Potentiometric pH Measurements in the Pressure Acid Leaching of Nickel Laterites.

摘要

An electrochemical cell consisting of a flow-through yttria-stabilized zirconia (YSZ) sensor and a flow-through Ag/AgCl reference electrode has been employed to measure pH of high-temperature acidic sulphate solutions relevant to the pressure acid leaching (PAL) of nickel laterites. In a previous study, this cell was used to measure pH of H2SO4, Al2 (SO4)3-H2SO4 and MgSO 4-Al2(SO4)3-H2SO4 solutions at 250°C. In this work, the solutions range in complexity from the binary MgSO4-H2SO4, NiSO4-H 2SO4, and Al2(SO4)3-H 2SO4, through the ternary MgSO4-Al2(SO 4)3-H2SO4 and NiSO4-Al 2(SO4)3-H2SO4, to the PAL process solutions, whereas the temperature ranges from 200°C to 250°C. The measured and theoretical pH values typically agree within less than 0.1 pH unit and 0.2 pH units in synthetic solutions and PAL solutions, respectively. This is an improvement over the results of the previous study in synthetic solutions, which show differences between theory and experiment as high as 0.4 pH units. The conversion of measured potentials into pH values is based on the new mixed-solvent electrolyte (MSE) speciation model of the OLI Systems software calibrated independently based on solubility measurements. Both Henderson's equation and the exact definition of the diffusion potential were employed in treating the obtained experimental data. Experimental pH values calculated using the diffusion potentials evaluated by either approach are essentially the same. This finding suggests that Henderson's equation, which is based on readily available limiting ionic mobilities, can be effectively used. Lithium chloride is found to be a suitable alternative to sodium chloride as the reference electrode solution for the measurement of pH of aluminium-containing solutions, because it did not induce precipitation of aluminium as an alunite-type compound. The experimental results indicate that the high-temperature behaviour of Ni, Co and Mn 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. The cell can be used for hydrometallurgical process research and development on a laboratory scale with very satisfactory performance, provided that a well-behaved YSZ sensor is available.