Metal substituted xerogels for improved peroxide bleaching of kraft pulps

Abstract

A novel process for bleaching cellulosic pulp is disclosed which provides improvements in pulp brightness and delignification without negatively impacting physical properties of the pulp. Specifically, a process is disclosed in which kraft pulp is treated with at least one of each of an oxidizing agent, an alkaline agent, and a metal substituted xerogel in a bleaching stage to improve brightness and delignification of softwood, hardwood, or recycled pulp. In a preferred embodiment, a process is disclosed which uses at least one metal substituted xerogel as a catalyst in an alkaline peroxide bleaching stage to improve kraft pulp brightness and delignification. Pulps bleached according to the process of the present invention are also disclosed.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of cellulosic pulp treatment. More particularly, the present invention relates to activated alkaline peroxide processes for bleaching kraft pulps to improve brightness and delignification without negatively impacting physical properties of the pulp.

[0003] 2. Description of the Related Art

[0004] Bleaching is a chemical process applied to cellulosic materials, such as pulp, to increase their brightness by decolorizing and/or removing lignin. Brightness is the reflectance of visible light from cellulosic cloth or pulp fibers formed into sheets.

[0005] The absorbence of visible light by cellulosic pulp fibers is mainly caused by the presence of lignin, a principal constituent of wood. Lignin-removing bleaching, or delignification, not only increases brightness, but also improves brightness stability because lignin is known to darken with age.

[0006] The bleaching of chemical pulp is typically a multi-stage process, with bleaching chemicals applied sequentially, often with intermediate washing between treatments (stages). In chemical pulp, the wood chips are cooked with appropriate chemicals in an aqueous solution at elevated temperature and pressure. The objective is to degrade and dissolve away the lignin and leave behind most of the cellulose and hemicellulose in the form of intact fibers. See G. Smook, Handbook for Pulp and Paper Technologists, Angus Wilde Publications, p. 38 (2nd Ed. 1994). The chemicals commonly used for pulp bleaching include oxidants (e.g. chlorine, chlorine dioxide, oxygen, ozone, and hydrogen peroxide), alkali (e.g. sodium hydroxide), and, for mechanical pulps, a reducing agent, such as sodium hydrosulfite. These chemicals, alone, or in simultaneous or sequential combination, are mixed with pulp suspensions at prescribed pH, temperature and consistency for a prescribed time, to achieve improvements in brightness and delignification. The two major alkaline processes for producing chemical pulps are the alkaline sulfate or "kraft" process and the soda process. In both these processes, wood chips are cooked with sodium hydroxide in order to dissolve the lignin which binds the fibers together. Sodium sulfide is an additional component of the pulping chemical mix in the kraft process.

[0007] The bleaching of pulp with hydrogen peroxide is known in the art. In C. Dence and D. Reeve, "Pulp Bleaching: Principles and Practice" (Tappi Press 1996), incorporated herein in its entirety, chapters 1 and 6 discuss the use of hydrogen peroxide in hardwood (HW) and softwood (SW) kraft and mechanical pulp bleaching processes. Moreover, U.S. Pat. No. 6,007,678 discloses a process for the delignification and bleaching of pulp in which the pulp is delignified with an organic peracid or salts thereof, treated with a complexing agent, washed, and subsequently bleached with a chlorine-free peroxide bleaching agent such as hydrogen peroxide or peracetic acid.

[0008] Due to environmental concerns, the use of totally chlorine-free and elementally-free chlorine bleaching sequences is increasing, consequently, the use of hydrogen peroxide in the bleaching of chemical pulps is also on the rise. This is because hydrogen peroxide is environmentally benign. After the oxidizing power of hydrogen peroxide is spent, only water and oxygen remain as by-products and no chloro-organic compounds are introduced to waste streams. Conversely, when chlorine based oxidants are used in pulp bleaching, chlorinated organic compounds remain in the bleaching effluent.

[0009] Unfortunately, conventional hydrogen peroxide bleaching processes have several drawbacks as compared to conventional processes using chlorine-containing bleaching agents. These drawbacks include providing inferior delignification to conventional chlorine processes as well as increased cost relative to conventional processes since larger amounts of expensive hydrogen peroxide may be required.

[0010] At least three technical approaches to improving the delignification properties of hydrogen peroxide are known in the art: 1) removing transition metals from the pulp prior to bleaching either by chelation or acid washing of the pulp; 2) adding a peroxide activator to the bleaching stage for kraft pulps; and 3) using zeolites in the bleaching and/or pre-bleaching of mechanical pulps.

[0011] Improvement in hydrogen peroxide bleaching through the removal of transition metals is disclosed, for example, in C. Dence and D. Reeve, "Pulp Bleaching: Principles and Practice," p. 353 (Tappi Press 1996). A variety of peroxide activators have been studied for use in chemical pulp bleaching operations. For example, U.S. Pat. No. 6,048,437 describes a process for bleaching chemical pulp by simultaneous use of chlorine dioxide, a peroxide, and at least one reaction catalyst selected from the group consisting of oxoacids of elements of Groups IV, V, and VI and salts of these acids. The use of TAED (tetraacetylethylenediamine) as a peroxide activator is disclosed in N. Turner and A. Mathews, "Enhanced Delignification and Bleaching Using TAED Activated Peroxide," 1998 TAPPI Pulping Conference, p.1269. The activation of hydrogen peroxide with nitrilamine is described in W. Sturm and J. Kuchler, "The Nitrilamine Reinforced Hydrogen Peroxide Bleaching of Kraft Pulps," 1993 Non-Chlorine Bleaching Conference, Hilton Head, also disclosed in German Patent No. 41 14 134 A1 discloses the use of cyanamide or cyanamide salts to activate hydrogen peroxide bleaching of alkali pulp.

[0012] Hydrogen peroxide activation using ammonium triperoxo-phenanthrolin- e vanadate (ATPV) on kraft pulps is described in M. Suchy and D. Argyropoulos, "Improving Alkaline Peroxide Delignification Using a Vanadium Activator," 1998 TAPPI Pulping Conference, p.1277. The use of silicoperoxomolybdate, sodium molybdate, ammonium molybdate, and molybdosilicate cluster ions as hydrogen peroxide activators are described, respectively in J. Jakara and J. Patola, "The Use of Activated Peroxide in ECF and TCF Bleaching of Kraft Pulp," International Non-Chlorine Bleaching Conference Proceedings, Amelia Island, (2) p. 38 (1995); V. Kubelka, R. Francis, and C. Dence, "Delignification with Acidic Hydrogen Peroxide Activated by Molybdate," J. Pulp & Paper Sci., 18(3), p. J108 (May 1992); R. Agnemo, "Reinforcement of Oxygen-Based Bleaching Chemicals with Molybdates," 9th Annual Symposium on Wood and Pulping Chemistry: 1997, Oral Presentations International Symposium on Wood and Pulping Chemistry Quebec CA, Conference Dates: Jun. 9, 1997-Jun. 12, 1997 (CPPA Canadian Society for Chemistry, China Technical Association of the Paper Industry, EUCEPA, Japan TAPPI, Paprican, TAPPI, and Technical Association of the Australian and New Zealand Pulp and Paper Association) pp. D2-1-D2-3 (Jun. 12, 1997); J. Barna, E. Ratnieks, and F. Souza, "Use of Activated Hydrogen Peroxide and Peroxyacids in ECF Bleaching," Trabalho apresentado No. 30, Congresso Anual De Celulose E Papel Da ABTCP Realizado EM Sao Paulo-SP-Brasil, De 03 A 07 De Novembro De 1997, pp.161-176.

[0013] Moreover, the use of binuclear manganese complexes to activate hydrogen peroxide bleaching is described in International Patent Application No. PCT/EP95/033088; in L. Kuhne, J. Odermatt and T. Wachter, "Application of a Catalyst in Peroxide Bleaching of Eucalyptus Kraft Pulp," Holzforschung, Vol. 54, No. 4, pp. 407-412 (2000); and in Y. Cui, P. Puthson, C. Chen, J. Gratzl, and A. Kirkman, "Kinetic Study on Delignification of Draft-AQ Pine Pulp with Hydrogen Peroxide Catalyzed by Mn(IV)-Me.sub.4DTNE," Holzforschung, Vol. 54, No. 4, pp. 407-412 (2000).

[0014] The use of macrocyclic tetramide iron (III) complexes as hydrogen peroxide activators is disclosed in both U.S. Pat. No. 6,099,586; and J. Hall, L. Vuocolo, I. Suckling, C. Horwitz, R. Allison, L. Wright, and T. Collins, "Development of the P.sub.Fe Process: a New Catalysed Hydrogen Peroxide Bleaching Process," APPITA, Annula General Conference, 2:455-461 (1999). Also, the use of polypyridines to activate hydrogen peroxide is described in T. Jaschinski and R. Patt, "The Effects of Polypyridines as Peroxide Activators in TCF Bleaching of Kraft Pulps," 1998 International Pulp Bleaching Conference Proceedings, 2, pp. 417-422, June 1-5, Helsinki, Finland; and in T. Jaschinski and R. Patt, "Process and Bleaching Solution for Bleaching Cellulosic Pulps," German Patent No.19,614,587, Oct. 16, 1997.

[0015] The use of zeolites A and P to improve hydrogen peroxide bleaching of mechanical pulps by inhibition of transition metal catalyzed peroxide decomposition, is described in K. Dyhr and J. Sterte, "Use of Zeolites in Hydrogen Peroxide Bleaching of Pulp," Nordic Pulp and Paper Research Journal, Vol. 13, No. 4 (1998). While improvements in the bleaching of mechanical pulps were effected by use of zeolites, the researchers observed no effect of the zeolite additives on the bleaching of chemical pulps.

[0016] Xerogels are zeolite-type structures. Prior to the present invention, the use of xerogels to enhance chemical bleaching was not known in the pulp bleaching art. Xerogels have found use in general oxidative chemistry. For example, In "An Effective Heterogeneous WO.sub.3/TiO.sub.2--SiO.sub.2 Catalyst for selective oxidation of cyclopentene to glutaraldehyde by H.sub.2O.sub.2," Catalysis Letters, Vol. 62, No. 2-4, pp. 201-207, 1999; R. Jin, X. Xia, W. Dai, J. Deng, and H. Li describe the synthesis of TiO.sub.2--SiO.sub.2 by the xerogel method and its use to prepare a WO.sub.3/TiO.sub.2--SiO.sub.2 catalyst by an incipient wetness method. The catalyst was employed as the first heterogenous catalyst in the liquid phase cyclopentene oxidation by aqueous H.sub.2O.sub.2 which exhibited higher selectivity (about 75%) to glutaraldehyde, and, in turn, higher glutaraldehyde yield than a WO.sub.3/SiO.sub.2 heterogenous catalyst or a tungstic acid homogeneous catalyst under the same reaction conditions.

[0017] In R. Neumann and M. Levinelad, "Metal-Oxide (TiO.sub.2, MoO.sub.3, WO.sub.3) Substituted Silicate Xerogels as Catalysts for the Oxidation of Hydrocarbons with Hydrogen Peroxide," Journal of Catalysis, Vol. 166, No. 2, pp. 206-217, March 1997, TiO.sub.2, MoO.sub.3 and W.sub.3 were dispersed in amorphous silica using the low temperature sol-gel procedure for xerogel preparation. The resulting metallosilicate compounds are catalytically active in the 30% aqueous H.sub.2O.sub.2 oxidation of alkenes and alcohols provided the metal oxide precursor in the xerogel synthesis is a metal-diclorodialkoxy compound yielding MO(.sub.x)(Cl)--SiO.sub.2.

[0018] In R. Neumann and M. Levinelad, "Vanadium Silicate Xerogels in Hydrogen Peroxide Catalyzed Oxidations," Applied Catalysis A-General, Vol. 122, No. 2, pp. 85-97, Feb. 16, 1995, vanadium silicate xerogels (V.sub.2O.sub.5--SiO.sub.2) were prepared by the sol-gel method by hydrolysis of vanadium and silicon alkoxides. The use of these xerogels as catalysts for oxidation of alkenes, alcohols and phenols was studied using 30% aqueous hydrogen peroxide as oxidant. This study found that the manner of xerogel preparation strongly influenced the catalytic activity of the xerogels.

[0019] Applicants have now surprisingly found that metal substituted xerogels can be used effectively to improve brightness and delignification in alkaline hydrogen peroxide bleaching of kraft pulps.

SUMMARY OF THE INVENTION

[0020] In accordance with the purpose of the invention in one of its aspects embodied and broadly described herein, there is disclosed a process for bleaching cellulosic pulp comprising treating the pulp with at least one oxidizing agent, at least one alkaline agent and at least one metal substituted xerogel, in amounts effective to improve brightness and delignification of the pulp. In another aspect, the present invention includes a process for bleaching kraft pulp in an alkaline peroxide stage comprising treating the pulp with hydrogen peroxide, sodium hydroxide, and at least one metal substituted xerogel in amounts effective to improve brightness and delignification of the pulp.

[0021] The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

[0022] Further advantages of the invention will be set forth in part in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

[0023] The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of the specification. The drawings illustrate embodiments of the invention, and together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 is a graph of data showing the delignification of SW kraft pulps using an alkaline hydrogen peroxide stage with a 0.2% charge of metal (Ti, Mn, V, Mo, W, Li, or Al) substituted xerogels.

[0025] FIG. 2 is a graph of data showing the delignification of SW kraft pulps using an alkaline hydrogen peroxide stage with 0.2% and 0.05% metal (Al, Mn, or W) substituted xerogels.

[0026] FIG. 3 is a graph of data showing the delignification of EDTA chelated SW kraft pulps using an alkaline hydrogen peroxide stage with 0.2% and 0.05% metal (Al, Mn, or W) substituted xerogels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention provides novel processes that use metal substituted xerogels to improve the delignification and brightening properties of kraft pulps, preferably in an alkaline peroxide bleaching stage. The use of metal substituted xerogels for improving alkaline peroxide delignification of kraft pulps has not been reported previously. Xerogels are zeolite-type structures, however, while zeolites have been shown to be ineffective in the bleaching of kraft pulps. The novel metal xerogel activated processes of the present invention surprisingly improve hydrogen peroxide bleaching of a variety of pulps. The term "bleaching" includes both decolorization and delignification bleaching processes. The terms "kraft pulps" and "chemical pulps" refer to pulps containing fibers that have been digested according to one or more of the sulphate, sulphite, soda, or organosolv processes, or mixtures thereof, and to recycled pulps, which may be a mixture of chemical and mechanical pulps. The pulp fibers may be of hardwood or softwood.

[0028] The inventive process can be effectively employed immediately after the chemical cooking or later in the pulp bleaching sequence. In fact, the inventive process can be used with a myriad of peroxide-based bleaching sequences. Non-limiting examples of suitable sequences include: the QPQP* and AZQP* sequences disclosed in C. Gustavsson, K. Sjostrom, W. Wafa Al-Dajani, "Influence of Cooking Conditions on the Bleachability and Chemical Structure of Kraft Pulps," Nordic Pulp and Paper Research Journal, Vol. 14, No. 1, pp. 71-81 (1999), incorporated herein by reference in its entirety; the (D(EOP)DP, D(EOP)PD and XD(EOP)DnD sequences disclosed in Y. Sun, "Kraft Bleach Plant ECF Conversion: Comparison of Sequences involving Enzymes, Chlorine Dioxide, Oxygen and Hydrogen Peroxide," Appita Journal, Vol. 52, No. 1, pp. 45-50 (January 1999), incorporated herein by reference in its entirety; the OD(EPO)DP, O(Z/D)(EPO)DP, (D/C)(EPO)DED, (D/Z)(EPO)DED, OD(EPO)DD, D(EPO)DED, and OD(EPO)D sequences disclosed in C. Courchene, B. Khandelwal, V. Magnotta, T. McDonough, A. Ragauskas and A. Shaket, "Comparative Evaluation of Low-AOX Hardwood Kraft Pulp Bleaching Sequences," Proceedings of the 85th Annual Meeting of the Pulp and Paper Technical Association of Canada, Part B, pp. B307-B314, (Jan. 28-29, 1999), incorporated herein by reference in its entirety; and the D(EOP)DPP, D(EOP)PDP, DEDP, and DEPD sequences disclosed in P. Froass, J. Hamilton, A. Ragauskas, J. Sealey, and D. Senior, "Interaction of Hydrogen Peroxide and Chlorine Dioxide Stages in ECF Bleaching," TAPPI Journal, Vol. 81, No. 6, pp. 170-178 (June 1998), incorporated herein by reference in its entirety. In all of the above-mentioned exemplary sequences, the inventive process should improve the performance of the EOP and P-stages (or P*-stage). Performance of the P-stage should also be improved where elevated temperatures and pressures are employed, as is disclosed in A. Audet, R. Berry, B. Roy, B. van Lierop, "High Temperature Alkaline Peroxide Bleaching of Kraft Pulps," Int'l. Non-Chlorine Bleaching Conference Proceedings, Orlando Fla., (Pulp & Paper and Emerging Technology Transfer Inc.) Session Advances in ECF/TCF Technologies-Part 2, Paper No. 3-3, p. 6 (Mar. 24-28, 1996), incorporated by reference herein in its entirety.

[0029] Suitable pulps for the inventive process include softwood pulps with an initial Kappa number between about 5 and about 40, and hardwood pulps with an initial Kappa number between about 5 and 25. Higher Kappa number pulps could be treated as well but peroxide costs could be prohibitive. The term "Kappa number" is a well known term in the art and is calculated to provide a measure of the lignin content of pulp. Preferably, the initial Kappa number of the softwood pulp is from about 20 to about 35; more preferably from about 25 to 30; most preferably from about 28-30. Preferably, the initial Kappa number of the hardwood pulp is from about 10 to about 25; more preferably from about 15 to about 25 and most preferably from about 20 to 25.

[0030] In the processing of pulp suspensions, the term "consistency" refers to the percentage of pulp in the total mass of suspension. Suitable pulps for the process of the present invention include pulps having a consistency of from between about 3% and about 25%; more preferably from about 10% to about 20%; and still more preferably from about 10% to about 15%. Most preferably, the pulp has a consistency of about 10%.

[0031] The process of the present invention comprises bleaching pulp with at least one oxidizing agent to facilitate delignification of the pulp. Suitable oxidizing agents may include but are not limited to hydrogen peroxide, and other chlorine-free oxidizing compounds. Preferably, the oxidizing agent is a chlorine-free agent selected from oxygen, ozone, hydrogen peroxide, peracetic acid, peroxo monosulfate (also known as Caro's acid) or mixtures thereof. Most preferably, the oxidizing agent is hydrogen peroxide.

[0032] The process of the present invention also comprises use of at least one alkaline agent during the inventive bleaching stage to facilitate lignin removal. According to the process of the present invention, the alkaline agent can be any alkaline agent suitable for use in pulp bleaching processes. Preferably, the alkaline agent is selected from NaOH, KOH, Na.sub.2CO.sub.3, Mg(OH).sub.2, and Ca(OH).sub.2. Most preferably, the alkaline agent is NaOH.

[0033] Bleaching reaction rates can be significantly affected by pH, consequently, alkali or acid may be required to achieve optimal pH. Preferably, the pH of the process of the present invention is from about 9 to about 12.5; more preferably from about 11 to 12.5. Most preferably, the pH is about 12.2.

[0034] Temperature and pulp residence time are also important conditions in the inventive bleaching process. Suitably, the temperature of the present invention can range from about 60.degree. C. to about 90.degree. C. Preferably, the temperature of the present invention can range from about 70.degree. C. to about 90.degree. C. Most preferably, the temperature of the present invention is about 70.degree. C. Usually, hydrogen peroxide is performed at medium consistency (8-15%). The process of the present invention can be batch or continuous. Residence time for the inventive bleaching process suitably can range from about 60 minutes to about 180 minutes. Preferably, the residence time ranges from about 90 minutes to about 180 minutes. Most preferably, the residence time is about 90 minutes.

[0035] The process of the present invention also comprises a novel use of at least one metal substituted xerogel to enhance the bleaching process. Suitable metals for substitution into the xerogel include, but are not limited to Al, V, W, Mn, Li, and Mo, and mixtures thereof. Preferably, the metal substituted xerogel ("Xgel") is an alkyl metal substituted xerogel and is selected from Mn-Xgel, Al-Xgel, and W-Xgel. Most preferably, the alkyl metal substituted xerogel is Al-Xgel. Optionally, the metal substituted xerogel used in the process of the present invention may be a recycled metal-substituted xerogel.

[0036] The manner of xerogel preparation may impact the reactivity of the xerogel. Different types of metal adapted for xerogel preparation have a different effects on the reactivity of the xerogel in peroxide bleaching. Xerogel pore structure can affect catalytic activity and ion exchange capacity in hydrogen peroxide bleaching. Appropriate preparation conditions for synthesizing xerogels would be apparent to the skilled artisan.

[0037] The metal substituted xerogels used in the present invention can be prepared by processes known to those of skill in the art. Most preferably, the metal substituted xerogels are prepared according to the procedure described in R. Neumann, M. Chava, and M. Levin, "Hydrogen Peroxide Oxidations Catalysed by Metallosilicate Xerogels," J. Chem. Soc. Che. Commum. pp. 1685-87 (1993), herein incorporated by reference in its entirety, and set forth in Example 1 of the present disclosure.

[0038] The oxidizing agent, alkaline agent, and metal-substituted xerogel used in the process of the present invention are provided in amounts effective to improve brightness and delignification of the bleached pulp. According to the present invention, improved brightness and delignification is achieved by increasing brightness and delignification above that present in the pulp prior to bleaching in accordance with the present invention. Preferably, the process of the present invention improves pulp TAPPI brightness from about 27.8 to about 37.8 and improves pulp delignification by from about 35% to about 38%. Most preferably, pulp brightness is improved by at least about 30% and pulp delignification is improved by at least about 38%.

[0039] The amount of oxidizing agent used in the process of the present invention is preferably from about 2% to about 4% by weight of oven dry pulp. Most preferably, the amount of oxidizing agent is about 2% by weight of oven dry pulp. The amount of alkaline agent is preferably from about 2% to about 4% by weight of oven dry pulp. Most preferably, the amount of alkaline agent is about 2% by weight of oven dry pulp. The amount of metal substituted xerogel is preferably from about 0.05% to about 2% by weight of oven dry pulp; more preferably from about 0.05% to about 0.2%. Most preferably, the amount of metal substituted xerogel is about 0.2% by weight of oven dry pulp. While various combinations of oxidizing agent, alkaline agent and metal substituted xerogel may be used, the process of the present invention most preferably includes a combination of hydrogen peroxide, NaOH, and Mn-Xgel. These components can be added simultaneously or sequentially, however, if added sequentially, the oxidizing agent is preferably added last.

[0040] Manganese and other metallic ions present in kraft pulps can have a particularly adverse effect on the bleaching efficiency of chlorine-free bleaching agents such as ozone and alkaline peroxide compounds. Thus a chelating treatment step, in which a compound is added to form complexes with metallic ions, may be useful prior to an alkaline peroxide bleaching step. Suitable chelating agents can include but are not limited to nitrogenous organic compounds, polycarboxylic acids, or phosphonic acids. Preferably, the chelating agent is selected from ethylenediaminetetraacet- ic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), nitrilotriacetic acid (NTA), oxalic acid, citric acid, hydroxyethylene diaminetetraacetic acid (HEDTA), diethylenetriaminepentamethylene-phospho- nic acid (DTPMPA), or tartaric acid. Most preferably, the chelating agent is EDTA.

[0041] The type and added amount of chelating agent depends on the type and amount of trace metal ions in the incoming pulp, as well as the conditions in the treatment such as pH, temperature and residence time. Preferably, the amount of chelating agent added is in the range of from about 0.3% to about 0.8% by weight of dry pulp, calculated as 100% chelating agent, and most preferably about 0.6% by weight of oven dry pulp.

[0042] After alkaline peroxide delignification, the pulp can be used for direct production of paper. Alternatively, the pulp may be finally bleached to a desired higher brightness in one or more stages. Final bleaching may be carried out by use of the chlorine free bleaching agents indicated above, including additional inventive metal substituted xerogel-catalyzed alkaline peroxide stages, with optional intermediate washing or extraction stages, or by use of chlorine containing bleaching agents, such as chlorine dioxide.

[0043] The invention is further discussed in conjunction with the following examples, which are merely illustrative of the present invention.

EXAMPLES

Example 1

[0044] To test the effectiveness of metal substituted xerogels in catalyzing an alkaline hydrogen peroxide bleaching stage, the following experiments were performed. SW kraft pulp (Brownstock, initial Kappa number=28.5) was bleached, with and without a metal substituted xerogel, using 2.0% hydrogen peroxide and 1.0% NaOH. The control hydrogen peroxide bleaching experiment employed no xerogel but included 0.20% MgSO.sub.4. Control hydrogen peroxide bleaching experiments were also conducted using 0.2% charges of the alkyl metals Ti(OiPr).sub.4, Mn(OEt).sub.2, V(OiPr).sub.4, Mo(OEt).sub.4, W(OiPr).sub.4, Li(OEt), and Al(OEt).sub.3, (where OEt=CH.sub.2CH.sub.3O and OiPr=CH(CH.sub.3).sub.2O) respectively, and test experiments were conducted with 0.2% charges of the following alkyl metal substituted xerogels, Ti-Xgel, Mn-Xgel, V-Xgel, Mo-Xgel, W-Xgel, Li-Xgel, and Al-Xgel, respectively. A control experiment using only alkyl metal without prepared xerogel was also performed. All of the experiments were performed at 10.0% consistency and a temperature of 70.degree. C., for 90 minutes.

[0045] The improvements in delignification are summarized in Table I and FIG. 1. A comparison of the delignification data indicates that the usage of 0.2% tungsten, lithium, or aluminum ethoxide substituted xerogels in an alkaline peroxide stage improves delignification by 44, 37, and 38% respectively. This extent of delignification is greater than what can be achieved with 4% hydrogen peroxide under comparable bleaching conditions as summarized in Table 2. Also, the additional control studies, with alkyl metals (no xerogel) and xerogel only, failed to yield improved peroxide delignification of SW kraft pulps, as shown in Table 1. These latter experiments demonstrate that the improved peroxide bleaching properties observed with metal substituted xerogels is due to the chemical composition of the catalyst and not to the individual components.

1TABLE 1 Delignification of SW kraft pulp using an alkaline hydrogen peroxide stage with and without alkyl metal modified xerogels or alkyl metals Kappa # after alkaline peroxide stage: Initial Kappa # was 28.5 Catalyst Ti Mn V Mo W Li Al Alkyl metal 22.2 22.1 20.1 20.6 20.4 20.2 20.1 Alkyl metal 19.1 19.1 19.5 19.0 18.3 18.8 18.7 xerogel No catalyst: 21.4 Xerogel: 21.0

[0046]

2TABLE 2 Alkaline hydrogen peroxide delignification of SW kraft pulp with a starting kappa number of 28.5 and TAPPI brightness value of 25.2. Peroxide Bleaching Kappa Number after TAPPI Brightness after Conditions.sup.1 Peroxide Stage Peroxide Stage 2% charge of H.sub.2O.sub.2 21.4 30.8 1% charge of NaOH 3% charge of H.sub.2O.sub.2 20.3 31.3 1% charge of NaOH 4% charge of H.sub.2O.sub.2 19.6 32.5 1% charge of NaOH .sup.10.5% charge Of MgSO.sub.4, experiments performed at 10% consistency, 70.degree.C. for 90 min.

[0047] The metal xerogels used in the above and below-mentioned experiments were prepared in the following manner: silicone tetraethoxide (57 mmol) was dissolved in absolute ethanol (17 ml) and 0.15 N HCl aqueous solution (0.11 ml). The mixture was heated to 60.degree. C. for 90 minutes and then cooled to room temperature followed by addition with stirring of 3.00 mmol of the appropriate metal alkoxide (i.e., metal=Ti, Mn, V, Mo, W, Li, and Al, respectively). The mixture formed a gel immediately and was left in an open-beaker allowing slow evaporation of solvent. A solid was formed within 12 -24 hours. The solidified xerogel was ground and dried at 100.degree. C. for 12 hours. The improved delignification accomplished with the metal substituted xerogels also yields improved pulp brightness values as summarized in Table 3. All metal substituted xerogels (i.e., metal=Ti, Mn, V, Mo, W, Li, and Al, respectively) improved pulp brightness properties with the tungsten, lithium, and aluminum substituted xerogels being the most effective, providing brightness gains of +30%. Control studies using 2% hydrogen peroxide indicated that the brightness gains achieved with 0.2% W, Li, or Al substituted xerogels was comparable to what could be achieved with 4% hydrogen peroxide. In the presence of hydrogen peroxide, the metal substituted xerogels may act as catalysts leading to the formation of inorganic peroxides and/or chelate unwanted metals present in the pulp.

3TABLE 3 TAPPI brightness of SW kraft pulp bleached with an alkaline hydrogen peroxide stage, with and without alkyl metal modified xerogels or alkyl metals. Tappi Brightness Catalyst Ti Mn V Mo W Li Al Alkyl metal 30.1 29.7 30.9 31.7 31.4 31.7 31.8 Alkyl metal 32.4 31.7 31.9 31.8 32.5 32.2 32.4 xerogel No catalyst: 30.8 Xerogel: 25.2

[0048] These experiments clearly show that the use of alkyl metal xerogels with an alkaline peroxide stage improves the delignification and brightening properties of a peroxide bleaching stage for kraft pulps.

Example 2

[0049] Experiments were also conducted to determine if a metal xerogel-catalyzed peroxide bleaching treatment had an detrimental effects on pulp strength properties. SW kraft pulp (Brownstock, initial Kappa number=21.2) was bleached, with and without a metal substituted xerogel, using 2.0% hydrogen peroxide and 2.0% NaOH. The control hydrogen peroxide bleaching experiment employed no xerogel but included 0.30% MgSO.sub.4 (This salt was not added to the xerogel experiments). Test experiments were conducted with 0.05%, 0.2%, and 0.3%, respectively, charges of the following alkyl metal substituted xerogels, Mn-Xgel, W-Xgel,and Al-Xgel, respectively. All of the experiments were performed at 10.0% consistency and a temperature of 70.0.degree. C., for 90 minutes. After bleaching, the pulps were washed with deionized water (it is not necessary that deionized water be used) and physical properties were determined using standard TAPPI T220 om-88 "Physical Testing of Pulp Handsheets" testing procedures.

[0050] FIG. 2 summarizes the observed changes in delignification for the pulp bleached with Al, W, and Mn xerogels (xerogels prepared as discussed above example 1). Table 4 summarizes the physical properties of the bleached pulps.

4TABLE 4 Physical Properties of SW Kraft Pulp Bleached with an Alkaline Hydroxide Stage with and without Selected Metal Substituted Xerogels (X-Gel). Tensile Tear Burst Freeness Density Index Index Index Pulp (CSF, ml) (kg/m.sup.2) (Nm/g) (m Nm.sup.2/g) (KPa m.sup.2/g) Brownstock 315 560 87.4 13.0 7.1 P-Bleached(No 320 541 86.5 13.0 6.7 X-gel) 0.05% W 321 536 90.9 12.9 6.7 X-gel 0.05% Al 300 537 91.3 12.5 7.0 X-gel 0.05% Mn 308 549 87.7 12.6 6.9 X-gel 0.2% W X-gel 324 488 87.9 12.7 7.1

[0051] All pulps are refined by PFI (revs. 4000). Pulp refining is a mechanical treatment of pulp fibers. This process increases the strength by increasing the surface area and improves the capacity of the pulp to absorb water. The treatment is described in C. J. Biermann, Essentials of Pulping and Paper-making, Academic Press, p. 137, (1993), incorporated herein by reference in its entirety.

[0052] The bleaching and physical property results summarized in FIG. 2 and Table 4 indicate that the metal xerogels improve delignification of a peroxide stage while yielding peroxide bleached pulps that exhibit equal or better physical properties to the control pulp bleached only with hydrogen peroxide.

Example 3

[0053] Experiments were also conducted to determine the effect of a Q (chelating) stage on a metal substituted xerogel peroxide stage. SW kraft pulp was chelated with EDTA at a pH of 5, for 30 minutes, at a pulp consistency of 2%. The consistency on pulp bleaching is usually calculated as the weight percent of pulp in given pulp suspension (CSC %=od pulp wt/wet pulp wt* 100) The required consistency is easily obtained by adjusting the amount of water in the pulp suspension. Sulfuric acid was used to adjust pH. The peroxide bleaching experiments of Example 2 were then repeated using a 2.0% charge of hydrogen peroxide at 10.0% consistency and a temperature of 70.degree. C., for 90 minutes.

5TABLE 5 Effect of Xerogel on Peroxide Bleaching of EDTA Pretreated Pulp H.sub.2O.sub.2 NaOH Xerogel Amt. Kappa Tappi Xerogel Type (%) (%) (%) No. Brightness Original 21.2 27.8 Control 2 2 0 14.7 46.3 4 4 0 12.4 50.0 W 2 2 0.05 13.4 47.1 0.2 14.0 43.7 4 4 0.05 11.1 53.8 0.2 12.3 51.3 Al 2 2 0.05 13.1 50.0 0.2 12.5 50.0 4 4 0.05 11.3 56.0 0.2 10.6 56.1 Mn 2 2 0.05 14.3 40.2 0.2 15.2 36.0 4 4 0.05 13.0 43.6 0.2 13.0 40.5 *At control bleaching, 0.3% MgSO.sub.4 is added. *Addition of xerogel is based on metal contents. *Bleaching conditions: 70.degree. C., 90 min.

[0054] The results of the peroxide delignification using a chelated pulp with and without metal substituted xerogels are summarized in FIG. 3 and Table 5. These results indicate that the tungsten, aluminum and manganese xerogels still enhance delignification, although the former two operate significantly better.

[0055] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims.

Claims

We claim:

1. A process for bleaching cellulosic pulp comprising treating said pulp with at least one oxidizing agent, at least one alkaline agent and at least one metal substituted xerogel, in amounts effective to improve brightness and delignification of said pulp.

2. The process of claim 1 wherein said pulp is a kraft pulp.

3. The process of claim 1 wherein said pulp has a consistency of from about 10% to about 20%.

4. The process of claim 3 wherein said pulp has a consistency of from about 10% to about 15%.

5. The process of claim 4 wherein said pulp has a consistency of about 10%.

6. The process of claim 1 wherein said treatment is practiced at a temperature of from about 60.degree. C. to about 90.degree. C.

7. The process of claim 6 wherein said temperature is from about 70.degree. C. to about 90.degree. C.

8. The process of claim 7 wherein said temperature is about 70.degree. C.

9. The process of claim 1 wherein said treatment is practiced for a time of from about 60 minutes to about 180 minutes.

10. The process of claim 9 wherein said time is about 90 minutes.

11. The process of claim 1 wherein said treatment is practiced at a pH of from about 9 to about 12.5.

12. The process of claim 11 wherein said pH is about 12.2.

13. The process of claim 1 wherein said at least one alkaline agent is selected from NaOH, KOH, Na.sub.2CO.sub.3, Mg(OH).sub.2 and mixtures thereof.

14. The process of claim 11 wherein said alkaline agent is NaOH.

15. The process of claim 1 wherein said at least one metal substituted xerogel is an alkyl metal substituted xerogel.

16. The process of claim 15 wherein said alkyl metal substituted xerogel is selected from Al-Xgel, W-Xgel, Mn-Xgel, V-Xgel, Mn-Xgel, Li-Xgel, and Mo-Xgel, and mixtures thereof.

17. The process of claim 16 wherein said alkyl metal substituted xerogel is W-Xgel.

18. The process of claim 16 wherein said alkyl metal substituted xerogel is Al-Xgel.

19. The process of claim 16 wherein said alkyl metal substituted xerogel is Mn-Xgel.

20. The pulp of claim 1 wherein said pulp is selected from hardwood pulp, softwood pulp, and recycled pulp, and mixtures thereof.

21. The process of claim 20 wherein said pulp is softwood pulp.

22. The process of claim 1 wherein said at least one oxidizing agent is selected from oxygen, ozone, hydrogen peroxide, peracetic acid, peroxo monosulfate and mixtures thereof.

23. The process of claim 22 wherein said at least one oxidizing agent is hydrogen peroxide.

24. The process of claim 1 wherein the initial kappa number of said pulp is from about 20 to about 35.

25. The process of claim 24 wherein the initial kappa number of said pulp is from about 20 to about 30.

26. The process of claim 25 wherein the initial kappa number of said pulp is about 28.5.

27. The process of claim 1 wherein said treatment improves pulp brightness by at least about 30% .

28. The process of claim 1 wherein said treatment improves pulp delignification by at least about 38%.

29. The process of claim 1 wherein the amount of said at least one metal substituted xerogel is from about 0.05% to about 0.2% by weight of oven dry pulp.

30. The process of claim 29 wherein the amount of said at least one metal substituted xerogel is from about 0.05% to about 0.2% by weight of oven dry pulp.

31. The process of claim 30 wherein the amount of said at least one metal substituted xerogel is about 0.2% by weight of oven dry pulp.

32. The process of claim 1 wherein said pulp is recycled.

33. The process of claim 1 wherein the amount of said at least one oxidizing agent is from about 2% to about 4% by weight of oven dry pulp.

34. The process of claim 33 wherein the amount of said at least one oxidizing agent is about 2% by weight of oven dry pulp.

35. The process of claim 1 wherein the amount of said at least one alkaline agent is from about 2% to about 4% by weight of oven dry pulp.

36. The process of claim 35 wherein the amount of said at least one alkaline agent is about 2% by weight of oven dry pulp.

37. A pulp treated in accordance with the process of claim 1.

38. The process of claim 1 further comprising, prior to bleaching, treating said cellulosic pulp with at least one treating agent to remove undesirable trace metal ions.

39. The process of claim 38 wherein said at least one treating agent is a chelating agent.

40. The process of claim 39 wherein said chelating agent is selected from nitrogenous polycarboxylic acids, nitrogenous polyalcohols, polycarboxylic acids and nitrogenous polyphosphonic acids.

41. The process of claim 40 wherein said chelating agent is selected from EDTA, DTPA, NTA, HEDTA, DTPMPA, or mixtures thereof.

42. The process of claim 39 wherein the amount of said chelating agent is from about 0.3% to about 0.8% per ton of oven dry pulp.

43. The process of claim 42 wherein the amount of said chelating agent is about 0.6% per ton of oven dry pulp.

44. The process of claim 39 wherein said chelating treatment is practiced at a pH of about 5.

45. The process of claim 38 wherein said treating agent is an acid.

46. The process of claim 38 wherein said pulp is washed between said chelating treatment and said bleaching treatment.

47. A process for bleaching kraft pulp in an alkaline hydroxide stage comprising treating said pulp with hydrogen peroxide, sodium hydroxide, and at least one metal substituted xerogel in amounts effective to improve brightness and delignification of said pulp.

48. The process of claim 47 wherein said at least one metal substituted xerogel is selected from W-Xgel, Al-Xgel, Mn-Xgel, and mixtures thereof.

49. The process of claim 47 wherein said at least one metal substituted xerogel includes at least one recycled metal substituted xerogel.

50. The process of claim 1 wherein said at least one metal substituted xerogel includes at least one recycled metal substituted xerogel.

51. The process of claim 47 wherein said pulp is washed after said treatment.

52. The process of claim 47 wherein the amount of hydrogen peroxide is from about 2% to about 4%, the amount of sodium hydroxide is from about 2% to about 4%, and the amount of metal substituted xerogel is from about 0.05% to about 0.2%, all by weight of oven dry pulp.

53. The process of claim 52 wherein the amount of hydrogen peroxide is about 2%, the amount of sodium hydroxide is about 2%, and the amount of metal substituted xerogel is about 0.2%, all by weight of oven dry pulp.

54. The process of claim 1 wherein said process comprises at least one stage in a multistage bleaching sequence.