The build-up of phosphorus in a kraft pulp mill and the precipitation of calcium phosphate from green and white liquors

Phosphorus is accumulated as calcium phosphate in the lime cycle of the chemical recovery system in a kraft pulp mill. This strongly affects the amount of available CaO in the lime. A concentration of 1% P by weight in the lime leads to a decrease of about 5% by weight in the amount of available CaO. The accumulation of phosphate in the lime cycle is due to the fact that calcium phosphate is more soluble in green liquor than in white liquor. It is not possible to decrease the concentration of phosphate ions in the green liquor to the same level as in the white liquor by precipitation of calcium phosphate, since it is mainly the carbonate ion concentration that determines the phosphate ion concentration in these liquors. This is because phosphate and calcium ions are replaced partially by carbonate and Sodium ions in the precipitated calcium phosphate through a substitution mechanism. Thus, it is necessary to keep the lime cycle partly open to avoid a large accumulation of phosphorus in the cycle. The amount of lime mud that has to be purged depends on the demands on lime quality, the intake of phosphorus with the wood, and the degree of system closure.