COOLING COOLING TOWERS DESALINATION GYPSUM MEMBRANES POLYMERS REVERSE OSMOSIS SCALING

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Abstract

The deposition of mineral deposits of calcium containing salts on heat exchangers and reverse osmosis (RO) membrane surfaces is a serious problem in many industrial operations, and poses operational challenges in the efficient operation of the systems. The four most commonly encountered calcium-containing salts in mineral deposits are calcium carbonate, calcium phosphate, calcium phosphonate, and calcium sulfate. The formation of these deposits on metallic surfaces of heat exchangers and equipment surfaces is favored by the decrease of their solubility with increasing temperature, and their deposition results in decreased heat transfer, causing increased energy costs. In addition, scaling is often accompanied with corrosion, which leads to damage of the metallic parts of heat exchangers, pipes, and pumps. Calcium sulfate is found in three different crystalline polymorphs; calcium sulfate dihydrate (CaSO4.2H2O, gypsum), calcium sulfate hemihydrate (CaSO4 ﾕ ﾽ H2O, plaster of Paris), and calcium sulfate anhydrite (CaSO4). The formation of calcium sulfate phases depends strongly on supersaturation, ionic medium, temperature, and impurities present in the aqueous system.

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