Recovery of valuable minerals from acid mine drainage using thermally activated cryptocrystalline magnesite

Abstract

Acid mine drainage (AMD) is a biorecalcitrant and toxic wastewater matrix that causes serious environmental, ecotoxicological, carcinogenic, and socio-economic stresses. It is typically encountered in countries with strong mining industry, including South Africa, the USA, Canada, and China. In recent decades, acid mine drainage (AMD) has been a topical issue of prime concern, primarily due to the magnitude of its environmental, ecotoxicological, health,and socio-economic impacts. Mining activities are notorious for their environmental footprint and AMD is the acidic water effluent that typically originates from metal (primarily gold) and coal mines. The problem persists in numerous countries with a strong mining industry. Even though AMD has a low to very low (acid) pH, it also contains dissolved toxic metals (e.g. copper and iron), both causing detrimental effects to receiving aquatic ecosystems. Practical and cost-effective solutions for the prevention of negative impacts and particularly for its treatment at large scale has yet to be introduced. Traditionally, active (driven by frequent input of chemicals, energy, and equipment) and passive (typically based on oxidation or reduction) treatment technologies have been employed. Active systems tend to be more efficient than passive ones in contaminants removal; however, at the expense of process complexity, cost, and energy consumption. More recently, and under the circular economy concept, the recovery of valuable minerals from AMD has been a topical issue of prime interest particularly for mine dominated regions that produce highly concentrated AMD. In light of the above, AMD in South Africa is rich in elevated levels of Al, Fe, Mn and sulphate amongst miniscule levels of heavy metals, metalloids, rare earth metals and radionuclides.

This study was developed with the aim of using activated magnesite for the recovery of Al, Fe, Mn, and sulphate from AMD. Batch experimental procedures were used to fulfil the goals of this study, specifically the one factor at a time (AFAAT) whereby one factor was varied with the rest fixed. The optimised parameters include contact time, and feed stock dosage on the recovery of valuable minerals. The metals were recovered through sequential and fractional precipitation at varying pH gradients. The obtained results were analysed using the state of the art analytical techniques, such as Inductively coupled plasma mass spectrometry (ICP-MS), Gallery™ Plus Discrete Analyzer photo spectrometer, HANNA Multi-parameter probe, Inductively coupled plasma - optical emission spectrometry (ICP-OES), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray fluorescence (XRF), which were used along with a high resolution scanning electron microscope (HR-SEM) coupled with energy dispersive X-Ray spectroscopy (EDS). The pH Redox Equilibrium (in C language) (PHREEQC) was used to substantiate aqueous and solids experimental results. The experiments were conducted in triplicate and results were reported as mean values. The samples were analysed in an ISO/IEC 17025:2017 accredited laboratory, i.e., at Magalies Water Services Laboratory, Brits, North West, South Africa.

Findings from this study confirmed the feasibility of recovering Al, Fe, Mn and sulphate from real AMD and their relativity to solution pH. PHREEQC confirmed that prevalent species existed as di-valents, tri-valents, and oxyanions in aqueous solution. The sensitivity study registered Fe to have been recovered at pH ≥ 3 - 3.5, gypsum at pH ≥ 4 - 9, Al at pH ≥ 6.5, Mn at pH ≥ 9.5, Cu at pH ≥ 7, Zn at pH ≥ 8, Pb at pH ≥ 8 and Ni at pH ≥ 9. Essentially, greater than 99% removal efficiency was achieved for all the metals (Al Fe and Mn) for given pH regimes except for sulphate that attained 40% removal efficacy. Other metals such as Zn, Cu, Ni, and Pb were also removed from AMD. The reduction of chemical species in the treated AMD further confirmed the attenuation of chemical species. This corroborated the sludge results. The experimental results corroborated the geochemical modelling and characterisation results. The fate of chemical species were verified using a synergy of HR-SEM equipped with EDS and PHREEQC amongst other analytical techniques. Lastly, but not least, the PHREEQC geochemical model suggested that chemical species were removed as (oxy)-hydroxides, (oxy)- hydro-sulphates, and metals hydroxides. Ca and SO₄²- were removed as gypsum (CaSO₄·2H₂O) and Pb, Fe, and Al as hydroxyl sulphate minerals. Mg formed a complex of MgSO4 in solution and it was removed as Mg(OH)2 at pH ≥ 9.5. Overall, this study proved the pragmatism and feasibility of recovering valuable minerals from AMD. Finally, feasible and practical avenues for minerals harvesting from AMD for potential valorisation and beneficiation were confirmed and scientifically proven. Future research studies should focus on the enhancement of the purity of minerals recovered from AMD and the beneficiation of the product water for drinking purposes.