Efficient recovery of potassium chloride from liquid effluent generated during preparation of schoenite from kainite mixed salt and its reuse in production of potassium sulfate

Abstract

Although bittern of oceanic origin can be utilized for the production of K₂SO₄ via kainite (KCl·MgSO₄·2.75H₂O) and schoenite (K₂SO₄·MgSO₄·6H₂O) double salts, certain limitations are encountered in practice. These include (i) difficulties in obtaining pure schoenite from crude kainite and (ii) the requirement of KCl in the K₂SO₄ forming process. When schoenite is prepared from kainite through a simple reaction-cum-leaching process and the liquid effluent is desulfated with CaCl₂ as part of a scheme to coproduce magnesia (Ghosh et al. U.S. Patent Publication No. 2005/0220698 A1, 2005 (Notice of Allowance); International Patent Publication No. WO 2005/063626 A1, July 14, 2005), the composition of the resultant effluent is found to be ideally positioned in the phase diagram for direct recovery of sylvinite (NaCl/KCl). The intermediate steps of carnallite crystallization and decomposition, encountered during sylvinite preparation from Dead Sea brine and desulfated oceanic bittern, are avoided as a result. In this manner, 70-80% of the KCl in the effluent can be recovered, which largely fulfils the requirement for K₂SO₄ preparation. The amount of water that must be evaporated is 6.5-7.5 kg per kg of KCl in the form of sylvinite.