Optimization of high-density sludge process for maximum value recovery

Abstract

The Republic of South Africa is one of the countries in the world that makes extensive use of its natural resources. Mining operators who are in charge of these heavy mining activities may abandon their mining sites when the mineral resources are depleted without following proper government procedures for mine closure. In this line of investigation, the abandoned pyritic rock tailings (in the abandoned mine) when exposed to water and air form a waste material known as acid mine drainage. The hazardous waste generated by mining activities is not limited to South Africa but is a global environmental issue. The South African government's policy for mining industries in the country encourages maximum exploitation of natural resources with near-zero waste discharge to the environment. The application of this policy entails the treatment of acid mine drainage (AMD), which is currently proving difficult due to the high costs associated with it. This challenge inspired this research, which aims to obtain a value-added material (nano-iron) from AMD and significantly cut down the amount of lime used for the neutralization of the pre-treated AMD as opposed to direct neutralization, thereby offsetting treatment costs. The field AMD samples used for beneficiation in this study were collected in South Africa and characterized both physically and chemically to determine the quality of the solutions. The physical characterization of the AMD samples revealed low pH, high electronic conductivity (EC), and total dissolved solids (TDS), whereas the chemical characterization revealed high dissolved metals and elevated sulphate concentration. These dissolved metals are toxic and include Fe, Ca, Mg, Al, Mn, and others. The complex chemical composition of acid mine drainage has exacerbated the problem of AMD treatment. Different approaches have been used to treat AMD, which could produce good-quality water even if it is unfit for human consumption. Chemical treatment is the most common and appropriate method for treating AMD, but it generates a large amount of sludge, which adds to treatment costs. The South African mine water used for this research was collected from three mining sites of the Navigation coal mine in Mpumalanga Province: Penstock (P), Toeseep (T), and Kopseer (K). These AMD samples were elementally analyzed to determine if they were iron-rich AMD samples. The pH values of these AMD samples were P=2.14±0.02, T=2.20±0.03, and K=2.08±0.02 respectively. The AMD samples were classified as acidic with poor water quality by TWQR guidelines for potable water. The IC and ICP analyses revealed that all three mine water samples (P, T, or K) contained SO2-4, Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, Na, Mg, Mn, Mo, Ni, Pb, Se, Be, B, Li, Hg, P, Sb, Si, Sr, Th, Y, and Zn in varying concentrations (see Table 4.6). As a reminder, the motivation for this study stemmed from the high cost of treating AMD, which contains toxic elements and has an acidic pH. Although the synthesis of nano-iron from AMD using an unfriendly chemical reductant (sodium borohydride) has been reported in the literature, the synthesis of nano-iron particles from AMD using a chemical and an environmentally friendly reductant (green tea extract) prior to lime neutralization of their corresponding supernatant has not been reported. The production of nano-iron from AMD prior to lime neutralization reduces the amount of lime used to neutralize AMD by nearly half, lowering its treatment costs. The aim of this study was achieved by first selecting which iron-rich field AMD sample from the three characterized Navigation coal mine AMD solutions was going to be utilized in this study. The choice was made due to the availability of the iron-rich AMD sample as well as economic considerations. In this case, the Penstock AMD was chosen as the iron-rich AMD sample used for this study. The production of nano-iron from the Penstock AMD solution using a chemical reductant (sodium borohydride) was optimized using time and concentration, whereas the production of nano-iron from AMD using an environmentally friendly reductant (green tea extract) was optimized using mass of extract, temperature, and time. Following that, the pre-treated AMD solutions were characterized using ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS) prior to lime neutralization. The pre-treated AMD neutralization process was optimized by varying the amount of lime used to achieve a specific pH and gypsum product. In summary, the first phase of this study was carried out to remove iron as nano-iron from the AMD solution, and the second phase was carried out to neutralize the pre-treated AMD solution with lime. This experimental approach was devised in such a way that the traditional sludge generation process during AMD liming could be avoided, and the mass of lime used for the neutralization process could be cut in half compared to direct neutralization of the AMD solution, resulting in cost savings. In order to identify the nano-iron product synthesized from AMD using reductants, the following characterization was done X-ray Diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM/EDS), scanning transmission microscopy (STEM), and Fourier transform infrared (FTIR). The pre-treated AMD neutralization process was optimized by varying the amount of lime used to achieve a specific pH and gypsum product. In summary, the first phase of this study was carried out to remove iron from the AMD solution, and the second phase was carried out to neutralize the pre-treated AMD, which was characterized using X-ray fluorescence (XRF) to determine the level of purity of the gypsum product. However, when neutralizing sodium borohydride pre-treated AMD with lime, the solid product contained more calcite than gypsum, and when neutralizing green tea pre-treated AMD with lime, the solid product contained more gypsum than calcite. The nano-iron products synthesized from AMD were applied as solid catalysts in the decolourization of methylene blue. The ultraviolet visible (UV-vis) analytical technique was used to characterize the decolorized methylene blue solution.