Stabilized Sodium Dithionite Solutions

1. Field of the Invention

This invention relates to bleaching and especially to bleaching of groundwood pulps with dithionite-based compositions. More particularly, it relates to aqueous sodium dithionite solutions which are stabilized by additives and to methods for preparation thereof.

High-yield wood pulps comprise groundwood pulps, either stone ground or refiner ground, and semichemical pulps. The former are respectively ground between stones or patterned steel disks without chemicals, and the latter are briefly softened by pressure cooking with chemicals before refiner grinding. These pulps are preferred for temporary usages requiring low strength, such as newsprint.

Groundwood pulps from certain woods, such as spruce, are often sufficiently bright for such uses without bleaching, but others need to be increased in brightness, particularly if semichemically cooked. Because groundwood pulps retain their original lignin content and because lignin darkens at a pH above 6.5, a lignin-preserving bleaching agent must be employed under acid conditions. Dithionites are highly suitable therefor.

Dithionites, commonly termed hydrosulfites or hyposulfites, have been used for years to bleach a wide variety of materials including straw, feathers, glue, textiles, and woodpulps. However, because dithionites are strong reducing agents, these problems have arisen:

1. WHEN IN POWDER FORM AND EXPOSED TO MOISTURE WHICH IS ALWAYS ABUNDANT IN PAPER MILLS, SODIUM DITHIONITE HAS THE POTENTIAL TO DECOMPOSE RAPIDLY AND EXOTHERMICALLY, EVOLVING SUFFICIENT HEAT TO IGNITE ADJACENT MATERIALS OR BY-PRODUCT SULFUR FROM THE DECOMPOSITION;

2. AS STORED SOLUTIONS, DITHIONITES DECOMPOSE BY REACTION WITH OXYGEN AND WATER UNDER AEROBIC CONDITIONS TO FORM BISULFITE AND BISULFATE IONS OR DECOMPOSE AUTO-OXIDATIVELY BY REACTION WITH WATER UNDER ANAEROBIC CONDITIONS TO LIBERATE BISULFITE AND THIOSULFATE IONS, BOTH DECOMPOSITION ROUTES CREATING ACIDITY SUCH THAT THE DECOMPOSITION PROCEEDS RAPIDLY TO COMPLETION; AND

3. DURING BLEACHING UNDER ACIDIC CONDITIONS, SODIUM DITHIONITE SOLUTIONS TEND TO DECOMPOSE SO RAPIDLY THAT A CONSIDERABLE LOSS IN BLEACHING EFFECTIVENESS RESULTS.

For solving the first problem, various additives may be mixed with sodium dithionite powder, but they must not interfere with stability of the dithionite solutions or with bleaching efficiency. As disclosed in U.S. Pat. No. 3,669,895, water-soluble inorganic carbonates and sulfites and borax have been added to sodium dithionite to form a stabilized powder composition. Another stabilized powder composition, disclosed in U.S. Pat. No. 2,450,034, comprises sodium dithionite, anhydrous sodium carbonate, and sodium diethylenetriaminopenta-acetate.

Zinc dithionite is a preferred substitute for sodium dithionite, as taught in U.S. Pat. Nos. 984,312 and 2,450,034, because it is relatively stable under normal transport and storage conditions, is not spontaneously flammable when added to water, and is storable for up to three hours as a 10% solution, but its toxicity in effluent waters is ecologically undesirable. A radically different approach is taken by the process of U.S. Pat. No. 3,167,515 in which sodium dithionite is generated in situ by adding sodium metabisulfite and sodium borohydride to water. However, sodium dithionite solutions generated thereby have the same extremely short storage life as solutions prepared from commercial sodium dithionite, as it has been reported that they decompose at about 20% per hour at pH 6. The most popular commercial form of sodium borohydride being offered to the pulp bleaching industry contains approximately 42-44% sodium hydroxide, but the excess alkali must be neutralized because it is critical that the pH not exceed about 7 at any stage in the bleaching process in order to avoid a pronounced yellowing of the pulp.

Stabilizing sodium dithionite solutions by maintaining a highly alkaline pH therein, thereby neutralizing sulfite and sulfate ions as fast as they are formed, has long been practiced. Sufficient sodium phosphate is added to the straw bleaching process of U.S. Pat. No. 1,027,953, for example, to render the bath distinctly alkaline till the end of the bleaching treatment. In U.S. Pat. No. 3,804,944, sodium hydroxide (up to 20%) is added to sodium dithionite to bring the initial dissolving pH to the range of 9.0 to 13.0 for storage stabilization at temperatures of 35.degree. F. to 135.degree. F. According to U.S. Pat. No. 3,653,804, durability of a sodium dithionite solution is considerably enhanced by addition thereto of at least 5% by weight of sparingly soluble zinc compounds, including zinc oxide, zinc hydroxide, zinc carbonate, zinc basic carbonate, zinc basic silicate, and zinc basic phosphate. Significant stabilization during anaerobic preparation and storage of sodium dithionite solutions by addition of zinc sulfate at 2.0% to 11.3% by weight is taught in U.S. Pat. No. 3,672,829.

An important consideration is that decomposition of dithionite solutions is catalyzed by heavy metals, such as iron, and that groundwood pulps contain such metal ions as iron, manganese, copper, nickel, and cobalt, with iron predominating. A further consideration of interest is that commercial sodium dithionite, such as that produced from zinc metal or amalgam as taught in U.S. Pat. No. 3,536,445, contains various impurities, such as sodium chloride, sodium sulfate, sodium thiosulfate, and other salts, totalling up to 24%. Crystalline sodium dithionite is also produced by the process of U.S. Pat. No. 3,576,598 from SO.sub.2 and sodium formate. The formate-derived sodium dithionite produces solutions having lower alkalinity than the zinc-derived solutions.

The prior art has long recognized that sodium dithionite solutions can be improved in stability and that bleaching efficiency can also be improved by sequestering these heavy metal ions. However, groundwood pulps themselves act as low-capacity ion-exchangers with a great affinity for heavy metal ions, less affinity for the alkaline earth metals, and very little affinity for the alkali metals, according to the Pulp and Paper Magazine of Canada, Technical Paper T174, pages 77-84, June 6, 1969. Although washing with water removes a large proportion of the alkali metals and washing with hydrochloric acid removes alkali and alkaline earth metals nearly completely but the heavy metals only partly, it requires a sequestering agent at a low pH for effectively deionizing groundwood pulps.

In U.S. Pat. No. 2,707,144, water-soluble phosphates, such as sodium pyrophosphate, sodium tetraphosphate, and sodium tripolyphosphate, are proposed for bleaching groundwood pulps. Sodium tripolyphosphate (STPP) is indeed the most commonly used complex forming agent at this time because of cost considerations.

Sodium citrate, with or without alkali tetraborate, is proposed in U.S. Pat. No. 2,826,478, and ethylenediaminetetraacetic acid, in amounts of 0.05% to 0.5% by weight, with or without water-soluble phosphates, is disclosed as a sequestering agent in U.S. Pat. No. 2,707,145. U.S. Pat. No. 3,709,779 suggests replacing STPP with sodium silicate as a sequestrant for enhancing bleaching efficiency.

Sodium dithionite-based bleaching compositions in powder form create problems in paper mills, textile plants, etc. (in addition to those caused by exothermic decomposition) because a powder is difficult to mix efficiently with a slurry. If a solution is preliminarily prepared in the paper mill, special equipment is needed, and personnel may be exposed to danger from SO.sub.2 gas and heat created by rapid decomposition of sodium dithionite. It is consequently desirable to have a prepared bleaching solution delivered to the paper mill and pumped directly from a tank truck into a storage tank ready for use.

This delivery procedure, however, requires at least five days for mixing chemicals with water, loading the bleaching solution, delivery to the paper mill, and brief storage therein near the point of application. Such a bleaching solution ideally should have the following properties:

1. less than 3% decomposition after five days storage at 50.degree. F.;

2. less than 25% total alkalinity as Na.sub.2 CO.sub.3 (on a sodium hydrosulfite basis) with no special requirements for soda ash;

3. 2-4% soluble zinc (sodium hydrosulfite basis);

4. a solution pH of approximately 10;

5. substantially no precipitate or turbidity;

6. sequestering activity (as calcium carbonate) equivalent to 15% sodium tripolyphosphate; and

7. total solids of approximately 20% by weight. Preliminary storage tests were made at 70.degree.-75.degree. F. With 14 alkaline agents such as sodium carbonate, sodium tripolyphosphate, borax, sodium phosphate, sodium acetate, and sodium hydroxide in the presence of sodium dithionite at concentrations of 10% to 13%, and similar tests were made with 8 sequestrants and complexformers combined with sodium hydroxide. However, none provided the stability desired with exception of caustic soda, and it caused too much alkalinity for use on groundwood pulp because it is critical that the pH not exceed about 7 at any stage in the process after the addition of any of the bleaching chemicals, in order to avoid a pronounced yellowing of the pulp, so that the excess sodium hydroxide must be neutralized before contact is made with the pulp slurry.

Stability tests at various temperatures and concentrations were also made on aqueous solutions prepared from three commercially available bleaching compositions having the following dry-basis formulations shown in Table I:

All solutions were stored under a protective layer of mineral oil in one-quart glass jars in order to maintain anaerobic conditions. All Composition A solutions contained a heavy white floc, apparently zinc carbonate, that was easily dispersed upon shaking. All Composition B solutions were clear with a slight yellow cast when initially dissolved. After storage for four days, the solutions were dark gray with some black floc precipitate. All Composition C solutions contained a slight floc. The stability characteristics of the Composition A solutions are given in Table II.

The stability characteristics of Composition B solutions are given in Table III:

The stability characteristics of the Composition C solutions are given in Table IV for a commercially useful concentration range:

These tests demonstrate that commercially available compositions, although containing the ingredients known to be helpful for stabilizing sodium dithionite solutions, are not adequate because of floc formation, low temperature requirements, or low concentration requirements. A need evidently exists for stabilized sodium-dithionite solutions and for methods to prepare them.

Practical commercial limitations for stabilized sodium-dithionite bleaching solutions appear to comprise a content of at least 12% Na.sub.2 S.sub.2 O.sub.4 by weight, 50.degree. F. storability, 5% maximum decomposition after 5 days storage, and substantially no floc, fines, or turbidity.

It is accordingly an object of this invention to provide an aqueous sodium dithionite bleaching solution comprising a combination of additives so that no more than 5% of the dithionite decomposes after five days storage at 50.degree. F. (10.degree. C.) of a solution containing at least 13% Na.sub.2 S.sub.2 O.sub.4.

It is another object to provide a method for preparation of this solution.

It is a further object to provide a bleaching solution that is substantially without floc, fines, or turbidity.

In accordance with these objectives and the spirit of this invention, stabilized sodium dithionite solutions are herein provided that have excellent stability with less than 5% decomposition during five days storage at 50.degree. F. (10.degree. C.), several also exhibiting similar stability at storage temperatures of 70.degree.-75.degree. F. (21.degree.-24.degree. C.). The solutions require no additional chemicals for pH adjustments prior to direct application to the pulps. The bleaching results are equal to those obtained from dry, proprietary sodium dithionite blends or commercial zinc dithionite when compared on a basis of pounds of bleaching agent per ton of pulp per point brightness gain.

In general, these stabilized solutions alternatively or additively comprise a chelating agent, a source of zinc ion, a complex former, and a source of hydroxyl ion in combined amounts discovered by methodical testing of individual additives, isolated and/or combined with other additives, to detect synergistic behavior and interactions involving ion-exchanging, chelating, and complex-forming.

Desirable storage and bleaching results are presented for solutions prepared from zinc-derived sodium dithionite as well as sodium formate-derived sodium dithionite using at least one additive selected from soda ash, sodium tripolyphoshate, nitrilotriacetic acid trisodium salt, zinc hydrosulfite, borax, sodium phosphate, and ethylenediaminetetraacetic acid tetrasodium salt.

Successful field trials have been concluded at five paper mills in South Carolina, Oregon, and Washington. On a five-day run in one western paper mill, the amount of one stabilized sodium-dithionite bleaching solution was 10% to 15% less than the homemade zinc hydrosulfite (100% assay basis). A similar report from the South Carolina paper mill indicated superiority to solutions prepared via a commercial bleaching system employing a solution of sodium borohydride and sodium hydroxide.

Following a methodical testing procedure, bleaching solutions were prepared according to various formulations planned for testing seven selected additives. The order of addition of the additives, when used in a formulation, to potable water was: a chelating agent, zinc dithionite, sodium tripolyphosphate, sodium phosphate, sodium metaborate, sodium carbonate, sodium hydroxide, and sodium dithionite. Samples for stability studies were stored in one-quart glass jars under a protective layer of mineral oil.

In was soon found that fines were initially present in solutions prepared from zinc-derived sodium dithionite; if these fines were removed by filtration immediately after preparing a solution, it remained clear for several days.

The methodical testing procedure broadly covered investigations into stabilizing of zinc-derived sodium dithionite and formate-derived sodium dithionite bleaching solutions. These investigations comprised:

1. stabilization testing of solutions having one to seven additives at various additive levels and at a cool storage temperature of 50.degree. F. (10.degree. C.) to identify those having satisfactory cool stability and to determine the need for chelating agents, coordination compounds, and alkalinity sources;

2. stabilization testing of selected cool-stable solutions at about ambient temperature (70.degree.-75.degree. F. or 21.degree.-24.degree. C.);

3. stabilization testing of selected formate-derived sodium dithionite solutions to determine the need for hydroxyl ions from NaOH, Na.sub.2 CO.sub.3, Na.sub.3 PO.sub.4, or Na.sub.5 P.sub.3 O.sub.10 ;

4. bleach tests of selected cool-stable and ambient-stable solutions and of standard bleach solutions on a groundwood pulp at three bleaching levels; and

5. mill trials with selected solutions.

As shown in Tables V and VI for Examples 1-19, a number of zinc-derived sodium dithionite bleaching solutions were storage-tested at 50.degree. F. (10.degree. C.) or at 70.degree.-75.degree. F. (21.degree.-24.degree. C.). Storage stability was obtained by using additives supplying a chelator, a source of zinc ion, a source of hydroxyl ion, and a complex former. In particular for investigating the effects of these additives, the zinc dithionite concentration was varied, zinc sulfate was alternatively used as a means of introducing the zinc ion, nitrilotriacetic acid trisodium salt monohydrate (NTA.Na.sub.3 H.sub.2 O) and ethylenediaminetetraacetic acid tetrasodium salt (EDTA.Na.sub.4) were evaluated as chelating agents, and the amounts of sodium carbonate and sodium tripolyphosphate were varied in order to establish the most satisfactory solution. The benefits of a chelating agent were clearly demonstrated with a formulation prepared from sodium dithionite manufactured by the zinc reduction process, as there was heavy floc formation in the three solutions without the nitrioltriacetic acid salt (Examples 8, 9, and 13), whereas the solutions with a chelating agent had only a slight amount of fines in the zinc-derived sodium dithionite (Examples 1-7, 10-12, and 14-16). Solutions of Examples 17-19 were also adequately stable although no chelating agent or zinc ion was added, but the alkalinity of the solutions was somewhat high and undesirable turbidity was noted.

Although the preferred form of NTA is the monohydrated trisodium salt (NTA.Na.sub.3.H.sub.2 O), the use of other forms of nitrilotriacetic acid (NTA), such as aqueous solutions thereof, is not excluded in the processes of this invention.

Other chelators that provide ligands for sequestration of metals, such as ethylenediaminetetraacetic acid (EDTA) are also useful, in the form of anhydrous salts, hydrates, or solutions, for making the storage-stabilized bleaching solutions of this invention.

Additional data illustrating the effects of the various additives on the stability of solutions derived from zinc-based sodium hydrosulfite is assembled in Table VII. For instance, in Example 22, wherein sodium tripolyphosphate and soda ash were omitted from Example 7, solution stability was extremely poor with 54.1% decomposition of the sodium dithionite after five days storage at 50.degree. F. When the sodium tripolyphosphate was included (Example 20), solution stability increased to 31.3% decomposition per five days. Likewise, the addition of soda ash (Example 21) provided improved stabilization, indicated by 4.2% decomposition per five days. However, only by combining both additives was the desired degree of stabilization acquired, as observed in Example 7.

With further reference to Table VII, the solution of Example 3 (in which the sodium tripolyphosphate was increased from 1.4% in Example 7 to 2.7% and the sodium carbonate level decreased from 2.61% in Example 7 to 1.29%) was more stable than when either additive was used individually at the levels shown in Examples 20 and 21. Since sodium carbonate normally exerts a greater influence on solution stability, the excellent stability achieved in Example 3 was somewhat unexpected.

On the basis of solution stabilities for formulations of Examples 3, 15, and 7, these solutions were chosen for groundwood pulp bleaching studies. Each solution was comparably used for bleaching a groundwood pulp against standards of zinc dithionite and a commercially available, dry sodium dithionite blend (Composition B). The data show that the formulation producing the solution of Example 3 produced brightness gains exceeding both standards. The solution of Example 15, which is somewhat similar to make-up to Example 3, was also equivalent to, or slightly better than, the commercially available sodium dithionite blend but it did not quite meet the brightness gains attained with zinc dithionite. The solution of Example 7 was disappointing on this particular pulp.

On the basis of storage and bleaching data, the formulation for the solution of Example 3 was selected for field evaluation at a paper mill in South Carolina. The results from this mill trial indicated that the formulation was superior to sodium dithionite solutions prepared from sodium borohydride in a solution of sodium hydroxide.

Utilization of formate-derived sodium dithionite introduces extra acidity above that from zinc-derived sodium dithionite that must be compensated for with caustic soda. In order to optimize stability of solutions prepared with this product without having excessive free alkalinity present, a series of solutions containing two levels of concentration of soda ash with various treatment levels of caustic soda were prepared and stored at 50.degree. F. As noted in Table IX, excellent solution stability was obtained when 1% sodium hydroxide (Na.sub.2 S.sub.2 O.sub.4 basis) was added to the test formulation containing, among other additives, 2.49% Na.sub.2 CO.sub.3 and 14.44% formate-derived sodium dithionite on a solution basis (Example 27). Likewise, comparable stability was obtained with 2.6% NaOH with the Na.sub.2 CO.sub.3 concentration being lowered to 1.29% and the sodium tripolyphosphate concentration being almost tripled, as demonstrated in Example 32 in Table IX and Example 40 in Table X.

In a series of storage tests which are assembled in Table XI with 14.44% formate-derived sodium hydrosulfite, 0.47% nitrilotriacetic acid trisodium salt monohydrate, and 0.18% zinc dithionite, the effects on solution stability of caustic soda, sodium carbonate and sodium tripolyphosphate are investigated further. Herein, the addition of 0.34% NaOH on a solution basis was found to be inadequate for acceptable solution stabilization as losses exceeded 40.0% within three days (Example 43). By combining 1.3% soda ash with the 0.34% caustic soda, stability of the stored solution increased to 4.7% decomposition of the sodium hydrosulfite after five days (Example 41). This trend towards increased stability continues through the addition of another alkaline agent, sodium tripolyphosphate, and upon combining each of the alkaline ingredients, solution decomposition dropped to only 1.4% after 5 days (Example 40). Removal of the caustic soda from this solution caused a significant increase in losses of from 1.4% to 9.1% after five days. Similar effects were evident in test formulations for Examples 44, 38, 45, and 26, where different concentration ranges were examined.

Evidence for the importance of sodium tripolyphosphate was further strengthened through bleaching studies with solutions containing different weight ratios of sodium tripolyphosphate to soda ash as present in Table XII. Here, there were no significant differences in solution stability but there was a definite trend toward increased brightness gains as the sodium tripolyphosphate concentration was increased from 0 to 2.67% (solution basis) in the formulation. Therefore, with solution stability comparable and bleaching ability enhanced by increasing the sodium tripolyphosphate content, the order of preference for large-scale use of these formulations is Examples 40, 38, and 35.

Because sodium tripolyphosphate reverts to the orthoposphate with time, sodium phosphate, with its readily available alkalinity, appears as a likely replacement for the tripolyphosphate. In addition, the alkalinity of the sodium phosphate presents an excellent opportunity for eliminating caustic soda from formate-sodium hydrosulfite based formulations.

At 50.degree. F., as shown in Tables XIII and XIV by Examples 50 and 52, the presence of 0.92% sodium phosphate stabilizes a formate-derived sodium dithionite solution at a level equivalent to 0.34% NaOH. Soda ash at 2.49% significantly improved the stability, but without caustic soda it was not until sodium phosphate and sodium carbonate were used in combination that an acceptable stability of 1.7% decomposition after seven days was attained in Example 51. Only slightly better stability, 1.2% decomposition per seven days, was demonstrated by Example 60 in Tables XV and XVI when caustic soda was present.

The use of sodium phosphate as a replacement for caustic soda and sodium tripolyphosphate was further researched at 70.degree.-75.degree. F., a temperature more suited for industrial applications because of the elimination of refrigeration for storage. Results of this phase of the investigation are assembled in Tables XIII-XVII. In Examples 55-57 at the 4% concentration level, sodium phosphate gave marginal stability to the solution at 75.degree. F., but of greater significance was the formation of a black floc, as noted in Example 56. The black floc was not noted, however, when zinc dithionite and nitrilotriacetic acid trisodium salt were included in the formulation and moreover, solution stability was slightly better as shown by Example 68.

Also shown in Tables XII-XVIII are other formulations such as Example 59 which have possible industrial applications. Bleaching results in Table XVIII with Examples 59 and 68 were found to be comparable to the bleaching standard based on zinc-derived sodium dithionite of Examples 3, 69, and 77 in Tables V, VI, VIII, XIII, XIV, XXI, and XXII, which was the very successful formulation field-tested initially at a South Carolina paper mill. Of the two newer formulations, Example 68 is preferable because it appears to be slightly more stable. However, one should not hesitate in using the formulation of Example 59 if soda ash were more readily available than sodium phosphate.

Examples 75 and 76 in Table XIX and XX demonstrate the importance of the zinc ion and the chelating agent. Without the two additives in question the decomposition tripled after five days at 50.degree. F.

In Tables XIX-XXII, sodium metaborate is substituted for sodium phosphate. Ambient decomposition tests results and brightness gains in Tables XX and XXII show good stability, as compared to Examples 8-14 in Table VI, Examples 53-59 in Table XIV Examples 66-68 in Table XVI, and Examples 69-71 in Table XVII. (18) Choices between the sodium metaborate and the sodium phosphate formulations can consequently be based entirely on raw materials availability, cost, and possible pollution problems that might be encountered through the use of either phosphate or boron because both provide adequate storage stability and bleaching effectiveness.

Upon examining the formulations of the successful examples, it is noteworthy that, in addition to sodium dithionite, only one (Example 57) contains two additives and only five (Examples 15, 21, 23, 35, 55) contain three additives. All others have at least four in addition to sodium dithionite.

It is apparent that the anaerobic storage stability of an aqueous sodium dithionite bleaching solution is sensitive to the relative proportions and interactions of a chelating agent, a source of zinc ion, and a complex former (e.g., Examples 9, 10, 13 compared to Examples 10-12 and 14) and that it is also critically dependent upon the source, quantities, and interactions of the complex former and the source of hydroxyl ion in ways that are not readily understood or predictable.

Because it will be readily apparent to those skilled in the bleaching art that innumerable variations, modifications, applications, and extensions of the discoveries hereinbefore disclosed can be made without departing from the spirit and scope of the invention, what is herein defined by such scope and is desired to be protected should be measured, and the invention should be limited, only by the following claims.