NSSC Process Optimization: II. Spent Liquors, 1998 Pulping Conference Proceedings

Modern trends indicate the use of pulping processes that require less chemicals being friendlier to the environment. This is the case of NSSC (Neutral Sulfite Semi-Chemical) process where there is a need for disposing spent liquors economically and ecologically. The main objective this part of the project, using a design of experiments approach (Central Composite Design: 29 cooks including 5 central points), was to determine the operating conditions at which spent liquors containing the lowest ratio of inorganic/organic solids can be obtained. These operating conditions had to be such that they maintain at the same time the pulp quality. Hybrid poplar (Populus deltoides) was used as raw material. Minimum and maximum limits of the independent variables were established using mill conditions as set point values. Variables studied were: cooking time (10 to 30 minutes), cooking temperature (154 to 186ºC), sodium sulfite (4 to 12% o.d.), and sodium carbonate charges on wood (0 to 3% o.d.). Each property was evaluated by comparison with the mill reference data (yield: 80%, pH: 7, IS/OS: 0.8). Optimum conditions minimizing the inorganic/organic solids ratio were: 26 min, 177 ºC, 6.0 % Na2SO3, 2.54% Na2CO3. Spent liquors organic solids compositions were also determined. Time, temperature and sulfite charge showed the greatest effects on organic material dissolution. Carbonate charge acts through interactions.