Saturday, 27 July 2013

Gold from the World's Deepest Mine

Gold egg
The richest known gold fields in the world are located in a comparatively small area on the southern tip of the African continent marked by layers of auriferous reefs in the Witwatersrand area near Johannesburg. They are worked these days at levels from 1,600 to 12,000 feet below the surface. Some of the forty-one South African gold mines are among the deepest in the world and the technology of deep mining is more highly developed in South Africa than anywhere else.

South African mining technology, rather than geophysical peculiarities, can be credited with making it possible for miners to work at all at such great depths in relatively tolerable temperatures. In most parts of the world, temperatures rise one degree centigrade for every 107 feet of descent. This means that at the deepest levels of the mines in South Africa, temperatures should reach boiling point, and beyond. But in the Witwatersrand area, the temperature increases much more slowly at the rate of one degree centigrade per 390 feet. Thus, at the deepest levels, temperatures are kept down to circa 50 degrees centigrade. Effective ventilation techniques can bring this down to forty degrees centigrade.

Gold jewelleryGold mining techniques do not differ extensively from those used in mining other substances, such as coal and iron. The big difference is the extra-ordinary depth at which gold ore has to be mined. To reach it, miners must cut through rock that is often harder than granite.

The Mine at East Driefontein

South African mining techniques are best illustrated by reference to the East Driefontein mine which was begun relatively recently. It is part of the mining complex near the town of Carletonville, one of seven such fields which form a 300-mile arc across central South Africa.

From 1963 to 1967, test 'borings were taken to determine the pattern of gold-bearing reefs in the area and to analyze the rock: The tests indicated that there was some seventy-seven million tons of gold-bearing ore in the region, with a yield of about eighteen grams of gold per ton. The area .boasted three reefs the Ventersdorp Contact Reef which circles practically the entire area; below it, the Main Reef which covers some sixty percent of the area; and, still further down, the Carbon Leader Reef which is almost the same size.

Gold smart carThere are three geological problems complicating access to the reefs a huge water-bearing dolomite sheet, a hard layer of lava above the reefs, and the depths of the reefs, some of which are about 13,000 feet.

Shortly after work began on the East Driefontein mine on October 26, 1968, there was serious flooding from the dolomite layer. Both the East and neigh-boring West Driefontein mines were flooded. Elaborate pumping systems had to be set up for both mines and it took more than a year to pump them dry. In the early stages, tens of millions of gallons of water were pumped to the surface daily'.

Both main shafts of the East Driefontein mine were deliberately sunk in the water resistant area of the Pretoria rock formation. It had been hoped that this would help avoid the danger of flooding and the necessary depth of 5,000 feet would therefore be easily attainable. That would have meant that the reef, which descended from 3,000 feet at an angle of twenty-two degrees, could be worked by means of galleries from the two main shafts. But, during preliminary stages, it was decided to sink a shaft to a depth of 6,500 feet. Each shaft had a diameter of twenty-four feet. Where they went through the dolomite layer, the shafts were reinforced with concrete.

Gold pieceFrom the bottom of the two main shafts, the two sub-vertical shafts were sunk, as a second phase, to 9,500 feet. The winding engines of the main shafts were above ground. Those serving the sub-vertical shafts were set up in subterranean caverns. The largest of these, in the hard Ventersdorp lava of East Driefontein mine, is the size of a church nave 110 feet long, 45 feet wide, and 48 feet high.

The roof prop method is employed to excavate these caverns. This entails gouging out a. gallery at the level of the cavern's eventual ceiling, with the ensuing excavation taking place on both sides and below the gallery. Particularly hard rock requires special blasting techniques which produce especially smooth walls. A great many blast holes are bored close together along the line of advance. Charges are all detonated simultaneously and the pressure of the explosion takes the line of least resistance, usually sideways rather than forward.

Smooth walls and gallery ceilings reduce the danger of subsequent rock falls. At various depths, horizontal galleries from the main shafts, often extending more than a half mile, connect to gold-bearing reefs. The heights of these reefs vary considerably, from a few millimeters up to, in exceptional cases, nineteen feet. The conglomerate is easily recognized as a yellow ocher band woven through the dark Ventersdorp lava.

Deep Mining

Extracting ore at great depths poses very special mining problems because pressure is considerable in deep mines. The process leaves comparatively shallow hollows, sometimes of substantial length. These are subject to the pressure of the entire rock mass above them. From horizontal galleries, other galleries are driven to follow the line of the reef. Numerous other galleries are cut at right angles to them. The height of the working faces, known as stopes, is kept to a minimum because the economics of mining have much to do with the relationship between the amount of rock that has to be extracted and the usable ore it provides. The galleries in which it was necessary to work doubled over or horizontally were once supported by timber pit props. These days, special hydraulic props are used. Once side galleries have been pierced, further galleries, parallel to the main gallery, are cut so that a rectangular network is formed. This permits the conglomerate to be removed in such a way that only thin support pillars need be left within the pattern of galleries. Areas that have been worked dry are filled in with barren rock. Thus, only a small proportion of such rock has to be brought to the surface.

Getting the Ore Out

Gold coinA large gold mine depends as much on an effective transport system as it does on efficient extracting processes. Mechanical loading devices are situated at the work face in modern mines to transfer the ore to conveyor belts after each blasting. The loading machines are designed to fit into shallow galleries.

Conveyor belts, which are constructed in sections, take the ore through the working galleries to the tracks of an underground railway in the main gallery. From there, the ore is transported in trucks to the hoisting shaft where it is emptied into buckets and raised to the surface.

The underground railway is generally the most extensive part of the mine's transportation system. It has been reckoned that the amount of track laid in South Africa's mines slightly exceeds that of its surface railroad system. 

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