New Process of Producing Highly Concentrated Polysulfide Liquor by Electrolysis of White Liquor, 1999 Pulping Conference Proceedings

Keigo Watanabe, Masahiro Shimizu & Yasunori Nanri

Iwakuni Research Laboratory

Tatsuya Andoh

Kawasaki Kasei Chemicals Ltd.

Tetsuji Shimohira

Research Center, Asahi Glass Co., Ltd.

A new electrolytic oxidation process to produce highly concentrated polysulfide (PS) liquor from the white liquor has been developed by a cooperative work of three Japanese companies. Advantages of this process have been verified through a long-term operation by using the bench scale cell. A specially-designed electrolytic cell makes the system suitable for actual mill operation. The white liquor is oxidized electrically to produce PS liquor in the anode compartment, while sodium hydroxide is formed in the cathode compartment simultaneously. Since the undesirable thiosulfate formation is easy to control in the electrochemical process, it was possible to produce highly concentrated PS liquor with a good current efficiency. Consequently, this process achieves to concentrate sodium sulfide in white liquor and to produce highly concentrated PS and purified sodium hydroxide simultaneously, while the sulfur balance in the kraft pulping process can be maintained. PS cooking of hardwoods was evaluated using the oxidized liquor produced by the bench-scale cell with the laboratory digester. Then, large increases of pulp yields and the kappa number reductions were observed. Therefore, this process has great possibility to improve the pulp yield and reduce AOX concurrently. The application of this process to modified cooking process is also discussed.