PRETREATMENT OF SOFTWOOD WITH SULFIDE-CONTAINING LIQUOR PRIOR TO KRAFT COOK

摘要

The sorption of sulfide in wood was studied underrndifferent process conditions and with differentrncompositions of the pretreatment liquor. This sulfidernsorption is of importance in the development ofrnmodified kraft pulping with improved selectivity.rnFurthermore different kinds of black liquor and blackrnliquor components were also studied. The aim was tornfind out whether or not the dissolved substances in thernblack liquor participate in the delignification processrnand thus increase the delignification rate.rnThe sorption of sulfide in wood chips was found tornincrease with increasing hydrosulfide ion concentration,rntime, temperature and concentration of positive ions,rnbut decreases with increasing concentration ofrnhydroxide ions. The ratio of hydrosulfide to hydroxidernions should be at least 6-7 in order to achieve a highrnsulfide sorption. The presence of polysulfide in therntreatment liquor almost doubles the sulfide sorption. Anrnincrease in sulfide sorption during pretreatment gives anrnincrease in the selectivity in the subsequent kraft cookrn(viscosity at a given kappa number). However, sulfidernsorption alone can not explain all of the improvment inrnthe selectivity of a kraft cook with pretreatment.rnOur results also indicate that the better selectivity of arnkraft cook with pretreatment is to great extent due tornthe low hydroxide ion and high hydrosulfide ionrnconcentrations in the pretreatment liquor and thernrelatively low temperature and long time during thernpretreatment. No difference in the selectivity of thernsubsequent kraft cook or in the physical properties ofrnpulp was observed when pretreating with different typesrnof black liquor or with an inorganic liquor containingrnno organic substances. This implies that the organicrnsubstances in the black liquor have no significant effectrnon the selectivity. However, the black organic liquorrnsubstances will affect the buffer capacity of therntreatment liquor during the pretreatment and thusrnindirectly affect the sulfide sorption and pulpingrnselectivity.rnThe unbleached pulps from kraft cooks withrnpretreatment have different tear index-yield and tearrnindex-viscosity relationships than conventional kraftrnpulps.