Mechanism and chemical control of CaCO3 scaling in the kraft process

Buildup of scale in the kraft digester and black-liquor evaporators is a ma jor contributor to lost pulp mill productivity. Scale deposition occurs in areas such as the heaters and extraction screens of continuous digesters an d the liquor side of heat-transfer surfaces in the evaporators. This reduce s the efficiency and control of pulping and evaporation processes and event ually forces the costly cleaning of equipment. Since kraft pulping conditio ns require high temperatures and a high concentration of calcium and carbon ate in alkaline liquors, the precipitation of CaCo3 is inevitable. Traditio nally, scaling tendencies have been managed using process control methods t o reduce deposition rates. This paper examines the thermodynamic principals behind such process modifications while introducing the kinetic steps of t he overall deposition pathway. Chemical additives that interfere with and r etard the individual steps of the overall scaling mechanism can help to fur ther reduce deposition rates.