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Investigation of the potential beneficiation of laterite leaching residues

Politis Georgios

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Summary

In the present thesis, attempts were made to beneficiate a laterite leaching residue through reduction roasting and wet magnetic separation. This residue obtained after column leaching tests used to recover Ni, Co and other useful metals, using sulfuric acid (H2SO4) as the leaching solution. It is known that the (by)products derived from mining and metallurgical processes are disposed of in the environment causing environmental pollution and significant effects on human health, so their valorization either through the production of building materials with beneficial properties or through the recovery of useful metals from them is considered necessary. Also, the reducing agents (e.g. coal) used in pyrometallurgical processes produce large amounts of CO2, SO2, while significant amounts of energy are consumed for their production. In this regard, in the present work, the reduction roasting process was carried out using either carbon or biochar derived from the pyrolysis of olive tree pruning wastes. Biochar is considered an alternative to carbon, as it contains a lower content of harmful elements (e.g. P, S, etc.).According to the experimental procedure, the Kopaida laterite leaching residue was first crushed, and then ground to obtain a suitable particle size for the beneficiation tests. The chemical analysis of the laterite residue showed that the nickel content was 0.30 %, while the iron and aluminium contents were 23 % and 2 %, respectively. The mineralogical analysis revealed that the main mineral phases present in the residue were quartz [SiO2], hematite [Fe2O3] and goethite [FeO(OH)], while the main nickel bearing mineral was willemseite [(Ni, Mg)3Si4O10(OH)2]. The reduction roasting was performed at specific residence times (30 min, 60 min, 90 min) and temperatures (800 °C, 1000 °C, 1200 °C) using different amounts of reducing agent (1.5 g and 3 g). As previously mentioned, the reducing agents used were activated charcoal and olive tree pruning-based biochar. The results showed that the optimum conditions for the reduction roasting process were: roasting temperature of 1200 °C, residence time of 90 min and reducing agent of 1.5 g. Under these conditions, the % contents of metallic iron [Fe], as well as the iron-based minerals hematite and magnetite increased, while the % contents of the non-ferrous minerals, such as quartz decreased. Then, magnetic separation tests were performed at current intensities of 0.2 A (one pass), 1 A and 2 A (two passes) for the samples 10C (10 g solids, 1.5 g charcoal) and 10B (10 g solids, 1.5 g biochar) corresponding to the optimal conditions (90 min / 1200 °C) to further study the potential beneficiation of the residue. The resulting magnetic separation products were subjected to chemical and mineralogical analyses. The chemical analyses of the magnetic separation products showed that for both reducing agents used, the % contents of iron and nickel increased in the magnetic product, while the % content of silicon increased in the non-magnetic product (one pass, 0.2 A current and two passes, 1 A and 2 A currents). Also, mineralogical analyses of the magnetic separation products with both reducing agents (charcoal, biochar) showed that the main mineralogical phases that appear are quartz [SiO2], cristobalite [SiO2], chromite [FeCr2O4] and fayalite [Fe2SiO4]. More specifically, for a current intensity of 0.2 A, the non-magnetic products contained a larger amount of cristobalite [SiO2] and fayalite [Fe2SiO4], while the magnetic products contained a larger amount of hematite [Fe2O3]. Also, metallic iron [Fe] appeared only in the magnetic products. In the two passes (1 A and 2 A), it was observed that the highly magnetic products contained a larger amount of chromite [FeCr2O4], hematite [Fe2O3] and metallic iron [Fe], the intermediate magnetic products contained a larger amount of fayalite [Fe2SiO4], while the non-magnetic products contained the highest amount of cristobalite [SiO2] and quartz [SiO2].

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