Rodney T. Jones
Pyrometallurgy Division, Mintek, Private Bag X3015, Randburg, 2125, South Africa
Mintek has developed and piloted a novel process for the treatment of nickel-copper and PGM (platinum group metal) sulphide concentrates. The ConRoast 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 PGMs. The environmental benefits with respect to sulphur emissions are considerable, in that essentially all of the sulphur is removed from the enclosed roasting equipment in a continuous stream of SO2 of an appropriate strength for feeding to a sulphuric acid plant. (In the case of low-sulphur feed materials, the gas could instead be scrubbed and neutralized.)
This process allows great flexibility with respect to the selection of ore types, and does not impose limits on the minimum quantities of contained base metals or sulphur, and can tolerate very high contents of chromite in the concentrate.
The dead-roasted concentrate is fed hot into the DC arc furnace. This reduces the energy requirement for the smelting process. The DC arc furnace is well known for its ability to handle fine feed materials. The furnace is operated under quite strongly reducing conditions at high temperature. This avoids the commonly experienced problem of magnetite or chromite spinel build-up in the furnace. (This removes the constraint on the permissible content or Cr2O3 in the concentrate, which in turn allows higher PGM recoveries to be achieved in the concentrator.)
The smelting process is alloy-based rather than matte-based, as there is effectively no sulphur present by this stage of the process. (This removes another constraint on the feed material to be processed, in that there is no minimum quantity of sulphur required. Even weathered oxidized ores can be processed in this way.) By adjusting the amount of reductant fed to the furnace, the amount of alloy produced can be varied, by reducing iron from iron oxide already present in the feed material. (This eliminates yet another constraint on the ore composition, in that there is no minimum amount of nickel and copper required to ensure good collection of the PGMs.) In fact, iron collection of PGMs is far more effective than matte collection. Very clean slags are produced in the furnace, containing small enough quantities of PGMs that the slags can be discarded or even sold for purposes such as road-fill or shot-blasting.
The alloy is water-atomized prior to leaching. Prior to the atomisation, the molten alloy can be refined, if required, (to remove small quantities of carbon, silicon, or chromium) in a ladle holding furnace. Although it is possible to use a converter to remove iron from the molten alloy, there is no specific requirement for Peirce-Smith converters or for a converter aisle (thereby eliminating the inherent scheduling problems of this batch process, as well as losses and spillages from the crane transport of ladles, and high labour costs). Instead, iron may be rejected from the alloy hydrometallurgically, by precipitation as hematite for example.
The alloy from the furnace differs from the conventional matte feed to the refinery, in that it contains virtually no sulphur, yet contains high amounts of iron. An iron-removal step is therefore required prior to the separation of the base metals (Ni, Cu, Co) and the precious metals. Mintek's preferred approach has been based on sulphuric acid leaching, with an atmospheric leaching step for the dissolution of Fe, Ni, Co, followed by oxidative pressure leaching for the dissolution of Cu. This has resulted in a high-grade PGM concentrate containing exceptionally low quantities of undesirable elements. This PGM concentrate is an eminently suitable feedstock for a precious metals refinery.
The ConRoast process achieves very high metal recoveries, and produces high-purity metals, and a clean high-grade PGM concentrate.
Further details are available at:
Phone: +27 (11) 709-4602
Fax: +27 (11) 793-6241
Copyright © 2001-2002 Rodney Jones, Mintek, firstname.lastname@example.org
27 June 2002