Model for the partition of metal ions in aqueous two-phase systems.

A model for the partition of metal ions in aqueous two-phase systems (ATPS) has been developed. The partition coefficient of a metal ion D(M), is a function of several variables of the ion (size, charge and electronegativity), characteristics of the ATPS such as type of salt, salt concentration and PEG concentration and additional inorganic salt present in the ATPS. The model has been tested for complex anions of BiX4- (BiCl4-, BiBr4- and BiI4-) and cations from groups I and II (Na+, Cs+, Ca2+, Sr2+ and Ba2+) giving a good correlation in both systems. It was found that for these systems partition coefficient increases with ion size and the variables Y which is a characteristic of the ATPS and Z which is a characteristic of the additional salt present in the system. The partition coefficient of BiX4- increases with the variable X which is a characteristic of the electrical interactions of the metal ion. The cations from groups II and I exhibit the opposite behavior, which is attributable to the ion charge.

Drowning-out crystallisation of sodium sulphate using aqueous two-phase systems.

A novel method to obtain crystals of pure, anhydrous salt, using aqueous two-phase systems was studied. A concentrated salt solution is mixed with polyethylene glycol (PEG), upon which three phases are formed: salt crystals, a PEG-rich liquid and a salt-rich liquid. After removal of the solid salt, a two-phase system is obtained. Both liquid phases are recycled, allowing the design of a continuous process, which could be exploited industrially. The phase diagram of the system water-Na2SO4-PEG 3350 at 28 degrees C was used. Several process alternatives are proposed and their economic potential is discussed. The process steps needed to produce sodium sulphate crystals include mixing, crystallisation, settling and, optionally, evaporation of water. The yield of sodium sulphate increases dramatically if an evaporation step is used.