Implications for the origin of secondary sylvite from a simulation of carnallite dissolution

• Low Br and high Rb concentrations occur in sylvite derived from carnallite.
• Sylvite and halite are separated from sylvinite by flotation method.
• Redistribution of Br and Rb between solid and liquor phases due to KCl re-crystallization.
• Driving force is a difference between Mg2 + dissolution rates and K+ crystallization rates.

The relationship between sylvinite (KCl + NaCl) and carnallite (KCl·MgCl2·6H2O) is important in potash mines containing carnallite, and especially where sylvinite overlies carnallite. It is generally believed that this reversal of the normal depositional sequence may be caused by dissolution of carnallite. To obtain sylvite (KCl) from carnallite ore in the present study, first carnallite ore dissolution was performed in an aqueous solution at 25 °C, and then the sylvite was separated from halite (NaCl) in the sylvinite by a flotation method. The Br and Rb concentrations in the sylvite derived from the carnallite ore were determined. The results showed that more than 4/5th of the Br and Rb in the carnallite ore transferred to the mother liquor, while the remainder of the Br and Rb was in the sylvinite. The sylvite separated from this sylvinite had purity of 94–96%. In the sylvite derived from carnallite, the Br concentration was obviously lower (by an order of magnitude) than the concentration in primary sylvite from evaporation of MgSO4-deficient sea water, and the Rb concentration was twice that in primary sylvite from evaporation of MgSO4-deficient sea water. Compared to the Br and Rb concentrations in secondary sylvite from Khorat Plateau, Sergipe, and Prairie evaporites, the concentrations in the sylvite derived from carnallite ore were similar. The redistribution of Br and Rb in the form of secondary sylvite was in a relative stable proportion according to the comparison of Br and Rb. The lower Br and higher Rb concentrations in sylvite derived from carnallite ore could be caused by KCl re-crystallization and redistribution of Br and Rb between the solid and liquid phases. The driving force behind changes in the partition coefficients of Br and Rb is that the dissolution of carnallite is much slower than the nucleation of KCl.