Disposal Of Mining Waste Vastly Improved
While there’s a mother lode of controversy about what should be done with waste from modern mines, no one disagrees that tailings disposal has vastly improved.
Historic ore-processing mills sometimes squatted directly over streams, so the leftovers of the mineral-extraction process would be swept downstream. Or tailings were simply piled up anywhere, left to blow or wash away.
In the Coeur d’Alene River drainage alone, scientists estimate, 72 million tons of tailings went into the waterways from the 1880s to the 1960s.
That material was contaminated with lead, zinc, cadmium and other metals. In high enough concentrations, those metals can threaten the health of humans and wildlife.
At first, zinc wasn’t valuable and was simply discarded. Crude crushing and sorting methods left half of the valuable mineral in the rock. A change in ore processing in the 1920s meant more than 90 percent of the metals were removed, but also greatly increased the amount of tailings produced.
Two things come out of a mine.
The ore is what the miners are after. The waste rock is what they must dig out to get to the ore - it’s piled up somewhere, but not processed.
Tailings are what’s left after the ore is pulverized and the valuable minerals are extracted with chemicals, or with chemicals and gravity.
Some tailings end up as sand. The rest, ground as fine as talcum powder, are called “slimes.”
“It’s not barren material,” explained Steven Hoffman, a scientist with the Environmental Protection Agency. “Tailings are everything that was in the earth, except the material the mining company was after.”
Ore processing plants are huge, noisy places. The equipment includes what look to be locomotive-sized garbage cans, turned on their sides. These are “ball mills,” filled with steel balls that grind the rock to powder. Water is added to that.
“The slurry is pumped into big ‘bathtubs’ with a paddle on top. These are the frothers,” said Hoffman. “Add chemicals, and it creates a foam that has an affinity for the metal you want to float. The microscopic metal attaches to the bubbles, and it’s skimmed off as foam. … Everything that isn’t skimmed off is tailings.”
The tailings can be refloated several times, using different chemicals to capture different metals. Those are shipped to smelters for refining.
The tailings - 60 percent solid, 40 percent water - are normally piped to a tailings impoundment. The water floats to the top of the pile, where it creates a pond. Water from the pond is then pumped to the mill for reuse.
, DataTimes