Black Liquor Oxidation as a Means of Efficient Chemical Recovery in Paper Mills

Abstract

The chemical recovery cycle is essential for every pulp mill producing pulp via chemical pulping. The purpose of this cycle is to recover inorganic chemicals used for pulping that can facilitate heat and electric energy cogeneration. New methods such as white liquor oxidation or black liquor gasification can increase the efficiency of the cycle and help to decrease the consumption of chemicals, thus contributing to more environmentally friendly pulp and paper production. This work focuses on assessing white-liquor-processing methods and evaluating their impact on chemical consumption in further pulp-processing stages. Model balances were established for a large paper mill with a capacity of 0.75 mil. tonnes of pulp and paper production, requiring around 100 tonnes per hour of white liquor for pulping. The results indicate that major savings on the purchase of chemicals can be realized, namely, more than 0.8 tonnes per hour and more than 1.2 tonnes per hour of pure sodium hydroxide in the cases of partial white liquor oxidation and full white liquor oxidation, respectively. Greenhouse gas emissions can be reduced by more than 10 thousand tonnes per year of CO2 equivalent as a result. The economics of the proposed technology’s implementation are favourable, indicating a simple payback period of less than three years for a certain combination of chemical and utilities costs.

Keywords

• recovery cycle
• white liquor
• oxidation
• pulping