Potassium Recovery from Purge Streams in Pulp Mills

Abstract

In pulp mills, non-process elements such as potassium and chloride accumulate over time and must be discarded through a purge stream to maintain process efficiency and prevent equipment degradation. Currently, the purge stream is directed to effluent treatment. It contains valuable material that could be recovered, but such a process is not implemented today. The aim of the project was to evaluate and develop a new process for recovering potassium from the purge streams of two different mills. The potassium is converted into a commercial product via an intermediate, and can be used as a fertilizer. This new process was compared to the conventional production of the commercial product, in terms of energy demand and process design. The project began with a literature study, followed by an experimental phase focused on cooling crystallization of the intermediate and re-crystallization of the final product. Analytical methods used were ICP-AES, XRD, SEM-EDX and dry solids analysis, to determine elemental composition and dry solid content. MAT-LAB was used as a simulation tool to estimate the energy demand of the cooling crystallization. The process of recovering the intermediate was found to be sensitive and highly dependent on the composition of the purge stream. The highest purity of the intermediate product was found at 15 °C with slow cooling of the purge stream from Mill 2. The re-crystallization to the commercial product was most effective with a leaching solution containing 50/50 wt% ML2 mimic and saturated solution of potassium chloride for both mills. The new process was shown to have a lower energy demand than the conventional process, especially with slow cooling applied. Further, it was proved to be safer with respect to by-products and operational parameters. In conclusion, the new process shows potential in recovering potassium, contributing to the pulp mills circular economy and sustainability. Nonetheless, further optimization of process conditions and an economic assessment are necessary to validate its industrial viability.