Abstract

Platinum group metal (PGM) matte, a nickel-copper rich sulphide melt with high iron and low cobalt contents, is tapped through an alumina-chrome brick lined tapping channel in a platinum smelter. Penetration of PGM matte into these tap-hole bricks leads to detrimental refractory wear which can cause furnace breakouts. Multiple factors affect the extent of penetration, with the superheat of the matte being a significant factor. PGM furnaces in South Africa typically smelt blends of concentrates from the Merensky Reef, UG2 Reef and the Platreef. As supplies of Merensky concentrate are decreasing, increasing amounts of UG2 concentrate (which is rich in chromite) are used. The high chromium content of the UG2 concentrate increases the slag liquidus temperature. As the smelters operate above the slag liquidus temperature, the slag operating temperatures can range between 1460-1650°C depending on the feed composition. These high slag operating temperatures result in matte superheats of 300-650°C, which implies the matte is extremely fluid, can easily penetrate the refractory bricks and thereby increase refractory wear. The extent to which matte penetration and chemical interaction takes place were investigated through a post mortem study of a tapping channel removed from the Anglo American Platinum Mortimer smelter in South Africa. Matte penetration in regions surrounding the hole in the refractory tapping module was the main focus. The investigation also included determining whether residual tap-hole clay is present in the tapping channel. The excavated bricks were analysed in centimetre sections from the tap hole towards the edge of the brick, using SEM-EDS analysis. XRD analyses were done on samples taken from the position of the new tapping module condition as well as the worn hole, both on the tap hole side of the brick (the hot face) and at the edge of the brick (the cold face). It was found that upon solidification, the penetrated matte crystallised to form sulphides with different concentrations of iron, copper and nickel, or combinations of these elements. Chalcopyrite (CuFeS2), pentlandite ((Fe, Ni)9S8) and pyrrhotite (Fe1-xS) were the dominant phases. A significant amount of iron oxidation was observed and to a lesser extent the oxidation of copper. This resulted in the formation of a CuFe2O4 – Fe3O4 solid solution spinel phase in the areas surrounding the worn hole, and a more complex Cu-Fe-Al-Cr-based spinel phase in the position of the original hole. Residual tap-hole clay could not be identified. SiO2 observed in the bricks is associated with CaO and MgO, which indicates that slag also came in contact with the matte tap-hole bricks. Matte penetration was more significant toward the bottom of the tap hole than towards the side, indicating that gravity aided matte penetration.