CROSS-REFERENCES The subject application is a continuation-in-part of U.S. Patent Application having serial number 08/273,539, filed on July 11, 1994, entitled Peroxide Bleaching Process for Cellulosic and Lignocellulosic Material.

BACKGROUND OF THE INVENTION 1.0 Field of the Invention The present invention relates generally to the alkaline peroxide bleaching of lignocellulosic materials for use in the pulp and paper industry, and cellulosic and synthetic polymeric materials for use in the textile industry. More particularly the present invention relates to materials used to construct pumps, mixers, transfer piping and reactors used in the bleaching of the aforementioned materials, thereby increasing the efficiency of the bleaching reagent. 2.0 Related Art The bleaching of lignocellulosic materials is generally comprised of a number of individual treatments with various reagents known to delignify and/or brighten the lignocellulosic material. There exist in the paper and pulp industry numerous combinations of various reagents arranged in countless different sequences, each of which is selected based on wood species, availability of bleaching reagents, product quality desired, economics, and a number of other factors. The treatments of the pulp in a multistage bleaching system are understood in the industry, and the following designations are recognized to represent the various reagents used, and to a certain extent, the proportions in which they are used in a given processing step, and whether or not the pulp is washed between the processing steps. A Acid

C Chlorine D Chlorine dioxide

Cd Chlorination, with a mixture of chlorine and a small amount of chlorine dioxide Dc Chlorination with a mixture of chlorine dioxide and a small amount of chlorine

P Peroxide

P _HT Peroxide high temperature process

E Extraction (alkaline treatment)

Ep Extraction with added peroxide H Hypochlorite

Eh Extraction with added hypochlorite

Eo Oxidative extraction (extraction with added oxygen)

Eop Extraction with added oxygen and peroxide

N Neutralization O Oxygen delignification

Q Chelation

Z Ozone

X Enzyme treatment In addition, the following terms have been coined to designate the use of the materials of construction of the present invention in a processing step which uses peroxide containing bleaching reagents:

P(Mod) Peroxide, modified to incorporate the materials of construction of the present invention;

Ep(Mod) Extraction with peroxide modified to incorporate the materials of construction of the present invention;

Eθ(Mod) Oxidative extraction (extraction with added oxygen), modified to incorporate the materials of construction of the present invention;

EopcMod) Oxidation extraction, modified to incorporate the materials of construction of the present invention; OoMod) Oxygen delignification, modified to incorporate the materials of construction of the present invention;

PxcMod) Caro's acid (peroxymonosulfuric acid), modified to incorporate the materials of construction of the present invention; and

PxA(Mod> Mixed peracids, modified to incorporate the materials of construction of the present invention.

The foregoing list of coined terms is not complete, either with respect to presently existing bleaching stages or those which may be defined in the future, which are modified to incorporate the materials of construction of the present invention. Historically, sequences used which were very economical for a specific pulp processing plant included the use of large quantities of chlorine gas, frequently combined with the use of hypochlorite, in sequences like CEH, CEHH, CEHEP, CEHED, CED, CEDED, CEHDED, CEoDED, CdEoDED, as well as any or all of the preceding sequences preceded by oxygen delignification, i.e. , OCEH, OCEHH, OCEHEP, OCEHED, OCED, OCEDED, OCEHDED, OCEoDED, OCdEoDED. The use of chlorine gas and hypochlorite, although very economical bleaching reagents, has recently become undesirable from the point of view of environmental concern about the effect of the bleaching effluent on the environment. One major concern has been the toxicity of chlorinated "dioxins" and "furans" which has led the industry toward the near complete elimination of the use of chlorine gas and hypochlorite, and in many cases the reduction of the lignin content of the pulp entering the bleaching process by the addition of a delignification step like oxygen delignification which reduces the quantity of bleaching reagent required. Currently, a shift towards the use of chlorine dioxide as the major bleaching reagent has been implemented in many mills, in sequences such as DED, DEpD, DEoD, DEopD, DEDED, DEoDED, DEopDED, XDEop(DN)D, DEop(DE)D, DEop(DN)D, ODED, ODEpD, ODEoD, ODEopD, ODEDED, ODEoDED, ODEopDED, ODEop(DE)D, ODEop(DN)D. The elimination of chlorine and replacing it with chlorine dioxide is attractive as it requires minimal changes to the pulp processing equipment in the pulp processing system, however, it requires substantial investment in chlorine dioxide chemical processing facilities, as the chlorine dioxide must be manufactured at the site of the pulp processing facility. An alternative reagent to chlorine dioxide has long been known to be hydrogen peroxide, but its use has been very limited due to the relatively high purchase cost of liquid peroxide, and its relative inefficiency when used in large quantities.

The use of peroxide is desirable since the by-products of its use in delignification and brightening, and its decomposition, are oxygen and water, which are benign to the environment, and the oxidized material dissolved from the pulp is not contaminated with undesirable chlorinated compounds. The equipment used for bleaching lignocellulosic materials with hydrogen peroxide under alkaline conditions has been typically constructed of stainless steels. Though the corrosion resistance of these materials is adequate under these conditions, their effect on hydrogen peroxide decomposition has not been studied extensively. It is generally assumed by those practicing the art that with a large pulp volume to equipment surface area ratio the effect of the reaction vessels, mixers, and peripheral equipment on hydrogen peroxide decomposition is negligible. While this may be true under relatively mild temperature conditions (40-70 °C), at high temperatures peroxide is more susceptible to decomposition. Limiting the extent of this non-productive peroxide decomposition will make bleaching with hydrogen peroxide much more efficient.

In the field of pulp bleaching it is known that metal ion management is a necessary step for the efficient use of hydrogen peroxide. Metal removal from pulp is commonly achieved by using either a chelation or acid washing step to remove metal ions from the pulp to be bleached (J. Basta, L. Holtinger, and P. Lundgren, International Pulp Bleaching Conf., Stockholm, June 11-14, 1991, p 23-33; N.

Troughton and P. Sarot, 1992, Tappi Proceedings 1992 Pulping Conference, p 519- 530; P. Earl and X.T. Nguyen, March 1993 Proc. Non-chlorine Bleaching Conf., Hilton Head, SC, USA). The purpose of this metal removal step is to prevent or at least minimize the losses of peroxide caused by decomposition reactions catalyzed by many transition metals (J.D. Sinkey and N.S. Thompson, 1974, Paperi ja Puu 5: 473- 486). Under conventional bleaching conditions, the removal of transition metals from the pulp is generally sufficient to allow for efficient bleaching of the pulp. Recently, it has been shown that more forcing conditions during alkaline peroxide bleaching improves chemical pulp bleaching (Copending application having U.S. Serial Number 08/273,539, B.P. Roy, B. van Lierop, R. Berry, W. Miller and L. Shackford, 1994;

U. Germgard and S. Norden, 1994 Int. Pulp Bleaching Conf. Proc, Vancouver, 1994, p 199-209). However, under these harsher conditions, the rate of alkaline peroxide decomposition is increased substantially.

The literature teaches that most of the observed hydrogen peroxide decomposition is due to metals which commonly contaminate chemical pulps.

However, reaction vessels which are commonly used in a bleach plant are typically constructed from materials containing metals known to decompose hydrogen peroxide (Fe, Mo, Mn, Cr, etc.). An alternative material of construction, titanium, is also unsuitable for alkaline peroxide bleaching because it is rapidly corroded under typical alkaline peroxide (pH> 9.0) bleaching conditions (Clarke and Singbeil, 1993; R.W. Schutz and M. Xiao, NACE, 1994). In contrast, zirconium is an appropriate material for the construction of hydrogen peroxide storage vessels since it has good corrosion resistance under most conditions (Bloom R. Jr., L.E. Weeks, and C.W. Raleigh, Corrosion 16: 100-106. 1960). Zirconium has excellent corrosion resistance to peroxide (5 weight % solutions) at 70 °C under moderately alkaline (pH 9.5) conditions (Yau, T.-L., 1990 Tappi Eng. Conf. Proc. Seattle, p 1-7; Yau, T.-L., March 1991 Tappi J 74(3), 149-153). Furthermore, zirconium ions (from the sulphate or nitrate salts) can moderately decrease the decomposition of hydrogen peroxide mediated by copper and iron ions at room temperature (Yau, T.-L., D.R. Holmes, and J. Fahey, Tappi Eng. Conf. Proc , 1993, p 1013-1020). Zirconium may be the material of construction of a hydrogen peroxide vaporizer operating at elevated temperatures (145 °C) to minimize iron contamination of the peroxide (Moniz, B.J., 1984, "Corrosion resistance of zirconium in chemical processing equipment", In Industrial Applications of Titanium and Zirconium, ASTM STP 830, R.T. Webster and C.S. Young, Eds., American Society for Testing and Materials, pl90-202). The advantages of zirconium as the material of construction for alkaline peroxide bleaching equipment have not been recognized or described, particularly for the more extreme conditions of pH and temperature currently recommended for alkaline peroxide bleaching of lignocellulosic materials.

SUMMARY Accordingly, the present invention is directed to a process for bleaching of lignocellulosic, cellulosic and synthetic polymeric fibrous materials using alkaline peroxides and having the steps of creating an aqueous slurry of about 0.25-50% by weight fibrous materials, adding alkali to increase the pH of the slurry to greater than 7.5, adding peroxide-containing solution to equal 0.10-50%, by oven-dried weight of the fibrous materials, heating the fibrous materials to a temperature of greater than 70 °C, and reacting the peroxide-containing solution with the fibrous materials, wherein the improvement comprises the steps of: conducting at least one of the process steps in at least one vessel having a contact surface, wherein at least said contact surface of said at least one vessel is constructed from one or more metals selected from the group consisting of zirconium, niobium, hafnium, tantalum or alloys thereof.

A main advantage of the incorporation of the materials of construction and/or the associated method steps of the present invention in a process for bleaching of lignocellulosic, cellulosic and synthetic polymeric fibrous materials is that the beneficial effect of the use of peroxide in the bleaching sequence is improved substantially, and the economics of the bleaching operation are improved since very little peroxide is lost due to decomposition, i.e. , a greater portion of the applied peroxide is beneficially used for delignification or bleaching.

BRIEF DESCRIPTION OF THE DRAWINGS The structural features and functions, as well as the method steps, of the present invention will become more apparent from the subsequent detailed description of the preferred embodiment when taken in connection with the accompanying drawings, wherein:

Figs. 1 and 2 are schematic diagrams illustrating a bleach plant incorporating the apparatus and process steps of the present invention according to a preferred embodiment;

Figs. 3A-3D are graphs illustrating the effects of multiple exposures on coupon-catalyzed alkaline peroxide decomposition, wherein: Fig. 3A illustrates the effects of multiple exposures of zirconium 702, zirconium 705, titanium and polyethylene coupons; Fig. 3B illustrating the effects of multiple exposures of 2205 duplex stainless steel, 2507 duplex stainless steel, 2304 duplex stainless steel and polyethylene coupons; Fig. 3C illustrates the effects of multiple exposures of AL- 6XN, Avesta 654 SMO and polyethylene coupons; and Fig. 3D illustrates the effects of multiple exposures of 304L stainless steel, 316L stainless steel, 317L stainless steel, glass and polyethylene; Figs. 4 is a bar graph illustrating the effect of various metals with and without

DTP A in the solutions on the amount of alkaline peroxide decomposed;

Figs. 5A-5E are schematic block diagrams, each illustrating the process stages of a particularly configured elemental chlorine free (ECF) bleaching plant, wherein at least one of the stages of each of the plants may incorporate the materials of construction and process steps of the present invention;

Figs. 6A-6E are schematic block diagrams, each illustrating the process stages of a particularly configured totally chlorine free (TCF) bleaching plant, wherein at least one of the stages in each plant may incorporate the materials of construction and process steps of the present invention.

DETAILED DESCRIPTION The best mode for carrying out the invention will now be described for the purposes of illustrating the best mode known to the applicant at this time. The examples are illustrative only and not meant to limit the invention. Referring now to the drawings, Figs. 1 and 2 are schematic diagrams illustrating a bleach plant 800 configured for the sequence OAEopZPm- which incorporates the apparatus and process steps of the present invention according to a preferred embodiment, as subsequently described. Bleach plant 800 includes an initial oxygen stage having an oxygen reactor (not shown) used to bleach, or brighten a pulp passing through the reactor and an upstream high density stock tower (not shown). The pulp discharges the high density

stock tower through pipe 802 to a compaction baffle filter 804. Water or an effluent may be supplied from a filtrate tank (not shown), associated with the downstream and subsequently discussed bleaching stages, to the compaction baffle filter, or washer 804 via a recirculation pipe (not shown). The pulp is washed using water and/or filtrate from subsequent stage(s) which displaces the oxygen-reacted-pulp reactants entering filter 804 through pipe 802, with the resultant filtrate discharging from filter 804 through pipe 810. As is known in the art, the filtrate discharging from washer 804 through pipe 810 (as well as the filtrate discharging from the subsequently discussed washers 828, 840, 870 and 904 through pipes 829, 848, 880 and 910, respectively) is largely reused for washing of pulp on prior stages of bleaching. There are numerous methods of recirculating these filtrates, which are known to be economical and practical, such that the amount of effluent which may be discharged to the receiving waters is minimized. For clarity of illustration, these bleach filtrate recirculation systems are not shown in Figs. 1 and 2 illustrating bleach plant 800. It is also recognized in the art that in a totally chlorine free bleach plant design, such as bleach plant 800, although most of the effluent may be recirculated to the recovery process in the associated mill, a small purge of effluent from the bleach plant is desirable to eliminate a large portion of the metals in the filtrate from carrying forward into the bleaching stages. The pulp, or pulp sheet which is being processed through bleach plant 800 comprises an aqueous slurry having about 0.25-50% by weight fibrous materials, typically 8-20% fibrous materials. The pulp, or aqueous slurry, discharges filter 804 through a transfer pipe, or conduit 814 to an A-stage pump 816 which preferably comprises a Clove Rotor ^® pump made by Ingersoll-Rand Company. It is noted that each of the transfer pipes, or piping conveying the pulp slurry between various components throughout bleach plant 800 have been designated with the reference numeral 814, regardless of the length and configuration of the piping used. The pulp is pumped through pipe 814 to a mixer 820, (which may comprise a Hi-Shear ^® mixer made by the Ingersoll-Rand Company) with sulfuric acid also being supplied to mixer 820 via pipe 822. The acid pulp then flows through pipe 814 to an acidification

reaction tower 826. The upflow pipe 814 and downflow reaction tower 826 are sized to assure a pulp retention time, within pipe 814 (disposed between mixer 820 and tower 826) and tower 826, ranging from 5-60 minutes, preferably 20-40 minutes. The pulp is brought to tower 826 at a temperature ranging from 30-90° C, preferably ranging from 50-60° C. It may be necessary to include a pulp heater (not shown in the A-stage) to accomplish the stated temperature ranges.

The pulp discharges from tank 826 through pulp flowpath conduit, or pipe 814 to an A-stage compaction baffle filter, or washer 828. Filter 828 is also supplied with water and/or filtrate from subsequent stages, for washing of the pulp. The displacement action of these filtrates and the pulp slurry within filter 828 creates an A-stage filtrate which discharges filter 828 through pipe 829. The pulp then flows from filter 828 through a pipe 814 to pump 830 of the caustic extraction stage, commonly referred to in the art as the Eop stage. Caustic, preferably sodium hydroxide, is introduced to the pulp slurry via pipe 832 at a location between filter 828 and pump 830. The purpose of the sodium hydroxide is to neutralize the acid pulp exiting the acid, or A-stage and to render the pulp alkaline so it is appropriate for the next alkali delignification and/or brightening stage. The pulp is then pumped to an oxygen/peroxide mixer 834 via one of the flowpath conduits or transfer pipes 814. Hydrogen peroxide and oxygen are also supplied to the peroxide mixer 834 via pipes or conduits 835 and 837, respectively. Accordingly, a peroxide-containing solution is added to the aqueous slurry such that the peroxide-containing solution is equal to 0.10-50%, by oven-dried weight of the fibrous materials in the aqueous slurry. Both the oxygen and hydrogen peroxide serve to delignify and brighten the pulp during the Eop stage. The pulp flows from mixer 834 to an upflow tube, or column 836 via a flowpath pipe 814. The upflow tube 836 is sized to assure a pulp retention time of 1-45 minutes, preferably ranging from 5-20 minutes. The temperature of the pulp within upflow tube 836 ranges from 60-100° C, and preferably ranges from 80-95 °C, and pressure within tube 836 ranges from 20-100 psig. It is likely that a pulp heater (not shown) will be required to achieve the desired temperature range within tube 836. A vent valve 837 is disposed in the upflow tube

836 used to convey the pulp to reaction tower 838. The pulp discharges upflow tube 836 to the downflow reaction tower 838. Tower 838 is sized to assure a pulp retention time ranging from 15-180 minutes, preferably ranging from 30-90 minutes, to achieve the desired delignification and/or brightness of the pulp. The peroxide- containing solution reacts with the fibrous materials of the aqueous slurry, so as to at least partially delignify and/or brighten the fibrous materials, during the periods of time when the pulp is retained within the upflow tube, or column 836 and reaction tower 838. The pulp is then discharged from tower 838 and flows to an Eop-stage washer, or compaction baffle filter 840 via flowpath pipe 814. Water or filtrate from a subsequent stage for washing of the pulp is also supplied to filter 840 as discussed previously. Due to the displacement action of the wash water entering filter 840, an alkaline effluent, or filtrate is produced which discharges filter 840 via pipe 848.

The pulp then flows from the Eop-stage washer 840 to a Z-stage pump 856 via a flowpath conduit, or pipe 814. As with pumps 830 and 816, pump 856 preferably comprises a Clove Rotor ^* pump. Sulfuric acid is introduced to the pulp at a location between filter 840 and pump 856 via pipe 858. As shown in Fig. 2 the pulp is then pumped to a mixer 860 via flowpath pipe 814. It is noted that the encircled letter A on Figs. 1 and 2 represents a common location between pump 856 and mixer 860. Ozone is also added to mixer 860 via pipe 862, for the purpose of further delignifying and brightening the pulp during the ozone, or Z-stage. The pulp flows from mixer 860 to an ozone reactor 864 via transfer pipe 814. The ozone reactor 864 is sized to assure a pulp retention time ranging from 1-5 minutes. The pulp temperature within the ozone reactor 864 ranges from 30-100° C, preferably ranging from 50-70° C. The pulp is discharged from ozone reactor 864 to an ozone separator 866 which is used to separate the unreacted gaseous ozone from the pulp. The pulp then flows to tank 868 via transfer pipe 814. The pulp then discharges tank 868 flowing to the Z-stage washer, or compaction baffle filter 870, via transfer pipe 814. Water and/or filtrate produced by the downstream stage is also supplied to the washer for washing the pulp. As a result of the water and filtrate entering washer 870, a filtrate is produced which discharges washer 870 via pipe 880.

The pulp discharges from a washer 870 to a hot peroxide, or ^~ P _m , stage pump 886 via transfer or flowpath pipe 814. A caustic, preferably comprising sodium hydroxide, is introduced to the pulp at a location between the Z-stage washer 870 and the hot peroxide, or Y _m , stage pump 886 via pipe 888. The pulp is then pumped to a pulp heater 890, which may comprise a steam mixer. Steam is added to pulp heater 890, via pipe 892, to raise the temperature of the pulp to the desired level for processing through the hot peroxide stage. The pulp exits pulp heater 890 and flows to a peroxide mixer 894 via flowpath pipe 814. Hydrogen peroxide is added to the peroxide mixer 894 via pipe 896. The peroxide-containing solution added to the aqueous slurry equals 0.10-50% , by oven-dried weight of the fibrous materials of the aqueous slurry. The pulp discharges the peroxide mixer 894 via flowpath pipe 814 to an upflow reaction tube, or column 898. The upflow reaction tube 898 is sized to assure a pulp retention time ranging from 1-45 minutes, preferably ranging from 5-20 minutes. The quantity of steam added to heater 890 is sized to assure a pulp temperature, within upflow tube 898, ranging between 70-150° C, preferably ranging from 100-140° C. A vent valve 900 is disposed in the flowpath pipe 814 used to convey the pulp from the upflow tube 898 to a peroxide reaction tower 902. The pulp flash cools as it flows through vent valve 900, for the second phase of the reaction of the hot peroxide stage, which occurs in downflow reaction tower 902. Tower 902 is sized to assure a pulp retention time ranging from 5-400 minutes, preferably ranging from 20-180 minutes. Due to the flash cooling of the pulp as it passes through vent valve 900, the pulp temperature within tower 902 ranges from 90-100°C, and preferably ranges from 95-100° C. Alternatively, the pulp may be cooled prior to discharge through valve 900, in which case flashing may not occur or, the pulp may not be hot enough to flash. The peroxide-containing solution reacts with the fibrous materials of the aqueous slurry so as to further delignify and brighten the fibrous materials during the times when the pulp, or aqueous slurry, is retained within the upflow reaction tube, or column 898 and the reaction tower 902. The pulp discharges from tower 902, flowing to the hot peroxide, or P _HT stage washer, or compaction baffle filter 904 via transfer pipe 814. Water and/or filtrate produced downstream is

supplied to the washer 904 for washing the pulp. The pulp is then supplied to a discharge pump 908, via a transfer pipe 814, and is then pumped to a storage tank (not shown). The filtrate which discharges washer 904 through pipe 910 is recirculated for washing on prior stages as discussed previously. One of the central features of the present invention is the use of the subsequently described materials of construction for a variety of vessels used in bleaching plant 800, as well as a variety of other bleaching plants as subsequently discussed, and the associated method steps. As set forth in the Background Section of the present invention, and further described in this disclosure, the decomposition of bleaching peroxide is particularly problematic at the elevated temperatures and pressures described in the parent application, U.S. Patent Application having Serial Number 08/273,539, which is herein expressly incorporated by reference in its entirety. Accordingly, the subsequently described, novel materials of construction have particular application for vessels used in the Eop and ^~ P _m stages of bleach plant 800, due to the pulp temperature within these stages and to the introduction of hydrogen peroxide at an upstream portion of these stages and the resultant alkaline sodium peroxide, having a pH of at least 7.5, resulting from the reaction of hydrogen peroxide with sodium hydroxide. For purposes of this application, a "vessel" may comprise any of the following pulp processing equipment: transfer pipes; pumps; pulp heaters including steam mixers; peroxide mixers; reaction tubes; and reaction towers. Accordingly, the subsequently described materials of construction have particular application in bleach plant 800 in constructing transport piping 814, pumps 830 and 886, mixer 834, pulp heater 890, peroxide mixer 894, reaction tubes or columns 836 and 898, and reaction towers 838 and 902, or any other equipment which comes in contact with the alkaline peroxide. Also, it should be understood that the subsequently described materials of construction may be advantageously utilized for other components of bleach plant 800 which do not come in contact with the alkaline peroxide although the advantages derived therefrom, specifically the reduction in the decomposition rate of peroxide, may not be justified from a cost standpoint due to the lower temperature of the remaining stages of the bleach plant 800. Each of the

aforementioned elements of the Eop and V _m stages of bleach plant 800, i.e., mixers 834 and 894, pipes 814, etc., in a preferred embodiment are preferably made from one or more metals selected from the group consisting of: zirconium-base castings as defined in specification ASTM B752-latest edition, entitled "Standard Specification for Castings, Zirconium-Base, Corrosion Resistant, for General Application"; zirconium and zirconium alloys as defined in specification ASTM B551 -latest edition, entitled "Standard Specification for Zirconium and Zirconium Alloy Strip, Sheet and Plate"; hafnium and hafnium alloys, as defined in specification ASTM B776-latest edition, entitled "Standard Specification for Hafnium and Hafnium Alloy Strip, Sheet, and Plate"; niobium and niobium alloys as defined in specification ASTM B393-latest edition, entitled "Standard Specification for Niobium and Niobium Alloy Strip, Sheet, and Plate" ; and tantalum and tantalum alloys as defined in specification ASTM B708- latest edition, entitled "Standard Specification for Tantalum and Tantalum Alloy Plate, Sheet and Strip". Other metals and/or their alloys which achieve substantially equivalent results are also envisioned to be within the scope of this invention.

Any of the aforementioned vessels, employing the material of construction of the present invention (i.e., mixers 834 and 894, transfer piping 814, reactor or reaction tubes 836 and 898, etc.) may be made entirely of zirconium, niobium, hafnium, tantalum, or alloys thereof, or may be fabricated such that they are made in part from these metals and in part from conventional materials of construction used in bleaching plants such as stainless steel, carbon steels or other materials. However, in this instance, when the vessels are fabricated from multiple materials, the surfaces which may be referred to as contact surfaces, of the vessels which are in direct contact with the pulp being processed through bleach plant 800 must be constructed of one of the materials of the group consisting of zirconium, niobium, hafnium, tantalum, or alloys thereof, while the remaining portions of the effected vessels may be constructed of the conventional materials. This may be accomplished by cladding or plating processes for instance, or other conventional means. It should be noted that the method for fabricating, casting or otherwise manufacturing the effected vessels does not comprise a portion of the present invention.

In conjunction with the use of the materials of construction of the present invention comprising, zirconium, niobium, hafnium, tantalum, and alloys thereof, the present invention includes a process for bleaching of lignocellulosic, cellulosic and synthetic polymeric fibrous materials using alkaline peroxide comprising the following steps, regardless of the processing stage of bleach plant 800, or other bleaching plants, which incorporate the apparatus and process steps of the present invention: creating an aqueous slurry consisting of about 0.25-50% by weight fibrous materials; and adding alkali to increase the pH of said slurry to greater than 7.5.

When the materials of construction are incorporated in the Pm- and Eop stages of bleach plant 800, or other peroxide-adding stages of bleaching plants which may incorporate the present invention as subsequently discussed, the process of the present invention includes the further steps of adding peroxide-containing solution to equal 0.10-50%, by oven-dried weight, of the fibrous materials and reacting the peroxide- containing solution with the fibrous materials. The "adding" step may take place in a mixer, such as mixers 834 and 894 of bleach plant 800, or within other vessels of a peroxide-adding stage of a bleach plant. It is noted that the "reacting" step may be accomplished in upflow tubes, columns, or a pipe such as reaction tubes 836 and 898, or in downflow reaction towers, such as towers 838 and 902. During the "reacting" step a portion of residual peroxide is consumed. The "reacting" step serves to at least partially delignify and to brighten the fibrous materials. The "adding" and "reacting" steps may also be included in the process of the present invention when the materials of construction of the present invention are incorporated in any one of the following stages of a bleach plant, wherein peroxide, or a peroxide-containing solution is added to the aqueous slurry: P; Ep; Eop. Additionally, the "reacting step" may be included in the process of the present invention when the materials of construction of the present invention are incorporated in a peroxide-generating stage of a bleaching plant such as an O or an Eo stage, wherein peroxide is generated as a by-product of reaction.

When the materials of construction of the present invention are incorporated in the P _HT stage of bleach plant 800, the process of the present invention further includes

the step of heating the fibrous materials to a temperature of greater than 70 °C. When the materials of construction of the present invention are incorporated in any one of the P, Eo, Ep, and O-stages of a bleach plant the process of the present invention further includes the step of heating the fibrous materials to a temperature greater than 60°C.

Regardless of the bleach plant stage in which the materials of construction of the present invention are incorporated, the process of the present invention includes the improvement of conducting at least one of the process steps in at least one vessel having a contact surface wherein the vessel is constructed from one or more metals selected from the group consisting of zirconium, niobium, hafnium, tantalum or alloys thereof.

The process of the present invention may further include at least one of the following steps: limiting the passivated degradation of peroxide to less than 1 g/min/cm ² xlO ^Jl ; limiting the abraded degradation of peroxides to less than 2g/min/cm ² xl0^ without the addition of a sequestering agent; limiting alkaline peroxide degradation to less than 20% of original volume. Specifically, an alkaline peroxide which can be used in the present invention is illustrated to be sodium peroxide having a pH of at least 7.5. More preferably, the alkaline peroxide used in the process of the present invention will be sodium peroxide having a pH ranging from 8-13. In some preferred embodiments, the pH of the peroxide, preferably sodium peroxide, will range from 11-13. However, it is anticipated to be within the scope of the invention to utilize other base alkaline peroxides, provided that the pH of the peroxide is at least 7.5.

The advantages of the present invention regarding the reduction in the degradation, or decomposition of an alkaline peroxide, is demonstrated in the subsequently discussed examples. Since the materials of construction of the present invention significantly reduce the rate of decomposition of alkaline peroxides, particularly at elevated temperatures (i.e., temperatures greater than 90°C), relative to prior conventionally used materials of construction such as stainless steel and titanium, the process of the present invention may eliminate the use of sequestering agents in

the peroxide-containing stages, such as DTPA (1, 1,4,7,7-

Diethylenetriaminepentaacetic acid), and EDTA (ethylenediaminetetraacetic acid), used in prior processes. For example, if bleach plant 800 were constructed entirely of prior conventional materials, it may be necessary to introduce chelating or sequestering agents at various locations in bleach plant 800, particularly in the peroxide mixer 894. Accordingly, the utilization of the materials of construction of the present invention results in a significant cost reduction in the present bleaching process compared to prior processes. Additionally, due to utilization of the materials of construction of the present invention, smaller vessels, particularly reaction tubes and reaction towers, may be utilized such that a significantly greater proportion of the processed pulp contacts the surface of the vessels without adversely affecting, or decomposing the alkaline peroxides used to brighten the pulp.

EXAMPLES The following examples are provided which demonstrate some of the benefits which can be achieved by following the teachings of this invention.

TJie effect ofpassivated metal coupons of different metallurgies on the decomposition of alkaline peroxide

Example 1: The surface area of each of the metal coupons was carefully measured and the coupon was secured with a teflon cord and immersed in the test solution in an open top reaction vessel. A bleaching liquor (100 ml) consisting of hydrogen peroxide (0.25 g), MgSO ₄ (0.005 g), DTPA (0.02 g), and NaOH (0.25 g) contained in a polyethylene vessel was heated to 121 °C in a pressurized autoclave and kept at temperature for 20 minutes. At the end of this period, the autoclave was vented and the alkaline peroxide concentration was determined by iodometric titration with potassium iodide and ammonium molybdate as catalyst. The rates of alkaline peroxide decomposition in the presence of the different coupons was calculated and tabularized in Table 1.

Table 1 The effect of coupons of various compositions on the decomposition of alkaline peroxide bleaching solutions

Bleaching Reactor Alkaline Peroxide Decomposition Rate (g/min/cm ² xl0-y

Pyrex glass container + bleach liquor ² 0.21

Control (polyethylene container + bleach 0.10 liquor) ²

+Zr702 coupon ³ 0.20

+Zr705 coupon ³ 0.34

+Ti coupon ⁴ 4.6

+304L ss coupon ⁵ 3.8

+316L ss coupon ⁵ 1.9

+317L ss coupon ⁵ 1.8

+2205 duplex ss coupon ⁵ 2.6

+AL-6XN ss coupon ⁶ 1.2

+2507 duplex ss coupon ⁵ 4.1

+2304 duplex ss coupon ⁵ 4.5

+Avesta 654 SMO ss coupon ⁵ 2.7

¹ The rate of decomposition was determined after keeping the samples at temperature for 20 minutes.

² The rates of decomposition for these materials (which were the containers for the other coupons) was based on the exposed surface of the containers.

³ As defined in specification ASTM B551-92, entitled "Standard Specification for Zirconium and Zirconium Alloy Strip, Sheet, and Plate".

⁴ As defmed in specification ASTM B265-93, entitled "Standard Specification for Titanium and Titanium Alloy Strip, Sheet, and Plate".

⁵ As defined in specification ASTM A240-93, entitled "Standard Specification for Heat-Resisting Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels".

⁶ As defined in specification ASTM B688-93, entitled "Standard Specification for Chromium-Nickel- Molybdenum-Iron (UNS N08366 and UNS N08367) Plate, Sheet, and Strip".

The rates of decomposition for each of the metal coupons was determined by subtracting the amount of decomposition due to the polyethylene container from the total decomposition observed in a particular sample.

The effect of surface abrasion of passivated metal coupons of different compositions on the decomposition of alkaline peroxide at elevated temperatures and pH values

Example 2: The reaction mixtures were prepared as in Example 1 but immediately before exposing the coupon to the bleach liquor at temperature each sample was abraded (using a material with the same composition as the coupon) and then exposed to the alkaline bleaching solution (Table 2).

It was unexpected that zirconium metal and alloys thereof would have a much smaller effect than other metallurgies on the decomposition rate of alkaline peroxide at high temperatures (> 100°C). This is further illustrated in Fig. 3A-3D, where the benefits of in situ passivation can be seen with the coupons with various metallurgies.

It is noteworthy that, of the metallurgies tested, only zirconium shows a consistently low level of peroxide decomposition, even when "fresh abraded" surfaces were exposed to alkaline peroxide. After the zirconium coupon was abraded, it was rapidly repassivated by the bleaching liquor and upon a second exposure to the bleaching liquor, the decomposition rate of the alkaline peroxide solution decreased from 0.89 x 10^ g/min/cm ² to 0.34 x 10 ^"4 g/min/cm ² for Zr705 and from 1.2 x 10 g/min/cm ² to 0.20 x 10" ⁴ g/min/cm ² for Zr702. This is a key property of this material, since in a typical application of this material of construction, the walls of the alkaline peroxide reactor will be subjected to continuous abrasion from the flow and turbulence of the lignocellulosic material.

To restore the maximum chemical resistance of zirconium alloys and to minimize hydrogen absorption during pickling, the following acid mixture is recommended. The acid solution should be composed of 25 to 50 volume % (350 to

700 g/1) of nitric acid (70%), and 3 to 7 volume % (36 to 84 g/1) of hydrofluoric acid (60%) at 49 °C, maintaining a ratio of ten parts nitric acid to one part hydrofluoric acid. Immersion time in the acid should be 30 to 60 seconds.

Table 2 Tlie effect of surface abrasion of coupons of various compositions on the decomposition of alkaline peroxide bleaching solutions

Bleaching Reactor Alkaline Peroxide Decomposition Rate(g/min/cm ² x 10 ^"4 ) ¹

Passivated ³ Abraded

Pyrex glass container + bleach liquor ² 0.21 0.35 control (polyethylene container + bleach liquor) ² 0.10 0.22

+Zr702 coupon 0.20 1.2

+Zr705 coupon 0.34 0.89

+Ti coupon 4.6 7.2

+304L ss coupon 3.8 7.3

+316L ss coupon 1.9 7.2

+317L ss coupon 1.8 6.9

+2205 duplex ss coupon 2.6 7.4

+AL-6XN ss coupon 1.2 6.7

+2507 duplex ss coupon 4.1 7.9

+2304 duplex ss coupon 4.5 7.9

+Avesta 654 SMO ss coupon 2.7 5.9

¹ The rate of decomposition was determined after keeping the samples at temperature for 20 minutes. The rates of decomposition for these materials (which were the containers for the other coupons) was based on the exposed surface of the containers.

³ Each coupon was abraded by passing a coupon of the same composition over the surface of the coupon to be evaluated with ten strokes, five forward and five back, the force being applied by hand pressure.

The rates of decomposition for each of the metal coupons was determined by subtracting the amount of decomposition due to the polyethylene container from the total decomposition observed in a particular sample.

The effect of the materials of construction on the efficiency of bleaching of alkaline peroxide at different temperatures

Example 3: An oxygen-delignified softwood kraft pulp (kappa number 9- 1; ISO brightness 35.0) was treated with a solution containing hydrogen peroxide at a charge of 2.5% calculated on oven-dry pulp basis, equivalent to a concentration of 0.25g H ₂ O ₂ /10g oven-dry pulp at a 10% pulp consistency. The peroxide was added to a bleaching solution consisting of 2.5 % NaOH, 0.05% MgS0 ₄ and 0.2% DTPA calculated on a pulp OD basis and then the bleaching solution was added to the pulp. Immediately after mixing the pulp and bleaching solution, the pulp samples were placed into reaction vessels of either stainless steel, polyethylene, or Pyrex and heated to temperatures of 90 or 110°C, kept at temperature for 60 or 240 minutes, then cooled. The pulp samples were washed, then the ISO brightness of the resulting pulps was determined (Table 3). This data clearly illustrates the benefits of having an appropriate reaction vessel material when bleaching lignocellulosic materials with alkaline peroxide, especially at higher temperatures.

Table 3 The effect of different reaction containers on the overall effectiveness of alkaline peroxide in bleaching softwood kraft pulp

Bleach Temp. (°C) Time (min) ISO Peroxide ¹ vessel Brightness (%) Charged Residual

Stainless 90 240 73 2.5 0.1 Steel ²

" 110 60 75 2.5 0.0

Polyethylene 90 240 80 2.5 0.4

Pyrex Glass ⁴ 110 60 82 2.5 0.5

¹ The peroxide charges and residuals are calculated on a weight % basis based on OD pulp.

² The pulp was kept at temperature in a stainless steel pressure vessel at 75 psig under oxygen (100%) and 90 or 100°C for the times indicated. The pulp was mixed continuously.

³ The pulp was kept at temperature in a polyethylene bag at temperature for the time indicated. The pulp was kept at temperature in a pyrex glass container at temperature for the time indicated.

A hardwood kraft brownstock pulp (kappa number 12.0; ISO brightness 30.0) was bleached in reaction vessels constructed from various materials

Example 4: The hardwood kraft brownstock pulp Kappa No. 12.0 was treated with a solution containing hydrogen peroxide at a charge of 2.5% calculated on an oven-dry pulp basis, equivalent to a concentration of 0.25g H ₂ O ₂ /10g oven-dry pulp at a 10% pulp consistency. The peroxide was added to a bleaching solution consisting of 2.5% NaOH, 0.05% MgSO ₄ and 0.2% DTPA calculated on a pulp OD basis. The pulp samples were heated to a temperature of 121 °C and kept at temperature for 5 or 20 minutes, then cooled. The pulp samples were washed, then the Kappa number of the resulting pulps were determined (Table 4). It was unexpected that the reaction vessel constructed of zirconium resulted in a pulp having a lower Kappa number.

Table 4

A comparison of the zirconium and 316 stainless steel reactors for bleaching hardwood kraft pulp

Reactor Metallurgy 316 S.S. Zirconium

Time at temperature, min 5 20 5 20

Kappa Number 7.7 7.0 6.5 6.1

Final pH 11.5 11.5 11.6 11.6

Peroxide Charge (%) ^* 2.5 2.5 2.5 2.5

Peroxide Residual (%) 0.5 0 1.0 0

' Peroxide charges are expressed on a weight % OD pulp basis.

Tlie effect of surface abrasion of passivated metal coupons of different compositions on the decomposition of alkaline peroxide at elevated temperatures and pH values

Example 5: The reaction mixtures were prepared as in Example 1 but immediately before exposing the coupon to the bleach liquor at temperature each sample was abraded (using a material with the same composition as the coupon) and then exposed to the alkaline bleaching solution (Table 5).

It was unexpected that hafnium, zirconium, tantalum, and, to a lesser extent, niobium metals and alloys thereof would have a much smaller effect than that of, for example, ferrous metal alloys commonly used in the construction of peroxide reactors, on the decomposition rate of alkaline peroxide at high temperatures (> 100°C). Furthermore, the benefits of in-situ passivation of these materials can be seen by comparing the amount of decomposition mediated by these coupons with various metallurgies. It is noteworthy that, of the metallurgies tested, only zirconium, tantalum, hafnium, and to a lesser extent, niobium show a consistently low level of

peroxide decomposition, even when "fresh abraded" surfaces were exposed to alkaline peroxide (Table 5).

After the zirconium, hafnium, tantalum, or niobium coupons were abraded, they were rapidly repassivated by the bleaching liquor and upon a second exposure to a fresh solution of bleaching liquor, the decomposition rate of the alkaline peroxide solution decreased. This is a key property of these materials, since in a typical bleaching application of these materials of construction, the walls of the alkaline peroxide reactor will be subjected to continuous abrasion from the flow and turbulence caused by the passage of the lignocellulosic material.

Table 5

TJie effect of surface abrasion of coupons of various compositions on the decomposition of alkaline peroxide bleaching solutions

Bleaching Reactor Alkaline Peroxide Decomposition Rate (g/min/cm ² x 10 ^"4 ) ¹

Passivated Abraded

Control (polyethylene container + bleach 0.10 0.22 liquor) ²

+Zr705 coupon ³ 0.34 0.89

+Ti coupon ³ 4.6 7.2

+317L ss coupon ³ 1.8 6.9

Tantalum ³ 0.10 0.10

Hafnium ³ 0.10 0.10

Niobium ³ 1.2 1.2

¹ The rate of decomposition was determined after keeping the samples at temperature for 20 minutes.

² The rate of decomposition caused by the polyethylene container for these materials (which was in the containers for the other coupons) was based on the exposed surface of the container. The polyethylene was unabraded and not passivated.

³ Coupon tested in an unabraded polyethylene container.

The rates of decomposition for each of the metal coupons was determined by subtracting the amount of decomposition due to the unabraded polyethylene container from the total decomposition observed in a particular sample.

Vie effect of passivated metal coupons with and without DTPA on the extent of alkaline peroxide decomposition

Example 6: The effect of passivated metal coupons of different compositions with and without DTPA added to the solution on the decomposition of alkaline peroxide at elevated temperatures (121 °C) and pH values was determined. The reaction mixtures with and without DTPA were prepared as in Example 1. The amount of peroxide decomposition was determined after keeping the immersed metal coupon samples in the peroxide solution at 121 °C for 20 minutes in a glass container. The amount of decomposition by each material was measured as the total decomposition less the decomposition in the glass container alone, with and without DTPA added. The results are shown in Fig. 4.

It was unexpected that the zirconium would have a much smaller effect than that of, for example, these ferrous metal alloys commonly used in the construction of peroxide reactors, on the decomposition rate of alkaline peroxide at high temperatures (> 100 °C). Further unforeseen benefits were observed by comparing the amount of decomposition mediated by these coupons in the presence of zirconium than was observed with either the stainless steels or titanium in the presence of DTPA (Fig. 4). The increase in decomposition that was observed with zirconium can be explained by the iron plus chromium content (0.2% max.) present in the commercial alloys used for this evaluation as described in bulletin ASTM B551-92.

Discussion While the foregoing description has set forth the preferred embodiments of the invention in particular detail, it must be understood that numerous modifications, substitutions, and changes can be undertaken without departing from the true spirit

and scope of the present invention as defined by the ensuing claims. For instance, while the disclosure has focused on the bleaching of lignocellulosic and cellulosic fibrous materials using alkaline peroxide, it is envisioned that the bleaching of non- naturally occurring polymers (i.e., synthetic) such as polyesters, polyamides, polyacrylates or polyolefins (e.g., polypropylene) would be equally applicable.

Additionally, while the materials of construction and associated process steps of the present invention have been illustrated in particular detail with respect to bleach plant 800, which comprises an OAEopZPm-, the materials of construction and associated process steps of the present invention may be advantageously incorporated in at least the peroxide stages of a wide variety of bleach plants, including the element chlorine free (ECF) plants illustrated schematically in Figs. 5B-5E, where peroxide is applied to reduce the amount of chlorine containing bleaching chemicals.

There is great concern for the possible increased pressure on the industry to further reduce the amount of chlorine containing compounds used in the bleaching of wood pulp, and the effect of the proposed USEPA "Cluster Rule" on the requirements for process changes in the bleaching of wood pulp. It is desirable to implement so called "totally chlorine free" (TCF) or "totally effluent free" (TEF) technology in order to eliminate the use of chlorine containing reagents and/or eliminate effluent from the bleaching operation. In this case, numerous sequences have been proposed for implementation into the pulp processing system, most of which have a relatively high cost of bleaching reagents. The improved efficiency of the use of peroxide according to the present invention makes TCF and TEF technology more attractive economically. Accordingly, the materials of construction and the associated process steps of the present invention may be advantageously utilized in at least one stage of each of the TCF bleach plants illustrated schematically in Figs. 6A-6E, with each plant using peroxide in combination with oxygen and ozone as shown.

Furthermore, the materials of construction and associated process steps of the present invention may be applied to other peroxide-utilizing processes, such as peroxide-utilizing stages in chlorine-containing bleaching sequences like but not limited to (DcEoDEDP; DEoDEDP; DEoDEpD; DEoDP; DPDP; ODEoDPD;

ODEoDP; ODEoPD; ODPD; ODPDP; OZEoDP; etc. , where Dc represents a mixture of chlorine and chlorine dioxide and D represents a chlorine dioxide stage); and mechanical pulp brightening, in a variety of configurations, including for example, tower bleaching, in-refiner bleaching processes and in processes such as APMP (alkaline peroxide mechanical pulp process).

Additionally, the beneficial effect of the use of the materials of construction of the present invention, with respect to preventing decomposition of peroxide species, can be extended to those processes in which peroxide is generated during the course of their application. One such example is oxygen delignification. It is well recognized (Sjostrom, E. 1981, The chemistry of oxygen delignification, Paperi ja Puu 63 (6-7), p 438-442; Gratzl, J.S., 1990, Reactions of polysaccharides and lignin in bleaching with oxygen and related species. Tappi Oxygen Delignification Symposium, Toronto, Canada, p 1-21) that hydrogen peroxide is produced during the course of the oxygen delignification process. This peroxide serves an important function in the effectiveness of oxygen delignification. By preventing the decomposition of this peroxide, oxygen delignification can be made more efficient and selective.

Likewise, other compounds containing the peroxy bond can be used under alkaline conditions. There are a variety of organic and inorganic peroxides, for example peracetic acid and peroxymonosulphuric acid which can be applied under alkaline conditions (Z.P. Geng, H. m. Chang, H. Jameel, 1993; Mixed peracids manufacture and use as non-chlorine delignification and bleaching agents, Proc. Tappi Pulping Conference, Atlanta, USA, p 353-362; R.T. Hill, P.B. Walsh, and J.A. Hollis, 1992, Part I: peracetic acid-an effective alternative for chlorine compound free delignification of kraft pulp. Proc. Tappi Pulping Conference, Boston, U.S.A. p 1219-1230). Under such circumstances zirconium, tantalum, hafnium, and niobium and alloys thereof would be expected to be similarly beneficial, i.e., these materials of construction would reduce the decomposition rate of peracids relative to that experienced when conventional materials of construction are used.

Thus, what has been shown in general, is the ability to inhibit the premature decomposition of the oxygen-oxygen (i.e., O-O) chemical bond through the judicious selection of reaction vessel construction materials. This property has been shown to be quite significant when the peroxide is hydrogen peroxide, or its alkaline derivatives, the specific derivative being dependent on the choice of alkaline compound, e.g., sodium, potassium, lithium, rubidium or cesium which is present in the system.

However, this type of chemical bond is generic to more than simply the peroxides, and can be generalized to peracid-containing compounds, such as persulfuric acids (Caro's acid), perchromic acid, and peralkyl acids. Specific examples of peralkyl acids which are included within the scope of this invention are shown in general formula (I)

wherein R is a lower saturated alkyl of from 1 to 3 carbons. Specific examples of such peralkyl acids would include peracetic acid, perpropionic acid, perbutyric acid, etc., and their alkaline salts thereof.

There are countless ways to incorporate the use of the present invention into the processing systems for lignocellulosic, cellulosic and synthetic polymeric fibrous materials, and the foregoing is not intended to limit the applicability of the present invention.

The invention is therefore not limited to specific preferred embodiments as described, but is only limited as defined by the following claims.