US2112191A - Process for purifying metallic cadmium - Google Patents

Description

Patented Mar. 22, 1938 STES ICE

PROCESS FOR PURIFYING METALLIC CADMHUM Adolphe Denis Henri Lon Fassotte, Neerpclt,

andRen Favre, Overpelt,

Belgium, assignors No Drawing. Application May 12,1936, Serial No. 79,402. In Belgium March 21, 1936 3 Claims.

' This invention relates to a process of purifying metallic cadmium and more particularly to the production of pure cadmium from alloys of cadmium and zinc.

Cadmium is generally associated with zinc. When ores and other analogous products containing zinc and cadmium are treated in the thermal way, that is by reduction and distillation in crucibles or retorts, alloys of zinc and cadmium or mixtures of zinc and cadmium dusts are gen- 'erally obtained. When ores, Cottrell dusts, or

other residues containing cadmium are treated in the wet way, the cadmium is separated from the solutions obtained by precipitation by means of metallic dust of zinc in excess, a cement of cadmium mixed with zinc being obtained.

When raw zinc is purified by the method of reflux redistillation, the cadmium is also separated in the form of an alloy of cadmium and zinc.

It may be said that generally an alloy of cadmium and zinc is obtained. This alloy is easily obtained in an almost pure state because the other elements have a boiling point quite different from that of the cadmium and are separated without difficulty from the latter, by distillation. On the contrary, it is not possible,

or very costly, to separate zinc'fromcadmium by successive distillations, because their boiling points are very near to each other (respectively 778 C. and 915 C.), and also because the zinc has a vapour tension which is already important at the boiling temperature of the cadmium.

The present invention provides a simple and cheap process for separating the zinc fro'm'the cadmium from alloys, which may contain practically any percentage of zinc.

According to the present invention, the alloy cadmium-zinc is treated in a molten state with a substance capable of forming with the zinc a compound which is insoluble in cadmium, the said substance having itself no action upon the cadmium. 1

Caustic soda (NaOH) is particularly suitable as a reagent, more especially when its action is combined with that of water vapour.

Caustic soda forms salts with the zinc, namely sodium zlncates, whlch are insoluble in cadmium; moreover, the caustic soda does not attack the cadmium.

One mode, particularly advantageous, of carrying the invention into efiect, is the following:

The alloy zinc-cadmium is molten in a receptacle of flat shape made of cast-iron or of another suitable metal. "The bath of metal is covered with an amount of molten caustic soda corresponding to from 1.5 to 3 times the amount which is theoretically necessary for converting the. whole of the zinc into zlncate. Either water under pressure, or a solution of caustic soda in water, or water vapour is afterwards injected into the molten alloy. In the three cases, there is an escape of water vapoiuin the midst of the metal, which water vapour stirs violently the metal and the bath of soda and mixes them intimately in the contact zone. There is thus a rapid renewal of the contact surfaces of the metal and of the soda, thus ensuring a rapid reaction. Moreover, the water vapour acts itself as an oxidizer towards the zinc and accelerates the reaction be- I tween the caustic soda and the zinc. The molten metal being constantly covered by a layer of molten soda, is protected from oxidation by the atmospheric air, and this is particularly important at the end of the operation, in view of the extreme oxidability of molten pure cadmium; this allows the regulation at will of the temperature of the bath without any danger of oxidation.

It is also of advantage to cover the receptacle with a tight joint hood, which collects the water vapour escaping therefrom and which, by maintaining a vapour atmosphere above the soda in reaction, avoids its progressive carbonation by the atmospheric CO2.

After refining the cadmium as said above, the spent soda situated'above the bath is collected with a large spoon or otherwise and the pure cadmium is cast into the usual moulds.

It is obvious that any other device permitting an intimate contact between the molten metal and the reagent may be used for carrying the process into efiect. For instance, the molten metal may be pumped in a short-circuit through a more or less thick layer of molten caustic soda, with or without injectioncf water vapour into the soda. It is also possible to produce the stirring and the intimate contact between the metal and the molten soda by causing them to rotate inside a closed drum. heated on the outside and provided on the inside with blades, buckets, sieves or similar mixing devices. The soda and the molten metal may be mixed in a tank by means of mechanical stirring means or of injectors or ejectors working with water vapour or by means of a gas which does not attack the cadmium, such as nitrogen or hydrogen.

On the other hand, it has been ascertained that when zinc is eliminated as completely as is allowed by the process according to the present invention, the iron present in the impure alloy is eliminated by the same operation.

Example of carrying the invention into effect: In a cast iron tank having a diameter 1 metre and a depth of cm., heated by means of a furnace, are molten 1000 kilos of an alloy cadmiumzinc containing: 4

Percent Zinc 25 Ir 0.02 Lead 0. 003 Cadmium the rest 500 kilos of molten caustic soda (NaOH) are added, preferably in two or three stages, care being taken previously to remove each time the spent reagent. Water vapour is injected, preierably superheated at 350 C., by means of twenty iron pipes pointed at their ends and traversing the sheet metal hood which covers the reaction bath.

The molten metal is maintained at a temperature of 380 C. to 450 C.

After about thirty hours of reaction, a cadmiumis obtained'.the analysis of which is as follows: I

' Percent 0 to 0.003 Ir 0.0004 to 0.0006 Lea 1 0.0015 Cadmium 99.995, or more The spent soda from the above described process contains: sodium zincate, oxide of due, a small amount of oxide of cadmium, of

iron and of lead in suspension, a small amountof sodium carbonate, an excess of caustic soda. This spent-soda is dissolved in water and the oxides of nine. of cadmium. of iron and of lead are separated by decantation or filtration from the solution. 'Ihe solution is afterwards carbonated by means of carbon dioxide (00:) obtained from any available source.

The soda passes entirely into the form of soluble carbonate whilst the zinc is precipitated in the state of very pure insoluble zinc carbonate.

The zinc carbonate may be used as such, or it may be used to manufacture a very pure zinc, for

instance by reduction and distillation in heated retorts.

The solution of sodium carbonate may be caustifled by means of caustic lime, by a known process and the caustic soda thus obtained may serve whilst water vapour under pressure is injected to stir the molten metal and soda, and then separating the zinc compoundproduced from the cadmium, as set forth.

3. A process of purifying metallic cadmium containing zinc comprising treating the impure cadmium in the molten state with molten caustic soda, the molten soda and molten metal being placed in superposed layers, and water'vapour' being injected to stir the molten under pressure metal and soda, and then separating the zinc compound produced from the cadmium as set forth. A

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