pulping liquor borne solids are concentrated within an aqueous solution by a successive plurality of freezing steps wherein a discrete quantity of the solution is crystallized in each step by freezing . the water crystals from each freezing step are subsequently separated from the remaining liquid solution by screening and applying axial pressure to the crystalline mass . greater efficiencies may be obtained in some cases by mixing or reslurrying a pressed , crystalline mass and pressing a second time . another auxiliary separation step comprises a quiescent thaw of the crystalline mass from the first two press steps to permit additional dissolved solids to be drained away from the purified water ice .