IMPROVED PROCESS FOR THE BLEACHING OF CELLULOSIC PULPS USING HYDROGEN PEROXIDE

Abstract: The bleaching of wood pulp is improved by the combination of a pretreatment of the pulp with a polyaminocarboxylic acid, e.g. ethylenediaminetetraacetic acid, prior to bleaching with an alkaline aqueous peroxide solution containing a stabilizing amount of an aminophosphonic acid derivative together with a polymer of an unsaturated carboxylic acid or amide or of an alkylsulfonic acid substituted amide.

Description: This invention concerns the bleaching of wood pulp during paper manufacture. The active bleaching agent used is hydrogen peroxide. Sodium silicate is normally employed as a stabilizer to prevent early depletion of the active bleaching agent. In the process of making the pulp, metal ions are picked up from the wood, the water and the machinery used to masticate the wood chips and pulp. While some of the metal ion content is lost in the deckering or dewatering step, it is sometimes advantageous to add a chelating agent. Of the commercially available chelating agents, the sodium salt of diethylenetriaminepentaacetic acid has been reported to be the most effective. This is found in an article "The Effect of DTPA on Reducing Peroxide Decomposition", D.R. Bambrick, TAPPI Journal , June 1985, pp. 96-100. The use of silicates as a hydrogen peroxide stabilizer in such systems, however, results in problems when insoluble silicates are deposited upon the fibers and the machinery employed. When the silicates are deposited on the pulp fibers the result is a harsher feel of the paper. The fouling of equipment can cause down-time and shortened life of the equipment. Because of this, silicate-free systems have been suggested. These silicate-free systems have been found to work well in the single stage hydrogen peroxide bleaching of Kraft pulps where the choice of stabilizer possibly influences the bleaching mechanism by changing the reaction pathway of hydrogen peroxide. In such systems, the addition of poly-(α-hydroxyacrylate) as a stabilizer also has been shown to improve pulp brightness. The use of this stabilizer is discussed in a paper "Hydrogen Peroxide Bleaching of Kraft Pulp and the Role of Stabilization of Hydrogen Peroxide," by G. Papageorges, et al . given at the ESPRA Meeting in Maastricht, Netherlands, May, 1979. GB-A- 1,425,307 discloses a method for preparing this stabilizer. In US-A- 3,860,391 the bleaching of cellulose fibers and mixtures thereof with synthetic fibers is accomplished by employing peroxide in a silicate-free system in the presence of an aliphatic hydroxy compound, an amino alkylenephosphonic acid compound and, alternatively, with the addition of a polyaminocarboxylic acid. Representative of the above are erythritol or pentaerythritol, ethylenediaminetetra-(methylenephosphonic acid) or 1-hydroxypropane-1,1,3-triphosphonic acid and ethylenediaminetetraacetic acid or nitrilotriacetic acid, respectively. US-A- 4,238,282 describes a pulp bleaching system employing chlorine (not peroxide) which uses various chelating agents, including poly acrylic acid (mol. wt. <2000), alkylene polyaminocarboxylic acids, and aminophosphonic acids and their salts. Other more recent U.S. patents which employ such phosphonates as indicated above, but in a peroxide bleaching system, include US-A- 4,239,643 and its divisional US-A-4,294,575. While, as noted above, various combinations of chelating agents are useful in stabilizing peroxide bleaching systems, the presence of metal ions, e.g. iron, manganese and copper, provides a catalytic effect with respect to the decomposition of the peroxide and also tends to reduce the brightness of finished mechanical pulps. While the chelants might be expected to take care of minor amounts of the metal ions, the presence of significant amounts of magnesium and/or calcium ions which may be present in the wood pulp or water or both tends to overwhelm the ability of the chelants to complex the iron, manganese and copper ions present. Certain combinations of the aminophosphonic acids together with polycarboxylic acids or polycarboxylic amides or a sulfonic acid derivative of a polyamide have been found to provide s ilization in the presence of significant amounts of magnesium and/or calcium ions and in the presence of small amounts of copper and the like metal ions which catalyze the peroxide decomposition. This stabilizer is disclosed in US-A- 4,614,646. It has now been found that improved bleaching results by treating wood pulp with a polyaminocar boxylic acid prior to contacting the pulp with the stabilized aqueous peroxide solution referred to above. This invention comprises an improvement involving two steps of the process of bleaching wood pulp for manufacture of paper products. The bleaching is accomplished in an alkaline aqueous peroxide system. Prior to the addition of the peroxide the pulp is dewatered to a solids content of from 10 to 40 percent by weight. In a process for bleaching wood pulp using hydrogen peroxide in an alkaline silicate-free aqueous system, the improvement comprises the steps of: (1) pretreating the pulp with a polyaminocarboxylic acid or salt thereof and (2) bleaching with hydrogen peroxide stabilized with (a) an aminophosphonic acid chelant or salt thereof and (b) at least one polymer of (i) an unsaturated carboxylic acid or salt thereof, (ii) an unsaturated carboxylic amide or (iii) an unsaturated carboxylic amide wherein the amide hydrogens are substituted with an alkylsulfonic acid group or salt thereof. The useful aminophosphonic acid derivatives include those corresponding to the formula wherein: M is independently H, alkali metal, NH₄, or an amine radical; R₁ is an aliphatic straight or branched chain, cyclic or aromatic radical having from 2 to 6 carbon atoms; n is 0 to 12; and m is 1 to 3. One example of a compound of Formula (I) is diethylenetriaminepentamethylene phosphonic acid or its ammonium, alkali metal or amine salt. The polymeric acids and amides useful in the invention include those of the formulae wherein: A is independently hydrogen or methyl; Z is independently NH₂ or OM wherein M has the previous meaning and wherein the Z substituents may be the same or different; and p is from 13 to 5,500, preferably from 25 to 250 and wherein: R₂ is an alkylene radical having from 1 to 6 carbon atoms; p′ is from 5 to 2,000, preferably from 10 to 350; and A and M have the above indicated meanings and wherein the M substituents may be the same or different. Copolymers of monomers of the above formulae are also useful, e.g. acrylic acid or its ammonium, alkali metal or amine salt. Thus a partially hydrolyzed polyacrylamide is effective. Such polymers have molecular weights of from 1,000 to 400,000. While the polyaminocarboxylic acids have previously been used in a silicate stabilized peroxide bleach system, e.g. see the previously mentioned Bambrick article, their use does not give the dramatic increase in brightness obtained by the present invention. Apparently, the addition of the polymer-aminophosphonic acid stabilized bleach, in the absence of silicate, creates an environment wherein pretreatment with a polyaminocarboxylic acid is not only highly desirable and efficient, but is critical to a superior bleaching of the pulp. The polyaminocarboxylic acids useful in the pretreatment step of the bleaching process include those alkylene-polyaminopolycarboxylic acids having the formula wherein A, B, C, D, E and F are independently hydrogen, CH₂COOR₄, CH₂CH₂OH or CH₂CH(CH₃)OH; R₃ is a hydrocarbon radical of the formula -CH₂CH₂-, -CH₂CH₂CH₂- or R₄ is hydrogen, an alkali metal, ammonium or an amine radical; a and b are each integers of 0-2. Representatives polyaminocarboxylic acids are ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), triethylenetetraminehexaacetic acid (TTHA) and N-hydroxyethylethylenediaminetriacetic acid (HEDTA). Mixtures of polyaminocarboxylic acids can be used, especially mixtures of the completely carboxylated polyamine with those in which one amine hydrogen is replaced with a hydroxyethyl group, the remaining hydrogens being replaced by carboxymethyl groups. A particularly preferred blend is HEDTA or its salts and EDTA or its salts. Representative of the amine salts of the polyaminocarboxylic acids are their mono-, di-or trialkanolamine salts, e.g. the monoethanolamine salt of EDTA. The following examples are illustrative of the present invention. To demonstrate the relative effectiveness of chelant pretreatment on both the polymer-phosphonate and silicate stabilized pulp bleaching systems, wood pulp from two mills was obtained. Samples of each pulp were first pretreated with a polyaminocarboxylic acid chelant. Then the treated pulp was bleached with an alkaline (initially pH >8) peroxide bleach liquor containing either silicate or the polymer-phosphonate stabilizer. After bleaching under the conditions shown in Table I, which are typical of those used in pulp mills, the bleach liquor was removed and the pH and residual peroxide were determined. The pH of the pulp was first adjusted to 4.5 to arrest the peroxide reaction and then the pulp was formed into a handsheet and dried. The handsheet was then measured for brightness (expressed in GE units). Where applicable, TAPPI Standard Methods were used. The pretreatment and bleaching conditions as shown above were employed with pulp #1, using three different polyaminocarboxylic acids for the pretreatment at a level based on the oven dry weight of the pulp of 0.12% (or 6 lbs./ton (3kg/tonne) of the commercially available 40% solution). Example A is a control in which no pretreatment was used prior to the bleaching step. Examples 1, 2 and 3 used the sodium salts of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and hydroxyethylethylenediaminetriacetic acid (HEDTA), respectively, for the pretreatment. Results are shown in Table II. The differences (delta) between the control and each of the resulting brightness and residual peroxide measurements are shown in Table III for the same examples. BRIGHTNESS AND RESIDUAL PEROXIDE RESULTS, PULP #1 PRETREATMENT BRIGHTNESS (GE) % RESIDUAL H₂O₂ Example Na₂SiO₃ DTPMP + NaPA Na₂SiO₃ DTPMP + NaPA A 62.2 60.6 15.3 5.9 1 63.8 65.3 30.4 33.8 2 62.7 64.9 31.4 27.4 3 63.0 64.9 31.1 31.1 BRIGHTNESS AND RESIDUAL PEROXIDE RESULTS, PULP #1 PRETREATMENT DELTA BRIGHTNESS DELTA RESIDUAL H₂O₂ Example Na₂SiO₃ DTPMP + NaPA Na₂SiO₃ DTPMP + NaPA 1 1.6 4.7 15.1 27.9 2 0.5 4.3 16.1 21.5 3 0.8 4.3 15.8 25.2 With pulp #1, the addition of a pretreatment does improve the brightness response and corresponding residual peroxide for both the silicate and polymer-phosphonate systems. The increase in brightness for the silicate system is only 0.5 to 1.6 units while the polymer-phosphonate system showed a 4.3 to 4.7 increase. The same procedure was followed with pulp #2 using the same conditions shown in Table I and employing the same chelants. Example B is a control and Examples 4, 5 and 6 employed EDTA, DTPA and HEDTA, respectively at 0.12% based on the oven dry weight of pulp. Table IV shows the results and Table V shows the differences of each of the examples from that of the control. BRIGHTNESS AND RESIDUAL PEROXIDE RESULTS, PULP #2 PRETREATMENT BRIGHTNESS (GE) % RESIDUAL H₂O₂ Example Na₂SiO₃ DTPMP + NaPA Na₂SiO₃ DTPMP + NaPA B 71.1 60.8 25.6 0.3 4 70.8 71.1 54.3 54.5 5 70.9 70.6 49.1 36.0 6 71.2 71.6 51.8 54.3 BRIGHTNESS AND RESIDUAL PEROXIDE RESULTS, PULP #2 PRETREATMENT DELTA BRIGHTNESS DELTA RESIDUAL H₂O₂ Example Na₂SiO₃ DTPMP + NaPA Na₂SiO₃ DTPMP + NaPA 4 -0.3 10.3 28.7 54.2 5 -.02 9.8 23.5 35.7 6 0.1 10.8 26.2 54.0 The effect of pretreatment on pulp #2 with the silicate system exhibited no benefit. On the other hand, the polymer-phosphonate system showed a dependence on pretreatment giving a 9.8 to 10.8 brightness increase. In another control in which no pretreatment and no stabilizer for the peroxide were used the brightness of pulp #1 was 55.4 units and the residual H₂O₂ was 0.7%.

Claims: A process for bleaching wood pulp using hydrogen peroxide in an alkaline silicate-free aqueous system, said process comprising the steps of: (1) pretreating the pulp with a polyaminocarboxylic acid or salt th of and (2) bleaching with hydrogen peroxide stabilized with (a) an aminophosphonic acid chelant or salt thereof and (b) at least one polymer of (i) an unsaturated carboxylic acid or salt thereof, (ii) an unsaturated carboxylic amide or (iii) an unsaturated carboxylic amide wherein the amide hydrogens are substituted with an alkylsulfonic acid group or salt thereof. A process as claimed in Claim 1, wherein the salts of the acids in steps (1) and (2) are independently an ammonium, an alkali metal or an amine salt. A process as claimed in Claim 1 or 2, wherein the aminophosphonic acid chelant used in step (2) has the formula wherein M is independently H, alkali metal, NH₄, or an amine radical, R₁ is an aliphatic straight or branched chain, cyclic or aromatic radical having from 2 to 6 carbon atoms; n is 0 to 12; and m is 1 to 3. A process as claimed in Claim 3, wherein m is 1 and n is 0, 1, 2 or 3. A process as claimed in Claim 3 or Claim 4, wherein R₁ is an aliphatic radical having 2 carbon atoms. A process as claimed in any one of the preceding claims, wherein the polymer has the formula wherein: A is independently hydrogen or methyl; Z is independently NH₂ or OM wherein M is independently hydrogen, an alkali metal, ammonium and an amine radical; and p is from 13 to 5,500; or wherein: R₂ is an alkylene radical having from 1 to 6 carbon atoms; p′ is from 5 to 2,000; and A and M have the previous meanings; or mixtures of said polymers. A process as claimed in Claim 6, wherein the polymer is of formula (II) wherein p is an integer of from 25 to 250. A process as claimed in Claim 6, wherein the polymer is of formula (III) wherein R₂ is an alkylene radical having 4 carbon atoms and p′ is an integer of from 10 to 350. A process as claimed in any one of the preceding claims, wherein the polyaminocarboxylic acid of step (1) has the formula wherein A, B, C, D, E and F are independently hydrogen, CH₂COOR₄, CH₂CH₂OH or CH₂CH(CH₃)OH; R₃ is a hydrocarbon radical of the formula -CH₂CH₂-, -CH₂CH₂CH₂- or R₄ is hydrogen, an alkali metal, ammonium or an amine radical; and a and b are each integers of 0-2. A process as claimed in Claim 9, wherein R₃ is -CH₂CH₂-, and (i) a is 0 and A, B, C and D are CH₂COOR₄; (ii) a is 0, one of the amine substituents is -CH₂CH₂OH and the remainder are CH₂COOR₄; or (iii) a is 1, b is 0 and A, B, C, D and E are CH₂COOR₄. A process as claimed in Claim 9, wherein the polyaminocarboxylic acid of step (1) consists essentially of a mixture of ethylenediaminetetraacetic acid and N-hydroxyethylethylenediaminetriacetic acid or alkali metal, ammonium, or amine salts thereof.

Inventors: CHRISTIANSEN, STEVEN H., MICHALOWSKI, ROBERT J., MYERS, JIMMY, WILSON, DAVID A.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1987
Main classification: D21C9
IPC codes: D21C9