Electrode carbons

Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. The Gzochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process. 

The purity of a sample of K3Fe(CN)6 was determined using linear-potential scan hydrodynamic voltammetry at a glassy carbon electrode using the method of external standards. The following data were obtained for a set of calibration standards. 

Electrodes. Because of the numerous different processes, there are many different types of electrodes in use (9), eg, prefabricated graphite, prefabricated carbon, self-baking, and composite electrodes (see Carbon). Graphite electrodes are used primarily in smaller furnaces or in sealed furnaces. Prebaked carbon electrodes, made in diameters of <152 cm or 76 by 61 cm rectangular, are used primarily in smelting furnaces where the process requkes them. However, self-baking electrodes are preferred because of thek lower cost. 

A typical large three-phase ferroalloy furnace using prebaked carbon electrodes is shown in Eigure 4. The hearth and lower walls where molten materials come in contact with refractories are usually composed of carbon blocks backed by safety courses of brick. In the upper section, where the refractories are not exposed to the higher temperatures, superduty or regular firebrick may be used. The walls of the shell also may be water-cooled for extended life. Usually, the furnace shell is elevated and supported on beams or on concrete piers to allow ventilation of the bottom. When normal ventilation is insufficient, blowers are added to remove the heat more rapidly. The shell also may rest on a turntable so that it can be oscillated slightly more than 120° at a speed equivalent to 0.25—1 revolution per day in order to equalize refractory erosion or bottom buildup. 

In the electrothermic part of the furnace, electrical energy introduced via three carbon electrodes, keeps the bath molten and completes the lead oxide reduction. Fumes generated in the electrothermic section are oxidized in a post-combustion chamber by adding ambient air, before the vapor is cooled, dedusted, and released to the atmosphere. 

Another important potential appHcation for fuel cells is in transportation (qv). Buses and cars powered by fuel cells or fuel cell—battery hybrids are being developed in North America and in Europe to meet 2ero-emission legislation introduced in California. The most promising type of fuel cell for this appHcation is the SPEC, which uses platinum-on-carbon electrodes attached to a soHd polymeric electrolyte. 

Aluminum is best detected quaUtatively by optical emission spectroscopy. SoHds can be vaporized direcdy in a d-c arc and solutions can be dried on a carbon electrode. Alternatively, aluminum can be detected by plasma emission spectroscopy using an inductively coupled argon plasma or a d-c plasma. Atomic absorption using an aluminum hoUow cathode lamp is also an unambiguous and sensitive quaUtative method for determining alurninum. 

Typical raw material mix to produce one metric ton of silicon consists of 2500—3000 kg quartz, 1200—1400 kg of low ashcoal and/or charcoal, and 1500—3000 kg wood chips. From 11 to 14 MWh of electrical power, and prebaked carbon electrodes of 90—140 kg, are consumed. 

A typical 20-MW, a-c furnace is fitted with three 45-in. (114.3-cm) prebaked amorphous carbon electrodes equdateraHy spaced, operating on a three-phase delta connection. The spacing of the electrodes is designed to provide a single reaction zone between the three electrodes. The furnace is rotated to give one revolution in two to four days or it may be oscillated only. Rotation of the furnace relative to the electrodes minimizes silicon carbide buildup in the furnace. 

Electric-Arc Furnace. The electric-arc furnace is by far the most popular electric steelmaking furnace. The carbon arc was discovered by Sir Humphry Davy in 1800, but it had no practical appHcation in steelmaking until Sir William Siemens of open-hearth fame constmcted, operated, and patented furnaces operating on both direct- and indirect-arc principles in 1878. At that early date, the avadabiHty of electric power was limited and very expensive. Furthermore, carbon electrodes of the quaHty to carry sufficient current for steel melting had not been developed (see Furnaces, electric). 

In 1891, a small amount of siUcon carbide was produced bypassing a strong electric current from a carbon electrode through a mixture of clay and coke contained in an iron bowl that served as the second electrode (1). The abrasive value of the crystals obtained were recognized and The Carbomndum Company was founded that year (2). About 10 years earlier tetratomic radicals of siUcon (Si2C202, Si2C2N) had been reported (3). That work also produced some SiC. 

Anthracite. Anthracite is preferred to other forms of coal (qv) in the manufacture of carbon products because of its high carbon-to-hydrogen ratio, its low volatile content, and its more ordered stmcture. It is commonly added to carbon mixes used for fabricating metallurgical carbon products to improve specific properties and reduce cost. Anthracite is used in mix compositions for producing carbon electrodes, stmctural brick, blocks for cathodes in aluminum manufacture, and in carbon blocks and brick used for blast furnace linings. 

Anthracite is calcined at appreciably higher temperatures (1800—2000°C). The higher calcining temperatures for anthracite are necessary to complete most of the shrinkage and to increase the electrical conductivity of the product for use in either Soderberg or prebaked carbon electrodes for aluminum, siHcon, or phosphoms manufacture. 

With the exception of carbon use in the manufacture of aluminum, the largest use of carbon and graphite is as electrodes in electric-arc furnaces. In general, the use of graphite electrodes is restricted to open-arc furnaces of the type used in steel production whereas, carbon electrodes are employed in submerged-arc furnaces used in phosphoms, ferroalloy, and calcium carbide. 

Carbon Electrodes. Carbon electrodes are rigid carbonaceous shapes deployed in electric furnaces. They are the final link in the chain of conductors from the energy source to the reaction zone of an electrically heated vessel. The gap bridged by the electrode is that between the contact plates that transmit current to the electrode and the discharge area at the arc end of the electrode. 

Two types of carbon electrodes are in widespread use. Prebaked carbon electrodes (Fig. 5) are those made from a mixture of carbonaceous particles and a coal-tar pitch binder. The electrode is formed by extmsion or mol ding from a heated plasticlike mix and subsequently baked. Final bake temperature is sufficient to carbonize the binder, ie, about 850°C. At this temperature the binder is set, all volatiles have left, and a significant portion of the product shrinkage has occurred. 

Self-baked carbon electrodes are those whose shapes are formed in situ (33). The carbonaceous mixture is placed into a hoUow tube-shaped metal casing. The upper end receives the unbaked mixture as a soHd block, small particles, or warm plastic paste. The casing contains inwardly-projecting longitudinal perforated fins that become surrounded by baked carbon as the casing is incrementally moved downward and through the contact plates. Casing and carbon are consumed in this furnace. 

A variety of products are made in submerged-arc furnaces. Among them are various alloys and compounds. Each uses a particular type or grade of carbon electrode to hold production costs at the lowest possible level. Graphite electrodes could be and are used in some submerged-arc furnaces. Such a choice is the result of special conditions that warrant use of the more expensive graphite in Heu of carbon. 

Several grades of carbon electrodes are available. The characteristics of each result from the raw materials and processes used ia manufacturiag. The generic descriptions and primary constituents are as follows ... 

AH carbon electrodes are amorphous. They are formed from a mixture of particles, fillers, and a biader, and they are baked to about 850°C. This is... 

Prebaked carbon electrodes are manufactured in all diameters up through 1500 mm. Some prebakes are produced as quadriforms to suit specific furnaces. Self-baking electrodes are in service through 2134 mm diameter. Electrode lengths are as needed for particular appHcations. Rounds are available in lengths up to 2794 mm and quadriforms as long as 3556 mm. Self-baked electrodes are continuous. 

Production of carbon electrodes is a capital-intensive business. Two suppHers dominate the prebaked market. Carbon paste producers are more numerous and tend to serve local markets. There is no international standard for the threaded joints on carbon electrodes. Manufacturers of straight pin carbon electrodes have followed the physical specifications adopted for graphite electrodes (37). Unified standards do not exist for pinless joints resulting in limited interchangeability among brands. Electrode diameters are offered in both English and metric sizes with no restrictions on new or unique diameters. 

The physical properties of carbon electrodes are deterrnined by the raw materials and processes used in their manufacturer. There are no universal grade designations and the pubHshed properties are quite broad. Table 3 shows ranges for some of the common commercially available grades. 

Table 3. Ranges of Physical Properties of Typical Carbon Electrode Grades...

Carbon electrodes are the normal choice for the link in the connection chain to deflver power to the arc tip. Graphite may be used in special apphcations, but the higher cost of graphite favors the use of carbon electrodes. Carbon possesses properties ideal to its appHcation as an electrode. These properties include no softening point, no melting point, electrical conductivity, strength increases with increasing temperature, resistivity drops as temperature increases, available in the size and purity desired, and cost effectiveness. 

Although carbon electrode production has been regarded as a mature business, the steady growth in demand and the need for improved electrodes has prompted ongoing development efforts in these areas (/) cost containment through raw material substitutions and process improvements (2) higher purity electrodes for those processes such as siUcon production (J) improvements in thermal shock resistance to enhance electrode performance and (4) better joining systems for prebakes. 

The Shawinigan process uses a unique reactor system (36,37). The heart of the process is the fluohmic furnace, a fluidized bed of carbon heated to 1350—1650°C by passing an electric current between carbon electrodes immersed in the bed. Feed gas is ammonia and a hydrocarbon, preferably propane. High yield and high concentration of hydrogen cyanide in the off gas are achieved. This process is presently practiced in Spain, AustraUa, and South Africa. 

Eig. 7. CycHc voltammograms for the reduction of 1.0 mAf [2,2 -ethylene-bis(nitrilomethyHdyne)diphenolato]nickel(II) in dimethyl formamide at a glassy carbon electrode, in A, the absence, and B and C the presence of 2.0 and 5.0 mAf 6-iodo-l-phenyl-l-hexyne, respectively (14). 

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