USAC began mining antimony at the Stibnite Hill Mine near Thompson Falls, Montana in 1969. Mining was from 23 different drifts using room and pillar methods, slushers, jack-leg drills, and electric trammers. A heavy media separator (HMS) plant and flotation mill were installed. Initial concentrates were sold to smelters until 1971.
From 1971 until 1976 the concentrates were converted to metal first using the English precipitation method and the using an electro-winning plant. From 1977 until 1983 the concentrates were converted to sodium antimonite hexa-hydrate and sold to the television industry as a fining agent for television bulbs. Thereafter, antimony metal, primarily from China, and metal converted from third party sodium antimonite to metal, was sold as metal or converted to oxide and sold fumed.
Mining operations in Montana were curtailed in 1983 due to low prices. During a three-year period, the Company, operating as a Mexican Subsidiary named USAMSA, mined antimony at Sierra Guadalupe in Zacetecas, Mexico and smelted at Madero in Coahuila, Mexico. Buying Chinese metal became uneconomic and USAC relied on smelter byproduct feed from Canada. When prices strengthened again in the early 2000’s, we returned to Mexico to begin operations to enhance our raw material supply.