Heat-transfer model for the acheson process
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I. INTRODUCTION
II. DESCRIPTION OF THE ACHESON PROCESS
SILICON carbide (SiC) has extreme hardness, sharpness, and good thermal chemical properties, which make it ideal for use as an abrasive and refractory material. It was discovered accidentally by Acheson in 1891. Later on, he produced SiC by carbothermal synthesis in a resistor furnace. The process is known as the Acheson process, and the overall reaction is given by
A schematic of a resistance furnace of the type used in the Acheson process is shown in Figure 1. A massive current is passed through the graphite rod, located in the center of the cylindrical furnace and surrounded by a coke-silica mixture. Heat is generated at the surface of the electrode, due to which the reaction occurs (Reaction [I]) between silica and coke to produce silicon carbide. Heat is continuously supplied by the electrode, and it travels from the center to the periphery of the furnace. In the Acheson furnace, the firebrick walls of the furnace are protected by a refractory lining of waste SiC. The raw materials (sand and finely ground petroleum coke) are mixed together and deposited in the furnace up to the level of the electrodes. A core of carbon or packed graphite powder is then tamped into place along the length of the trough between two electrodes. Additional charge material is piled over the core until the furnace is full. A voltage is applied to the furnace electrodes so that massive currents pass through the resistance core, converting electrical power into heat energy. This heat is transferred to the surrounding charge. During firing, the furnace temperature of the core ranges between 2000 ⬚C and 2700 ⬚C, depending on the raw material used, additives added, etc. Typically, once the temperature at the core reaches 1500 ⬚C or more, various chemical reactions start taking place in the surrounding mixture to form the SiC. The CO produced during the process, according to Reaction [I], is burnt at the top of the charge, which is open to the atmosphere. Firing is done for about 40 hours and then, after cooling, the side walls are removed. An outer layer of uncombined mixture is broken away, exposing the cylindrical mass of sharp, brilliant crystals. This is the silicon carbide. The theoretical requirement of energy for the production of silicon carbide[6] from silica and carbon is about 2.2 kWh/ kg. However, the actual power consumption in industry[2] is ranges between 6 and 12 kWh/kg of newly produced SiC.
SiO2 ⫹ 3C ⫽ SiC ⫹ 2CO
[I]
Quartz (sand) and coke are used as the major raw materials to produce silicon carbide in a resistance-heating furnace. Since the discovery of SiC, there have been many attempts to produce silicon carbide in bulk via other routes[1] such as by using other solid-phase reactions, liquid-phase reactions, or gaseous-phase reactions. Only solid-phase reactions have been found to compete with the Acheson process in the mass production of silicon carbide. More than 95 pct of the world production of silicon carbide comes via the Acheson process. The Acheson pr
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