Why is open pit mining done

Mining explained Open-Pit Mining Definition. What is open-pit mining? Benefits of open-pit mining include: Ease-of-use for mass production Small shut-down expense Ability to mine selectively for certain grades of ore Comparatively small crew size Elimination of safety hazards that can accompany complex underground mining operations Easy drainage of subsurface water No machinery restrictions - even heavy and bulky machinery can be utilized Lower capital and operating costs Materials commonly extracted from open-pit mines include: Bitumen Clay Coal Copper Coquina Diamonds Iron Gold Gravel and Bedrock Granite Gritstone Gypsum Limestone Marble Molybdenum Silver Uranium Phosphate Open-pit Mining Operations 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.

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Also, open pit has virtually an unlimited ability to create and use high-performance large-sized mining and transportation equipment that can provide the highest technical and economic parameters. Open pit mining has higher productivity 3—5 times of underground methods , lower production costs, more safe and hygienic working conditions, more complete recovery of a mineral, and lower per unit production cost.

Open pit mining is characterized not only by its high share of total minerals production, but it is also considered as one of the surface mining methods that contributes to the construction of powerful performance quarries — million tons of rock a year reaching to a depth of m.

Therefore, decisions on the construction of new or existing quarries should be economically justified. Table 1 shows the advantages and disadvantages of open pit mining method [ 2 , 3 ]. Advantages and disadvantages of open pit mining method [ 2 , 3 ]. Open pit mining is widely used with metallic ore bodies aluminum, bauxite, copper, iron , and nearly all nonmetallic coal, uranium, phosphate, etc.

It is a traditional cone-shaped excavation although it can be of any shape, depending on the size and shape of the ore body that is used when the ore body is typically pipe-shaped, vein-type, and steeply dipping stratified or irregular [ 4 ]. The major open pit and ore body configurations are classified into the following: Flat lying seam or bed, flat terrain e. Massive deposit, flat terrain e. Dipping seam or bed, flat terrain e.

Massive deposit, high relief e. Thick-bedded deposits, little overburden e. Flat lying seam or bed, flat terrain. Massive deposit, flat terrain. Dipping seam or bed, flat terrain. Massive deposit, high relief.

Thick-bedded deposits. The parameter known as the stripping ratio represents the amount of uneconomic material that must be removed to uncover one unit of ore, i.

Also, the ratio of the total volume of waste to the total volume of ore is defined as the overall stripping ratio. A lower stripping ratio means that less waste has to be removed to expose the ore for mining which generally results in a lower operating cost [ 5 ]. The major types of stripping ratios are overall, instantaneous, and break-even. In order to specify the maximum allowable stripping ratio SR max of a surface mine, break even ratio can help to establishes the pit limits. SR max defined as the ratio of overburden to ore at the ultimate boundary of the pit, where the profit margin is zero.

It can be calculated as:. The maximum allowable stripping ratio enables us to locate the ultimate pit boundary or limit based on prevailing economic, physical, and geometric conditions in the pit. Ore occurring in the ore body beyond this maximum stripping ratio will have to be left or mined underground. A copper pit designed in this manner with varying ore grades and critical SR max. Example A seam of coal has a density of 1. It is covered by 27 m of shale which has a density of 1.

Calculate the stripping ratios? The head assay of a copper ore is 0. Overburden is a waste rock material that must be removed to expose the underlying ore body. It is preferred to extract as little overburden as possible in order to access the ore of interest, but a larger volume of waste rock is removed when the mineral deposit is deep.

Most removal operations which includes drilling, loading, blasting, and haulage are cyclical. This is true for hard rock overburden which must be drilled and blasted first. 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.

In the North, open-pit mining comes with a unique set of challenges. For example, at the Diavik Diamond Mine, the diamond bearing orebodies, known as kimberlite pipes volcanoes which brought diamonds to surface 55 million years ago , were under shallow waters of a large lake called Lac de Gras. To safely open-pit mine the kimberlite, water retention structures dikes were constructed in the lake to hold back the lake waters during mining.

The main cost advantage of open-pit mining is that it allows mining with more efficient larger equipment. This means faster production and mining of lower grades of ore is possible. It also means potentially bigger revenues early as the mine prepares to go underground and mine the deeper parts of the orebody. The open-pit mining cycle includes drilling ore and waste rock, loading drill holes with explosives, blasting, ore loading and hauling to stockpiles for processing, waste rock loading and hauling to the waste rock storage pile.