Background of the Invention

Field of the Invention

The invention relates to the field of paper manufacturing and, more particularly, to a process and composition for delignifying a lignocellulosic material, such as chemical wood pulp, using a mixture of monopersulfuric acid and a ketone.

Information Disclosure

Pulp is the raw material for the production of paper, paperboard, fiberboard and the like. In purified form, it is a source of cellulose for rayon, cellulose esters and other cellulosic products. Pulp is obtained from plant fiber such as wood, straw, bamboo and sugarcane residues. Wood is the source of 95% of the pulp fiber produced in the United States.

Dry wood consists of 40 to 50 percent cellulose, 15 to 25 percent other polysaccharides known as hemicelluloses, 20-30 percent lignin, a biopoly er which acts as a matrix for the cellulose fibers, and 5 percent of other substances such as mineral salts, sugars, fat, resin and protein. Lignin is composed primarily of methoxylated phenyl propane monomeric units interconnected by a variety of stable carbon- carbon and carbon-oxygen (ether) linkages. The lignin of conifers is apparently an oxidative polymerization product of coniferyl alcohol [3-(3'~ methoxy-4 '-hydroxyphenyl)allyl alcohol], while the lignin of deciduous trees appears to be derived from

coniferyl alcohol and sinapyl alcohol [3-

(3• ,5•dimethoxy-4 ^» -hydroxyphenyl)allyl alcohol] .

The strength of paper ultimately produced from pulp is dependent upon the chemical integrity of the cellulose, while the color arises from the lignin. The desired selectivity will be reflected in a low kappa number (little unbleached lignin) and a high viscosity of the residual pulp (little cleavage of long-chain celluloses) .

Chemical pulp is manufactured by dissolving the lignin with hot solutions of (1) sodium hydroxide, (2) calcium, magnesium, or ammonium bisulfite, or (3) a mixture of sodium hydroxide and sodium sulfide (made from lime and reduced sodium sulfate) . The products, known as soda pulp, sulfite pulp or sulfate (kraft) pulp, respectively consist of impure cellulose. In the chemical process, most of the hemicelluloses are also dissolved. Thus, the yield for chemical pulping is typically 40-60% based on wood weight. Mechanical pulps are characterized by their high yield and high lignin content. These pulps are called "mechanical" because a significant amount of mechanical energy (grinding and refining) is required to breakdown the wood chips. Chemical pulps contain about 5% lignin (weight basis) while mechanical pulps typically contain greater than 15% lignin. In order to make a white sheet from a chemical pulp almost all of the residual lignin must be removed. This is normally achieved by multistage bleaching using oxidants, some of which [chlorine (Cl ₂ ) , chlorine dioxide (C10 ₂ ) , and sodium hypochlorite (NaOCl) ] contain chlorine. Presently

bleached chemical pulp producers are seeking ways of decreasing or eliminating the use of chlorine- containing chemicals, the use of which leads to the formation and subsequent discharge of organochlorine compounds. Regulations to limit the discharge of adsorbable organic halogens (AOX) have already been established in several countries.

Replacement chemicals presently being used commercially or in research include oxygen, ozone, hydrogen peroxide and other peroxides. Oxygen is less selective than chlorine and chlorine dioxide and can therefore only be used for partial lignin removal. The cellulose is strongly affected, especially when the lignin content is low; therefore, the oxygen treatment must be of short duration.

Monopersulfuric acid or Caro's acid and its caroate anions have features that are attractive for kraft pulp bleaching: 1) Caro's acid is a more efficient solubilizer of lignin than is H ₂ 0 ₂ , 2) it is only marginally more expensive than H ₂ 0 ₂ because H ₂ S0 ₄ is the only reactant needed to generate it from H ₂ 0 ₂ , and 3) the sulfate anions in the resulting bleaching effluent can be recycled to the kraft recovery system. The use of Caro's acid for bleaching pulp is disclosed in U.S. Patents 4,404,061; 4,475,984;

4,756,800; 4,773,966; 5,004,523 and European Patent 415 149. The formation reactions for Caro's acid and caroate anions are summarized in the following equations:

X H ₂ S0 ₄ + H ₂ 0 ₂ <* H ₂ S0 ₅ + (X - 1) H ₂ S0 ₄ + H ₂ 0 H ₂ S0 ₅ + H ₂ 0 + ** HS0 ₅ ^' + H ₃ 0 ⁺ pKa<0

HSO _s - + H,0 SO, + H,0 ⁺ pKa = 9.4

Selected ketones react with caroate to form a dioxirane intermediate as shown for acetone in Scheme A:

Scheme A

HO

CH, CH,

II \

H-O-O-S-0 ^" -0-0- s-o ^"

CH.

H* OH ^"

Dioxiranes are capable of transferring an oxygen atom to a variety of donor compounds yielding an oxidized product and the ketone precursor. [See

Jeyaraman and Murray J _^ . Am. Chem. Soc. 106. 2462-2463 (1984) ] .

Lee (PCT application WO 91/12369) discloses that dimethyldioxirane (DMD) is effective in delignifying kraft pulps. However, because both DMD and acetone are highly volatile, they are isolated together from a caroate/acetone mixture. Therefore, the use of DMD as an oxidant according to Lee requires that the pulp be treated with large quantities of acetone. In WO

91/12369 the pulps were treated with anywhere from 2.72 to 5.28 times their weight of acetone. In a commercial situation, all of the effluent from such a treatment or stage would have to be channeled to a recovery system for the acetone. If only 5% of the solvent escapes recovery then a minimum of 136 kg of acetone/ton of pulp will have to be removed by secondary treatment or be discharged to the environment. The process described in WO 91/12369 is therefore not presently attractive on a commercial scale.

There is thus a need for a process for bleaching chemical wood pulp that minimizes or avoids the use of chlorine-based bleaching agents while at the same time not creating waste disposal problems of its own.

There is a further need for a process and a composition that are highly selective for solubilizing lignin but have very little effect on cellulose. Summary of the Invention

It is an object of the present invention to provide a process for bleaching chemical wood pulp that avoids the use of chlorine-containing bleaching agents.

It is a further object to provide a process and composition that are selective for lignin and minimize degradation of cellulose.

It is a further object to provide a process and composition for bleaching chemical pulp that minimize waste-disposal problems.

It is a further object to provide a process and composition that are economically attractive compared to existing systems.

These and other objects, features and advantages are provided by the present invention.

In one aspect the invention relates to a process for delignifying a lignocellulosic material, in particular for bleaching wood pulp, most particularly for kraft wood pulp. The process comprises exposing the pulp at pH 6.0 to 9.5 to a mixture of monopersulfuric acid and a ketone of formula

wherein R ¹ and R ² are independently selected from the group consisting of alkyl and aryl, or R ¹ and R ² together form a carbocycle. Preferably the ketone is present at from 1 to 4% of the dry weight of the pulp; preferred ketones are acetone, methyl ethyl ketone and cyclohexanone; and the pH is optimally maintained at about 7.0. An embodiment is characterized in that the kappa number of the wood pulp is reduced by ten or more while the viscosity of the pulp is reduced by less than 5 cp.

In other aspects of preferred embodiments the consistency is from about 1% to about 35% in water and monopersulfuric acid furnishes from 0.1 to 2.0% active oxygen based on the dry weight of the pulp.

In another aspect, the invention relates to a

composition for delignifying a lignocellulosic material comprising in combination:

(a) water;

(b) from about 0.05 to about 0.3 moles per liter of a ketone of formula

o II

wherein R ¹ and R ² are independently selected from the group consisting of alkyl and aryl, or R ¹ and R ² together form a carbocyclic ring;

(c) from about 0.0008 to 0.50 about moles per liter of monopersulfuric acid; and

(d) sufficient buffer to maintain the pH of the composition between about pH 7 and about pH 8.5. Preferably the ketone is selected from the group consisting of acetone, methyl ethyl ketone and cyclohexanone and the buffer is sodium bicarbonate.

In other aspects the invention relates to pulps that have been delignified by the processes described above and to the corresponding delignification mixtures. Chemical pulps may contain acetone and monopersulfuric acid which generate dimethyldioxirane within the pulp.

Brief Description of the Drawings

Fig. 1 is a graph of kappa number versus % active oxygen based on the weight of the pulp for a composition of caroate only and a composition according to the invention, containing ketone. The

peroxidic compounds involved each contain one active oxygen atom per molecule.

Fig. 2 is a graph of kappa number versus % ketone based on the weight of the pulp for two ketones: acetone and methyl ethyl ketone (MEK) .

Detailed Description of the Invention

The basis of the invention is the discovery that a combination of monopersulfuric acid and ketone provide a superior bleaching composition and process for chemical wood pulp.

The caroate/ketone delignification process of the invention involves several interrelated and competing reactions which can be represented schematically in Scheme B:

Scheme B

ketone dioxirane caroate

C ID

C2 l ignin 3D I ϊ g π i n

C 1 ntermed I ates D I ntermed I ates

C y oxidized

useful ly oxidized products of l ignin

(1) caroate reacts with ketone to generate dioxirane;

(2) dioxirane reacts with lignin to oxidize the lignin; (3) caroate reacts with lignin to oxidize the lignin; (4) dioxirane react with products of the caroate/lignin reaction to further decompose the lignin; (5) caroate reacts with products of dioxirane/lignin reaction to further decompose the lignin; and (6) caroate reacts with dioxirane to liberate molecular oxygen and decompose the dioxirane according to Scheme C:

Scheme C

a nd

Reaction (1) in scheme B is known and is discussed by Jeyaraman and Murray [J_j_ Am. Chem. Soc. 106, 2462- 2463 (1984)]. Reaction (2) is known and is the basis of the published PCT application of Lee (WO

91/12369) . Reaction (3) is also known and is the basis of patents cited above. The enhanced efficiency and selectivity, as well as the reduced requirements for ketone, of the present invention are believed to arise from the heretofore unknown facility of pathways involving reactions (4) and (5) . However, because reaction (6) consumes oxidant without producing product, the facility of reactions (4) and (5) are only observed and realized when reaction (6) is suppressed. (It must be remembered that in addition to the lignin decomposition shown in scheme B, there is a parallel series of competing cellulose decomposition reactions occurring simultaneously.) Thus the key to successful delignification is the discovery of conditions that take advantage of reactions (4) and (5) and maximize the rate of overall conversion of lignin to usefully oxidized lignin. By "usefully oxidized lignin" is meant that the oxidation and hydrolysis have proceeded far enough to yield products that will be washed out of the cellulose matrix (e.g. carboxylic

acids) . The "usefully oxidized" products of the pure dioxirane and the pure caroate processes are not necessarily the same and, in fact, are probably different from each other as well as being different from the products of the crossover oxidations. The most facile route to usefully oxidized lignin appears to be an initial attack by dioxirane on lignin (equation 2) followed by caroate oxidation of the intermediates (equation 5) .

The amount of residual lignin in a pulp is measured by its kappa number; 0.15 times the kappa number is the weight percent of lignin. Softwood pulp coming out of a kraft process has a kappa number of 20 to 30; hardwood pulp is somewhat lower: 10 to 20. It is desirable for most uses to reduce the kappa number as much as possible, but in any event to below about 10 for softwood and about 5 for hardwood. At the same time, the production of paper having a desirable degree of strength requires that the cellulose be minimally degraded. The integrity of cellulosic structures is measured by determining the viscosity of a cupri-ethylenediamine solution according to the procedures described in TAPPI standard method T230. Softwood pulp coming out of a kraft process has a viscosity, in this test, of about 22 to 40 centipoises (cp) . It is desirable to maintain the viscosity above 15 during delignification. The measure of a selective delignification process is thus a high ratio of viscosity to kappa number.

The results in Table 1 (using 0.47% active oxygen on pulp) show that caroate by itself is a

fairly efficient delignifier of pulp. Southern pine kraft pulp was treated with oxidant at 0.47% active oxygen on pulp at 2.67% consistency, pH 7.0 to 7.5 at 25° C for 2 hours followed by extraction with 2% NaOH on pulp at 12% consistency for 2 hours at 80° C. Acetone, when present, was at 15% on pulp. Dimethyldioxirane (DMD) is significantly more effective than caroate, resulting in a kappa number decrease of 14.7 as compared to 8.3 for caroate alone. The extent of delignification with the caroate/ketone mixture of the invention falls in between that of pure caroate and pure DMD.

Table 1

Treatment Kappa Number Viscosity, cp

1. Untreated 28.9 25.5

2. Caroate 20.6 25.1

3. Caroate/Acetone 17.9 22.6

4. DMD (pH«6) 14.2 20.1

5. DMD (pH 8.0) 14.8 22.1

With the small quantity of ketone that is preferred in the compositions and processes of the invention, it is unlikely that the concentration of dioxirane will be high enough to provide enough oxidizing power to completely oxidize a lignin molecule. What is more likely is that a low steady- state concentration of dioxirane will initiate the oxidation of the fully etherified aromatic structures in lignin which react only slowly with caroate. Once the aromatic nuclei have been ruptured, then the secondary structures are oxidized to carboxylic acids by the caroate which is present at a much higher

concentration. We have observed that DMD is quite effective at initiating de-aromatization of phenolic and fully etherified lignin model compounds.

Evidence in support of the hypothesis that the dioxirane concentration is low during caroate/ketone delignification can be found in Table 2 where a low kappa number pulp was afforded similar treatments to those described in Table 1. This pulp was manufactured from southern pine by modified kraft pulping followed by oxygen delignification. As before, acetone was used at 15% on pulp. If an oxidant depolymerizes cellulose, then the effect is normally more pronounced as the concentration of lignin is decreased. Lignin is much more reactive and serves to protect cellulose.

It can be seen that DMD caused a significant decrease in pulp viscosity while caroate alone did not. The caroate/ketone result is similar to that of caroate alone indicating that caroate and not DMD supplied most of the oxidizing power.

Table 2

Treatment Kappa Number Viscosity, cp

1. Untreated 14.8 23.8

2. Caroate 8.8 21.0

3. Caroate/Acetone 6.4 21.8

4. DMD 3.8 16.7

As mentioned above, the suppression of the

reaction of dioxirane with caroate (reaction 6) while still allowing the presence of oxidatively effective levels of both (for reactions 4 and 5) , is critical to obtaining the advantages of the process of the invention. It has been found that with respect to the equations in Scheme C above, reaction C2 is twenty-five times faster than reaction Cl. Since the pK _a of HS0 ₅ - + H ₂ 0 _* * S0 ₅ ⁼ + H ₃ 0 ⁺ is 9.4, keeping the pH of the reaction below pH 8.5 will minimize the amount of S0 ₅ ⁼ present and thereby decrease the rate of loss of oxidant. The effects of pH on the caroate/ketone delignification of the invention were examined. The untreated pulp had a kappa value of 27.0. Using acetone as the ketone, a kappa number of 7.2 (73% delignification) was obtained at pH 8.0. Lowering the pH to approximately 7.0 resulted in a pulp with kappa number 4.3 (84% delignification). Sodium bicarbonate has been found ideal for buffering the system at pH 7, but sodium carbonate, sodium hydroxide or sodium acetate could also be used. In contrast, and consistent with our understanding of the chemistry involved, pH plays only a minor role in DMD delignification (Note entries 4 and 5, in Table 1.)

The process of the invention, while sensitive to pH, does not appear particularly sensitive to temperature. Reaction temperatures of 10° C, 25° C, and 50° C were investigated in caroate/acetone delignification of a kraft pulp (kappa, 27.0; viscosity, 27.7 cp) . In all cases, the reaction was complete after 30 minutes and led to equivalent kappa numbers and pulp viscosities after alkaline extraction.

Most delignification processes based on peroxidic chemicals are sensitive to transition metals. Therefore, a softwood kraft pulp with kappa number 27.0 and viscosity 27.7 cp was acid-washed and, together with the unwashed pulp, was analyzed for transition metals. (Table 3) .

Table 3 Transition Metals in Souther Pine Kraft Pulp

Sample Transition Metals (ppm)

Fe Cu Mn Co V

Unwashed 24.3 5.2 75.5 0.048±0.008 2.45±0.02 Acid-Washed 13.2 1.5 5.0 0.023+0.008 0.23+0.02

When the pulps were treated with caroate (0.94% active oxygen on pulp) and acetone, the kappa numbers after alkaline extraction were identical (11.1) and the viscosities were nearly identical (25.0 cp for the unwashed and 25.4 cp for the acid-washed pulp). Similar results were obtained for another softwood kraft pulp which was not analyzed for transition metals. Surprisingly, it appears that the transition metals at the concentrations normally found in kraft pulps have little if any effect on caroate or caroate/ketone delignification. However, this does not appear to be the case with DMD delignification. When unwashed and acid-washed samples of an oxygen- delignified softwood kraft pulp (kappa no. 21.4; viscosity 26.9) were reacted with DMD (0.47% active oxygen) , the kappa number and viscosity were respectively 5.5 and 17.1 cp for the acid-washed sample and 7.3 and 20.8 cp for the unwashed sample.

What is most likely indicated is that transition metals in the unwashed sample catalyzed the decomposition of DMD to oxygen and acetone, resulting in a loss of oxidizing power.

The effect of caroate charge on delignification, with and without acetone, is shown by the plots in Figure 1. Without acetone, the delignification was conducted at 12% consistency (hand-mixing in plastic bags) and 40° C for 2 hours to ensure full consumption of the caroate. A charge of 0.94% active 0 ₂ produced a pulp with kappa number of 15.8 (41% delignification) . When the caroate charge was doubled, the kappa number fell only three additional units (53% delignification) . When acetone was added, according to the invention, (0.94% active 0 ₂ at 2.67% consistency and 25° C) the pulp had a kappa number of 11.1 (59% delignification), while in the presence of acetone, 1.88% active 0 ₂ produced a kappa number of 4.3 (84% delignification). Therefore, beyond 40% delignification, caroate/acetone was effective while caroate alone was not.

The effect of varying the nature of the ketone has also been examined. In 1974, Montgomery [J___ Am. Chem. Soc. 96. 7820-7821 (1974) ] first reported in situ generation of dioxiranes by caroate oxidation of ketones. In that paper the author reported that acetone, di-2-pyridyl ketone, l-(4-oxocyclohexyl) tri ethylammonium nitrate, N,N-dimethyl-4- oxopiperidinium nitrate, and cyclohexanone were effective catalysts in the oxidation of chloride ion and of polar blue, an anionic dye. Compared to acetone, however, the other ketones catalyzed caroate

decomposition more than they catalyzed oxidation (Table 4) . Therefore, it is unlikely that they would delignify to the same extent as acetone (along with caroate) ; however, they offer the potential advantage of a lower ketone charge because they are more effective at catalyzing oxidation reactions (Table 4).

Table 4 Catalysis of Caroate Reactions by Ketones

Relative Rates of Reactions

Ketone Decomposition Chloride Polar Blue of Caroate Oxidation Oxidation

None < 0.1 < 0.1 < 0.1

Acetone 1.0 1.0 1.0

Cyclohexanone 9.4 6.1 8.8

Di-2-pyridylketone 32 25 25

1 -(4-oxocy clohexyl) trimethylammonium nitrate, 1,000 501 630

N,N-dimethyl-4-oxo- piperidinium nitrate 1,400 1,300 930

Results of the testing of representative examples of ketones in caroate-ketone delignification are shown in Table 5. The process was carried out at 1.88% active 0 ₂ , pH 7.0 to 9.0, 2.67% consistency at 25° C for two hours on softwood kraft pulp having an initial kappa number of 27.0 and a viscosity of 27.7 cp.

Table 5

Comparison of Ketones as Catalysts in Caroate

Delignification of Softwood Kraft Pulp

Ketone Caroate Ketone Kappa Viscosity Consumed, % % on pulp Number cp

None 56 0 17.7 27.6

None* 96 0 12.8 25.0

Acetone 99 15 4.3 23.5

Methyethyl ketone 99 15 6.2 23.6

2-Pentanone 65 75 17.4 -

3-Pentanone 57 75 18.6 -

Cyclohexanone 99 90 7.0 25.7

92 2.5 10.6 —

_" 82 1.25 12.1 -

Di-2-pyridyl ketone 99 5.0 19.7 20.9

N , N-dimethy 1-4-oxo- piperidinium bromide 99 5.0 15.6 27.5

^♦ medium consistency 12%; 40° C

From the standpoint of ketone charge, cyclohexanone appears to be the most promising. A charge of 2.50% on pulp at 2.67% consistency corresponds to an aqueous phase concentration of 0.007 M, ten times lower than the minimum effective concentration for acetone. However, acetone and MEK were more effective than cyclohexanone in lowering the kappa number. Di-2-pyridyl ketone and N,N- dimethyl-4-oxopiperidinium nitrate were ineffective

Early delignification experiments were conducted with 75-100% ketone on pulp by dry weight. At 2.67% consistency, the acetone or methyl ethyl ketone (MEK) charge could be reduced to a minimum of 15% on pulp. By increasing the pulp consistency to 13.9 % in a Quantum Mark IV reactor, the acetone charge could be

further decreased to 3% on pulp while the MEK could be decreased to 4% on pulp (Figure 2) . A charge of 15% on pulp at 2.67% consistency corresponded to an aqueous phase acetone concentration of 0.071 M which is roughly equivalent to the 0.083 M concentration obtained with an acetone charge of 3% at 13.9% consistency. At 25% consistency, only 1.44% acetone on pulp would be required to give an aqueous phase concentration of 0.083 M. A charge of 3-4% acetone or MEK on pulp is lower than the minimum amount of ketone heretofore described for a delignification process by a factor of 68.

When the pulp was oxygen-delignified to kappa number 18.8 prior to caroate treatment (pH 7.0 to 7.5, 2.67% consistency, 2 hours at 25° C, 0.94% active 0 ₂ ) , acetone addition (15% on pulp) lowered the kappa number from 9.1 to 5.3 (Table 6). The pulp was easily bleached with C10 ₂ and suffered only minimal strength loss. The caroate/acetone pulp with its low kappa number and high viscosity would be ideal for final bleaching with ozone and hydrogen peroxide thus eliminating all chlorine-containing compounds.

Table 6

Caroate Delignification of Oxygen- Prebleached Southern Pine Kraft Pulp

Acetone in Residual Caroate Kappa Viscosity, Caroated Stage (% of Applied) Number cp

Untreated - 18.8 22.3

No 26 9.1 18.5

Yes 0 5.3 20.3

One skilled in the art will also recognize that in addition to the wood kraft pulp illustrated, one may apply the process of the invention to other known lignocellulosic materials preferably in comminuted form such as chips or to the pulps produced from such lignocellulosic materials to obtain analogous results.

While the preferred lignocellulosic species are woody materials, especially tree woods including softwoods and hardwoods, other lignocellulosic species commonly employed in making pulp and paper may be employed. Illustrative of these non-woody species are such materials as grasses, cereal straws, bamboo, cornstalks, sugar cane bagasse, kenaf, hemp, jute, sisal, esparto, reeds and the like.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that other changes in form and details may be made therein without departing from the spirit and scope of the invention.