Fractional solidification process and apparatuses for use therein

Abstract

The present invention introduces a process for effectively washing a crystal-liquid (mother liquor) mixture with a mass of wash liquid to thereby form a mass of highly purified crystals and also introduces apparatuses for use therein. The purification is accomplished by properly conducted alternative stationary and agitated crystal washing operations and substantially countercurrent transfer of the solid phase and the free liquid in a purification zone. The zone contains a set of stationary washing sub-zones and a set of agitated washing sub-zones that are laid alternatively along the vertical direction. The mass of crystals in a stationary sub-zone is compacted by a compacting means to form a bed with an enhanced degree of compaction. The masses of crystals and liquid in each agitated sub-zone are properly agitated so that the impurities in the mother liquors retained in the crystal mass are released to the free liquid. It is important either to reduce the amount of liquid that is transferred with the crystals or counterwash the crystals during an inter sub-zone transfer of the solid phase. Means for accomplishing these functions are incorporated in the apparatuses of this invention. Because of the properly conducted alternative stationary and agitated washing operations and the properly conducted inter sub-zone transfers of the solid phases and the free liquids, the impurity concentrations of the liquids retained in the solid phases transferred between sub-zones decrease sequentially, nearly following a geometric sequence rather than approaching a limiting value.