SEPARATION AND CONCENTRATION OF CALCIUM AND MAGNESIUM FROM SEA-WATER BY CARBOXYLIC RESINS WITH TEMPERATURE-INDUCED SELECTIVITY

Processes of concentration and separation of calcium and magnesium fro m artificial and natural sea water by carboxylic ion-exchange resins o f acrylic and methacrylic types at different temperatures have been in vestigated. The values of equilibrium separation factor alpha for Ca2-Na+, Mg2+-Na+ and Ca2+-Mg2+ exchanges in ternary systems have been de termined in the temperature range of 10 degrees C to 80 degrees C. A s ignificant increase of alpha values at elevated temperatures has been observed in the first two cases while for Ca2+-Mg2+ exchange less rema rkable temperature dependence of alpha can be distinguished. This effe ct has been shown to allow a selective thermostripping of Ca2+ and Mg2 + from the resins equilibrated at 80 degrees C with sea water in apply ing cool sea water at 10 degrees C. The thermostripping leads to a sel ective desorption of both Ca2+ and Mg2+ while Na+ ions remain sorbed, resulting in the increase of Ca2+ and Mg2+ concentration in the eluate up to 50% (in comparison with the initial sea water) and a decrease o f 10% for Na+ concentration. These results may be considered as unique in polythermal concentration in comparison with, e.g. conventional ev aporation technique. The results of consecutive sorption-thermostrippi ng cycles have shown the possibility to concentrate calcium and magnes ium from natural sea water more than three times by applying reagentle ss (and wasteless as a result) ion-exchange technique. The results of frontal separation of Ca2+ and Mg2+ on acrylic resin in Na+-form from natural sea water and thermostripping solutions obtained are also pres ented. The novel approach for forecasting temperature dependences of t he resin selectivity has been proposed. The approach is based on a the rmodynamic interpretation of the results obtained that allows to predi ct the temperature dependences of both alpha (for binary Mg2+-Na+ exch ange) and the apparent equilibrium constant of ternary Na+-Ca2+-Mg2+ e xchange.