The Hall-Heroult Process

This historical review is not to be understood as comprehensive. Instead only some significant developments are discussed to explain how the overall cell performance has been improved over the years. 

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Many metals are extracted from their compounds, as found in ores, by electrolytic processes. By far the most important is the Hall-Heroult process, invented in 1886, for producing aluminium from alumina, itself refined from bauxite ore. Alumina is dissolved in molten cryolite, Na3Alp6, and electrolysed, using carbon anodes and the aluminium itself as cathode. While various details are being steadily improved, the basic process is still the same today. 

Aluminum metal is produced from aluminum oxide by electrolysis using the Hall-Heroult process, whose story is detailed in our Chemical Milestones Box. The melting point of AI2 O3 is too high (2015 °C) and its electrical conductivity too low to make direct electrolysis commercially viable. Instead, AI2 O3 is mixed with cryolite (Na3 AlFfi) containing about 10% CaF2. This mixture has a melting point of 1000 °C, still a high temperature but not prohibitively so. Aluminum forms several complex ions with fluoride and oxide, so the molten mixture... 

Chemical Milestones The Stoiy of the Hall-Heroult Process... 

Successful electrolysis of aluminum requires a liquid medium other than water that can conduct electricity. The key to the Hall-Heroult process is the use of molten cryolite, Na AlFg, as a solvent. Cryolite melts at an accessible temperature, it dissolves AI2 O3, and it is available in good purity. A second important feature is the choice of graphite to serve as the anode. Graphite provides an easy oxidation process, the oxidation of carbon to CO2. ... 

The first production of aluminum was by the chemical reduction of aluminum chloride with sodium. The electrolytic process, based on the fused salt electrolysis of alumina dissolved in cryolite, was independently developed in 1886 by C. M. Hall in America and P. L. Heroult in France. Soon afterwards a chemical process for producing pure alumina from bauxite, the commercial source of aluminum, was developed by Bayer and this led to the commercial production of aluminum by a combination of the Bayer and the Hall-Heroult processes. At present this is the main method which supplies all the world s needs in primary aluminum. However, a few other processes also have been developed for the production of the metal. On account of problems still waiting to be solved none of these alternative methods has seen commercial exploitation. 

The Hall-Heroult process is a prodigious consumer of electrical energy. The energy required to produce 1 ton of aluminum from ore is more than twice that required to produce 1 ton of copper and ten times that for 1 ton of steel. More than 75% of this energy is consumed in the reduction of alumina to aluminum metal. The reasons for this high energy consumption have been presented in Table 6.18. The theoretical energy requirement for... 

K. Grjotheim, C. Krohn, M. Malinovsky and J. Thonstad, Aluminum Electrolysis The Chemistry of the Hall-Heroult Process, Aluminum-Verlag GmbH, Dtisseldorf, West Germany, 1977. 

The Alcoa chlorine process uses about 30% as much electrical energy as the Hall-Heroult process. A1C13 melts at a much lower temperature than the Al203/Na3[AlF6] mixture, so less energy is required to heat the electrolysis container. The product, chlorine gas, is recycled in the Alcoa chlorine process, which keeps the cost down. Also the electrodes do not have to be replaced, as they do in the Hall-Heroult process. However, the main disadvantage is that the Alcoa chlorine process is more dangerous to workers since chlorine is a toxic gas. 

ALCOA A process proposed for manufacturing aluminum metal by the electrolysis of molten aluminum chloride, made by chlorinating alumina. It requires 30 percent less power than the Hall-Heroult process and operates at a lower temperature, but has proved difficult to control. Developed by the Aluminum Company of America, Pittsburgh, in the 1970s and operated in Palestine, TX, from 1976 abandoned in 1985 because of corrosion problems and improvements in the efficiency of conventional electrolysis. 

Compagnie AFC A variation of the Hall-Heroult process for making aluminum metal, in which the electrolyte is a mixture of aluminum fluoride, sodium fluoride, and barium chloride. The process is operated at a lower temperature than the Hall-Heroult process, and the product has a purity of 99.99 percent instead of 99.5 percent. Developed in 1934 by the Cie. des Produits Chimiques et Electrometallurgiques Alais, Froges et Carmarque, in France, and still operated by that company. 

Cowles An electrothermal process for making aluminum alloys. A mixture of bauxite, charcoal, and the metal forming the alloy (usually copper), was heated in an electric furnace and the molten alloy tapped from the base. The process cannot be used for making aluminum alone because in the absence of the other metal the product would be aluminum carbide. Invented by the Cowles brothers and operated in Cleveland, OH in 1884 and later in Stoke-on-Trent, England. The electrical efficiency was poor and the process was superseded by the Hall-Heroult process. 

Deville (1) The first commercial process for making aluminum metal. Molten sodium aluminum chloride was reduced by heating with metallic sodium. Invented by H. E. St-Claire Deville and operated around 1854 to 1890. Superseded by the Hall-Heroult process. See also Cowles. 

Grjotheim, K., Krohn, C., Malinovsky, M., Matiasovsky, K., and Thonstad, J., Aluminium Electrolysis—Fundamentals of the Hall-Heroult Process. Aluminium-Verlag GmbH, Dtisseldorf, 1982. 

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]. 

Paul-Louis-Toussaint Heroult. 1863-1914. French metallurgist. Independent discoverer of the electrolytic method of preparing aluminum now known as the Hall-Heroult process. He designed electric furnaces, and made many important contributions to the electrometallurgy of iron and steel... 

In the Hall-Heroult process, a molten alumina is dissolved in cryolite and the mixture is electrolyzed to produce molten aluminum. 

Molten salts or ionic liquids (also referred to as fused salts by some authors) were among the very first media to be employed for electrochemistry. In fact, Sir Humphrey Davy describes electrochemical experiments with molten caustic potash (KOH) and caustic soda (NaOH) [1] as early as 1802 A wide variety of single molten salts and molten salt mixtures have been used as solvents for electroanalytical chemistry. These melts run the gamut from those that are liquid well below room temperature to those melting at more than 2000°C. The former present relatively few experimental challenges, whereas the latter can present enormous difficulties. For example, commercially available Teflon- and Kel-F-shrouded disk electrodes and Pyrex glass cells may be perfectly adequate for electrochemical measurements in ambient temperature melts such as the room-temperature chloroaluminates, but completely inadequate for use with molten sodium fluoroaluminate or cryolite (mp = 1010°C), which is the primary solvent used in the Hall-Heroult process for aluminum electrowinning. 

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. 

Similarly, passing 1 mol of electrons in the Hall-Heroult process produces 1/3 mol (9.0 g) of aluminum, because 3 mol of electrons are required to reduce 1 mol of Al3+ to aluminum metal ... 

What constant current (in amperes) is required to produce aluminum by the Hall-Heroult process at a rate of... 

Today we use aluminium in very large quantities. The annual production in the world is 19.5 million tonnes. The commercial extraction of aluminium has been made possible by two scientists, working independendy of each other, who discovered a method using electrolysis. The two scientists were Charles Martin Hall (USA), who discovered the process in 1886, and the French chemist Paul Heroult, who discovered the process independendy in the same year. The process they developed, often called the Hall-Heroult process, involves the electrolysis of aluminium oxide (alumina). The process involves the following stages. 

The principal method used in producing aluminum metal involves three major steps refining of bauxite by the Bayer process to produce alumina, electrolytic reduction of alumina by the Hall-Heroult process to produce aluminum, and casting of aluminum into ingots (Browning 1969 Dinman 1983 IARC 1984). 

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. 

Energy efficiency is stated to be about 10 per cent better than the Hall-Heroult process. 

Due to the high energy consumption and the high investment and operating cost of the Hall-Heroult process, several other ways of making aluminum have been studied over the years. Some features of a process based on electrolysis of aluminum chloride are treated in the following [230],... 

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