Hot Caustic Extraction of Spruce Sulfite Dissolving Pulp

Abstract: The purpose of this thesis was to optimize the hot caustic extraction stage on a spruce sulfite dissolving pulp. The pulp was subjected to hot alkaline extraction at different temperatures and alkaline charges. The consistency of the pulp was always at 8%. The pulp sample was 40g dry weight of and the effect of the extraction temperature and sodium hydroxide concentration after two hours of extraction on the viscosity, yield and R18 values of the pulp were examined. The viscosity and R18 values after the extraction process were found to increase as the temperature and sodium hydroxide concentration increased while the yield was found to be reduced. Also, it was found that at 600C and 800C an increase in sodium hydroxide charge had a low influence on the yield and viscosity compared to the influence at 1000C and 1200C. An increase in Sodium hydroxide charge from 35 to 120kg/ton of pulp had a higher influence on the R18 value than the temperature effect going from 600C to 1200C; hence alkaline charge was a more important parameter controlling the purification during hot alkaline extraction. The result also shows a correlation between yield loss and purification; an increase in the degree of purification gives a lower yield because of the degradation of the hemicelluloses. Also, the effect of hydrogen peroxide and oxygen addition in the hot alkaline extraction stage was investigated. It was found that hydrogen peroxide reduced the viscosity of the dissolving pulp when compared to the viscosity of the pulp treated without peroxide. Addition of oxygen in the hot alkaline extraction stage increased the R18 values and the viscosity of the pulp compared to the pulp extracted without oxygen.