Controlled Decomposition and Selective Dissolution of Carnallite: Phase Equilibria, Reaction Mechanisms, and Potassium–Magnesium Separation—A Critical Review

Carnallite (KCl• MgCl₂•6H₂O) provides a potentially important potassium resource, but potassium recovery is constrained by strongly coupled phase equilibria, incongruent dissolution, secondary crystallization, and the circulation of concentrated magnesium chloride brines. This critical review examines the thermodynamic and mechanistic basis of controlled decomposition, cold and temperature-assisted treatment, compositional effects, kinetics, solids handling, and integration with flotation and crystallization. The evidence is organized to distinguish established measurements from process-level inference, with emphasis on the KCl–MgCl₂–H₂O system and the influence of sodium, calcium, and other impurities. The analysis shows that equilibrium determines which selective transformation is feasible, whereas nucleation, mixing, interfacial transport, crystal morphology, and brine recycle determine whether the attainable selectivity is achieved operationally. We propose a material-balance framework to report potassium recovery and magnesium rejection consistently across solids, mother liquor, wash streams, and recycled brine. Research priorities include phase-model validation on representative feeds, time-resolved in situ observation, robust separation of fine KCl crystals, and quantified water–energy trade-offs. The resulting framework links chemical selectivity to industrially meaningful recovery and product quality without conflating equilibrium potential with demonstrated plant performance.

Carnallite; Incongruent dissolution; Solid–liquid equilibrium; Potassium chloride; Magnesium chloride; Selective crystallization

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