Liquid phase coking of spent kraft pulping liquors

In a proceess for treating spent alkaline Kraft pulping liquors, the steps of flowing said liquor to an absorbing zone and adding SO.sub.2 to said liquors in said zone to lower the pH thereof to below 7;

• coking said liquors in the liquid phase under autogenous pressure of about 1000 to about 3000 psi at 450.degree. F to 700.degree. F for 0.5 minutes to 6 hours in a coking zone thereby forming coke, gases, and an aqueous effluent;
• separating in a separating zone said coke from said effluent;
• burning said gas to produce said sulfur dioxide;
• drying said coke with flue gas;
• the improvement comprising preheating hydrogen to 700.degree. to 2000.degree. F. passing said hydrogen over said coke to reduce the sulfur content of said coke and produce H.sub.2 S and adding lime and said H.sub.2 S to said effluent in an absorbing zone to form new Kraft cooking liquor.