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In order to understand the Placer Dome story, basic knowledge of the principles behind the recovery of mineral resources is necessary. The following "mining primer" will be supplemented periodically with articles addressing technology issues of concern to the mining industry.

Cortez Gold Mines, Nevada, USA Mining is the process of removing ore from its original site beneath the earth's surface ("ore" being material that can be mined at a profit). However, since there are no "standard" orebodies, there are no "standard" mine workings.

The nature of the deposit will indicate whether open pit or underground extraction will be used. Where an orebody is covered by a relatively shallow layer of overburden, open pit mining may be practical in the initial stages. This type of mining uses highly mechanized methods of earth moving. Its economics are based on moving large tonnages possible only in surface operations with giant trucks and shovels. Ore trucks of 90- to 180-tonne capacities can be loaded in a few minutes by shovels lifting 11.5 cubic metres or more at a time. This kind of equipment has significantly changed the economic feasibility of previously uneconomic mineral occurrences.

Where a thick layer of overburden renders open pit mining impractical, the more traditional underground method must be used. Even here, the rapid implementation of modern technology has changed the economics, making concentrations of lower grade ore practical to mine. However, the costs of sinking shafts or driving adits (a nearly horizontal entrance from the surface) to access ore remain high and require that underground mines work with generally higher grade ore than that of open pit mines.

Milling is the primary step in the treatment of ore and is usually done at the mine. In certain cases, as with some gold ores, this treatment results in the extraction of gold doré bars (which must still be refined) on site. Most base metals, however, are found in a complex chemical form which cannot be economically converted into metal at the mine. In such cases milling simply extracts or concentrates the mineral so that it can be economically shipped to smelting and refining facilities.

All milling processes begin with the crushing and grinding of ore to a size suitable for subsequent operations. These processes produce a particle that is just small enough to ensure liberation or unlocking of the mineral from associated host rock.

Crushing, the first stage of this process, reduces the coarse material from the mine to pebble-size in preparation for grinding, the term used to describe the final steps of particle size reduction.

Grinding occurs in large, rotating cylinders in which tumbling steel shapes, and often ore chunks, crush the pebbles to a fineness which assures mineral liberation.

Separation processes are designed to remove the fine mineral particles from their host rock. Different processes have been designed for different minerals, but all are efficient at separating small amounts of minerals from masses of waste. For instance, typical gold ore in today's mines would average one twentieth of a troy ounce (14.583 troy ounces per pound) of the precious metal in 2,000 pounds of ore. This is a very small ratio indeed, being approximately equal to two parts of gold per million parts of rock. Yet the modern recovery process is so efficient that many mines can drill, blast, load, haul, grind, and treat 20 tonnes of ore to recover one ounce of gold, sell it at a market price of, say, US$365 per ounce, and still record a profit.

Flotation is the most common method of separating base metal minerals, many of which were deposited in sulphide compounds. The finely ground ore is mixed with water to form a slurry. Certain chemicals or reagents that coat only the desired mineral components are added in small amounts. Rising air bubbles capture the coated mineral particles and float them to the surface where they are skimmed off to a separate circuit. The waste rock, now called tailing, remains in the slurry. This process is capable of increasing the concentration of minerals many times and of separating several different minerals from a complex orebody.

The shipment of concentrates to smelting and refining plants is the final step in the series of events that began with a prospector's interest in a rock sample or the flicker of a needle on an electronic instrument.

But the story does not end here. Eventually every mine exhausts its ore. The wind-up phase then begins: its objective is to prepare the site so that it will remain in a safe and stable condition, not very different from the surrounding area.

  • Nuts & Bolts of a Placer Dome Project
  • A New Way of Mining

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    This page last modified:  August 11, 1997