US3384534A

US3384534A - Bleaching of wood pulps with thiourea dioxide

Info

Publication number: US3384534A
Application number: US418911A
Authority: US
Inventors: Robert R Kindron; George W Houg
Original assignee: FMC Corp
Current assignee: FMC Corp
Priority date: 1964-12-16
Filing date: 1964-12-16
Publication date: 1968-05-21
Anticipated expiration: 1985-05-21
Also published as: FR1461003A; DE1546252A1; ES320608A1; FI43670B; GB1095279A

Description

nited Sttes Ptent 3,304,534 Patented May 21, 1868 3,384,534 BLEACHING OF WOOD PULPS WITH THIOUREA DIOXIDE Robert R. Kindron, Pennington, and George W. Hoi g,

Princeton, N.J., assignors to FMC Corporation, New

York, N.Y., a corporation of Delaware No Drawing. Filed Dec. 16, 1964, Ser. No. 418,911 17 Claims. (Cl. 162-71) This invention relates to the bleaching of wood pulps which contain substantial amounts of non-cellulosic materials, and particularly to the bleaching of these difiicultly bleached pulps with a reducing-bleaching agent under mild conditions.

Wood pulps are classified generally into two main classes. One class is the so-called purified, or chemical, pulps which are prepared by pulping methods which involve chemical removal of much of the non-cellulose wood materials, such as lignins and other impurities, to leave a relatively purified pulp composed of on the order of 80 to 100% of cellulose. These pulping methods involve a chemical digestion, and are typified by the kraft, sulfite and the like methods. The purified wood pulps are quite similar to cotton, which also is of relatively high purity, ordinarily containing about 80 to 95% by weight of cellulose.

The other basic class of wood pulps is prepared by methods which leave the bulk of the non-cellulosic constituents of the wood in the pulp. These methods normally are referred to as mechanical and chemi-mechanical methods, in which the separation of the wood material into fibers operates primarily through mechanical attrition of the wood, either in the form of chips or as logs. The lack of a strong chemical attack on the wood material leaves the bulk of the non-cellulosie constituents in the fibers ultimately produced. Such fibers contain up to about 60% of cellulose, with the remaining elements in the pulp being on the order of 40 to 60% of non-cellulose wood mate= rials.

The strictly mechanical methods for producing pulps are represented by the so-called groundwood process, in which logs or other large pieces of wood are ground on a grinding stone, and the refining methods in which the wood chips are mechanically subdivided in disc or the like refiners. Chemi-mechanical methods involve softening of the wood with aqueous softening agents such as sulfites, bisulfites and the like (without substantial ex= traction of non-cellulosics) preliminary to mechanical subdivision of the wood into fibers. Pulps produced by either of these essentially mechanical means are referred to herein as low-cellulose wood pulps.

The low-cellulose wood pulps are particularly desired because of their low cost and generally satisfactory phys ical properties. Their preparation involves very little loss of the original wood, and methods of producing them are generally less expensive to operate than are the so-called chemical methods. The particular utility of these pulps is in the preparation of printing papers, newsprint, molded products, corrugated paper, boxboards and the like.

While retention of the bulk of the wood material in mechanical pulps is favorable from an economic point of view, however, it gives rise to a dark color in the pulp, and because of the large amount of non-cellulose material in these pulps it is difficult to bleach them to reasonably light colors.

Generally, methods found very eifective for bleaching the comparatively purified cellulose fibers, including cotton and the so-called chemical pulps, have not been found effective for the low-cellulose wood pulps. One example is the method of caustic-extracting a pulp, and then bleaching it with peroxygen chemicals. While this method is effective in bleaching kraft and sulfite pulps and cotton, the method is largely ineffective with the unpurified wood pulps having cellulosic contents on the order of 60%.

Another example involves treatment of cotton with aqueous caustic-formamidine sulfinate (a salt of thiourea dioxide) preliminary to alkaline-peroxide bleaching of cotton. The caustic, preferably used in the amount of 2 to 6% by weight in the treating solution, serves as a solubilizer and extractant for non-cellulose constituents in the cotton, and the combination of the caustic and formamidine sulfinate prepares the cotton fibers for subsequent alkaline peroxide bleaching, imparting some brightness itself. This method is not suitable for use with low cellulose wood pulps. Strongly caustic solutions characteristically do not bleach these impure pulps efiectively. Other methods of treating relatively pure textile cotton and chemically purified wood pulps likewise often are not useful with low-cellulose wood pulps.

It is a feature of this invention to provide a method of bleaching low-cellulose wood pulps which markedly brightens them, yet does not extract substantial amounts of non-cellulosics from the pulps It is a further feature of this invention to provide such a method which operates with an inexpensive, nonoxidizing bleaching agent. i:

It has now been found that thiourea dioxide is an ex cellent bleaching agent for low-cellulose wood pulps when it is used in very low concentrations and under moderate conditions not leading to damage of the pulp or extraction of undue amounts of non-cellulosics from the pulps. The low-cellulose wood pulp is treated for up to about 4 hours, and preferably 2 to 3 hours, at about to 300 F. and preferably 90 to 212 F. and most preferably at to 180 F., in an aqueous slurry at a pulp concentration of about 1 to 20%, and preferably about 3%, by weight at a pH of about 5.5 to 8.0 with thiourea dioxide in an amount to provide about 0.1 to 2.0% by weight of thiourea dioxide on the basis of the dry pulp.

In a preferred embodiment of this invention, the bleach= ing effectiveness of the thiourea dioxide solution is im= proved by incorporation of either or both of sodium bisulfite and sodium tripolyphosphate or other polyphos= phate in the bleaching system. The sodium bisulfite is used in a weight ratio to the thiourea dioxide of up to about 20 to 1, with a weight ratio of approximately 6 to 1 being preferred, and when this additive is used the preferred pH for the pulp slurry is about 5.5 to 7. The polyphosphate is used in an amount up to 1% on the weight of the pulp, with an amount of about: 0.5% being preferred.

The amount of thiourea dioxide employed, and the pH of the bleaching solution, are critical to the obtaining of optimum brightness in the pulp. Use of less than about 0.1 weight percent of thiourea dioxide on the basis of the pulp weight, or operation ata pH of less than about 5.5, results in poor bleaching. It is interesting that operation at a pH above about 8.0 provides a substantial dropofi in bleaching effect, even providing substantial darken-= ing of the fiber; this would not be expected from the prior art which is applied to the bleaching of pure cellulose fibers such as cotton fibers. The upper limit on the amount of thiourea dioxide expressed above, namely about 2.0%, is established on an economically practical basis. Effective bleaching can be achieved at substantially lower levels than the 2%, and operation with 0.1 to 1.0% is preferred.

The thiourea dioxide employed in this bleaching process is employed either as the acid or as an alkali metal salt of the acid, particularly the sodium and potassium salts. The acid has the following formula:

HN OH The thiourea dioxide sometimes is named aminoiminomethanesulfinic acid or formamidine sulfinic acid.

The pulps which are treated by the herein process are the low cellulose wood pulps, a term which is used herein to define wood pulps containing not more than about 60% by Weight of cellulose, with the remaining material being non-cellulose wood constituents. These pulps are typified by the mechanical and so-called chemi-mechanical pulps. The mechanical pulps are represented by the groundwood pulps and the refiner pulps, produced respectively by grinding wood logs on grinding stones, and by passing wood chips through a disc or other refiner to mechanically subdivide the wood into fibers. The chemimechanical pulps are provided by softening the wood prior to mechanical subdivision of the fibers. The softening agents may be aqueous sulfite, bisulfite or other common softening agents, used in a fashion to avoid extraction of more than on the order of to 20% of the non-cellulosics in the wood during the softening and pulping stages. These low-cellulose wood pulps are produced from a variety of wood sources, for example, spruce, fir, hemlock, pine, cottonwood, aspen and poplar.

The low-cellulose wood pulps are treated in the present process in the form of an aqueous slurry having a concentration of about 1 to 20% by weight, and preferably of about 3% by weight. Treatment of slurries containing substantially less than 1% of the pulp is uneconomical, requiring excessive equipment investment, while treatment of pulps having concentrations greater than about 20% is difiicult because of the mechanical problems as sociated with providing good mixing of the treating chemicals with the pulp. The process is a reducingbleaching process and accordingly is carried out under non-oxidizing conditions. In small scale operation it is sometimes desirable to establish a non-oxidizing, inert atmosphere over the pulp, e.g. by creating a nitrogen or other inert atmosphere in the bleaching chamber. In normal commercial scale operation this is unnecessary as normal plant operation conditions involving large batches are not such as to lead to oxidation.

The bleaching process of this invention is carried out at a temperature of about 90 to 300 F., preferably about 90 to 212 F., and most preferably about 160 to 180 F. These temperatures are such as to assure re= tention of good pulp properties, while at the same time providing effective bleaching in a reasonable time. Operation at temperatures below about 90 F. provides for too low brightness increases and for too long bleaching times, while operation at above about 300 F. is unnecessary and uneconomical. Normally the operation is carried out no higher than 212 F. except in such processes as the high consistency refiner bleach process.

The herein bleaching treatment is carried out for a time up to about 4 hours, and preferably about 2 to 3 hours. Operation for less than one hour normally does not provide the desired degree of brightness improvement, whereas operation for more than about 4 hours is unnecessary.

The pH of the aqueous phase of the pulp slurry is about 5.5 to 8.0. To provide this pH when the thiourea dioxide is used without sodium bisulfite, about 0 to 0.2% of an alkali, for example sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate or the like, is provided in the aqueous phase of the pulp slurry. More alkali, sometimes as much as 0.4% of the aqueous phase of the slurry may be required when sodium bisulfite is present, since this material reacts with and neutralizes some of the added sodium hydroxide. The

actual amount of alkali within this range employed is based on the acidity or alkalinity of the pulp being treated and its concentration in the pulp slurry, and is the amount required to provide the desired pH within the range of 5.5 to 8.0 pH.

Addition of sodium bisulfite and/or sodium tripolyphosphate or other polyphosphates materially improves the bleaching effect of the thiourea dioxide solution. The sodium bisulfite is used in amounts expressed in terms of the weight ratio of sodium bisulfite to thiourea dioxide, up to about 20 to 1. An amount of sodium bisulfite providing a weight ratio of about 6 parts of the bisulfite to 1 part of the thiourea dioxide is preferred. Use of more than the expressed upper amount of. 20 to 1 of bisulfite to thiourea dioxide is wasteful, not providing added bleaching advantage. The polyphosphate is employed in amounts up to 1% by weight. based on the dry weight of pulp, with 05% being preferred. Typical useful polyphosphates are sodium tripolyphosphate, tetrasodium pyrophosphate, sodium hexametaphosphate and the like.

Use of the sodium bisulfite is best carried out at a pulp slurry pH near the lower end of the herein range of 5.5 to 8. When bisulfite is employed addition of sufficient sodium hydroxide to raise the pH to upwards of 8.0 is wasteful of both sodium hydroxide and the bisulfite since these chemicals are partially consumed in a neutralization reaction. Accordingly, it is preferred to operate at a pH of about 5.5 to 7 when using sodium bisulfite.

The pH of the pulp is normally adjusted upwardly by addition of sodium hydroxide. However, equivalent alkalies such as sodium carbonate, potassium hydroxide, potassium carbonate and the like may be employed in place of the sodium hydroxide to adjust the pH to the desired level.

The bleaching process preferably is carried out by slurrying the pulp in water at the desired concentration, and adding the thiourea dioxide and if necessary the sodium hydroxide or other alkali, together with any desired ad= ditives such as the sodium bisulfite or the polyphosphate, and the like, to the pulp slurry either separately or to gether. Mild agitation is provided initially to provide even distribution of bleaching chemicals. The temperature is raised to the desired level by external heating means, by passing steam through the pulp slurry or the like. Follow= ing the bleaching treatment, if desired although not necessarily, the pulp is washed with water and either dried or used directly in producing end products.

The following examples are presented by way of illus-- tration of this invention and are not to be considered as limiting the scope thereof in any Way.

Example 1 A southern pine groundwood having an unbleached GE brightness of 61.0% was slurred with boiling dilution water in a slow-speed paddle mixer to a concentration at which, following addition of bleaching chemicals, the final concentration became 3% by weight of pulp. This concentration of 3% in the final slurry was used in the following examples, with the exception of Example 8, in which concentration Was varied. The pulp slurry was then transferred to a four-liter screw top polyethylene container, and the air in the container was replaced with nitrogen gas, after which the following chemicals, in the indicated amounts and dissolved in water, were added and the cap placed on the bottle. Thereafter the bottle containing the pulp slurry and added chemicals was shale en, and 50 ml. of pulp slurry was removed for pH determination. The bottle was then placed in a water bath held at F. to provide the desired bleaching temperature, and left there for 4 hours, Following the treatment the pulp was withdrawn, and its brightness determined on a Gardner refieetometer, with the brightness values being corrected to GE standards.

Thiourea dioxide was employed in the amount of 0.75 weight percent. based on the dry weight of pulp,

sodium hydroxide was present in the water phase of the pulp slurry at a concentration of 0.006 weight percent to provide a pH in the pulp of 7.1.

The brightness of the bleach pulp was 71.2%, a 10.2 point increase over the initial GE brightness.

Example 2.CornpariSOn with sodium hydrosulfitea known reducing agent bleach The procedure employed in this example was the same as that followed in Example 1. In this case a southern pine groundwood having an unbleached brightness of 61.0% was treated in a series of tests, each providing for a 4 hour treatment at 160 F., with chemicals indicated in the following table (Table 1) in the indicated amounts.

1 Percent by weight based on the dry weight of the pulp. 2 Sodium hydrosulfite.

3 Thiourea dioxide.

4 Sodium bisulfite.

Example 3 This example demonstrates the effect of the amount of thiourea dioxide employed in the process of this invention. A southern pine groundwood having a GE brightness of 61.0% was treated according to the procedure of Example l, again over a period of 4 hours at 160 F. The pH of the slurry of pulp was 7.1 to 7.2, which condition was achieved by providing sodium hydroxide in the aqueous phase of the slurry at a concentration of 0.005 to 0.006% by weight. The increases in brightness provided by the treatments are shown in column 2 of table 2 which follows.

TABLE 2 TDO percent Brightness 0.17. Pulp ln rease Basis Points Example 4 This example demonstrates the effect of sodium bisulfite when it is employed together with the thiourea dioxide bleaching agent. A southern pine groundwood having an unbleached GE brightness of 61.0% was treated as described in Example 1. The pH of the system was 6.6 to 7.2, and was obtained by providing sodium hydroxide at a concentration of 0.003 to 0.008% by weight of the aqueous portion of the pulp slurry. Thiourea dioxide was employed in the amount of 0.25% on the dry weight of the pulp, together with amounts of sodium bisulfite providing the weight ratios to the amount of thiourea dioxide indicated in the first column of Table 3 which follows. The brightness increases are shown in the last column of this table.

6 Example 5 This example demonstrates the effectiveness of a typical polyphosphate, sodium tripolyphosphate, in improving the bleaching effect of thiourea dioxide. A northern spruce/ fir groundwood having an unbleached GE brightness of. 59.7% was treated by the method described in Example 1. The treating chemicals were thiourea dioxide employed in the amount of 0.5% on the basis of the dry weight of the pulp, and sodium tripolyphosphate, again on the basis of the dry weight of the pulp, in the amounts indicated in the first column of Table 4 which follows. The pH of the system was 7.7 to 7.8, provided by employing sodium hydroxide in the aqueous system at a concentration of about 0.006 to 0.009% by weight. The brightness increases obtained after 4 hour treatments at 160 F. are shown in the last column of Table 4.

TABLE 4 Wei ht Parent Brightness STPP, pereznt pare nt pH N .0 H In rease 'lDO solu. b.=sis Points 1 Sodium tripolyphosphate.

Example 6 The effect of the pH on the bleaching effect of the thiourea dioxide is demonstrated in this example. A southern pine groundwood having an unbleached GE brightness of 61.0% was treated as described in Example 1. Thiourea dioxide was employed in the amount of 0.5% by weight based on the dry weight of the pulp, and the concentrations of sodium hydroxide in the aqueous system shown in Table 5 which follows, were employed to vary the pH. Brightness increases are shown in the third. column in Table 5.

The effect of temperature on the bleaching process is demonstrated in this example. A southern pine groundwood having an unbleached GE brightness of 61.0% was treated by the method shown in Example 1. Five-tenths percent of thiourea dioxide on the dry weight of the pulps was employed in these experiments, together with an amount of sodium bisulfite to provide a 6:1 weight ratio thereof to the thiourea dioxide. The pH was 6.6 to 6.7 (sodium hydroxide being present in the aqueous phase of the slurry at a concentration of 0.016 weight percent). Samples of this pulp were treated for 4 hour periods at the temperatures indicated in column 1 of Table 6 which follows, with the results shown in column 2 as brightness increases in GE percentage points.

TABLE 6 Weight Weight per 'ent Patio Brightness TDO TDO/ Increase 0. 5 6/ l 4. 8 0. 5 (ill 7. 8 0. 5 6/1 10. 2 0. 5 (5/1 10.8

Example 8 The effect of the slurry concentration of the pulp on the bleaching process is demonstrated in this example. A southern pine groundwood pulp having an unbleached GE brightness of 61.0% was prepared as aqueous slurries whose concentrations following addition of bleaching chemicals were those expressed in Table 7 which follows. The slurries were treated in accordance with the procedure described in Example 1. Five-tenths percent by 61A method of bleaching mechanical and chemimechanical low-cellulose wood pulps containing up to 60% cellulose, in which said low-cellulose wood pulp is treated at a temperature of 90 to 300 F. for up to 4 weight of thiourea dioxide on the dry weight of the pulp, 5 hours in an aqueous pulp slurry having a pH of 5.5 to 7.0 and sodium bisulfite in an amount to provide a 6:1 weight and containing 1 to 20% by weight of said pulp, thiourea ratio thereof to the thiourea dioxide, were employed. A dioxide in an amount to provide 0.1 to 2.0% of thiourea pH of 6.3 to 6.7 was provided by employing sodium hydioxide on the weight of the pulp and sodium bisulfite in droxide in the aqueous phase of the slurries at the conan amount up to 20 times the weight of said thiourea dicentmtions set forth in Table 7. The brightness increases 10 oxide. which were achieved are shown in this Table 7. 7. The method of claim 6 in which the sodium bisulfite TABLE 7 Weight percent. Pul Weight Ratio NaOli weight Brightness Concentration TDO Nmszo rno percent in Increase Aqueous Phase Points It can be seen from the above examples that thiourea dioxide is a very eifective bleaching agent for the lowcellulose wood pulps when employed in the amount of 0.25% or more by weight based on the dry weight of pulp at a pH of 5.5 to 8.0. It is shown also that the effectiveness of this process is dependent upon careful observance of certain critical limitations on amounts of ingredients, pH, temperatures and the like. It is also shown that thiourea dioxide is better than one of the best prior art reducing bleaches, sodium hydrosulfite, in its bleaching efiectiveness. These excellent bleaching results are con= firmed with other low-cellulose wood pulps, and the method of this invention provides a very useful means for bleaching these diificultly bleached pulps.

Pursuant to the requirements of the patent statutes, the principle of this invention has been explained and exemplified in a manner so that it can be readily practiced by those skilled in the art, such exemplification including what is considered to represent the best embodiment of the invention. However, it should be clearly understood that within the scope of the appended claims, the invention may be practiced by those skilled in the art, and having the benefit of this disclosure, otherwise than as specifically described and exemplified herein.

What is claimed is:

1. A method of bleaching mechanical and chemimechanical low-cellulose wood pulps containing up to 60% cellulose, in which said low-cellulose wood pulp is treated at a temperature of 90 to 300 F. for up to 4 hours in an aqueous pulp slurry having a pH of 5.5 to 8.0 and containing 1 to by weight of said pulp and thiourea dioxide in an amount to provide 0.1 to 2.0% of the thiourea dioxide on the weight of the pulp.

2. The method of claim 1 in which 0.1 to 1.0% of thiourea dioxide on the weight of pulp is used.

3. The method of claim 1 in which the temperature of the bleaching treatment is 90 to 212 F., and the time of said treatment is 2 to 3 hours.

4. The method of claim 1 in which the temperature of the bleaching treatment is 160 to 180 F., and the time of said treatment is 2 to 3 hours.

5'. The method of claim 1 in which the pulp concentration in the slurry is about 3%.

is used in the amount of about 6 times the weight of thiourea dioxide employed.

8. The method of claim 6 in which 0.1 to 1.0% of thiourea dioxide on the weight of the pulp is used.

0. The method of claim 6 in which the temperature of the bleaching treatment is to 212 F., and the time of said treatment is 2 to 3 hours.

10. The method of claim 6 in which the temperature of the bleaching treatment is to F., and the time of said treatment is 2 to 3 hours.

11. The method of claim 6 in which the pulp concentration in the slurry is about 3%.

12. A method of bleaching mechanical and chemimechanical low-cellulose wood pulps containing up to 60% cellulose, in which said low-cellulose wood pulp is treated at a temperature of 90 to 300 F. for up to 4 hours in an aqueous pulp slurry having a pH of 5.5 to 8.0 and containing 1 to 20% by weight of said pulp, thiourea dioxide in an amount to provide 0.1 to 2% of thiourea dioxide on the weight of the pulp and a polyphosphate in an amount of up to 1% of the weight of the pulp.

13. The method of claim 12 in which the polyphos= phate is sodium tripolyphosphate and said sodium tripolyphosphate is employed in an amount of about 0.5% of the weight of the pulp.

14. The method of claim 12 in which 0.1 to 1.0% of thiourea dioxide on the weight of the pulp is used.

15. The method of claim 12 in which the temperature of the bleaching treatment is 90 to 212 F., and the time of said treatment is 2 to 3 hours.

16. The method of claim 12 in which the temperature of the bleaching treatment is 160 to 180 F., andthe time of said treatment is 2 to 3 hours.

17. The method of claim 12 in which. the pulp concentration in the slurry is about 3%.

References Cited UNITED STATES PATENTS 2,860,944 11/1958 Young 16276X DONALL H. SYLVESTER, Primary Examiner. H. R. CAINE, Examiner.

Claims

1. A METHOD OF BLEACHING MECHANICAL AND CHEMIMECHANICAL LOW-CELLULOSE WOOD PULPS CONTAINING UP TO 60% CELLULOSE, IN WHICH SAID LOW-CELLULOSE WOOD PULP IS TREATED AT A TEMPERATURE OF 90* TO 300* F. FOR UP TO 4 HOURS IN AN AQUEOUS PULP SLURRY HAVING A PH OF 5.5 TO 8.0 AND CONTAINING 1 TO 20% BY WEIGHT OF SAID PULP AND THIOUREA DIOXIDE IN AN AMOUNT TO PROVIDE 0.1 TO 20% OF THE THIOUTEA DIOXIDE ON THE WEIGHT OF THE PULP.