Analysing the influence of compressed air pressure on gold production

Abstract

Unreliable energy provision in South-Africa led to various high energy-consuming entities searching for new ways to optimise energy distribution. Due to the increasing need for energy saving initiatives on deep-level gold mines, scope exists to implement various optimisation techniques on high energy-consuming systems. One of the highest energy-consuming systems on deep-level gold mines is the compressed air network. Various studies have been conducted on the optimisation of a deep-level gold mines’ compressed air networks, however, most of the studies conducted primarily focused on the shaft’s compressed air network energy inefficiencies. Studies which include both optimisation of compressed air on the shaft and the plant are uncommon. A gold plant requires compressed air mainly for instrumentation operation and for agitation purposes in the leaching tanks. A shared compressed air network between the shaft and the plant usually limits potential for electrical cost savings through control of the pressure setpoints at the shaft throughout the day. The plant receives fluctuating pressures from the shaft compressor house due to its dynamic nature. Fluctuating pressure delivery to agitation leaching tanks may lead to settling of ore within the leaching tanks. The settled ore contains unprocessed amounts of gold which do not progress further than the leaching tanks and is required to be reprocessed with the next batch. A hypothesis was drawn up which states an increased and non-fluctuating pressure delivery to leaching tanks would have a positive effect on residue quantities in the leaching tanks. The goal of the study is, therefore, to establish whether an increased, non-fluctuating pressure delivery will have a positive effect on the residue quantities in leaching tanks and gold production totals. In addition to potential benefits on the plant, electrical cost savings scope increases significantly on the shaft. A study conducted on mine A showed the compressed air network was operating inefficiently. The plant used the air supplied from the shaft compressor house for agitation purposes in the leaching tanks. The delivered air pressure fluctuated greatly and may affect the agitation potential of the leaching tanks. The plant’s instrumentation operated on one 90 kW stand-alone compressor. The shaft had to adjust the compressors’ setpoint according to the demand needed for plant operation. This hindered lowering the overall setpoint during shaft off-peak mining hours when drilling did not occur. The compressed air network of mine A was thus severely over budget and created a need to investigate different methods of optimisation where both the shaft and the plant could accumulate energy savings and possibly production increases. Various studies focusing on production enhancement techniques were analysed but very few focused on the influence of compressed air agitation pressure on the performance of leaching tanks. A gap in the literature was thus identified. One of the solutions identified was to install a 315kW stand-alone compressor on the plant. The compressor installation enabled the plant to operate their compressed air network independently from the shaft, and adjust the pressures needed for agitation and instrumentation operation. The 90 kW compressor used for instrumentation operation was thus switched off. The main optimisation benefit on the plant’s side is concluded as non-fluctuating pressures being delivered to the leaching tanks, which may lower the gold residue quantities trapped in the leaching tanks due to insufficient agitation. In addition to a decrease in gold residue quantity within leaching tanks, and elimination of fluctuating pressures delivered to the plant, electrical cost savings on both the shaft and plant was achieved. The electrical cost savings on the shaft is calculated as R 3.3 million per annum, while the monetary value of the residue quantity decrease is quantified as R 2.43 million during the 28-day analysis period immediately after compressor installation during the 2018/2019 financial year. An overall verification of whether the consequence of pressure delivery change to the plant is solely responsible for the residue decrease could not be made due to various important parameters not being kept constant (due to the industrial-scale of the plant) throughout the study.