Pit planning and layout
The overall economic goal in surface mining is to remove the least amount of material while gaining the greatest return on investment by processing the most marketable mineral product. The higher the grade of the mineral deposit, the greater the value. To minimize capital investment while accessing the highest valued material within a mineral deposit, a mine plan is developed that precisely details the manner in which the ore body will be extracted and processed. As many ore deposits are not a uniform shape, the mine plan is preceded by extensive exploratory drilling to profile the geology and position of the ore body. The size of the mineral deposit dictates the size and layout of the mine. The layout of a surface mine is dictated by the mineralogy and geology of the area. The shape of most open-pit mines approximates a cone but always reflects the shape of the mineral deposit being developed. Open-pit mines are constructed of a series of concentric ledges or benches that are bisected by mine access and haulage roads angling down from the rim of the pit to the bottom in a spiral or zigzag orientation. Regardless of size, the mine plan includes provisions for pit development, infrastructure, (e.g., storage, offices and maintenance) transportation, equipment, mining ratios and rates. Mining rates and ratios influence the life of the mine which is defined by depletion of the ore body or realization of an economic limit.
Contemporary open-pit mines vary in scale from small privately-operated enterprises processing a few hundred tonnes of ore per day to expanded industrial complexes operated by governments and multinational corporations that mine more than one million tonnes of material per day. The largest operations can involve many square kilometres in area.
Overburden is waste rock consisting of consolidated and unconsolidated material that must be removed to expose the underlying ore body. It is desirable to remove as little overburden as possible in order to access the ore of interest, but a larger volume of waste rock is excavated when the mineral deposit is deep. Most removal techniques are cyclical with interruption in the extraction (drilling, blasting and loading) and removal (haulage) phases. This is particularly true for hard rock overburden which must be drilled and blasted first. An exception to this cyclical effect are dredges used in hydraulic surface mining and some types of loose material mining with bucket wheel excavators. The fraction of waste rock to ore excavated is defined as the stripping ratio. Stripping ratios of 2:1 up to 4:1 are not uncommon in large mining operations. Ratios above 6:1 tend to be less economically viable, depending on the commodity. Once removed, overburden can be used for road and tailings construction or may have non-mining commercial value as fill dirt.
Mining equipment selection
The selection of mining equipment is a function of the mine plan. Some of the factors considered in the selection of mine equipment include the topography of the pit and surrounding area, the amount of ore to be mined, the speed and distance the ore must be transported for processing and the estimated mine life, among others. In general, most contemporary surface mining operations rely on mobile drill rigs, hydraulic shovels, front-end loaders, scrapers and haul trucks to extract ore and initiate ore processing. The larger the mine operation, the larger the capacity of equipment required to maintain the mine plan.
Equipment is generally the largest available to match the economy of scale of surface mines with consideration for matching the capacities of equipment. For example, a small front-end loader can fill a large haul truck but the match is not efficient. Similarly, a large shovel can load smaller trucks but requires the trucks to decrease their cycle times and does not optimize utilization of the shovel since one shovel bucket may contain enough ore for more than one truck. Safety may be compromised by attempting to load only half of a bucket or if a truck is overloaded. Also, the scale of equipment selected must match the available maintenance facilities. Large equipment is often maintained where it malfunctions due to the logistical difficulties associated with transporting it to established maintenance facilities. When possible, the mine’s maintenance facilities are designed to accommodate the scale and quantity of the mine equipment. Therefore, as new larger equipment is introduced into the mine plan, the supporting infrastructure, including the size and quality of haul roads, tools and maintenance facilities, must also be addressed.
Conventional Methods of Surface Mining
Open-pit mining and strip mining are the two major categories of surface mining which account for more than 90% worldwide surface mining production. The primary differences between these mining methods are the location of the ore body and the mode of mechanical extraction. For loose rock mining, the process is essentially continuous with extraction and haulage steps running in series. Solid rock mining requires a discontinuous process of drilling and blasting prior to the loading and hauling stages. Strip mining (or open-cast mining) techniques relate to the extraction of ore bodies that are near the surface and relatively flat or tabular in nature and mineral seams. It uses a variety of different types of equipment including shovels, trucks, drag lines, bucket wheel excavators and scrapers. Most strip mines process non-hard rock deposits. Coal is the most common commodity that is strip mined from surface seams. In contrast, open-pit mining is employed to remove hard rock ore that is disseminated and/or located in deep seams and is typically limited to extraction by shovel and truck equipment. Many metals are mined by the open-pit technique: gold, silver and copper, to name a few.
Quarrying is a term used to describe a specialized open-pit mining technique wherein solid rock with a high degree of consolidation and density is extracted from localized deposits. Quarried materials are either crushed and broken to produce aggregate or building stone, such as dolomite and limestone, or combined with other chemicals to produce cement and lime. Construction materials are produced from quarries located in close proximity to the site of material use to reduce transportation costs. Dimension stone such as flagstone, granite, limestone, marble, sandstone and slate represent a second class of quarried materials. Dimension stone quarries are found in areas having the desired mineral characteristics which may or may not be geographically remote and require transportation to user markets.
Many ore bodies are too diffuse and irregular, or too small or deep to be mined by strip or open-pit methods and must be extracted by the more surgical approach of underground mining. To determine when open-pit mining is applicable, a number of factors must be considered, including the terrain and elevation of the site and region, its remoteness, climate, infrastructure such as roads, power and water supply, regulatory and environmental requirements, slope stability, overburden disposal and product transportation, among others.
Terrain and elevation: Topography and elevation also play an important role in defining the feasibility and scope of a mining project. In general, the higher the elevation and rougher the terrain, the more difficult mine development and production are likely to be. A higher grade of mineral in an inaccessible mountainous location may be mined less efficiently than a lower grade of ore in a flat location. Mines located at lower elevations generally experience less inclement weather-related problems for exploration, development and production of mines. As such, topography and location affect the mining method as well as economic feasibility.
The decision to develop a mine occurs after exploration has characterized the ore deposit and feasibility studies have defined the options for mineral extraction and processing. Information that is necessary to establish a development plan may include the shape, size and grade of minerals in the ore body, the total volume or tonnage of material including overburden and other factors, such as hydrology and access to a source of process water, availability and source of power, waste rock storage sites, transportation requirements and infrastructure features, including the location of population centres to support the labour force or the need to develop a townsite.
Transportation requirements may include roads, highways, pipelines, airports, railroads, waterways and harbours. For surface mines, large land areas are generally required that may have no existing infrastructure. In such instances roads, utilities and living arrangements must be established first. The pit would be developed in connection with other processing elements such as waste rock storage areas, crushers, concentrators, smelters and refineries, depending on the degree of integration required. Due to the large amount of capital necessary to finance these operations, development may be conducted in phases to take advantage of the earliest possible saleable or leasable mineral to help finance the remainder of the development.
Production and Equipment
Drilling and blasting
Mechanical drilling and blasting are the first steps in extracting ore from most developed open-pit mines and are the most common method used to remove hard rock overburden. While there are many mechanical devices capable of loosening hard rock, explosives are the preferred method as no mechanical device can currently match the fracturing capability of energy contained in explosive charges. A commonly used hard rock explosive is ammonium nitrate. Drilling equipment is selected on the basis of the nature of the ore and the speed and depth of the holes necessary to fracture a specified tonnage of ore per day. For example, in mining a 15-m bench of ore, 60 or more holes will generally be drilled 15 m back from the current muck face depending on the length of the bench to be mined. This must occur with enough lead-time to allow for site preparation for subsequent loading and haulage activities.
Surface mining is now typically conducted utilizing table shovels, front-end loaders or hydraulic shovels. In open-pit mining loading equipment is matched with haul trucks that can be loaded in three to five cycles or passes of the shovel; however, various factors determine the preference of loading equipment. With sharp rock and/or hard digging and/or wet climates, tracked shovels are preferable. Conversely, rubber-tyred loaders have much lower capital cost and are preferred for loading material that is low volume and easy to dig. Additionally, loaders are very mobile and well-suited for mining scenarios requiring rapid movements from one area to another or for ore blending requirements. Loaders are also frequently used to load, haul and dump material into crushers from blending stock piles deposited near crushers by haul trucks.
Hydraulic shovels and cable shovels have similar advantages and limitations. Hydraulic shovels are not preferred for digging hard rock and cable shovels are generally available in larger sizes. Therefore, large cable shovels with payloads of about 50 cubic metres and greater are the preferred equipment at mines were production exceeds 200,000 tonnes per day. Hydraulic shovels are more versatile on the mine face and allow greater operator control to selectively load the from either the bottom or top half of the mine face. This advantage is helpful where separation of waste from ore can be achieved at the loading zone thereby maximizing the ore grade that is hauled and processed.
Haulage in open-pit and strip mines is most commonly accomplished by haul trucks. The role of haul trucks in many surface mines is restricted to cycling between the loading zone and the transfer point such as an in-pit crushing station or conveyance system. Haul trucks are favoured based on their flexibility of operation relative to railroads, which were the preferred haulage method until the 1960s. However, the cost of transporting materials in surface metal and non-metal pits is generally greater than 50% of the total operating cost of the mine. In-pit crushing and conveying through belt conveyor systems has been a primary factor in reducing haulage costs. Technical developments in haul trucks such as diesel engines and electrical drives have lead to much larger capacity vehicles. Several manufactures currently produce 240 tonne capacity trucks with expectation for greater than 310 tonne capacity trucks in the near future. In addition, the use of computerized dispatch systems and global satellite positioning technology allow vehicles to be tracked and scheduled with improved efficiency and productivity.
Haul road systems may use single or dual direction traffic. Traffic may be either left or right lane configuration. Left lane traffic is frequently preferred to improve operator visibility of tyre position on very large trucks. Safety is also improved with left hand traffic by reducing the potential for driver-side collision in the centre of a road. Haul road gradients are typically limited to between 8 and 15% for sustained hauls and optimally are about 7 to 8%. Safety and water drainage requires long gradients to include at least 45-m sections with a maximum gradient of 2% for every 460 m of severe gradient. Road berms (elevated dirt borders) located between roads and adjacent excavations are standard safety features in surface mines. They may also be placed in the middle of the road to separate opposing traffic. Where switch-back haul roads exist, increasing elevation escape lanes may be installed at the end of long steep grades. Road edge barriers such as berms are standard and should be located between all roads and adjacent excavations. High-quality roads enhance maximum productivity by maximizing safe truck speeds, reduced down-time for maintenance and reduced driver fatigue. Haul-truck road maintenance contributes to reduced operating costs through reduced fuel consumption, longer tyre life and reduced repair costs.
Rail haulage, under the best of conditions, is superior to other methods of haulage for transport of ore over long distances outside the mine. However, as a practical matter, rail haulage is no longer widely used in open-pit mining since the advent of electrical and diesel-powered trucks. Rail haulage was replaced to capitalize on the greater versatility and flexibility of haul trucks and in-pit conveyor systems. Railroads requires very gentle grades of 0.5 to a maximum of 3% for up-hill hauls. Capital investment for railroad engines and track requirements is very high and requires a long mine life and large production outputs to justify return on investment.
Ore handling (conveyance)
In-pit crushing and conveying is a methodology that has grown in popularity since first being implemented in the mid-1950s. Location of a semi-mobile crusher in the mine pit with the subsequent transport out of the pit by a conveyor system has resulted in significant production advantages and cost savings over traditional vehicle haulage. High cost haulage road construction and maintenance is reduced and labour costs associated with haul truck operation and truck maintenance and fuel are minimized.
The purpose of the in-pit crusher system is primarily to allow transport of ore by conveyor. In-pit crusher systems may range from permanent facilities to fully mobile units. However, more commonly, crushers are constructed in a modular form to allow some portability within the mine. Crushers might be relocated every one to ten years; it may require hours, days or months to complete the move depending on the size and complexity of the unit and the relocation distance.
Conveyors’ advantages over haul trucks include instantaneous start up, automatic and continuous operation, and a high degree of reliability with up 90 to 95% availability. They are generally not impaired by inclement weather. Conveyors also have much lower labour requirements relative to haul trucks; operating and maintaining a truck fleet may require ten times as many crew members as an equivalent-capacity conveyor system. Also, conveyors can operate at grades up to 30% while maximum grades for trucks are generally 10%. Using steeper grades lowers the need to remove low-grade overburden material and may reduce the need to establish high cost haulage roads. Conveyors systems are also integrated into bucket wheel shovels in many surface coal operations, which eliminates the need for haulage trucks.
Solution Mining Methods
Solution mining, the most common of two types of aqueous mining, is employed to extract soluble ore where conventional mining methods are less efficient and/or less economical. Also known as leaching or surface leaching, this technique can be a primary mining method, as with gold and silver leach mining, or it can supplement the conventional pyrometallurgical steps of smelting and refining, as in the case of leaching low-grade copper oxide ores.
Environmental aspects of surface mining
The significant environmental effects of surface mines attract attention wherever the mines are located. Alteration of terrain, destruction of plant life and adverse effects on indigenous animals are inevitable consequences of surface mining. Contamination of surface and underground waters often presents problems, particularly with the use of lixiviants in solution mining and the run-off from hydraulic mining.
Thanks to the increased attention from environmentalists around the world and the use of planes and aerial photography, mining enterprises are no longer free to “dig and run” when the extraction of the desired ore has been complete. Laws and regulations have been promulgated in most of the developed countries and, through the activities of international organizations, are being urged where they do not yet exist. They establish an environmental management programme as an integral element in every mining project and stipulate such requirements as preliminary environmental impact assessments; progressive rehabilitation programmes, including restoration of land contours, reforestration, replanting of indigenous fauna, restocking of indigenous wild life and so on; as well as concurrent and long-term compliance auditing (UNEP 1991,UN 1992, Environmental Protection Agency (Australia) 1996, ICME 1996). It is essential that these be more than statements in the documentation required for the necessary government licenses. The basic principles must be accepted and practised by managers in the field and communicated to workers on all levels.
Regardless of the necessity or economic advantage, all surface solution methods share two common characteristics: (1) ore is mined in the usual way and then stockpiled; and, (2) an aqueous solution is applied to the top of the ore stock which reacts chemically with the metal of interest from which the resulting metal salt solution is channelled through the stock pile for collection and processing. The application of surface solution mining is dependent on the volume, the metallurgy of the mineral(s) of interest and the related host rock, and available area and drainage to develop sufficiently large leach dumps to make the operation economically viable.
The development of leach dumps in a surface mine in which solution mining is the primary production method is the same as all open-pit operations with the exception that the ore is destined solely for the dump and not a mill. In mines with both milling and solution methods, ore is segregated into milled and leached portions. For example, most copper sulphide ore is milled and purified to market grade copper by smelting and refining. Copper oxide ore, which is not generally amenable to pyrometallurgical processing, is routed to leach operations. Once the dump is developed, the solution leaches the soluble metal from the surrounding rock at a predictable rate that is controlled by the design parameters of the dump, the nature and volume of the solution applied, and the concentration and mineralogy of the metal in the ore. The solution used to extract the soluble metal is referred to as a lixiviant. The most common lixiviants used in this mining sector are dilute solutions of alkaline sodium cyanide for gold, acidic sulphuric acid for copper, aqueous sulphur dioxide for manganese and sulphuric acid-ferric sulphate for uranium ores; however, most leached uranium and soluble salts are collected by in-situ mining in which the lixiviant is injected directly into the ore body without prior mechanical extraction. This latter technique enables low-grade ores to be processed without extracting the ore from the mineral deposit.
Health and safety aspects
The occupational health and safety hazards associated with mechanical extraction of the ore in solution mining are essentially similar to those of conventional surface mine operations. An exception to this generalization is the need for non-leaching ore to undergo primary crushing in the surface mine pit before being conveyed to a mill for conventional processing, whereas ore is generally transported by haul truck directly from the extraction site to the leach dump in solution mining. Solution mining workers would therefore have less exposure to primary crushing hazards such as dust, noise and physical hazards.
The leading causes of injuries in surface mine environments include materials handling, slips and falls, machinery, hand-tool use, power haulage and electrical source contact. However, unique to solution mining is the potential exposure to the chemical lixiviants during transportation, leach field activities and chemical and electrolytic processing. Acid mist exposures may occur in metal electrowinning tankhouses. Ionizing radiation hazards, which increase proportionally from extraction to concentration, must be addressed in uranium mining.
Hydraulic Mining Methods
In hydraulic mining, or “hydraulicking”, high pressure water spray is used to excavate loosely consolidated or unconsolidated material into a slurry for processing. Hydraulic methods are applied primarily to metal and aggregate stone deposits, although coal, sandstone and metal mill tailings are also amenable to this method. The most common and best known application is placer mining in which concentrations of metals such as gold, titanium, silver, tin and tungsten are washed from within an alluvial deposit (placer). Water supply and pressure, ground slope gradient for runoff, distance from the mine face to the processing facilities, degree of consolidation of the mineable material and the availability of waste disposal areas are all primary considerations in the development of a hydraulic mining operation. As with other surface mining, the applicability is location specific. Inherent advantages of this method mining include relatively low operating costs and flexibility resulting from the use of simple, rugged and mobile equipment. As a result, many hydraulic operations develop in remote mining areas where infrastructure requirements are not a limitation.
Unlike other types of surface mining, hydraulic techniques rely on water as the medium for both mining and conveyance of the mined material (“sluicing”). High pressure water sprays are delivered by monitors or water cannons to a placer bank or mineral deposit. They disintegrate gravel and unconsolidated material, which washes into collection and processing facilities. Water pressures may vary from a normal gravity flow for very loose fine materials to thousands of kilograms per square centimetre for unconsolidated deposits. Bulldozers and graders or other mobile excavating equipment are sometimes employed to facilitate mining of more compacted materials. Historically, and in modem small-scale operations, the collection of the slurry or runoff is managed with small volume sluice boxes and catches. Commercial-scale operations rely on pumps, containment and settling basins and separation equipment that can process very large volumes of slurry per hour. Depending on the size of the deposit to be mined, the operation of the water monitors may be manual, remotely controlled or computer controlled.
When hydraulic mining occurs underwater it is referred to as dredging. In this method a floating processing station extracts loose deposits such as clay, silt, sand, gravel and any associated minerals using a bucket line, drag line and/or submerged water jets. The mined material is transported hydraulically or mechanically to a washing station which may be part of the dredging rig or physically separate with subsequent processing steps to segregate and complete processing. While dredging is used to extract commercial minerals and aggregate stone, it is best known as a technique used to clear and deepen water channels and floodplains.
Health and safety
Physical hazards in hydraulic mining differ from those in surface mining methods. Due to the minimal application of drilling, explosives, haulage and reduction activities, safety hazards tend to be associated mostly often with high pressure water systems, manual movement of mobile equipment, proximity issues involving power supplies and water, proximity issues associated with collapse of the mine face and maintenance activities. Health hazards primarily involve exposure to noise and dusts and ergonomic hazards related to equipment handling. Dust exposure is generally less of an issue than in traditional surface mining due to the use of water as the mining medium. Maintenance activities such as uncontrolled welding may also contribute to worker exposures.