Synthetic abrasives manufacture

Abrasives have evolved iato an essential component of modem iadustry. Sandstone, emery, and comndum were the abrasives of choice until the late 1800s when artificial materials were developed. Today synthetic abrasives offer such improved performance that the natural ones have been largely replaced except for jobs where cost is paramount. In 1987 U.S. statistics (4) showed natural abrasive production to be about 7 million while that of cmde manufactured abrasives was over 182 million. Total value of abrasives and abrasive products worldwide is estimated to be over 6 biUion dollars. 

As shown in Figure 2.33, the mill consists of a flat rotor and stator manufactured in a chemically inert synthetic abrasive material, and the mill can be set to operate at clearances from virtually zero to 1.25 mm, although in practice the maximum clearance used is about 0.3 mm. When duty demands, steel working surfaces may be fitted, and in such cases the minimum setting between rotor and stator must be 0.50-0.75 mm, otherwise pick up between the steel surfaces occurs. 

The chief contaminant is 0.3-0.5% sodium oxide, which fortunately does not affect electrolysis, with <0.05% calcium oxide, <0.025% of silica or iron oxide, and <0.02% of any other metallic oxide [4]. Apart from metal production, some of this high temperature alumina is used for the manufacture of synthetic abrasives and refractory materials. Activated alumina destined for adsorptive uses is produced in the same way, except that more moderate calcining temperatures of about 500°C are employed, which produces a highly porous product with excellent surface activity. The volume of alumina from the world s major producers is listed in Table 12.3. Australia has been the largest producer for many years (Table 12.3). 

Diamonds found in nature in the earth s crust are called natural diamonds. Some of them (familiar as the precious stones used for jewelry) reach gem-quaHty standards for clarity, color and size others may be used as industrial diamonds to make various tools for abrasive machining of hard materials, etc. (Figure M51). The main part of the latter, however, are synthetic diamonds, manufactured from graphite, and used for many purposes in industry. Diamond is the hardest material known and has the highest thermal conductivity of aU materials at room temperature. 

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. 

Pulpstones. Improvements have been made in the composition and speed of the grinding wheel, in methods of feeding the wood and pressing it against the stone, in control of power to the stones, and in the size and capacity of the units. The first pulpstones were manufactured from quarried sandstone, but have been replaced by carbide and alumina embedded in a softer ceramic matrix, in which the harder grit particles project from the surface of the wheel (see Abrasives). The abrasive segments ate made up of three basic manufactured abrasive siUcon carbide, aluminum oxide, or a modified aluminum oxide. Synthetic stones have the mechanical strength to operate at peripheral surface speeds of about 1200—1400 m /min (3900 to 4600 ft/min) under conditions that consume 0.37—3.7 MJ/s (500—5000 hp) pet stone. 

The bulk of synthetic industrial diamond production consists of the smaller crystal sizes up to 0.7-mm particle size (25 mesh). This size range has wide utihty in industry, and a significant fraction of the world s need for diamond abrasive grit is now met by synthetic production yielding thousands of kilograms per year. Because the raw materials are plentiful, synthetic production could, if necessary, supply the world demand for diamond abrasive. Development work continues in order to improve size and utility of the manufactured product and to realize the full potential of diamonds at minimum cost. An appreciable increase in performance has been obtained by coating the diamonds with a thin layer of nickel or copper, before incorporating them into wheels. The thin layer of metal apparendy improves adhesion and heat transfer. 

Manufactured abrasives include silicon carbide, fused aluminum oxide, sintered aluminum oxide, sol-gel sintered aluminum oxide, fused zirco-nia-alumina, synthetic diamond, cubic boron nitride, boron carbide, slags, steel shot, and grit. 

Just like everything else, erasers have changed with the times. Most of us, when we think of polyvinyl chloride, or pvc, think of vinyl car roofs or the old-fashioned lp record album. But this synthetic polymer can also be formulated into erasers. To make the material soft and pliable, manufacturers add a plasticizer such as dioctyl phthalate to the mix to improve abrasion, they stir in some calcium carbonate, or chalk. 

Olefin fiber n. A manufactured fiber in which the fiber-forming substance is any long chain synthetic polymer composed of at least 85% by weight of ethylene, propylene, or other olefin units. Olefin fibers combine lightweight with high strength and abrasion resistance, and are currently being used in rope, indoor-outdoor carpets, and lawn furniture upholstery. 

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