Abstract
Polysulfide (PS) cooking is one of the most efficient technologies available today to increase pulp yields. The PS has to be produced from Na2S in the white liquor (WL) to keep Na+ and S2- balance in the WL recovery cycle. To produce the PS in high efficiency, a WL electrolysis using ion-exchange membranes has been developed by Nippon Paper Industries CO., LTD.
Modified cooking is characterized by sequential charges of WL to the cooking digester. Recently, Iso-thermal cooking (ITC) , which is one of the modified cooking, characterized by low temperatures and long-cooking times has been industrialized and installed in a lot of kraft pulping processes. However, not only NaOH concentrations but also Na2S concentrations at a start of the cooking are decreased, because the initial charge of WL was decreased by the modified cooking.
In this study, the combinatorial use of ITC cooking and PS cooking were evaluated, expecting a process in which the mill scale pilot electrolyzers were installed. A yield gain caused by the PS cooking was decreased in cases of the modified cooking conditions. However, when the PS was produced by the electrolyzers, the yield gain was increased contrary to the case by the air oxidation. By the electrolyzers, the PS concentrations became double and a lack of initial Na2S concentration was minimized.
In addition, the electrolyzers, which have a function of electro dialysis using ion-exchange membranes, enabled sulfide elements in WL to be concentrated as PS and Na2S. Consequently, much more yield gain was expected by using the new modified cooking.
