Monday, March 25, 2013

A Mini-Rant about Mine Dumps

"The briefest description of waste rock dumps is from Western Australia—all you need to know in two pages. [May not be original link; also condensed here to one page.] The most comprehensive description of a waste rock dump is in the EduMine course Design and Operation of Large Waste Dumps. In practice no definition is needed; simply see the picture on the front page of this review."

— Jack Caldwell, from Waste Rock Dumps, rev. 28 June 2006
A mine dump, or waste rock dump, by definition consists of waste rock from a mine — whether it be an underground or surface mine — that has been dumped somewhere, usually quite close to the mine, sometimes even inside a surface or underground mine, so as to minimize motorized or track haulage costs.

Old mine dump.
A heap leach pile (or leach pile or heap) is not a mine dump: the rock consists of ore that is (or was at some time) being processed by some form of leaching. A leach pile sits on a leach pad liner, and can't be moved without placing it on another liner (environmental requirements).

Tailings should not be called mine dumps, either: they are the tails that have come out the back end of the ore processing plant; the heads went in the front end. Tailings (or tails) were once ore, not waste, or they wouldn't have gone into the mill for milling and then processing. They are usually found behind tailings dams (although not always). Tailings shouldn't really contain any ore, but sometimes — perhaps if the mill or recovery plant isn't operating properly or if the ore going into it is of an unexpected or unusual character — some amounts of the ore mineral may get through to form ore-bearing lenses behind the tailings dam. Sometimes — usually in the future when (if) the price has gone up, unless your mill is currently letting through too much unprocessed mineral all the time (therefore right now and also in the future) — the entire tailings mass may comprise a low-grade ore deposit. Many tailings deposits have been sampled or drilled as potential ore. Tails near Kalgoorlie have been mined; Eldorado Gold plans reprocessing of tailings at its Olympias, Greece, project.

Mine dump inside old, inactive open pit.
Many mines stockpile various kinds of ore, often ore that is in an intermediate low-grade category below one cutoff grade and above an absolute minimum cutoff grade. For these "low-grade" stockpiles, the plan is to process them through the mill or on a heap leach pad at some later date, either when (if) the price goes up or toward the end of the mine when many up-front mining and processing costs have been paid off. Sometimes these stockpiles become waste (and therefore become part of a larger mine dump or waste pile) simply because they don't get mined: the price doesn't go up, or end-of-mine costs aren't low enough to make them worth re-hauling, or mining is shut down for whatever reason before the low-grade stockpiles are processed. Other types of ore stockpiles include those made of high-grade ore or some particular type of ore that is recalcitrant to your usual processing. These piles may require special processing or blending with other types or grades of ore.

Mine dump in fog at sunrise.
Waste on mine dumps can become ore at a later date if mining and milling technologies improve or if the price of the commodity of interest goes up. Material on leach pads theoretically becomes non-ore as the metals of interest are leached out, but again, the material is subject to future price and technology changes, and leach pads fail to produce sometimes, leaving the original metals intact or partly intact (it could be argued that the leach piles with unfavorable recovery rates were technically not ore in the first place). I suppose that one could argue that the material on already leached pads is now waste rock, and therefore the old leach pile is a mine dump, but as I mentioned before, the pile is probably on a liner, and it therefore has to be handled differently from any other piles of rock around the now inactive mine.

One could also, perhaps, argue that the tails or tailings are now waste and therefore a mine dump, but I cringe at this idea, because there really is a specific name for that material: tails or tailings. Tails can become ore after their creation if mining technologies improve or if the prices of commodities contained within go up. One hopes that a mill isn't putting too much metal out in the tails; that usually implies a poor recovery rate, and poor recovery rates are usually unfavorable to profitable mining. And mining has to be profitable, or the individual mine owner or individual miner (yes, there are a few), the mining company, or the shareholders of a mining company would be better off putting their money into a bank or a mattress instead of investing in the mine.

Old mine dump.
Most of the mine dumps I see out and about are waste piles and dumps near an old working: a shaft, decline, adit, glory hole, or other old and inactive complex. When I'm working at or near an active mine, I can usually recognize the mine dumps because the haul trucks are actively dumping loads of waste. It can, however, be difficult to look at any given mine and know which piles of rock are currently in the waste or ore category.

I do get tired of hearing every single pile of rock near a mine being called a mine dump.



mrsalphageek said...

During my college years, I took a summer job at a copper mine in Arizona. I was assigned to the SXEW plant. Our feed for the plant was re-pulped tailings. Technology and economics had made it feasible to remove more of the previously inaccessible copper. Thanks for the intersting article.
Dianna J.

Silver Fox said...

Glad you enjoyed it!

Desert Survivor said...

Thanks, i didn't know the diference!

Utemike said...

Excellent concise rant. My favorite came from an Oklahoma employee who could not appreciate Nevada dumps or Nevada.

Passing the Newmont dump north of Carlin he observed, "they won't be happy until they turn this whole state upside down, and it will look better when they get done."

Silver Fox said...

But probably still wants mining to stop.