The limitation of kraft recovery unit capacity in pulp and paper making can be overcome by using wet air oxidation (WAO) as pre-treatment stage to treat spent black liquor. WAO is a process in which organic and/or oxidizable inorganic components are oxidized using air or pure oxygen in a liquid phase at elevated temperature (150-370°C). In order to keep the reaction in the liquid phase, high pressures are required (2-20 MPa).;All experiments were performed in a highly pressurized steel reactor using oxygen to oxidize kraft black liquor. In the first part of the experimental work, the performance of the reactor and interaction between the operating parameters were studied. The interaction between temperature, oxygen charging pressure and initial pH of the solution were found to be negligible. Furthermore, the effects of these operating conditions on the degradation of the organic compounds in black liquor were investigated. Results showed that 70% reduction of total organic carbon amount can be achieved even without the addition of a catalyst. As expected, the reaction temperature has a positive effect on both the rate and the actual amount of organic removal. Moreover, controlling the initial pH of the solution showed a great improvement especially at the first thirty minutes. Lowering the initial pH to 4 showed a significant improvement with reducing the chemical oxygen demand (COD), which was 88% reduction as compared to 76% at same temperature and oxygen charging pressure after an hour and a half of treatment.
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