reading: open-pit mining | geology
This form of mining differs from extractive methods that require tunneling into the earth, such as long wall mining. Open-pit mines are used when deposits of commercially useful minerals or rocks are found near the surface; that is, where the overburden (surface material covering the valuable deposit) is relatively thin or the material of interest is structurally unsuitable for tunneling (as would be the case for sand, cinder, and gravel). For minerals that occur deep below the surfacewhere the overburden is thick or the mineral occurs as veins in hard rockunderground mining methods extract the valued material.
Open-pit mines are typically enlarged until either the mineral resource is exhausted, or an increasing ratio of overburden to ore makes further mining uneconomic. When this occurs, the exhausted mines are sometimes converted to landfills for disposal of solid wastes. However, some form of water control is usually required to keep the mine pit from becoming a lake, if the mine is situated in a climate of considerableprecipitation or if any layers of the pit forming the mine border productive aquifers.
Open-cast mines are dug on benches, which describe vertical levels of the hole. These benches are usually on four to sixty meter intervals, depending on the size of the machinery that is being used. Many quarries do not use benches, as they are usually shallow.
Most walls of the pit are generally dug on an angle less than vertical, to prevent and minimize damage and danger from rock falls. This depends on how weathered the rocks are, and the type of rock, and also how many structural weaknesses occur within the rocks, such as a faults, shears, joints orfoliations.
The walls are stepped. The inclined section of the wall is known as the batter, and the flat part of the step is known as the bench or berm. The steps in the walls help prevent rock falls continuing down the entire face of the wall. In some instances additional ground support is required and rock bolts, cable bolts and shotcrete are used. De-watering bores may be used to relieve water pressure by drilling horizontally into the wall, which is often enough to cause failures in the wall by itself.
Ore which has been processed is known as tailings, and is generally a slurry. This is pumped to a tailings dam or settling pond, where the water evaporates. Tailings dams can often be toxic due to the presence of unextracted sulfide minerals, some forms of toxic minerals in the gangue, and oftencyanide which is used to treat gold ore via the cyanide leach process. This toxicity can harm the surrounding environment.
After mining finishes, the mine area must undergo rehabilitation. Waste dumps are contoured to flatten them out, to further stabilise them. If the ore contains sulfides it is usually covered with a layer of clay to prevent access of rain and oxygen from the air, which can oxidise the sulfides to producesulfuric acid, a phenomenon known as acid mine drainage. This is then generally covered with soil, and vegetation is planted to help consolidate the material. Eventually this layer will erode, but it is generally hoped that the rate of leaching or acid will be slowed by the cover such that the environment can handle the load of acid and associated heavy metals. There are no long term studies on the success of these covers due to the relatively short time in which large scale open pit mining has existed. It may take hundreds to thousands of years for some waste dumps to become acid neutral and stop leaching to the environment. The dumps are usually fenced off to prevent livestock denuding them of vegetation. The open pit is then surrounded with afence, to prevent access, and it generally eventually fills up with ground water. In arid areas it may not fill due to deep groundwater levels.
Gold is generally extracted in open-pit mines at 1 to 2ppm (parts per million) but in certain cases, 0.75ppm gold is economical. This was achieved by bulk heap leaching at the Peak Hill mine in western New South Wales, near Dubbo, Australia.
Nickel, generally as laterite, is extracted via open-pit down to 0.2%. Copper is extracted at grades as low as 0.15% to 0.2%, generally in massive open-pit mines in Chile, where the size of the resources and favorable metallurgy allows economies of scale.
opencast mining - an overview | sciencedirect topics
Opencast mining operations involve huge quantities of overburden removal, dumping and backfilling in excavated areas. A substantial increase in the rate of accumulation of waste dumps in recent years has resulted in greater height of the dump for minimum ground cover area and also given rise to danger of dump failures. Further, steeper open-pit slopes are prone to failure. These failures lead to loss of valuable human life and damage to mining machinery. There is a need for continuous monitoring of dump and pit slopes, as well as for providing early warning before the occurrence of slope failure. Different technologies have been developed for slope monitoring. After studying the features and limitations of existing slope monitoring systems, it determined that there is a need to provide a reliable slope stability monitoring and prediction system by using a solar power-based long-range wireless sensor network for continuous monitoring of different prevailing parameters of slope stability. An accurate prediction of slope failure using a multiparameters-based prediction model is required for giving warning per the danger levels of impending slope stability. Considering the requirement, a slope failure monitoring and prediction system has been developed by the authors, using a wireless sensor network for the continuous monitoring of slope failure and to provide early warnings. The chapter describes details of slope stability mechanism, parameters affecting slope failure and triggering aspects, monitoring systems, prediction software, and laboratory experiments for calibrating geosensors and field installation of the developed system.
Opencast mining operations involve the removal of huge quantities of overburden, dumping, and backfilling of the excavated area. These overburden deposits of waste material containing low-grade ores are represented as mine spoil dumps. The degraded land due to mining activities and overburden of stockpiling of mine discards results not only in the formation of the unsightly landscape but also is associated with the pollution of terrestrial and aquatic ecosystems. The gradual increase in such landscapes created due to extensive mining activities may endanger the agro-forest productivity of the country as well as the ecological balance (Juwarkar et al., 2015, 2016).
Mining activities generate a large number of waste rocks and tailings which get deposited at the surface. The land surface is damaged and the waste rocks and tailings are often very unstable and will become sources of pollution. Land degradation due to mining has reached alarming proportions mainly due to over-exploitation and mismanagement of natural resources. One of the consequences of the ever-increasing human population, supported by accelerated land degradation is the lowering of the man to land ratio. Therefore, there is an urgent need to address the major environmental issues which are the outcome of the mining activities. The issues include the destruction of habitat and biodiversity at the mine site, ecosystem/habitat/biodiversity protection in adjacent land, landscape/visual impact/loss of land-use, site stabilization and rehabilitation, mine waste/tailings disposal, air emissions, dust, climate change, siltation and changes in river regimes, effluent discharges and acid drainage, groundwater alteration or contamination, workplace health and safety, the impact of metals toxicity and settlement issues around mines. Considering all these issues, the overall reclamation of overburdened/wasteland is a challenging task. The main challenge in the restoration of these degraded/wastelands are sustainable biodiversity and management of the mine spoils dumps and the adjoining area, keeping in view the local requirement of the vicinity (Bradshaw, 1993; Ekka and Behera, 2011; Juwarkar et al., 2016) (Fig. 18.2).
Mining and opencast mining in particular affect all components of ecosystems (Bradshaw, 1997). During opencast mining, material overlying the mined minerals, called overburden, is excavated and deposited in a heap. This heap can be located either inside the mine pit (an internal heap) or outside (an external heap). As a consequence of this, whatever ecosystem that was present at the time prior to mining activity or external heap deposition is basically destroyed, either excavated or buried (Bell and Donnelly, 2006). Additional areas can be impacted or degraded by processing of minerals forming tailings and other deposits. In addition to these immediate effects, the surrounding environment can be affected in many different ways, for example, water pumped into mines can affect local water table depths, resulting in the release of acid mine drainage or other mining water into surface waters. In this chapter, we will focus only on reconstruction of upland ecosystems in overburden heaps. Overburden differs substantially from previous local soils, and some maybe even toxic for plants (Bradshaw, 1997; Frouz et al., 2011a). Overburden soil typically is composed of fragmented mixed and dumped parent material, which often weathers intensively after dumping. These soils do not have horizons, and they generally also lack soil structure. In terms of other soil properties, overburden soils differ substantially from native soils in surrounding ecosystems; they typically have extreme pH (either too alkaline or too acidic) and extreme soil texture (Table 6.1). Because sediments that often form overburden materials may be sorted by texture, overburden materials are often characterized by being too sandy or being mixed with a large sand and gravel component or on the other hand having too much clay for proper soil functioning. Finally, and critically, overburden soils lack recent organic matter input, which is the foundation of many important processes such as aggregate formation, sorption of nutrients, and water-holding capacity. Postmining soils also typically have low content of available nutrients, namely, nitrogen (N) (Bradshaw, 1997). This may hamper plant growth, but on the other hand, this is one of the reasons for high species richness and frequent occurrence of rare and endangered species in postmining land. In most of the developed (industrial) world, ecosystems surrounding mined sites are characterized by having excess N availability (due to deposition) that leads to increasing plant cover but also to a shift in the plant community to less competitive species and consequently to a reduction of species number. In contrast to this general high N availability, in postmining sites where species-rich topsoil has been removed, availability of nutrients is lower, which reduces overall plant biomass but favors less competitive species.
When we restore soil in postmining sites, there is a wide range of possible actions that fall between two extreme scenarios. The first option can be called complete reconstruction, whereas the other extreme is to do nothing and wait for soil formation to occur on its own. Complete reconstruction of soil involves covering overburden with either some suitable material, which does not hamper plant and then layer of A horizon (sometimes A and B horizons) materials transplanted from another location that will be mined in the near future. These are usually transplanted in a way that resemble as much as possible the organization of target soils. The other extreme option, as mentioned previously, is to do nothing and wait for ecological succession to do the work of rebuilding soil. In actual practice, we typically do something that falls in the middle of these extremes, and we prepare some kind of suitable rooting medium and assist in vegetation development that then feeds back to accelerated soil formation.
It is important to note that the goals of postmining restorations may vary considerably depending on socioeconomic conditions in a given location. In some cases, there is more emphasis on restoring the production function of postmining land in terms of agricultural production, while in other cases the restoration goal is to return lands to more natural ecosystems. A final consideration is the type of vegetation to be restored, for example, forest or grassland vegetation, and each of these may require different substrate quality and can benefit from manipulations of the substrate. Clear decisions about the objectives of a particular restoration are essential, because as will be shown later, conditions that may hamper the restoration of highly productive land may actually provide suitable habitat for many rare and endangered species and vice versa.
In this chapter, we will deal with various approaches to restoration of postmining soils, discuss which approaches to choose depending on our target and environmental conditions, and explore how the soil restoration corresponds with whole ecosystem recovery.
Land, in various forms, is a finite natural resource. The necessity of land is ever increasing due to rapid population growth in the developing countries and per capita enhanced industrial growth. One must analyze and understand that the quantum of land requirement during actual mining and beneficiation (Table 14.1) is small in comparison to other industries and urbanization. The minerals are mined at the sites where they exist. In general, mining activity occurs in remote places far away from cities. The possibility of land and soil degradation is expected at these remote locations only.
In case of open cast mining there will be complete loss of agricultural land and deforestation in and around the pit. Underground mining uses limited surface land for the entry system and infrastructure development. In either situation, adequate compensation is provided to the landowners by cash, employment and rehabilitation. New agricultural land is developed and aforestation is done under overall land-use planning. Mining in the unreserved forestland is replaced by enough plantations in nearby areas. Under normal circumstances no mining is permitted in reserve forest area.
Various mining activities, particularly open pit, affect the topsoil and subsoil to a great extent by changing the natural soil characteristics e.g. texture, grain size, moisture, pH, organic matter, nutrients etc. In an ideal open-pit situation it is desired that the soil horizons within the selected mining limits are clearly defined. Topsoil and subsoil are removed separately, preferably by scraping, and stockpiled at an easily accessible stable land. These soils can selectively be relaid simultaneously to reclaim degraded land for agriculture or can be reused in future at the time of mine closure. As far as practicable the removed vegetation from the mining zone should be replanted at suitable areas.
The effect of unplanned mining and mine waste dumping will change the surface topography and thereby the local drainage pattern. The damage of natural drains and waste dumps may act as a barrier to the natural flow of rainwater resulting in water logging and flash floods which in turn will cause damage to agriculture and to local properties downstream. It will also affect the seasonal filling of nearby reservoirs and recharging of the groundwater around the area. The changes in the drainage pattern can be anticipated from the expected post-mining surface contours. Action plan for the surface drainage pattern can be designed accordingly. This planning is required particularly from the view of total water management and erosion control.
Opencast mining on hill slopes, particularly in areas of heavy rainfall, is vulnerable to landslides causing loss of human life, property and deforestation. This can be controlled by geo-technically designed slope of the mine and adequate support system.
Mining activity changes the land-use pattern and alters the surface topography by increased surface erosion and excavations. If proper reclamation is not done, this can result in unaesthetic landscape. Open-pit mines must be filled with mine waste rock as reclaimed land. It can be filled with rain or floodwater for fisheries, water sports etc.
The methods and procedures of land use are planned in detail before the actual mining starts. The status is periodically compared during active mining to maximize the benefits of better land use and to incorporate remedial measures in case of deviation. The mine area should be reclaimed to the best possible scenario at the time of mine closure. It is the responsibility of the mining company to take into account the cost of reclamation in the project cost. The reclaimed land should preferably be reverted back to the erstwhile landowners under a mutual agreement. If it does not work, the land can be developed for the local society based on the overall planning of the region. The modus operandi can be decided by representatives from the mining company, local inhabitants, local authorities and state planning department.
The open pit or open cast mining method is the obvious choice for a property with a wide area of mineralization exposed or existing close to the surface and continuing to greater depths. The open pit exposes the orebody from the surface by separate removal of ore and associated waste rocks. It is the most economic option for a deposit up to that depth where the economic ratio of ore and waste can be sustained. There are many advantages to the open pit mining method, namely:
The land owners, often tribal population, and contractual farmers living within the Mining Lease area are rehabilitated and compensated by cash, separate housing, employment, health care, education, and other facilities.
Surface mining operations generate excessive waste rocks from overburden, footwall, and hanging wall. These huge waste rocks need to be removed carefully, transported and stacked by scientific and systematic manner at safe location.
A short- and medium-term plan within the framework of a long-term plan is prepared based on surface topography and 3D configuration of the orebody with respect to its shape, size, inclination, depth, grade distribution, hydrology, etc. The first task in open pit mining is to remove the top soil, subsoil, and overburden rocks in sequence. The overburden is dumped separately. It can be reverted for replacement in reverse order. The next stage is to open the mineralized ground as the first slot, known as a box cut. The slot is then expanded to form a bench system. Mining continues by advancing the benches horizontally within the broad framework of the ultimate mine layout. The benches have two components, i.e., a floor for easy movement of labor and materials, and a face (wall) to prevent collapse. The pit maintains a critical slope angle, both at the footwall and hanging wall side, not exceeding 45 degrees from the horizontal. The slope at the footwall corresponds to the inclination of the orebody. The limit will be close to the orebody with minimum waste rock generation. The hanging wall slope is relatively shallow to reach the deepest level of the pit bottom (Fig.12.3). The hanging wallbenches generate the maximum waste rock from overburden.
Figure12.3. Schematic projection of open pit benches, footwall and hanging wall slope angle at technical stability and safety of miners and machineries. It depicts the ultimate pit bottom in surface mining beyond which ore production will be uneconomic. Mining will continue by underground methods if orebody persists in depth.
The vertical height of benches varies between 5 and 10m depending on the width of the orebody, type of machinery deployed, and to minimize footwall dilution. The minimum width of the benches is 18m. The haul roads are 4560m wide at a gradient of 1 in<9, and permit easy movement of dumpers and other heavy machinery. The haulage road is connected to the main road for shifting ore to the surface stockpile, and subsequently to the process plant. The waste rock is moved to the waste dump at an appropriate location. The haul road runs along the periphery of the pit. It proceeds downward from upper to lower bench by developing ramps at suitable turnings making a total haulage system (Fig.12.4).
The overburden waste rock is dumped as a heap at a suitable location beyond the ultimate pit limit. The heap spreads both horizontally and vertically. The preferred location would be at the shortest distance over nonagricultural, nonforest land, with a nondrainage slope at the footwall side of the deposit. The dump material can be backfilled to the abandoned pit as the pit progresses or at the closure of the mine. The mine backfill process is the environmental reclamation of the worked-out area. A panoramic view of a large working rock phosphate mine is given in Fig.12.6.
Figure12.6. View of Jhamarkotra rock phosphate mine with haulage road and series of overburden benches. The mine is planned to be 7km long, 700m wide, and 280m with an ultimate open pit limit of 2Mt ore and 16Mt overburden waste per annum capacity (December 2008).
The open-pit or open cast mining method is the obvious choice for a property with wide area of mineralization exposed or exists close to the surface and continues to greater depth. The open pit is opening the orebody from the surface by separate removal of ore and associated waste rocks. It is the most economic option for a deposit up to that depth where the economic ratio of ore and waste can sustain. There are many advantages in open-pit mining method namely:
There are few disadvantages like acquisition of surface right and rehabilitation of inhabitant people, loss of production due to extreme summer and winter, rain and snow, and handling of excessive waste rock.
A short, medium-term plan within the framework of a long-term design is prepared based on surface topography, 3D configuration of the orebody with respect to its shape, size, inclination, depth, grade distribution, hydrology etc. The first task in open-pit mining is to remove the topsoil, subsoil and overburden rocks in sequence. The overburden is dumped separately. It can revert back for replacement in reverse order. The next stage is to open the mineralized ground as first slot and known as box-cut. The slot is then expanded to form bench system. The mining continues by advancing the benches horizontally within the broad framework of ultimate mine layout. The benches have two components i.e. floor for easy movement of man, machinery and materials and face (wall) to prevent collapsing. The pit maintains a critical slope angle both at footwall and hanging wall side, not exceeding 45 from the horizontal. The slope at footwall is in correspondence with the inclination of the orebody. The limit will be close to the orebody with minimum waste rock generation. The hanging wall slope is relatively shallow to reach to the deepest level of ultimate pit bottom (Fig. 11.3). The hanging wall benches generate the maximum waste rock from overburden.
The vertical height of benches varies between 5 and 10m depending on width of the orebody, type of machineries deployed and to minimize footwall dilution. The minimum width of the benches is 18m. The haul roads are 45-60m wide at a gradient of 1 in <9 and permit easy movement of dumpers and other heavy machineries. The haulage road is connected to the main road for shifting ore to the surface stockpile and subsequently to the process plant. The waste rock is moved to the waste dump at appropriate location. The haul road runs along the periphery of the pit. It proceeds downward from upper to lower bench by developing ramps at suitable turning making a total haulage system (Fig. 11.4).
The total pit development and production activities include drilling, blasting, excavation, loading and transportation of broken ore (Fig. 11.5). The drill units are Jackhammer, Wagon drill and DTH hammers. The excavation and loading equipments are scrapers, bulldozer-ripper combination, front-end loaders, power shovels, draglines, bucket wheel excavators-bucket chain excavators and graders. Transportation is done by various capacity trucks, dumpers, trains and belt conveyors.
The overburden waste rock is dumped as heap at a suitable location beyond the ultimate pit limit. The heap spreads both horizontally and vertically. The preferred location would be at the shortest distance over nonagricultural, non-forest land and non-drainage slope at the footwall side of the deposit. The dump material can be backfilled to the abandoned pit as the pit progresses or at the closure of the mine. The mine backfill process is the environmental reclamation of the worked out area. A view of a large working rock-phosphate mine is given in Fig. 11.6.
FIGURE 11.6. View of Jhamarkotra rock-phosphate mine with haulage road and series of overburden benches. The mine is planned for 7km long, 700m wide and 280m ultimate open-pit limit at 2Mt ore and 16Mt overburden waste per annum capacity (December 2008).
Topsoil removal, as occurs in open-cast mining areas and during the preparation and use of high-mountain ski slopes, heavily impoverishes protozoa and soil life in general (Figure 8). However, colonization occurs within a few months. Abundances and biomasses in 2- up to 46-year-old afforested mine soils are in the same order as in undisturbed forest soils, but typical humus-inhabiting, large-sized testacean species are lacking or occur rarely, showing that reclamation was only partially successful. Six ubiquists out of 48 taxa contributed 6187% to the overall abundance. Generally, all test sites had a distinct testacean community, whose structure depended on age, substrate, and stocking.
Figure 8. Cluster based on similarity (Soerensen index, UPGMA-linkage) of numbers and species of testate amoebae in various immigration test sites, minicontainers (sites L, A), and recultivated forests. The distances are rescaled to fall in the range of 125. Quality and development of the substrates exposed for different time intervals were more important for immigration and colonization success than the adjacent source habitats.
Mining of coal is undertaken broadly using two methods: surface and underground. In the case of underground mining, losses are high compared with those of opencast mining; the main causes of these losses are enumerated in the following subsections.
where, l1, losses of coal in the roof and floor of the seam; l2, losses of coal while cleaning the roof of the bench; l3, losses of coal during selective mining of seams with bands more than 1m in thickness; and l4, losses of coal during blasting and transportation.
where l1, losses of coal left in the roof or floor of the seam in the working panel; l2, losses of coal left as barriers between the panels; l3, losses of coal due to unfavorable geological conditions such as odd-shaped areas and faults; and l4, losses of coal resulting from the need for different types of protective pillars required for surface support.Losses in both types of coal mining are graphically presented in Figure3.11. It is clear that 100% recovery of coal deposits is not possible. The losses, however, need be minimized by the application of improved extraction techniques.
The use of remote sensing techniques can help monitor the effect of opencast mining at local to regional scales. The major advantage of remote sensing is the availability of past data, thereby helping us in reconstructing the effect of mining in the past few decades. Although the idea of change detection analysis is not new, the increased accessibility to new remote sensing data sets acquired at varying spatial, spectral, and temporal resolutions, along with the advancements in image processing technologies has given a big impetus to LULC change studies. The LULC change detection workflow implemented in this study has proven to be very effective in understanding change trajectories and the current status of the LULC in the study areaa piece of information that is vital for mining area management and for setting policies for further coal extraction.
everything you need to know about open pit mining - positive points
Open pit mining is one of mankind's greatest achievements in the ongoing search forprecious ores such as copper, silver, iron, and gold. It describes the process of systematically excavating land and digging out the rocks and minerals in search of metallic ores and removing them via borrow or open pit. The open pit mining method (also known as opencast, open-cut, or strip mining) is not extractive; meaning that most of the time there no need to tunnel directly into the earth. Rather, it is a technique used when rocks and/or ore are discovered near the surface.
Due to a thinner layer or covering of sand, gravel or cinder, these mineral-rich areas produce rich mineral resources. When miners locate valuable minerals in hard rock veins below the surface covered by a thick or extra-heavy overburden, they may then resort to underground mining routines.
Open pit mining results in waste products collected from all sides and the bottom of a pit which leaves a huge canyon-like hole. These "quarries" are open pit mines that produce dimension stone and materials used in buildings and construction. Mining usually continues until they have realized all available mineral resources. Until that time, however, open pit mines are made larger than necessary. Their enormous size makes them suitable locations for landfills.
In most cases, water control is important to prevent the mine pit from turning into a lake. The mine becomes vulnerable if it is located in an area susceptible to heavy rainfall and/or large amounts of precipitation. Open pit mining is an operation that relies on digging an open pit as an effective way of accessing the desired ores and materials. It is a method of mining the Earth's surface to extract minerals and various substances that exist close to the mining site's surface.
Open pit mining (also known as strip mining) is the process of extracting ore, minerals and/or fossil fuels that occurs on the surface of a particular mining site. When considering all the mining operations in the world, at least 40 percent of mining takes place at the surface reports Greenpeace International. When compared to underground mining methods, open pit surface mining is considerably more efficient.
One of the biggest benefits open pit mining produces is the growth in the overall efficiency when compared to deep-shaft mining methods. Mining occurs on the surface, so there aren't any space restrictions arising from narrow tunnels and shafts. Thus, the extraction rate is unaffected producing more ore at a faster rate. Also, sampling each "bench" or level within an open pit is easier when making the determination whether to mine deeper. Surveyors can quickly analyze the ore's potential and yield avoiding injuries and safety hazards.
Mining companies realize increased quantities of organic and inorganic materials when using open pit mining techniques due to the large extraction vehicle size which increases the amount of ore harvested per day. The bottom-line shows increased efficiency and reduced operating costs when mining the open pit method.
The numbers are impressive. Stats show that open pit mining is proven safer than shaft mining. Anytime you engage in underground mining; there's always a risk. Whether it's a cave-in or toxic gas release, people can die or suffer terrible injuries. There was a time when the most common way to extract ore was shaft mining resulting in thousands of deaths. Cave-ins, noxious fumes, gas events and accidental traumas related to equipment were on the rise. Over 3200 people died in 1907 due to mining incidents. Today, the following changes have drastically influenced the mining business, making it a much safer occupation:
The mining company's typically store collected waste materials (waste rock or overburden) close to the open pit. When the layer parallel to the soil's crust is revealed (the ore horizon), several benches (steps) are cut into it making waste removal easier. Depending on the mine's size, there could be one or more roads cut into the sides which are great for navigating the gigantic earth/ore haulers.
Sometimes, pumps are necessary for getting rid of water in the pit so, the extra room for crews and vehicles facilitates smoother and faster results. Open pit mining remains the preferred method for surface mining offering better ventilation, increased mineral production and larger profits.
The correct way to mine using open pit techniques starts and ends with thorough planning. Your mission involves exposing and mining valuable minerals and ore. This means it is necessary to excavate and move large amounts of waste rock. The focus is to gather the mineral deposits while being cost effective.
Carefully consider and select physical design parameters and schedule the ore extraction and waste relocation so that vehicles and drivers are available for each. Extraction programs require meticulous planning as they are complex. The scheduling demands you make decisions that might cost the company huge sums of money.
Engineering know-how and experience is the key to a successful open pit mining operation. It is a daily lesson in economics, geology, and mining engineering criteria. A bench, for example, is a ledge forming an individual level of operation. Miners send the mineral and/or waste materials collected back to the bench's face where the mineral or waste awaits relocation in successive layers, each of which is a bench. It takes planning and scheduling to operate the many benches simultaneously; all in different locations and elevations throughout the mine.
Conscientious planning must include environmental factors pertaining to protection and public relations which can negatively or positively impact the profit margin. It starts with the first day of exploration and progresses to encompass reclamation. It is crucial that the planning include the prevention of design errors instead of correcting them later on.
This is critically important due to the cost of environmental protection and the effects bad press has on a mining operation. An experienced planner knows they must give careful consideration to the regulatory (legal) framework in mining affairs. Proactive management avoids the high cost of compliance by taking the law into account when designing or planning to mine.
From the initial onset of the open pit mine design planning stage, the gathering of data and getting permits, addressing the impact of mining on the environment are of the utmost importance. Beginning with exploration, the core holes must be sealed, and the site reclaimed. The savvy planner knows of impacts such as:
Waste processing is a huge consideration requiring knowledge of the infrastructure underground and the surface. The planner must be familiar with the processing plant responsible for the minerals, roads needed for transport and mine access and any remote facilities, etc. If the mining process causes any deterioration of either the surface or groundwater, they must be ready to implement both remedial and treatment measures that meet discharge standards.
The plan put in place to govern the mine's operations must include every technical measure deemed necessary to handle all environmental problems that may arise from the initial data gathering to the mine closure including reclamation of the excavated or disturbed surface areas. Reclamation plans include:
At the heart of every responsible mining endeavor, there is the desire to leave the land in a healthy, environmentally sound state. At the conclusion of the mining project, the company restores the disturbed area. Land rehabilitation comprises stabilizing waste dumps by flattening them out. If the ore contains sulfides, the mining company covers it with a layer of clay to prevent rain and oxygen from coming into contact with it which could oxidize the sulfides producing sulfuric acid; a phenomenon known as acid mine drainage.
The next step is to cover the area with soil and seed it with vegetation to help merge the materials. Eventually, erosion takes place. The process of leaching slows because of the soil cover's ability to protect the environment. Thus, the land suffers no damage by the acid or heavy metals. There aren't any long-term studies or findings on the soil cover's success because of the relatively short time that the large scale open pit mining has been in operation.
It could take hundreds of years for some areas where waste is to become "acid neutral" no longer contaminating the environment. Reputable mining company's fence off certain areas preventing livestock from feeding off the restorative vegetation. They then make the open pit inaccessible by surrounding it with a fence. In many areas, they have converted the land into recreational parks or even residential communities.
opencast mining (quarrying): methods, advantages & disadvantages | mining technology
In this article we will discuss about the opencast mining (quarrying):- 1. What is Opencast Mining? 2. Methods of Quarrying 3. Quarriable Limit 4. Formation of Benches 5. Manual Opencast Working in Coal 6. Removal of Overburden 7. Extraction of Coal 8. Advantages 9. Disadvantages.
Opencast mining or quarrying of minerals is easier than mining by underground methods. During quarrying the alluvium and rocks below which the minerals lie, are removed and dumped, in the initial stages, in a place which is not required in future for quarrying, residential or other purposes.
The mineral exposed is completely extracted. Opencast mining is also known as open-put mining, open-cut mining, surface mining and also as strip mining, the later term being commonly used in the U.S.A. for opencast mining of coal.
In this case manual labour is employed. Small drilling machines, drilling 1.2 m to 1.8 m deep holes, 37 mm diameter, are used and the holes are blasted with gun-power or other explosives. The overburden and mineral are manually loaded into tubs which are hauled by rope haulages or locomotives. Tipping trucks are also sometimes employed and manually loaded.
In this method heavy earth moving machinery like draglines, power shovels, rear-dumping trucks (common type being Haulpaks), well-hole drills etc. are used. The blast holes are 6 m to 18 m deep, and 125 mm to 250 mm diameter. The rock is blasted by liquid oxygen, open cast gelignite or other high explosives.
Mineral and overburden are transported by locomotives, belt conveyors or large trucks known as dumpers. Bucket wheel excavators are used in some mines for soft rocks e.g., at Neyveli lignite project.
A method of surface mining known as placer mining involves mining and washing together of generally unconsolidated or semi-consolidated rock near the ground surface and the method is normally not treated as opencast mining but a variation of it.
The cost of removing overburden to extract mineral lying below it goes up as the quarrying operations extend to the dip side of the property and the thickness of overburden increases. The stripping ratio, thickness of overburden; thickness of mineral deposit therefore decides the economic working limit of quarrying, i.e., the quarriable limit.
The overburden and mineral deposit can be extracted by formation of benches or by keeping the surface sloped so that the angle of slope does not exceed 45 from the horizontal. A bench has two elements, the floor and the face (high wall).
In alluvium, morrum, loose earth, etc. which is likely to slide the bench height should not exceed 1.5 m. During the rainy season, there is possibility of land slide of the loose debris and it is desirable to keep the width of the floor much larger than the height of the bench.
Benches in sand stone and hard rocks are rarely vertical but generally sloping at a small angle with the vertical. The mining regulations do not stipulate definite bench heights in hard rocks except that the bench width has to be more than the bench height. In manual quarry the bench height is usually 3 metres to 4.5 metres and in mechanised quarries, more than 5.5 metres and depends upon the height of the boom of shovel above the bench floor.
Suitable bench height for a 2 m3 shovel is 6 to 8 m and for a 3.5 m3 shovel, about 12 m. The slope of the high wall is usually 20 off vertical and depends upon the travel of the bucket during loading. The width of bench floors in mechanised quarries is usually 15 metres and preferably more for movement of dumpers, tractor loaders and other equipment.
The quarriable area is divided into sections along the strike so that overburden extraction takes place in some sections and coal extraction in others which have the coal already exposed after overburden removal. Small pillars of the rocks excavated are left for measurement.
These are called witness or Sakhi in Hindi. They are removed after measurement of excavated area, usually once a week is over. The height of such sakhi should not exceed 2.5 m and where the height of such pillar exceeds 1.25 m its base should not be less than 1.5 in diameter.
The soft material like earth and weathered rock is cut by earth cutting picks. A team of workers consists of 3 or 4 members, one cutting and two loading. As female workers are allowed in quarries, a team of 3 workers usually includes one or two females members who are normally permitted to work only between 5 a.m. and 7 p.m. under the Mines Act. Each team is allowed a small plot usually 4.5 m x 4.5 m. The average output per worker in soft rock or earth is about 2.8 m3 (in situ) per day.
Method (a) is used in a small quarry where the output is only 50 to 100 te of coal per day, electricity or compressed air is not available and labour is cheap. A hole is 1.2 m to 1.5 m deep and one man can drill 5 to 8 holes, each 1.2 m deep in a shift (8 hrs.) Method (b) is now- a-days commonly employed.
Compressed air is supplied by steel pipes 50 to 100 mm dia, up to central places and branch pipes supply air to drills through lose pipes. Two workers (one driller and one helper) drill 40 to 50 holes, each 1.5 m deep, in one shift. The holes one placed 1.2 to 1.5 m apart and are blasted with gun powder or other suitable explosives. Blasting is done during the rest interval of the workers to prevent frequency interruption of work.
The overburden is loaded into tipping tubs (0.73 m3 capacity) which are hauled by direct or endless haulages. In seams of mild gradients locomotives may be used. The haulage track is taken to each bench or alternate benches. Blasted overburden of higher bench is sometimes dropped on the lower bench for loading into tubs.
Overburden is dumped to the rise of the outcrop or beyond the quarriable limit but as the coal extraction proceeds, the overburden may be dumped in the area from which coal is extracted an operation known as back filling. The overburden should be so dumped that it does not roll down at the coal benches when it assumes its angle of repose, nor should it choke water courses or damage paddy fields, other agricultural area or water reservoirs.
The coal which is exposed after removal of overburden is blasted after drilling holes. The same drills which are used for stone may be utilised for coal also but if the compressor has a limited capacity electrically operated drills are used for coal due to their relative lightness and better performance.
The spacing of holes in coal is 1.5 m to 2.2 m, the depth varying from 1.2 m to 2 m. The coal available per kg of high explosive like special gelatin (60 to 80%) is generally 10-12 tonnes. With blasted coal, the average loading performance per loader is nearly 4 tubs (1.1 m3 capacity).
After it has advanced some distance along the dip. The entrance to the working places is by steps and inclined roads. The permanent installations like haulages, compressors, power transmission lines, etc. are installed in such places that their frequent shifting is not necessary when the stone benches or coal benches advance.
On a level track hand pushing of empty tubs having pedestal bearing is not uncommon for a distance of 100 m. If the same track is used for loads as well as empties, the gradient of the track should be nearly 1 in 80 in favour of loads. These factors limit the length of benches.
The property is divided into blocks, each 120 to 180 m long along the strike. Each block has a direct or endless haulage in the middle and the benches are formed along the strike on either side of the haulage. Block A which is shown more advanced than Block B, raises coal from the coal benches CB1, CB2, CB3, CB4.
Each bench is 3 m high and has a haulage track on its floor. From the clipping point PI to the junction P2 the haulage track is either level or slightly rising in bye at 1 in 80 or so. The stone benches SB1, SB2, are ahead of the coal benches so that the exposed coal lasts for 2-4 weeks. Stone from the higher bench SB2, is dropped on the lower bench SB1 which has haulage tracks on its floor.
The stone is taken in tipping tubs along a level track TA by hand pushing for dumping in the de-coaled area. The track TA is along the barrier. The tubs cross the direct haulage track over a bridge K1 to fully utilise the decoaled area for dumping.
As the stone benches advance the position of the bridge K1 has to be shifted to the dip side. Another level haulage track is taken from the stone bench SB1 to the decoaled area over a ledge LL1. This ledge is 3m wide at the top and is a solid barrier of coal and stone left between blocks A and B. The coal of the ledge is not recoverable.
In block B, there is emphasis on removal of overburden. The direct haulage track is along the floor of the coal seam and more stone benches than in block A are provided for employment of a large number of workers on overburden removal.
The stone benches SB5, SB6, SB8 are served by the central direct haulage. Stone benches SB3 and SB4 are worked by a level track Tb and another level track passing over the ledge LL1. stone is taken over these level tracks in tipping tubs for dumping in the decoaled area in the same manner as described for block A. The bridge K2 serves the same purpose as bridge K1 in block A and is advanced towards the dip at intervals.
During rainy season coal raising may be suspended in block A which is on the dip side, and only overburden removal may take place from the higher stone benches which are free from water. From block B, only coal raising may take place during monsoon and the block can be kept free from water and making a through connection 1.8 m high in the ledge LL1 for the water to gravitate to block A.
Drains for water, called garland drains are cut to minimise inflow of surface water into the quarry. Pumps are installed at possible places of heavy accumulation of water. The installation of compressed air pipes and the overhead power lines.
There is no problem of roof control or ventilation. Full extraction of mineral is possible. No mineral is blocked in shaft pillars, support of main roadways, etc., as in underground mining. Quick return on capital and early extraction are possible without the need to wait for a long period of development work or unproductive work like shaft sinking, etc. Large output is available from a small area and supervision is easy owing to concentrated work. Artificial lights are necessary only after dark.
Dangers and hazards are less as compared to underground mining. There is no risk of gas explosion. Very few stringent mining regulations are applicable to quarries. For example, compared to underground mining less number of competent persons is to be appointed and less number of statutory inspections is necessary.
Training of operatives is easier. Large scale mechanisation is possible as there is no restriction on the dimensions of machines to be used. Unlike in underground mines, machinery working at high voltage can be employed.
Work is affected by weather. During winter nights, and summer mid-days efficiency of workers is very low. During rainy season unless effective steps are taken to dewater the mine mineral which is at the lower levels, cannot be worked and mining comes practically to a standstill.
Where quarrying aims at quick return on capital, outcrop mineral is also mined. As it is inferior in quality due to weathering and percolating of water, the overall quality available to the consumers is affected in earlier phases of mining.
The quarried area and the OB heaps present an unpleasant sight. In some foreign countries the mining law requires that the quarried area should be filled up with overburden and restored to the pre-quarry state fit for agriculture.
Marshy land, after extraction of underlying mineral and restoration of surface has, in a few cases, resulted in a good agricultural area. Such low requiring surface restoration to pre-quarry stage does not exist in India.
A property with extensive area on the strike and containing a thick mineral bed moderately inclined, lying at shallow depth, is ideal for quarrying. In India coal seams with inclination as steep as 1 in 3 have been worked by mechanised opencast mining methods in Karanpura field.
Seams at shallow depths which are actively gassy, liable to spontaneous heating or with bad roof should preferably be extracted by quarrying as the mining legislation for underground working of such seams is stringent.
Before the quarrying operations are undertaken it is necessary to vacate buildings and divert electric overhead lines, aerial ropeways, water mains, telephone lines, roads, railway lines, streams, etc. from the area which has to be quarried.
The trees have to be cut. Sufficient space for dumping of overburden, not far from the quarry, has to be considered. Where mechanised opencast mining is to be adopted, plans should be prepared to show the contour lines and the thickness of coal and overburden and good roads without steep inclinations and sharp curves should receive attention.
Dumpting yard for OB should be so selected that wind does not carry the dust to residential colony. By dumping of OB if the ponds, paddy fields, mango/coconut groves, not under the ownership of the mine owner, are likely to be affected, the compensation payable should not be ignored.
If heavy explosive charges have to be blasted as in mechanised quarries, the quarry site has to be far away from residential area (beyond 300 m). Such heavy blasting may cause cracks in old buildings resulting in demand for compensation.
On the surface reference lines have to be marked on a square pattern, every 30 m apart, for monthly measurement of the excavation and they should extend 50 m beyond the limits of the proposed quarry. Junctions of the squares should be marked by permanent pegs in brick pillars.
7 different types of mining processes newsmag online
Mining is used to extract many highly used resources in the world today. Mining unveils valuable minerals like coal, which is a source of electricity and heat, as well as in the manufacturing of cement and steel. As such, these minerals must be mined to supply the high demand for this resource, and there are many ways to do so.
Most mining operations combine different methods to extract minerals most efficiently. Initially, the mineral closer to the surface would be mined using a surface mining method. After the minerals have been exhausted that way, the coal found deeper in the seam would be mined using an underground mining method. This allows for the most minerals in an area to be extracted, using the most efficient methods.
The type of mining is based off of a variety of factors. The thickness and depth of the coal seam are very important in deciding which method will be used, as well as the terrain that is found on the surface above the coal. Economic factors also come into play, as some methods are more or less costly than others.
There are two main types of mining every method can be divided into either surface or underground. Logically, surface mining is used to gain access to minerals located closer to the earths surface. The place where minerals can be found is often called a seam. A seam is considered shallow if it is located closer to the earths surface, and deep if it is located further away.
Underground mining can be used for minerals in more shallow seams, but is more often used for deeper deposits.In order to access the minerals underneath, blasting is done. Blasting is an key part of surface mining, as it helps to prepare the terrain for mining. The rocks are broken up using explosives, and then cleared away using machinery.
The shaft may be an exact vertical or a near vertical drop into the mine, and an elevator or lift is installed to bring workers into and out of the mine. The mine will extend from this initial spot in all directions.
Also known as open pit mining, this is a method of surface mining where a large pit is made in the ground, and the coal is mined directly from that pit. This is a great method to use for a shallow coal seam and it is extremely productive.
Using specialized machines, longwall mining is an underground mining method that cuts out panels of the coal, which are then sheared down and brought to the surface. This method is highly mechanized, and is therefore a very safe type of mining.
It is a highly productive method as well, with a very low cost per ton as compared to some other methods. However, longwall mining does require highly specialized machinery, which can be very costly to start off. It also doesnt allow for very great selection of the grade of the ore.
The title given to room and pillar mining is very fitting, as it is an underground method of clearing out coal by creating roomlike spaces. Some material is left in pillars, which helps to support the roof so that ore around it can be removed.
These types of mining offer good ventilation a sought after feature in underground mining and high productivity. Its not a very flexible method, and doesnt offer a lot of selectivity, however it is better than some other methods. The room and pillar method does, however, limit the depth at which the ore can be mined.
With hydraulic coal mining, water is pushed at high pressure against the ores surface, breaking away the chunks bit by bit. The water, with the bits of coal in tow, are then pushed through a pipe to the surface, where the coal is filtered out of the water.
One of the major advantages of hydraulic coal mining is that the water minimizes the chances of fire and explosion, and it reduces the amount of dust, making it a safer and more healthy working environment. However, power consumption for these types of mining is very high.
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open-pit mining definition | anglo american
Open-pit mining, also known as opencast mining, is a surface mining technique that extracts minerals from an open pit in the ground. Open-pit mining is the most common method used throughout the world for mineral mining and does not require extractive methods or tunnels. This surface mining technique is used when mineral or ore deposits are found relatively close to the surface of the earth. Open-pits are sometimes called quarries when they produce building materials and dimension stone.
Open-pit mines are dug on benches that are between four and sixty-meters in size, depending on the size of machinery used to excavate. The walls of most open-pit mines are dug at an angle and include steps to prevent avalanches from occurring inside the build site. The incline section of the wall is called the batter, and the flat part of the step is called the bench or berm.
Waste rock is piled up near the edge of the pit and spreads both horizontally and vertically. This is known as the waste dump. The waste dump is also tiered and stepped, to keep rocks from falling into other parts of the site.
After mining finishes, the mine area must undergo rehabilitation to minimize environmental damage. This step in the mining process is critical to ensuring the sustainability of the land for future use.
First, waste dumps are contoured to flatten them out and stabilize them. If the ore contains sulfides, it is covered with a layer of clay to prevent rain and oxygen from oxidizing the sulfides into sulfuric acid, which is also known as acid mine drainage.
Then, the waste dump is covered with soil, vegetation is planted, and the area is fenced to prevent livestock from eating the newly planted vegetation. This layer will eventually erode but in the meantime, it will allow the leaching of heavy metals to occur slowly enough for the surrounding environment to absorb them.