A technical and economic appraisal for optimal exploitation of the H sub-incline complex at Nchanga underground mine
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Nchanga Underground mine uses a block caving variant to extract the lower ore body of the Nchanga syncline. The deepest part of the ore body, the H Sub-Incline complex, is thin with 4.39m average thickness, 5.27% weighted average grade of Copper and hosts 1,219,314t in-situ. Operational challenges were being faced including early dilution discharge and pre-mature collapse of secondary developments. In addition, the tramming equipment was failing to meet planned production targets by discrepancies of 35.8% for availability, 25.4% for utilisation and 63.3% for trammed ore. Caved mass evaluations showed that inclined undercutting and a 63.5o angle of draw based on friction angle of the ore zone are suitable for exploiting the complex. Total dilution entry point tonnage of 774,553t was computed for the ore body so as to determine the tonnage which will be drawn before diluted material starts discharging into scraper drifts. Maximum buffer reserve tonnage between development and production was calculated as 192,553t after considering a block recovery factor of 140%, minimum stand-up time for secondary developments (i.e. 0.93years) and annual production rate of 308,500t per year. The calculated buffer reserves will save Konkola Copper Mines $173,231.46 in support costs for developments which are outside the just-in-time range. Additionally, sub systems of dump trucks which had the most impact on loss of machine availability were identified by reliability analysis so as to propose maintenance intervals which coincide with 75% on each reliability curve for the respective trucks. Following this, it was observed that mean time-to-repair of the Sandvik EJC533 truck’s transmission was significantly higher at 82.80hrs than that of the other Atlas Copco trucks which averaged at 2.34hrs. It was inferred that this difference was caused by a lack of proficiency to maintain and repair Sandvik EJC533’s transmission. Sending artisans to Sandvik for transmission debugging and diagnosis training is expected to reduce repair time and improve reliability. Correspondingly, preliminary investigation showed that Longwall and Cut-and-Fill stoping are the technically superior methods to exploit the complex. Longwall mining was selected as the most appropriate method for extracting future reserves of the complex after a review of documentation on these methods was done using the criteria of operating costs, ore recoveries and productivity per man hour shift. The study established that extracting the complex will increase the life-of-mine by at least 2.42 years, yielding net discounted earnings of $63,006,898.32 excluding interests, taxes and depreciation.
The University of Zambia