Formation of Soluble Calcium in Alkaline Pulping of Acacia

Calcium is the primary inorganic element in wood. During chemical pulping, in which wood is delignified to free cellulose fibers for pulp, a significant portion of the calcium in wood is released. The released calcium is mostly soluble at first, going through a metastable limit before reacting with carbonate to form calcium carbonate. The precipitation of calcium forms scale in the pulping digesters and in downstream processes such as bleaching and black liquor evaporation. This work provides new data for the tropical hardwood Acacia and gives equations for the release of calcium as a function of H factor in Kraft and soda pulping. Higher levels of soluble calcium were seen in the pulping of Acacia than has been seen in the batch pulping of pine in earlier studies, and this soluble calcium forms a more stable intermediate than is formed during the pulping of pine. The implication is that scaling in mills pulping tropical hardwoods such as Acacia can be more severe and more of this soluble calcium may leave the digester to result in fouling of black liquor evaporators.