Recovery of water and inorganic chemicals from kraft black liquor using membrane separation processes

Recovery of water with inorganic compounds in kraft black liquor was studied by pressure carbonation followed by different combinations of membrane separation processes. Experiments were carried out to observe the effect of carbonation on black liquor, in terms of conversion of lignin-bound alkali compounds into carbonate salts. Further, the reaction rate during carbonation was studied and a correlation for decrease of active alkali with time is presented, which is equivalent to a first order rate equation. Different combinations of membrane processes used were (a) ultrafiltration of the carbonated liquor followed by nanofiltration, (b) nanofiltration of the carbonated liquor and (c) direct nanofiltration of untreated black liquor. Comparisons were made in terms of recovery of the inorganics, quality of the permeate, and permeate flux. Of the three combinations, the first scheme yields higher rejection and better recovery as well as reasonable permeate flux. Recovery was higher for higher feed concentrations.