Open pit mining at its best

Staff Geology for Investors in Geology BasicsMine Planning and ManagementMining and Mine DesignMining Methods Once a potentially economic mineral deposit has been found many different challenges face the company preparing to open a mine. Examples of the two main categories of mining. Mining techniques can be divided into two broad categories: Surface mining consists of stripping soil and vegetation away and possibly a limited amount of rock and then removing ore in large quantities.

Open pit mining at its best

View Larger Image Open-pit mining methods are applicable to mining ore deposits that apex at or near the surface.

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If the deposit apexes below the surface, the overburden and barren capping overlying the ore must be removed in advance of open-pit mining. The removal of this material is known as stripping. The stripping-pit limits must be extended beyond the limits of the ore pit to provide a bench, and the pit-wall slopes must be such as to prevent sloughing of overburden onto the ore area.

Where the ore is to be hauled from the pit by locomotive or trucks, additional excavation is required to provide an approach on a grade suitable for haulage purposes.

The cost of this preparatory work is obviously a charge against the cost of mining the ore, each ton of which must bear its proportionate share. As the stripping-ore ratio volume or tons of overburden and capping to tons of ore made available for open-pit mining increases, obviously the cost of stripping per ton of ore increases.

The question of whether to mine by open-pit or underground methods is an economic one. The economic limit for open-pit mining is reached when the preparatory and stripping costs per ton of ore plus the open-pit mining cost per ton of ore plus the cost of equipment, interest, and amortization charges equal the cost per ton by mining underground plus equipment and development cost and interest and amortization charges.

The stripping-ore ratio is a basic factor used for determining whether to employ open-pit or underground methods and for determining the economic limits of open-pit mining. Other factors influencing the choice between open-pit and underground methods may be inferred from the advantages and disadvantages of open-pit mining, some of which are listed below.

In specific instances consideration of these may affect the choice of method or result in revision of pit limits established by cost estimates based upon stripping-ore ratios. Among the advantages of open-pit mining are its flexibility, the ability to obtain mass production with it, and the ease with which rate of production can be increased or decreased once the pit has been developed fully; small shut-down expense; the ability to mine selectively to meet requirements for certain grades of ore except in glory-hole or milling pits ; complete extraction of the ore inside the pit limits except as limited by ore benches for haulage tracks and maintenance of safe pit slopes, but this ore can usually be recovered finally by employing special clean-up methods ; comparatively small number of men employed, a large proportion of whom are skilled labor in mechanized operations; and elimination of hazards inherent in underground mining operations.

On the other hand, certain disadvantages may outweigh the advantages and affect direct economic considerations in some instances.

Open pit mining at its best

Thus, large open-pit operations involve heavy capital outlay for equipment, and where the amount of overburden to be removed is extensive, correspondingly high capital expenditure is required for stripping.

This capital is nonproductive until ore mining is begun, and during the stripping period heavy interest charges often accumulate. The time elapsing before production begins may in itself be a serious disadvantage, especially if exploitation is undertaken at a time when ore prices are favorable and the demand for ore is strong.

Disposal of the waste from stripping operations sometimes occasions a serious problem, especially when the terrane is flat or where exorbitant prices must be paid for dump area near the mine. Climatic conditions may limit the operation to being seasonal and necessitate cessation of operations during certain months, and in areas where torrential rains are prevalent frequent flooding of the pit may be a serious obstacle to pit mining.

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Simple forms of open-pit ore mining include excavation and loading of overburden and ore by hand into wagons or trucks and excavation and removal of overburden and ore by means of animal-drawn slip or drag scrapers.

Such methods are applicable to deposits that are too small to warrant capital investment in mechanical equipment and will not be discussed.

Overhead slackline cableways are used extensively in quarries, for sand and gravel excavation, and for other purposes but are not used very extensively for ore mining in this country. This kind of work has been discussed by Thoenen in relation to sand and gravel excavation in an earlier Bureau of Mines publication.

Discussion in this bulletin is limited to two principal forms of open-pit mining—glory- hole or milling and power-shovel and dragline mining. The pit is thus in the form of an inverted cone or a series of cones the rims of which finally intersect, and the ultimate rim of the pit will conform in shape to the outlines of the ore body or of that portion of the ore body that is to be mined by this method.

The sides of the pit sloping toward the mill hole or raise often are mined in narrow benches, the broken ore being blasted into the pit, whence it is drawn off through the mill holes into cars and trammed or hoisted to the surface. When the slopes of the pit become too flat for the ore to run into the mill holes it becomes necessary to use drag scrapers or other means to transfer it.

The glory-hole method has been used extensively for mining relatively small pipe- or wedge-shaped ore bodies or the upper parts of bodies that apex at or near the surface, and for mining the ore remaining in track benches and in the bottom of power-shovel pits below the elevation where locomotive haulage becomes uneconomical because of adverse track grades.

Some stripping of loose overburden or barren cap rock may have to be done before glory-hole mining can be started, possibly by the use of scrapers, dragline excavators, or power shovels, by hydraulicking, or by glory-holing.

The cost of equipment for glory-hole mining usually is much less than for power-shovel work, and for relatively small-scale operations the production cost may be less.

For large-scale operations, mining by power shovel usually is cheaper. Glory-hole work is not well-adapted to selective mining as all material within the pit must be blasted down into the mills. Thus, lean ore or waste inclusions within the ore can be taken out and disposed of separately only if the ore is first cleaned from the slopes and bottom of the pit and the mill holes emptied, and then the pit is cleaned and mill holes are emptied again after the waste is blasted out.

At the Britannia mine the glory-hole method was employed in the Bluff ore body fig.

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Glory holes were opened, likewise, in the upper Fairview ore bodywhere closely spaced parallel veins with some mineralization between them made this method preferable to more expensive selective mining methods. Working benches were 3 feet wide and varied in length.

The ore was drawn through grizzly chambers into transfer raises leading to the underground transportation system. In the East Bluff ore body low-grade ore around the glory holes and old stopes was mined by shooting the ore out into the pit, using large charges of explosive placed in coyote drifts driven toward the side of the pit from underground workings.mining: Open-pit mining Deposits mined by open-pit techniques are generally divided into horizontal layers called benches.

The thickness (that is, the height) of the benches depends on the type of deposit, the mineral being mined, and the equipment being used; for large mines it. mining: Open-pit mining Deposits mined by open-pit techniques are generally divided into horizontal layers called benches.

The thickness (that is, the height) of the benches depends on the type of deposit, the mineral being mined, and the equipment being used; for large mines it. At its maximum, the area covered by the Hull Rust open pit mine is five miles (8km) long, 2 miles (3,2km) wide and feet (m) deep.

It looks like a man-made Grand Canyon of .

Conclusion

Open pit, open cast or open cut mining is a surface miningtechnique of extracting rock or minerals from the earth by their removal from an Get Price Differences between underground mining and open pit mining.

Choosing between open-pit mining and underground mining is a decision that every mining company makes, but it is not based on the miner’s whim and predilection. Selection always depends on the geometry properties of the deposit, its position, location, and type.

Open pit mining does offer some advantages over traditional deep shaft mining.

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Pit mining is more cost effective than shaft mining because more ore can be extracted and more quickly. The working conditions are safer for the miners because there is no risk of cave in or toxic gas.

Open pit mining is the preferred.

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