Studies on Polysulfides Cooking (IV) -Effect of cooking liquor impregnation on PS cooking-

Abstract:

Polysulfide (PS) cooking is a well-known method of increasing pulp yield. The higher PS concentration results in higher yield-up. Excessive PS generation, however, often causes a lack of sulfidity, because PS is made from Na2S in the white liquor (WL). High Liquor/Wood ratio sometimes causes low Na2S concentrations as well. Anthraquinone addition compensates such negative effects of low Na2S concentration on PS cooking. By using the efficient PS generation methods, such as WL electrolytic method, wasting of Na2S can be avoided. These key factors for more effective PS cook were discussed in previous reports. In this report, the effect of cooking liquor impregnation on PS cooking was investigated. It was found sufficient impregnation is indispensable to obtain the good yield improvement effect with PS. The yield-up effect on PS cooking was not observed without impregnation stage. PS is unstable under the high temperature and it must be distributed uniformly into wood chips prior to the cooking reaction. Further, cracking treatment of woodchip was evaluated. This effect itself was not so large but was effective for the PS cooking without impregnation stage and especially for low permeability wood chips. Therefore the cracking treatment is one of the suitable methods to help PS cooking.