Process for bleaching and delignification of lignocellulosic materials

Abstract: Delignification and bleaching of lignocellulosic material is enhanced after the pulp has been treated with peroxomonosulfuric acid.

Description: Background of the Invention Bleaching of lignocellulosic materials can be divided into lignin retaining and lignin removing bleaching operations. In the case of bleaching high yield pulps like Groundwood, Thermo-Mechanical Pulp and Semi-Chemical pulps, the objective is to brighten the pulp while all pulp components including lignin are retained as much as possible. This kind of bleaching is lignin retaining. Common lignin retaining bleaching agents used in the industry are alkaline hydrogen peroxide and sodium dithionite (hydrosulfite). Hydrogen peroxide decomposes into oxygen and water with increasing pH, temperature, heavy metal concentrations, etc. The decomposition products, radicals like HO · and HOO · , lead to lower yields by oxidation and degradation of lignin and polyoses. Therefore, hydrogen peroxide is stabilized with sodium silicates and chelating agents when mechanical pulps (high yield pulps) are bleached. The bleaching effect is achieved mainly by the removal of conjugated double bonds (chromophores), by oxidation with hydrogen peroxide (P), or reduction with hydrosulfite (Y). Other bleaching chemicals more rarely used are FAS (Formamidine Sulfinic Acid), Borohydride (NaBH₄), Sulfur dioxide (SO₂), Peracetic acid, and Peroxomonosulfate under strong alkaline conditions. Pretreatments including electrophilic reagents such as elemental chlorine, chlorine dioxide, sodium chlorite and acid H₂O₂ increase the bleaching efficiency of hydrogen peroxide bleaching as described in Lachenal, D., C. de Chondens and L. Bourson. "Bleaching of Mechanical Pulp to Very High Brightness." TAPPI JOURNAL, March 1987, Vol. 70, No. 3, pp. 119-122. In the case of bleaching chemical pulps like kraft pulp, sulfite pulps, NSSC, NSSC-AQ, soda, organosolv, and the like, that is to say with lignocellulosic material that has been subjected to delignifying treatments, bleaching includes further lignin reducing (delignifying) reactions. Bleaching of chemical pulps is performed in one or more subsequent stages. Most common bleaching sequences are CEH, CEHD, CEHDED, CEDED, CEHH. (C chlorination, E caustic extraction, H alkaline hypochlorite and D chlorine dioxide). In all of these bleaching sequences, the first two stages are generally considered as the "delignification stages". The subsequent stages are called the "final bleaching". This terminology describes the main effects that can be seen by the specific chemical treatments. While in the first two stages the most apparent effect is the reduction of residual lignin, in the subsequent stages the most distinguishable effect is the increased brightness. With the development of new mixing devices like high shear mixers at medium consistency, oxygen delignification and oxygen reinforced extraction stages have been commercialized in numerous mills (Teuch, L. Stuart Harper. "Oxygen-bleaching practices and benefits: an overview". TAPPI JOURNAL, Vol. 70, No. 11, pp. 55-61). Although oxygen delignification; i.e. application of oxygen prior to the chlorination (C) stage, could be implemented because of economical advantages, environmental concerns arise. This is due to the considerable amount of chlorinated organic compounds such as dioxins in the paper mill effluent and in the resulting product. These problems have highly accelerated the implementation of oxygen stages to avoid the chlorination products. Oxygen delignification stages can yield delignification rates of up to 65% on kraft and sulfite pulps. In the industry, however, most mills operate oxygen stages with delignification rates between 40 and 45%, because the reaction becomes less selective at higher delignification rates. As a consequence, pulp viscosity and pulp strength properties drop steeply when operating beyond a delignification rate of about 50%. As environmental regulations by the authorities in Europe, Canada and in the U.S. are becoming increasingly stringent, extensive research and developments throughout the industry are focused on the enhancement of oxygen delignification. All of these studies have one goal in common; increasing the selectivity of oxygen by increasing the reactivity of the residual lignin prior to the oxygen stage. Several pretreatments have been explored and published. (Fossum, G., Ann Marklund, "Pretreatment of Kraft Pulp is the Key to Easy Final Bleaching", Proc. of International Pulp Bleaching Conference, TAPPI, Orlando 1988, pp. 253-261). All of these pretreatments with elemental chlorine, chlorine dioxide, ozone, nitrogen dioxide, acid hydrogen peroxide, etc. convert lignin to more easily oxidizable substances and make the subsequent oxygen stage more selective towards delignification. At the same time, viscosity loss of the oxygen delignified pulp is reduced. As the main driving force for the implementation of pretreatments is the reduction of chlorine containing bleaching agents, all processes which use chlorine containing agents are anticipated to have very little viability for the future. Some known pretreatments without chlorine such as Prenox®, PO A or ozonation involve heavy capital investment and are therefore unattractive from the commercial standpoint. It is generally presumed that during the acid hydrogen peroxide pretreatment with and without oxygen, the aromatic ring is hydroxylated. This hydroxylation action weakens the ring stability so that the subsequent oxygen treatment can cleave the aromatic ring more easily. The relatively extreme reaction conditions as described by Suess, H. U. and O. Helmling, (Acid hydrogen peroxide/oxygen treatment of kraft pulp prior to oxygen delignification. Proc. International Oxygen Delignification Conference, TAPPI, pp. 179-182, 1987) show that the effect of acid hydrogen peroxide on enhancement of oxygen delignification is very limited. The effect can be enhanced with organic peracids but organic peracids have the disadvantage that transportation of quantities needed in the pulp and paper industry would be too expensive to be feasible. On-site manufacturing is also not practicable because of the very large sized reaction vessels that would be required. This is due to the fact that long residence times are needed to reach equilibrium. Another disadvantage of using organic peroxides would be that after the reaction, the organic acid and residual peracid in the filtrate would drastically increase the TOC, BOD and COD concentration in the effluent with all its negative environmental impacts. EP-A-O 190 723 discloses a process for the improvement of mechanical properties of a chemical or semi-chemical bagasse pulp by treating these materials with peroxomonosulphuric acid or a salt of this acid. No desired level of the final pH is mentioned. Summary of the Invention An object of the invention is to provide a process for the treatment of lignocellulosic materials using peroxomonosulfuric acid (Caro's acid) and/or its salts in combination with oxygen and an optionally a peroxide. Caro's acid has the advantage over hydrogen peroxide in that it reacts faster, at milder reaction conditions, and by far more selectively towards lignin oxidation. It has been found that the treatment of lignocellulosic materials with peroxomonosulfuric acid and/or its salts at a wide range of reaction conditions in accordance with claim 1 yields an extraordinary enhancement of subsequent delignification and bleaching in combination with oxygen delignification and oxidative stages containing oxygen and an optionally a peroxide. The present invention is characterized by the synergistic effect that at the same time, pulp viscosity is maintained at comparable levels of commonly run oxygen delignification stages and strength properties are even improved. Detailed Description of the Invention Lignocellulosic materials such as untreated wood, wood chips and annual plants like corn stalks, wheat straw, kenaf and the like can be used in accordance with the invention. Especially suitable is material that has been defiberized in a mechanical, chemical processes or a combination of mechanical and chemical processes such as GW, TMP, CTMP, kraft pulp, sulfite pulp, soda pulp, NSSC, organosolv and the like. It is this kind of material in an aqueous suspension, hereinafter referred to as pulp, which is treated in accordance with the present invention with peroxomonosulfuric acid and/or its salts and subsequently subjected to an oxygen and an optionally peroxide stage. Peroxomonosulfuric acid can be applied by dissolving commercial grades of its salts such as Caroat® (Degussa AG) or by on-site generation e.g. by mixing high strength hydrogen peroxide with concentrated sulfuric acid or SO₃ prior to the addition point. Peroxomonosulfuric acid and/or its salts can be used alone or simultaneously together with H₂O₂ and/or molecular oxygen, preferably without molecular oxygen. The consistency of the pulp can range from 0.01% to 60% preferably from 1% to 30%. The peroxomonosulfuric acid and/or its salts contains more or less excess acid, depending on its source. Therefore, it is customary that a chemical base such as NaOH, MgO, etc. be added to the pulp in order to control the acidity at a desired pH level. Any suitable alkaline material can be used to control acidity provided it does not adversely effect the process or product. Any sequence of chemical addition, including the simultaneous addition, can be carried out. Typically, the starting pH (after addition of caustic and addition of peroxomonosulfuric acid and/or its salts) is between 7 and 11. With the course of the reaction, the pH drops to a final pH mainly because of the liberation of sulfuric acid. As the sulfuric acid being released derives from the peroxomonosulfate anion, the higher the peroxomonosulfuric acid charge is, the greater is the drop in pH. The final pH is between 3 and 5. The Caro's acid treatment is carried out with 0.01% to 3% (based on oven-dry weight of pulp) of active oxygen contained in the peroxomonosulfuric acid and/or salt. Preferred chemical charge is 0.05% to 1.5% AO (active oxygen). Trials have shown that the treatment (peroxomonosulfuric acid stage) is very little effected by temperature; that is, the reaction is not very temperature dependent. Thus, the peroxomonosulfuric acid (and/or salt) is effective at low temperatures such as 5°C as well as at temperatures of up to 100°C. Preferable temperatures for the treatment are however in the range of 15°C and 70°C. Depending on temperature, pH and chemical charge the residence time required is between 1 second up to 10 hours. It is to be noted that the peroxomonosulfuric acid (and/or salt) stage can be applied to any kind of treated (bleached) or untreated (e.g. brown stock) pulp. Advantageously, one or more heavy metal and organic contaminants eliminating process steps can be carried out to favorably impact the delignification efficiency of the aforesaid stage. Peroxide stabilizing agents (such as silicate, chelating agents like Na₅DTPA, Na₄EDTA, DTPMPA, etc.) and cellulose protecting agents like urea, magnesium salts, etc. are favorable for the process. The actual synergistic effects of treatment with peroxomonosulfuric acid (and/or salt) under the described conditions are not immediately apparent right after the treatment. The synergistic effects thereof however become apparent once the pulp is subsequently subjected to oxygen delignification or oxidative extraction with oxygen, or a combination of oxygen and peroxide described by Epo, EoP, Op, Eop. Thus, according to the invention, the beneficial and synergistic effects achieved by the Caro's acid treatment described hereinafter become apparent after further process steps are carried out; The effects are dramatically enhanced delignification and bleaching without additional pulp viscosity losses. This result could not have been predicted from what has gone before. As described in "The Chemistry of Delignification", Part II by Gierer J., Holzforschung, 36 (1982), pp. 55-64, acid hydrogen peroxide and organic peracids like peracetic acid hydroxylate the aromatic rings of lignin through the formation of perhydroxonium cations H₃O₂⁺; that is, HO⁺. It is known in the art that hydrogen peroxide does not react readily with Kraft lignin. An explanation can be found in Blaschette A. and D. Brandes Chapter VII, "Nichtradikalische (polare) Reaktionen der Peroxogruppe", pp. 165-181. "Wasserstoffperoxid und seine Derivate", Editor W. Weigert, Huthig Verlag 1978. Electrophilic substitution on the aromatic ring with a peroxide can also be described as a nucleophilic substitution on the peroxidic oxygen of the peroxygen compound. The π-electrons of the aromatic group attack nucleophilically the peroxidic oxygen. In the transition state, the YO⁻ is removed quicker the less basic YO⁻ is (see reaction below). Applying this to the reaction of acid hydrogen peroxide and peracetic acid, it is believed to present an explanation of why hydrogen peroxide is a weaker hydroxylation agent than peracetic acid. In the case of H₂O₂, the removed molecule is water (H₂O), a relatively weak acid; in the case of peracetic acid it is acetic acid, a moderately strong acid. As peroxomonosulfuric acid removes sulfuric acid (a very strong acid), the hydroxylation occurs more rapidly. The hydroxylation of the aromatic rings, however, is not enough in order to extract the lignin from the pulp. In a subsequent alkaline oxygen stage, the biradical molecule oxygen or radicals deriving from decomposition of H₂O₂ are trapped by the anions of the hydroxylated lignin, which are then oxidized to the quinonoid forms. Under the reaction conditions of these stages quinones are easily further degraded. As a consequence, oxygen and/or H₂O₂ is consumed more completely by the additionally hydroxylated lignin. Less attacks of the cellulose are possible which lead to less fiber damage, i.e. higher viscosities, more lignin degradation and bleaching. The relatively small brightening effect that results from this treatment stage with peroxomonosulfuric acid (and/or its salts) alone is believed likely to arise as a consequence of also partly hydroxylated aliphatic double bonds, partly removal and/or destruction of lignin and lignin fragments and other reactions as described by Gierer, J. The reason why this treatment stage also enhances subsequent alkaline peroxide bleaching stages can be traced back to the same mechanism. The treatment stage in which peroxomonosulfuric acid and/or its salts is used can be designated by the symbol "X". The new process which is the subject of this invention features a combined application of the X stage with any other kind of oxygen and/or peroxide stage, generally described by the symbol [OX]. The new process can be abbreviated by "X-[OX]" whereby "[OX]" can stand for O (oxygen delignification, Eo, Ep, Eop, (extraction stages reinforced with oxygen, peroxide, oxygen and peroxide, and P (peroxide stage). The process can be used repeatedly and in combination with other bleaching stages commonly used in order to delignify and bleach to required levels. The two treatments, step X and [OX] can be conducted with and without intermediate washing. If intermediate washing is applied, any kind of wash water not negatively affecting the overall effects of this process can be used, i.e. [OX] filtrate. It is, however, indispensible that the X step is performed prior to the [OX] step. The following examples serve to illustrate the present invention without limiting it in any way. Example 1 Unbleached southern hardwood kraft pulp was subjected to an acidic pretreatment in order to eliminate heavy metals from the pulp. The pretreatment was performed at pH 2.0 (adjusted with H₂SO₄) 50 ° C, 2 % cons. in the presence of about 0,2 % of Na₂SO₃ and 0,2 % Na₅DTPA for 30 minutes. The pulp was dewatered to 30 % consistency without additional washing. The pulp was then divided into 8 even samples of 50g O.D. each. Reaction conditions and pulp properties are outlined in Table 1. Between the oxidative pretreatment and the oxygen stage thorough washing with deionized water was applied to the pulp in order to prevent interferences due to carry-over of different amounts of residual chemicals The results of these trials show that oxygen delignified by far more selectively after treatment with Caroat (peroxomonosulfate). The difference compared to acid hydrogen peroxide (pretreatment trial 21) is not only even higher delignification in the O stage it is the superior selectivity of oxygen in the O stage that is dramatically improved by the X pretreatment. Compared to the standard oxygen stage (trial #1) delignification could be improved in trial 8 by 84% rel. At the same time, viscosity dropped by only 9%. Additional trials were performed identical to trial #4 except that the NaOH charge in the X stage was varied in order to see the effect of pH in the X stage on delignification efficiency of the following O stage. Trial No. 9 10 11 12 13 14 NaOH charge - 0.10 0.80 2.00 2.80 3.60 pH initial 1.40 3.1 3.7 9.3 10.4 10.5 pH final 1.40 2.4 3.2 4.8 7.7 9.8 brightness after O₂ 50.9 50.6 51.0 53.4 57.0 57.9 Kappa after O₂ 6.9 6.9 5.9 5.4 5.9 6.1 Viscosity after O₂ 16.0 15.9 16.2 16.6 15.6 15.7 These trials showed the applicability of the X stage over a wide pH range. An optimum in efficiency could be found around a final pH of 3 to 5. Example 2 The same unbleached hardwood kraft pulp was acidic washed as described under Example 1. Afterwards, the pulp was bleached in a X₁-O-X₂-Eo-P to a final brightness of 76.5 and a final viscosity of 13.1. Bleaching the pulp in X₁-O-X₂-Eo-D, final brightness and viscosity was 85.3 and 12.8, respectively. Chemical charges and reaction conditions were (X = 0.5% AO (Caroat); 1.8% NaOH; 0 = 3.2% NaOH, 0.3 MPa O₂; X₂ = 0.25% AO (Caroat); Eo = 1.6% NaOH, 0.3 MPa O₂ and P = 0.47% H₂O₂ and 0.8% NaOH). A final brightness of 86.3% ISO and final viscosity of 12.2 could be achieved bleaching the same raw material in a X₁-O-X₂-Eop-D sequence. All chemical charges were the same as in trial 1. 1.0% active chlorine as ClO₂ was applied in the final D stage and in Eop: 0.4% H₂O₂. This example demonstrated that repeated application of the "X-[OX]"-Process led to fully bleached pulp brightness levels. Example 3 Unbleached southern pine kraft pulp was treated according to Example 1. The reaction parameters are outlined in the table below. This example should compare the effects the X-[OX] process has on strength properties compared to a common oxygen delignification. The "X-[OX]" process (trial 2), compared to regular oxygen delignification (Trial 1), yielded a 53% higher delignification rate and a pulp with a brightness of 4.4 points higher, a tear index of 42% higher, the burst index was 3% higher aid the Tensile index was 14% higher. Compared to all other known processes that enhance oxygen delignification, these results were surprising and unexpected. Trial No. 1 Reference 2 Raw material Kappa 23.7 23.7 Acid wash + + Pretreatment AO (%) (Caroat®) - 0.5 NaOH (%) - 1.8 Consistency (%) - 15 Temperature (°C) - 40 Time (min.) - 60 pH initial - 8.8 pH final - 3.6 Residual AO (%) - 0.03 Oxygen stage MgSO₄ (%) 0.5 0.5 O₂ (MPa) 0.3 0.3 NaOH (%) 3.2 3.2 Consistency (%) 20 20 Time (min.) 60 60 Temperature (°C) 100 100 pH initial 12.3 12.5 pH final 10.6 10.5 Brightness (%) 32.2 36.6 Kappa 15.1 10.5 Delignification (%) 36.3 55.7 Tear index (mNm²/g) 7.10 10.09 Tensile index (Nm/g) 6.75 7.69 Burst index (kPam²/g) 4.95 5.09 Breaking length (km) 11.2 12.0 CSF (ml) 500 500 In a relative recent paper ("Pretreatment of Kraft Pulp is the Key to Easy Final Bleaching", by Greta Fossum and Ann Marklund, TAPPI, Proc. 1988 International Pulp Bleaching Conference, pp. 253-261), a variety of pretreatments are compared. Example 4 In order to find out the contribution each chemical (HSO₅⁻, O₂ and NaOH) has in the overall effect, another series of trials was conducted. Unbleached southern pine kraft pulp was treated according to Example 1 prior to performing various bleaching trials, as described in Table 5. In order to identify each chemical contribution to the overall effects of the "X-[OX]" treatment, the following procedure was chosen. The prewashed raw material was split into two even parts of pulp. One part was subjected to the X treatment, the other part was subjected to the same treatment but no active oxygen was added. After completion of the first step, both pulp samples were diluted with deionized water to 2% consistency, dewatered on a Buchner funnel, thoroughly washed with even parts of water and thickened to 30% consistency. Both samples were divided again into two even parts of pulp. All samples were subjected to oxygen delignification conditions (even in the same reactor), except that one of each pair of samples was charged with nitrogen instead of oxygen. By that, the effect of oxygen, together with caustic soda and the effect of caustic soda alone, could be investigated. Trial 1 2 3 4 Raw Material E O X-E X-O Kappa # 27.8 27.8 27.8 27.8 Viscosity [MPa.s] 30.9 30.9 30.9 30.9 Brightness [%] 27.6 27.6 27.6 27.6 1st Stage AO (Caroat) (%) - - 0.25 0.25 NaOH (%) 0.25 0.25 0.80 0.80 Consistency 15 15 15 15 Temperature (°C) 40 40 40 40 Time (min) 60 60 60 60 pH Initial 4.5 4.5 6.8 6.8 pH Final 4.5 4.5 3.3 3.3 Residual AO (%) - - 0.10 0.10 Brightness (%) 27.5 27.5 29.3 29.3 2nd Stage O₂ (MPa) - 0.3 - 0.3 N₂ (MPa) 0.3 - 0.3 - Consistency (%) 20 20 20 20 Time (min) 60 60 60 60 Temperature (°C) 100 100 100 100 NaOH % 3.2 3.2 3.2 3.2 pH Initial 12.8 12.9 12.8 12.9 pH Final 12.5 12.5 12.5 12.2 Brightness (%) 31.7 37.2 33.5 40.6 Kappa (%) 24.7 22.0 17.2 13.0 Viscosity (%) 30.8 20.3 27.7 22.4 The results provide the synergistic effects of the combined (sequential) treatment of pulp with, first, peroxomonosulfuric acid and, second, an oxygen delignification stage. The results demonstrate that although the delignification rate achieved with X-O was clearly higher than in O, the viscosity loss was much less than expected. The "X-[OX]" process proved to have synergistic effects on brightness increase, delignification, viscosity preservation and strength characteristics. Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the appended claims.

Claims: A process for bleaching and delignification of lignocellulosic pulp with a source of peroxomonosulfuric acid and subsequently with another agent, characterized in that an alkaline acting material is added during the treatment with peroxomonosulfuric acid and/or its salts in order to control the acidity until a final pH of 3 to 5 is obtained, and subsequently subjecting said pulp to an oxygen treatment to obtain the desired degree of delignification and/or brightness without significant cellulose degradation or increase in viscosity loss, while strength properties of the pulp are improved. A process according to claim 1, wherin said pulp is subjected to an oxygen and peroxide treatment. A process according to claim 1 or 2, wherein a peroxide stabilizer is added during the treatment with peroxomonosulfuric acid. A process according to claim 1 or 2, wherein the stabilizer are DTPA, EDTA, DTPMPA, silicate or Mg salts. A process according to claim 1 or 2, wherein 0,01 % AO (active oxygen) to 3 % AO (active oxygen) is used in the peroxomonosulfuric acid treatment. A process according to claim 1 or 2, wherein the subsequent stage contains a combination of oxygen and peroxide commonly described by Eop, Epo, EoP and Op. A process according to claim 1 or 2, whereby no intermediate washing is carried out between the peroxomonosulfuric acid treatment and the subsequent oxygen or oxygen/peroxide treatment. A process according to claim 1 or 2, whereby one or more intermediate washing steps are carried out between the peroxomonosulfuric acid treatment and the subsequent oxygen or oxygen/peroxide treatment. A process according to claim 8, whereby fresh water is used as dilution and/or wash water. A process according to claim 8, whereby the filtrate of the subsequent oxygen or oxygen/peroxide stage is used as dilution and/or wash water. A process according to claims 1 to 10, whereby the source of the peroxomonosulfuric acid is a triple salt of approx. 45 % KHSO₅, 25 % KHSO₄ and 30 % K₂SO₄ approx. formula is 2 KHSO₅.KHSO₄.K₂SO₄-(Caroat®).

Inventors: ARNOLD GERHARD, HELMLING OSWALD DR, MEIER JUERGEN, ARNOLD, GERHARD, HELMLING, OSWALD, DR., MEIER, JUERGEN, Meier, Jürgen 
Applicants: DEGUSSA, DEGUSSA AKTIENGESELLSCHAFT
Application year: 1990
Main classification: D21C9
IPC codes: D21C9