Mill-scale implementation of Paprican's process for Polysulfide liquor production in kraft mill causticizers, Part I: Batch trials and process optimization

The prototype mill-scale installation of a new, low-cost process for the pr oduction of polysulfide (PS) liquor is described. The system was retrofitte d into a kraft mill by modifying only one of the mill's four causticizers. Batch trials were conducted using two different impellers-with oxygen, oxyg en-nitrogen mixtures, and air-to convert a portion of the sodium sulfide in white liquor to PS. The mill tial results were very similar to those obtai ned 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 ox ygen, over 7 g/L of PS (as sulfur) was produced with only 45 min retention time. Using air, 6.8-8.4 g/L of PS were produced in 90-120 min. Process liq uor and lime analyses indicated that he added Mn stayed with the lime mud a nd was retained in the lime cycle. The added Mn caused no operating problem s in recaust. Numerous batch tests were completed with each gas to optimize process conditions and to develop engineering data that will facilitate ef fective process control and process installation in additional mills. PS li quors produced in the batch trials were used in pilot-plant pulping trials that confirmed the expected yield increases relative to conventional kraft pulping.