Mining is the process of excavating and recovering ore and associated waste rock from the earth’s crust. If the economic evaluation of a mining project offers positive results, pointing to a high probability that the exploitation of the mineral deposit will produce benefits, mining will be the next step.
The selection criteria of the mining method and the type of mine to be built are based on the competence of the rock, the distance to the surface of the deposit, the characteristics of the deposit and economic conditions.
Conditioned by the distance to the surface, extraction methods are broadly grouped into open pit and underground mining.
About 85% of global tonnage is produced from open pit mines, including placer operations, while the remaining 15% from underground mines.
Operating mines range from small underground operations to large open pits, some moving tens of thousands of rocks per day.
Open pit or surface mining
Open pit or surface mining is a form of operation aimed at extracting minerals that are close to the surface.
In recent decades, surface production has spread as open pit mining is less expensive than underground mining, due to the higher cost of underground extraction methods.
The depletion of the richest ore bodies, combined with the development of new technologies, makes it necessary to work in open pit mines with lower ore grades.
The current trend is large-scale open pit mining using the economy of scale, that is, the savings in production cost that is due to mass production.
In mining parlance, a large mine will produce significantly more production per unit of input than a small mine.
Therefore, large-scale equipment is used to make operations efficient and economical.
For example, today the trucks are huge and carry up to 500 tons per load.
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Underground mining
If the depth of an ore deposit is such that overburden removal makes open pit mining uneconomical, underground methods should be considered.
Underground mining refers to the extraction of raw materials from below the surface of the ground.
A logical procedure for classifying underground mining methods is to divide them into the following three groups:
- methods that produce openings that support naturally or require a minimum of artificial support (for example, pillars)
- methods that require substantial artificial support (eg, cut and fill)
- cave-in methods where rock collapse is an integral part of the extraction process (eg, block caving).
For example, in the pillar method, very common in underground mining, ores are obtained from large voids (rooms) and pillars are left between the rooms to support the overlying rocks.
In general, the mineral body included in the pillars remains at the end of the extraction and is not recovered.
Combined mining
If the ore body extends from the surface to great depth, mining sometimes begins near the surface from an open pit and then mining continues with underground mining for the deeper parts of the ore body.
This method is generally called merged mining.
Use of explosives
The use of explosives is often indispensable in the exploitation of mineral resources. Therefore, blasting is usually part of the mining cycle in mining.
Blasting is the process of breaking the material by using a quantity of explosive charged in special holes. There are many different types of explosives used today, such as ammonium nitrate fuel, slurries, and emulsions.
The holes drilled for the blast are loaded so that each is fired in a sequence designed to obtain the desired breakup of the rocks. The explosives are detonated in the drill holes.