Reclamation of water and the synthesis of gypsum and limestone from acid mine drainage treatment process using a combination of pre-treated magnesite nanosheets, lime, and CO2 bubbling

Abstract

In this study, an integration of pre-treated magnesite, lime, and CO2 bubbling (MLC) was used for the treatment of acid mine drainage (AMD). The primary aim was to reclaim clean water and synthesize valuable minerals. This treatment process comprises three steps which include neutralisation (i) using magnesite, gypsum synthesis (ii) using lime and limestone synthesis (iii) using CO2 bubbling. Reactors at a semi-pilot scale system were used to fulfil the goals of this study. AMD was mixed with magnesite and lime at 1 g: 100 mL S/L and 8 g: 100 mL S/L ratios respectively. Pilot results revealed that amorphous hydroxides of Fe, gypsum, and limestone can be obtained from the secondary sludge/product. The obtained materials were of high purity (>75%). This was further confirmed by X-ray Diffraction, X-ray Fluorescence, and Fourier Transform Infrared Spectrometer analytical techniques. The product water was suitable for irrigation, industrial and agricultural use as per South African standards. Furthermore, it was observed that the initial pH of AMD was 2.5 and it was increased to pH ≥ 10 and >12 after contacting magnesite and lime respectively. To stabilise the pH, CO2 was bubbled and the pH was reduced to ≤ 7.29 which was suitable for a number of applications. Moreover, ≥ 99% and ≥ 95% of metal species and sulphate were removed from an aqueous system, respectively. The techno-economic evaluation indicated that it can cost R806.40 (66 USD) to treat 3.5 KL of acid mine drainage and have a return of R11263.60 (933 USD) from the selling of the recovered materials, thus making this technology economically viable. From the findings of this study, it can be concluded that the application of MLC process can neutralise AMD and produce valuable products. More so, this novel and self-sustainable project will therefore go a long way in curtailing the impacts of AMD by valorising the product minerals and exploit the resultant commercial value hence aiding in off-setting the running costs of the treatment process. Keywords: Acid mine drainage, Magnesite, Gypsum, Limestone, Carbon dioxide