Missouri has three world-class lead/zinc subdistricts (Old Lead Belt, Mine La Motte-Fredericktown, and Viburnum Trend) and several minor subdistricts that are in a region referred to as the Southeast Missouri Lead District. Arsenic, cadmium, cobalt, copper, lead, nickel, and zinc are the primary trace elements associated with the sulfide minerals of the Mississippi Valley Type ore deposits present in the district.

The MVT deposits of the Southeast Missouri Lead District are metal-sulfide deposits that are hosted in Paleozoic dolostone, limestone, and to a lesser extent sandstone. The primary metal-sulfide minerals of these deposits are pyrite–marcasite (FeS2), galena (PbS), and sphalerite (ZnS). In localized areas, mixed-metal sulfide minerals also occur in minor amounts. These include copper sulfide minerals such as bornite (Cu5FeS4), chalcopyrite (CuFeS2), and enargite (Cu3AsS4); nickel sulfide minerals such as millerite (NiS) and vaesite (NiS2); and iron sulfide minerals such as arsenopyrite (FeAsS) (Rakovan, 2007).

SEMO’s mining operations take place up to 1,250 feet underground. The mines extract more than 20,000 tons of lead-bearing ore every day.

Millions of dollars worth of equipment operates underground, including hydraulic drills, trucks, loaders, charging rigs and scalers – all of which are disassembled to transport down the shaft and reassembled underground.

SEMO uses the room-and-pillar mining method, in which rock pillars (up to 35 feet wide and 120 feet tall) support the ground above.

Primary recovery of lead, zinc, and copper bearing ores takes place after the mineral-containing ore has been crushed and hoisted by shaft to the surface Mill facilities. The ore is then ground to thesize of beach sand with a rod and ball mill.

There, the ground ore is fed into flotation tanks where tiny air bubbles are introduced. Added to the mixture are chemicals known as a reagents, which bonds to the tiny lead and copper minerals particles and lifts them to the surface where a bubbly froth forms. The froth is skimmed for final separation of the lead and copper minerals. Any remaining materials, such as zinc minerals and dolomite, sink to the bottom of the tank. The zinc minerals are separated from the dolomite in a similar fashion.

The wet minerals are concentrated in large tanks called thickeners. The concentrates are then dried with large vacuum filters that remove the majority of the moisture. Three different concentrates are produced, lead, zinc, and copper. The resulting lead concentrate contains approximately 77 percent lead.

At this stage, the lead concentrate is transported to Doe Run’s Herculaneum Smelting Division for smelting or sale to foreign traders. The copper and zinc concentrates are sold to interested buyers.