WO1998007922A1

WO1998007922A1 - Method for lowering the level of oxalic acid

Info

Publication number: WO1998007922A1
Application number: PCT/SE1997/001365
Authority: WO
Inventors: Nils-Olov Nilvebrant; Anders Reimann; Filipe De Sousa
Original assignee: Svenska Traforskningsinstitutet; STFI Skogsindustrins Tekniska Forskningsinstitut AB
Current assignee: Svenska Traforskningsinstitutet; STFI Skogsindustrins Tekniska Forskningsinstitut AB
Priority date: 1996-08-20
Filing date: 1997-08-20
Publication date: 1998-02-26
Anticipated expiration: 1999-02-20
Also published as: EP0920552A1; SE9603029D0

Abstract

One or several oxalic acid or oxalate decomposing enzymes are used to lower the concentration of oxalic acid and/or oxalate precipitates in the production of pulp and paper. The method is particularly useful in processes using oxidative bleaching and in particular in processes where the process liquids are recirculated within the plant. Thus the forming of incrusts and sediments, comprising sparingly soluble oxalate compounds, is reduced or prevented.

Description

Title : Method for lowering the level of oxalic acid

The present invention pertains to the lowering of the concentration of oxalic acid in process liquids in the production of pulp and paper. The present invention concerns the lowering of the concentration of oxalic acid and in particular the prevention of the formation of calcium oxalate incrust and/or the degrading of precipitated calcium oxalate in the production of pulp and paper.

Background of the invention Environmental concerns are gaining importance in all fields, including the production of pulp and paper. The recent transition to oxygen containing bleaching agents together with the efforts to essentially close the processes - i. e. to minimize the effluents from the plants by recirculation of process liquids and reusing process chemicals - has given rise to new problems. One of these is the accumulation of organic and inorganic material. A particularly urgent problem is the formation of sparingly soluble oxalate compounds, which precipitate to form sediments and incrusts e. g. in digesters, evaporators, heat exchangers, pipe lines and washing filters, a phenomenon also known as scaling. These incrusts lead to lower heat transfer and change the flow characteristics, thus impairing the process economy. In some cases, parts of said incrusts can break off in chunks, which can travel down-stream and seriously damage valuable process equipment and cause costly shut-downs.

Oxalic acid is the smallest among the dicarboxylic acids, as it contains only two acid moieties, linked together. Oxalic acid dissociates in water according to the following:

^H2^C2°4 ^*•*** ^{H+ +HC}2°4^" P^Kal = ²³

HC₂O₄- ≠ H⁺ + C₂O₄ ²- pK_a2 = 4.19

The dissociation coefficient is temperature dependent. It decreases with increasing temperatures. Oxalic acid occurs abundantly in various plants, the most well known examples being rhubarb and spinach. Nevertheless it also occurs in significant amounts in wood, in concentrations in the range of about 0.1 to 0.4 kg/ton. The bark may contain up to 10-fold higher concentrations and hardwood bark can contain even up to about 15 kg oxalic acid per ton. This underlines the importance of using thoroughly debarked wood for the production of pulp. The main portion of the problem caused by oxalic acid is however related to its formation in the process, namely during pulping and bleaching. Examples of the underlying reactions will be given further in the description.

The calcium involved in the forming of calcium oxalate enters the process as a

SUBSTITUTE SHEET during digestion and bleaching. Examples of the underlying reactions will be given further in the description.

. The calcium involved in the forming of calcium oxalate enters the process mainly as a component of the wood raw material. In trees most of the calcium is contained in the bark, but it is found also in the wood and in the foliage. Pine can contain about 0.3 to 0.5 kg/ton and contents of about 0.3 to 0.6 kg/ton are found for hardwood. The concentrations in the bark are, as for oxalic acid, again considerably higher.

Oxalic acid is shown to form in all bleaching steps that involve oxidative bleaching. The source is thought to be aromatic ring structures with free phenolic hydroxy moieties, split in the decomposition of the lignin. This reaction seems to be most pronounced in bleaching with oxygen, hydrogen peroxide and ozone. In particular ozone bleaching seems to cause the formation of considerable amounts of oxalic acid. Oxalic acid formation is also observed in bleaching with chlorine dioxide and peracetic acid.

Another source of oxalic acid is though to be carbohydrates. The decomposition of carbohydrates is, however, an unwanted effect. An example of the reaction of lignin with

H₂0₂, R=CH₃ or CH(OH)CH₃ is presented below:

The role of xylane deserves attention. Xylane, one of the polysaccharides joining the lignin and the cellulose in the wood, is though to be a major source of oxalic acid. One intermediate compound formed in the decomposition of xylane is hexenuronic acid, which in turn is sensitive for bleach chemicals. degrades such structures to equimolar amounts of oxalic acid.

Calcium oxalate is a solid with very low solubility product in water (2.57 x 10-9 at 25°C and 1.78 x 10-9 at 18°C). Oxalic acid also forms salts with other metal ions than calcium, e. g. sodium, potassium and ammonium. These are, however, more soluble in water and do not cause problems to any greater extent in comparison with calcium oxalate, while other counter ions like barium and manganese yield salts with low solubility. The solubility of calcium oxalate is temperature and pH dependent and accordingly precipitates are formed in places where the pH or temperature changes rapidly. Locations liable for precipitation are thus e g washing filters after acid bleaching, white water tanks and on filters and heat exchangers. Calcium oxalate scaling can naturally also form in other places, sometimes very locally, provided that the surrounding conditions favour precipitation. Also precipitation directly on the pulp occur.

State of the art The above presented problem is currently attacked mainly in two ways, on one hand mechanical removal of the incrusts and, on the other hand various methods for changing the conditions governing the formation of incrusts. The former includes time consuming, often manual or semi-manual cleaning or scrubbing of the inside of equipment, e g pipes, screens and filters with water under high pressure. Sometimes this cleaning is supplemented with acid pretreatments or the use of complexing agents like EDTA. This approach is, under any circumstances, both cumbersome and costly. Costs are incurred both due to the added work required, the shut-down periods and increased wear on the equipment, e g screens. The latter approach, i. e. changing the conditions governing the formation of incrusts, includes the adjustment of pH, temperature and the addition of chemicals. Said chemicals either form a complex with one of the ions involved or inhibit the crystal -growth of calcium oxalate. The possibilities of manipulating the process parameters such as pH and temperature are restricted by process requirements and economical considerations. The addition of chemicals, among which in particular EDTA, DTPA, sodium tripolyphosphate, sodium pyrophosphate and methylene phosphoric acid are preferred, is followed by an increased COD (chemical oxygen demand), higher costs for chemicals and possible secondary process problems e g due to the accumulation of unwanted compounds.

The use of oxalic acid decomposing enzymes is known from medical applications, mainly for the determination of oxalate in urine samples. Problems caused by the presence

SUBSTITUTE SHEET of oxalic acid have surprisingly also been encountered in the brewing industry. US 4 652 452 deals with this problem. According to US 4 652 452, the level of oxalic acid must be lowered to less than about 15 pp in finished beer to prevent "gushing", whereby beer gushes from the bottle when opened. The precipitate of calcium oxalate, formed when calcium is added to stabilize and activate amyiases during mashing, is also unwanted as the precipitating "beerstone" harbours contaminating microorganisms. US 4 652 452 consequently describes the introduction of oxalate decarboxylase (EC 4.1.1.2) during mashing and/or fermentation to avoid the production of beerstone. There is no mentioning of the applicability of this approach to other processes and it is very unlikely, that a practitioner in the field of pulp and paper production would assume, that a method developed for the production of beer, would be applicable in a completely different industry. Hitherto, no approach to solve the problem of oxalate incrusts or scaling in the pulp and paper industry, known for at least two decades, has involved the use of enzymes. At present, there is no satisfactory method for the prevention of the formation of calcium oxalate incrusts in the production of pulp and paper. The problem ^'s urgency is increasing in pace with the ongoing attempts to minimize the effluents from pulp and paper plants and maximize the recirculation of chemicals and water.

Summary of the invention The problem is now surprisingly and efficiently solved according to the present invention as specified in the attached claims. The present invention will be illuminated in further detail in the following description and examples with attached figure, which nevertheless are not to be construed as limiting the scope of the invention.

Description of the figures Figure 1 illustrates the enzymatic degradation of oxalic acid using different doses of oxalate decarboxylase and oxalate oxidase, respectively.

Description of the invention The present inventors have surprisingly shown, that the influence of an effective amount of an oxalic acid decomposing enzyme or mixture of such enzymes on at least one process fluid or flow of such fluid, solves the above problems. In the following description, the term "effective amount" refers to an amount that, under the given conditions, leads to a reduction of the amount of oxalic acid present in a liquid to a level below the level where oxalates can precipitate in any partial volume of said liquid under the given conditions. The term "conditions" refers to the conditions influencing both the activity of the oxalic acid decomposing enzyme and the formation of oxalate precipitates, i e temperature, pH, concentration of enzyme inhibitors, concentration of ions capable of forming sparingly soluble oxalate salts, other components reacting with the oxalic acid or oxalate, components influencing the crystal growth of the oxalates and the like.

The term "process liquid" refers to any stream or volume of process liquids in a plant for the production of pulp and/or paper where oxalate precipitation occurs. The term "process liquid" naturally also includes any stream or volume, diverted or separated from the process, e.g. for complete or partial recirculation to the process. The oxalic acid decomposing enzyme is preferably an enzyme or a mixture of enzymes which exhibits optimal activity under the conditions present in the process liquid, or at conditions to which the process liquid can be adjusted without undue cost or complication. Said enzyme or mixture of enzymes can be extracted from natural sources or synthesized, for example using existing or genetically modified microorganisms expressing said enzyme or enzymes. Preferably the microorganisms are modified, by selection or genetical engineering or a combination of both, to express an enzyme with suitable pH and temperature tolerance, in amounts making the production economically feasible.

Non-limiting examples of oxalic acid decomposing enzymes, suitable for use in a method according to the present invention, are oxalate decarboxylase (EC 4.1.1.2) and oxalate oxidase (EC 1.2.3.4). A combination of the above enzymes can also be used. The oxalic acid is efficiently decomposed to either carbon dioxide or formic acid or a combination of both. Any excess of CO₂ is then released in a gaseous form. However, if the process conditions are such, that the CO is trapped as carbonates and the risk of CaCO formation arises, the-choice of oxalate oxidase is preferred. Formic acid, being highly soluble in water, poses no problem. It can be released with the effluent from the plant, without adding unduly to the COD-load. Unless a decrease in COD load is more critical, in which case the use of oxalate decarboxylate becomes the preferred choice. Hydrogen peroxide is also formed when oxalic acid or oxalate is degraded by the enzyme oxalate oxidase. The production of H O₂ is potentially beneficial, since it can be utilized as a bleaching agent.

Non-limiting examples of natural sources of oxalic acid decomposing enzymes are for example beet shots, beet roots, barley roots and/or seedlings, grain sorghum and

SUBSTITUTE SHEET banana peel. Microorganisms presently known to express suitable enzymes include fungi, such as Aspergillus, Myrothecium and Collybia, e g Collybia velutipes and selected bacteria, such as Pseudomonas sp OX-53. Singular species known to produce suitable enzymes further include Postia placenta and Cariolus versicolor. The fungi and bacteria listed above are only given as examples. Presently microorganisms of the species Aspergillus are considered practically suitable for the production of the present enzymes but the scope of the present invention is not limited to the use of the named microorganisms. It is very possible that other enzyme producing microorganisms suitable for the present invention already exist or will be developed using mutation and selection or methods of genetical engineering. It is also likely, that the enzyme producing capabilites of an existing microorganism can be further enhanced through genetical engineering.

The oxalic acid decomposing enzyme is added in an effective amount under the given conditions, preferably in an amount in the interval of 1 to 10 000 U/l, wherein 1 U is a measure of enzyme activity, correspondning to the activity needed to decompose 1 mmol oxalic acid / min.

According to a preferred embodiment of the present invention, the enzyme is present in the process liquid in a stabilized form. The enzyme can be stabilized according to conventional methods, known to a practitioner, skilled in the field of enzyme technol- ogy. Examples of methods of stabilization are, but not limited to, the following: chemical modification, cross-linkage (polymerization) of enzyme molecules, enzyme bonding to inert carriers, incorporation of enzymes within semi-permeable membranes, microencapsulation and the like.

According to one embodiment of the present invention, the oxalic acid decompos- ing enzyme is added- to the process liquid in a stabilized but soluble form. The enzyme is preferably combined with suitable additives or chemically modified to better tolerate the chemical and physical conditions in the process liquid.

According to another preferred embodiment of the invention, the oxalic acid decomposing enzyme is encapsulated in microspheres and the process liquid or a diverted stream of process liquid is brought to pass in intimate contact with said microencapsulated enzyme. In this embodiment, the microencapsulated enzyme is preferably contained in a reactor or other suitable vessel, through which the process liquid or diverted stream thereof is brought to pass. Preferably the conditions in said vessel is adjusted to the

TE SHEET optimum conditions for the enzyme or enzyme mixture in question.

The environmental conditions during the enzymatic treatment are not critical for the scope of invention, but can of course be used to control the enzymatic reaction. The environmental conditions of the enzymatic treatment are to a certain extent governed by the normal process parameters of the pulping and paper making processes. Simultaneously, the requirements of the enzyme or enzyme mixture have to be considered. A pH in the interval of 2 to 13 is possible, while an interval of 2.5 to 8 is preferable, depending on the enzymes used. The temperature of the reaction mixture is of considerable importance as it affects the reaction rate of the enzymes. The reaction rate directly influences the time needed for the desired reaction to take place and thus the necessary storage volumes. Depending on the enzymes used, their thermotolerance and thermal optimum, the temperature can be in an interval from about 5 - 95 °C, theoretically even higher e.g. about 100 °C, but preferably about 30 - 60 °C and most preferably about 30 - 45 °C. One can anticipate a future development of highly thermotolerant enzymes, which would enable the enzymatic treatment to be performed at considerably higher temperatures. It is possible that this would give rise to unexpected synergistic effects.

A skilled worker can, given the requirements of the enzymes in question, determine more exactly the optimum environmental conditions for application of the enzyme or enzyme mixture. The presence of heavy metals can also influence the en- zymatic activities and should therefore be avoided. A skilled worker with knowledge of both the manufacturing of paper and the usage of enzymes is able to adapt the process to accommodate the enzymatic treatment according to the present invention.

Examples Data from the degradation of oxalic acid in solution and in the form of precipi- tated calcium oxalate- are given in the tables below as parts per million (ppm) or in % of initial concentration.

Example 1. Degradation of oxalic acid using oxalate oxidase

Oxalate oxidase from barley seedling was added to an aqueous solution containing 40 ppm oxalic acid, at pH 4 and 37 °C. The amount oxalate oxidase added was 0.14 units per μtnol oxalic acid. Samples were taken after 5, 9 and 33 minutes, respectively. The results are presented in table 1.

SUBSTITUTE SHEET Table 1. Degradation of oxalic acid using oxalate oxidase

Reaction time Oxalic acid Oxalic acid

(min). (ppm) (% of initial amount)

0 40 100

5 18.5 46

9 2.0 5

33 0.5 1

Example 2. Degradation of calcium oxalate using oxalate oxidase

The oxalic acid, present in an aqueous solution at pH 4 and 37 °C, was precipitated with calcium. Subsequently oxalate oxidase from barley seedlings was added, in an amount corresponding to 0.18 U per μmol oxalic acid. Samples were taken after 40, 90 and 140 minutes, respectively. The remaining calcium oxalate was determined as oxalic acid after dissolution of the salt with a strong cation exchanger (Amberlite IR-120 in proton-form). The results are presented in table 2.

Table 2. Degradation of calcium oxalate using oxalate oxidase

Reaction time Calcium oxalate Oxalic acid

(min) (ppm) (% of initial amount)

0 38.2 95.5

40 26.0 65.0

90 15.1 37.8

140 11.8 29.5

Example 3. Degradation of oxalic acid using oxalate decarboxylase

The degradation of oxalic acid was investigated using the following concentrations of oxalic decarboxylase (from Aspergillus); 0.14, 0.45 and 0.21 U per μmol oxalic acid at pH 5 and 37 °C. The results are presented in tables 3, 4 and 5. The acids were separated by ion chromatography and oxalic acid was quantified using standard solutions, while the quantification of formic acid was relative.

I TE SHEET Table 3. Degradation of oxalic acid using 0.14 U oxalate decarboxylase per μmol oxalic acid

Reaction time Oxalic acid Oxalic acid Formic acid

(min) (ppm) (% of initial cone.) (% of cone, at time 118)

0 40 100 0

5 30 75 44

20 16 40 84

36 11 29 89

51 10 24 96

67 9 22 97

100 8 20 100

118 8 20 100

Table 4. Degradation of oxalic acid using 0.45 U oxalate decarboxylase per μmol oxalic acid

Reaction time Oxalic acid Oxalic acid Formic acid

(min) (ppm) (% of initial cone.) (% of cone, at time

20)

0 40 100 0

5 14 35 64

20 2 5 100

Table 5. Degradation of oxalic acid using 0.21 U oxalate decarboxylase per /-.mol oxalic acid

Reaction time Calcium oxalate Oxalic acid

(min) (ppm) (% of initial amount)

0 38 96

40 25 62

90 15 37

140 11 28

SUBSTITUTE SHEET Example 4. Enzymatic degradation of oxalic acid in process water sample

The applicability of the enzymatic oxalic acid degradation according to the present invention was confirmed by a test on an authentic sample taken from a kraft pulp bleachery; the process water after an ozone stage (Z). The sample initially contained 41 ppm oxalic acid. After less than 15 minutes at room temperature in the presence of 2 U oxalate decarboxylase, no oxalic acid remained.

The results of the experiments are summarized in Fig. 1. Although the invention has been described with regards to its preferred embodiments, which constitute the best mode presently known to the inventors, it should be understood that various changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention which is set forth in the claims appended hereto.

Claims

1. Method for loweπng the level of oxalic acid and/or the amount of precipitated oxalate in process liquids in the production of pulp and paper, in particular in preventing the forming of incrusts and sediments of oxalate compounds in the production of pulp and paper using oxidative bleaching, characterized in that the concentration of oxalic acid, present in the process liquid or liquids, is lowered to a value where no precipitation occurs at the given conditions by subjecting said process liquid or liquids to the action of at least one oxalic acid or oxalate decomposing enzyme or a mixture of such enzymes. 2. Method according to claim 1, characterized in that said oxalic acid or oxalate decomposing enzyme is one chosen from the following group; oxalate oxidase (EC 1.2.3.4), oxalate decarboxylase (EC 4.1.1.

2) and a mixture thereof.

3. Method according to any one of the preceding claims, characterized in that said enzyme or enzymes is/are brought in contact with the process liquid or liquids by leading said process liquid or liquids through a vessel containing said enzyme or enzymes in an immobilized form.

4. Method according to claim 2, characterized in that said process liquid or liquids are subjected to at least one species of viable microorganisms expressing said enzyme or enzymes.

5. Method according to claim 2, characterized in that said process liquid or liquids are subjected to biological material containing said enzyme or enzymes.

6. Method according to any one of the preceding claims, characterized in that the oxalic acid is converted to any one of formic acid, hydrogen peroxide, carbon dioxide and a mixture thereof.

7. Use of at least one oxalic acid or oxalate decomposing enzyme for decomposition of oxalic acid or oxalate present in the process liquid or liquids in the production of pulp and paper.

8. Use of at least one oxalic acid or oxalate decomposing enzyme according to the previous claim, characterized in that said enzyme is chosen from the group consisting of oxalate oxidase, oxalate decarboxylate and a mixture thereof.

SUBSTITUTE SHEET