ConRoast
Jubilee has exclusive rights to develop and commercialise the unique ConRoast smelting technology until 2020. The technology has been proved to be capable of recovering platinum group elements (PGEs) from difficult to treat ores, smelter residues, chromite tailings and high chrome platinum concentrate. Apart from the commercial potential that this affords, reprocessing old tailings dumps also contributes to significant environmental improvements in South Africa’s extensive platinum mining areas.
Conventional ‘six-in-line’ platinum smelting technology, though well understood, is at least 30 years old and is highly polluting, particularly from emissions of fugitive sulphur dioxide gas.
However, the principal drawback of conventional technology is that it is largely unsuited to the processing of chrome-rich UG2 reef, nickel-rich Platreef ores and chrome rich LG and MG reefs, without mixing concentrates from these resources with those from the Merensky reef. The UG2’s higher concentration of chromite affects conventional smelting process by gradually choking furnaces with insoluble chrome spinels with collateral damage to furnace linings.
While the platinum majors are largely locked into their conventional plants, emerging producers have until now been constrained to having their UG2 concentrates smelted by the majors after blending with Merensky concentrates. This has significant limitations on the new producers’ capacity. Various other experimental smelting technologies have been tried over the years, but with limited success.
The ConRoast process developed, tested, proved and patented by Mintek, South Africa’s state-owned minerals technology laboratory - represents a major advance. It opens up the possibility of mining the vast tonnages of UG2 reef that form part of the platinum-rich Bushveld Complex but that otherwise may remain unexploited as the relatively easy access Merensky reef is depleted and no longer available for blending wth UG2.
Jubilee has completed a design and engineering study for its first 5 MW DC arc ConRoast smelter. This 5MW facilty is under construction at Jubilee’s Middelburg smelting facilty in Middelburg It will be the first commercial (as opposed to experimental or research) ConRoast plant and is envisaged to be used for processing some current PGE concentrates and other PGE-bearing materials from Northam Platinum and Sylvania Platinum.
Fundamentally, the ConRoast process removes sulphur from metal sulphide concentrate prior to smelting in a DC arc furnace, which under reducing conditions produces molten iron droplets to capture PGEs, nickel, copper and cobalt and settles out into an iron alloy prior to refining for recovery and separation of the individual metals.
Sulphur dioxide is removed from the concentrate and captured in the roasting plant for conversion into sulphuric acid, a process that has obvious environmental advantages. As a result, ConRoast is not limited by the sulphide content of metal ores and can, therefore, treat a wide range of ores. As importantly, the DC arc furnace is not adversely affected as is the conventional ‘six-in-line’ process by the chromite that characterises platinum concentrates form UG2, LG and MG reefs.
Chromite in concentrates is difficult to smelt in conventional lower-temperature AC furnaces as it forms high-density spinels with high melting points that solidify on furnace walls and in the slag. This reduces furnace volumes and efficiencies. As a result, conventional furnaces can only accept feeds containing 1.5% to 2.5% chromite. In contrast, ConRoast furnaces can accept any proportion of chromite.