The extensive deposits in the coastal areas north of the Orange River lie in marine terraces under a layer of from 30 to 60 feet of sand. The Consolidated Diamond Mines Company operate over a 50-mile stretch at Orangemund, and this is the richest producer of gems in the whole world. Further south they also mine extensively at Kleinzee. The whole project is a mighty earth-moving operation, easily the biggest earth-moving activity in the whole world, for the great 60-foot overburden of sand has to be removed to uncover the diamond-bearing gravels. At Kleinzee alone, the diamondiferous gravel itself which is handled amounts to half a million tons a year, and il diamonds won amount to 1 carat per 10 tons of gravel treated. This mass of material involved is in addition to the huge quantity of sand overburden bulldozed away.
These alluvial diamonds have been dispersed from former pipes and tend to be concentrated in river-beds and along beach gravels. They were probably brought to coastal sea regions by river, and the sand then deposited over them. The sea must have retreated or the sea-bed rose, leaving high and dry the diamond-bearing marine terraces. Drift sand accumulated to the extent of some 60 feet in most regions and has to be shifted first.
The first step in mining is to sink prospecting bore holes, which show the thickness of the sand overburden and also reveal the geological character of the sought-after diamondiferous gravel. Then comes the first major mining operation, that of removal of overburden. This is carried out by formidable teams of really massive bulldozers. As may be imagined, the moving of a 60-foot depth of sand is a big operation.
When the diamond-bearing gravels are exposed, prospecting trenches 1 yard wide and perhaps up to 1 mile long are then cut and from these the effective richness of the deposit is assessed by on-the-spot recovery of any diamonds present, J using simple direct hand-sorting. At this point in the proceedings the workmen are very carefully watched, for if they try to pocket some of the diamonds (a common practice) then a seriously low figure of richness may be assessed from what is in fact really a lucrative region.
Having decided that a region is to be worked, the formidable earth-moving operation begins. The giant bulldozers operate in teams of three, each shifting overburden sand at the rate of nearly 200 tons an hour. In some regions, where the sand is too soft to support such heavy machines, drag-line excavators are used. The removal of the sand exposes the gravel diamond-bearing terraces which are quite shallow in depth, usually being no more than a yard thick. That is all that is available for mining, a shallow but very extensive bed.
The gravel is completely removed and stockpiled. All gullys and pot holes below the average level need to be searched carefully by hand and by sweeping, since very often the heavy diamonds have concentrated in such pot holes in the past and the richest finds are made in these.
The gravels containing the diamond later used for manufacturing white gold engagement rings are screened in situ locally where mixed vibrating screens reject 80 % of the gravel as waste and the diamond-rich residue is taken by truck or train to a central treatment plant.
nedelja, 24. avgust 2008
četrtek, 24. julij 2008
The grease table
The grease table is a remarkable invention, discovered by F. Kirsten in 1897. Kirsten was an employee at de Beers and when washing some ores containing diamond he accidentally spilt the contents of his wash-pan, which contained both diamond and crushed ore, over the well-greased axle hub of A cart-wheel. He noticed that the diamonds stuck to the grease, but not the ore. He thereupon designed a simple inclined table of corrugated iron and thickly coated this with grease. When diamond-containing crushed ore concentrates were mixed with water and then washed over the tables (which were shaken), the diamonds were found to be trapped on the grease, whilst everything else rolled off with the water stream. It soon transpired that the trapping action was extraordinarily efficient and almost all diamonds (to within less than 1 %) were trapped within the first 12 inches of grease.
This grease-table system is now used on a big scale, the tables being mechanically vibrated. Every hour the grease is scraped off (with diamond adhering) and packed in cans covered with gauze and then heated. The grease melts, is removed and re-used, the diamonds accumulating in the can.
The saving in labor, compared with the old hand-sorting methods of last century is enormous. The grease table acts because of a curious property of diamond surfaces. Neither diamond nor grease are wetted by Water, so the diamonds adhere to the grease. All other minerals are effectively coated (in the water) by a water layer and so slide off the sloping grease table, assisted by a jerking vibrating movement. The modern grease table is an aluminium sheet 4 feet wide, with steps on it, and with surface sloping down at 15 degrees to the horizontal. Petroleum jelly is the best grease medium. Used grease is melted down and a ¼-inch-thick layer first spread over the table. On this is usually spread a thin layer of new refined jelly.
We have described above the mining and extraction of diamonds later used for diamond jewelry, diamond wedding bands from the great South African deep pipes, those which are under the control of the de Beers Group. These mines constitute a most important source of gems and also, of course, produce a large quantity of industrial diamond. A great deal of capital is sunk into these mines. The de Beers Group has an annual production of diamond worth over £50 million. The Premier mine alone during the past 60 years has yielded nearly 200 million tons of blue ground, and clearly the excavating, hauling and processing of all this is a major engineering feat. It has produced perhaps 10 tons of diamond. Yet, in terms of total output, more diamonds are recovered from the great alluvial deposits, especially those of South West Africa, Congo, Orange River and Angola. Both the mining methods and the recovery techniques for alluvial diamonds are entirely different to those used in deep pipes, and in the next section we shall briefly review these.
These diamonds appear either in what are called marine terraces, or in river-beds. For our purpose we can classify both types as alluvial deposits. Secondary alluvial deposits are also mined, off-shore from the sea-bed itself, these being called 'marine' diamonds.
This grease-table system is now used on a big scale, the tables being mechanically vibrated. Every hour the grease is scraped off (with diamond adhering) and packed in cans covered with gauze and then heated. The grease melts, is removed and re-used, the diamonds accumulating in the can.
The saving in labor, compared with the old hand-sorting methods of last century is enormous. The grease table acts because of a curious property of diamond surfaces. Neither diamond nor grease are wetted by Water, so the diamonds adhere to the grease. All other minerals are effectively coated (in the water) by a water layer and so slide off the sloping grease table, assisted by a jerking vibrating movement. The modern grease table is an aluminium sheet 4 feet wide, with steps on it, and with surface sloping down at 15 degrees to the horizontal. Petroleum jelly is the best grease medium. Used grease is melted down and a ¼-inch-thick layer first spread over the table. On this is usually spread a thin layer of new refined jelly.
We have described above the mining and extraction of diamonds later used for diamond jewelry, diamond wedding bands from the great South African deep pipes, those which are under the control of the de Beers Group. These mines constitute a most important source of gems and also, of course, produce a large quantity of industrial diamond. A great deal of capital is sunk into these mines. The de Beers Group has an annual production of diamond worth over £50 million. The Premier mine alone during the past 60 years has yielded nearly 200 million tons of blue ground, and clearly the excavating, hauling and processing of all this is a major engineering feat. It has produced perhaps 10 tons of diamond. Yet, in terms of total output, more diamonds are recovered from the great alluvial deposits, especially those of South West Africa, Congo, Orange River and Angola. Both the mining methods and the recovery techniques for alluvial diamonds are entirely different to those used in deep pipes, and in the next section we shall briefly review these.
These diamonds appear either in what are called marine terraces, or in river-beds. For our purpose we can classify both types as alluvial deposits. Secondary alluvial deposits are also mined, off-shore from the sea-bed itself, these being called 'marine' diamonds.
četrtek, 10. julij 2008
MARINE DIAMONDS
Much publicity of late has been devoted to the new marine method of extraction of diamond. This has not proved very profitable, however. In connection with marine terraces and with river-bed sources, it was evident that such sources did not automatically stop at coast-lines; there must be some diamondiferous-bearing gravel off-shore on the sea-bed itself. Such marine diamond 'fields' have been sought out and are being worked. Special ships have been designed and built. Huge suction pumps have been installed, and the gravel front the sea-bed is sucked up, sorted through and then discharged some distance away in order not to foul the region being worked.
There are great difficulties in the way. Only in calm weather can the great suction tubes be trailed on the sea bottom. The depth to be 'fished' is limited by the weight of the suction tube. The tubes are constantly jamming through being blocked by large boulders loosened in the operation. The results of the sea mining are disappointing in terms of the costs of the operations. Diamonds are certainly recovered, as predicted, but the volume acquired makes the operation work at a loss, In spite of this, the method is being pursued doggedly.
There are great difficulties in the way. Only in calm weather can the great suction tubes be trailed on the sea bottom. The depth to be 'fished' is limited by the weight of the suction tube. The tubes are constantly jamming through being blocked by large boulders loosened in the operation. The results of the sea mining are disappointing in terms of the costs of the operations. Diamonds are certainly recovered, as predicted, but the volume acquired makes the operation work at a loss, In spite of this, the method is being pursued doggedly.
ponedeljek, 30. junij 2008
TREATMENT OF ORE
The recovered ore must be processed in order to obtain the diamonds. It is first crushed with crushers, set l|-inch apart. It is reckoned that this method is good enough to pass most diamond ring without damage possibilities. The reduced ore is then passed by conveyor belt to a washing plant capable of dealing with 1000 tons of blue ground per hour. The ore is I he re mixed with a puddle of muddy water and then transferred to an annular rotating pan 14 feet across, where it is mixed by rakes.
Now, diamond is heavy (density 3-5 times that of water), whilst the density of the puddle liquid is only 1-3, so the diamond easily sinks to the bottom of the pan from which it is raked out by helical raker arms. The lighter mass is then re-processed with f-inch crushers to catch the smaller diamonds, and this again later re-crushed to trap still smaller sizes.
This crushed material is in each case sent to a second pan and is further treated. The concentrates obtained from these treatment pans are now ready for the more refined process needed for final recovery. Between the years 1907 and 1952 I he diamonds were extracted with a machine called the ‘pulsator’, but this has now been replaced by a new method of treatment which is more effective and cheaper to operate.
In the newer method the output from the pans is first screened for small-size material (fines) by being passed over small-mesh stainless steel screens which are vibrated. The lines which pass through the screen are treated with a grease table in the manner to be briefly described later. The main concentrates then pass on to what is called the 'heavy media separation cone'. This is essentially a conical flotation vessel which has in it a high-density liquid, a puddle of ground metal powder of ferro-silicon which as a puddle ,has the high density of 3-0.
The diamonds sink in this vessel, and as it is conically shaped they concentrate at the lower tip. In due course this concentrate meets a vibrating grease table, for complete extraction.
Now, diamond is heavy (density 3-5 times that of water), whilst the density of the puddle liquid is only 1-3, so the diamond easily sinks to the bottom of the pan from which it is raked out by helical raker arms. The lighter mass is then re-processed with f-inch crushers to catch the smaller diamonds, and this again later re-crushed to trap still smaller sizes.
This crushed material is in each case sent to a second pan and is further treated. The concentrates obtained from these treatment pans are now ready for the more refined process needed for final recovery. Between the years 1907 and 1952 I he diamonds were extracted with a machine called the ‘pulsator’, but this has now been replaced by a new method of treatment which is more effective and cheaper to operate.
In the newer method the output from the pans is first screened for small-size material (fines) by being passed over small-mesh stainless steel screens which are vibrated. The lines which pass through the screen are treated with a grease table in the manner to be briefly described later. The main concentrates then pass on to what is called the 'heavy media separation cone'. This is essentially a conical flotation vessel which has in it a high-density liquid, a puddle of ground metal powder of ferro-silicon which as a puddle ,has the high density of 3-0.
The diamonds sink in this vessel, and as it is conically shaped they concentrate at the lower tip. In due course this concentrate meets a vibrating grease table, for complete extraction.
torek, 24. junij 2008
Mining of Diamond
We shall consider now some of the problems met with ill winning the mineral diamond from the earth. There are in effect two major different kinds of mining operation for re¬covery of diamond. In the one case, the mineral is won from the original virgin deep pipes. In the other, it is recovered from alluvial deposits on the surface (marine off-shore mining is an extension of this). Considerable capital is involved it) both procedures. Historically, diamonds as first found in India and later in Brazil were essentially alluvial deposits, The mining of deep pipes first began round Kimberley in South Africa. Although more diamond is now recovered from alluvial beds than from deep mine pipes, we shall describe pipe-mining operations first by reviewing the operations of the main mines, the de Beers Group in South Africa. Buy wedding bands diamond wedding rings
When the South African diamond rush started (in about 1871) by 1872 there were 50 000 diggers around Kimberley and each was allocated a small plot, a mere 30 feet by 30 feet, separated by 15-foot-wide roadways. Diggers dug down by open-cast mining and inevitably soon began undercutting the neutral roadways, which quickly collapsed. An extraordinary spidery web of rope haulages was constructed to raise ore, but the situation soon became chaotic. Cecil Rhodes quickly realised that the only way to work a mine was to buy out all the prospectors and introduce a central engineering control, This he achieved in 1888, by which date his de Beers Consoli¬dated Mines Company controlled the de Beers and Kimberley mines. They soon acquired the new Dutoitspan, Bultfontein and Wesselton mines, and these five became a centre of development for new mining techniques. The Kimberley and de Beers mines are now closed, but in their place the Jagers-fontein, the Premier and, lately, the Finsch mines have come into operation for the group. Check diamond jewelry if you need diamond rings.
PIPE MINING
Diamonds are found in volcanic pipes of blue rock (kimber-lite), such a pipe being roughly cylindrical, maybe 1000 yards across and going down to an enormous unknown depth. Pipes are almost vertical, penetrating the surrounding rock. As the initial uncoordinated early mining progressed, a deep hole into the earth was created. However, progressively the side walls of the surrounding rock (‘reef’ is the miner’s name
for this) began to fall in and to overburden the blue rock ('blue ground' in miners' terminology). Soon the whole bottom of each vast pit was one enormous accumulation of reef. The only solution was to attack the blue ground from the side and so come in below the collapsed reef. The main shaft is cut about 1000 feet from the side of the mine. At 600-foot intervals tunnels are run from the main shaft, These join a secondary shaft. From this a complex of short tunnels at 40-foot intervals is driven into the solid blue ground which is intact below the mass of reef rubble. These tunnels are developed into a geometrical system of chambers which are created through excavating the blue ground which is carrying the diamond. The excavated ore is loaded into rope, hauled cars and eventually reaches a shaft and then is brought to the surface.
Instead of 'chambering', as it is called, in some mines a modified arrangement called 'block caving' is adopted. In this, material is cut away leaving inverted cones, which in due course collapse and supply the blue ground needed. This material is withdrawn along tunnels by mechanical scrapers, being then pulled into cars for haulage to the surface. Block caving is faster and safer as a method of mining than chamber¬ing and is also more economical to operate. So far, levels up to 2000 feet deep are being exploited. The bigger mines pro¬duce about 4800 tons of ore a day, yet such a quantity of ore may produce a mere 350 carats of diamond, say about 3 ounces of diamond, and this represents total output from a big mine into which much capital has been sunk and which needs a big labour force.
When the South African diamond rush started (in about 1871) by 1872 there were 50 000 diggers around Kimberley and each was allocated a small plot, a mere 30 feet by 30 feet, separated by 15-foot-wide roadways. Diggers dug down by open-cast mining and inevitably soon began undercutting the neutral roadways, which quickly collapsed. An extraordinary spidery web of rope haulages was constructed to raise ore, but the situation soon became chaotic. Cecil Rhodes quickly realised that the only way to work a mine was to buy out all the prospectors and introduce a central engineering control, This he achieved in 1888, by which date his de Beers Consoli¬dated Mines Company controlled the de Beers and Kimberley mines. They soon acquired the new Dutoitspan, Bultfontein and Wesselton mines, and these five became a centre of development for new mining techniques. The Kimberley and de Beers mines are now closed, but in their place the Jagers-fontein, the Premier and, lately, the Finsch mines have come into operation for the group. Check diamond jewelry if you need diamond rings.
PIPE MINING
Diamonds are found in volcanic pipes of blue rock (kimber-lite), such a pipe being roughly cylindrical, maybe 1000 yards across and going down to an enormous unknown depth. Pipes are almost vertical, penetrating the surrounding rock. As the initial uncoordinated early mining progressed, a deep hole into the earth was created. However, progressively the side walls of the surrounding rock (‘reef’ is the miner’s name
for this) began to fall in and to overburden the blue rock ('blue ground' in miners' terminology). Soon the whole bottom of each vast pit was one enormous accumulation of reef. The only solution was to attack the blue ground from the side and so come in below the collapsed reef. The main shaft is cut about 1000 feet from the side of the mine. At 600-foot intervals tunnels are run from the main shaft, These join a secondary shaft. From this a complex of short tunnels at 40-foot intervals is driven into the solid blue ground which is intact below the mass of reef rubble. These tunnels are developed into a geometrical system of chambers which are created through excavating the blue ground which is carrying the diamond. The excavated ore is loaded into rope, hauled cars and eventually reaches a shaft and then is brought to the surface.
Instead of 'chambering', as it is called, in some mines a modified arrangement called 'block caving' is adopted. In this, material is cut away leaving inverted cones, which in due course collapse and supply the blue ground needed. This material is withdrawn along tunnels by mechanical scrapers, being then pulled into cars for haulage to the surface. Block caving is faster and safer as a method of mining than chamber¬ing and is also more economical to operate. So far, levels up to 2000 feet deep are being exploited. The bigger mines pro¬duce about 4800 tons of ore a day, yet such a quantity of ore may produce a mere 350 carats of diamond, say about 3 ounces of diamond, and this represents total output from a big mine into which much capital has been sunk and which needs a big labour force.
MISCELLANEOUS USES OF DIAMOND
The number of other industrial uses of diamond are almost too numerous to record. Diamond plates have been used as bearings in watches and instruments, fine-pointed diamonds are used as engravers and scribers; the tips of gauges have been faced with diamond. Widespread is the use of diamond sty 11 for record-players. In the scientific study of hardness (important to the engineer) this is tested by indenting a shaped diamond cone or pyramid into the object under study, Diamonds are used in the glass industry for edging, for scribing, for bevelling, and so on.
There is a complete range of dentist drills made of diamond. Diamond knives are used for cutting very thin biological sections, for sawing thin wafers of germanium for transistors in nuclear engineering, for drilling and sawing ferrite magnetic cores for the electronic industry … the list is endless.
Even in the home the diamond will be found to have been used in a hundred and one places - if not at first hand, then at second hand. It will have been used on the windows, the electric lamps, the vacuum-cleaner motor bearings and the bathroom tiles.
It will have trued the profile grinders which made the threads on taps and bathroom fittings and on numerous odd mass-produced pieces of metal around the house. II will have played a dominant part on the car in the garage, and much of the electric wiring may have passed through n diamond die, not only lighting wiring but that in the vacuum cleaner, the sewing machine, the television set, and even in the electric razor. In fact, the influence of the diamond is everywhere. There is no single one item in modern industry which plays a more widespread and strategic part Hum this hard little stone, the diamond engagement rings.
There is a complete range of dentist drills made of diamond. Diamond knives are used for cutting very thin biological sections, for sawing thin wafers of germanium for transistors in nuclear engineering, for drilling and sawing ferrite magnetic cores for the electronic industry … the list is endless.
Even in the home the diamond will be found to have been used in a hundred and one places - if not at first hand, then at second hand. It will have been used on the windows, the electric lamps, the vacuum-cleaner motor bearings and the bathroom tiles.
It will have trued the profile grinders which made the threads on taps and bathroom fittings and on numerous odd mass-produced pieces of metal around the house. II will have played a dominant part on the car in the garage, and much of the electric wiring may have passed through n diamond die, not only lighting wiring but that in the vacuum cleaner, the sewing machine, the television set, and even in the electric razor. In fact, the influence of the diamond is everywhere. There is no single one item in modern industry which plays a more widespread and strategic part Hum this hard little stone, the diamond engagement rings.
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