Mechanism and chemical control of CaCO3 scaling in the kraft process

Buildup of scale in the kraft digester and black-liquor evaporators is a major contributor to lost pulp mill productivity. Scale deposition occurs in areas such as the heaters and extraction screens of continuous digesters and the liquor side of heat-transfer surfaces in the evaporators. This reduces the efficiency and control of pulping and evaporation processes and eventually forces the costly cleaning of equipment. Since kraft pulping conditions require high temperatures and a high concentration of calcium and carbonate in alkaline liquors, the precipitation of CaCo3 is inevitable. Traditionally, scaling tendencies have been managed using process control methods to reduce deposition rates. This paper examines the thermodynamic principals behind such process modifications while introducing the kinetic steps of the overall deposition pathway. Chemical additives that interfere with and retard the individual steps of the overall scaling mechanism can help to further reduce deposition rates.