An exception to the cyclical effect is dredging method used in hydraulic surface mining and some types of loose material mining soil with bucket wheel excavators. The percentage of waste rock to ore excavated is defined as the stripping ratio. Stripping ratios of up to are common in large mining operations. Ratios above tend to be less economically feasible depending on the type of ore extracted. In selecting a particular stripping method and its corresponding equipment, the ultimate aim is the removal of material waste and burden at the least possible cost [ 6 ].
Stripping methods are classified into: declining;. In this method, each bench of ore has to be mined in sequence, and the waste in the particular bench has to be removed to the pit limit. The ore is easily accessible in the subsequent benches and the operating working space is widely available. Furthermore, all equipment usually work in the same level and so no contamination from waste blasting is left above the ore body.
This method is highly productive especially at the beginning where equipment required is at minimal toward the end of the mine life. The primary disadvantage of this method is that the overall operating costs are at maximum during the initial years of operation when the maximum repayment of capital is needed and so cashflows are required to handle interest and repayment of capital see Figure 7.
Declining stripping method. In this method, stripping of overburden is performed as needed to uncover the ore. The working slopes of the waste faces are essentially maintained parallel to the overall pit slope angle. This method also allows for maximum profit in the initial years of operation and greatly reduces the investment risk in waste removal for ore to be mined at a future date.
It is considered as a very popular method whereas mining economics or cutoff stripping ratio is likely to change in a very short time. This method is sometimes impractical because of its small spaces narrow benches. It is available for operating a large number of equipment especially at the beginning of stripping see Figure 8. Increasing stripping method.
This method aims to remove the waste at a rate estimated by the overall stripping ratio. The working slope of the waste faces starts very shallow, but increases as mining depth increases until it equals the overall pit slope.
This method has the advantage of removing the extreme conditions of the former two stripping methods outlined. Equipment fleet size and labor requirements throughout the project life are relatively constant. In this method, a good profit can be generated initially to increase cash flows. The labor and equipment fleet can be increased to maximum capacity over a period of time, and then, they can decrease gradually toward the end of the mine life.
Distinct mining and stripping areas can be operated simultaneously, allowing flexibility in planning see Figure 9.
Constant stripping method. Open pit mines are constructed of series of benches that are bisected by mine access and haulage roads angling down from the rim of the pit to the bottom.
The bench height is the vertical distance between each horizontal level of the pit. The elements of a bench are illustrated in Figures 10 and 11 , unless geological conditions dictate, otherwise all benches should have the same height. The bench height should be designed as high as possible within the limits of the size and type of the machine or equipment selected for the required production.
The bench should not be so high that it will cause safety problems. The bench height in open pit mines will usually range from 15 m in large mines e. The slope angle of the pit walls is a critical factor. If the slope angle is too steep, the pit walls may collapse.
If it is too shallow, excessive waste rock may need to be removed. The pit wall has to remain stable as long as mining activity continues. The stability of the pit walls should be examined as carefully as possible. For example, rock strength, faults, joints, and fractures are key factors in the evaluation of the proper slope angle.
Main mine elements and parameters. Detailed mining operation in an open pit. The main economic goal in open pit mining is to remove the smallest amount of material while obtaining the greatest return on investment by processing the most marketable mineral product. The higher the grade of the ore, the greater the value received.
To reduce the capital investment, an operation plan has to be developed in order to precisely dictate the way in which the ore body has to be extracted. Open pit mines vary in scale from small private enterprises processing a few hundred tons of ore a day to large companies operated by governmental and private corporations that extract more than one million tons of material a day.
The largest mining operations can involve many square kilometers in area. The production cycle also referred to as the mine unit operation that consists of ripping and dozing, drilling, blasting, loading, and hauling see Figure Open pit operations cycle. Typically, bulldozer, wheel dozers, and motor graders are the most common equipment used, in which material transport distance is short and it can be pushed by a blade.
The dozer has a large blade capacity and it is designed specifically for bulk material excavation, whereas the motor grader is a machine with a long blade used to create a flat surface during the grading process. The ore deposit can be mined by means of drilling and blasting in order to fracture the rock into a loadable size. Blasting parameters should be matched with mechanical machines for drilling of blast holes and charging of explosives.
Blast holes are drilled in well-defined patterns, which consist of several parallel rows. In bench blasting, the normal blast hole patterns are square, rectangular, and staggered, Figure The most effective pattern is the staggered pattern, which gives the optimum distribution of the explosive energy in the rock. Blast holes drilling patterns. Nowadays, surface mining is conducted using shovels, front end loaders or hydraulic shovels. In open pit mining, loading equipment is matched with haul trucks that can be loaded in 3—5 cycles of the shovel.
Many factors determine the preference of loading equipment. For example, with a hard digging rock, tracked shovels are more advisable. On the other hand, rubber-tyred loaders have lower capital cost and are better for loading materials that are low in volume and easy to dig. Furthermore, loaders are very mobile and well applicable for mining scenarios requiring rapid movements from one area to another. Loaders are also often used to load, haul, and dump material into crushers from blending stock piles placed near crushers by haul trucks, Figure Loading material using front end loader.
Hydraulic shovels and cable shovels are common equipment used in open pit mining. Communities Our blueprint for the future of sustainable mining 13 March Communities A day in the life of Nicinha: a local cheese producer in Brazil 8 August Environment Three ways biodiversity awareness is critical to mining 10 September Environment Behind every successful mining company is a strong biodiversity strategy 4 July Environment Our blueprint for the future of sustainable mining 13 March Environment How critical is innovation to the mining sector?
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We have almost exhausted our natural resources and there are huge What are the differences between open pit mining, underground mining, and strip mining? What is the environmental impact of diamond mining? See all questions in Mining. Waste rock is piled up at the surface, near the edge of the open pit.
This is known as the waste dump. The waste dump is also tiered and stepped, to minimize degradation. 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 often cyanide which is used to treat gold ore via the cyanide leach process.
This toxicity can harm the surrounding environment. Figure 2. Opencut coal mine loadout station and reclaimed land at the North Antelope Rochelle coal mine in Wyoming, United States. 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 produce sulfuric acid, a phenomenon known as acid mine drainage.
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