Interest in production of salt and other minerals from desalination reject has been accelerated with the recent popularity of themes such as sustainable development goals, critical materials, etc. Although about 20’000 desalination plants are in operation worldwide, none of them has been producing salt and other minerals from the rejected brine on a large scale commercially. The stumbling blocks are numerous, including not only high investment and operating cost but also the difficulty to produce salts of marketable quality.
Most desalination plants produce drinking water using reverse osmosis membranes. Seawater contains calcium sulphate. Due to its low solubility, calcium sulphate begins to crystallise when about a half of the water is removed and the seawater concentrate reaches 7% salinity. Unless avoided, calcium sulphate crystals would form scale on the membranes and stop the desalination process. To avoid this, antiscalants are added. The
antiscalants cause the calcium sulphate to crystallise in microscopic crystals, which are harmless. The problem is that the antiscalants also cause other salts to crystallise as tiny crystals. Such products are not marketable.
Recently introduced new membranes facilitate a process, which is capable of producing water from seawater without antiscalants. Applying pressure lower than about 80 bar, drinking water is produced while concentrating the seawater up to about 22% salinity. This new concentration capability reduces the investment and operating cost to less than a half of the similar systems previously proposed. From the saline concentrate, not only sodium
chloride of up to 99.99% NaCl purity, but also marketable calcium sulphate, magnesium oxide, potassium chloride, sodium sulphate, bromine, etc., can be produced, while no effluents are being discharged. The presentation will focus on factors which determine the economy of potential application of the process in cases where conventional salt production is problematic or where discharge of desalination reject to the sea is
undesirable.
