Monitoring the free lime content in lime mud using zeta potential

The presence of unreacted lime (free lime) in the lime mud can cause many problems in the chemical recovery operation of kraft pulp mills. There are currently no standards for free lime determination. Conventional methods used in mill laboratories require extensive preparations and give inconsistent and sometime erroneous results. A systematic study was performed to determine if zeta potential, a dispersion characteristic of the lime mud and liquor slurry, can be used to indirectly indicate the amount of free lime in the lime mud. Lime mud with different free lime contents was produced in the laboratory by causticizing green liquors with lime at different liming ratios. The results show that zeta potential changes from negative to positive when the free lime in lime mud exceeds a critical value. This unique characteristic enables a rapid detection of free lime.