The synthesis of carnallite from the spent electrolyte of magnesium cells and a chlorine-magnesium solution is studied. The process parameters that ensure the formation of synthetic carnallite that meet the requirements of the magnesium industry are determined. Specifically, the synthesis temperature is 100-130°C, the initial concentration of magnesium chloride in a solution is 290-400 g/l, the consumption of spent electrolyte is 105% of the stoichiometric composition, and the particle size is 0. 315 mm. The sequence of synthesis operations is as follows: heating of a chlorine-magnesium solution and a spent electrolyte to 110-120°C, their mixing in a reactor, electrolyte dissolution, the evaporation of a reaction mass during agitation, and drying. © 2011 Allerton Press, Inc.