The recovery of nickel from laterites by chelate formation and reduction with hydrogen

Abstract

The interaction between alkaline solutions of the sodium salts of ethylenediaminetetraacetic acid (E.D.T.A.) and nickel-bearing laterites has been studied at temperatures between 25°C and 90°C. Experiments were carried out with a serpentine ore containing 1.65% nickel, 6.10% iron, 20.2% magnesium and a limonite ore containing 1.51% nickel, 49.7% iron, 0.66% magnesium. Reduction with 400 p.s.i. or 800 p.s.i. hydrogen at 120°C or 140°C has been examined as a means of recovering nickel from the leach solution and regenerating the leachant. For both ores, an increase in pH from 8 to between 11 and 13, or a rise in temperature, gave increased nickel dissolution rates and improved selectivity for nickel dissolution over that of iron, manganese, magnesium, aluminum, chromium and calcium. After 48 hour leaches at 90°C and pH 13 with 1.5 moles E.D.T.A. per mole of nickel in the ore, nickel extractions from the serpentine and limonite were 87% and 27% respectively. Rates of nickel dissolution from both materials were markedly increased by precalcination at temperatures which gave partial decomposition of the mineral structures. Tests showed that reduction with hydrogen could be used to recover nickel from leach solutions at pH 13 but not at pH 12. Reduction from a leach solution at pH 13 with 800 p.s.i. hydrogen at 140°C for 3 hours gave 91% nickel recovery and a solution that was used effectively as recycle leachant. The substantial increase in nickel dissolution rates that resulted from precalcination suggests that leaching characteristics are strongly dependent on laterite mineralogy. With further information covering additional materials of differing mineralogy, the feasibility of any process based on E.D.T.A. leaching of raw laterite could be assessed more readily.