Environmental Code of Practice for metal mines: chapter 2
2. Mine Life Cycle Activities
- 2.1 Exploration and Feasibility
- 2.2 Planning and Construction
- 2.3 Mine Operations
- 2.4 Mine Closure
In this document, the mine life cycle is described in the following steps, or phases. These phases and the associated key activities are illustrated in Figure 2.1, and consist of:
- the exploration and feasibility phase;
- the planning and construction phase;
- the mine operations phase; and
- the mine closure phase.
Figure 2-1: Activities of the Mine Life Cycle
This figure illustrates the main activities of the mine life cycle. The first phase is exploration and feasibility, where activities include reconnaissance, locating mineral anomalies, discovery, sampling, and economic feasibility decisions. The second phase is planning and construction, where activities include mine planning, environmental/social planning, closure planning, environmental and other permits, clearing, stripping, blasting and infrastructure. The third phase is operations, where activities include ore crushing, grinding, concentrating, waste rock, tailings and wastewater management, and progressive reclamation. The final phase is closure, where activities include site clean-up, reclamation, rehabilitation, maintenance, and environmental monitoring.
This section provides a brief overview of the activities that take place during each phase of the mine life cycle. Associated environmental concerns are discussed in Section 3.
2.1 Exploration and Feasibility
The objective of initial exploration is to identify and assess mineralized areas to determine whether more intensive exploration is warranted. The methods used in initial exploration include the following.
- Geophysical Surveys: Geophysical survey techniques include magnetic, electromagnetic, electrical, radiometric and gravity techniques, and surveys can be conducted from the air or on the ground. These surveys provide information on potential targets for ground-based exploration.
- Prospecting and Geological Mapping: This can involve the mapping and sampling of targets identified in airborne geophysical surveys, regional-scale mapping and more detailed mapping of areas of particular interest. The objective is to provide a preliminary assessment of the potential for mineralization over a relatively large area.
- Geochemical Surveys: A range of materials may be sampled, most commonly rocks and soil. Samples are sent for chemical analysis for metals of interest. Results of the analyses are compiled and compared with the results obtained from other exploration methods.
- Diamond Drilling: Diamond drills recover a core of rock, and cores from several holes allow geologists to build a three-dimensional picture of the local geology. Core samples are also subjected to chemical analysis.
- Trenching: Trenches may be dug or areas of outcrop stripped of vegetation and soil to enable mapping of near-surface geology and for bulk sampling where ore and other geologic units may be very near the surface.
In areas where the results of initial exploration are positive, advanced exploration may commence. The primary goals of advanced exploration are to define the quantity and quality of potential ore and the geometry of the deposit and to determine the most appropriate mining and processing methods. The establishment of small-scale underground or open pit mine workings are essential to provide the information needed to make decisions regarding further development at a site. Larger amounts of rock are removed during bulk sampling as part of advanced exploration. Valuable information can be obtained concerning rock quality, mineralogy and geochemistry. Bulk sampling is commonly accompanied by extensive diamond drilling, the results of which are used to improve the understanding of the geometry of the mineral deposit, as well as the quantity, characteristics and delineation of the potential ore body.
If the quantity and quality of potential ore present are adequate to proceed to a feasibility study, the data from advanced exploration are used for preliminary planning of mine layout, ore processing design, and estimating the cost of developing and operating a mine.
Mineral deposits that are worthy of further evaluation following advanced exploration are subjected to a rigorous process to determine the feasibility of developing a mine at the site. This process involves an assessment of the technical, legal and economic feasibility of the envisaged project, including assessments of the mineral reserve and investment returns. The mineral reserve is estimated based on the results of advanced exploration. Mining methods are determined on the basis of safety, economics, practicality and environmental considerations.
Mineral exploration targets that are demonstrated to be viable and that receive the necessary funding and permits are ultimately brought into production. Once a decision has been made to proceed with production at a site, final site planning and engineering studies are completed in preparation for the beginning of mine construction.
2.2 Planning and Construction
During the planning phase, which in practice may overlap with the completion of feasibility studies, all aspects of the mine are planned in detail. This includes planning related to mining and ore separation processes, as well as site infrastructure needs, schedules for construction and commissioning of facilities, and all planning associated with environmental aspects of operations.
The most significant activity during mine construction is the establishment of underground or surface mine workings to provide direct access to the ore body. Related activities include the construction of ore processing facilities, waste management areas, and site infrastructure. The scope and complexity of the works to be completed during this phase vary considerably from project to project; however, some elements are common to all mine construction projects. These key activities are briefly described below.
Site Preparation - Clearing, Stripping and Grading: The clearing and stripping of overburden is completed in preparation for the construction of various facilities on site. The overburden is typically stockpiled if it is suitable for later use in mine reclamation.
Construction of Mine Infrastructure: Most of the on-site facilities and utilities associated with the mine are developed during the construction phase. Depending on a number of factors, including the size of the operation, the location, and the proposed mining and milling processes to be used, infrastructure may include:
- transportation facilities, including access roads to the site, on-site roads, and in some cases an airstrip, rail line or port facility;
- ore handling and processing facilities;
- mine waste disposal facilities;
- water management and wastewater treatment systems;
- power infrastructure, including power distribution system and any on-site generation facilities;
- shops, offices, warehouses and accommodations;
- fuel supply and storage;
- vehicle storage and maintenance facilities;
- explosives storage facility;
- water supply, potable water treatment and distribution system; and
- sewage and waste disposal (including incinerators, landfill and land farm).
Establishment of Mine Workings: During the construction phase, underground or surface mine workings are established to provide direct access to the ore body. Surface mines, also known as open pit mines, are preferred for the extraction of ore close to the surface. Deeper or more irregularly shaped ore bodies are generally mined by underground methods. The mine workings are excavated by drilling and blasting. Drills are used to drill patterns in the rock that, upon blasting, will fragment the rock. To fragment the rock, explosives are injected into drill holes and detonated. Once the rock is fractured it is removed from the mine. Most of the material removed during the construction phase is waste rock, and any ore that is removed is stockpiled for later processing. Mine construction may also include some ore production for use in testing the ore handling and processing facilities.
2.3 Mine Operations
The mine operations phase represents the period during which a mine produces and processes ore to produce a product for market. At some sites, the mine operations phase may extend continuously over a period of several years to decades while at other sites, the mine operations phase may include short or extended periods of inactivity due to changes in market conditions. The mine operations phase includes both ore extraction and ore processing and associated activities.
The key activities of the mine operations phase are illustrated in Figure 2.2.
Figure 2-2: Typical Activities of the Mine Operations Phase
This is a flowchart showing the relationship between the five main stages of the mine operations phase. Stages shown in time-sequential order are mining, crushing, grinding, ore separation and concentrate dewatering. From the mining phase two outputs are waste rock to the waste rock pile and waste water from both mining and the waste rock pile. Water and reagents are added at the grinding and ore separation stages. Tailings are produced from the ore separation stage and go to the tailings management facility. Water is recycled between the grinding, ore separation and concentrate dewatering stages, and the tailings management facility. The final product is the ore concentrate, which goes to further processing.
2.3.1 Ore Extraction
Open Pit Mines
Open pit mining is the preferred method for the extraction of ore from deposits that are close to the surface, since the cost per tonne of ore mined is generally lower than that for underground mining. Other factors that may influence the decision about whether to mine using open pit or underground methods include the ore grade, the geometry of the deposit, other physical characteristics, and site characteristics such as topography. Open pits are generally much wider than they are deep to ensure the stability of the pit walls (see Figure 2.3). The stripping ratio (the ratio of waste rock to ore) varies dramatically over the life of an open pit mine and depends on the geometry of the ore body, ore grades, slope stability, site geology, and variations in the price of the metal.
Figure 2-3: Cross-section Through a Typical Open Pit Mine
A cross-section through a typical open pit mine is illustrated. Shown here is the large ore body below the land surface. Surrounding the ore is the waste rock. The waste rock pit walls slope up and outward from the bottom of the ore body, the angle of this being the final pit slope. Steps in the pit slope are called benches and the haul roads follow these. Surrounding the waste rock is the host rock, which is not removed.
In underground mines, the ore is extracted through a series of vertical shafts and ramps and horizontal drifts and adits (see Figure 2.4). Extraction is more selective than in open pit mining, and the ratio of waste rock to ore generated is much lower. In about one half of Canadian underground mines, waste rock is used as mine backfill to provide roof and wall support underground. Waste rock that is not used for construction or as backfill is disposed of on the surface.
Figure 2-4: Cross-section Through a Typical Underground Mine
A cross-section through a typical underground mine is illustrated. The mine headframe sits on the surface of the overburden and houses the main shaft containing the skip, which acts as an elevator. A sump sits at the bottom of the shaft. At various levels horizontal channels from the main shaft lead to the mining area within the ore body, or the stope. Ramps are used to access various levels and ore is sent to the underground crusher and ore bin via ore passes. Exploration drifts are dug to sample lower parts of the ore body using diamond drilling. A ventilation shaft leading to the surface allows for fresh air exchange.
2.3.2 Ore Processing
Once ore is extracted from a mine it is processed to recover the valuable minerals. Ore typically consists of small amounts of valuable minerals in close association with much larger amounts of waste minerals of no economic value (gangue). The valuable ore minerals are separated (liberated) from the gangue in milling operations to obtain higher quality metal. Major steps in ore processing include grinding and crushing, chemical/physical separation and dewatering.
Grinding and Crushing
Grinding and crushing of ore is undertaken to physically liberate valuable minerals prior to separation by physical and chemical processes. Crushing is done dry, and is used for coarse size reduction. Grinding is used to achieve finer size reduction. Grinding is conducted wet, and chemicals such as lime, soda ash, sodium cyanide, and sulphur dioxide may be added in the grinding circuit in preparation for ore separation. Ore must be ground fine enough to liberate the ore minerals from the gangue, or subsequent separation methods will not be as effective.
Ore separation may be done using physical or chemical separation methods. The end product of ore separation is an ore concentrate. After separation, some ore concentrates are sent for further processing, such as smelting, to produce pure metal for sale.
A by-product of ore separation is tailings, which are a mixture of water and finely ground rock from which most of the minerals of value have been removed. Tailings may still contain metal-bearing minerals, and the mixture may also contain residues of reagents used in ore processing.
Physical Separation Processes: Physical separation processes exploit differences in the physical properties or behaviour of mineral particles, such as size, density and surface energy. The bulk of the mineral is not chemically altered, although chemical reagents may be used to help in the separation process. Commonly used physical separation processes are as follows:
- Gravity Separation: Minerals can be separated on the basis of differences in density, particularly for iron ore and gold, as well as tungsten, tantalum and niobium. Gravity separation may also be used to pre-concentrate metallic minerals prior to further processing. Gravity separation tends to require the use of smaller amounts of process reagents than some other ore separation methods.
- Magnetic Separation: Minerals can be separated on the basis of differences in magnetic susceptibility. Magnetic separation has been used in Canada to separate iron ore from waste minerals, to remove magnetite (iron oxide) and pyrrhotite (iron sulphide) from base metal ores prior to flotation, and to recover magnetite from copper concentrates. Like gravity separation, magnetic separation tends to require the use of smaller amounts of process reagents than some other ore separation methods.
- Flotation Separation: Flotation is used for the separation of a wide variety of minerals on the basis of differences in surface properties of minerals in contact with air and water. It is the dominant process for the recovery of base metal ores and is also used in uranium and gold processing operations. To separate minerals using flotation, fine air bubbles are introduced into a mixture of ground ore in water, known as a slurry. In this slurry, mineral particles collide with air bubbles, and minerals that favour contact with air attach to the air bubbles and float to the surface of the flotation cell. As air bubbles accumulate at the surface, a froth forms and eventually overflows as the flotation cell concentrate. Minerals that favour contact with water remain in the slurry and go to the flotation cell tailings. A number of chemical reagents are used to aid the process.
Chemical Separation Processes: Chemical separation processes involve the preferential leaching of one or more minerals, particularly for the recovery of gold, silver and uranium and in some cases copper. A number of chemical processes are used for ore separation:
- Leaching with Cyanide: This is the dominant method for recovery of metallic gold or silver. A dilute solution of calcium or sodium cyanide is used to dissolve the metal. Following leaching, metals are recovered from the solution by absorption directly from the leach slurry onto activated carbon granules or by the addition of zinc dust to the solution which causes the precious metals to precipitate from the solution.
- Leaching with Sulphuric Acid: Uranium ores are processed using sulphuric acid to dissolve the uranium. The uranium is then removed from the solution using ion exchange or solvent extraction, which results in the adsorption of uranium on a resin or organic solvent. The uranium is then removed from the resin or solvent. In some cases, copper ores are also leached with sulphuric acid.
The ore concentrates obtained from most physical ore separation processes are slurries with high water content that must be dewatered prior to further processing. Dewatering involves two processes, i.e., thickening and filtration. In thickening, slurries are thickened by gravity settling. The excess water is decanted off and may be recycled in the milling processes. After thickening, the slurry is passed through a vacuum filter, which traps the particulates. Most of the remaining water is removed.
2.4 Mine Closure
Mines are closed when the ore minerals are completely exhausted or when it is no longer profitable to recover the minerals that remain. In some cases, mines may be closed temporarily and put into a status called "care and maintenance," also known as temporary suspension. This is frequently done during periods of low commodity prices in the expectation that higher prices in the future will make further commercial operations financially viable. Eventually, ore reserves are depleted, and mines are permanently closed.
Since much of the work conducted at a mine site during the mine closure phase is related to environmental protection and rehabilitation, mine closure is discussed in further detail in Section 3.
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