Open-pit mining: This method is adopted when mining hard or solid rock, which requires a discontinuous process of drilling and blasting prior to the loading and hauling stages. Open-pit mining is therefore employed to remove hard rock ore that is disseminated and/or located in deep seams and is typically limited to extraction by shovel and truck equipment. Some of the metals mined by open-pit method are gold, silver, diamonds, copper and so on. The operation in this method of mining is oval in shape.
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 is on the order of 12 to 15 metres. Open-pit mining is generally conducted on a number of benches at any one time. The top of each bench is equivalent to a working level, and access to different levels is gained through a system of ramps. The width of a ramp depends on the equipment being
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used, but typical widths are from 20 to 40 metres. Mining on a new level is begun by extending a ramp downward. This initial or drop cut is then progressively widened to form new pit bottom.
The walls of a pit have a certain slope determined by the strength of the rock mass and other factors. The stability of these walls, and even of individual benches and group of benches, is very important – particularly as the pit gets deeper. Increasing the pit slope angle by only a few degrees can decrease stripping costs tremendously or increase revenues through increase ore recovery, but it can also result in a number of slope failures on a small or large scale.
Millions of tons of material may be involved in such slides. For this reason, mines have ongoing slope stability programs involving the collection and analysis of structural data, hydrogeologic information, and operational practices (blasting in particular), so that the best slope designs many be achieved. It is not unusual for five or more different slope angles to be involved in one large pit. As a pit is deepened, more and more waste rock must be stripped away in order to uncover the ore. Eventually there comes a point where the revenue from the exposed ore is less than the costs involved in its recovery. Mining then ceases. The ratio of the amount of waste rock stripped to ore removed is called the overall stripping ratio. The break –even stripping ratio is a function of ore value and the costs involved.
Quarrying: it is a term used to describe specialized open-pit mining techniques wherein solid rock with a high degree of consolidation and density is extracted from localized deposits.
Quarried materials are either crushed and broken to produce aggregate or building stone, such as dolomite and limestone, or combined with other chemicals to produce cement and lime.
Dimension stone such as flagstone, granite, limestone, marble, sandstone and slate represent a second class of quarried materials.
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Although seldom used to form entire structure, stone is greatly valued for its aesthetic appeal, durability, and ease of maintenance. The most popular types include granite/ limestone, sandstone, marble, slate, gneiss, and serpentine. All natural stone used for structural support, curtain walls, veneer, floor tile, roofing, or strictly ornamental purpose is called building stone, and building stone that has been cut and finished for predetermined uses in building construction and monuments is known as dimension stone. The characteristics required of good dimension stone are uniformity of texture and colour, freedom from flaws, suitability for polishing and carving, and resistance to weathering.
Pit geometry: although quarrying is also done underground, using room-and-pillar techniques, most quarries involve the removal of blocks from hillsides or from an open-pit type of geometry. The first in developing such a quarry is the removal of the vegetative cover of trees and underbrush. Next, the overburden of topsoil and subsoil is removed and stockpiled for future reclamation.
The rock is quarried in a series of benches or slices corresponding to the thickness of the desired blocks. This is often on the order of 4.5 to 6 metres, but, since it is actual quarry practice to take advantage of any natural horizontal seams, block thickness may vary.
The quarrying process consists of separating large blocks, sometimes called loafs, from the surrounding rock. These blocks may be 6 metres high by 6 metres deep and 12 to 18 metres long, and they may weigh in the range of 1,200 to 2,000 tons. (Such large blocks are subsequently divided into mill blocks weighing 15 to 70 tons). The removal of blocks from the quarry has traditionally been done by one or more fixed derricks. As a result the plan area of a quarry has been determined not only by the geometry of the deposit and the amount of overburden but also by the reach of the derrick boom. However, derricks are gradually being replaced by highly mobile front-end loaders of sufficient capacity to remove, lift and carry
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30-ton mill blocks, and the layout, design, and operating procedures of quarries are being modified accordingly.
There is a very high waste factor in the quarrying of dimension stone. For some quarries, the amount of usable stone is only 15 to 20 percent of that quarried. For this reason, an important aspect of quarry planning is the location of the waste or “grout” pile.
Strip Mining (open-cast mining): Strip mining or open-cast methods is applied in processing non-hard rock deposit and refers to extraction of ore bodies that are near the surface and relatively flat or tabular in nature and mineral seams. Unlike open-pit mining, strip mining or loose rock mining process is essentially with extraction and haulage steps running in series. It uses a variety of different types of equipment including shovels, trucks, drag lines, bucket wheel excavators and scrapers. Coal is the most common commodity that is strip mined from seams. The primary differences between these mining methods are the location of the ore body and the mode of mechanical extraction.
In strip mining a long narrow strip of mineral is uncovered by a dragline, large shovel, or similar type of excavator. After the mineral has been removed, an adjacent strip is uncovered and its overlying waste material deposited in the excavation of the first strip. Strip Mining method is scarcely adopted in Ebonyi State as all the mines visited are Open-pit and Quarries.