Effects of pH and MgSO4 on Lignin Redeposition and Retention during Brownstock Washing

Abstract Process closure in kraft pulp mills requires increased recycling of washing liquors, which can alter liquor composition and promote lignin redeposition during brownstock washing. This study investigates the effect of pH and magnesium ions (Mg2+) on lignin retention and removal from pulp fibers in controlled washing experiments. Unbleached and bleached softwood kraft pulps were washed with industrial liquors collected from different stages of the kraft process at a Swedish mill. Lignin redeposition and retention were assessed using kappa number, brightness, and alkaline leaching. Within the alkaline pH range investigated, decreasing pH was generally associated with increased lignin retention on the fibers. NaOH addition improved lignin removal, but the effect plateaued beyond a liquor-dependent threshold. MgSO4 addition had little effect at the mill-relevant concentration of 1 g/L, whereas the deliberately elevated concentration of 10 g/L increased lignin retention. Overall, the results demonstrate that liquor chemistry influences the balance between lignin removal and retention during brownstock washing. Because conventional mill-scale indicators such as chemical oxygen demand (COD), dissolved solids carryover, and wash loss were not measured, the results are interpreted specifically in terms of lignin redeposition and retention rather than overall washer performance.