(57) Releasing impurities from a calcium-based mineral to produce pure double salt crystals for producing high quality gypsum and chalk – A method for releasing impurities from a calcium-based mineral with an aqueous solution of one or more ionic salts at a temperature of approximately 85°C or above, preferably 100°C, e.g. at 1-2 atmospheres, wherein the calcium based mineral is any mineral having a significant calcium component in the form of calcium carbonate (CaCO3), or calcium sulphate (CaSO4), or calcium oxide (CaO) and the one or more ionic salts comprise any of ammonium sulphate; ammonium chloride; and sodium sulphate, at least one of the calcium-based mineral and the one or more ionic salts comprises sulphate and at least one of the calcium-based mineral and the one or more ionic salts comprises ammonium. The concentration of the solution is approximately 25% or higher mass fraction such that double salt crystals, e.g. koktaite ((NH4)2SO4•CaSO4•H20) are formed and impurities are released. The double salt crystals may be separated and converted to chalk; and may be reacted in an aqueous solution to precipitate gypsum which may be converted to chalk. The method is designed to produce high quality gypsum, e.g. primarily calcium sulphate dehydrate.
(71) Carbon Cycle Limited, 248 Sutton Common Road, Sutton, Surrey, SM3 9PW, United Kingdom