Prebake cells

Furnaces used to bake anodes for prebake cells use the cooling anodes to preheat combustion air. Hot combustion gases from the baking 2one are used to preheat incoming anodes. Using these techniques, about 4.2 MJ/kg (1004 kcal/kg) of anode carbon or only 2520 MJ/1 (6.03 x 10 kcal/t) of aluminum is required to produce anodes. 

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 different cell technologies for aluminum production depend on the nature of the carbon anodes and the current load. However, the electrolyte composition and the operation of the electrolysis is very similar for all technologies. Information about innovations and performance data related to aluminum electrolysis has traditionally been very open. Figure 3 shows a schematic drawing of a modem prebaked cell. 

There are mainly two types of cells, a Spderberg and a prebake cell. The main difference between these two types of cells is the anode (see Chapter 3.2 and Figure 1.11.1). The cells are then subdivided based on where the alumina feeding takes place (side or center). The Spderberg cells are also named based on how the studs (anode collector bars) are oriented in the anode (sideways or vertically Figure 1.11.2). 

The cathode is basically the same, but the width/length ratio might be a little bigger for the Spderberg cell. Generally a modern prebake cell is much larger than a Spderberg cell. 

Figure 1.11.2 Sketches showing a Soderberg cell (a) and a prebake cell (b). Reprinted from Ref. [7]. Copyright (1993) Aluminium-Verlag CmbH...

Taberaux A, Brown JH, Eidridge IG, Alcorn TR. Erosion of cathode blocks in 180 kA prebaked cells. Light Met. 1999 128 187. 

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. 

The electrolytic cells are large containers (usually steel), and each is a cathode compartment lined with either a mixture of pitch and anthracite coal or coke baked in place by the passage of electric current or prebaked cathode blocks cemented together. 

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.

In this context, a review is presented of the complex chain of events affecting anode performance, ranging from the properties of precursors for filler cokes and binder pitches, through production of these raw materials and their fabrication into anode carbon, and concluding with anode performance evaluation in full-size prebake and Soderberg cells of different designs. 

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 ... 

There is considerable variety in the design of both prebake and Soderberg cells. For Soderberg cells, the electrical conductor pins, or studs, may either be inserted almost-horizontally (HS) into... 

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].

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