Hydrogeochemical assessment, water treatment and revalorization of dumps, tailings and drainages produced at Phalaborwa Industrial Complex
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The Phalaborwa Industrial Complex is formed by several mines and factories that extract value out of the geological formation named Phalaborwa (or Palabora) Igneous Complex (PIC). The industrial activity started in 1950’s with the extraction of phosphate rocks and Cu. Since then, more than 4500 Mt of solid waste enriched in magnetite, Zr, Ni, Au, Ag, Pt and rare earth elements (REE), the latter unexplored yet, have been accumulating in PIC area in the form of tailings and waste rock dumps, as well as above 3 Mm3 of industrial wastewater, including rock drainages and process water, as described in chapter 1. Due to the industrial activities, the water quality of the aquifers underneath PIC has been deteriorated reaching up to 10 g/L of sulphate, particularly surrounding the impoundment dams of the phosphoric acid plant from the fertilizer industry. Chapter 2 introduces the study area and addresses the groundwater quality at PIC and the efforts to restrain the contamination plume by using abstraction boreholes, which resulted in a continuous rise of pollution within PIC facilities but it helped to control the migration of the plume beyond the industrial area. In addition, chapter 3 describes the state of the art in passive water treatments that might deal with such water pollution. However, this study goes beyond the environmental assessment, it is a comprehensive evaluation of PIC’s wastes, which led to the revalorization of the mining wastes as potential REE resources and as neutralization reagent and culminated with the design of a system that would bring benefit from both characteristics of the wastes. Chapter 4 encompasses a mineralogical and geochemical study of the PIC’s mine wastes that assessed all the waste rock dumps and tailings. The study was conducted under the hypothesis that the abundance of REE from the ore and host rocks mined from PIC might be preserved or even enriched in the mining wastes. The abundance of REE minerals (mainly monazite) and REE-bearing minerals (mainly fluorapatite, calcite and dolomite) confirmed that hypothesis and suggests the economic potential of PIC wastes as secondary source of REE. The most profitable REE are Nd, Dy, Pr and Tb (87% of net value). The tailings are economically more attractive than the WRDs because the mineral processing has generated tailings of mostly monomineralic particles enriched in REE. The environmental characterization of PIC wastes, described in chapter 5, was carried out in order to evaluate its potential as neutralizing reagent for passive water treatment. The mining waste used to treat acid industrial wastewater (AIW) need to accomplish two main characteristics i) high neutralization potential and ii) low toxicity. National and international procedures were carried out to assess the neutralization potential and the toxicity of the leachate that could be released from each rock and each tailing. The results of this investigation showed that none of the PIC rocks have the potential to produce acid rock drainage. It also demonstrated that the carbonatite rocks and the tailings from the copper plant (herein named East tailing) exhibit the highest neutralization potential (up to 800 kg CaCO3 eq/t). According to the National Environmental Management Waste Act (59/2008) of South Africa, PIC wastes classify as Type 3 waste (non-hazardous). PIC wastes would mostly release non-toxic elements such as Ca, Mg, SO4, Na, P, K and Fe. Although there are radionuclides such as U and Th in the non-labile fraction of PIC wastes, leachable concentrations were always below 0.006 mg/L. Among PIC wastes, East tailing would the best option as alkaline reagent to neutralize AIW because of its neutralization potential and non-harmful leachate composition predicted. The knowledge acquired at this point of the investigation served to develop a system that could remediate the extremely acidic wastewater from the neighbouring phosphoric acid plant. This system would be a near-zero waste if the substrate used get enriched in REE and become a marketable by-product. In chapter 6, the material from East tailing was selected for its abundance of REE minerals and REEbearing minerals, as well as for its neutralization potential (reactor A). A BDAS reactor (Barium carbonate Dispersed Alkaline Substrate) was added to the system to reduce the hardness and to further improve the water quality (reactor B). The system, developed at bench scale, was able to remediate the extremely AIW from the phosphoric acid plant and, at the same time, to concentrate the REE contained in both the water and the tailing. The treated water complies with WHO (World Health Organization) guideline for drinking water for all the parameters except Ni, Cd and occasionally Ba. Mineralogical and geochemical analyses showed that the REE concentration increased from the initial1.3 g/kg up to 2.1 g/kg in the central area of reactor A. Most REE precipitated as newly formed REE-rich Ca-Al-F phosphate. Minor concentrations of REE were found in reactor B, together with most of the radionuclides. Altogether, the findings of the thesis bring to the table an eco-friendly and sustainable alternative to concentrate REE in a circular economy approach, while improving the quality of the extremely acidic wastewater. Therefore, the implementation of this system in PIC would have positive impacts to both the economy and the environment of Phalaborwa and the surroundings. This approach to an environmental problem caused by industries could be extrapolated to other carbonatite deposits in South Africa and abroad as a feasible environmental solution with little to none economic imbursement implications. Further feasibility studies of the marketable substrate are recommended in order to estimate if the remediation of acid water using East tailing could be a profitable and sustainable activity.