Prebaked anode

Piebaked anodes aie produced by molding petroleum coke and coal tar pitch binder into blocks typically 70 cm x 125 cm x 50 cm, and baking to 1000—1200°C. Petroleum coke is used because of its low impurity (ash) content. The more noble impurities, such as iron and siUcon, deposit in the aluminum whereas less noble ones such as calcium and magnesium, accumulate as fluorides in the bath. Coal-based coke could be used, but extensive and expensive prepurification would be required. Steel stubs seated in the anode using cast iron support the anodes (via anode rods) in the electrolyte and conduct electric current into the anodes (Fig. 3). Electrical resistivity of prebaked anodes ranges from 5-6 Hm anode current density ranges from 0.65 to 1.3 A/crn. ... 

The waste materials produced during the primary production of aluminum are fluoride compounds. Fluoride compounds are principally produced during the reduction process. One reason that prebaked anodes are favored is that the closure of the pots during smelting facilitates the capture of fluoride emissions, although many modern smelters use other methods to capture and recycle fluorides and other emissions. 

Two types of cells are used in the Hall-Heroult process those with multiple prebaked anodes (Fig. 1), and those with a self-baking, or Soderberg, anode. In both types of cell, the anodes are suspended from above and are connected to a movable anode bus so that their vertical position can be adjusted. The prebaked anode blocks are manufactured from a mixture of low-ash calcined petroleum coke and pitch or tar formed in hydraulic presses, and baked at up to 1100°C. 

Figure 15 Hall-Heroult electrolysis cell using prebake anode technology.

The anode carbon for the cell is usually a baked composite of calcined petroleum-coke filler bound with coal-tar pitch coke. The carbon composite may either be compacted into blocks which are baked before use in the cell (prebake anode), or be baked in place (as a single block) above the cell as the green paste moves downward toward the anode electrolytic face (Soderberg anode) (1,2.,.2D. For prebake cells, electrical connection is made by inserting a steel conductor rod, or pin, into the top of the anodes, Soderberg anodes may have either vertical (VS) or near-horizontal (HS) conductor rods. 

The third mechanism of carbon consumption is airburn of prebake anode tops and the bottom edges of Soderberg anodes during cell operation. This mechanism typically accounts for about 17% of total prebake carbon consumption, but can vary (for different cell designs) from less than 10% to about 40% during severe airburn problems. The following equation represents such airburn reactions ... 

The objective of making good anode carbon, which has the property values already specified, involves four important operations which are applied differently for Soderberg and prebake anode fabrication. These four operations are (1) filler aggregate sizing (2) paste pitching level determination, (3) paste compaction, and (4) compacted composite baking. 

The most important objective for anode filler aggregate sizing is to obtain a high vibrated aggregate bulk density (23). For prebake anodes, maximum aggregate bulk density is required to achieve maximum baked carbon density. For Soderberg anodes, this requirement is somewhat modified by paste rheological requirements... 

Zoric, J., Rousar, I., Thonstad, J., Mathematical modelling of industrial aluminium cells with prebaked anodes. Part I Current distribution and anode shape. Journal of Applied Electrochemistry, 27, pp. 916-927,1997. 

Fig. 3 Three types of Hall-Heroult cell in commercial use. The upper two use Soderberg anodes while the last uses prebaked anodes. From McGravie et al. [13].

Assuming that this is pure carbon, as is nearly true with prebaked anodes, and the stoichiometry of Eq. 12.19. 

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... 

C02)- The prebaked anodes are produced in more controlled conditions leading to better-defined structures. These lead to lower cell voltages and higher current efficiencies. In addition, external thermal treatment allows much superior control of emissions to the environment. 

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