Surface mining methods - an overview

Surface mining methods-an overview

Gideon Gipmai Yowa
(July, 2017)

Senior Instructor, Mining Eng. Dept., PNG University of Technology. PNG

     1.    Overview
Surface mining involves the extraction of mineral commodity on the earth’s surface and in the open air. Surface mines can be fully mechanized, partly mechanized, or labour intensive. Large scale operations with high production rates are normally fully mechanized while small scale operations as with alluvial and artisanal mining require manual laboring.

The common fully mechanized surface mining are the open pit (open cut), quarry, auger, and, the open cast methods. Variations of open cast can be terrace or contour mining especially on coal deposits on hilly topography. Partly mechanized and labour intensive methods are common in placer and artisanal operations. Another category of surface mining is called the aqueous methods and involves leaching, solution, and alluvial mining. Aqueous methods use a solvent to extract the mineral.

With the advancement in technology, and safety regulations, small to large scale operations, are mechanizing their operations.

     2.     Open pit (open cut)
Open pit (open cut) mining method is applied on shallow, non-stratified, and massive deposit like porphyry copper-gold, iron ore, uranium. The overburden is stripped to expose the ore. Multiple benches are established to reach the economic pit limit. When economic conditions improve in the case of increase in the metal price, or reduction in the operational cost, or extension of mineralisation, push back (cut back) is done to extract the remaining ore.

Fig1.   Open pit mine

Determining the stripping ratio and the stable pit slope angle are the main challenging task in the design and optimization process. Also the location of ramps, haul roads, and type of equipment, fleet capacity, and dewatering are important in mine planning.

The initial capital expenditure is usually high, however, the production rate is usually high and that results in a low operating cost compared to underground mines.

    3.    Quarry
Quarry refers to mining of non-metallic minerals (industrial minerals) for use in civil and architectural constructions.

Quarry nowadays does not only refer to cutting of rock blocks or dimension stones, but also refers to mechanical excavation of loose rocks or drill and blast for rock aggregates. Quarrying is now becoming unpopular for dimension stones and popular for industrial minerals, mostly because concrete and bricks have replaced the requirement of dimension stones. Dimension stones and aggregates come from various rocks and common ones are the granite, limestone, marble and slate.

    4.    Open cast (strip)
Open cast is applied on flat deposits like coal and potash. Most flat deposits are sedimentary in nature and are less competent for serious drilling and blasting operations. Other variations to open cast are the terrace, and contour mining which are applied on coal deposits on hills, or deposit that have thick over burden, or are undulating and not flat (complex geology).

A walking dragline is common for stripping the waste overburden and casting the waste (spoil) into the mined out panel in an open cast mine. In a terrace or contour mine an excavator and fleet of trucks are used to move the waste to dump in mined out areas. Reclamation is done immediately after mining.

Fig2.   Open cast mining

Fig3.   Terrace coal mining

There are fewer benches in a open cast than terrace stripping.
    5.    Auger
Auger method is applied to recover outcrop and thin seams by boring openings on high walls.  No stripping of overburden is done and is usually in conjunction with contour stripping.

Fig4.   Auger mine

     6.    Placer-hydraulicking
Hydraulicking involves shooting high pressure water streams into the face of unconsolidated wall of sediments. The walls are disintegrated and heavy minerals such as gold, platinum, diamond are liberated.

Fig5.   Hydraulicking

The slurry is channelled into sluice boxes along the toe. The heavy metals are trapped along the mats or the ripples of the sluice boxes which are then removed and further panned to recover the minerals.

     7.    Placer-dredging
Dredging can be a continuous rotating bucket line, rotating bucket wheel, rotating cutter head, or suction line. The vessel floats on the water while the components that scoop dirt to the vessel are submerged in the water. The dirt is processed on the floating dredge. Dredge is applied on loose materials beneath the water and is used to mine heavy metals such as gold.

Fig6.   Dredging

    8.    Excavator & trommel
Mechanized alluvial mines also employ trommel and excavator. The excavator feeds the trommel with dirt and the trommel segregates the dirt. The coarse are discharged as tails and finer ones travel through the sluice box which then traps the heavier metals like gold.

Fig7.   Trommel & excavator

    9.    Solution-borehole
Solution mining is done by dissolution, melting, leaching, or slurrying.

Water is injected into the minerals and is pumped out to the surface in solutions. Evaporite minerals like salt and potash are dissolved, sulphur is melted through Frasch process, and other minerals are pumped out through leaching and slurring.

Major engineering challenges with this method are ground water contamination and ground subsidence or movement.

    10.  Solution-leaching
Leaching involves the addition of chemicals to extract the metals. In some instances bacteria has been used to leach metals. When leaching takes place on an in situ ore, it is termed in situ leaching, leaching in ore dumps is called heap leaching, and leaching in tanks or vats is called vat leaching. Leaching is carried out on copper, uranium, gold and silver metals.

Some of the common chemicals (solvents) used in leaching are; ferric sulfate (solvent) to leach copper sulfide ore, sulphuric acid (solvent) to leach copper oxide ore, hydrogen peroxide (solvent) to leach uranium ore and , sodium cyanide (solvent) to leach gold.

Engineering challenges are faced in process control, proper design of walls or tanks, and environmental contamination.

     11.  Artisanal & small scale
Local miners engage in small scale mining for subsistence living use simple manual methods such as sluice box, rocker box, and panning dish.

Further reference
Hartman. H (1987), Introductory Mining Engineering, John Wiley & Sons, pp177-250

Surface mining methods - an overview Surface mining methods - an overview Reviewed by Gideon Gipmai Yowa on July 19, 2017 Rating: 5

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