Mill Scale Implementation of Paprican's Process for Polysulfide Liquor Production in Kraft Mill Causticizers - Part 1: Batch Trials and Process Optimization, 1998 Pulping Conference Proceedings

The prototype mill scale installation of a new, low cost process for the production of polysulfide (PS) liquor is described. The system was retrofitted into a kraft mill by modifying only one of the mill’s four causticizers. Batch trials were conducted using two different impellers, with oxygen, oxygen–nitrogen mixtures, and air, to convert a portion of the sodium sulfide in white liquor to PS. The mill trial results were very similar to those obtained in the laboratory using samples of the mill’s green liquor and reburnt lime. Using small amounts of manganese to improve the process selectivity with oxygen, over 7 gpl of PS (as sulfur) was produced with only 45 minutes retention time. Using air, 6.8 to 8.4 gpl of PS were produced in 90 to 120 minutes. Process liquor and lime analyses indicated that the added Mn stayed with the lime mud and was retained in the lime cycle. The added Mn caused no operating problems in recaust. Numerous batch tests were completed with each gas to optimize process conditions and to develop engineering data that will facilitate effective process control and process installation in additional mills. PS liquors produced in the batch trials were used in pilot plant pulping trials that confirmed the expected yield increases relative to conventional kraft pulping.