Chemistry: electrical and wave energy – Processes and products
Patent
1986-05-29
1988-04-19
Niebling, John F.
Chemistry: electrical and wave energy
Processes and products
204 69, 204 71, C25C 302, C25C 336
Patent
active
047387599
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for producing calcium or calcium alloys of high purity by electrolysis of a calcium derivative in a molten salt bath, the derivative being dissolved in the bath in ionic form.
Numerous methods of electrolysis in molten salts are known which allow various metals to be obtained, such as calcium, silicon, magnesium and sodium, etc, which are deposited on the cathode.
These methods do not always allow substances of adequate purity to be obtained without awkward and costly purification stages.
A "liquid cathode" method can also be adopted, and this process involves using as cathode a liquid or molten metal which is generally deposited on the base of the electrolytic tank.
On the same lines, methods for producing silicon by electrolysis or by electro-refining are also known, a good example of these methods being described in French Pat. Specification No. 2 480 796. In this method, which relates to the refining of silicon, some silicon is dissolved in a molten salt bath based on alkali and/or alkaline earth halides and nitrides from a silicon anode. The silicon thus dissolved is deposited in the cathode such that, even if this refining operation leads to high purity silicon, it is impossible to guarantee that it is free from traces of metals which are themselves also deposited on the cathode.
The desire to obtain high purity silicon which is free from metals need not be emphasised since it is known that this element is one of the main constituents of certain electronic memories and circuits.
The object of the invention is specifically to obtain high purities both with regard to the calcium and to it partner in the derivative. It involves causing the calcium or the alloy to be deposited on a cathode by electrolysis of a calcium derivative in a bath of molten salts based on calcium halides, the calcium derivative being dissolved in the bath of molten salts in ionic form.
It has been observed that the deposition of pure, that is non-alloyed calcium necessitated the application of high electric powers. Furthermore, the metal calcium is soluble in the bath of halides and has to be removed from this bath before dissolving in it. Otherwise, the electrical conductivity of the bath becomes too great and the electrolytic cell will short-circuit after a certain period of operation.
It is therefore advantageous to work with a liquid metallic cathode and the calcium formed in situ is alloyed with the metal constituting the cathode.
Aluminium, tin, copper, lead, bismuth, nickel, etc, for example, may be selected as metal constituting the liquid cathode. The metal constituting the cathode is, for example, in the molten state. These metals may be used in pure form or in a mixture. They may also be pre-alloyed with calcium to lower their melting point.
In a first variation, the metal cathode is truly liquid, that is to say the cathode is liquid from the beginning before electrolysis is initiated.
In a second variation, with a nickel cathode for example, the cathode may be solid at the beginning and may pass gradually to the liquid state during the electrolysis process as the Ca/Ni alloy is being formed.
Suitable calcium derivatives in ionic solution in the molten calcium halides include calcium nitride, calcium hydride, calcium carbide, calcium silicide or silico-calcium, calcium boride, calcium cyanide and calcium cyanamide.
CaSi.sub.2 and CaC.sub.2 are of particular interest as they allow silicon, respectively graphite carbon of high purity to be produced at the anode, obviously in addition to high purity calcium or calcium alloys at the cathode.
Calcium halides is the group comprising calcium chloride, calcium fluoride and mixtures thereof are selected for the electrolytic bath. Dehydrated industrial CaCl.sub.2 could be used, for example. If mixtures are used, eutectic mixtures will preferably be selected. The bath may also contain other halides, for example alkaline halides, in particular chlorides or fluorides.
Molten salt baths at temperatures of between 650.degree. and 1100.degree.
REFERENCES:
patent: 3226311 (1965-12-01), Van Diest
patent: 3284325 (1966-11-01), Gibson
Bonomi et al., "Electrowinning of calcium metal by electrolysis of molten salt solution in calcium carbide", Molten Salt Electrolysis Met. Prod., Int. Symp., 1977, 14-20.
Bienvenu Gerard
Dubruque Dominique
Extramet S.A. Zone Industrielle
Hsing Ben C.
Niebling John F.
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