| A new chemical model for laterite leach solutions (ie., solutions produced in an autoclave during pressure acid leaching of laterites for nickel extraction) is introduced. The major electrolytes in these solutions are H2SO4, Al2(SO4)3, and MgSO4. A hybrid ion-association-interaction approach is implemented to describe the chemistry and thermodynamics of these ternary solutions at 250°C. The complexes in the solution and the ion-interaction parameters (in the Pitzer model) are identified through processing solubility data in the binary, H2SO4-Al2(SO4) 3 and H2SO4-MgSO4, as well as the ternary, H2SO4-Al2(SO4)3-MgSO 4, electrolyte solutions at or near 250°C. The identified aluminum-bearing species are Al3+, Al(SO4)+, and Al 2(SO4)30, with Al2(SO 4)30 as the dominant species at moderate to high H2SO4 concentrations. Also, Mg2+ and MgSO40 are the identified magnesium-bearing species, with Mg2+ being dominant, except at low concentrations of H2SO4. To verify the identified chemistry of H 2SO4-Al2(SO4)3 solutions independently, the electrical conductivities of these solutions at 250°C, measured through a novel conductivity cell and set-up, are analyzed. These measurements show that the conductivity at constant H2SO4 molality drops with increasing Al2(SO4) 3 molality. This behavior is caused through a decrease in H+ molality and an increase in ionic strength. Furthermore, the slope of the conductivity vs. Al2(SO4) 3 molality is much steeper at low Al2(SO4) 3 molalities than at higher values. This is attributed to a much higher dissociation of Al2(SO4)3 at low molalities than at high values. After this confirmation of the solution chemistry, a typical laterite leach solution is simplified to a ternary electrolyte solution that contains H2SO4, Al2(SO4) 3, and pseudo-MgSO4 (i.e., a lumped entity containing all divalent metal sulfates and having the properties of MgSO 4). A chemical model for these solutions at 250°C is developed (based on the ion-association-interaction approach) and then verified against the measured solubility of aluminum in real leach solutions. It is found that a higher concentration of H2SO4 when processing high-Mg ores is required than that when processing low-Mg ores. This is due to the requirement to maintain the same level of true acidity (i.e., H+ molality rather than H2SO4 molality) "at temperature" in order to obtain comparable Ni extraction rates. Finally, based on the true acidity "at temperature", a new general rate equation for nickel extraction is also developed. |
