Platinum group metals behaviour during iron precipitation and goethite seeding in nickel sulphate solution

ENGLISH ABSTRACT: Platinum producers’ base metal refineries leach converter matte to solubilise base metal sulphides and produce a platinum group metal residue. Nickel, cobalt and some of the iron present in the matte are solubilised during atmospheric sulphate leaching. The pregnant leach solution also contains dissolved other precious metals (OPMs, ruthenium, rhodium and iridium). The solution passes through holding tanks prior to being fed to heat exchangers and crystallisers for the production of nickel sulphate hexahydrate as by-product. An iron sludge precipitates during long residence times in tanks and on contact with hot surfaces. This sludge entrains OPMs and could potentially be used as a mechanism for recovery of dissolved OPMs. The behaviour of OPMs needed to be established during impurity precipitation in the nickel sulphate solution. Goethite material was also seeded, as a means to provide a growth surface during precipitation. The solution was heated to 70 °C or 90 °C and the pH was adjusted to pH 2.5 and pH 4.0, with caustic soda (NaOH). The total metal concentration was varied between the total metals of the first stage leach, 65 g/ℓ, and that of upgraded solution, 95 g/ℓ. Goethite seeding (10 g/ℓ, 50 g/ℓ and no seed) and iron valence (ferrous and ferric) were varied in six hour experiments. It was found that the OPMs could be selectively precipitated via neutralisation, with pH and temperature being crucial factors. At pH 2.5, 89 % ruthenium and rhodium precipitated and 14 % iridium precipitated. All ruthenium and rhodium precipitated at pH 4.0 and iridium precipitation increased to 88 %. OPM precipitation was 34 % faster at high temperature experiments. The order of readability to precipitate was ruthenium, followed by rhodium and then iridium. OPM precipitation was also possible without iron in synthetic solutions. Different iron oxide phases formed. Spherical, aggregated crystals between 50 nm – 100 nm in size were observed with elemental compositions agreeing well with that of synthetic ferrihydrate and schwertmannite. Sulphate inclusion was more prominent during the rapid precipitation in ferric runs. Ruthenium and rhodium were finely dispersed within the iron oxide precipitates of lower densities. Iridium precipitated in a clearly distinguishable phase consisting of iridium (50 – 80 wt.%), chloride and oxygen. The observation of the acicular rod crystals that are associated with goethite, the absence of spherical crystals, the uniform density observed with back-scatter images and the observed increase of overall particle size strongly suggest that seeding did induce the targeted growth of goethite in nickel sulphate solution with ferric iron. Goethite seeding did not increase the rate of iron precipitation or OPM recovery.