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Missouri is a State of Mines
Lead Milling

by Susan Ward Aber

Buick Lead Milling Plant

Mining the mineral simply begins the process of changing rough material into a useable product. For structural materials, such as sand and gravel, once the material is mined, washing and separating is all that is required for a useable product. Mineral extraction is much more complex and expensive. After the mineral and rock is mined from the ground, the ore begins processing in a mill. The ore rock is first crushed and ground up, then minerals physically separated by processes such as leaching, solvent extraction, or milling and flotation. Gold is often processed by heap leaching or bioleaching. Leaching involves spreading the ore over lined pads and spraying a weak cyanide solution on the "heap," or in the case of bioleaching, exposing the ore to a concentrated number of bacteria. The gold is recovered from the cyanide solution by an electrical process and refined into pure bullion. Solvent extraction-electrowinning is a process used in copper production, whereby electricity is used to extract the metal from a solvent solution. An example of milling and flotation is in the extraction of lead and the process utilized at the Buick Milling plant.

Doe Run operates four mills that process lead in southeast Missouri and the group toured the plant associated with the Buick Mine. The four mills have an aggregate production capacity of 30,000 tons of ore per day. The mills are for the most part automated and run from a central control room overlooking the milling operation. The milling sequence involves crushing, grinding, flotation, and filtering, which are shown with the images below!

Our guide detailed the operation before we entered the milling plant,
because once inside the noise level was too high for lecturing.

Crushing is the first operation in the milling process. The rod and ball mills are big drums containing steel balls (left). The raw ore material is tumbled in the drums and as they rotate, the ore is crushed into progressively smaller sizes. The crushed ore on average contains 5% lead, 1% zinc, and 0.3% copper, and looks like this (right). After the ore is ground and screened, it moves onto the flotation process.

The ground ore is mixed with water and a foaming agent.
This resulting slurry is agitated, which forms bubbles. The
mineral attaches to bubbles, floats to the top, and is skimmed off.

The flotation process separates the metal-containing minerals from the waste rock. The waste rock is known as tailings and is eventually discarded. The lead concentrates are transported to the Herculaneum smelter for refining into the finished 99.99% pure lead metal product. Zinc, copper, and silver are also recovered from the ore and sold to companies that will treat and refine them.

These "flotation cells" use both chemicals and air to separate the valuable metal ores. In a bulk flotation, zinc sulfide, lead sulfide, and copper sulfide, are separated. Some of the tanks were separating lead sulfide from the copper sulfide, while other tanks were separating zinc sulfide from the other minerals.

Filters remove 95% of the water and the concentrate is now over 75% lead
and nearly ready to move onto the smelting plant.

The entire milling operation is controlled and viewed from a central location in the plant. This elevated room has a bank of computers and a few employees who monitor the efficency of each step in the process. After the milling process is complete, the mineral concentrates are transported to the smelter or sold to other companies for further processing. The smelting process obtains the desired degree of purity and the mineral is then ready for commercial and
end use.

Return home or visit the lead mine or stone quarry sites.

Created July 10, 2000; last update, June 9, 2005. © 2000-2005 Susan Ward Aber. All rights reserved.