Production of Natural Diamond

The worldwide production of natural diamond has risen steadily since World War II, in answer to increasing demand (mostly for gemstones) and as a result of improved prospecting and mining techniques and the opening of new mines. Production, which was only two tons in 1947, reached an estimated twenty tons in 1992. 

The major diamond-producing countries in 1990 are shown in Table 12.4.0 H I 

Central Africa 6 tons nearly all industrial grade 

Russia 2 tons gem and industrial, export only, domestic use unknowi 

South Africa 1.5 tons gem and industrial production declining 

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World annual production of natural diamonds, the cubic form of carbon, is about 110 million carats (1 carat = 200 mg). Almost all is derived from kimberlite or its weathered remnants, but Australian production is from the Argyle mine, at which the host rock is lamproite. Kimberlites are olivine- and volatUe-rich potassic ultrabasic rocks of variable geological age that typically form near-vertical carrot-shaped pipes intmded into Archean cratons. The volatile-rich component is predominantly CO2 in the carbonate minerals calcite and dolomite, and the texture is characteristically inequigranular, with large grains (macrocrysts), usually of olivine [Mg2Si04], in a fine-grained, olivine-rich matrix. 

Table 1.2 Annual production of natural diamond in 2003 (accordng to US Geological Survey).

Diamond size is expressed in carats 1 carat = 0.2 g. The mines at Kimberley have produced a total of more than 200 million carats since the 1870s. Almost half of South Africa s diamonds are of gem quality. The largest producer of gemstone diamonds in southern Africa - and in the world - however is Botswana. The world production of natural diamond in the year 2001 is shown in Table 39.3 [39.6]. 

Diamond is not thermodynamically stable at atmospheric pressure, but its transformation into graphite is extremely slow below 800°C. Its synthesis, achieved for the first time in 1955, takes place by treating graphite at 2,700°C, under 12 GPa pressure, in the presence of a catalyst (1% to 2% of B, Be, Si, etc ), in an equipment (anvil, punches) which besides can be made only in sintered hard metal WC-Co. Present global production reaches 400 million carats, which is more than three times the production of natural diamonds (a carat is 1/5 g). 

Diamond. Diamond [7782 0-3] is the hardest substance known (see Carbon, diamond, natural). It has a Knoop hardness of 78—80 kN/m (8000—8200 kgf/m ). The next hardest substance is cubic boron nitride with a Knoop value of 46 kN/m, and its inventor, Wentorf, beheves that no manufactured material will ever exceed diamond s hardness (17). In 1987 the world production of natural industrial diamonds (4) was about 110 t (1 g = 5 carats). It should be noted that whereas the United States was the leading consumer of industrial diamonds in 1987 (140 t) only 260 kg of natural industrial diamonds were consumed this is the lowest figure in 48 years (4), illustrating the impact that synthetic diamonds have made on the natural diamond abrasive market. 

Soon after the first successful diamond synthesis by the solvent—catalyst process, a pilot plant for producing synthetic diamond was estabUshed, the efficiency of the operation was increased, production costs declined, and product performance was improved while the uses of diamond were extended. Today the price of synthesized diamond is competitive with that of natural diamonds. 

Diamond, the hardest material known, is the most representative inorganic product of natural and artificial high pressure synthesis. As illustrated later, it can be obtained from graphite at 5.4 GPa and 1667 K, typically, while it is not diamond but graphite that is thermodynamically stable at ambient conditions. Though solid hydrogen cannot be quenched to ambient conditions, diamond can be and remains stable, in effect, to about 600 K. From practical viewpoints, high pressme synthesis is of value if the product has a particular... 

Today about 540 megacarat (1081) of industry-grade diamond are synthesized per year. In terms of cost per amount, the production of artificial diamond can easily compete with the hauling of natural material and so the first clearly surpasses the latter in the armual output Main supphers in 2003 have been Russia (16t), Ireland (121), South Africa (12t), Japan (6.8t), and Belarus (5t) (according to US Geological Survey)(Table 1.2). Recently, China entered the market on a large scale as well. 

The very complex nature of the macroscopic and microscopic structures as they affect strength and the behavior of diamond abrasive/particles still requires extensive work to elucidate fully. However, from a crystallization point of view, gaining control over the crystallization behavior is the key to the production of optimal diamond abrasives. This, of course, may be achieved by choice and manipulation of the pressure/temperature conditions, source carbon structure and solvent/catalyst metal type, leading to control over nucleation and growth rates. 

However, diamond is scarce and costly and this has motivated researchers, in the last one hundred years or so, to try to duplicate nature and synthesize it. These efforts are finally succeeding and the scarcity and high cost are now being challenged by the large-scale production of synthetic diamond. The properties of these synthetic diamonds are similar (and in some cases superior) to those of natural diamond at a cost which may eventually be considerably lower. 

Industrial demand for diamond is met in part by synthetic diamonds. The scale of production of synthetic diamonds is significantly greater than that of mining natural material. 

Ultrasonic Microhardness. A new microhardness test using ultrasonic vibrations has been developed and offers some advantages over conventional microhardness tests that rely on physical measurement of the remaining indentation size (6). The ultrasonic method uses the DPH diamond indenter under a constant load of 7.8 N (800 gf) or less. The hardness number is derived from a comparison of the natural frequency of the diamond indenter when free or loaded. Knowledge of the modulus of elasticity of the material under test and a smooth surface finish is required. The technique is fast and direct-reading, making it useful for production testing of similarly shaped parts. 

Other advances in material science have helped humans mimic nature in the production of certain materials. For example, in the last half of the twentieth century we have learned how to produce synthetic diamonds. Diamonds were first produced commercially in the 1950s by General Electric by subjecting graphite to temperatures of 2,500°C and pressures approaching 100,000 atmospheres. Currently, well over a hundred companies produce synthetic diamonds. 

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