Application of ammonium-citrate-thiosulfate leaching on awaruite-bearing serpentinite ores

Nickel is an important alloying element of stainless steel and non-ferrous alloys. Awaruite as an alternative nickel source is gaining attention at the depletion of sulfide nickel and the high cost of laterite nickel, which are traditional nickel resources. In view of the absence of a nickel recovery method from awaruite resources, UBC hydrometallurgy laboratory developed an in-house hydrometallurgical technology of atmospheric leaching in ammonium-citrate-thiosulfate solution. Ammonium selectively dissolves nickel in the presence of oxygen while citrate acts as a ferric chelator and thiosulfate a depassivation agent of nickel-iron alloy. In order to prepare the technology for pilot testing, in-depth batch leaching tests as well as preliminary solvent extraction tests were conducted and reported in this study. The leaching tests on an alternative source had an optimized result of ~85% nickel extraction. The nickel leaching reactions were believed under surface reaction control as indicated by the estimated activation energy of 54.4kJ/mol through kinetic model fitting of leaching test data. However, the leaching tests with high pulp density (>10%) and recycled liquor simulating industrial settings demonstrated hindered nickel extraction due to iron accumulation. Specifically, a targeted comparison test showed that in order to retain 80% nickel extraction at 30% pulp density, simultaneous increases to four times the original ammonia concentration and six times the original citrate concentration are required in comparison with the 10% pulp density baseline test. Therefore, this technology needs to be improved on iron removal and economical leach reagents recycle for commercialization. In the solvent extraction study, ACORGA K2000 as an ideal reagent candidate based on hydroxyoxime was tested on synthetic PLS containing 1 g/L Nickel (II), 0 – 0.8 g/L Fe (III), 0.1 – 0.5 M citrate and 1.5 – 6 M total ammonia. Hydroxyoxime has been proven effective in separating base metals from ammoniacal media. Results revealed that complete separation of nickel from iron was achieved at 5 v/o reagent solution concentration. Nickel can also be readily eluted with diluted sulfuric acid at pH = 1. The extraction kinetics was found to be relatively slow with 30 min of mixing/settling time required to reach equilibrium.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat