DRD fine-tunes gold liberation
Junior gold mining company DRDGOLD has finally justified its brave decision to upgrade the Ergo processing plant two years ago, writes Leon Louw.
When CEO Ni?l Pretorius and his technical team at DRDGOLD decided to embark on an expansion and optimisation programme in 2014, the gold mining industry was not in the best space. The gold price was about to start a steady decline to levels that many analysts would later describe as unsustainable. To make things worse, economists were struggling to find reasons why gold would regain its popularity. The murky outlook left many CEOs — especially those heading up marginal operations — contemplating whether it was worth keeping the mills running. In the midst of all this doom and gloom, DRDGOLD decided to increase production by “extracting maximum value” as Pretorius put it. This would be done by “liberating previously unrecoverable gold encapsulated in sulphide materials”.
The R320-million upgrade was a massive gamble. Although Pretorius is often accused of being too conservative when it comes to expenditure, his saving habit had left the company with enough capital to pay for the project in cash. This, in addition to the fact that DRDGOLD is an extremely low-cost, mechanised operation, gave the proposal some impetus when the board decided to give it the green light. However, a number of shareholders didn’t take so kindly to the proposal and sold their shares. Those shareholders that remained probably did so more in hope than in belief. Those hopes were dealt a further blow when the newly introduced technology failed to perform as expected.
Facing the headwinds
Initial test results on the new flotation and fine-grind (FFG) circuit and the high-grade carbon-in-pulp (CIP) circuit that came online in January 2014 were dismal. The circuit was suspended in April 2014 to determine the cause of the metallurgical problems, so more shareholders fled. The share price took a tumble and traded at an all-time low in early 2015.
But Pretorius and his team remained resolute. They started up the plant once again towards the end of 2014, and the new additions to the plant slowly started finding a rhythm. Almost a year later, Ergo was processing nearly 1.8 million tonnes per month (Mtpm) of sand and slurry from historical mine dumps across Gauteng. Buoyed by the success of their new technology, the company increased volumes by starting to mine a new asset: the Van Dyk Dam close to Brakpan, which is a stone’s throw from Ergo. The inclusion of the Van Dyk Dam in the Ergo set-up reduced the risk of being dependent on the feed material from sources on the west Witwatersrand and the Elsberg complex only. At full tilt the plant performed better than expected and with exceptional gold recoveries, DRD set its new target at 2Mtpm and started talking about improving the technology to recover even more gold.
At the same time that the DRDGOLD volumes being pumped through Ergo’s processing plant started increasing, the gold price improved and the rand made a nosedive. So in February 2016, as other commodity prices were lingering at the bottom of the cycle, gold was heading north of USD1 300 per tonne. Gold mining companies became the beneficiaries of poor local political decisions and unexpected international events, such as the Brits deciding to turn their backs on the European Union. As a result, Pretorius, his technical team, and those brave shareholders who unfalteringly believed, were richly rewarded. “What a year it’s been,” says Pretorius. “In October 2015 DRDGOLD had a market capitalisation of R700-million. Three to four months later that figure stood at R4.9-billion,” says Pretorius.
Looking to increase throughput
With extra cash in the bank, DRDGOLD has not only been able to reward its shareholders regularly, but they are also looking at mining additional dumps. Although the Ergo plant capacity is sufficient to process the envisaged 2Mtpm, the big constraint up to now has been the capacity of the tailings facility to store the extra material. This problem, however, has been solved with the company recently receiving approval from the Department of Water and Sanitation (DWS) for its Withok tailings dam, located near the Ergo plant. “The ability to store more residue material will enable us to process material for a longer time,” says Jaco Schoeman, head of operations at DRDGOLD. As the Ergo plant increasingly evolves into an exclusively high volume lower grade facility, the question is whether the plant will be able to process the increased volumes. “We can increase the capacity of this plant to 2.4Mtpm without spending too much money. The constraint, however, is not so much the capacity of the plant, but the ability to deposit the residues. The rate of deposition is largely determined by the size of the tailings dam, and the infrastructure to get the tailings to the dam. This is what we have now put in place with the development of Withok,” says Pretorius.
The ensuing question is whether the company has access to enough sources to remain a sustainable operation for the next 20 years. Even if DRDGOLD increases the monthly throughput to as much as 2.7Mtpm, Pretorius says that enough sand and slime will be available to keep Ergo’s wheels turning. A map of DRDGOLD’s asset and license areas indicates the extent of past gold mining activity and the large number of historic dumps that are still scattered throughout the Johannesburg area. Although DRDGOLD has access to the majority of these, some will never really be worth mining. The cost of establishing the infrastructure at some of these sites will just be too expensive to justify the mining thereof. However, Pretorius says that there is more than 800 million tonnes of sand and slurry available to mine. “We have more or less 200 million tonnes at about 0.3 gram per tonne; 600 million tonnes that’s hovering between 0.25 grams and 0.27 grams per tonne; and then plenty more at just below 0.24 grams per tonne. At the right volume and mix this will ultimately give us sustainability,” says Pretorius.
Pumping through the volumes
Schoeman says the floatation plant has stabilised now and while the company still wants to ensure that it achieves consistent volumes, they want to explore opportunities presented by new technologies. “There is room for improvement in our plant. We are debating how we can further improve our current process and reduce the gold that ends up in the tailings dam,” says Schoeman.
At the moment, the Ergo plant is fed by three pipelines through which sand and slurry from various old tailings dams and mine dumps are pumped. Two-thirds of the material is sourced from a number of reclamation sites, and the rest is mined from the historic Crown and City complexes towards the west of Johannesburg. The slime, which makes up 85% of Ergo’s material, is mined with hydraulic high-pressure hoses. The sand — residue from old stamp mills used more than 60–100 years ago — is loaded with front-end loaders onto conveyor belts, and then pumped as slurry to the plant at Ergo.
The process of recovery
When the material reaches the plant, it reports to the newly constructed flotation section, made up of 108 flotation cells. The high-grade material is upgraded in the high-grade circuit. “In this process the material entering the plant is upgraded about 10 times. At arrival, it measures about 0.3 grams per tonne, while the float concentrate runs at about 3 grams per tonne,” says Schoeman. The pyrite is pumped to two concentrate thickeners before entering the mill circuit to liberate the gold further by grinding it finer.
The mill circuit includes four vertically stirred mills. This is quite a unique aspect of the Ergo plant, as the traditional gold processing plant normally uses horizontal ball mills. Vertical mills are more commonly used in platinum plants. The vertical mills, supplied by Johannesburg-based FLSmidth, use beads of about 3.2mm in diameter. The beads are made with an inert material that resembles porcelain. The beads are produced in South Africa by Dakot Milling Media (DMM), a company that manufactures high alumina ceramics. The specific beads used by DRDGOLD were manufactured for an entirely different application, but after discovering their unique abilities and low wear rate, the team at DRDGOLD and FLSmidth decided to test the beads in the vertical mills. Not only did it work like a charm, but also it was cheaper and more efficient than exotic beads previously used by the company. “We found that the wear rates of these beads were a lot better than the imported beads — and they were cheaper. This is a cost intensive part of our operation, and the new beads actually reduced our operational costs,” says Henry Gouws, manager of Ergo. The beads in the mills break the outer shell of the pyrite to get to the gold and make it amenable to the normal cyanisation process. According to Schoeman, one of the four mills is supposed to be a standby unit. “However, it does look as if we will have to use all four units. That’s because we are pushing to put more pyrite through the circuit — more pyrite will require more mill capacity,” says Schoeman.
From the mills the material proceeds to the carbon-in-leach (CIL) tanks where the gold is exposed to cyanide, oxygen, and lime; liberated; and put into solution. “We are looking at improving the leach circuit even further by introducing highly tense agitators. By increasing the power into these circuits we can maybe improve the kinetics in terms of metallurgical efficiencies,” says Gouws. The residue from the high-grade circuit is pumped to the four 138m thickeners. One of these thickeners is in the process of being refurbished. The thickeners at Ergo are unique because of their size and the fact that they have a peripheral bogey system that scrapes the slime into the centre of the thickener. From the thickeners the material is passed through the low-grade circuit, where it proceeds through a very long leach circuit that enables the process to recover more low-grade gold. From there the residue is pumped to the tailings facility.
Through thick and thin
The recovery process at Ergo is highly technical. Continuous research and the development of better technologies will eventually result in more gold being liberated. DRDGOLD’s Ergo FFG plant is not labour intensive: it is quiet — almost eerie — when you walk through the plant. At any point in time no more than four people are active in the FFG plant. The nature of DRDGOLD’s operations has eliminated one of the highest cost drivers of any mining operation: labour. This, together with the weak rand, the recent gold rally, the stabilisation of the recovery circuits, and the large number of tailings dumps that the company has access to, will ensure that those shareholders who persevered with the company through the bad times will be generously rewarded during the good times.