Types of Surface Mining Operations

Where a bedded-type mineral deposit occurs at or near the surface, it can be mined at a lower cost and with fewer problems by surface mining. Coal seams lying within 100 feet of a level surface can be conveniently mined in this way. This is known as strip mining, because the overburden is first stripped off and cast aside in order to expose the seam of coal in strips. After the coal is removed, the overburden is replaced and the area is revegetated.

Strip mining is usually carried out on a large scale. For this purpose, gigantic machines like walking draglines, excavators, power shovels, or bucket wheel excavators are used to strip and dump the overburden. The coal is then mined with smaller power shovels, loading into trucks. In most cases a certain amount of drilling and blasting is required to loosen hard bands of overburden materials and therefore to facilitate the action of the excavators. Bedded deposits of the softer minerals, occurring near the surface, can be handled in the same way.

The harder minerals of the metalliferous disseminated or replacement deposits may also be handled by surface mining, generally called open cut work. These deposits are often about 40 feet thick, under overburden of a maximum depth of, say, 50 feet. In these cases hard, rocky overburden and ore are dealt with. Much more complicated methods of drilling and blasting must be used and a modern type of loading machine like a power shovel is called for. Many porphyry copper mines have adapted to these procedures.

For steeply dipping wide lodes with a considerable vertical extent, additional considerations are necessary. After preliminary overburden removal to expose the whole of the ore body, a series of benches is developed. These are stepped in heights limited to about 50 feet for safety reasons. The horizontal part of the bench is designed to provide space for the broken ore from each blast, to deploy drilling machines to drill blastholes for blasting on the next lower bench, to allow space for mobile power shovels to move in and load the ore into transport vehicles, and to provide a roadway for the latter. However, as open cut mining proceeds to greater depths, the exposed hanging wall and the footwall would become a source of hazard unless these walls were stripped back (by a similar benching procedure) to provide a safe general slope angle to the bottom of the cut. The waste rock mined in the walls is transported to a dump area, whereas the ore is taken to the crushing plant.

As mining proceeds to deeper horizons, increasing tonnages of overburden are mined from the walls, adding considerably to working costs. A point is reached where it is better to cease open cut work and to continue to mine the deposit by converting it to an underground mining operation.

Quarrying is another type of surface mining operation, where rock is being mined (or quarried) for the production of construction stone or building stone.

Another phase of surface mining relates to getting salt and other minerals from dry salt lakes or from lake brines. A number of minerals corresponding to sodium, po­tassium, and magnesium chlorides and sulphates are produced by solar evaporation, fractional crystallization, and by mechanical scraping and loading.

Eventually the material is mined from blocked out areas and segregated into waste, low grade, and ore with the ore going to stockpiles conforming to established categories. Shipments to the mill are controlled by blending from the stockpiles to obtain the desired ore grade for any given period.

After the ore zones have been delineated, the open pit crest limits are established using an average slope of 2/3 to 1 from the top of the ore to the surface. Overburden currently varies from 100 to 175 ft, with ultimate plans involving depths up to some 250 ft.