Tall oil production from black liquor: Challenges and opportunities

Tall oil is an important by-product of Kraft pulping processes. One possibility to improve the economic feasibility of Kraft pulp mills is to consider tall oil production from their wasted tall oil soaps. This review paper describes the current technology practiced to produce tall oil from the black liquor of Kraft pulping processes. Moreover, alternative processes to separate tall oil soap from black liquor, produce tall oil from the separated soap, purify tall oil, and to produce value-added products from tall oil are reviewed. The main perspectives and challenges associated with each process are comprehensively described. Currently, soap is separated from black liquor via decantation. To improve the efficiency of this separation, the application of neutral soluble colloids in dilute black liquor reported to be promising. Tall oil soap is converted to crude tall oil in the acidulation process via treating with sulfuric acid. However, the use of sulfuric acid forms calcium sulfate as a by-product of the process. The replacement of sulfuric acid with a sodium sesquisulfate solution is beneficial for decreasing the direct use of sulfuric acid in the tall oil production process. Crude tall oil can be used without purification as fuel. Its purification and subsequent reaction with methanol can lead to biodiesel production. However, this process may be complicated to implement in industry. The production of valuable lignin-based products from the tall oil production process may have benefits for the mills as these processes can be fully integrated into the Kraft pulping and tall oil production processes.