Precipitation of impurities from synthetic laterite leach solutions

The precipitation of metals from a synthetic laterite leach solution was studied within the pH range of 4–7 at 40 °C with MgO and NaOH as neutralisation reagents. It was found that the precipitation order was Cr(III) > Al(III) > Cu(II) > Fe(III) as pH below 4.5, and the order changed to Cr(III) ∼ Fe(III) > Al(III) > Cu(II) as pH above 4.5. The precipitation of chromium was almost complete (< 0.2 mg/L) and only about 1 mg/L Al remained in the neutralised solution at pH 5.2. The precipitation of iron was almost complete (< 0.2 mg/L) when the pH was 5.6, and the precipitation of copper and zinc was almost complete (< 0.2 mg/L) only at pH 7.0. It was found that nickel and cobalt were present in the precipitate even at a low pH of 4, which was much lower than their precipitation pH of 7.0. The presence of nickel and cobalt in the precipitate was assumed in two chemical forms, one as sulphate resulting from chemical adsorption, which is predominant below pH 5.7; and also as the hydroxide which resulted from co-precipitation and is predominant above pH 5.7. No significant precipitation was observed for manganese, magnesium and calcium within the pH range of 4.0–7.0.