Scale formation on reactor walls remains a major operational challenge in high-pressure acid leaching (HPAL) of nickel laterites, leading to reduced heat transfer efficiency, increased maintenance, and process downtime. This study investigates the influence of slurry solid content and acid-to-ore (A/O) ratio on autoclave scale formation during laterite leaching. Experiments performed under typical HPAL conditions (265 °C and ~50 bar) with laterite ore examined how these parameters affect metal extraction, scale quantity, and mineralogical composition. Scale deposits were quantified and analysed to determine their composition and to evaluate the precipitation tendency of potential scale-forming minerals through solution speciation and supersaturation behaviour during HPAL leaching. The results show that increasing slurry solids significantly promotes scale formation, producing denser and more strongly adherent deposits, while higher acid dosage further enhances precipitation of sulphate-bearing phases. Mineralogical analyses indicate that the scales are primarily composed of hematite, hydronium alunite, and magnesium sulphates, whose formation is driven by solution supersaturation during leaching. Elevated solids content also promotes incorporation of valuable metals into the scale matrix, leading to reduced nickel and cobalt recovery. In contrast, operation at moderate solids content and near-stoichiometric acid addition limits scale accumulation while maintaining high metal extraction. These findings highlight the coupling between HPAL operating conditions, solution chemistry, and fouling behaviour, and suggest an operational window for reducing scale formation without the use of chemical additives that may interfere with downstream processing. • Experimental study of autoclave wall scale under HPAL conditions (265 °C). • Scale mineralogy and morphology linked to solution chemistry evolution. • High solids promote denser scale with strong wall adhesion. • Operating window suggested to limit fouling without chemical additives.
Dickson et al. (Sun,) studied this question.