Extraction of major elements from PGE tailings in view of nanoparticle synthesis for environmental technological applications

Abstract

Mining operations associated with the primary production of PGE’s in South Africa have generated and continue generating large quantities of mineral waste and by-products, for instance in the form of tailings. Mine tailings (MT) have been identified as a major source of environmental impact for many mining operations and changes in legislation are enforcing more stringent requirements with regards to the management of tailings storage facilities (TSF). Mineral beneficiation of PGE’s has become a key focus area in South Africa to ensure sustainable growth of the economy beyond mining. This initiative could potentially be expanded to include associated mineral waste products which could create a suitable platform to minimize the volume of tailings stored in a TSF and reduce associated environmental impact. Magnesium-rich PGE mine tailings have been previously assessed for mineral carbonation technologies, although researched processes were unsuccessful in extracting significant amounts of magnesium. PGE tailings also contain significant amounts of other major elements (e.g. Al, Fe).Therefore, they offer the potential to serve as secondary mineral sources which can be reprocessed and converted into value-added products, provided that a suitable economically-viable technological process can be developed.

This study demonstrated that major elements contained in PGE mine tailings (e.g. Fe, Al, Mg, Cr) can be extracted using a suitable technological process. A multi-stage process was developed, which involved a thermochemical solid solid treatment using ammonium-based reagents as activators followed by aqueous dissolution. Potential uses for the major elements have been identified, such as the conversion of Fe and Al to nanoparticles, with potential applications in contaminated water remediation.

The variability of the bulk elemental compositions and mineralogy of the tailings was expected to have a direct effect on the elemental extraction efficiency from PGE tailings. The Two Rivers tailings sample, obtained from the eastern limb of the BIC was used as a first case study to establish the experimental design and develop the process. The Two Rivers tailings sample is typically fine grained and relatively enriched in Fe, Mg, Al, and Cr. The primary minerals present are generally unaltered, comprising of spinel, plagioclase, enstatite and amphibole mineral species. Preliminary studies have qualitatively evaluated the effect of particle size of the tailings and treatment temperature on the thermochemical solid-solid treatment using ammonium sulphate ((NH4)2SO4; AS). Results have shown that the transformation of the mineral phases into metal sulphate based compounds is dependent on tailings particle size and on treatment temperature. Subjecting the 45-75 µm tailings size fraction to the thermochemical treatment at temperatures below 500 °C favoured the formation of ammonium metal sulphate based compounds. At 550 °C it favoured the formation of metal sulphate based compounds. The reaction product generated at 550 °C was identified by XRD as mikasaite (Fe2(SO4)3). Morphological examination and elemental analysis indicated that the mikasite phase was made up of hexagonal structures which contained Fe, Al, Cr and Mg. This suggested that elements such as Fe, Al, Cr and Mg may compete for AS during thermochemical treatment. Since a temperature of 550 °C for the thermochemical treatment of the 45-75 µm fraction favored the formation of metal sulphates rather than ammonium metal sulphates, this temperature was selected for all future experiments.

The efficiency of the thermochemical treatment step could only be evaluated if dissolution step of the process was optimized in terms of temperature, pH and duration of dissolution. Elemental extraction efficiency for all elements in ultra-pure water improved significantly with an increase in temperature from ambient temperature to 80 °C; however under the latter conditions, elemental extraction was hindered by the secondary precipitation of Fe during the dissolution procedure, an observation which had not been made at ambient temperature. The dissolution procedure was further examined in 0.63M HNO3 in an attempt to prevent the secondary precipitation of Fe. Baseline dissolution experiments at ambient temperature confirmed the absence of secondary precipitation; however elemental extraction efficiencies for Al, Ca, Cr, Fe, and Mg remained relatively low. The effect of dissolution temperature on elemental extraction efficiency in 0.63M HNO3 was investigated by subjecting the thermochemically-treated MT (MTro) formed using the selected thermochemical treatment to the dissolution procedure at 80 °C from 30 minutes up to 17 hours. The percentage mass loss endured by MTro following dissolution indicated that the optimal dissolution probably occurred within 6 hours, after which further dissolution may be negligible. Higher elemental extraction efficiencies were obtained for Al (56 %), Cr (20 %), Fe (24 %), Mg (23 %) and Ca (79 %) within the first 6 hours of dissolution at 80 °C. The effect of dissolution temperature on elemental extraction efficiency was further investigated by subjecting MTro to the dissolution procedure at 60 °C and 95 °C. Higher elemental extraction efficiencies were achieved for Al (59 %), Cr (27 %) and Fe (35 %) except for Ca (80 %) and Mg (25%) which were similar after dissolution at 95 °C. Dissolution at 60 °C and 80 °C yielded much lower extraction efficiencies for Al (33 – 56 %), Fe (18 24 %), Mg (21 – 23 %), Ca (50 – 79 %) and Cr (12 – 20 %). The co-extraction of the major elements Fe, Cr, Al, Mg and Ca demonstrated their competitive chemical affinity for AS during thermochemical treatment, which constrained the preferential extraction of Fe. The optimum dissolution conditions established for maximum elemental extraction efficiency, particularly for Fe, was achieved using 0.6M HNO3 solution at a temperature of 95 °C for 6 hours. Using the optimal dissolution parameters, the effect of different ammonium salts (ammonium chloride (NH4Cl); ammonium nitrate (NH4NO3); ammonium bisulphate (NH4HSO4), ammonium sulphate (NH4)2SO4) and thermochemical treatment temperature on major elemental extraction efficiency was also investigated. (NH4)2SO4 proved to be the most suitable activator, yielding the highest elemental extraction for Al (59 %), Fe (35 %),Cr (27 %), Ca (79 %) and Mg (25 %) between 500 °C and 550 °C. NH4Cl, NH4NO3 and NH4HSO4 yielded lower elemental extraction efficiencies for all temperatures tested. However, NH4Cl and NH4NO3 were more selective for Ca at a lower thermochemical treatment temperature (450 °C) compared to all other elements yielding up to 79 % Ca extraction which was similarly achieved using (NH4)2SO4. In an attempt to investigate the effect of Fe:Cr ratio on the elemental extraction efficiency (case study 2), a series of tailings samples with varying mineralogy and Fe:Cr ratios were selected from the Amandelbult, Marico Chrome and Rhovan operations TSF’s located along the western limb of the BIC. The mineralogical composition of the different tailings are different and correlate to the local variations in mineralogy observed along the limb which changes from a harzburgitic rock-type in the north to a pyroxenetic rock type further south and, finally a noritic rock-type further southeast. The tailings comprise of varying proportions of spinel and plagioclase mineral phases, with minor amounts of alteration minerals (serpentine, talc, chlorite, and calcite). Experimental results indicated that Cr depleted mine tailings yielded higher Fe (ca. 72 %), Al (ca. 100 %), Ca (ca. 100 %) and Mg (ca. 40 %) extraction efficiencies in comparison to Cr-enriched tailings. The correlation between the elemental Fe:Cr ratio in the starting materials (i.e. MT45-75) and the one in solution following thermochemical treatment and optimal dissolution was assessed. In the main, the higher the Fe:Cr ratio in the starting material, the higher the Fe:Cr ratio in solution. The correlation followed a linear relationship (R2 ≈ 1) with slopes of about 1.5 and 2 for 350⁰C and 550⁰C respectively. This linear correlation was unexpected; given the complex mineralogical composition of the samples originating from different tailings generated from varying mineral deposits, and could not be explained at this stage. The result would suggest that Fe and Cr were co extracted from similar mineral phases. However, Fe and Cr are contained in different types of minerals for the different tailings. A detailed mineralogical investigation is required in order to better understand the extraction of these two elements from tailings. It can however be speculated that following thermochemical treatment, Fe and Cr are present in a number of similar sulphate based Fe-Cr solid solutions (which may also contain additional substituting (compatible) elements such as Mg and Al), which are then dissolved during the dissolution process. Results from this study have shown that the multi-stage process under development has the potential to extract major elements from PGE tailings. The best extraction efficiencies obtained for Fe, Al, Ca and Mg were 72 %, 100 %, 100 % and 40 % respectively, although those were not obtained for the same tailings or using the same activator. Further studies will be required to further enhance the elemental extraction efficiency of Fe and Al.