Characteristics of scales formed from pressure leaching of Yuanjiang laterite

The characteristics of scales developed on autoclave interiors during pressure acid leaching of Yuanjiang laterite were investigated, using X-ray diffraction, scanning electron microscopy/X-ray energy dispersive spectroscopy and particle size analysis. The effects of ore mineralogy and leaching conditions on scale formation were also studied. Amorphous silica and basic ferric sulfate preferentially deposit to the side wall of autoclave and stirring rod, constituting the inner layer of scale. The outer layer of scale contains mainly hematite, growing onto the surfaces of scales initially formed. Some scales formed from the coarser ore feed contain undissolved maghemite, which is coated by hematite and basic ferric sulfate, and deposit at the bottom surface of autoclave. Aluminum-containing sulfate only forms from the ore feed with high aluminum contents. Scales have smaller particle sizes than residues, and residues than ore feeds. Scales are softer when processing ore feeds ground to − 80 μm compared to − 140 μm. Amorphous silica, amorphous or microcrystalline aluminum-containing sulfate, and finer particles of basic ferric sulfate are more prone to form scales than to deposit as residues. Porous silica can accommodate finer iron products, resulting in an increase of scale amount. The relatively low amount of scale formed from the magnesium-rich ore may mainly arise from the precipitation of magnesium sulfate and its bondage to hematite at high temperatures. Increasing slurry density and leaching time, and decreasing agitation speed lead to aggravation of scaling.