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Exploration Drilling

Core drilling is carried out using special drill rigs, equipped with a tube drill string and impregnated diamond bit (IDB). The open-centre IDB cuts a cylindrical core of rock, which is stored in a core barrel inside the tube, and retrieved as a sample. For tunnelling applications, core drilling provides essential information to determine the optimum excavation method and type of rock reinforcement to be adopted.

 

Figure 1. Atlas Copco HOBIC impregnated diamond coring bit.

The core is recovered from the core barrel by pulling up the complete tube string. All tubes are then disconnected and stacked close to the drillrig. Alternatively, a wireline arrangement can be used, which recovers the core barrel by pulling a wire inside the tube string.

The core is a proof of underground geology, which can be inspected, and split in halves. Samples of special interest are sent to laboratories for analysis of metal content. Cores from exploration drilling are stored in spe­cial boxes, and kept in retrievable archives. Boxes are marked to identify in which hole, and what depth, the sample was taken.

The information gathered by core drilling is important, and represents substantial capital investment, so max­imum use is made of the information from each core. The borehole may also be used a second time, for another form of geophysical exploration.

Diamond drilling is expensive, and core drilling programmes are time-consuming undertakings. Reverse circulation drilling is a complementary exploration technique, faster, but less accurate. Reverse circulation drilling is carried out with standard percussion rockdrills, using a special technique where the flushing media is introduced through a casing at the hole collar. Drill cuttings are pushed up through the drill string, discharging through a pipe outlet on the front head of the rock drill. Drill cuttings are sampled to identify variations in the rock mass. Exploration by reverse circulation drilling provides a comparatively fast and inexpensive way to explore rock down to 30 m depth. Results are, however, not of the same quality as for core drilling.

Reverse circulation drilling is nowadays the most common method for surface exploration of gold, especially in alluvial deposits.

 

Mineral Prospect

While the geophysical survey may indicate presence of minerals, such evidence is not sufficient to identify the prospect ore. Miners need proof of what minerals are there, and the metal grades, and confirmation that volumes of mineralized rock are large enough to continue the search. The first check would be to look for an outcrop of the mineralization. Where the ground cover is only a shallow layer of alluvial, trenches can be dug across the mineralized area, to expose the upper surface of the bedrock. A prospector will identify the discovery, measure both width and length, and assess the mineralized area. Rock samples from trenches sent to the lab for analysis may provide evidence to merit drilling some coring holes.



After outlining borders of the min­eral deposit on the ground surface, the next step is to confirm that the miner­alization continues down into the rock. The surface area multiplied by the extension at depth provides a first estimate of ore volume. Core drilling is used to map the geology inside a rock mass, and several thousand metres may be required.

Sets of diamond-drilled holes are laid out to intersect the mineralized zone along the strike, in parallel vertical sections. The geology can then be traced at different levels underground, and presented in three-dimensional models. Analysis of core samples will give further detail to the geology, and the mineralization can be divided in high- and low-grade sections.

An underground exploration programme may then be undertaken, using a temporary mine arrangement, with hoist and shaft. Strategic development of horizontal drifts will allow the diamond drillrig to core from the most promising locations. Core samples of the mineralization arc often used to establish the economics and viability of mineral dressing processes. In some cases, despite a high grade, the flotation process might be difficult, due to grain size after milling.

 

Figure 2. Samples of cores obtained from exploratory drillholes.

Before a mining company takes the decision to convert a prospect into a producing mine, the company's Board of Directors must be convinced that the mine will yield an acceptable return on capital invested. The feasibility study presents the time schedule, necessary capital, return rates and payback period.

Underground Mining

The underground mine aims for maximum economic recovery of minerals contained in the bedrock. The orebody is the volume containing valuable minerals, while the rock around it is waste.

Waste dilutes the ore, so miners try to leave it in place, wherever possible.

Ore close to the surface is mined by open pit techniques, in which the waste rock can be separated by loading, and trucked to the waste dump instead of the concentrator. Subsurface orebodies are exploited by underground mining, for which techniques are more complex.

The mining method is adapted to the rock conditions, and the shape, dimensions, strength and stability of the orebody.

In order to work the underground rock mass, infrastructure is required for access to work places, ore production, power supply, transport of ore, and maintenance of equipment.

 

 

Figure 3. Basic infrastructure required for a typical underground mine.

The shaft forms the access to the underground levels, and is the mine's main artery for anything going up or down. Shaft stations, drifts and ramps connect stopes with orepasses, tramming levels, and workshops for movement of miners and equipment.

Efficient ore handling is important. The blasted ore is loaded from production stopes, via orepasses to a main level, and thence to the crusher at the hoisting shaft. The crushed ore is then stored in a silo before transfer by conveyor to the measuring pocket at the skip station, from where it is hoisted to the surface stockpile.

Electric power is distributed throughout the mine, and is used to illuminate work places and to power drillrigs, pumps and other machines. A compressor plant supplies air to pneumatic rock drills and other tools, through a network of pipes.

Water reticulation is necessary in the mine, wherever drilling, blasting and mucking takes place, for dust suppression and hole flushing. Both ground water and flushing water are collected in drains, which gravitate to a pump station equipped with high-lift pumps to surface.

Air quality in mine workings must be maintained at an acceptable health standard.

The mine needs a ventilation system, to remove smoke from blasting and exhaust gases from diesel-powered machines, and to provide fresh air for the workers. This is normally provided via downcast fresh-air shafts. High-pressure fans on surface extract foul air through the upcast shafts. Ventilation doors control the underground airflow, passing fresh air through active work areas. Polluted air is collected in a system of exhaust airways for channelling back to the ventilation shaft.

 

 


Date: 2015-12-24; view: 810


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