The metallurgical process
The Ergo operation currently has two metallurgical plants in operation:
- Knights plant in Germiston is treating the Cason dump and depositing waste on the Brakpan tailings facility (BTF);
- The Ergo plant in Brakpan is treating material from a variety of sources, the majority of which – some (1.2Mtpm + 300 000t) – is fed to the plant via two feeder lines from the Elsburg tailings complex and the L/29 dump. Another 600 000ktpm is fed to the Ergo plant from Crown Mines and City Deep via the 50km pipeline. The Ergo plant also deposits its waste on the BTF, some 12km away. The targeted throughput at the Ergo plant is some 2.1Mt, with the potential to increase this figure to 2.4Mtpm if required.
- The Crown Mines and City Deep plants were, until recently, metallurgical plants in their own right. However, due to the streamlining of operations and the construction of a 50km pipeline, they have been re-engineered as milling and pump stations in support of the Ergo plant.
Tailings retreatment in South Africa began in the 1970s incorporating a technique known as flotation to recover pyrite uranium and gold. A CIL circuit was later added to enhance gold production. Three decades had passed before DRDGOLD acquired the Brakpan plant in 2007 with the aim of re-establishing the Ergo project. During this period, many variables had changed, including the closing of the flotation circuits at all plants. The Brakpan plant was therefore re-established using CIL.
Research and development (R&D)
From the start, the DRDGOLD board identified several key objectives as part of its Ergo project strategy – one of which was improving gold recovery. Research was commissioned, a pilot plant was constructed and it was discovered that pyrite particles, containing some 40% of the gold, were not responding as well as expected to the CIL process. A dual process solution was identified: re-introducing flotation and adding an additional stage – fine-grind or milling of the flotation concentrate.
In early 2012 the DRDGOLD board approved a capital amount of R250 million for the flotation/fine-grind circuits. Work began immediately on refurbishing unused sections of the existing Brakpan plant, together with the construction of the necessary additional plant to implement the new process. This work was completed towards the end of 2012, and was followed by commissioning of the new circuits.
Integrating the new circuits
The new circuits became fully operational in January 2014 and the first gold bar from the high-grade or CIP circuit was produced at the end of the month.
Unfortunately a number of metallurgical problems arose which, coupled with excessive rain during the first quarter of 2014, affected the entire plant. In April 2014, production through the new circuits was temporarily suspended to allow the operational team to restore stability to the plant and identify the problems. The low-grade or CIL section of the plant continued to operate as in the past.
In essence, the new technology was successful in liberating more gold and increasing recoveries but it masked the metallurgical instability and inefficiencies downstream in the existing part of the circuit. Thus the gains in one area were being lost elsewhere and the same amount of gold was being produced at a higher cost.
During the temporary suspension the CIL circuit continued to perform as expected. The FFG circuit was recommissioned in a three-stage process and by year-end the operation had stabilised. FY2015 performance is within the parameters anticipated from the onset.
Up to 20% more gold
We expect to see an improvement in recovery efficiencies of between 16% and 20%, which may result in an increase in recoveries of up to 8%, which in turn could offset progressively diminishing grades.
R&D: an ongoing requirement
Investment in R&D continues as the focus on increasing and optimising the volume to be treated and the recovery of gold from the resource continues. Ergo has a target of improving recovery efficiencies by 20% by the third quarter of FY2017.
Flotation/fine-grind and CIL: how it works
The slurry material from the reclamation sites feeds into the plant via a complex pipe network. Here it passes over linear screens to remove organic material and debris. The slurry then enters a flotation section consisting of banks of flotation cells. After the material is conditioned with reagents (collectors and frothers) it enters the cells where it is separated into two streams. One stream, the flotation concentrate, contains the sulphides which are enriched with gold while the second stream, the flotation tails, is made up of lower-grade siliceous material. This stream is treated by the conventional CIL process, which is the treatment process that has been used for the past 30 years with an extraction efficiency of 39% to 40%.
The concentrate stream is subjected to the new fine-grind process which research and development has demonstrated is more efficient in recovering the gold. Fine-grind involves milling the slurry material with tiny beads. At this stage the milled product, 80% of which is smaller than 24 microns, has been liberated from the sulphides, making recovery of the previously encapsulated gold easier as it comes into contact with cyanide during the process that follows. Research shows the extraction efficiency of this process to be 75% to 76%.
The dissolved gold is adsorbed onto activated carbon, and the ‘loaded carbon’ in each circuit enters the carbon treatment section where the gold is eluted from the carbon. The carbon then returns to the CIL circuit via a regeneration kiln. Once the gold has been eluted, it undergoes a process called electro-winning where the gold is precipitated, calcined and smelted in the existing smelthouse. The two eluate streams are kept separate to allow for accurate accounting of the gold produced by the respective high- and low-grade circuits. The leach residues are combined before being pumped to the BTF.