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Minimising the formation of acid rock drainage through management of the disposal of sulphidic waste rock and tailings
Expanded Title:The ideal approach to handling of sulphidic waste rock is to prevent the potential for generation of acid rock drainage (ARD) through the removal of the sulphide phase before its disposal, thus avoiding the need for long-term mitigation strategies. In the best case, this processing of waste rock prior to disposal should result in an increase in the recovery of values from the starting material and the re-allocation of waste materials as feedstocks for other uses. This report focussed on the goal of establishing feasible approaches for the prevention of the formation of ARD from mining wastes by the removal of the risk rather than its delay. Current ARD prevention strategies focus on covers and coatings. While these are effective, the life span of their effectiveness remains in question. The potential for the removal of sulphides from tailings was demonstrated in WRC research report 1831/1/11, using tailings and waste rock from a base metal operation as a model system. The removal of sulphide by both separation and by reaction was demonstrated for the tailings and waste rock respectively, with the former showing the most promise. In this report, the general application of sulphide removal by separation to reduce the risk of ARD generation is presented across tailings and finely divided mineral wastes from various sources, including a variety of coal fines and tailings from the gold industry. Demonstration of the removal of sulphide is presented with the associated reduction in potential for ARD generation. Further, the cost implications and disposal routes for the sulphide and benign fractions removed are considered. With respect to removal by reaction, accelerated bioleaching has, to date, shown limited value; however new approaches, using a broader spectrum of reaction systems, have been highlighted for further study. To extend this work, it is necessary to recognise the need for both acceleration of the reaction of sulphide under controlled conditions as well as deceleration or closing off of these reactions by restricting the supply of reactants through restricting access of water, oxygen, leach agents as well as microbial colonisation. The closing off of these reactions may be a treatment in itself or may follow the accelerated reaction to remove readily reactive sulphur. The role in waste rock dump characteristics, including permeability, as a means to manipulate this is addressed. An experimental study was conducted to establish methodology and provide proof of concept of the ‘mingling’ approach through analysis of flow. Here, permeability decreased, residence time increased, and degree of saturation increased with decreasing particle size for uniform sized particles. Most importantly, the model system demonstrated that, by including a fraction of fine particles with the waste rock material in the dump, permeability can be manipulated and flow restricted, thereby restricting access of reactants to the reactive sulphide minerals present. A substantial reduction in permeability resulted on homogeneous mixing of large waste rock particles with fines; however, the layering of fines (<2 mm) and waste rock provided a greater reduction. Leaching of a low grade base metal ore in such layers was compared to standard leach conditions using agglomerated ore and illustrated that the leach rate, under forced leaching conditions, was slowed, but not eradicated. Investigation of the co-mingling of waste coal with a benign fine tailings sample is recommended; however it was not possible to source the appropriate inter-burden sample, with samples received being of inappropriate composition. The impact of flow rate on colonisation and leaching was also studied experimentally. Here it was shown that rapid irrigation rates result in greater flow of micro-organisms through the bed while reduced irrigation rates encourage microbial colonisation of the waste rock bed. Preliminary data on associated leach rates of low grade ore with irrigation rate are available. Further irrigation of the waste rock bed from a single point demonstrated that regions of differing moisture content develop. In ‘drier’ regions, with moisture content of around 7%, colonisation was not detected with methods used (<103 cells per kg), whereas in higher moisture zones, of around 10%, colonisation was significant (109 – 1011 cells per kg). This illustrates the importance of restricting moisture content for avoidance of ARD formation. Recent studies on the removal of sulphide from tailings by separation were reviewed. This demonstrated that significant strides forward, independent of this study, have not been reported in the open literature since the publication of WRC 1831/1/11. In this study, we have extended the sulphide removal studies to a range of finely divided materials, including the tailings from the preparation of pyritic gold ore concentrate, the BIOX® tailings, and coal fines. In addition to demonstrating the preparation of the bulk of the material for disposal such that the ARD generation potential is small, potential uses for the residues are considered, as is the process costing. Using a series of five samples of coal fines, the proof of concept was demonstrated of sulphide removal from the bulk of the waste tailings by separation, in this case flotation, in order to eliminate or decrease ARD generation potential. Further, it was demonstrated that biodegradable oleic acid was an excellent collector, yielding improved performance over dodecane. A similar study on tailings from the gold industry has illustrated mixed results. Using flotation to upgrade the tailings from the laboratory bioleaching of pyritic gold concentrates has been successful with decreased sulphide grades reporting to the bulk tailings. Here ARD generation potential was reduced but not eliminated as with other mineral systems. On treatment of the tailings (collected from the tailings dam) from the concentrator circuit by further flotation, no significant upgrading was achieved; however the sulphide associated with the solid tailings was already low. The study of gold tailings samples was limited by those samples attainable. It is recommended that further representative samples be sourced for study to further assess the generalised nature of the findings. Potential uses for both the sulphide-rich and sulphide lean tailings samples have been identified and illustrate a range of potential applications. For the use of the sulphide rich stream, consideration is required for the quality of material needed as well as the impact of associated gangue fractions. To address this, the development of flowsheets for handling these streams and a system for their rating in selection of use is underway and will contribute to the project emanating from WRC proposal 1003088. In order to establish a framework for the economic costing of the sulphide removal from tailings and fine waste materials, a flotation desulphurisation flowsheet has been proposed to follow the traditional coal processing flowsheet. This has been used as the basis for an order of magnitude estimate of a new fine coal desulphurisation plant using flotation and the performance estimated in Section 5. Based on assumptions specified, this preliminary costing has suggested potential for economic viability. A sensitivity analysis is presented which targets, among others, value of the resource recovered, yield and reagent costs as key considerations for optimisation of the approach.
Date Published:01/02/2013
Document Type:Research Report
Document Subjects:Mine water - Mine water treatment
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:2015/1/13
ISBN No:978-1-4312-0461-8
Authors:Harrison STL; Franzidis JP; Van Hille RP; Mokone T; Broadhurst JL; Mbamba CK; Opitz A; Chiume R; Vries E; Stander HM; Jera M
Project No:K5/2015
Organizations:University of Cape Town
Document Size:3 448 KB
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