Platinum Jubilee – processing developments

Materials World magazine
,
1 Oct 2009

Michael Forrest investigates mine and processing developments in the Bushveld, South Africa.

Platinum demand in 2008 fell by five per cent to 6.35Moz, according to London, UK-based Johnson Matthey’s Platinum 2009 publication. The biggest decline was in autocatalysts, which fell half a million ounces to 3.81Moz, reflecting a 20% decrease in automotive production in the USA and similar cuts in Asia and Europe. Supply, however, dropped even further, a result of falling production in South Africa, which is the largest producer – accounting for 76% of global production.

The country’s shortfall of 0.54Moz resulted from power outages, skilled labour shortages, bad weather and poor smelter performance. The former is estimated to have cost 60,000oz, leaving nearly half a million ounces to the remaining causes. Over the past year, there have been a number of smelter breakdowns, including the Polokwane smelter commissioned in 2003, the only one on the eastern limb of the Bushveld Igneous Complex (BIC) in South Africa.

This igneous complex is host to the world’s largest platinum resource and has been exploited since the 1920s. The horizons containing the platinum group metals (PGM) are the Merensky and UG2 reefs, with the former containing more sulphides and gold credits, while the latter has higher amounts of chromium as well as more consistent and usually lower platinum levels. The centre of platinum mining has been at the southern part of the western limb, where the Merensky reef is nearer the surface. This was determined by geology and mineralogy as the UG2 ore was refractory – difficult to treat. It was not until the 1980s that these high chrome ores could be exploited.

Modern view

A number of companies are operating and developing mines in the eastern limb of the complex, including AIM-listed Jubilee Platinum of London. Its flagship project, Tjate, is located in the northern part of the BIC, down dip of mines operated by South African companies Implats and Angloplat. Here, both the Merensky and UG2 reefs are present under the 51km2 of licence area at depths of 600-1,200m, compared with mainstream operations that average around 1,400m, for example Implats and Union Deeps in the south of the western limb.

Jubilee has been exploring since 2004 and recently received an independent South African Mineral Resources reporting standards report on its resources. In the area where the initial mine, referred to as the First Mine, is planned, an indicated and inferred resource of 132.5Mt has been identified, grading 5.24g/t of platinum, palladium, rhodium and gold. This value is based on 42 cored boreholes (65,000m cumulative total) that intersect the reefs 154 times in the First Mine area.

The amount of UG2 ore used in the Bushveld smelters has steadily increased as new areas became available. Its use, however, has resulted in a number of problems, stemming from the lack of base metal sulphides in UG2 ore to form a sulphide matte in the traditional six-in-line AC furnace smelters. In addition, excessive chromite in concentrate tends to build up inside the furnace as a highly viscous and refractory mineral spinel. Furthermore, it has been difficult to contain sulphur dioxide gas from the sulphide concentrates during smelting.

Mintek, the South African Research Establishment, has a track record in developing DC arc smelting and has, over the past decade, advanced its ConRoast technology. This process is based on the removal of sulphur by roasting, followed by smelting of the dead-roasted concentrate in a DC arc furnace using an iron-based alloy as a collector for nickel, copper, cobalt and PGM.

It is designed to overcome some of the problems associated with smelting high chrome-bearing concentrates (including those of the UG2 reef), together with sulphide concentrates via the established matte route, including the need to operate at high temperatures, and, in turn, increased corrosive sulphide melts.

Hot in here

The first part of the ConRoast process is the removal of sulphur from the mineral concentrate. This is achieved by roasting a dried concentrate in a thermally-efficient fluidised-bed roaster. Sulphur dioxide typically accounts for eight to 15% of the off-gas and can be neutralised or captured to produce sulphuric acid.

The heated concentrate is then fed into a DC arc furnace operated at high temperatures under reducing conditions that can be adjusted by the amount of reductant fed into the furnace. Iron contained in the ore, or added as required, forms an alloy (instead of traditional matte) with the platinum and other metals. The PGM and base metal recovery rate to alloy is high and obviates the need for nickel or copper to capture platinum, as in traditional furnaces. It also overcomes the chromium problem by maintaining similar liquidus (melting) temperature of melt and slag, ensuring that the chromium reports to the slag.

The DC arc furnace can use fine-grained feed material and is well matched to the fluidised bed roaster. Such furnaces can operate at higher temperatures than traditional AC furnaces, as heat is supplied by the DC arc and not the thermal resistivity of the slag, which can be optimised for metallurgical recovery rather than its electrical properties. The final step to a fine-grained product for refining (leaching) can be obtained by subjecting the molten iron-rich melt to atomisation with a water jet. This results in fine-grained material with a high surface area.

Development of the process has taken over 15 years, mainly constrained by the availability of test material on a scale that would satisfy the major platinum companies. This required large-scale roasted concentrates, an expensive source. However, revert tailings with similar chemistry have been identified and used in a multi year demonstration in a 1.5MW DC furnace at the rate of 1,000t/month, beginning in 2004. After these successful trials, Mintek determined that a partner with sufficient funds was required to complete and commercialise the technology.

Following an unsuccessful approach to the major platinum producers, Mintek formed Independence Platinum Ltd with the specific objective of bringing its technology into commercial use. In 2006, Independence was acquired by Australian organisation Braemore Resources plc, a technology company whose objective was to use its exclusive licence of the ConRoast technology to develop an independent and vertically-integrated platinum company, as well as offering toll smelting and offtake agreements within the sector.

By July this year the companies had merged to create an enlarged Anglo-South African platinum mining group with sole-rights to innovative and cost effective technology. In the same month, Jubilee announced the placing of shares to London institutional investors, raising £13.25 million. On approval of the placing by Jubilee shareholders, and the completion of the final legal steps of the merger with Braemore, the new company will apply to trade on AIM and the Johannesburg stock exchanges, and Braemore will cease to be listed.

Further information: Jubilee Platinum