Chemical reactivity of Bushveld Igneous Complex mine residues for mineral beneficiation

Abstract

Background The mining sector in South Africa plays a crucial role in the country's economy, contributing approximately R 202.05 billion South African rand (around 11.18 billion U.S. dollars) to the country's Gross Domestic Product (GDP) in 2023. This sector is a significant contributor to economic activity, job creation and foreign investment. The Bushveld Igneous Complex (BIC), a large geological formation, hosts the world's largest known reserves of Platinum group metals (PGM), the country’s largest produced commodity. Mining in the BIC generated ca. 57% of the accumulated revenue from South African mining activities. Although mining contributes significantly to the economy, it also contributes significantly to the generation of toxic waste streams, including acid mine drainage (AMD) and large volumes of solid mine residues, such as mine rocks, tailings, and slimes. Managing these mine residues has become a major environmental concern for mining operations worldwide and has resulted in the implementation of stricter regulations to address the environmental and social risks associated with the disposal and management of these waste materials. Consequently, there is a constant demand for innovative and sustainable solutions, including developing valorisation technologies for recovering metals and minerals, which will assist in minimising their disposal and make the treatment processes more economically viable.

Mine residues from the BIC have been previously assessed as a secondary resource of major elements (e.g., Fe, Al) via a thermochemical solid-solid treatment using ammonium-based extracting agents followed by aqueous dissolution. This process has been considered for the extraction of elements and conversion into products for various applications such as mineral carbonation technologies and wastewater remediation. However, the valorisation of the residues into value-added products was not investigated. This study demonstrated that strategic elements such as Al, Ca, Cr, Fe and Si can be extracted from different BIC mine residues via extractive acid leaching, and via thermochemical treatment using ammonium sulphate (NH4)2SO4 as an extracting agent followed by acid leaching and converted into value added products such as calcium aluminate (CaAl2O4) and silica (SiO2) nanoparticles for various industrial applications. This study also examined the reactivity of specific mineral phases contained in BIC mine residues, such as Platinum Group Metal (PGM) tailings and mineral-rich concentrates and slimes, during thermochemical treatment and acid leaching processes to better understand their contributions to elemental extraction efficiency.

PGM tailings PGM tailings obtained from the Two Rivers mine were used to improve the previously conceptualised thermochemical and leaching process using ((NH4)2SO4) as an extracting agent. The effect of thermochemical treatment and duration was investigated, as well as the effectiveness of HNO3 vs H2SO4 as leaching agents. Lowering the thermochemical treatment temperature and increasing the duration from 550 ⁰C for 45 mins to 420 ⁰C for 6 h achieved a comparable extent of mineralogical transformation and could favour the possible scaling-up of the process and improve the overall extent of reagent recovery for potential reuse during the thermochemical treatment step. H2SO4 was found to be as suitable as HNO3 as a leachant, with no negative effect on elemental extraction efficiency. The use of commercial reagents for the synthesis of Fe nanoparticles was initially investigated and successfully yielded Fe nanoparticles with properties consistent with those reported in the literature. This method was then applied to the filtrates, obtained after thermochemical treatment using (NH4)2SO4 and acid leaching of the tailings, but proved unsuccessful due to the presence of Al, Cr, Mg and Si contaminants, which were co-extracted during the leaching step. This result led to the development of a selective separation scheme aimed at recovering Fe from the filtrates. Precipitation experiments indicated that co precipitation of Al, Cr, and Fe impeded the formation of magnetic Fe-based nanoparticles. These results identified the complexities involved in nanoparticle synthesis from mine tailings containing diverse mineral phases. Further investigation was conducted into the reactivity of major individual mineral phases, such as plagioclase, enstatite and hematite, during direct acid leaching compared to thermochemical treatment. Mineral-rich concentrates and slimes The reactivity of plagioclase-rich concentrates and slime, enstatite and hematite were assessed during thermochemical treatment using (NH4)2SO4 and a direct acid leaching processes. Plagioclase was found to have limited reactivity with (NH4)2SO4 when subjected to thermochemical treatment under the experimental conditions tested. Thermochemical treatment at 550 ⁰C for 45 mins improved the extraction of Al, Fe, Na and, to a lesser extent, Ca from the other mineral phases present in the plagioclase-rich concentrates. In comparison, direct acid leaching achieved high extraction efficiencies, particularly for Al, Ca and Na, which were derived from the plagioclase phase. Direct acid leaching in HNO3 at 95⁰C allowed for the complete leaching of the plagioclase concentrate. However, longer durations of up to 24 h were required to achieve stoichiometric leaching of elements into solution. The enstatite concentrates showed limited reactivity during thermochemical treatment with (NH4)2SO4, as indicated by similar extraction efficiencies obtained for Mg and Fe, which are the main elements present in enstatite. Direct acid leaching achieved minimal extraction of Fe and Ti from the hematite concentrate; however, the extent of reactivity of the hematite mineral during thermochemical treatment could not be ascertained due to the amorphosity of the concentrate. Based on these results, the focus shifted towards the use of the plagioclase-rich slime, which had increased reactivity, for conversion into value-added products.

Plagioclase-rich slime The Ca and Al-rich filtrates obtained after direct acid leaching of the plagioclase-rich slime were used to investigate for the initial synthesis of CaAl2O4 nanoparticles. The synthesis was conducted using the solution combustion synthesis method with urea and glycine as fuels. Characterisation of the resulting products indicated a product mixture comprising Ca, Al, Mg, Fe, and Si-rich oxides, spinel, and silicate phases. The presence of these multiple mineral phases confirmed the challenge related to the heterogeneity of the precursor solution, which contained Mg, Fe and Si, which were co-extracted from the plagioclase slime. This highlighted the need to further purify the filtrate to enhance homogeneity, particularly for the synthesis of CaAl2O4 products from a plagioclase-rich mine slime. To address this challenge, purification of the plagioclase slime filtrate was successfully achieved via a multistage process. This process entailed a calcination pre-treatment followed by direct acid leaching for elemental extraction. Subsequent precipitation and separation processes were used to produce SiO2 nanoparticles with high levels of purity. The remaining solutions were then used to synthesise Ca and Al rich products. Through these improvements, Ca and Al-based products were successfully synthesised. The most promising products contained only three calcium aluminate phases, which were much less compared to the number of phases identified in the other synthesised products. Results from this study have demonstrated the potential of elemental extraction and the sustainable synthesis of nanoparticles from a BIC mine residue. Future work will be required to improve the physical properties of the products. The economics of the process will also need to be considered to ensure its feasibility and long term sustainability.

Implications of this study The exploratory lab-scale study presented in this thesis demonstrates the potential utilisation of mine residues, particularly those from the BIC. The extraction of valuable elements and the synthesis of nanomaterials from these residues, could potentially offer a sustainable approach to mineral processing and waste management and could reduce environmental damage associated with traditional mining and tailings disposal, while also creating new value-added products. If these lab-scale processes can be scaled up to industrial level, the results could contribute to South Africa’s economic output. Secondary mineral extraction techniques could enhance the recovery of metals from tailings, increasing the overall yield of valuable resources from existing mining operations. Second, the synthesis of nanomaterials from mine residues could create new industries and products.