Study On Controlling Of Optical Properties Of Calcium Carbonate From White Mud In Straw Pulp Alkali Recovery

The solid waste treatment technology was later and weaker than wastewatertreatment technology of papermaking industry. And the solid waste secondarypollution was more serious day after day, and the key point of papermakingindustry was end-treatment and energy conservation and emission reduction.Alkali recovery white mud which can be caldcined for reusing is one of the solidwastes of papermaking industry. The feasibility of aldcining of straw alkalirecovery white mud decreased year by year, because the high silicon content andhigh price energy. White mud couldn’t be applied massively, because the pulpmill was spread and quantity of white mud was small, and the white mud used aspaper filler was the most important resource utilization method. However, thebrightness of white mud was low and chromaticity was unsteady, and it notmeets the paper filler requirement. So it’s important to increase the brightnessand stabilize the chromaticity in white mud refined calcium carbonate process.This study focused on optical properties of alkali recovery white mud. Weresearch the effect factors of the optical properties of white mud, and we hopethe result can help to promote the optical properties in refining process, andreduce the solid waste and reuse as paper filler better.To compare the white mud with commercial PCC, we found that thebrightness was8.76%low, the settlement volume was77.91%low, but theparticle size was24.47%high. The brightness and the settlement volume couldnot meet the industry standard. The white mud had more elements Si and Mgbecause the raw materials were different.We analyzed the difference of the physical properties and causticzing rawmaterials of white mud, the results showed that the effect of moisture onbrightness was18.74%, and the effect of drying temperature, particle size andcrystal form were not insignificant; the effect of green liquor and lime were 11.28%and15.17%respectively.And more researches on lime showed that the effect of hydrochloric acid（HCl） insoluble on brightness was3.35%. And the particle size and settlementvolume could affect the optical properties indirectly. We filtrated the lime withfilter screen after it’s slaking to decrease the effect on optical properties of whitemud. And with400mesh filter screening the slaked lime, the brightness of whitemud could increase nearly3%ISO.The effect of the insoluble of green liquor was7.31%, we promoted theoptical properties by filtrating the green liquor with500mesh filter screen，andthe brightness increased9.4%by filtrating the green liquor with filter paper. Thedifferent white mud which reacted by different treated green liquor and lime haddifferent optical properties, the white mud made by clarify green liquor and limewas better than the other two treatment ways, the brightness was14.44%ISOhigher than the green liquor without any treatment.We studied on minor constituent of raw materials, the results showed thatwhen there were only elements S, Si, K, Cl existed in the green liquor, there wasno significant effect on optical properties. The insoluble of green liquor was darkgreen and the main reason for its color was the Fe³⁺and S^2-. And the Fe³⁺and S^2-could form the dark green precipitate——Fe2S3, and the reacting caused theliquor turning dark green. So we can use hydrogen peroxide to treat the S2-in thegreen liquor, in order to avoid the S2-react with Fe3+again. We bleached thewhite mud with hydrogen peroxide, the results suggested that the brightness canincrease4.18%ISO, it was5%higher than the unbleached.Our conclusion were that the causticizing raw materials were the mainreasons for the optical properties of white mud and the Fe3+and S^2-caused thegreen liquor’s color and its precipitate. The precipitate of green liquor could beremoved by500meshes or more filter screen filtrating and flocculating by someagents. To avoid the effect of the S^2-meeting the Fe3+to produce the dark greenprecipitate, we can use hydrogen peroxide to remove or change the S2-to othervalence state, and the white mud we can bleach it with hydrogen peroxide. Sothe optical properties of white mud can be improved by the treatments above.