Selective removal of calcium from sulfate solutions containing magnesium and nickel using aqueous two phase systems (ATPS)

•The ATPS proved efficient separation of calcium from nickel and magnesium.•Staged operations removed calcium from sulfate solution at pH 1–2.•No extractant was required to remove calcium from synthetic solutions.•The ATPS can be used for both dilute and concentrated solutions.

The present study investigated the removal of calcium from sulfate solutions containing magnesium and nickel using aqueous two phase systems (ATPS) at room temperature. In an attempt to evaluate the application range of this separation method, batch scale tests (bottom/top phase weight = 1) were carried out using synthetic test solutions at two concentration levels (low and high). The test solution of high concentration used as the bottom phase consisted of a solution containing [Ca] = 0.44 g·L− 1, [Mg] = 1.42 g·L− 1 and [Ni] = 80.0 g·L− 1, whose concentration level is similar to those typically found in high pressure acid leach liquors. The test solution of low concentration used as the bottom phase consisted of a solution of sodium tartrate in which a given volume of the solution of high concentration was added, resulting in a dilution of approximately 80 fold. The performance of some cationic extractants (Cyanex 272, 1N2N and PAN) at changing concentrations and the effect of the pH of the aqueous system (1, 6 and 11) were investigated in the dilute condition. This study found that, when ATPS operated in at least 3 contact stages, it proved to be a highly efficient and selective method to separate calcium from magnesium and nickel, using no extractant and at low pH values (around 1–2) for both dilute and concentrated solutions. The extraction of nickel depends on the pH and the type/concentration of extractant, while the extraction of magnesium was not affected by the studied operating variables.

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