AtHydrothermal iron removal Rustenburg Base MetalsMetals Refinery, the primary nickelNickel leach liquor is processed through a neutralisation–hydrothermal autoclaveAutoclave treatment at 140–150 °C using caustic soda to remove solubilised iron as a hematiteHematite/natrojarositeNatrojarosite residue. A dynamic model of the process was developed to evaluate a pH control strategy for this process with the objective of minimising co-precipitationPrecipitation of nickelNickel and copperCopper. A transient reactive mass-balance model was developed to describe measured data by data fitting and a simplified version of the mass balance was used for control algorithm modellingAutoclave modelling. The simplified version is less accurate and only provides pH as an output, whereas the transient mass balance can output pH, aqueous flows of base metalsMetals, and flows of phases formed in the residue. The fitted dispersion parameters in the mass-balance model suggest that dispersion of caustic soda within the autoclaveAutoclave is orders of magnitude slower than that of the aqueous feed, which is a consequence of the relative flows, compounded by a surface dosing configuration. Slow dispersion of caustic soda amplifies its excess requirement, hence promoting copperCopper and nickelNickel hydroxide formation in the residue. Elevated autoclaveAutoclave temperature was found to depress iron concentration in the autoclaveAutoclave aqueous product stream. Simulation of a dependent-gains controller illustrates that adequate control of the autoclaveAutoclave discharge pH is possible by automatic adjustment of the caustic:feed flow ratio; however, the response lag associated with the manipulated variable is of the order of 30 min and warrants an advanced process control solution.