Kraft pulping process

A process of producing cooking liquor for addition to a digester for enhancement of the yield derived from kraft pulping of wood chips in which a portion of the black liquor is treated with lime under certain conditions and then reused as cooking liquor. The causticizing of the black liquor results in the generation of polysulfide ions and allows, through the formation of calcium-organo complexes, for the carryover of soluble calcium with the black liquor. The presence of calcium ions synergistically enhances the effectiveness of polysulfide such that the use of the new cooking liquor results in pulp yield increases of several percentage points for the same lignin content when compared to conventional kraft pulping.