Black Liquor Oxidation As a Means of Efficient Chemicals Recovery in Paper Mills

Chemicals recovery cycle is essential for every pulp mill producing pulp by chemical pulping. Its purpose is to recover inorganic chemicals used for pulping with the possibility of 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 chemicals consumption, thus contributing to more environmentally friendly pulp and paper production. This work is focused on assessing white liquor processing methods and on evaluating the impact on chemicals consumption in further pulp processing stages. Model balances are set up for a large paper mill with a capacity of 0.75 mil. tonnes of pulp and paper production, requiring around 100 tonnes per hour white liquor for pulping. Results indicate that a major saving can be realized on chemicals purchase: more than 0.8 tonnes per hour and more than 1.2 tonnes per hour of pure sodium hydroxide in case of partial white liquor oxidation and full white liquor oxidation, respectively. Greenhouse gases emissions can be reduced by more than 10 thousand tonnes per year of CO₂ equivalent as a result. Economics of proposed technology implementation is favorable, indicating a simple payback period of less than three years for a certain combination of chemicals and utilities costs.