Closed Cycle Kraft Mill With On-Site NaOH Preparation for Delignification and TCF Bleaching, 1998 Pulping Conference Proceedings

Chemical recovery with internal NaOH preparation in a closed cycle kraft mill producing TCF bleached pulp is outlined. Three different kraft cooking methods are compared: normal kraft cooking, impregnation type kraft cooking, and impregnation type polysulphide cooking. In impregnation type kraft and polysulphide cooking, sulphide-rich alkali is used in the impregnation stage and sulphur-free caustic soda in the cooking stage. The main parameters used for comparison include alkali charges in cooking, oxygen delignification and TCF bleaching, pulp yield, heat generation in the recovery boiler, and steam consumption. An open cycle TCF kraft mill is used as a reference case.

If sulphur-free NaOH is used not just in bleaching and oxygen delignification but also in cooking, the only way it can be prepared is via some method based on separation of carbonate from green liquor. Such a method allows sulphur-free NaOH to be prepared in a closed cycle kraft mill without changing the sulphur to sodium ratio or without increasing the dead load of the recovery cycle. For a closed cycle softwood kraft mill, cost estimates show savings over open cycle pulping at kappa number 25, based on costs of chemical recovery, purchased chemicals and wood. The saving is greatest when causticized mother liquor from carbonate crystallization is used in impregnation type kraft cooking to kappa number 40 and in conventional kraft cooking to kappa number 30, and slightly less for impregnation type cooking to kappa number 40 using either uncausticized mother liquor or polysulphide liquor for impregnation.

Estimates show that the cost of on-site NaOH preparation based on green liquor crystallization could be covered by the savings achieved by optimizing delignification during cooking and oxygen delignification. At a closed cycle mill the saving on purchased caustic soda would cover a significant proportion of the cost of recycling the bleach plant filtrates.