Wettable cathode

Fig. 19 Plot of interpolar resistance versus anode-cathode distance from the work of Dorward and Payne on a pilot cell equipped with wettable cathodes. The difference between the broken and solid lines is due to gas bubbles [47],...

Kvande [106] has reviewed recent work on inert anodes (and wettable cathodes). 

In the early 1980s Kaiser Aluminum investigated the use of titanium di-boride as a wettable cathode material for Hall-Heroult cells [47]. Similar investigations by Reynolds Metals Co. continued until that company s recent merger with Alcoa [107]. The Reynolds work, and earlier research and development by Martin Marietta Aluminum [108], involved TiB2-C composites. Approaches to the... 

Mention was made of physical modeling of Hall-Heroult cells and of the possibility of radical improvement in cell performance resulting from new materials for inert anodes and wettable cathodes. 

Aluminium Electrolysis with Inert Anodes and Wettable Cathodes and with Low Energy... 

With the purpose of reducing the electrical energy consumption and eliminating the emissions of gases that are toxic or have a greenhouse effect, inert anodes [5, 6] and wettable cathodes [4, 6, 7] can be used. 

Wettable or inert cathodes are prepared from TiB2 as the main component. It was introduced as a wettable cathode material by Ransley in 1962 [8], The electrical resistivity of TiB2 is of metallic type, being quite low (about 60 pQ cm at 1000 °C, i.e. about two orders of magnitude lower than that of carbon) [8]. 

Due to the fact that TiB2 is an expensive material, which is difficult to sinter, very brittle, and sensitive to mechanical and thermal shock, one has also tested composite materials, such as TiB2 mixed with carbon or borides (ZrB2, BN, etc.) or carbides (TiC, SiC, etc.). Composites can be sintered into various shapes or be applied as coatings on regular carbon blocks. Wettable cathodes (or inert cathodes) are wetted by liquid aluminium, so that a thin film of aluminium can be maintained on the cathode surface [4-7],... 

The most economical solution could be to retrofit inert anodes into existing Hall-Heroult cells. If that turns out to be feasible, it would greatly promote rapid conversion to inert anode technology, once a workable inert anode system is developed. However, the ultimate goal is to develop a cell with vertical or slanting inert anodes and wettable cathodes [6],... 

The present paper suggests two ways to use inert anodes and wettable cathodes in cells for aluminium production. 

Figure 1.4.1 shows a possible way to use carbon anodes and wettable cathodes in commercial cells for aluminium production [11]. In this electrolysis cell, the anodes are carbon anodes and the cathodes are wettable cathodes, made from a material containing T1B2 or ZrBj, in the form of a perforated plate, underneath the anodes, which can be moved vertically. At the bottom of the electrolysis cell is a pool of molten aluminium. Aluminium is deposited on the wettable cathode and, being molten, it flows through the cathode holes into the molten aluminium pool at the bottom of the cell. The molten aluminium, forming a thin film on the cathode surface, wets the cathode. 

Wettable cathodes can also be used - made of materials containing T1B2 or ZrB2 - as perforated plates, through the holes of which aluminium drips down into the aluminium pool at the bottom of the cell. 

Figure 1.4.1 Cell design of an electrolysis cell with carbon anodes and wettable cathodes, made from a material containing TIB2 or ZrB2- (I) Carbon anode, (2) wettable cathode, (3) molten electrolyte, (4) aluminium pool, (5) carbon lining, (6) thermal insulation...

Both the inert anodes and the wettable cathodes are placed horizontally, suspended from a device connected to the current source that can adjust the interpolar distance. The anodes and cathodes can be removed from the cell and replaced when needed. The film of liquid aluminium formed on the cathode surface flows down through the cathode holes. Since oxygen forms small bubbles on the anode surface [13], a short interpolar distance of about 1 cm or even less can be used. A current yield of 95-97% and low electrical energy consumption should be achievable. 

Figure 1.4.3 Details of the construction of inert anodes and wettable cathodes in Figure 1.4.2...

The electrolysis cell with vertical electrodes shown in Figure 1.4.4 also uses inert anodes and wettable cathodes [12], Several cells with vertical electrodes have been proposed in the literature [6, 14-19] but all these cells have electrodes with a rectangular shape and cathodes immersed into the aluminium pool. The electrodes proposed in Figure 1.4.4 are of trapezoid shape, by which the oxygen escape and aluminium flow are facilitated. It also provides better separation between the electrolysis products, and therefore the current efficiency is higher. Another advantage of the proposed cathodes is that they are not in contact with the aluminium pool, are movable in the horizontal and vertical direction, and are easily changeable. 

Figure 1.4.5 Aluminium electrolysis cell with vertical inert anodes and wettable cathodes, on many rows...

Figure 1.4.5 shows another type of electrolysis cell with vertical inert anodes and wettable cathodes, as in the cell from Figure 1.4.4, but those are on many rows. In this cell, as in the cell shown in Figure 1.4.4, the heat quantity obtained by current passing is enough to maintain the thermal equilibrium of the cell. 

Voltage drop Hall-Heroult cell normal ACD =4.45 cm Cell in Figure 1.4.1, carbon anodes and wettable cathodes, ACD = 2 cm Cells in Figures 1.4.2 and 1.4.4, inert anodes and wettable cathodes ACD = 1cm ACD = 0.7 cm ... 

Wettable cathodes are in contact mainly with liquid aluminium, so the solubility of TiB2 in liquid aluminium is important in this case. Titanium diboride has a low solubility in liquid aluminium. According to Finch [31] the solubility product at 1000 °C (on a wt% basis) is ... 

An attempt has been made to use the inert anodes in large cells, but apparently it was unsuccessful, probably due to cracking and corrosion of the anodes, contaminating the aluminium metal. Likewise the use of wettable cathodes in commercial cells has so far been unsuccessM due to material problems [4, 7]. 

Our proposal in this paper to use sets of anodes and cathodes that can easily be exchanged, and to use a low temperature of electrolysis, could be a good solution for using inert anodes and wettable cathodes for aluminium production. 

The aluminium pool is not serving as cathode, the wettable cathode being independent of the cell body. The geometry of the electrode arrangement facilitates a good separation of the electrolysis products (oxygen and aluminium). Thus the current efficiency will be high. 

An electrolyte of low melting point can be used, permitting electrolysis at the temperature of700-750 C. This could be crucial for the application of inert anodes and wettable cathodes, resulting in prolonged anode, cathode, and cell life. 

Galasiu, L, Galasiu, R. (2011) Cell designs for aluminium electrolysis with inert anodes and wettable cathodes. Proceedings of the International Congress Non-Ferrous Metals - 2011 , Krasnoyarsk, Rnssia, pp. 233-240. 

The cost of dense titanium boride plates (porosity below 5 %) is 10-20 times higher than the cost of the cathode carbon materials, even if we are speaking about the most costly graphitized impregnated blocks. Dense titanium boride materials have excellent strength, wear, corrosion resistance, and wettability, yet their application may be economically efficient to implement the construction of the reduction cell with inert anode and wettable cathode. 

Alcorn TR, Stewart DV, Taberaux AT. Recent Advances in wettable Cathodes for Aluminium Smelting. Light Met. 1990, TMS, 413-8. 

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