Smelting
Robert Peele’s 1916 Mining Engineering Handbook defines ore: “An ore is a metal-bearing mineral, or aggregate of such minerals, mixed with barren matter, called ‘gangue,’ and capable of being mined at a profit.”
The metal-bearing minerals in the Butte Hill include copper sulfides as well as the sulfides of other base metals, like zinc and lead. The copper sulfides are chemical compounds consisting of atoms of copper and sulfur as well as other elements, like iron and arsenic, and with names like chalcocite (Cu2S), bornite (Cu5FeS4), enargite (Cu3AsS4), and chalcopyrite (CuFeS2). To convert ore into metallic copper, those molecules of copper sulfide must be broken apart to yield pure copper and to dispose of the other elements. Doing that requires smelting, usually in furnaces to raise the temperature so that oxygen in the atmosphere can combine with the sulfur and other elements to liberate the copper.
Some of the first copper ores found in the Butte Hill were rich enough in copper and other metals that they could be shipped to distant smelters for treatment and still yield a profit, but miners knew that most of Butte’s ores would have to be smelted locally to be profitable. The Montana Copper Company built Butte’s first successful copper smelter in 1879. Within a few years, there were another half dozen copper smelters in Butte. Operators of each of these smelters recognized that they could be more profitable if they built concentrators to mechanically separate much of the gangue from the metal-bearing minerals before sending the concentrates to the smelter. Through the end of the nineteenth century and into the twentieth, Butte was a major smelting center.
When Marcus Daly launched his Anaconda Company, he recognized that water, needed for concentrating, was scarce in Butte, and that Butte’s valley location had a limited capacity for the smoke that smelting produced. He decided to build a smelter at his new town of Anaconda twenty-six miles west of Butte along Warm Springs Creek. The first Anaconda smelter went into operation in 1884 with a capacity five times greater than any of the Butte smelters. He built another smelter at Anaconda in 1889. Both of those smelters were replaced in 1902 by the more-modern Washoe Reduction Works. Gradually, the Butte smelters closed, with the last one closing in the 1920s. Ore was sent to Anaconda for smelting, where that smelter had grown to giant proportions. Consequently, Butte came to be known as the “Mining City” and Anaconda as the “Smelter City.”
Almost all traces of Butte’s once prominent smelting industry have been wiped from the landscape. The one exception is the “slag canyon” through which Silver Bow Creek passes just west of Montana Street, where the Butte Reduction Works once stood. Smelters produce two sets of waste products: smoke, which carries sulfur dioxide, arsenic, flue dust, etc.; and slag, which is a molten waste from the smelting furnaces carrying mostly silica and iron but also other metals. When the Butte Reduction Works was operating, it had to control the flow of Silver Bow Creek through its property and also address the complaints of downstream farmers who claimed that tailings from the concentrator were damaging their lands and crops along the stream. The Butte Reduction Works used molten slag, poured into formwork, to build the walls that form the canyon, and also to build slag walls visible further west to impound the tailings and prevent them from flowing downstream. The tailings were reprocessed in the late 1910s, leaving the slag walls as the only physical remains of Butte’s once important smelting industry.
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