Characterization of Scales Obtained during Continuous Nickel Laterite Pilot-Plant Leaching

The processing of nickel by pressure acid leaching nickel laterite ores at 250°C results in liquors supersaturated with respect to aluminum, iron, and silica. These can precipitate from the liquor and onto the autoclave interior (scaling). This reduces the availability of the autoclave. This article presents a methodology for characterizing scale using X-ray powder diffraction (XRPD), Rietveld analysis of the XRPD trace, and scanning electron microscopy/microprobe. This methodology is then used to characterize three scales formed during the continuous (pilot plant) pressure acid leaching of nickel laterites. Scales formed during the leaching of Western Australian ore, from an acid environment in hypersaline process water, contain mainly sodium alunite/jarosite as a solid solution together with minor hematite and amorphous silica. Scales formed during leaching of tropical nickel laterite, in low salinity process water, consist of an aluminum-rich hydronium alunite with hematite and low concentrations of amorphous silica. Ore mineralogy, leach liquor composition, acidity, and temperature, as well as precipitation kinetics, all appear to influence the scale mineralogy. This article discusses the influence of these factors on the formation of three scales.