Mineralogy and environmental geochemistry of slag in Lower Area One, Butte, Montana.

摘要

Butte, Montana is the host to a Cu-Mo porphyry deposit and has been a location of mining interest since 1864. Once copper ore is removed from this deposit it goes through a smelting process. Copper smelting processes consist of roasting, smelting, concentrating, and fire refining. During smelting, metals are separated from the waste, or gangue material. Slag is the waste material from these processes. Non-ferrous slag, such as a copper ore, removes iron and silica from the original ore. There is much interest in slag postproduction for there is often still a significant weight percent of metals trapped within a silica matrix. Slag in this study is from the Butte Reduction Works (BRW) copper smelter that started operation in 1886. The slag walls were constructed alongside Silver Bow Creek starting in 1894, in a now superfund site called Lower Area One, and have never been geochemically analyzed. The main goals of this study were to characterize the mineralogy, chemical composition, and environmental geochemistry of the slag walls in Lower Area One. The composition of BRW slag is dominated by Fe, Ca and Si. Silicates in the olivine and pyroxene groups, glass, and magnetite are the most abundant solid phases found in BRW slag. Olivine occurs as feather-shaped crystals that often grew within interstitial spaces. Some samples contain more than one olivine type: an Fe-rich olivine approximating fayalite (Fe2 SiO4) and an Fe-Ca olivine approximating kirschsteinite (CaFeSiO4). Pyroxenes occur as prismatic crystals that often radiate out from sulfide droplets like sunrays. More than one pyroxene type was consistent throughout these samples, a hedenbergite (CaFeSi2 O6) as well as a Ca-Fe rich pyroxene with the approximate formula Ca3FeSi4 O12. The olivines and pyroxenes are rich in metal impurities, including Zn, Mn, and Cu. Magnetite is abundant as euhedral grains disseminated through the slag, and the slag itself is magnetic. Sulfides and other metallic compounds occur as tiny spheres, or "prills", which are remnants of the molten Cu-sulfide matte that failed to separate from the slag during smelting. The main sulfides in the prills are bornite and chalcocite, commonly intergrown in an exsolution texture. Other prill phases include chalcopyrite, sphalerite, galena, and pyrrhotite.;Based on bulk chemical composition, some metals in the slag, including Fe, As, Co, Mn, Pb, Zn, and W, exceed USEPA screening levels for residential and/or urban soils. The BRW slag is not acid-generating, and instead has significant acid neutralizing potential in the form of Ca-olivine and Ca-pyroxene. TCLP and SPLP tests indicate an overall low potential to leach metals from the slag, although one sample exceeded the TCLP standard for lead. Concentrations of dissolved As and W were elevated in the SPLP leachates, possibly due to the high pH of the solutions after interaction with the slag. Interaction of BRW slag with Silver Bow Creek water gave variable results: some metals (e.g., Zn, Fe) showed a decrease in dissolved concentration whereas others (e.g., As, W) increased. Dissolved copper concentration increased after interacting Silver Bow Creek water with one slag sample but decreased slightly in the other two samples. These contrasting results are explained by heterogeneity in the composition and mineralogy of the BRW slag. Because Silver Bow Creek passes directly through the BRW slag walls, there is a possibility of release of certain metals from the slag to the stream water.