Modern Mining March 2015

GOLD

by Anglo American when the plant was first commissioned but subsequently discontinued) and adding an additional stage – fine grind or milling of the flo- tation concentrate followed by treatment in a small dedicated CIP circuit. The FFG project was approved in 2012 and imple- mented during 2012 and 2013 by DRDGOLD using an in-house project team.

Although the FFG or high- grade circuit came on line in January last year, it operated for less than three months before DRDGOLD decided to suspend the new section – a decision that Pretorius described as one of the biggest and most difficult deci- sions he has had to take since joining DRDGOLD ten years ago. “Suspending something that was brand spanking new was difficult,” he said. At the time the decision was announced, DRDGOLD said that while the float and milling sections had performed well, the CIP had not yet stabilised and appeared to be contributing to metallurgical instability and carbon inefficiencies down- stream in the CIL or low grade section. By September last year, DRDGOLD had completed an analysis of the problems and opti- mised the circuits and was confident enough to reintroduce one stream of the three-stream FFG circuit, with generally positive results. It fol- lowed up in January this year by restarting the other two streams. Commented Pretorius: “The plant has its own personality, its own tempo, but we are now starting to see the upside of the flotation and fine grind coming through.” With the FFG now fully operational, all the slurry entering the Brakpan plant enters the flo- tation section after passing over linear screens to remove organic material and debris. After the material is conditioned with reagents, it enters the float 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 material which is treated in a 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

well as an extensive network of pipelines. The Brakpan plant is DRDGOLD’s flagship and is responsible for the majority of production, with most of its feedstock (roughly two thirds) being sourced from the Elsburg tailings complex and the L/29 dump. The overall resource being exploited by Ergo amounts to 11 Moz of gold. DRDGOLD has been involved with the Ergo assets since 2007 (initially in joint venture with Mintails, whose stake it purchased in two deals in 2008 and 2010) and has since mod- ernised and extended the operation, among other things constructing a 50 km long, 600 000 tonne/month capacity, HDPE-lined pipeline to link Crown and the Brakpan plant. The FFG circuit is the latest enhancement of the opera- tion and is designed to increase the efficiency of extraction by roughly 0,03 g/t – representing an approximate 10 % increase in recovery. The need for the FFG was identified sev- eral years ago when research by DRDGOLD (including trials in a pilot plant) revealed that pyrite particles, containing some 40 % of the gold entering the Brakpan plant, were not responding as well as expected to the CIL process. A dual process solution was identi- fied – re-introducing flotation (originally used

The concentrate stream is subjected to the new fine-grind process, which involves milling the slurry material with tiny beads using four vertical stirred mills, three of which are seen here (photo: Arthur Tassell).

22  MODERN MINING  March 2015