Electrode prebaked

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. 

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. 

Pitch. The principal outlet for coal-tar pitch is as the binder for the electrodes used in aluminum smelting. These are of two types. Older plants employ Sn derberg furnaces, which incorporate paste electrodes consisting of a mixture of about 70% graded petroleum coke or pitch coke and 30% of a medium-hard coke-oven pitch. This paste is added periodically to the top of the monolithic electrode as it is consumed. The more modem smelters employ prebaked electrodes requiring less binder, about 18%. 

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. 

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. 

Fig. 5. Prebaked electrodes. Courtesy of UCAR Carbon Technology Corp.

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. 

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. 

Prebaked carbon electrodes, 12 752, 755 Prebaked cathode blocks, 12 765-766 Prebaked electrodes, 12 758 Precautionary Principle (PP), 10 245-246, 24 188... 

The use of electrodes in aluminum reduction operations is associated with the generation of several types of wastes (Dinman 1983 IARC 1984). In aluminum reduction facilities using the prebake process, PAHs are generated. In aluminum reduction operations using the Soderberg cell process, considerable amounts of volatiles from coal tar pitch, petroleum coke, and pitch, including PAHs, are generated. 

Gulf Canada has been active in this area for sometime and recently has developed a process that produces a high quality electrode binder pitch from a petroleum-based feedstock (7). This pitch has been used on a commercial scale by the aluminum industry in both vertical stud Soderberg and prebaked electrodes. 

Anode characteristics For prebaked Resistivity Current density For Soderberg Resistivity Current density Electrode spacing... 

Aluminium Production. Primary aluminium production is based on two different technologies basically using Soderberg or prebaked anodes. PAH are emitted from the carbon electrodes containing tar and pitch as binder (22). The emissions from the prebaked electrodes are usually 1-10% of that of the Soderberg electrodes. 

Except for the anodes, cell design has changed little since the introduction of the process in the last century. The cells have got bigger, however, as the materials and construction techniques have improved, and typically a cell will now be 3 m X 8 m X 70 cm deep with eighteen prebaked anodes. The inter-electrode gap must be rather wide, about 5 cm, (a) to prevent shorting because the small difference in density between the melt (2.1 g cm ) and the molten aluminium... 

A. Vatland, W. Bruff (1995) Development of an in situ prebaking electrode system for silicon metal production in INFACON 7, ed. J. Kr. Tuset, H. Tveit, I. G. Page, FFF (the Norwegian Ferroalloy Research Organisation, SINTEF, Trondheim), p. 431-440. 

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