South Africa’s DRDGOLD has announced that it has temporarily suspended its recently constructed high grade section – a flotation circuit, a set of fine-grind mills and the cyanide leach and carbon in pulp (CIP) circuit – to determine the cause of the metallurgical problems that have led to lower than expected gold production for Q3 FY14. The company primarily derives its revenues from very large Ergo Mining recovery and reprocessing operations on the enormous tonnages of gold mine tailings left by some of South Africa’s now-closed down gold mining operations in and around Johannesburg.
Ergo (originally an acronym for East Rand Gold and Uranium) works and reprocesses a vast footprint of old tailings dumps adjacent to Johannesburg on the central and eastern Witwatersrand of South Africa. Its assets cover an area approximately 62km from east to west and 25km from north to south.
Ergo’s flagship metallurgical plant, some 50km east of Johannesburg in Brakpan, and the Knights plant in Germiston together comprise what is the world’s largest gold surface tailings retreatment facility. Together with the milling and pump station at the old Crown Mines and City Deep properties (both former gold plants), the new consolidated Ergo operation processes 2.0 – 2.1 million tonnes of low grade gold-bearing material a month.
The technical problem the principal operation is trying to get to grips with is that although the new FFG (Flotation Fine Grind) technology, introduced to liberate more gold and increase recoveries by between 16% to 20% and commissioned in January this year, has achieved positive float and grinding results, the CIP section has not yet stabilised and appears to have also contributed to metallurgical instability and carbon inefficiencies downstream in the existing carbon in leach (CIL) circuit or low grade section.
Suspending the new section will enable the operational team to stabilise the low grade circuit and to determine the factors that could be affecting carbon efficiencies in both circuits. The company expects the original lower-grade CIL circuit to be settled within a month. The subsequent test work is likely to be completed within a further three to four month period.
The company also comments that incessant rains during February and March and surges, dips and interruptions in the power supply during this period have also negatively impacted both the high grade and low grade sections, contributing to the metallurgical instability of both sections.
The Ergo team will therefore make use of the down-time to install drain valves to prevent silting up of the float cells during future power fluctuations, and auxiliary power units to thickener underflow pumps to prevent silting up of thickeners during these trip-outs.
The FFG and high grade CIP circuit came into full operation in mid-January 2014 following the commissioning of a third thickener. Initial results were positive and utilisation was generally good. Flotation efficiencies and the performance of the fine-grind mills were consistent with targets. Sampling results show that the amount of gold remaining in solids (the so-called “washed residue grade”) reduced to within target range. This suggested the gold that previously remained inert within the pyrites was now driven off and had become soluble.
However, loaded carbon values remained well below the target range in both the CIP and CIL circuits. This indicated that the gold remained in solutionand was not being adsorbed onto carbon. To some extent this may be attributable to poor carbon efficiency –caused by volatile density levels in the feed at various stages in the process – which in turn is attributed to the thickeners that manage the flow of feed into the main CIL section. It may also be a knock-on effect of the reagents used in the flotation cells.
Currently both carbon circuits of the plant – the higher grade CIP and the lower grade CIL – are unstable and are operating well below target range.
As a result of these metallurgical inefficiencies and the resultant high dissolved gold losses, gold production for the quarter ended March 2014 is estimated to be 14% lower compared to the preceding quarter, and all-in sustaining unit costs are estimated to be up by 24% quarter on quarter.
Because of a higher gold price and lower total operational and capital expenditure, the company’s cash balance remained virtually unchanged quarter on quarter.
Simplified Summary of the FFG process and its interaction with the low grade CIL circuit
Slurry is sourced from various reclamation sites and is delivered to Ergo’s Brakpan plant. The slurry, or feed, is pumped through three banks of flotation cells to create a concentrate that contains gold-bearing pyrites. The concentrate enters a set of fine-grind mills to break down the pyrites and release gold which would otherwise have remained shielded from the subsequent metallurgical extraction process. (Gold trapped in these pyrites is the main target of the FFG and the reason for its construction.) This fine pulp then enters a high grade leach circuit, where cyanide is added to dissolve the gold in the pulp. The concentrate is then pumped to the CIP section where activated carbon is added and the dissolved gold is adsorbed. This loaded carbon is processed through an elution circuit, where the goldis stripped off the carbon into solution. This solution then flows through a series of electro-winning cells where the gold is electrically deposited onto electrodes. Periodically the gold is washed off the electrodes, calcined and smelted into doré bars.
The remaining 96% of the feed leaves the flotation circuit and enters the low grade circuit. It first enters a set of three thickeners to increase its density before being pumped into the main CIL tanks. It is essential that the thickeners are in good working order and stable, because the high grade tail cannot be processed by the low grade circuit until it has been thickened. Their purpose is therefore to increase the density of the feed, otherwise the carbon that is introduced later in the process will sink to the bottom of the CIL tanks, and not remain in suspension. From this point onwards in the low grade CIL circuit, the feed is subjected to a similar process to the one described above – cyanidation, carbon loading, elution, electro-wining and finally smelting into dorébars.