ALKALINE FILTRATE RECIRCULATION TO WASH LONG FIBER PULP IN THE OD(EPO)DED SEQUENCE

Abstract

Circuit closure can reduce water consumption, but negative effects on pulp quality and equipment wear may make it unfeasible. This study aimed to evaluate how the alkaline filtrate of the recirculation stage (EP) affects bleaching, pulp quality and characteristics of the filtrates produced. Pre-delignified cellulose pulp from a mixture of three coniferous woods was used. Bleaching followed the D(EPO)DED sequence, with the addition of five, 10 and 15 kg/ odt of the alkaline filtrate (EP) to the pre-O₂ pulp. The physical and mechanical cellulosic pulp properties were evaluated in the control and with 10 kg/odt of the alkaline filtrate. The inorganic compound accumulation in the system was evaluated in the control pulp and with 15 kg/ odt of the alkaline filtrate. The filtrate use increased the ClO₂ consumption for bleaching and sulfuric acid and NaOH to adjust the pH of the stages. The pulp tensile index was higher and the tear index lower with the filtrate use in pulp without refinement, however the properties of refined pulp were similar between treatments. The filtrate increased the calcium, chloride, sodium and sulfate levels in the D stage and that of sodium and sulfate after the (EPO) stage. Filtrate recirculation can reduce water use, but it increases bleaching costs and metal accumulation in the system