Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Solid
Reexamination Certificate
1995-03-15
2001-11-20
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Solid
C205S771000, C205S772000
Reexamination Certificate
active
06319391
ABSTRACT:
The present invention relates to the removal of metal from graphite and in particular to the decontamination of graphite bodies coated with metallic uranium or other actinides or actinide-containing materials.
BACKGROUND OF THE INVENTION
Graphite is used in large quantities in the casting of metallic articles, especially uranium fuel rods for nuclear reactors. In such use the graphite becomes contaminated with the metal being cast and is essentially scrap. It is necessary for the metal contaminant which typically forms no more than ten per cent by weight, eg 2 to 6 per cent by weight of the scrap, to be separated from the graphite so that the graphite can be safely disposed of. The metal may be recovered, and, where appropriate, re-used. One known method of separation comprises incinerating the graphite and collecting the metallic contaminant as ash. This method is expensive and it is harmful to the environment because of the large quantities of carbon dioxide produced.
DESCRIPTION OF THE INVENTION
According to the present invention a method of treating scrap graphite having a metal contaminant adhered thereto go as to separate the metal from the graphite comprises placing the scrap graphite in an aqueous oxidising electrolyte and passing through the electrolyte an electric current. The electric current may be a directional electric current.
The graphite body disintegrates in the electrolyte and the metal dissolves at an accelerated rate under the influence of the electric current. The metal can also break off from the graphite and may dissolve over a longer time period in the electrolyte. The graphite so treated may therefore be separated from the electrolyte eg by filtering and washing. Where the graphite has been contaminated with uranium the separation by this process is sufficiently successful to allow the graphite to be disposed of in a conventional manner rather than special means required for hazardous, radioactive materials.
The scrap graphite may contain less than 40 per cent, in most cases less than 10 per cent by weight, eg from 2 to 6 per cent by weight of contaminant metal so that the metal is a minor by product of the separation process (in terms of its quantity).
The electrolyte is desirably a strong acid, eg nitric and/or sulphuric acid. Its concentration is preferably in the range 5 to 70 per cent by weight of acid:aqueous solution. In general, the process works more rapidly as the concentration of the acid increases. The process speed also increases with the assistance of (a) an elevated electrolyte temperature, eg 30 to 80 degrees Celsius; also with (b) mechanical agitation or stirring of the electrolyte and also with (c) an increase in applied electric current or (d) input of additional energy from other sources, eg ultrasonic devices, or sparging.
The mean applied electric current needs to be greater than the minimum current required for the reaction, which is typically 10 milliamps per cm
2
but may be greater than 100 milliamp per cm
2
.
Where mechanical stirring of the electrolyte in applied this may be by the use of a conventional paddle or agitator. Alternatively, ultrasonic stirring may be used.
The electrolytic system containing the electrolyte may comprise an acid bath into which the graphite is placed. The graphite may be contained in a basket, eg made of plastics material. One part of the basket, eg its upper body part (which resides out of the electrolyte), may be made of metallic material to act as an electrode conductor when the interconnected graphite pieces are immersed in the electrolyte. The conduction path is therefore from the metallic material through the graphite mass to the electrolyte. Alternatively, the current may be provided through one or more large solid blocks of graphite which are placed on top of the graphite stack and act as the said electrode conductor. Alternatively, or in addition, the positive electrode for applying electric current may be provided by a block of metal, eg stainless steel, in contact with the scrap graphite and/or by a collar of metal, eg stainless steel, inside the basket, eg slidably located against the inner wall thereof, in contact with the scrap graphite. The other electrode may be provided by a metallic, eg stainless steel, wire gauze positioned around the basket. Several baskets of graphite may be treated in this way together in the same bath.
The electrolyte employed in the bath may be circulated in and out of the bath in a known way. The used electrolyte containing graphite and dissolved metal may be filtered to remove the graphite and thereafter may be recycled for re-use. The concentration of the electrolyte may be maintained by distilling the acid or by sparging it with air. The process for supplying, extracting and treating electrolyte may be a continuous process or a batch process.
The electrolytically treated graphite may be washed and removed to a dump as non-hazardous waste.
The present invention provides an environmentally safer and cleaner method of separating metal, especially uranium, from graphite, prior to disposal of the graphite, than the method used in the prior art.
The method of the present invention may be applied to the separation of uranium from graphite employed to cast the uranium or alternatively to the separation of precious or semi-precious metals from graphite electrodes.
Electrolytic methods of separation of carbon from metals is known in the prior art, eg as described in prior patent specifications GB 497,835, GB 1,273,170, U.S. Pat. No. 4,385,972 and EP 0,221,187. However, in these references the carbon is not present as graphite, ie is either a minor impurity or is present as a compound, eg tungsten carbide. The object in these cases is to remove contaminant carbon to recover metals. In contrast, in the present invention the object is to remove contaminant metal from graphite, the contaminant metal forming only a small part of the material to be separated. The present invention provides a method of breaking down the carbon matrix and this problem is not faced or dealt with in the prior art. As noted above, the invention provides a more efficient, cost effective and environmentally friendly way of dealing with metal contaminated scrap graphite than the method conventionally used in the prior art described above and therefore beneficially and surprisingly provides a significant industrial advance in, for example, the nuclear industry where such scrap graphite is produced in large quantities.
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Palamalai et al., “Development of an Electro-Oxidative Dissolution Technique for Fast Reactor Carbite Fuels” (1991) (Abstract Only).
Holderness Stewart Garfield
Lewin Robert Glyn
British Nuclear Fuels
Nixon & Vanderhye
Phasge Arun S,.
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