State-owned research organisation Mintek is demonstrating its Retreatment Flowsheet for Au and Uranium Tailings, or Reflaut, tailings treatment process at gold miner Gold Fields’ South Deep mine, in Johannesburg. The plant recovers gold, harmful sulphides and radioactive uranium from tailings, significantly reducing the mineral content responsible for generating acid mine drainage.

Mintek Minerals Processing division head of flotation Kevin Pillay notes that there are about 517 mine tailings in Gauteng, covering about 18 000 ha with harmful and radioactive minerals. These tailings have generally undergone gold leaching processing, but still contain gold, trapped in silicate or pyrite.

A number of tailings retreatment projects have failed, as the cost of the flotation reagents and the low volumes of gold recovered made the projects unfeasible. “We have actually reduced the cost of the reagents quite drastically in terms of the uranium and the sulphide collectors we use,” he asserts.

During the Reflaut process, tailings are processed through a flotation circuit, which separates concentrates, comprising uranium, sulphides as well as gold. Gold can be recovered from silicate rock by undergoing ultrafine milling and another gold leaching process; however, the pyrite requires a different process.

The Reflaut process involves the concentrate undergoing an oxidation process, which oxidises the sulphides, thereby breaking down pyrite to free up the gold for leaching.

“The sulphur forms an acid, which is then used to leach the uranium in the sample. “This will then go to solid-liquid separation – solids being the gold and liquids being the uranium, which will then be neutralised,” Pillay explains. The liquid is then combined with flotation tailings, and the uranium is selectively removed from the solution.

The resulting tailings have a sulphide content of less than 0.3% and less than 40 parts per million of uranium, which is the generally accepted safety standard. The demonstration plant at South Deep has a tailings retreatment capacity of about 200 kg/h. About 50% of the gold is recovered in 7% of the mass from the tailings retreated at South Deep.