Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Radioactive metal
Reexamination Certificate
1999-05-05
2001-05-22
Langel, Wayne (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Radioactive metal
C423S393000, C423S400000
Reexamination Certificate
active
06235252
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for recovering, in the form of nitric acid, the nitrate ions contained in the aqueous effluents from the nuclear industry, by thermal decomposition of these nitrate ions in solution and recovering in an aqueous medium the NO
x
vapours generated by this thermal treatment.
The invention also relates to a process for recovering, in the form of nitric acid, the nitrate ions contained in the aqueous effluents from the nuclear industry, by thermal decomposition of these nitrate ions and recovering in an aqueous medium the NO
x
vapours generated, the nitric acid being intended to be recycled into the nuclear industry:
for dissolving the uranium-bearing concentrates coming from the processing of uranium-bearing ores;
or for dissolving nuclear fuel pellets:
before they are implemented in a nuclear reactor, i.e. originating from their production line, in the form of pellets exhibiting defects or in the form of waste originating from stages such as forming or pressing of uranium oxide or of a mixed oxide for the preparation of raw pellets, the sintering under reducing atmosphere, or else the precision grinding of the pellets obtained making it possible to give them their dimensions and to disclose possible texture defects such as cracks;
and/or at the end of life, after their irradiation in a nuclear reactor, their withdrawal from reactor, their cooling in pool, their grinding and conversion of their components into nitrates.
STATE OF THE ART
In the nuclear industry there are numerous stages of purification and/or of conversion of compounds into metal nitrates, such as those of uranium, plutonium, thorium, caesium, ruthenium, gadolinium, cerium or others.
These nitrates are subsequently decomposed thermally into metal oxides with the occuring of a gaseous effluent which contains steam and NO
x
nitrous vapours.
In its complete cycle, ranging from its mining extraction to its implementation as fuel in atomic power stations, uranium originating from uranium-bearing ores is supplied in the form of a uranium-bearing concentrate of various compositions, such as uranate of magnesium, sodium, potassium or ammonium, or still uranium-bearing concentrates made up of oxides such as U
3
O
8
or peroxides such as UO
4
,nH
2
O. These uranium-bearing concentrates still contain many disturbing impurities which must be removed because of their incompatibility with the uranium cycle comprising conversion, enrichment and production of the fuel.
In fact, before its fuel qualities are obtained, uranium may pass through a hydrofluorination stage (conversion into UF
4
). From this stage, the uranium tetrafluoride obtained may lead either to the production of uranium metal feeding the natural-uranium nuclear reactors or feeding the enrichment by laser way, or to another fluorination stage (conversion of UF
4
into UF
6
), the uranium hexafluoride obtained being the source of material feeding circuits for uranium enrichment by gas diffusion or ultracentrifuging or laser enrichment.
Consequently, the conversion of the uranium originating from a uranium-bearing concentrate may require, before any other chemical conversion, a stage of purification to remove the impurities which are present.
This purification consists in treating the uranium-bearing concentrate with nitric acid, which not only dissolves the uranium in the form of a uranyl nitrate, but also the impurities which accompany the uranium-bearing concentrate. In the case of uranates these impurities are, for example, for the most important ones, sodium, potassium, ammonium, calcium and magnesium. In the case of uranium-bearing concentrates in the form of oxides, the impurities are present in much smaller quantity.
The solution of impure uranyl nitrate is then treated by the mean of an organophosphorus solvent such as tributylphosphate, which allows a pure uranyl nitrate to be extracted and obtained.
At the term of this purification stage an acidic aqueous effluent is obtained which is practically without uranium, but containing all the impurities which have been converted into soluble nitrates, initially present in the uranium-bearing concentrate.
At present, this effluent which, in this form, cannot be discharged into the natural environment, is neutralized, generally with lime, and then stored in this state in leakproof ponds in which a separation of solid and liquid phases takes place: the liquid phase concentrates naturally by a slow evaporation which is, however, partially compensated by rainfall. However, the surface of these ponds must continually increase to collect and be capable of confining this effluent as soon as it occurs.
It is known from FR-A-2,410,870 a process for the treatment of a residual solution containing ammonium nitrate originating from the nuclear industry. This process consists in spraying the aqueous solution of ammonium nitrate at the top end of a furnace into a region having the decomposition temperature of the said nitrate and in discharging the decomposition compounds at the bottom end.
This process, however:
essentially concerns, the treatment of the ammonium nitrate alone which is decomposed at low temperature, yielding gaseous decomposition residues into the region heated at the decomposition temperature, and not a mixing of metal nitrates in which each nitrates is dependent of a decomposition temperature which is different from the other and much higher than the decomposition temperature of ammonium nitrate and that leads in solid and gaseous decomposition residues;
carries out the spraying of the solution of nitrates to be decomposed through a nozzle. This spraying gives rise to droplets of various sizes which fall into the furnace at various speeds. This phenomenon causes an incomplete decomposition of the nitrate partly because of the sizing difference of the droplets and consequently of a changing kinetics of reaction;
leads in disturbing deposits which clog up the walls of the furnace and which require their regular elimination;
is embodied with means of mounted heating elements, such that a sufficient and homogeneous heat exchange is not allowed.
Because of all those disadvantages, this process can not be applied to the treatment of aqueous effluents containing a mixing of nitrates in solution.
This is why, in order to limit the expansion of these ponds, the invention pursues the objective of treating:
not only the aqueous phase of these ponds;
but also, directly, the aqueous effluents which are rich in impurity nitrates which are generated by the dissolving of uranium-bearing concentrate and the extraction of uranium with an organophosphorus solvent.
Consequently, the subject of the invention is:
a process for recovering the NO
x
nitrous vapours by converting them into HNO
3
and recycling the HNO
3
in the nuclear fuel cycle;
a process for thermal decomposition of an aqueous solution of nitrates of metal impurities or other, which are effluents from the nuclear industry;
a process allowing the heat treatment of the liquid phase of the ponds for storing the aqueous effluents originating from the nuclear fuel cycle;
a process allowing the direct heat treatment of an aqueous phase in the uranium cycle, this aqueous phase resulting from the separation of two phases, one organic, containing the purified uranium, the other aqueous containing all the impurities;
a process allowing the direct treatment of the aqueous phase containing all the impurities by thermal decomposition, without clogging the walls of the plant by the solid phase.
SUMMARY OF THE INVENTION
The invention aims hence at overcoming the disadvantages of the prior art by providing a process which:
decomposes the nitrates contained in an aqueous solution whatever the decomposition temperature of each nitrate present in the said solution;
operates an instantaneous decomposition of those nitrates;
achieves the said decomposition at high temperature;
avoids all disturbing deposit over the walls of the corresponding device.
The process hereby created pursues the objective of carrying out an instant
Davied Sylvie
Floreancig Antoine
Schall Gilbert
Comurhex (S.A.)
Langel Wayne
Millen White Zelano & Branigan P.C.
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