US2022873A - Manufacture of pulp - Google Patents

Description

C. K. TEXTOR MANUFACTURE OF PULP Dec. 3, 1935.

Filed sept. 1e, 192e LMSSQ bss `Patented Dec. 3, 1935 UNITED STATES PATENT OFFICE MANUFACTURE F PULP l l Clinton K. Textor, Cloquet, Minn., ass'ignor to Northwest Paper Company, Cloquet, Minn., al

corporation of Minnesota Application september 1s, 1926, sensi No. 135,930

5 claims. (01.92-1'1) This invention relates'to improvements in wood digestion with normal suliites,` the pulp produced, the recovery of the chemicals in the resulting liquor, and the preparation of the cooking liquor. woods in general, the present application is directed more particularly to the preparation of pulp from woods, such as poplars, gumwoods, maples, oaks, elms, basswood, and all such woods as are generally included under the term hardwoods, and loosely characterized as short-- with sodium sulte liquor, the regulation of the alkali carbonate in the cooking liquor to minimize the corrosionof the equipment without adversely affecting the economy or the efficiency of the process, and the treatment of the so-called black liquor, resulting from the cooking operation so that it may be highly concentrated in eicient tubular evaporators without plugging these evaporators by separation of solids.

Other objects and advantages of my invention will become apparent to those skilled in the art from the following specification. The accompanying gure shows, more or less diagrammatically, one arrangement of apparatus capable of carrying out the process of this invention, but it is to be understood that this illustration is given only for the purpose of'facilitating an explanation ofthe process, and it is not to be construed as a limitation of the invention short of its scope as comprehended in the appended claims.

In 1880, Cross made known the utility of sodium sulfite as a Wooddigestant. Since that time a number of modifications have appeared in the literature and in patents of the United States, Germany, Canada and Great Britain. This art shows that it is consideredrnecessarywhen using an essentially sodium sulte cooking liquor, comprising, principally, the normal sodium salt (NazSOa), to have present caustic soda (NaOH) to dissolve 'the silicates and aluminates found in cellulose bearing materials, such as wood. Schachts German Patents Nos. 122,171 and 131,118 specifically require the presence of small Although these improvements apply to amounts Iof caustic soda. Other investigators have deemed it necessary to have still larger amounts of caustic soda present in the normal sodium sulte cooking liquor, especially when highly resinoufsl Woods are used. I havefound 5 that it is not necessary to have caustic soda (NaOH) present in such a sulte liquor to produce a satisfactory pulp, and that there are certain advantages in operation and certain improvements in the pulp resulting directly from replacing thestronger caustic soda of the prior art with a milder alkali carbonate,l such as sodium bicarbonate, or sodium carbonate.

By my method of cooking, which is described in detail hereinafter, I obtain a high yield or well cooked pulp, in some cases as' high 60% (for short-bered wood) Vof the weight of the wood employed, both being calculated on oven driedV material. The product thus produced is of specially good strength, has an unusuallyy White color for unbleached pulp, and possesses those characteristics essential for the manufacture of strong, durable paper of the highest grade. The pulp is readily bleached to a good white color,

the bleached pulp retaining practically all of the ture and the proportions of material to wood may be varied according to the type of wood, the moisture content of the wood, the kind of pulp desired, and other factors hereinafterdescribed as carried out with aspen.

When cooking a short-bered wood, such as aspen, I prefer to use the following procedure. The digester I is filled to the desired height with 40 aspen, chipped to the usual size, and the cooking liquor is added in volumes similar to those employed for cooking wood by the soda or modied -soda process, as is well known to those versed in the art. I find it desirable to add from one-quarter to three-quarters cubic footof cooking liquor per cubic foot of chips as they lie in the digester, depending upon the moisture content of the wood, the method of circulation, the method of heating, and other factors. The preferred cooking liquor is of such a concentration that there is added for every pounds of oven-dried chips from 30 to 40 pounds of sodium sulte (NazSOa) and 1.3 to 2.6 pounds of sodium bicarbonate (NaHCOs). When it is necessari? to dilute the l cooking liquor for use in the dlgester, a waste black liquor, more particularly described hereinafter, or black liquor washings may be used in place of Water Without any detrimental result. The detailed method of preparing the cooking liquor either as a fresh solution or as a regenerated solution is stated in detail hereinafter in the ensuing description. Such a cooking liquor has been found to have an alkalinity correspondingv bonate to produce the desired safeguard against excessive` corrosion. In using greater quantities of carbonate, no additional advantages have been observed. It vis important that the carbonate used be in addition to the amount of sodium sulte required and not in substitution therefore. The quantity of sodium sulte considered necessary for the process is deemed otherwise important in at least one particular circumstance.' I have found that the preferred liquor in the quantity stated must contain sucient sodium suliite (NazSOs) to prevent the production of black or dark brown pulp by any sodium thiosulfate (NazSzOs) which may be present as an impurity, as hereinafter explained. It appears that if the normal sulte becomes exhausted during the digesting process, the thiosulfate then acts predominantly upon the pulp disadvantageously, producing a dark colored product.

The cooking process is preferablycarried out in an unlined steel digester by a procedure similar to that of the soda or modif-led soda process. By a soda process, I refer to one which employs caustic alkali (NaOH) as the pulping agent, while by a modified soda process I refer to a soda process in which the liquor consists chiefly of caustic alkali but to which either sodium sul. fide (Na2S) or sodium sulte (NazSOs) or other materials have been added. A temperature of 175 to 188 C. may be employed and the corresponding saturated steam pressure is maintained within the digester. With the above quantities of materials, the digester is heated directly, steam being injected thereinto. By a suitable change in proportions, the digester may be heated indirectly, if desired. The time of cooking varies from three to seven hours, being generally shorter for this type of wood than for coniferous woods. The first one or two hour period is used for attaining the desired temperature and pressure. This period is also considerably shorter than when the same process is used for coniferous woods,\and this is believed to be a result of the more rapid sorption of liquor by various woods which are classed with aspen. The digester is relieved of non-condensing, gases throughout the cooking operation eitherperiodically or continuously. At the end of the cooking operation, the digester is blown through a blowline 2 into a receiving vessel 3. f

The next step in the process is to free the pulp from the liquor, resulting from the digestion, this liquor hereinafter being referred to as black liquor. The separation is herein accomplished by means of a continuous filter, indicated at 4,

from which the pulp is discharged in one direction and the black liquor in another. The pulp is then subjected to a thorough washing and screening operation, as indicated generally at 5, to remove undigested pieces of wood and to free 5 it from traces of the discoloring black liquor. Thorough Washing lis essential for the production of a pulp with a good natural color which is easybleaching. This wmed pulp may, if desired, be used for paper without further treatment, as in- 1o dicated at 6. A bleaching operation is represented at 1, and the subsequent washing at 8, after which the pulp is suitable for a high grade paper, as shown at 9. The unbleached pulp is of a color almost white and as good as some coml5 mercially bleached pulp now found on the market. It is much lighter and stronger than when a pulp is made from the same wood by the soda or a modified soda process. It has a color lighter than unbleached spruce sulte pulp and when 2O made into a sheet it has a bursting strength equal to spruce sulte pulp. By a sulfite pulp, I refer to one made by an acid sulte solution having from 4% to 6% total sulfur dioxide in the solution of which 1% to 11/% is combined with the base, 25 such as sodium, calcium or magnesium, the remainder being present as free sulfurous acid.

The pulp of my process has been only slightly acted upon by the cooking, and is readily bleached toa brilliant white with slightly less bleaching 30 powder than is required when the same wood is pulped by the soda or modified soda process.

Another feature of the process is the cyclic operation as regards the base metal of the digestion liquor. Sincesodium isy by far the cheap- 35 est and the preferred base, the process'will be explained in detail with reference to the said liquor. In general, the preferred steps of regeneration comprise concentration of the black liquor resulting from the digestion, incineration to burn 40 out the organic matter, leaching of the ash from the incineration, giving substantially a sodium carbonate (NazCOa) solution, and sulfiting of the leach liquor with sulfur dioxide (SO2) to form sodium sulflte (NazSOs) and to leave a predetermined amount of unsulted soda, preferably as sodium bicarbonate (NaHCOa).

In the concentration of the black liquor, I have used a quadruple effect high speed tubular evaporator in which the upper part of the body of the liquid being evaporated is contained in the lower ends of a `series of tubes, as represented in the drawing at I0. Considerable trouble has been experienced in so evaporating this liquor because it deposits solids during the concentration. The solids deposited are, primarily, organic compounds which adhere to the tube walls and gather on'the bottom'of the evaporator, making it necessary to clean the latter several times a day.

Such a condition prevents continuity in concentration and is generally very undesirable, whatever type of evaporator is used.

I have found that the addition of a caustic al kali, such as sodium hydroxide (NaOH), to the black liquor prevents the deposition of such solids and permits continuous evaporation to be carried out in the system described to a concentration of 36 to 40 B. (45% to 55% solids) "measured at 60 F. The evaporator can thus be operated continuously for several weeks as compared to a few hours. The addition of alkali may be made just prior to the entry of the black liquor into the evaporator, rat the point I I in the drawing. l

The addition of caustic alkali leads to numerlindicated at u. I have toune that sawdust ls ous unexpected advantages. It is customary in most cyclic processes to supply at some point fresh' a general practice to employ a waste liquor from, vsome other process whichv will provide the required caustic soda, such, for example, as the black liquor from some soda process of pulp formation.

The eect of the caustic soda isA to increase the alkalinity of the black liquor to be concentrated. Tests on several black liquors show an increase in alkalinity from pH 8.5 to about pH .9.2 upon addition of caustic soda through the medium of'bla'ck liquor from 'a caustic process and a further increase to pH 9.6 when additional caustic alkali (NaOH) is added to the mixed black liquors. The amount of caustic to be added is immaterial so long as a sufficient amount is added to avoid the troublesome conditions above described. The result desired is the guide to the amount required and this varies with the type of` wood for which the liquor has been used, the

time of cooking', and other specic conditions.

The syrupy mass or concentrate from the evaporation may be dried or run directly to an 'incineraton `various types of which are well known to the art. Since the organic content of the concentrate is sufficient to provide, upon combustion, the necessary heat to operate the mill, it is quite important that it be in the socalled soluble condition eilectedby the caustic condition. The alkaline concentrate, acting as a. vehicle for the organic matter, permits it to be pumped and conducted by pipe lines to the incinerator, whereinto it may be sprayed as a liquid fuel. This not' only increases the eiiioiency of the equipment, but it also eliminates the necessity of handling the solids which otherwise Wouldsettle out on concentration, the danger of liquor concentrate is to pass it through an incinerator I2 without approaching the fusion state. This results in an ash which is primarily sodium carbonate with some free carbon, depending in amount upon the extent towhich it has been burned. Furthermore, it may contain sodium suliide '(NazS) formed by the .reduction of certain of the sulfur compounds and small amounts of other substances. 'I'he incinerator ash is leached, as indicated at I3, with water or with some suitable solution. which. is to be enriched in sodium carbonate (NazCOs) 'I/'he leach liquor is then ltered through a suitable lter, as

is a most satisfactory filtering medium. It has the advantageous property of' removing suspended solids, chiefly carbon, without clogging. It is more eiiicient than sand in this respect, being 6, lighter, more porous, and has a higher specific surface. I have used the sawdust ina bed to a depth of about three feet, the sawdust bed being confined or weighted down, as at I6, to guard against floating when immersed. 'I'he tendency 10 of sawdust to float, being greater than that of sand, causes the bed to rise or expand rather than to pack when used in a tank with a drainer bottom Il.

In practice, I have found it economical and 15` otherwise 'beneflcial to prepare the filtered leach liquor from the ash of the concentrated combined black liquors obtained both from the di` gestion step of the present invention and from the old and well known caustic soda digestion 20V procesces. In Table I, following, there are shown analyses of two samples, designated A and B, each being a composite of forty samples of such leach liquor.

Table I Pounds per cubic loot Sodium carbonate 13. 2O l 12. 75 Sodium suliite 37 44 Sodium thiosulfat 21 l5 S um sulfide... 1. 09 1. 43 Sodium sulfate l. 20 1. 04 a5 Working under ideal conditions of incineration, all sulfur would be roasted out leaving a pure sodium carbonate ash which would yield an ideal leach liquor containing only sodium carbonate (Nazcoa): Practically, such an ideal 40 condition is not attainable, and, furthermore, it is not neces ary. However, when desired the eiect of these impurities m-ay be minimized by a special treatment of the leach liquor when it is, for any reason, desired toapproach the ideal condition. The sullting of the leach liquor with sulfur dioxide (SO2) initiates a series of reactions forming sodium thiosulfate (NazSzOs) from sodium sulfide (NazS). This formationof sodium thio-ulfate may be prevented by treating the leach liquor with carbon dioxide (CO2) taken, for example, from flue gases forming hydrogen sulfide (HzS) and sodium bicarbonate. The nor-- mally gaseous but water-soluble hydrogen sulfide is retained by the soutien being treated. This condition effects a counter-reaction requiring the use of a considerable excess of carbon dioxide to remove the hydrogen sulfide (HzS) from solution and thereby to permit continued conversion of the sodium sulfide into the bicarbonate. As

- there are large amounts of carbon dioxide available from flue gases, the excess required is no practical disadvantage to this operation from a chemical standpoint. From the standpoint of mechanical operation, however, it is possible to view this carbon dioxide treatment as an extra operating step in the regeneration. Such is not the case, for I have found it posfible to combine this step with the sulting step.

The leach liquor containing the sulfide is run against a counter-current of gas containing sulfur dioxide. and carbon dioxide, the latter being vpreferably in excess, so that substantially all the sulfur dioxide'is absorbed whereby to permit carbon dioxide to flow on.' Thus, the excess of 5 asf carbon dioxide meets the incoming'leach liquor, whichat this stage maybe rapidly re-circulated in the path of the carbon dioxide. The carbonic acid gas converts both the sodium sulde and the sodium carbonate to bicarbonate, and removes the resulting hydrogen sulfide. The sodium bicarbonate then absorbs the sulfur dioxide forming the normal sulte, the extent of this reaction being controlled to form .the desired composition of the emergent solution.

In the drawing, this treatment is represented as optional by dotted lines. `.4in-absorption tower I8 receives the leach liquor at the top and a stream of mixed gases enters the bottom of the tower. These gases are preferably taken from the incinerator I2, thus to eiect a substantial recovery of the .sulfur employed in the process, and an advantageous use of the carbon dioxide from the combustion of the black liquor concentrate.

The better practice, however, is to so effect the incineration of the black liquor that the impurities occurring in the leach liquor may be neglected. In such a case, the leach liquor is sulted directly by passing sulfur dioxide gas into the absorption tower I8, as represented at I9, until the liquor exhibits a predetermined composition of sodium sulflte (NazSOa) and of unsulfited soda. A part of the original sodium carbonate will not be sulfited, but it will be acidied to sodium bicarbonate by action of the sulfurous acid (H2503), formed upon the solution of the sulfur dioxide gas in the water. When the bicarbonate content is in the region of 0.50 to 0.75 pounds per cubicfoot of solution, the sulting is deemed complete for the preparation of the preferred cooking liquor, above described, and it is, consequently, lead to a storage tank 20.

For the purpose of explaining more 1n detail the various changes which occur during sulting, the following reactions are given:

In view of the above and of the preceding description, it should be understood that all the reactions are allowed to go to completion, except reaction 4. I'he supply of SO2 forming HzSOs by reaction 1 is out off sufliciently early to leave the required amount of sodium bicarbonate (NaI-1G03) formed by reaction 3, and to prevent its being completely sulted by reaction 4.

The above described operation of sulting results in a suitable suli'lte cooking liquor for use in the digesting step of this invention. Table 1I gives actual analyses of two samples, C and D,

" of such cooking liquor, each being a composite of forty samples.

Such a cooking liquor containing impurities, as

above displayed, is quite suitable for the cooking of wood by my method. It has not been observed that impurities to the amount above stated have any appreciable eiect upon -the quality of the pulp produced over that which 4is produced in. the absence of such impurities. No claim is made to the presence of these contaminating materials,

and it is to be understood that they are merely 5 incidental to a practical operation of the process hereinafter more particularly defined in the appended claims.

It should be explained at this point that heating a bicarbonate solution will cause reaction 3 10 to proceed in the reverse direction forming carbonio acid which breaks down into Water and gaseous water-soluble carbon dioxide (CO2) Since sodium bicarbonate is used in the digestion and is under heat, it is evident that it breaks down 15 forming carbon dioxide'and sodium carbonate (NazCOa). The carbon dioxide is released from the digester from time to time along with other gases formed in the processes. It is diflicult then to ascertain the ratio of normal carbonate to bi- 20 carbonate during the cooking process. The escape of the acid constituent (CO2) during digestion thus permits an increase in the alkalinity due to sodium carbonate (NazCOs). Such an increase has been observed, as will hereinafter be 25 explained.

The cooking liquor containing essentially sodium sulfite and sodium bicarbonate, and incidentally a number of impurities, as exemplied by Table II, initially has a measured alkalinity of 30 about 7.5 to 9.5. After it has been employed in Wood digestion and is recovered as black liquor, its pH value is about 8.5 to 10.5. 4At no time during the cooking operation does the pH value fall below substantially 1 which is the dividing line 35 between alkalinity and acidity. This is an important feature of the invention since liquors which are below pH 7 are considered as being acid and to have a corrosive action on steel digesters.

I have found by experience that when the al- 40 kalinity is maintained above pH 7.5 there is a minimum of corrosion. It has been observed that under these conditions, a black adherent lm forms on the steel digester, and this film apparently aids in preventing the cooking liquor 45 from attacking the underlying steel. It has further been observed that if the liquor has an inp. itial pH less than 7.5 the alkalinity drops to a value during certain stages of the cooking, which permits corrosion in spite of the black nlm. Long 5o experience has shown that this lowering of the alkalinity during the first part of the cooking process is not permanent, and that the alkalinity subsequently rises again during the continuance of the cooking operation. The rise is due in all 55 probability to the escape of the acid constituent CO2, acetic acid and other gases, probably acidic in character. The decrease or drop during the initial stages may be explained by a variety of hypotheses, which, however, will be omitted. It is 60 suillcient to state that the alkalinity decreases and corrosion occurs if the initial alkalinity is too low. Consequently, sufficient alkali carbonate is initially present to give a pH of 7.5 or greater. A larger quantity may be used, but apparently no 65 additional protection is obtained.

The foregoing description of the cooking liquor has been directed generally tb that prepared from leach liquor. It is, of course, to be understood that it may be prepared by sulfiting a sodium car- 70 bonate solution prepared from the commercial salt (NazCOa). For example, a solution containing eleven pounds of sodium carbonate (NazCOa) per cubic foot is sulted by passing in sulfur dioxide (SOz) until there remains only 0.50 to 0.75 75 l :,oaas's pound of sodium bicarbonate (NaHCO:) per thebicarbonate is produced from the normal carbonate as an incident to the sulting which cannot easily be avoided. However, when the salts are mixed without a sulting operation the cooking liquor may contain normal sodium carbonate Yand sodium sulte, the sodium bicarbonate not being essential when the normal carbonate is' present.y

In the practical operation of the process it is most desirable to regenerate the liquor, but the process, chemically, is not dependent upon this type of liquor. A fresh liquor may be used when desired, and may be made by any method which arrives at the desired proportions of alkali carbonate and normal alkali sulflte. One such method is to sulte a fresh sodium carbonate solution (NazCOs) until the requisite amount of soda remains unsulted. Another' method is to sulflte a pure, or a carbonate-containing, caustic soda. (NaOH), making up any deficiency in carbonate by an addition thereof.. Still another feature of the process is the flexibility of its recovery steps, not only to receive waste materials from other processes,- but also to provide material for use in other processes. This permits conjoint operation of several distinct digestion processes with a mingling of materials between them, and permits, further the use of the same apparatus for the conjointly operated processes. Not only may the several processes be simultaneously operated, but one may be readily followed by another because of this advantageous relation. Such a condition reduces the amount of equipment required and-gives a Well equipped plant a wide flexibility in the use of apparatus and processes. For example,the soda process which employs caustic soda (NaOH) as the basis of the digestive liquor may utilize the leach liquor for causticizing with lime. The leach liquor, which is high in sodium sulde (NazS), may be causticized with advantage for the sulfate rocess. Thus, at any time when the leach liquor s deemed too high in sodium sulfide for the better requirements of .the present invention, it may be readily diverted into the sulfate process, without economic loss, thus to elimivan acid sulflte process,having the same base as used for the process ot this invention.

In the fore part of this description reference was made to certain disadvantages of the soda or modied soda processes which are overcome by the present invention, especially those using caustic soda and sodium sulfitef One of these is the danger to humans of caustic solutions, which are very corrosive on the skin as compared to carbonates. Another is the diculty of preparing the sulflte-caustic soda solution.

On regeneration of cooking liquor for such caustic soda processes from black liquor the carbonate is formed. This carbonate requires in addition to the sulting operation a causticizing operation. I eliminate this causticizing step by retaining the carbonate unsuliited. The causticizing step is usually necessary even though in the preparation of the cooking liquor a caustic soda isv used initially instead of carbonate, be-

cause commercial caustic usually contains or readily forms a carbonate as impurity. It is customary to .causticize this impurity. These causticizing operations -when conducted in the presence of alkali sulilte present technical difll- 5 culties, thedescription of which b e omitted as forming no part of this invention. When an attempt is made otherwiseto 'eliminate the cans-'- ticizing operation, by over-suliltlng a soda solution and then adding caustic, anew condition is 1 encountered. The sulting of the soda liberates carbonio acid gas (CO2) which remains dissolved in the solution. This gas can only be removed with great operating dimculty and its removal is necessary for economy in the addition to the causl5 tic soda. Thus, the present invention, besides providing a new and advantageous pulping process, also eliminates causticizing operations and the C02 removal required by other processes, and, furthermore, simplies the preparation of the 20 cooking liquor by the regeneration method.

The light color of the product is one of the prominent features recommendingtlie use of this process. The black liquor contains 4a compound which 1s a chemical indicator, like litmus, cnang- 25y the latter colors the pulp and the color cannot be readily washed out. If the pulp is washed prior to a darkening'of the indicator, the latter is quite readily removed. Hence, to produce a light pulp it is important to prevent a high alkalinity 35 l in the presence of black liquor. The elimination of strong caustic soda from the alkaline processes and its replacement by a milder carbonate contribute largely to the causes of the light color of the pulp. As a measure of the pulp color, the 40 following examples are given in Table III, in which E is a sample of unbleached tamarack,V and F is a sample of semi-bleached tamarack. The method used to determine the color consists in forming a sheet of the pulp sample and meas- 45 urlng the percent o f light reilected by the sheet at different colors, expressed as their wave lengths in ngstrom units.

Table II f Percent reiiection Wave length 55 460 (blue and indigo) 49 66 iso 4s se 48 so 48 to 52 e1 57 65 60 5s o5 5s oo 61 6c sa 65 The aspen pulp is even lighter than the tama- 65 rack, above exempliiled.

It is to be understood that the process is not limited to use with one kind of wood, such as aspen, as herein described. ,Not only may different woods of the same class, but woods of different classes may be cooked as mixtures, such as mixed poplar and pine. -For example, I have successfully cooked birch and tamarack. the former a short-ilbered hardwood, and the latter a longiibered conifer. The cooking operationis ex- 75 tended tothe period requiredI for the slowest cooking wood without apparent damage to the pulp of the more rapidly cooking wood. 'I'he pulp of such a mixture gives rise to certain operating advantages, especially when it is processed with a continuous filter discharging a cake or heavy sheet of pulp froma suction roll. The long iibers alone produce a sheet which readily breaks, whereas the presence of short fibers tends to strengthen the sheet by iilling in the smaller spaces.

Another object in mixing woods forcommon digestion is to improve. the color of the darker and stronger type of pulp by the presence of a lighter and weaker type of pulp. For example, when ten to' twenty percent of birch is used with eighty to ninety percent of tamarack, the resultant pulp has been found to be as strong within commercial limits, as the tamarack pulp alone, and to have a color considerably lighter than the tamarack pulp alone. It is not at all customary to mix short-fibered hardwood pulp into longbered coniferous pulp for this purpose. Not only does this hold true for birch and tamarack, but it has been found to follow from mixed aspen and pine pulps.

It has also been found that mixed tamarack and pine, woods of the same class, have a strength disproportionately nearer that of the stronger pulp when both are made by my process, and that they have a strength almost proportionate when both are made by the sulfate or kraft process.

Other advantages of cooking mixed woods oi different classes will be obvious to those skilled in the art.

In the foregoing, I have described the process with reference to a sodium base, but it is understood that the chemically equivalent bases or the mixed bases are contemplated, as will appear from the character of the appended claims.

What I claim as new is:

1. The method of making substantially ligninfree cellulose pulp from so-called hardwoods or short-bered wood which comprises digesting the wood until cellulose is released with an alkaline sodium sulfite liquor, containing suillcient excess of sodium sulte to prevent staining and a sufficient amount of a sodium carbonate salt to give an alkalinity of about pH 7.5 to 9.5, at a temperature of from 175 to 188 C. and at the corresponding saturated pressure for a period of about three to seven hours, of which the flrst one to two hours is used in attaining said temperature and pressure.

2. The method of making substantially ligninfree cellulose pulp from wood chips of the socalled hardwoods or short-bered woods which comprises digesting the wood until cellulose is released with an alkaline sodium sulflte cooking liquor of an alkalinity not less than pH 7.5, which solutionfcontains 30 to 40 pounds of sodium sulte and a suiicient quantity of a sodium salt of carbonic acid to produce the required alkalinity 5 per pounds of oven dried chips, the concentration of the liquor being such that 1/4 to cubic foot of liquor is used per cubic foot of chips, at a temperature of from to 188 C., and at the corresponding saturated steam pres- 10 sure for a period of from three to seven hours, of which the first one to two hours is used in attaining said temperature and pressure.

3. The method of making substantially ligninfree cellulose pulp from so-called hardwoods or 15 short-fibered wood which comprises digesting the wood until cellulose is released with an alkaline sodium suliite cooking liquor of an alkalinity of a pH of about 7.5 to 9.5, which liquor contains sufficient excess of sodium sulte to prevent 20 staining, digesting it at a temperature of from 175 tol 188 C., and at the corresponding saturated steam pressure for a period of about three to seven hours, of which therst one to two hours is used in attaining temperature and 25 pressure.

4. The method of making substantially ligninfree cellulose pulp from wood chips of the socalled hardwoods or short-flbered woods which comprises digesting the wood until cellulose is 30 v released with an alkaline sodium sulfite cooking liquor of an alkalinity not less than pH 7.5, which contains 30 to 40 pounds of sodium sulte and `1.3 to 2.6 pounds of sodium carbonate per 100 pounds of oven dried wood, the concentration 35 of the liquor being such that 5/4 to 1%, cubic foot of liquor is used per cubic foot of wood chips in the digester, at a temperature from 175 to 188 C., and at the corresponding saturated steam pressure for a period of from three to seven hours, of which the irst one to two hours is used in attaining said temperature and pressure.

5. The process of making substantially ligninfree cellulose pulp from so-called hardwood or short-bered wood which comprises digesting 45 the wood until cellulose is released, using a cooking liquor containing in solution from 30 to 40 parts of normal alkali metal sulflte per 100 parts oi oven-dried wood so as to have suillcient excess ofvsaid sulte present in solution to prevent staining wh'en the cellulose is released, maintaining the solution alkaline with a carbonate of an alkaliv metal throughout the digestion, and continuing the digestion for from 3 to 7 hours at an elevated temperature and pres- 55 sure of which the rst 1 to 2 hours is used in attaining said temperature and pressure.

CLINTON K. TEXTOR.

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