Factors affecting phosphorus uptake/ dissolution during slaking and causticizing

Hydroxide is regenerated in the recovery cycle of kraft pulp mills by the addition of lime (CaO) to green liquor. Phosphate in green liquor can react with the lime during slaking/causticizing. Total titratable alkali (TTA), sulfidity, the concentration of phosphate in the green liquor, temperature, and the liming ratio were all variables explored in this work to determine their influence on phosphorus uptake and dissolution. Experiments were also run in which the lime was slaked before being added to the green liquor to separate reactions with phosphate during slaking and reactions that occur during causticizing. Both reburnt lime and technical grade CaO were used. The experiment results indicate that phosphorus primarily reacts with slaked lime (Ca(OH)(2)), and that the final concentration of phosphate in the white liquor at the end of slaking and causticizing is nearly independent of the initial concentration of phosphorus and only mildly dependent on the carbonate concentration in the green liquor. There do appear to be differences in the rate at which phosphate reacts with reburnt lime and technical grade CaO, though the reason for this was not determined. Application: Phosphorus that enters pulp mills with the wood to the digester is ultimately purged from the mill with the purged lime mud. This work can be used by mills to predict how much lime needs to be purged if considering higher phosphorus inputs to the mill through combustion of biomass fuels in the lime kiln or combustion of bio-sludge with black liquor in the recovery boiler.