Process development by Dynatec for the Ambatovy Nickel Project
The Ambatovy Nickel Project involves the proposed recovery of nickel and cobalt from a laterite deposit in Madagascar. The project has the potential to produce 60,000 t of nickel and 5,600 t of cobalt annually. In support of the bankable feasibility study, laboratory and mini pilot plant testwork was carried out at the Dynatec laboratory in Fort Saskatchewan. This paper describes the major findings of the test program.The Ambatovy ore is primarily ferralite (limonite). Saprolite is present at the base of the deposit and a transition zone, referred to as low magnesium saprolite, has also been defined. Separate bulk samples of ferralite and ferralite/saprolite blend were prepared for the testwork; the blend contained 1.29% Ni, 0.11% Co, 3.11% Al, 1.81% Cr, 46.2% Fe, 1.03% Mg, 0.83% Mn, and 2.53% Si.The preferred process includes acid pressure leaching, neutralization of excess acid with limestone, recovery of the soluble nickel and cobalt values as mixed sulphide intermediate by precipitation with hydrogen sulphide, and subsequent refining of the mixed sulphide to nickel and cobalt metals. Pressure vessels are used for the ore leach and sulphide precipitation steps. A Dynatec mini pilot plant autoclave, in operation during the Ambatovy ore leach testwork, is shown in the figure. The operating volume of this autoclave is about 30 L, corresponding to a scale-up factor of about 20 000 : 1 in comparison with commercial ore leach autoclaves.The mini pilot plant program included a 13-day unit operations campaign to test the ore leach and neutralization steps. The sulphide precipitation conditions were established in separate unit operations campaigns. Based on the results of the unit operations campaigns, a 9-day continuous process demonstration campaign of the ore leach circuit was then carried out, followed by a 6-day continuous demonstration of the sulphide precipitation circuit, using product liquor from the 9-day campaign.The Ambatovy ore responded very favourably to leaching. Nickel and cobalt extractions in the range of 95.5% to 97.0% and 94.0% to 97.0%, respectively, were achieved in the demonstration campaign. The optimal conditions in the ore leach step were sulphuric acid addition in the range of 220 to 300 kg/t of ore, 260°C, and a 75 min. retention time in the autoclave. The pressure leach discharge also proved to be amenable to a combination of slurry and solution neutralization with satisfactory rejection of impurities. The leach residue displayed excellent settling characteristics, both before and after neutralization with limestone, resulting in exceptionally good recoveries of metal values in the counter-current decantation wash circuit. Nickel and cobalt losses in the neutralization and washing steps were less than 1%.Approximately 99.5% of the nickel and 99.2% of the cobalt contained in the neutralized leach solution were recovered in the sulphide precipitation step, using relatively mild conditions of 105°C and 200 kPa H2S overpressure. The mixed sulphide product was then treated in a series of steps, simulating a commercial refinery, to ultimately produce separate nickel and cobalt metal products with better than acceptable purity.The successful testwork program has demonstrated a technically viable process for treatment of the Ambatovy ore and has provided essential data for the capital and operating cost study.