Chemistry: electrical and wave energy – Processes and products
Patent
1979-09-20
1990-04-24
Niebling, John F.
Chemistry: electrical and wave energy
Processes and products
204294, C25C 306
Patent
active
049197711
DESCRIPTION:
BRIEF SUMMARY
There are essentially two processes known for the production of aluminium by molten salt electrolysis.
The first of these processes is based on the electrolysis of aluminium oxide, which is dissolved in molten cryolite at temperatures of 950.degree.-970.degree. C. Apart from cryolite, no other substance has yet been found in which aluminium oxide is sufficiently soluble to enable aluminium to be produced by electrolysis below 1000.degree. C. In electrolytic cells run under technical operating conditions, the aluminium oxide content varies between approx. 2% and 8% by weight. If the aluminium oxide content in the cryolite melt is too low, e.g. less than 1-2%, the so-called anode effect occurs at the anode, which manifests itself as an increase of the cell voltage to several times normal value. The anode and cathode are of carbon. The oxygen set free by the decomposition of the aluminium oxide combines with the carbon of the anode to form carbon dioxide and carbon monoxide. In the case of prebaked carbon, electrodes are consumed approximately 0.43 to 0.5 kg of carbon per kg of aluminium produced.
The second process concerns the molten salt electrolysis of aluminum chloride. Since aluminium chloride sublimes at 183.degree. and is poor ionic conductor, it is usually dissolved in molten alkali chlorides. In order to be able to separate the aluminium in liquid form, electrolysis temperatures of approx. 700.degree. C. are chosen. The material principally used for the anode and cathode is graphite. Gaseous chlorine is released at the graphite anode. Several processes have been suggested for carrying out the electrolysis of aluminium chloride.
The electrolysis of aluminium chloride is beset with many difficulties. The collection and leading away of the gaseous chlorine at the node at approximately 700.degree. C. is a problem of construction materials. The vapor pressure of the aluminium chloride dissolved in the molten salt is relatively high, so that in drawing off the chlorine gas noticeable quantities of aluminium chloride are also taken out from the cell. As the concentration of aluminium chloride in the melt increases, the electrical conductivity decreases. It is likewise difficult to feed the aluminium chloride, which is in the form of a gas, into the melt. Experience shows that the aluminium chloride and the melt must be free of oxidic impurities, since the decomposition of the oxides causes the carbon of the graphite anodes to be consumed and their life to be reduced. A particular disadvantage is, however, that no simple way has yet succeeded in producing pure aluminium chloride directly by reductive chlorination of the aluminium ores, e.g. bauxite. It has, therefore, been suggested that initially pure aluminium oxide should be produced by means of the known Bayer process, and that this be subsequently converted to aluminium chloride and carbon dioxide using chlorine and carbon or carbonyl chloride (phosgene).
The process described above does lead to pure aluminium, but the process contains an additional stage and is correspondingly more costly.
The task of the invention here presented is to produce aluminium by electrolysis using an electrolyte consisting mainly of chlorides. Thereby not only are the deficiencies described above of the aluminium chloride electrolysis and aluminium oxide electrolysis processes avoided, but in addition the manufacture of aluminium chloride as a raw material for the process is also avoided.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further described with respect to the accompanying drawings wherein
FIG. 1 is a section through an electrolytic cell of the present invention;
FIG. 2 illustrates a longitudinal section through a multi-chamber electrolytic cell of the present invention;
FIG. 3 is a horizontal section through the plane (A-B) of the electrolytic cell depicted in FIG. 2;
FIG. 4 is a horizontal section of a five cell assembly or battery according to the present invention;
FIG. 5 shows a vertical section through the plane (C-D) shown in FIG. 4;
FIG. 6 is
REFERENCES:
patent: 503929 (1893-08-01), Hall
patent: 3009863 (1961-11-01), Angevine
patent: 4049511 (1977-09-01), Alder et al.
patent: 4115215 (1978-09-01), Das et al.
patent: 4118292 (1978-10-01), Fray et al.
Gorgos Kathryn
Niebling John F.
VAW Vereinigte Aluminium-Werke AG
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