A techno-economic evaluation of the production of hard coking coal from Tshikondeni coal discards
Powell, David Mark
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Tshikondeni Coal, operated by Exxaro Resources Limited on behalf of the owners, ArcelorMittal South Africa, is currently the only operational hard coking coal mine in South Africa. Therefore the yield from the plant needs to be maximised at the correct product specification, whilst losses to discard prevented as far as possible. The entire plant product is supplied to the ArcelorMittal South Africa’s Van Der Bijl Park works. Any shortfall in production is made up for in costly imports from either Australia or New Zealand. This thesis describes research work undertaken to: 1) Quantify the losses of product to discard of the process plant in terms of the dense medium cyclone and flotation circuits by efficiency testing, sizing analyses and discard washabilities. It was determined that the major contributors to the losses were in the -3mm size fraction for the dense medium cyclone and +1.0mm size fraction in the flotation circuit. 2) An extensive borehole drilling campaign was undertaken on the discard dumps on the mine to characterize the material on the dumps. 3) Samples were taken from the arising discard as produced by the normal operation of the plant and comprised of routine monthly discard samples, i.e. 1 tonne bulk samples from each of the operating shafts. 4) For the samples collected in 2) and 3) above, each were examined in detail by the application of appropriate technologies to effect upgrading by destoning, followed by milling and froth flotation. Solid-liquid separation and briquetting were also briefly examined. It was found that the discard could be effectively destoned by two potential methods, firstly by dense medium separation alone, or secondly by removal of the +8mm material by screening followed by dense medium separation. Thereafter the material could be milled to -212μm prior to froth flotation to produce the final product. A technical evaluation methodology was developed during the project which can be directly used on any given coal discards dump for future dump retreatment studies using the proposed stepwise approach. The final yields obtained for the dump borehole samples after froth flotation were noted to be low at approximately 15% for a product specification of 14% ash content. The yields obtained on the monthly composite samples were found to be worse with yields of approximately 6% to 11% being obtained. A plant design, based upon the application of the proposed technical evaluation methodology, was developed by using the main plant once ROM sources had been depleted and modified to include milling as well as the addition of a filter press and briquetting plant. A financial evaluation was performed in accordance with the Exxaro Project Evaluation Tool developed by the Corporate Finance Department. This revealed that a positive net present value (NPV) was achieved and that both the internal rate of return and the modified internal rate of return (which allows for risk within the project technically and also the project phase) were well above the hurdle rate set. Moreover, the payback period was found to be just under 2 years relative to a project life of 10 years. Simplification of the flowsheet to allow for only destoning using screening and dense medium separation also produced favourable results at similar yield of approximately 10% and the relatively minor modifications to the plant would require marginally over 1 year for payback. Yield in this instance may also be improved upon by optimization of the flotation circuit to process the fines fraction Unfortunately the project was not pursued further due to lack of strategic fit for both Exxaro and ArcelorMittal South Africa. However, the dump could be considered for sale to a third party for implementation of the project. Given the lack of industrial development, and consequentially the high unemployment rate, in this remote and rural area of South Africa, the latter could make for a good local economic development project for the local community. An alternative approach was also considered in parallel to the above work: could the plant be reconfigured to improve the yields? Five alternative plant circuits were examined and it was found that the overall yield could be improved by between 3% and 4%. This section of the work was scheduled to proceed to a bankable feasibility study level including the detailed design, but was ultimately stopped due to the global economic down turn which began in 2008. During 2015 the mine will enter the mine closure phase and the dump will be rehabilitated according to the mine closure plan, which requires covering in topsoil and grassing. This will maintain the integrity of the material on the dump and allow for future processing of the dump should the project be reconsidered.
- Engineering