Pressure acid leaching of a Chinese laterite ore containing mainly maghemite and magnetite

The high pressure acid extraction of nickel and cobalt from a Chinese laterite containing mainly maghemite and magnetite was studied. X-ray diffraction (XRD) and scanning electron microscopy/X-ray energy dispersive spectroscopy (SEM/EDS) were employed to characterize the residues. The factors influencing the dissolution of maghemite and magnetite, nickel and cobalt extractions and iron precipitation were investigated. The results show that after 75 min at 270 °C with an acid/ore ratio of 0.55, maghemite and magnetite completely dissolved, liberating 98% Ni and 88% Co into the leach liquor. EDS analysis reveals that some nickel may be associated with the amorphous silica and/or basic ferric sulfate, resulting in a minor loss of nickel. The presence of a cobalt-containing phase in the residues, believed to be ringwoodite, is mainly responsible for the incomplete extraction of cobalt. Both maghemite and magnetite dissolved gradually with the increase in temperature from 200 to 270 °C. Maghemite dissolved more slowly than magnetite at 270 °C which also produced ferrous sulfate in the leach liquor and increased the total iron extraction. Increasing temperature and/or agitation accelerated the hydrolysis of ferric sulfate. The leaching of maghemite and magnetite corresponds to a dissolution–precipitation mechanism. In both high and low acidic environments, the precipitation of ferric sulfate proceeds through the initial formation of basic ferric sulfate and its conversion to hematite. The extent of conversion depends largely upon residual acidity and reaction time.