Pennies dated before 1982 are 95 percent copper. When copper prices soared, pennies became 97.5 percent zinc, with a thin copper coating. Today, Doe Run harnesses the value of copper and zinc to deposit its own pennies in the bank.

To understand the local origin of these metals, consider Missouri’s mineral deposits. Deep underground, Doe Run employees free rock containing lead, zinc, copper and silver among other trace metals.

PHOTO CAPTIONS: (LEFT) In 2007, Doe Run built a copper flotation circuit at its Sweetwater Mine and Mill. (RIGHT) Zinc recovery occurs at Doe Run’s four mills, employing a differential flotation process.

With lead as Doe Run’s main product, employees go to great lengths to remove other materials in the ore. After metal byproducts are collected, they are used or sold whenever feasible. However, it is difficult to completely separate lead from all metals, particularly copper.

To access mineral deposits, Doe Run employees must travel deep underground where rock containing lead, zinc, copper, silver and other trace metals are methodically freed, crushed and conveyed to the surface for processing.

Primary copper recovery takes place at the mills after the mineral-containing ore from the mines has been crushed to a sand-like substance. In 2007, Doe Run built a copper “flotation circuit” at its Sweetwater Mine and Mill with existing materials and supplies. The end result was a cost-effective way to process copper ore on-site instead of transporting it to another mill. With the addition of this circuit, all four of SEMO’s mills can now effectively recover copper from ore.

“We can efficiently remove most of the copper by chemically floating it away from the finely crushed rock, making a saleable copper concentrate,” explained John Boyer, Doe Run general mill manager.

Ore tests in June 2008 showed that copper extracted from Sweetwater was indeed high-grade enough to process and sell. “Copper provides us with another revenue stream within our lead-mining and milling process,” Boyer said. “We also see environmental and economic benefits. By removing copper at the mills, we truck less material to Herculaneum [for removal].”

Because it is difficult to completely separate lead from by-product metals during the milling process, another copper separation process must take place at the Herculaneum Smelter. Viewed by lead smelters as an impurity, copper is separated from lead by using a heating and cooling process that causes copper to rise to the top of liquefied lead. This copper “dross” is carefully skimmed off the top, dried and sold.

Zinc is another byproduct of lead mining. Doe Run’s zinc recovery occurs at the four mills, employing the same “differential flotation” process.

A zinc flotation circuit has two parts: “roughers” that recover the majority of the zinc minerals, and “cleaners” that help purify the final product. This combination allows zinc to float, be collected and dried. The result is a sandy, concentrated material that contains approximately 60 percent zinc.

A penny for your thoughts …

• Doe Run’s production in 2008 included 21,586 dry short tons of copper concentrates which contained 28.76 percent copper.
• Missouri ranks fifth in U.S. copper production.
• The standard benchmark for electrical conductivity, copper conducts electrical current better than any other metal except silver.
• More than half of the zinc consumed is for galvanizing metals to protect against rusting.

 

 

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