Soderberg cells, anode

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 principal anode performance problem of Soderberg cells is the low-baked anode carbon. This results in preferential attack on binder coke and creates some level of filler dust problem as a standard operating condition. For VS Soderberg anodes, there is additional performance loss for the lower-quality pinhole carbon, which fills the space created when pins are reset. This is due to porosity created when pinhole paste is baked in place by the existing excessive heatup rates, VS Soderberg anodes are also adversely affected if the carbon has a high sulfur content. Conductor pin tips will become coated with an iron sulfide scale, which interferes with electrical conduction in the anode. 

Alternative Processes for Aluminum Production. In spite of its industrial dominance, the HaH-HAroult process has several inherent disadvantages. The most serious is the large capital investment requited resulting from the multiplicity of units (250 —1000 cells in a typical plant), the cost of the Bayer aluniina-puriftcation plant, and the cost of the carbon—anode plant (or paste plant for Soderberg anodes). Additionally, HaH-HAroult cells requite expensive electrical power rather than thermal energy, most producing countries must import alumina or bauxite, and petroleum coke for anodes is in limited supply. 

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. 

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 ... 

This paper reports the mathematical modelling of electrochemical processes in the Soderberg aluminium electrolysis cell. We consider anode shape changes, variations of the potential distribution and formation of a gaseous layer under the anode surface. Evolution of the reactant concentrations is described by the system of diffusion-convection equations while the elliptic equation is solved for the Galvani potential. We compare its distribution with the C02 density and discuss the advantages of the finite volume method and the marker-and-cell approach for mathematical modelling of electrochemical reactions. 

Kuang Z., Thonstad, J., Current distribution in aluminium electrolysis cells with Soderberg anodes. Part I Experimental study and estimate of anode consumption. Journal of Applied Electrochemistry, 26, pp. 481-486, 1996. 

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].

Anode elements are commonly prebaked low ash carbon blocks, since any ash residue ends up in the electrolyte. These are electrically connected to copper or aluminum bus bars (heavy electrical conductors) suspended over the cell, which also provide mechanical support and a means for vertical adjustment of the anode elements. An anode variant is the Soderberg paste option, which uses powdered petroleum coke formed into a paste with hard pitch. Electrical contact is established and mechanical adjustment provided by using specially shaped steel pins (Fig. 12.3). As the baked portion of this anode (Fig. 12.3) is gradually consumed, the paste approaches the molten electrolyte and the volatile components in the paste vaporize to leave a hard baked working anode element. Either type of anode element is consumed at the rate of 1-2 cm/day during normal operation, requiring periodic vertical adjustment to maintain an anode-aluminum metal pool spacing of about 5 cm. 

Sumitomo Chemical technology consists of a number of small changes to conventional Hall-Heroult electrolysis cells, which combine to cut power consumption to some 14,000 kWh/tonne from the normal experience of 16,000 to 18,000 kWh/tonne [16]. In addition to reduced power consumption, better emission control, extended cell life, and decreased labor requirements are also achieved by these changes. Better external cell insulation, changes in what is basically a Soderberg anode design, and better fume containment by cell skirt construction modifications provide these improvements. 

Fig. 4. Aluminum electrolyzing cell with Soderberg anode.

Self-baking anodes (known as Soderberg anodes), usually one per cell and therefore of much larger dimensions. Such anodes are fed at the top with the ground carbon and pitch binder and this bakes in situ as it gradually descends into the molten electrolyte to form a hard, dense material which acts as the anode surface. 

Soderberg anodes, use of which is declining, are baked directly in the aluminum reduction cell. In this case, petroleum coke is mixed with 25 to 30% binder the mixture is carbonized by the heat of the electrolysis bath (940 to 980 °C). 

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