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2 The crystallization of K2SO4 in the system K+, Mg2+/Cl!, SO4 / H2O involving the reaction 2KCl + MgSO4 K2SO4 + 2KCl was studied. Independently of the experimental procedure applied, it was confirmed that schoenite (K2SO4.MgSO4.6H2O) is an inevitable intermediate product of the considered transformation. The double decomposition reaction kinetics depend on formation and dissolution rates of intermediate salt and potassium sulphate, respectively. The formation of schoenite outside of its crystallization field might indicate a metastable space extension of this domain to cover the K2SO4 one. The formation kinetics rate of MgSO4.K2SO4.6H2O is quicker than the K2SO4 one.

The considered transformation can be carried out in one or two stages, mode A or mode B. We showed that the second mode allows a better output of the process by partial or integral recycling of solution “R”. The increase in this output is proportional to the amount of recycled solution. In addition the experimental study showed that whatever the adopted procedure, schoenite is formed as an intermediate product. The formation of the intermediate compound led us to think about the existence of a metastable field of formation of the schoenite which partially or completely covers the stable field of existence of K2SO4. A systematic study of the transformations showed that the reaction time is about 6 h for mode A and 2 h for mode B.