Use of calcium sulfite and air to bleach a delignified aspen kraft pulp

Edward L. Springer and James D. McSweeny

USDA Forest Service, Forest Products Laboratory, Madison, Wis. 53705-2398

KEYWORDS: work on a calcium sulfite-air system. Model B spectrophotometer2 with a Bleaching • Kraft pulps • Populus A reactor was set up wherein a dilute reflectance attachment. Magnesium tremuloides • Calcium sulfite • Air • pulp slurry (about 1% consistency) 2The use of trade, firm or corporation names in Potassium peroxymonosulfate • could be vigorously stirred and aerated this publication is for the information and conve­ Copper sulfate • Hydrogen peroxide while a dilute suspension of freshly nience of the reader. Such use does not constitute an official endorsement or approval by the U.S. • Sodium bisulfite prepared calcium sulfite was metered Department of Agriculture of any product or in simultaneously. Temperature, pH, service to the exclusion of others. and rates of addition were controlled. Devuyst and others found that a strong Pulp was pretreated with cupric ion. I. Reaction conditions and final bright­ oxidizing agent can be produced by After many trials, we were able to ness for simultaneous oxidant genera­ adding calcium or sodium sulfite to establish conditions for bleaching a tion and bleaching of delignified aspen vigorously aerated water1. In trying to chlorinated and alkali-extracted aspen kraft pulp reproduce their work (using sodium (Populus tremuloides Michx.) kraft sulfite without pulp present) we ob­ pulp (original pulp yield 54% initial tained highlyvariable results. At times kappa no. 16; kappa no. 6 following we could generate the oxidant, and at chlorination and alkaline extraction). other times no generation occurred. The results from our best two trials We traced this inconsistency to varia­ are listed in Tables I and II. For tions in the cupric ion content of the efficient oxidant generation and use, distilled water used in the experiment. rate of addition of calcium sulfite No oxidant was generated when dou­ slurry to the aerated pulp suspension ble-distilledwater,containing0.2parts had to be carefully controlled in accor­ per billion cupric ion, was used in the dance with the schedules in Table II. experiment. By adding cupric sulfate The pulp was pretreated by soaking todouble-distilled waterwe found that overnight (16 h) at 1.0% consistency in a maximum amount of oxidant was 0.10% copper sulfate. It was then generated when the cupric ion content washed with distilled water. was greater than 0.2 parts per million The bleached pulp was acidified (ppm). Our distilled water sometimes with sodium bisulfite. The pulp from contained about 0.2 ppm cupric ion. the reactor was first posttreated with This catalytic effect of cupric ion had 18.6% (on o.d. pulp) bisulfite at 1.2% notbeen encountered previously. consistency for 3 min and then washed II. pH and CaSO3 addition schedule Becausepulpspossession-exchange with distilled water. It was then post- for simultaneous oxidant generation capacity, and because we simultane­ treated with 4.7% (on o.d. pulp) bisul­ and bleaching of delignified aspen ously generated the oxidant and re­ fite at 1.2% consistency for 3 min and kraft pulp acted it with pulp in a single reactor, it again washed with distilled water. was necessary to soak the pulp to be Finally it was made into a handsheet bleached in a dilute solution of cupric in accordance with TAPPI Test Meth­ sulfate. This pretreatment prevented od T218 om-83. the pulp from absorbing cupric ions This double bisulfite treatment was from the solution and thus inhibiting used on all pulps after bleaching and oxidant generation. on the original pulp. The original pulp Removal of sulfur dioxide from brightness was 51.6%; brightness after power plant flue gases may soon pro­ two-stage bisulfite treatment was vide a cheap source of calcium sulfite. 53.4%. All reported brightness values We thus focused our initial bleaching were measured subsequent to this 1Devuyst. E.A.P., Ettel, V.A., and Mosoiu, M.A., two-stage bisulfite treatment. Bright­ Chemtech 9(7): 426(1979). ness was determined with a Beckman April 1986 Tappi Journal 129 carbonate, the standard, was set to III. Reaction conditions and final brightness for potassium peroxymonosulfate 100%. a To estimate the efficiency of oxidant and hydrogen peroxide bleaching generation, we compared potassium Potassium peroxymonosulfate bleaching with the peroxymonosulfate Hydrogen peroxide results from our best two trials. Run Run Run Run Run Run Devuyst and others1 hypothesized that 1 2 3 1 2 3 the oxidant was the peroxymonosul­ Weight, % on pulp 4.8b 9.5b 14.3b 1.1 2.1 3.2 -2 fate anion (SO5 ); we confirmed this Active oxygen, % on pulp 0.5 1.0 1.5 0.5 1.0 1.5 in work to be reported. This anion is Reaction temperature, °C 50 50 50 80 80 80 available in “Oxone,” a triple salt Reaction time, min 120 90 75 90 75 45 containing potassium peroxymonosul­ Alkali, % on pulp 3.3 5.0 8.3 1.3c 1.7c 1.7c fate, sold by DuPont2 (“Oxone” = Initial pH 11.5 11.5 11.7 10.7 10.8 10.7 Final pH 10.8 9.5 10.7 10.2 10.5 10.6 2KHSO5 × KHSO4 × K2SO4). A com­ parison was also made with hydrogen Residual oxidant, % 6.7 6.4 8.4 19.0 9.6 3.2 Brightness,%d 67.7 71.6 74.2 62.0 64.0 peroxide bleaching. 67.3

a b c For both peroxymonosulfate and Pulp consistency= 1.0%. As KHSO5. lncludes 5.0% sodium silicate (41° Be), 0.1% MgSO4 on hydrogen peroxide bleaching, the total pulp. Control=53.4%. quantity of oxidant was added to the pulp at the beginning of the trial. Results from “Oxone” and alkaline air system. Other pulps, other sulfites, hydrogen peroxide bleaching are giv- 1. Pulp brightness as a function of the and other types of catalysts warrant active oxygen content of the oxidants. en in Table III. The low consistency investigation. Increased consistency used (1%) facilitated efficient mixing and shorter bleaching times should be of pulp, calcium sulfite, and air in the studied. The calcium sulfite-airsys­ reactor. Brightness results were plot- tem, or some improvement thereon, ted vs. percentage of active oxygen of might prove to be an economically the oxidant (based on o.d. pulp) in Fig. viable bleaching system. 1. In calculating the percent active oxygen for simultaneous generation The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. and bleaching, a 100% conversion of This article was written and prepared by U.S. calciumsulfite to calciumperoxymono- Government employees on official time and is therefore in the public domain (i.e., it may not be sulfate was assumed. copyrighted). At equal levels of active oxygen, the calcium sulfite-airsystem gave bright- Received for review Sept. 30, 1985. ness values just a little lower than Accepted Nov. 20. 1995. those from potassium peroxymonosul­ fate (Fig. 1). These results suggest that conversion of calcium sulfite to peroxymonosulfate was high. We have studied only delignified aspen pulp using the calcium sulfite­

130 April 1986 Tappi Journal