Low temperature recovery of kraft black liquor

Abstract

A kraft black liquor recovery system utilizing three separate reactors for liquor pyrolysis, sulfate reduction and carbon plus organics combustion, respectively. Oxidized black liquor is pyrolyzed in a fluid bed reactor. The temperature in the fluid bed reactor is 600.degree. C. or lower. The resulting char, containing Na.sub.2 CO.sub.3 and Na.sub.2 SO.sub.4 and a significant amount of carbon, is separated from the pyrolysis gases and introduced in an indirect heated reactor where reduction of Na.sub.2 SO.sub.4 to Na.sub.2 S takes place in the solid state under an atmosphere generated by the reduction. The reduced char is cooled and leached to produce green liquor. The leached char and gases from the pyrolysis and reduction reactors are burned in a fluid bed combustion unit operating below the melting point of mixtures of Na.sub.2 CO.sub.3 and Na.sub.2 SO.sub.4. The fluid bed particles, consisting mainly of Na.sub.2 CO.sub.3 and Na.sub.2 SO.sub.4, serve to remove the volatile oxidized sulfur species formed by combustion of the pyrolysis gas. The overflow of pellets are ground and dissolved in the incoming heavy black liquor feed.