Abstract: A method is disclosed for production of a sulfate-containing chlorine free salt and anhydrous gaseous HCl from a metal chloride (MClx), oleum and water. MClx and oleum are mixed together with a water-containing liquid, forming gaseous HCl and a solution of a sulfate-containing salt. The salt is precipitated from the solution, and in a preferred embodiment, the supernatant liquid from the precipitation is recycled to the reaction mixture as the water-containing liquid in subsequent reaction cycles. The exothermicity of the reaction between the water-containing liquid and the oleum is sufficient to remove chlorine in the form of substantially pure useful HCl and enables the process to proceed without additional heating of the reaction mixture. When the metal is potassium, this method produces high-purity novel sulfate salts with high potassium content K3(NH4)(SO4)2, 3K2SO4.MgSO4 and 3K2SO4.CanMg1-nSO4.