A comparison of cold and hot caustic extraction of a spruce dissolving sulfite pulp prior to final bleaching

A magnesium sulfite pulp producer modified its pulping process to dissolving pulping in 2011 and needed a purification stage after the digester to obtain a sufficiently low hemicellulose content. The dissolving pulp was intended for viscose production and an R18 value of at least 95% and an intrinsic viscosity below 550 dm3/kg was requested. The results showed, as could be expected, a correlation between lower dissolving pulp yield and a higher degree of purification with respect to cellulose. The R18 values and the intrinsic viscosities of the dissolving pulps were found to increase with increasing sodium hydroxide concentration and/or higher temperature in the extraction stage. The pulp yield was found to be higher with higher temperature for the cold caustic extraction stage (CCE); for hot caustic extraction (HCE) the pulp yield became lower with higher temperature. The specified R18 value was easiest to reach using hot caustic extraction.