Monday, 29 July 2013

Underground Hazards of gold


Fire at underground mineUnlike coal and iron mines; where the greatest perils are explosions caused by dreaded firedamp, the worst hazards in South African gold mines are cave-ins at great depths.

Every gallery that is driven into the ground causes some disruption of the geological structure in its immediate surroundings. In the deepest South African mines, it is possible to encounter traction and pressure more than twenty-five times greater than the stress factor of ferroconcrete. It would be reasonable to expect that the greatest pressure would be on the gallery roof. But that is not generally true. The greatest stress tends to be within the rock, between ten and thirty feet above the roof.

 In a rock burst, events can happen with the speed in of an explosion. Whole galleries have been known to w collapse. However, a gallery usually comes down in re bursts of about eight inches at a time. The collapse and shattering noise that accompany it are merely re the visible and audible elements of a developing cataclysm. A cave-in is much like an earthquake, although the mechanical cause is different. There is never total quiet in a mine, even when it is not worked. There le is always some sound of cracking, sometimes muffled, at sometimes very sharp. Miners say "the mine is talking". It is the stress in the rock that they hear. For years, the South African mining authorities have financed a comprehensive program of research to study the causes of cave-ins and to develop countermeasures. The results of this research are similar to those of Y, seismologists.

It seems that in high pressure zones, microscopic ally fine alterations in the rock take place when mining disturbs the rock structure. These disturbances n can be relieved by the cutting of a gallery. But, such disturbances can also result in the accumulation of pressure within the rock mass. When the forces involved exceed the limits of elasticity of the rock, micro fissures develop which increase the volume of the rock. These add to the build-up of pressure. The forces finally become so powerful that they can only be relieved by movement of the entire surrounding re mass of rock. The gallery roof sinks. (In an earthquake, it is the lithosphere, the earth's outer crust, which at rises as faults are formed. However, in the case of earthquakes, it can take months or years, or even enormously greater spans of time, before the increase. of pressure within the rock produces visible consequences.)

The causes and courses of terrestrial upheaval have by now been so thoroughly researched that geologists are able to forecast the frequency and nature of micro fissures in rock with considerable accuracy. There are three main determining factors: depth, size of gallery, and capacity of gallery roof to absorb movement. These factors can be combined in a single equation which covers the amount of energy released by working the subterranean rock.

The South African Chamber of Mines has had mathematical models of these processes developed and programmed for computers so that mining engineers need only feed plans for work programs into computers to receive, within minutes, analysis of the safety factors involved in their projects. It has, for example, been calculated that when dealing with reefs more than a foot-and-a-half thick, at least fifteen percent of the reef must be left intact to support the network of galleries.

Such measures reduce the possibility of cave-ins, but they do not altogether eliminate the danger. Preventive measures are still required to protect miners from cave-ins which, despite precautions, take place from time to time at locations where pressure has, unnoticed, accumulated gradually. The answer to these persisting hazards seemed simple propping up the galleries. But, in the early 1960's, when South African mines began, to use quantities of hydraulic pit props, employed so widely in European mines, they experienced an unpeasant surprise. Pressure either made the props collapse or simply punctured the gallery roofs.

It took years to develop a high performance, rapid yielding hydraulic prop to do the job. It is remarkably similar to the hydraulically sprung undercarriage of an airplane. Today, over twenty thousand such props are in use in mines. They are set up in galleries at intervals of about three feet to offer miners protection from cave-ins and rock falls. Since these props were introduced, accidents due to falling rock have been reduced by ninety percent.

Automation Underground

The technology of gold mining continues to be steadily improved. Efforts these days concentrate principally on mechanization and improved management techniques above all the use of electronic data processing.

Mechanical improvements are principally concentrated on efforts to improve efficiency at the rock face. South African mining is very labor intensive. But machines have been developed which require less manning and which can operate in smaller spaces. There are now drilling and loading machines which are able even to continue functioning during blasting. Such contraptions have in places made it possible for the work force to be cut by half, with increased profitability, corresponding to a production increase of four hundred percent. Such developments are, however, not without social and political ramifications.

The shallower the mine, the more profitable it tends to be, even when the difference is only a matter of inches. This is because the extraction of non-gold rock is just as expensive as the extraction of gold-bearing ore. Wherever it has proved possible I to alter the ore-to-rock ratio in favor of ore, there has been a marked increase in return over investment.

Aside from isolated experiments, all South African mines have used labor intensive methods involving blasting. This is because at such great depths the rock is extremely hard. Moreover, the method is eminently suitable for work at such levels. At the same time, conditions rule out the use of the kind of new machinery employed in American and' European mines, with their large, relatively shallow galleries. But much effort is being devoted by the South Africans to designing equipment capable of operating productively in subterranean confined spaces.

There are considerable gold losses during blasting and loading of ore onto conveyors. The pressure of the explosion separates some of the gold particles from the ore and transforms them into fine dust which escapes into the air. After blasting, the entire area is carefully scored with hand brushes, but much of the gold dust is lost forever in rock cracks and fissures.

Mine Safety and Rescue
Some gold dust affixes itself to particles of rust, found in all mines, and is also lost. Methods still in the experimental stage are being developed for retrieving whatever gold dust now tends to be overlooked. In addition, new mining machines are designed to release no dust at all. The whole thrust of new techniques is to lose none of the precious metal in the process of acquiring it.

If a mine is to operate with maximum profitability, there must be an efficient, comprehensive plan for its overall operation. Development of the mine, excavating, transportation in shafts and galleries and all other aspects of getting the gold from the ground must not only be calculated to produce as much as possible, but also to produce it in steady quantities. The extremely expensive chemical and mechanical equipment used in processing the metal has limited capacities. If more ore is extracted than the plant can handle at any given time, expensive storage space must be provided. But underproduction is also expensive because the plant does not then operate at full capacity. 

Planning departments of the mining industry construct models to study such problems. These models simulate the extraordinarily complex processes involved in mine operations. Appropriate computer programs give engineers an opportunity to test their operations with their models and examine all possible consequences and alternatives. They are thus able to pinpoint weak spots and bottlenecks and to develop procedures for overcoming them. 

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