Preparation of mineral matrices by cold crucible induction...

Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath

Reexamination Certificate

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C205S360000, C205S362000, C588S012000, C588S014000, C588S015000

Reexamination Certificate

active

06436271

ABSTRACT:

The present invention relates to the preparation of mineral matrices by cold crucible induction melting. More specifically, the present invention relates to a process for preparing a mineral matrix by melting, which is implemented according to the method of direct cold crucible induction melting, said method comprising an initiation step for initiating the melting.
Within the context of the present invention, an innovation is in fact proposed of the implementation of said method of direct cold crucible induction melting; a method known per se. This innovation is found in the initiation of said melting.
Said method of direct cold crucible induction melting, for preparing mineral matrices, can be implemented in various contexts, and notably in that of the preparation of glass matrices. Such glass matrices fall more particularly within the context of methods of vitrifying radioactive waste materials. The present invention is described especially with reference to this context. The present invention is however in no way limited to said context.
In the method of vitrifying radioactive waste materials, said waste materials are mixed with molten glass inside a crucible which is heated, notably by induction. Said crucible is continuously fed with glass frit and with waste materials.
In the method of hot crucible melting, the power is transmitted, by conduction, to the load (molten glass+waste materials) which is in contact with the wall of the crucible. Said wall is itself heated by the Joule effect; this Joule effect is produced by the passage of Foucault currents which are induced in said wall. A device which is suitable for the implementation of such a method is described in the EP-A-0 196 809 patent application. Such a process is currently carried out in the Cogema (Compagnie Générale des Matiéres Nucleaires) vitrification workshops in La Hague, France.
In the most recent process of direct cold crucible induction melting, the wall of the crucible is cooled by a current of cooling water which circulates in channels set up to this end within said wall. The power is transmitted directly to the load (molten glass+waste materials) by the Joule effect, this Joule effect being produced by the circulation of Foucault currents which are induced directly into the molten glass, which is an electric conductor in this (molten) state.
Thus, for the initiation of such a process of melting, by direct cold crucible induction, said cold crucible containing (for said initiation) cold glass, the heating cannot be obtained by induction in said cold glass, which is a non-conductor in this state. It is therefore necessary, a priori, to rely on additional heating means for said initiation.
It is recommended to introduce, into the bed of cold (non-conductor) glass, an electric conductor element (of the metal ring or metal turnings type) in which element the currents induced can circulate causing, by the Joule effect, a progressive warming up until the initiation of the combustion of the metal which spreads closer and closer in causing the melting of the neighbouring glass, this glass undergoing melting in which induced currents develop progressively in turn, until the complete melting of said glass. The introduction of such a conductive element into a radioactive environment is susceptible to give rise to a few difficulties, mainly mechanical difficulties. A process of this type is described in the EP-A-0 640 992 patent application: a part of the waste materials to be vitrified themselves constituting the electric conductor element and another part of these consisting of vitreous substances.
It has also been envisaged, for the initiation of a process of direct cold crucible induction melting, to have recourse to heating means which are different from inductive heating means: burner(s), microwaves, electrodes, arc
The intervention of such additional heating means also poses problems:
said means hinder the inside and/or the outside of the cold crucible,
they complicate the procedures of use and of maintenance of said cold crucible.
The two approaches which are presented above require, in any case, particular fittings as regards the structure of the cold crucible and as regards its use.
Confronted with this technical problem of the initiation of the processes of vitrifying radioactive waste materials by melting, by direct cold crucible induction, and more generally with the technical problem of the initiation of the direct cold crucible induction melting, the Applicant proposes, within the context of the present invention, an original solution. This original solution is based on the prior intervention, within the cold crucible, of a conductive liquid.
The process of the invention—a process for preparing a mineral matrix by melting, which is implemented according to the method of direct cold crucible induction melting and which includes an original initiation step, for initiating said melting—characteristically comprises, during said initiation step, the constitution, in said cold crucible, of a conductive mineral load, by introduction of constituent elements of said matrix, into a conductive liquid bath, brought by induction to a suitable temperature. By virtue of the temperature rise of said conductive liquid bath, it is possible, by using only the means of inductive heating, to directly constitute a conductive mineral load in the cold crucible. This result can only be obtained if said intervening conductive liquid is liquid and a conductor, over a relatively wide temperature range between a temperature &thgr;
1
of less than 600° C., advantageously of between 100 and 500° C., and a temperature &thgr;
2
at least equal to the temperature at which the constituent elements of the matrix melt to generate said matrix.
The interest of such a wide temperature range is conceived insofar as it is sought:
on the one hand, to easily and rapidly obtain said conductive liquid in the cold crucible. This condition fixes the lower value &thgr;
1
of said temperature range. It has been seen that, reasonably, &thgr;
1
is of less than 600° C., that it is advantageously of between 100 and 500° C. The lower said temperature 0, the easier it is to implement the initiation of the process according to the invention;
and on the other hand, to melt the added constituent elements of the matrix in said conductive liquid; it being necessary for such a melting to be operated before the degradation and/or evaporation of said conductive liquid. This condition fixes the upper value &thgr;
2
of said temperature range. It is easily understood that this value &thgr;
2
is a function of the nature of the intervening constituent elements, and hence of the mineral matrix under melting sought after. It will be seen later on that the process of the invention is more particularly adapted for preparing matrices of glass, of enamel and of ceramic. It may be indicated here in an illustrative manner that within the context of the preparation of a matrix of glass undergoing melting, &thgr;
2
is at least equal to 1,000° C., advantageously at least equal to 1,300° C. It is altogether possible at these temperatures to prepare a glass undergoing melting from its constituent elements, of the mineral oxide type: SiO
2
, B
2
O
3
, CaO, . . . .
According to the invention, in order to initiate the melting process, use is made of a bath of such a liquid which is conductive over the whole of the temperature range: [&thgr;
1
-&thgr;
2
]. In said bath, which is subjected to a suitable heating power, (the) constituent elements of the matrix sought after—elements which melt
are added until the conductive mineral load sought after is obtained.
According to various embodiments:
said load is constituted of said added constituent elements; or
said load is constituted of said added constituent elements and of other constituent elements which were found initially in the bath of conductive liquid; or
said load is constituted of said added constituent elements and of at least one other constituent element which is obtained directly from said conductiv

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