Pressure acid leaching of nickel laterite

Pressure acid leaching is an acceptable process for the treatment of nickel laterites. This process offers many advantages such as low energy consumption, low capital cost and high nickel and cobalt recoveries. The dissolution process is coupled to selective thermal hydrolysis of iron and aluminum. The dissolution of laterite and precipitation of iron and aluminum are strongly dependent on the temperature and the acid-to-ore ratio, which is determined by ore compositions and mineralogy. Because of the complex nature of the laterites, a knowledge of the mineralogy of laterites, the nickel association with the various constitutional minerals and the reactivity of these mineral phases is fundamental to an understanding of the kinetics of nickel extraction and to the process optimization. The mineralogy of the laterite ore samples from two ore bodies in New Caledonia and a variety of locations in an Ivory Coast ore body was examined by X-ray diffraction, scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDX). X-ray diffraction and SEM-EDX studies showed that goethite, maghemite, quartz and lithiophorite are common minerals in these laterite samples. Spinel and gibbsite are present as minor phases in the New Caledonia laterite samples. Silicates such as talc, kaolinite and clinochlore were found in the Ivory Coast laterites. EDX results indicated that goethite is the major host mineral for nickel. Nickel also showed association with maghemite, lithiophorite and silicates (to various extents in Ivory Coast laterites). Interruptive autoclave leaching experiments were carried out on a blended sample from the Ivory Coast ore body. With a 30% pulp density slurry treated at 260°C, with acid/ore = 0.19, nickel and cobalt extractions reached 96% within 15 minutes as goethite and maghemite had completely dissolved. Hydronium alunite was detected by XRD in the leach residue after 30 minutes of leaching, indicating that the amount of alunite increased with leaching time. Interruptive pressure acid leaching experiments were also performed at 250°C on the samples from five different locations of the Ivory Coast ore body. Process conditions such as acid/ore ratio and pulp density were calculated from a spreadsheet provided by Falconbridge Limited. Nickel and cobalt extractions obtained from 30 minute leaching tests as well as the free acid concentrations in the filtrates differed significantly among the five samples. The goethite dissolution rate also varied among samples and showed a dependence on free acid concentration. Goethite fully dissolved from samples LD-97-13, LD-97-18 and LD-97-32 within 30 minutes at 250°C when the free acid concentration was >32 g/1 in the filtrate. However some of the goethite was detected in the leaching residue of LD-97-24 after 1 hour of leaching with a filtrate free acid concentration of 27.15 g/1. In the experiments, maghemite exhibited the same reactivity as goethite. Clinochlore appeared to be dissolved at free acid concentration of approximately 40 g/1 from sample LD-97-13. Talc, kaolinite, and quartz showed resistance to pressure acid leaching at prescribed leaching conditions. The results of the free acid measurement showed that acid consumption during leaching was dependent not only on the chemical composition, but also the mineralogy of the ore.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat