Converting practices at the stillwater precious metals smelter

Abstract

At the Stillwater Mining Company's precious metals smelter in Columbus, Montana, flotation concentrates containing copper, nickel, and platinum-group metals are smelted in a submerged-arc electric furnace. The matte, containing 30% copper plus nickel, is granulated, dried, remelted in a top-blown rotary converter, and blown with oxygen to "white metal" containing approximately 75% copper plus nickel and 2.5% precious metals. For the first month of converter operation (July 1990), conventional fluxing with quartz was practiced. An excessive number of slag foams and heavy accretion formation at the mouth of the converter led to a switch to lime fluxing as a replacement for silica. This resulted in eliminating slag foams and decreasing accretion buildup. It is believed that the Stillwater works is the only commercial smelter in the world following this practice.

Converter processing of platinum group metals

“…Chuquicamata (Warner and Diaz 2003) Copper Cliff (Warner and Diaz 2003) Thompson (Warner and Diaz 2003) Severonickel (Reznik 1993a) KNS before reduction (Baldock et al 1993) KNS after reduction (Baldock et al 1993) KNS (Hidayat et al 2018) Experimental (Toscano and Utigard 2004) Falconbridge (Bustos, Ip et al 1988) ACP Waterval Smelter PSC -Pre-2002 Jinchuan (Liu 1991) Experimental DON (Piskunen et al 2018) DON (Paakkonen and Mattelmaki 1996) Increasing extent of conversion In a pilot plant TBRC with clearly excessive matte entrainment, Matousek and Whellock (1990) report far lower partitions L matte/slag from 26.2, 26.0, 28.3 and 24.4 with decrease to 7.2, 6.8, 6.8 and 6.9 for Pt, Pd, Rh and Au, respectively with progress of the converter blow from 28% to 4% Fe content. From 4 to 10 times higher PGM partitions can be inferred for the commercial TBRC (Roset et al 1992), still with matte entrainment. Also with matte entrainment, PGM partitions in other commercial converter operations can be of the order of 30-50.…”

“…This presents a challenge to match converter capacity to a lower furnace matte supply (see TBRC later). Primary producers typically produce converter matte containing 1500-6000 g/t PGM content (local to Southern Africa), but this can even go higher to 25 000 g/t (Roset et al 1992), and boosted further when considerable quantities of secondary materials are treated together with the primary feed.…”

“…The technology evolution then progressed innovatively to permit the first non-ferrous application of top blown rotary converters (TBRC) to blowing PGM-containing nickel feeds directly to metal, as developed and commercialised by secondary producers (Copper Cliff -Table 3) (Queneau et al 1969;Thoburn and Tyroler 1979). This development was subsequently adopted especially when converting smaller volume PGM-bearing feeds, by smaller primary producers (first at Crocodile River, South Africa and then at Stillwater, USA -Table 2) (Roset et al 1992). The TBRC continues as the workhorse of many secondary PGM recyclers; at least four PGM recyclers in three countries are known to adopt TBRCs (usually in pairs).…”

“…Oxygen (O 2 )-enrichment: progressively towards 30% in both the primary and secondary producers, and up to 50% O 2 -enrichment sonic shrouded tuyere injection implemented on secondary producers (Bustos et al 1995(Bustos et al , 1999Kapusta et al 2005) (Table 3) and trialled on primary producers (Kapusta et al 2012) (Table 2). TSL bath converters and TBRCs also typically operate with 40-99% O 2enrichment (Roset et al 1992…”

Oxygen Potentials of Copper Smelting Slags

“…The first are those furnace, again with limestone as flux and coolant [2]. At the resulting from the production of mattes from ores or con-Stillwater Mining Company's Precious Metals Smelter in centrates, characterized by iron plus silica contents of approxiColumbus, Montana [3], lime, rather than limestone, is used mately 750/0 and iron to silica ratios of 1. Converting slags as the fluxing agent in the top blown rotary converter (TBRC), typically have iron plus silica contents also of 750/0,but iron to producing a platinum group metals bearing "white metal" from silica ratios of 2.…”