Aluminium Hall-Heroult process

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. 

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

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. 

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

Invented by H.Y. Castner and operated by the Aluminium Company at Oldbury, England in 1888, in order to supply sodium for the manufacture of aluminum. It was abandoned soon after, when the Hall-Heroult process for aluminum was developed. 

K. Grjotheim, H. Kvande (Eds.), Understanding the Hall-Heroult Process for Production of Aluminum, Aluminium-Verlag, Diisseldorf, 1986. 

Grjotheim, K., Krohn, C., Malinovsky, M., Matiasovsky, K. Thonstad, J. (1977) Aluminium Electrolysis. The Chemistry of the Hall-Heroult Process, Aluminium-Verlag GmbH, Diisseldorf. Grjotheim, K., Krohn, C., Malinovsky, M., Matiasovsky, K. Thonstad, J. (1982) Aluminium Electrolysis. Eundamentals of the Hall Heroult Process, 2nd Edition. Alnmininm Verlag GmbH, Diisseldorf. 

Thonstad, J., Fellner, P., Haarberg, G.M., Hives J., Kvande, H. Sterten, A. (2001) in Aluminium Electrolysis. Eundamentals of the Hall-Heroult Process 3rd Ed. Aluminium-Verlag, Diisseldorf, p. 230. 

Figure 4.1 Cell for the Hall—Heroult process for the extraction of aluminium.

The cell for the electrolysis consists of a bipolar stack of horizontal carbon anodes with an inter-electrode gap of 1.5 cm. The electrodes and electrolyte flow is designed to ensure the minimum contact between the electrolysis products since the molten aluminium and gaseous chlorine would otherwise react rapidly. The aluminium falls to a pool below the electrodes while the chlorine is pumped out the top of the cell to be used in the reaction with alumina. A simplified cell is shown in Fig. 4.3. It may be noted that the overall process again uses carbon in at least stoichiometric quantities although now in a chemical step. Overall the process run at 1 A cm has an energy efficiency which is currently claimed to be about 10% better than that for the Hall—Heroult process. 

Boiko OI, Delimarskii YK, Tchemov RV (1985) Ukrainian Chem J 51 385-388 Gtjotheim K, Krohn C, Malinovsky M, Matiasovsky K, Thonstad J (1977) Aluminium electrolysis the chemistry of the Hall-Heroult process. Aluminium Verlag, Dusseldorf Watanabe N, Fudjii Y, Yoshisawa S (1963) J Electrochem Soc Japan (Denki Kagaku) 31 131-135... 

Thonstad J, Fellner P, Haarberg GM, Hives J, Kvande H, Sterten A (2001) Aluminium electrolysis. Fundamentals of the Hall-Heroult process. Alumin-ium-Verlag, Diisseldorf... 

The Hall-Heroult process is used to produce aluminium (Figure 30). It uses an electrolyte of 3NaF.AlF3 (cryolite) + CaFa + AIF3 and about 3-8% AI2O3 is dissolved in the cryolite. The AI2O3 is the source of the aluminium. The process uses a carbon cathode and a consumable carbon anode. 

Fig. Al Steps in the Hall-Heroult process for aluminium extraction.

Aluminium is produced industrially only by the Hall-Heroult process, which was patented in 1886. The overall primary cell reaction is ... 

Phosphorus is one of the most significant impurity elements in the Hall-Heroult process for producing aluminium. High levels of phosphorus affect some important properties of aluminium and aluminium alloys. Also the current efficiency for aluminium is lowered due to cyclic red/ox reactions at the electrodes involving different oxidation states of phosphorus. Several studies in industrial cells have shown that a content of 100 ppm phosphorus in the bath reduces the current efficiency by about 1% [1]. In a more controlled laboratory study [2] it was found that the current efficiency was reduced by 0.72% per 100 ppm P in the electrolyte. Thisted and coworkers carried out similar experiments and obtained similar results [3,4]. Figure 1.7.1 shows the reported results of the effects of phosphorus on the current efficiency for aluminium deposition. 

A cell for the Hall-Heroult process is shown in Fig. 4.1, and a block diagram of the whole process is shown in Fig. 4.2. Cell design is determined largely by the need to contain molten cryolite at high temperatures and to withstand attack by molten aluminium and also by sodium and fluorine formed as minor products at the cathode and anode respectively. Hence, the cell is a strong steel box lined first with alumina to act as a refractory, thermal insulator and then with carbon. 

In a Hall-Heroult process, the electrolyte is cryolite, Na3AlF6 (3NaF AIF3), with dissolved alumina and additives (up to 16-18 %) of aluminium fluorite, calcium fluorite, magnesium fluorite, and sometimes other fluorites [1-4]. The melting point of pure cryolite is 1,010 ""C (Fig. 2.1), but the temperature of electrolyte may be... 

Electrolysis of aluminium, reduction of aluminium, smelting of aluminium, electro winning of aiuminium, Hall-Heroult process process of fabrication of molten primary aluminium due to high-temperature ( 950-980 °C) electrochemical reaction of reduction of alumina, dissolved in electrolyte, by carbon. 

Smelter, aluminium smelter the plant producing primary aluminium according to the Hall-Heroult process. May also produce secondary aluminium. 

Currently, it is possible to say that for the existing Hall-Heroult process of aluminium reduction, the service life of the metallurgical equipment has reached a certain satisfactory level, and significant investments in R D of refractory materials for aluminium are not likely. The exception is the materials science of carlxMi cathode materials, where certain efforts could be made. 

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