Characterization of precipitate formed during the removal of iron and precious metals from sulphate leach solutions
CITATION: Coetzee, R., Dorfling, C. & Bradshaw, S. M. 2017. Characterization of precipitate formed during the removal of iron and precious metals from sulphate leach solutions. Journal of the Southern African Institute of Mining and Metallurgy, 117(8):771-778, doi:10.17159/2411-9717/2017/v117n8a7.
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ENGLISH ABSTRACT: Nickel sulphate leach solutions produced in the first leaching stage of base metal refineries contain impurities such as iron as well as precious metals (Rh, Ru, and Ir). Iron precipitation results in sludge formation, which needs to be controlled for efficient operation of downstream nickel recovery processes. Recovery of precious metals from the leach solution is also desired. This study aimed to evaluate the characteristics of the precipitate produced from a nickel sulphate leach solution containing 62.5–89.3 g/L Ni, 2.5 – 3.57 g/L Fe, and 10 mg/L of each of Rh, Ru, and Ir. Seeded precipitation from ferric-containing solutions resulted in precipitates with a d₅₀ particle size of 100.6 μm, which was two orders of magnitude larger than the reference goethite seed d₅₀particle size of 1.3 μm. The particle size distributions of the precipitates formed from ferrous solutions were similar to that of the reference goethite seed. The precipitates formed from ferrous-containing solutions at pH 2.5 and at pH 4 had increased micropore areas (72.8 m²/g and 87.1 m²/g, respectively) and decreased external specific surface areas (53.4 m²/g and 49.0 m²/g, respectively) compared to the goethite reference material (micropore surface area of 66.2 m2/g and external surface area of 64.8 m²/g). For ferric-containing solutions at pH 2.5, a decline in specific surface area from 131.0 m2/g to between 82.0 m²/g and 100.6 m²/g was caused by aggregation and molecular growth inside micropores. Instantaneous iron precipitation from ferric solutions at pH 4 resulted in an increased Brunauer-Emmett-Teller (BET) surface area of 156.5 m²/g due to poor ordering of crystal structure and a more amorphous surface structure. Iron oxide phases present in the precipitates had elemental compositions similar to ferrihydrite and schwertmannite. Sulphate inclusion was more prominent during the rapid precipitation from ferric solutions than during precipitation from ferrous solutions. The precipitate formed at pH 2.5 was overall more crystalline than the precipitate formed at pH 4.0; nickel entrainment also increased with an increase in pH. Rhodium- and rutheniumcontaining species were finely dispersed throughout the iron phases in the precipitates. Iridium precipitated primarily without the inclusion of iron or other precious metals; particles consisting of iridium (50–80 wt%), chloride, and oxygen were formed.