Basic ideas for open pit mining

In the mining design, depending on the natural factors of the deposit, the following three situations may be encountered:

(1) All deposits should be underground;

(2) The mining bed should be used for open pit mining, or the lower part can only be used for underground mining;

(3) All deposits should be open-pit mining, or the upper part should be used for open-pit mining and the rest should not be mined temporarily.

For the latter two cases, it is necessary to determine the reasonable open-pit mining boundary, including determining the reasonable mining depth, the bottom perimeter of the open pit and the final slope angle of the open pit.

The determination of the open-pit mining boundary is essentially the control of the stripping ratio. Because of the deepening and expansion of the open-pit mining realm, the recoverable reserves have increased, but the amount of exfoliated rock has also increased accordingly. A reasonable open-pit mining boundary means that the stripping ratio controlled by the company does not exceed the economically reasonable stripping ratio.

In the open-pit mine boundary design, the stripping ratio that needs to be controlled has the average stripping ratio, the boundary stripping ratio and the production stripping ratio. The unit of stripping ratio can be expressed by m3/m3, m3/t or t/t.

(1) The average stripping ratio (np) refers to the ratio of the total amount of rock in the open-pit mining boundary to the total amount of ore.

(2) The boundary stripping ratio (nj) refers to the ratio of rock increment to ore increment caused by each unit depth added to the open pit mining boundary.

(3) Production stripping ratio (ns) refers to the ratio of the amount of rock stripped in a certain period of open pit mine to the amount of ore mined. The equilibrium production stripping ratio is calculated according to several stages, and the equilibrium production stripping ratio is called, and its value is considered to be fixed during a certain equilibrium period.

(4) Economically reasonable stripping ratio (njH) refers to the economically the maximum allowable stripping ratio of open pit mining. It is a theoretical limit value and an important technical and economic basis for determining the final boundary of open pit mines.

At present, there are many methods for determining the economically reasonable stripping ratio. There are two main types of comparison: one is the comparative method, which is calculated by comparing the economic effects of open pit mining and underground mining to divide the open-pit mining of deposits and The boundary of underground mining, such as the raw ore cost comparison method, the product cost comparison method, the reserve profit comparison method, the conversion cost comparison method, etc.; the other is the price method, which is calculated by comparing the open-pit mining cost with the ore price, in the deposit. It is only suitable for open-pit mining where it is used to divide the boundary between the open-pit mining part of the deposit and the temporarily unexploitable part, such as the minimum profit comparison method, the highest cost comparison, and the recovery investment comparison method. Their calculation methods, their various advantages and disadvantages, and applicable conditions.

The main factors affecting the open-pit mining realm are:

(1) Natural factors. Including deposit conditions, such as ore body shape, size, thickness, dip angle and other ore types and grades, ore and surrounding rock properties, topography, rivers near the mining area, engineering and hydrogeology.

(2) Technical organizational factors. Including open pit and underground mining technology, equipment level, in the vicinity of the mine railway, major buildings and structures and so on.

(3) Economic factors. Including infrastructure investment, infrastructure time and production time, mining cost and sales price of ore, depletion and loss of ore during mining, and the level of national economic development.

The above factors have different influences on different deposit conditions, and should be considered comprehensively when determining the open-pit mining boundary.

There are several main principles for determining open-pit mining:

1. The boundary stripping ratio is not greater than the economically reasonable stripping ratio

The principle is nj≤njH, which is essentially that the open-pit mining cost of the boundary layer ore in the mining boundary does not exceed the underground mining cost, so that the total cost of the open and underground joint mining of the whole deposit is the smallest or the total profit is the largest. It is simple and convenient, and has been widely adopted for the design of open pit mines at home and abroad. The biggest shortcoming is the determined open-pit mining boundary, which cannot directly control the investment and production cost of the open-pit mine (production stripping ratio). For example, for some discontinuous deposits or deposits with thin or thick deposits, or deposits with thick or complex topography (such as isolated hills in the boundary), when the principle is applied to determine the boundary, boundary stripping may occur. The ratio exceeds the requirement, and the initial stripping ratio exceeds the allowable value. In order to prevent false conclusions, it is generally necessary to use the principle that the average stripping ratio is not greater than the economically reasonable stripping ratio, that is, np≤njH.

Second, the average stripping ratio is not greater than the economically reasonable stripping ratio

The average stripping ratio is not greater than the economically reasonable stripping ratio, ie np≤njH. The essence of this principle is that the total cost of open-pit mining of all reserves in open-pit mining is less than or equal to the total cost of underground mining. The open-pit mining boundary delineated by this principle is larger than that delimited by the principle of nj≤njH, which may cause large amount of infrastructure stripping, large investment, long construction time, and can also produce stripping ratio in a certain period of open-pit mining. Exceeding the allowable value, the company is in a state of loss for a long time. For some valuable non-ferrous and rare metals deposits, etc., in order to expand open-pit mining in order to reduce ore losses and dilution, this principle can be used to delineate the realm of open-pit mining, in order to expand open pit mining reserves. This principle can also be used to determine the open-pit mining boundary for near-horizontal ore bodies or nests.

Third, the production stripping ratio is not greater than the economic reasonable stripping ratio

The production stripping ratio is not greater than the economically reasonable stripping ratio, ie ns ≤ njH. The essence of this principle is that when any production period of open-pit mines is produced according to the normal working work angle, the production stripping ratio does not exceed the economic reasonable stripping ratio, which reflects the change of production stripping ratio in open pit mining. law. The open-pit mining boundary delineated by this principle is larger than that delimited by the principle of nj≤njH, and is smaller than the principle of np≤njH, which can better reflect the superiority of open-pit mining. However, the production stripping ratio can usually only be determined after the open-pit mining boundary is determined, and the development method and mining procedure are determined accordingly. Therefore, the time, place, value and variation law of the maximum production stripping ratio are very The great uncertainty brings certain difficulties to the determination of the mining realm.

4. Determine the open-pit mining boundary according to the demand for ore and the degree of exploration.

This principle is generally applicable to non-metallic deposits with small stripping and large reserves such as limestone and dolomite, and iron deposits with extra thick and deep.

It should be pointed out that the principles for determining the open-pit mining boundary are based on economically reasonable stripping ratios, and the economic rational stripping ratio is closely related to the national economy and the level of science and technology, and its value is changing. Therefore, the bounded open-pit mining boundary in the design is only a reasonable value under certain conditions for a certain period of time.

With the advancement of science and technology and the continuous development of the national economy, the economic benefits of open-pit mining continue to improve, and the economic rational stripping ratio tends to increase. The open-pit mining boundary originally designed will also expand and deepen. Some long years of service in the latter part of the economic open pit mining need to expand or help delay the second phase of deep mining, such as Daye iron ore, copper, iron ore and other Green Mountain copper will happen.