The abrasives industry is composed of approximately 400 companies engaged in the following separate types of manufacturing: abrasive grain manufacturing, bonded abrasive product manufacturing,and coated abrasive product manufacturing.
Abrasive grain manufacturers produce materials for use by the other abrasives manufacturers to make abrasive products. Bonded abrasives manufacturing is very diversified and includes the production of grinding stones and wheels, cutoff saws for masonry and metals, and other products. Coated abrasive products manufacturers include those facilities that produce large rolls of abrasive-coated fabric or paper, known as jumbo rolls, and those facilities that manufacture belts and other products from jumbo rolls for end use.
The process description is broken into three distinct segments discussed in the following sections: production of the abrasive grains, production of bonded abrasive products, and production of coated abrasive products.
Abrasive Grain Manufacturing –
The most commonly used abrasive materials are aluminum oxides and silicon carbide. These synthetic materials account for as much as 80 to 90 percent of the total quantity of abrasive grains produced domestically. Other materials used for abrasive grains are cubic boron nitride (CBN),synthetic diamonds, and several naturally occurring minerals such as garnet and emery. The use of garnet as an abrasive grain is decreasing. Cubic boron nitride is used for machining the hardest steels to precise forms and finishes. The largest application of synthetic diamonds has been in wheels for grinding carbides and ceramics. Natural diamonds are used primarily in diamond-tipped drill bits and saw blades for cutting or shaping rock, concrete, grinding wheels, glass, quartz, gems, and high-speed tool steels.
Other naturally occurring abrasive materials (including garnet, emery, silica sand, and quartz) are used in finishing wood, leather, rubber, plastics, glass, and softer metals.
The following paragraphs describe the production of aluminum oxide, silicon carbide, CBN,and synthetic diamond.
- Silicon carbide.
Silicon carbide is manufactured in a resistance arc furnace charged with a mixture of approximately 60 percent silica sand and 40 percent finely ground petroleum coke.A small amount of sawdust is added to the mix to increase its porosity so that the carbon monoxide gas formed during the process can escape freely. Common salt is added to the mix to promote the carbon-silicon reaction and to remove impurities in the sand and coke. During the heating period, the furnace core reaches approximately 2200°C (4000°F), at which point a large portion of the load crystallizes. At the end of the run, the furnace contains a core of loosely knit silicon carbide crystals surrounded by unreacted or partially reacted raw materials. The silicon carbide crystals are removed to begin processing into abrasive grains.
- Aluminum oxide.
Fused aluminum oxide is produced in pot-type, electric-arc
furnaces with capacities of several tons. Before processing, bauxite, the crude raw material, is calcined at about 950°C (1740°F) to remove both free and combined water. The bauxite is then mixed with ground coke (about 3 percent) and iron borings (about 2 percent). An electric current is applied and the intense heat, on the order of 2000°C (3700°F), melts the bauxite and reduces the impurities that settle to the bottom of the furnace. As the fusion process continues, more bauxite mixture is added until the furnace is full. The furnace is then emptied and the outer impure layer is stripped off.
The core of aluminum oxide is then removed to be processed into abrasive grains.
- Cubic boron nitride.
Cubic boron nitride is synthesized in crystal form from hexagonal boron nitride, which is composed of atoms of boron and nitrogen. The hexagonal boron nitride is
combined with a catalyst such as metallic lithium at temperatures in the range of 1650°C (3000°F) and pressures of up to 6,895,000 kilopascals (kPa) (1,000,000 pounds per square inch [psi]).
- Synthetic diamond.
Synthetic diamond is manufactured by subjecting graphite in the presence of a metal catalyst to pressures in the range of 5,571,000 to 13,100,000 kPa (808,000 to
1,900,000 psi) at temperatures in the range of 1400 to 2500°C (2500 to 4500°F).