Appendix: THE PHASES OF MINING

(From C1 Policy Papers Conservation International Lightening the Lode – A Guide to Responsible Large-scale Mining – Amy Rosenfeld Sweeting and Andrea P. Clark)

The specific steps in metals mining operations may vary, depending on the size and shape of the ore body, the specific metal being mined, the extraction method and the scale of the planned operation. Nevertheless, all mining operations follow roughly the same course, consisting of two, inter-linked cycles. The mine cycle includes exploration, project development and construction, mine operation and ore extraction, closure and reclamation, while the mineral production cycleis the actual processing of the extracted ore, from crushing to concentration to final refining.

A.1 THE MINE CYCLE

The mine cycle begins with both basic and more complex exploration techniques, designed to pinpoint the location of a mineral ore body. Once the deposit has been found, the mine project is developed and the mineral is extracted using either underground or surface mining techniques.

A.1.1 Exploration

The first step in the mine cycle is finding the exact location of a mineral deposit. The exploration, or prospecting, process is based on the search for a combination of various geologic conditions that might indicate an ore deposit. This exploration might begin with a preliminary survey using remote sensing techniques such as aerial photography or satellite imagery, to identify specific geologic features.

Once a general location has been identified, prospectors complete a more detailed geologic mapping of the area, involving on-the-ground studies and analysis of surface samples through geochemical or geo-physical testing. Geochemical surveys use chemical principles to detect trace chemicals in samples from streams, soil, rock, groundwater and vegetation. Geophysical surveys use physical principles to measure the electrical and magnetic characteristics of rock from the area, in order to determine their mineral potential.

If the results of these surveys are promising, conventional methods of exploration such as trenching, excavation of small pits or shaft sinking may be used to determine the shape and character of a deposit. Shafts, which may be up to 1-2 meters square and 10 meters deep, or trenches, which may be 20-30 meters long and 1-4 meters wide, are dug with backhoes or other equipment to expose the mineral rock for further testing. Bulk sampling, which involves removing a large sample (10-50 tonnes) of the mineral body from the ground and processing it, may take place at the end of this phase.

Finally, after the approximate location of the deposit has been narrowed down, drilling is used to more precisely determine the direction and shape of the deposit. Drilling, which involves small, narrow-diameter drills, may include rotary drilling, which brings up broken up rock cuttings, or diamond drilling, which produces intact core samples. Drills may be stand-alone or truck mounted, and may require the construction of roads for access.

A.1.2 Feasibility studies and project development

Once a company has located an ore body, the next steps in the mine cycle include feasibility studies, basic project development and preparation for mine operation. Before making a final decision to continue development of a mine site, the company conducts a feasibility study to evaluate options and needs for development, and applies for mining permits and licenses. This study includes environmental and social impact assessments based on primary studies of the local ecosystem and consultation with stakeholders.

General project development involves several steps, including engineering of the basic mine structure and methods, and construction of infrastructure, including treatment plants, waste disposal facilities, access roads, storage facilities and staff facilities. Project development also includes ensuring stable and adequate sources of fixed and emergency power and water, and securing financing for all stages of the project.

A.1.3. Mine operation

The mine operation phase of mining is when the mineral is actually extracted from the ground. The two basic methods for mineral extraction are underground and surface mining. While the choice of method depends on the size, shape and depth of the ore body, all operations involve the basic steps of ore breaking, loading and hauling to a mill for treatment. The amount of rock removed depends on the grade of the ore body being mined. While about one-third of iron ore extracted eventually becomes a final refined product, refined copper represents only about 0.6 percent of the rock removed for its production. For gold, this percentage can be even lower: each ton of ore removed from a gold mine will result in an average of only about 1.5 grams of refined gold.

A.1.3.1. Underground mining

Underground mining is used when a deposit is located deep in the ground (up to 3,000 meters in some South African mines) and is of high enough grade to merit the development of shafts and associated infrastructure. Lead, zinc and gold are often mined using underground techniques, while silver and copper are also occasionally of high enough grade to be mined profitably underground. To gain access to an underground ore body, tunnels and shafts are drilled into the ground using explosives and heavy machinery. From these main tunnels, drifts and crosscuts provide access to mining areas called stopes. The area being mined at any one time is called the face.

Once the ore body has been reached, the rock is broken up using explosives. This broken rock, called muck, is then hauled to the surface by trains, trucks, loaders, trams or conveyors, in a process called mucking. An extraction/hauling system at the surface, composed of a hoist, skip (elevator) and towers, is sometimes used to lift the rock out of the ground vertically.

Once a stope has been mined out, it is often backfilled to provide stability and revent surface subsidence. Waste rock, crushed surface rock or even tailings may be used for backfill. These substances are sometimes mixed with cement and water to provide even more support.

A.1.3.2 Surface mining

Surface mining, which is considerably cheaper than underground mining as well as safer for miners, is used to extract minerals that are located within a few hundred meters of the surface and have little waste rock over them. Two-thirds of the world’s solid mineral production comes from surface mines.

Most surface mining of metals is done from open pits, which involves digging a large pit to expose the ore body below the surface. The first step in developing an open pit mine is removing surface vegetation. The topsoil is then removed and can be stored for later use or used to reclaim previously mined areas. Next, the overburden, which consists of subsoil and rocks above the mineral-containing ore is removed to expose the ore body.

An open-pit mine generally is constructed in the form of an upside-down cone, with large flat steps, called benches, about five meters wide with vertical sides as much as 12 to 18 meters high, winding down along the walls. These steps are built to make the walls more stable and to provide working surfaces and haul roads. The world’s largest open-pit mine, the Bingham Canyon Copper Mine in Utah, is one-half mile deep and more than 2 miles across.

Once the overburden has been removed to expose the hard rock below, miners drill blast holes in the rock and load them with explosives, which are set off to break up the rock. The rock is then loaded into large trucks, conveyors or trains to be hauled to crushing facilities for the first stages of mineral processing. (See section A.2)

During this process, the mineral-containing ore is removed along with waste rock that does not contain economic quantities of the target mineral. The ore is separated for further processing, while the waste rock is generally piled in waste dumps next to the pit. These dumps can be very large, depending on the stripping ratio of a mine. The stripping ratio is the amount of waste rock generated for each ton of ore produced. In an underground mine, this ratio can be as little as 1:1, but in an open pit mine, in rare instances, it might be as high as 40:1, meaning that, for every ton of ore removed, up to 40 tons of waste rock are removed from the ground.

A.1.4 Mine site reclamation

Once a pit has been mined out, the mining company closes and reclaims the mine area. Reclamation generally begins with the dismantling and removal of buildings and other infrastructure. Next the waste dumps and tailings impoundments are contoured and a revegetation program is begun. Open pits are rarely backfilled because of the great expense of such an operation and because changing economics might make the pit viable again sometime in the future. Backfilling a pit would rule out the possibility of reopening a mine by digging deeper into an existing pit. More often, the waste rock dumps are con-toured and replanted, often using stored topsoil.

The main goals of reclamation, which is discussed in further detail in Chapter 3, are ensuring that there is no future contamination from the pit, returning the land to a productivity level equal to or higher than before the mine, and making the area safe and esthetically pleasing for surrounding communities.

A.2 THE MINERAL PRODUCTION CYCLE

While some non-metallic minerals, such as sand, marble, limestone or clay, are used virtually in the same form in which they are extracted from the ground, requiring little processing, metals are usually found in small particles scattered throughout the ore, and require several steps of processing to produce a valuable, purified form of the metal. The mineral production cycle, which begins after the ore is hauled out of the open-pit or underground mine, utilizes several physical and chemical processes to concentrate and refine the desired metal.

A.2.1 Comminution

Before the ore can be processed to produce a purified metal, the minerals must be liberated from the waste material, or gangue, surrounding them. To do so, the ore is crushed and ground to reduce its particle size, in a process called comminution. Primary comminution is done in a large jaw or gyratory crusher that is used to crush large rocks into coarse pieces. This is often done right at the pit. Next, the ore undergoes secondary crushing or grinding, usually in a mill located away from the mine itself. In a grinding mill, the coarse pieces are ground into finer particles. Some mills are “charged” to quicken the grinding using steel balls or rods. During each stage, the material is sorted by size, using screens and classifiers.

A.2.2 Concentration or beneficiation

Once the ore has been crushed and ground to the necessary size, the minerals are separated from the gangue through a process called concentration, or beneficiation. This process leaves a concentrate of the target mineral that is then ready for further refinement. The concentration process for most metals is usually done on site in a processing plant, except for some nickel ores, which may be transported directly to a smelter off-site. There are two basic types of beneficiation: physical separation and chemical separation.

A.2.2.1 Physical separation

Physical separation processes rely on physical properties of the target minerals to separate them from the gangue. The three most common methods are flotation, gravity separation and magnetic separation. No matter which process is used, the final step in physical separation is dewatering, to separate the metal from water.