Study on the filtration characteristics of green liquor dregs

Separation of dregs from green liquor is a major challenge in kraft pulp mills, and it results in compressible filter cakes with high specific cake resistance. In industrial applications, the use of lime mud as a filter aid is a practical approach to reduce the resistance of the dregs cake. However, the amount of solid waste then increases considerably, because the lime mud has to be disposed with the dregs. In this study, green liquor sludges from two different pulp mills were filtered without filter aids by using laboratory-scale pressure filters. The effect of some variables, such as the permeability of the filter cloth, the temperature of filtration, the filtration pressure, and the storage period on the filterability of the green liquor sludges was investigated. The results provide information for optimizing the process parameters to improve the filterability of the sludge. The lowest cake resistance (2.20 · 1012 m/kg) was obtained at filtration the temperature of 80 °C. However, the higher operating temperature did not necessarily have a positive effect on the recovery of sodium (valuable element for the process) from the cake. The results showed that higher amounts of certain elements, such as Na, K and B were recovered from the dregs cake at the filtration temperature of 60 °C. It was also observed that the storage period had a negative effect on the filtration properties. In addition, elemental analysis of the filtrate and dregs cake samples showed that harmful metals, such as Al, Cd, Fe, Mn and Zn were mostly insoluble and remained in the dregs.