CaSO4 solubility in water–ethanol mixtures in the presence of sodium chloride at 25 °C. Application to a reverse osmosis process

Nowadays, the most common way to desalinate seawater is by reverse osmosis. As the degree of conversion during this process increases more freshwater is recovered from the feedwater. As a result, the salt concentration in the feed increases up to a point where the solubility limit could be reached. Experimentally, it is known that adding an organic substance such as ethanol to salty water induces salt precipitation. This work investigated the solid–liquid equilibrium of the system water–ethanol–NaCl–CaSO4 at 25 °C. Results show that as the ethanol content is increased CaSO4 solubility decreases. On the other hand, brine from the reverse osmosis plant at the University of Alicante was treated with ethanol to precipitate calcium sulfate and produce brine containing less calcium and sulfate. The treated brine was analyzed and its calcium content was compared with the predicted value based on the experimental data. The results suggest that it is possible to use ethanol to precipitate the salts from brine in order to obtain a higher degree of conversion in a reverse osmosis process. The obvious limitation of the method is the cost of recovering the ethanol by separation.