The mining process consists of three parts: mine development, resource extraction and initial processing.
Mine development is typically the most capital-intensive phase, but it is also the part which has the greatest effect on how reclamation will be performed. Mine development consists of setting up the infrastructure and controls that allow for the systematic and cost effective extraction of the resource to be mined. The mine infrastructure includes transportation on and around the mine; the utility lines necessary to move water, wastewater, power, and ventilation air for underground facilities; storage and stockpile areas; onsite processing facilities; and buildings for personnel. The procedures and facilities necessary to minimize post-mining and offsite environmental impacts are also components of the mine infrastructure. Mining controls include those practices and procedures that guide the safe and efficient extraction of the resource to be mined in compliance with regulatory permits and environmental standards.
While the reclamation standards and end use for a mine are set forth during the exploration and permitting phase of a mine’s life, the actual methods and timing for the reclamation are determined during the development process. During mine development, the operator will determine whether mining will allow for reclamation of areas where mining is completed concurrent with on-going mining in adjacent areas. Reclamation efforts, including, but not limited to, backfilling, grading, topsoil replacement and revegetation of all land that is disturbed by surface coal mining operations are required to be completed as contemporaneously as practicable with mining operations. Specific timeframes for rough backfilling and grading are required by law for all surface coal mining to ensure that reclamation efforts do not lapse. While not required by regulation, operators of mines extracting metals, sand and gravel, and industrial minerals often perform reclamation concurrent with mining for economic reasons. In some cases, such as where a post-mining pit may be converted to a water storage reservoir, development may include installing the barrier that will act as the pit liner before mining occurs to isolate the mining from the adjacent groundwater.
Resource Extraction and Initial Processing
The actual mining process consists of resource extraction and the initial material processing that is conducted on the mine site. Resource extraction does not begin until the necessary safety and environmental controls are in place. Mined deposits can loosely be lumped into one of two categories; soft rock deposits, and hard rock deposits. Resources that are typically mined from soft rock deposits in Colorado include coal, sand and gravel, clay, placer gold, and gypsum. Resources that are typically mined from hard rock deposits in Colorado include crushed rock, dimension stone, molybdenum, lode gold and silver, and limestone. Mineral extractions from soft rock deposits are generally performed by using earth-moving equipment to excavate the material directly from the ground. Hard rock material extraction generally requires the use of explosives to excavate the material.
In the case of a surface mine, the first step in the mining process is the stripping of vegetation from on top of the area to be mined. Trees, stumps, and other large vegetation may be shredded for later use as compost, or may be placed in an onsite disposal location. The topsoil and other overlying geologic materials that are not part of the body being mined are also typically stripped from the top of the resource to be mined. Topsoil stripped following vegetation removal may, and in the case of coal mines must be stockpiled onsite for replacement at the approximate same thickness from which it was removed. After the mined resource is excavated, it is either loaded onto trucks for transport to an onsite processing location or for haulage offsite. If the mined material will be processed onsite, it may also be loaded directly onto a conveyor belt that takes it to the processing location. If sub-economic rock material is encountered during mining, it is typically placed into stockpiles on the site, and the waste material stockpiles may be shaped to help blend them into the surrounding topography, or kept for additional reclamation fill.
For coal, waste banks or refuse piles have to be permitted and approved. Onsite disposal areas are designed, constructed, inspected and maintained throughout the life of mine project until bond release. Specific water control measures and construction requirements are strictly enforced.
The purpose of onsite processing of mined material is to either produce a finished product that can be sold directly from the mine site, or to minimize the amount of material that has to be transported off of the site for final processing. Onsite processing of the mined material typically starts with the material being classified by size, and if necessary, crushing of oversize material to the size required for sales or additional processing. Crushing may be the only processing that is done onsite. If additional processing occurs onsite, it may include washing, separation of specific minerals from the overall mined material mass, or chemical extraction of specific compounds. Coal used for power generation (thermal coal), may go directly from the mine to the power plant by conveyor.
The waste material generated onsite during mineral processing may be used as reclamation fill, or placed in stockpiles that are revegetated or otherwise stabilized. Topsoil stockpiles must be protected from wind and water erosion at all times. Subsoil on coal mine sites is also segregated and analyzed for suitability as reclamation fill. The remaining spoil material on surface coal mine sites is stockpiled in ridges for backfilling the open pits prior to subsoil and topsoil replacement.
That’s mine development followed by extraction and processing in a nut shell. But that is not the end of the life of the mine! Read more on the important next phase, RECLAMATION.