Mixed salts—scaling limits and propensity

Abstract

The main aim of desalination is removal of dissolved species from water either by thermal or membrane processes. In either process, the saline solution becomes concentrated, and once the concentration of salts exceeds its saturation limit, they form an unwanted deposit referred to as a fouled or scale layer. Even though precipitating salts coexist in industrial water systems, due to the complexity of the fouling process, research has primarily been concentrated on a single salt precipipation. The effect of other ionic species has been taken into account through the solution's ionic strength and use of the Debye Huckle Theory or a modified version of that. This paper critically assesses the current practices and consolidates the recent and ongoing work done by the author and her students [1–5] in thermodynamics and scaling limits in mixed salt solutions. The results indicate that the thermodynamics of single salt crystallization are not applicable to practical conditions of feed water when mixtures of salts are present. It is shown in various systems that the interactive effects of coexisting salts and the presence of a common ion cannot be ignored. Co-precipitation of salts with or without a common ion affects the solubility constants of the salts, and therefore application of the solubility data obtained under a single salt precipitation are questionable in mixed salt precipitation. Even the presence of minute amounts of another precipitating salt affects the thermodynamics and the scale structure and strength, which renders inapplicable the solubility data for the pure salts. The effect depended on both the dominance and type of salt. It should be emphasized that these results were obtained despite the fact that these assessments took into account the effect of solution ionic strength. Thus, considering the effect of ionic strength—though necessary—is not sufficient in mixed solutions. The presence of one salt does indeed affect the structure of the other salt. The effect of other salts on the scale morphology should not be ignored, and the author hypothesizes that to be the reason for inapplicability of the pure salt thermodynamics to that of mixed solutions. This paper establishes through thermodynamic relationships and through the Gibbs free energy of formation that the thermodynamic solubility constant of a salt in pure solution is different from that in a mixed solution if the morphology is affected by the presence of other salts.