The Hidalgo Smelter
— Ibrahim H. Gundiler

When U.S. Congress passed the 1970 Clean Air Act Amendment, the Environmental Protection Agency established discharge limits for point sources of pollution that were universally adopted by the states. Among the regulations that mainly concerned the copper mining states in the Southwest was the requirement for recovery of 90% of the sulfur input at the copper smelters. The existing reverberatory-furnace smelters were able to capture only 50-60% of sulfur if they were also equipped with an acid plant. Most smelters were not. In 1976, Phelps Dodge Corporation built the first flash furnace smelter in the US. in Hidalgo County, New Mexico. The Hidalgo smelter, equipped with two acid plants, was also the first smelter able to meet the new air quality standards in the nation. This energy-efficient flash smelter was capable of smelting 2400 short tons of copper concentrate a day while capturing 96% of sulfur input and producing sulfuric acid. The smelter was closed and mothballed in September 1999.