Measurement of pH in high-temperature nickel laterite pressure acid leach process solutions

The performance of a flow-through electrochemical cell with an yttria-stabilized zirconia (YSZ) sensor for pH measurement in high-temperature nickel laterite pressure acid leach (PAL) process solutions was evaluated. Very good agreement was observed between the measured pH values and those theoretically predicted using the OLI Systems software package calibrated independently based on solubility measurements. PAL process solutions were simplified to ternary MgSO4–Al2(SO4)3–H2SO4 solutions by combining all the divalent metal sulphates into one with the properties of MgSO4. The experimental results support the postulation that the high-temperature behaviour of nickel, cobalt and manganese sulphates can be satisfactorily approximated with that of MgSO4. The experimental findings also support the postulation that acid should be added to a PAL process so that the solution pH is around 1 at the leach temperature, regardless of the feed composition.

Research Highlights
► Direct pH measurement of PAL solutions at temperature.
► Use of a YSZ ceramic electrode coupled with a Ag/AgCl external pressure balanced reference electrode.
► All divalent metals behave similarly and supply SO4 which acts as a proton sink increasing the pH.
► Solution pH at temperature should be around 1 for effective leach regardless of divalent metal concentration, such as Mg.