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Hardrock Mining Overview



Definition

Hardrock mining involves uncovering and extracting non-fuel metal and mineral deposits of solid ores or eroded deposits in streambeds. Eleven common minerals mined are copper, gold, iron ore, lead, molybdenum, phosphate rock, platinum, potash, silver, uranium, and zinc.


Economic Value

The combined value of the eleven metals and minerals listed above was $12.15 billion, less than 1% of the gross national product in 1993 (U.S. Department of Commerce 1994). These metals and minerals are the primary raw materials used in many industrial applications and are thus essential to the American and world economies. The mineral industry also contributes to the national economy by virtue of its production of exports and reduction of industrial dependence on certain minerals that the United States would otherwise import.


Location

As the maps indicate, most mining for base and precious metals occurs in the western United States while mining for industrial rocks and minerals is more common in the eastern United States.


Mining Activities

Hardrock mining involves three basic steps: exploration, extraction, and beneficiation.

Exploration
Exploration involves locating and evaluating a suitable ore. Several mineral exploration methods are:

Surveys help identify conditions indicating the potential for an economically recoverable mineral deposit. A desk top survey includes a review of aerial photographs, geologic and geophysical maps, and published reports. A site survey often includes stream, sediment, and rock sampling. One valuable form of site surveys is remote sensing. Most naturally occurring materials absorb sunlight at specific wavelengths. Each mineral has a unique spectral signature related to its chemical composition, grain size, degree of cystallinity, and temperature of formation. The spectral signature is a measure of how reflected sunlight interacts with a surface. Aircraft or satellite systems can remotely collect spectral information over vast areas and thus determine the potential locations of ore deposits.

Drilling is typically undertaken if geologic conditions are promising and surface sampling indicates a possible ore body. Drilling is used to obtain core samples of the ore body at various depths for inspection and analysis. If further exploration of the potential deposit is necessary, drilling can be used to delineate the boundaries of the deposit, as well as the size and grades of the ore. Sophisticated statistical methods are used to infer the characteristics of ore bodies based on a limited number of samples.

Seismic Surveys rely on shock waves produced by small explosives detonated at or near the surface. Electronic receivers known as geophones are placed in strategic locations to measure the speed of shock waves, which travel at different speeds through different geological formations. A seismic survey measures the time it takes shock waves to reflect from various points between formations of different densities. This information may indicate the existence of certain geologic formations of interest.

Although not directly related to exploration, other activities are critical in establishing a mining operation, including the following:

  • Site development includes construction of access and haul roads, rail lines,
    barge terminals, and utilities.

  • Construction of mining facilities involves construction of shafts, support
    buildings, and housing facilities.

  • Construction of beneficiation facilities includes construction of pipelines,
    conveyance systems, leach pads (if needed), processing ponds, and a mill (if
    necesscary).

    Extraction
    Extraction involves removing ore deposit from the ground. The two basic types of extraction are surface mining and underground mining.

    Surface Mining, the most common form of hardrock mining in use today, consists of extracting the ore by digging. The costs of machines and maintenance for surface mining are low compared to those for underground mining. The basic steps of surface mining are:

    1. Overburden, or waste rock, removal: The amount of waste rock that must be
    removed varies, but is often enormous. The less valuable materials normally
    associated with the more valuable ore deposits are termed gangue materials.

    2. Blasting: Explosive charges are detonated to break up the waste rock and ore.

    3. Mucking: Heavy machinery removes and transports the ore.

    4. Primary crushing: The ore passes through primary crushing to reduce the size of
    the material to transport to the mill.

    5. Hauling: Trucks, rail cars, or conveyors move the ore from the mine to the mill.

    The two main forms of surface mining are open pit and strip mining:

  • Open pit mining is the most common method of extraction and involves a
    concentated excavation. The size of an open pit mine can vary from a small
    and superficial site to a pit more than 1000 feet deep and several square
    miles in area. Although the same techniques are used to develop mines, each
    mine is unique and its final structure depends on topography, the location
    and size of the ore, the mineralization of the ore, and other factors.
  • Strip mining is used to extract horizontal, near surface ore deposits (e.g.,
    phosphate). Strip mining progresses linearly and yields long disturbances.
    Figure 2 illustrates strip mining.

    Underground mining involves digging vertical shafts and horizontal tunnels, or adits, to recover ore deposits. Underground mining is more expensive and requires more skilled workers and specialized equipment than surface mining. Underground mines must enable workers to access, break, and remove the ore. Figure 3 illustrates an underground mining scene.

    Beneficiation
    Beneficiation is the processing of ore to separate the target mineral from the waste rock. Before beneficiation, nearly all crude ore must be reduced in size. Primary crushing reduces ore from 2-4 feet boulders to rocks 8-10 inches in diameter. Secondary crushing reduces the ore into pieces less than 1 inch in diameter. Large rotating cylinders grind the material, creating finely ground mill. Slurry is formed when water is added to finely ground mill.

    The type and extent of beneficiation depend on the desired product. Some common types of beneficiation are:

  • Gravity separation relies on large differences in density between the target
    metal (e.g., gold) and the materials in which it is found. It also requires that
    the target metal cannot be bound to the surrounding rock matrix. Some
    devices used in gravity separation are trommels, sluices, spiral
    concentrators, cyclones, jigs, and shaker tables.
  • Floation is adaptable to small particals and commonly used to recover lead,
    copper, platinum, and zinc. In floatation, the addition of a reagent chemical
    to ore slurry causes the minerals to become less dense than the gangue and
    rise to the top of the tank.
  • Leaching is the process of extracting a soluble metallic compound from ore. It
    involves dissolving the ore with a suitable solvent, such as sufuric acid or
    sodium cyanide solution.


    References

    U.S. Department of Commerce. Bureau of the Census, 1994. Statistical
    Abstract of the United States. Washington, D.C.

    U.S. Department of the Interior, Bureau of Mines. 1992. Minerals Yearbook,
    Volume I: Metals and Minerals. Washington, D.C.

    U.S. Environmental Protection Agency, Office of Water. 1997. EPA's National
    Hardrock Mining Framework. Washington, D.C.

    U.S. Environmental Protection Agency, Solid Waste and Emergency Response.
    1997. Introduction to Hardrock Mining: A CD-ROM Application
    Washington, D.C.