Hall-cell cathodes

TiB2 coatings for electrodes for aluminum production (Hall-cell cathodes). TiB2 has high resistance to molten aluminum yet it is readily wetted by the molten metal and good electrical contact is assured. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

More than one boride phase can be formed with most metals, and in many cases a continuous series of solid solutions may be formed. Several methods have been used for the relatively large-scale preparation of metal borides. One that is commonly used is carbon reduction of boric oxide and the appropriate metal oxide at temperatures up to 2000 °C. Fused salt electrolysis of borax or boric oxide and a metal oxide at 700-1000 °C have also been used. Small-scale methods available include direct reaction of the elements at temperatures above 1000 °C and the reaction of elemental boron with metal oxides at temperatures approaching 2000 °C. One commercial use of borides is in titanium boride-aluminum nitride crucibles or boats for evaporation of aluminum by resistance heating in the aluminizing process, and for rare earth hexaborides as electronic cathodes. Borides have also been used in sliding electrical contacts and as cathodes in Hall cells for aluminum processing. 

Suppose (hai a cell was formed by immersing a silver anode in an analyte solution ihai was 0.0250 M Cl. Br. and I ions and connecting the hall-cell to a sat-uratcil calomel cathode via a salt bridge. 

FIGURE 14.24 In the Hall process, aluminum oxide is dissolved in molten cryolite and the mixture is electrolyzed in a cell with carbon anodes and a steel cathode. The molten aluminum flows out of the bottom ot the cell. 

Shao Z and Halle SM. A high-performance cathode for the next generation of solid oxide fuel cells. Nature 2004 431 170-173. 

Aluminum is produced according to the Hall-Heroult process [42-44]. At the cathode, AlxFy species are reduced and lead to liquid aluminum. As the electrolysis proceeds, the metal from the aluminum oxide precipitates at the bottom of the cell. At the anode, oxygen evolution takes place producing carbon dioxide/monoxide and hence resulting in current and performance losses [42-44]. 

Refractories in the Aluminum Industry. Carbon materials are used in the Hall-Heroult primary aluminum cell as anodes, cathodes, and sidewalls because of the need to withstand the corrosive action of the molten fluorides used in the process (see Aluminumand aluminum alloys). 

Electiolytic production of aluminum by the Hali-Heroulf process consumes -5% of fhe electrical output of the United States Al34 in a molten solution of Al203 and cryolite (Na3AIF6) is reduced to Al at the cathode of a cell that typically draws 250 kA. This process was invented by Charles Hall in 1886 when he was 22 years old, just after graduating form Oberlin College.2... 

Charles Hall discovered that by mixing the mineral cryolite, Na,AlF6, with alumina he got a mixture that melted at a much more economical temperature, 950°C, instead of the 2050°C of pure alumina. The melt is electrolyzed in a cell that uses graphite (or carbonized petroleum) anodes and a carbonized steel-lined vat that serves as the cathode (Fig. 14.28). The electrolysis half-reactions are... 

FIGURE 18.18 An elec- trolytic cell for production of aluminum by the Hall-Heroult process. Molten aluminum metal forms at the graphite cathode that lines the cell. Because molten aluminum is more dense than the Al203-Na3AlFg mixture, it collects at the bottom of the cell and is drawn off periodically. 

Hall-Heroult cell The electrolysis cell in which aluminium is extracted from purified bauxite dissolved in molten cryolite at 900°C. This cell has both a graphite anode and a graphite cathode. 

The most used liquid metal pool electrode is the aluminum cathode (m.p. 660°C) in the Hall-Heroult aluminum extraction cell. Alkali metals and alkaline-earth metals are also used as liquid cathodes in their molten salt extraction processes. 

The aluminum industry consumes much more carbon, as baked anode composites, than the total of all other industrial uses for baked and graphitized carbon products. The free world s total annual aluminum production capacity is approximately 16 million short tons, about one-third being produced in the United States. World aluminum production involves the consumption (oxidation) of about eight million tons of anode carbon. Production occurs by electrolytic deposition from cryolite-alumina melts using a process patented simultaneously, but independently, in 1886 by Hall in America and Heroult in France. While minor process modifications have been made in the intervening years, and productivity greatly increased, substantially the same process is still used. The industrial electrolytic cell consists of a shallow carbon vessel about 10 ft. wide by 30 ft. long, and 1-2 ft. deep, which acts as the cathode and contains the fused salt bath and molten aluminum product. The carbon anodes are supported above the cathode and lowered into the cell at the rate of... 

A Aluminum is produced by the Hall-Heroult process in cells similar to this one. Note that the cathode is the carbon (graphite) lining of the cell itself. o Every ton of aluminum that is recycled saves huge quantities of electrical energy that would be spent to produce new aluminum from ore. 

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. 

Figure 22-1 (a) Schematic drawing of a cell for producing aluminum by electrolysis of a melt of AI2O3 in Na3[AlFg]. The molten aluminum collects in the container, which acts as the cathode, (b) Casting molten aluminum. Electrolytic cells used in the Hall-Heroult process appear in the background. 

A Hall-Heroult electrolytic cell Is used to produce aluminum metal. It is made of a steel shell lined with carbon that forms the cathode. Anodes of carbon hang down into the solution of aluminum oxide dissolved in cryolite. ... 

The electrolytic production of aluminum is carried out in Hall-Heroult cells that have changed little in nearly 100 years [39], The Hall-Heroult process operates at a high temperature (about 1250 K) and utilizes a molten salt electrolyte of alumina (AI2O3) and cryolite (Na3A102), with additives such as calcium fluoride and aluminum trifluoride. The cathode reaction is the reduction of AP+, with a consumable carbon anode. The overall reaction in the Hall-Heroult cell (shown schematically in Figure 26.15) is... 

The cells are strong steel boxes, lined with alumina (to act as a refractory), a thermal insulator, and carbon. The cathode is a liquid pool of aluminum that lies at the base of the cell, above a current collector consisting of a number of carbon blocks inlaid with steel bars. A frozen crust of electrolyte protects the cell housing from erosion. The cell has ports for the periodic addition of alumina through the crust, for the removal of A1 metal, and an extractor to vent anode gases (mainly CO2). As the carbon anode is consumed, it is lowered to maintain a constant anode/cathode gap (about 5 cm). In a typical plant for the production of 70,000 tons of A1 per year, 200 Hall-Heroult cells, each 3 m X 8 m in size with 15 m of anode area, are arranged in series. The operating current density is... 

Anhydrous aluminum oxide, or corundum, is reduced to aluminum by the Half process. Figure 20.20 shows a Hall electrolytic cell, which contains a series of carbon anodes. The cathode is also made of carbon and constitutes the lining inside the cell. The key to the Hall process is the nse of cryolite, or Na3AlFg (m.p. 1000°C), as the solvent for aluminum oxide (m.p. 2045°C). The mixture is electrolyzed to produce aluminum and oxygen gas ... 

In the modern version of the Hall-Heroult process, aluminum metal is obtained by electrolysis of aluminum oxide, which is refined from bauxite ore (AI2O3 2H2O). The aluminum oxide is dissolved at 1000°C in molten synthetic cryolite (NaaAlFe), another aluminum compound. The cell is lined with graphite, which forms the cathode for the reaction, as shown in Figure 20.22. Another set of graphite rods is immersed in the molten solution as an anode. The following half-reaction occurs at the cathode. 

Figure 20.22 The Hall-Heroult process operates at 900°C in smelters similar to this one. Note that carbon (graphite) serves as both the anode and the cathode. Recycled aluminum is often fed into the cell with the new aluminum. 

ZrB2 is useful as a crucible material for metal melts because of its excellent corrosion resistance. It is also used in Hall-Heroult cells (for A1 production) as a cathode and in steel refining where it is used as thermowell tubes. 

The electric resistivity in the solid solution system TiB2-ZrB2 was studied by Rahman et al. [273]. Billehaug and 0ye [274] present a study of several transition metal diborides for cathode materials in Hall-Herould cells and come to the conclusion that TiB2 should be an excellent candidate because of its stability against the... 

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