according to p, characterized in that at the stage of washing the practically delignified mass, the mass is washed with water from the washing of the mass essentially free of lignin, the mass is separated from the obtained washing water and at least a portion of the said washing water is directed to the step washing partly delignified mass.  121. The method according to p. 120, characterized in that at the stage of washing the partially delignified mass, the mass is washed with water from the washing of the essentially delignified mass, the mass is separated from the resulting washing water and at least a portion of the said washing water is directed to the mass washing step.  122. The method according to p. 121, characterized in that at the stage of washing the mass, the said mass is washed with water from washing the partially delignified mass, the mass is separated from the obtained washing water, the said washing water is collected and concentrated before calcination in the recovery boiler.  123. The method according to p. 122 or 122, characterized in that at the stage of bleaching using chlorine dioxide with a minimum chlorine content.  124. The method according to p, characterized in that the discharged waste water has a colorant of not more than 2 pounds per ton and the total amount of organic chlorides is not more than about 2.

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