EFFECTS OF LIMING RATIO ON LIME MUD SETTLING AND FILTERABILITY, 2010 TAPPI PEERS Conference

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Effective separation, washing and dewatering of lime mud are of great importance in lime kiln operation in kraft pulp mills. They are known to depend strongly on the amount of lime (or liming ratio) used in the slaker. A systematic laboratory study has been carried out to examine the effect of liming ratio on the settling rate and filterability of lime mud produced by causticizing an aqueous solution of Na₂CO₃ with pure lime and reburned lime from two different kraft mills. The results show that the mud settling rate and filterability decrease with an increase in liming ratio. The decrease is much more noticeable as the liming ratio exceeds a critical level determined by the causticizing equilibrium. Poor mud settling and filterability are not caused by the small particle size of Ca(OH)₂ (free lime) as commonly believed, but by the increase in positive zeta potential of Ca(OH)₂-containing mud particles. The presence of impurities in the reburned lime may have a significant effect on the particle zeta potential, and hence, on the lime mud dewatering efficiency.