Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/34—Electrolytic production, recovery or refining of metals by electrolysis of melts of metals not provided for in groups C25C3/02 - C25C3/32
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B60/00—Obtaining metals of atomic number 87 or higher, i.e. radioactive metals
  • C22B60/02—Obtaining thorium, uranium, or other actinides
  • C22B60/0204—Obtaining thorium, uranium, or other actinides obtaining uranium
  • C22B60/0213—Obtaining thorium, uranium, or other actinides obtaining uranium by dry processes
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B60/00—Obtaining metals of atomic number 87 or higher, i.e. radioactive metals
  • C22B60/02—Obtaining thorium, uranium, or other actinides
  • C22B60/0204—Obtaining thorium, uranium, or other actinides obtaining uranium
  • C22B60/0286—Obtaining thorium, uranium, or other actinides obtaining uranium refining, melting, remelting, working up uranium

Definitions

• the invention relates to a process for obtaining uranium metal in stages, from an oxidized compound, for example UO3, U3O8, using a chloride route.
• a process is usually used which successively involves reduction to the state of UO2 at high temperature using hydrogen or a carrier gas d hydrogen, such as NH3 for example, followed by fluoridation using hydrofluoric acid at high temperature or in the aqueous phase, to obtain UF4, and metallothermic reduction, for example with Mg or Ca, to obtain on the one hand l uranium in the form of ingots, on the other hand a byproduct which is a fluoride (for example Mg or Ca) which it is necessary to decontaminate before rejection.
• the Applicant has sought to develop a process which makes it possible to avoid the use of expensive and particularly dangerous products, such as hydrofluoric acid, the production of a byproduct which is also expensive to treat and to eliminate. It also sought a process which, preferably, is continuous and insensitive to the presence of impurities in the starting oxide or better operates a purification.
• the starting product is any oxidized compound of uranium, pure or impure, for example an oxide such as UO2, U3O8, UO3, UO4 or a mixture thereof, usually U3O8 and rather UO3 is used, or a uranate, preferably ammonium diuranate because the presence of alkali or alkaline earth is not always desirable.
• the starting uranium-containing compound is preferably mixed in dry and divided form (powder, scales, granules, etc.) with carbon (coke, carbon, graphite, etc.) also in divided form.
• This mixture is introduced as it is, or possibly after granulation or agglomeration, in a high temperature reactor where it reacts with chlorine gas, diluted or not in an inert gas such as argon, helium, nitrogen, introduced preferably against the current when operating continuously and / or so that it percolates through the load.
• an inert gas such as argon, helium, nitrogen
• the reaction generally produces UCl4 and is written: UO3 + 3C + 2 Cl2 ⁇ UCl4 + 3 CO (and / or CO2) but it can also form UCl5 and UCl6.
• One operates at a temperature above about 600 ° C and preferably between 900 and 1100 ° C, to preferably obtain UCl4 and limit the formation of UCl5 or UCl6, and at any pressure; however for practical reasons, it is easier to use a pressure close to atmospheric pressure.
• the proportion of CO and / or CO2 obtained depends on the reaction temperature. The reaction is complete.
• the reaction can be conducted in a wide variety of modes.
• the molten salt bath is then regularly fed with the mixture of said uranium-oxidized compound and carbon, while bubbling the chlorine.
• Such a process is particularly advantageous when the starting uranium-containing compound is an impure concentrate, containing in particular troublesome elements such as alkali or alkaline-earth, rare earths or others.
• This bath containing UCl4 can optionally be used for electrolysis, but it is preferable to recover UCl4 in gaseous form.
• the uranium-bearing compound alone or preferably in admixture with carbon, can then be introduced directly into a reactor containing a carbon bed which ensures the excess of carbon.
• reactor or furnace can be suitable, for example belt, rotary, sliding bed furnace ..., but the most interesting is the fluid bed reactor, comprising a carbon bed fluidized by chlorine and the reaction gases, which is supplied with said mixture of uranium compound and carbon preferably in the form of powder.
• the supply of the different types of reactors can also be in the form of granules, tablets, balls, etc. This type of process is especially advantageous when the uranium-containing compound contains few alkaline elements and preferably a low level of impurities.
• Sublimated UCl4 obtained during the reaction is filtered at the outlet of the reactor, for example on quartz cloth or silica.
• UCl4 contains volatile impurities
• UCl4 contains higher chlorides such as UCl5, UCl6, it is possible to carry out a disproportionation operation consisting in demoting said higher chlorides to UCl4.
• This operation consists simply in heating the mixture of chlorides either in the solid phase at a temperature of 150 to 500 ° C under reduced pressure, generally about 6 mm of mercury, or in the gas phase at a temperature at least equal to 800 ° C . Downshifting can also be done by electrolysis as will be seen below.
• the reduction is carried out in order to obtain the uranium metal according to any of the variants already mentioned.
• An igneous electrolysis is carried out in the medium of molten salt preferably in a bath based on chlorides, for example alkaline and / or alkaline-earth, making it possible to recover the solid uranium at the cathode and a release of chlorine at the anode.
• chlorides for example alkaline and / or alkaline-earth
• NaCl or a mixture of Na Cl + KCl is used.
• a bath containing exclusively fluorides, although possible, is not recommended, since it tends to stabilize the presence of oxyfluorides which are difficult to reduce without significantly increasing the oxygen content of the deposited metal.
• the composition of the bath can vary within wide limits. In general, it is adjusted so that the molten bath has a low vapor pressure in UCl4, and so that the temperature corresponds to the desired morphological structure of the uranium deposit at the cathode. Indeed, the crystalline morphology and the quality of the cathodic deposit depends to a large extent on the temperature at which it is carried out, on the chemical constitution of the bath and on its concentration of UCl4 and / or UCl3. The average uranium content of the bath is very variable.
• UCl4 is introduced in solid, liquid or gas form.
• a diaphragm between the anode and the cathode to prevent the recombination of the elements and facilitate the collection of chlorine. It must be sufficiently porous (10 to 60% of vacuum preferably 20 to 40%) and is made of a material resistant to temperature and to corrosion of the bath. It is preferable to use a conductive material, for example a metal, or better a graphite material which can be cathodically polarized to avoid any migration of U towards the anode and re-formation of chloride. A metal deposit can be made there tending to clog it; the metal deposited is then redissolved by depolarization. The polarization of the diaphragm leads to different concentrations in the anode (anolyte) and cathode (catholyte) compartments.
• the metal deposited on the cathode must be sufficiently adherent not to fall to the bottom of the tank and be unrecoverable, but not too much either to be easily recoverable.
• the crystal form of the deposit and its characteristics are a function, as has already been said, of a certain number of factors such as the nature of the bath, its composition, its concentration, its temperature, the current density, etc.
• the interpolar distance between electrodes is variable and depends largely on the form in which the metal is deposited; it is advantageous to fix the electrolysis conditions so as to avoid strong growths of said metal, therefore to deposit it in a fairly collected form, while preventing it from being too compact to then facilitate harvesting.
• the interpolar distance is between 50 and 200 mm.
• the cathode is sufficiently charged with a deposit of uranium contaminated by bath inclusions, it is washed and it is harvested either by mechanical means, such as scraping, machining, etc., leading to a metal. in divided form which is washed with acidulated water and / or melted to eliminate the said inclusions, either by physical means such as fusion ..., leading to a purified ingot surmounted by a layer of slag coming from the bath inclusions .
• the chlorine obtained at the anode is recycled in the previous step, after a possible addition of fresh Cl2 intended to compensate for the losses.
• Metallothermic reduction methods for obtaining uranium metal are well known, in particular the reduction of UF d'U by Mg or Ca where the reaction products go through a molten state. Given the thermal balances, such a process is not possible in the context of the reduction of UCl4. It is therefore preferable to operate in the following manner using the reaction: UCl4 + 4M ⁇ U + 4M Cl M represents a fusible metal capable of reducing UCl4 at temperatures below about 1100 ° C, if necessary with an external energy supply.
• Mg, Ca, but also Na, K or one of their mixture is used.
• This step of the process according to the invention consists in reacting the liquid reducing metal contained in a closed reactor or crucible generally made of ordinary steel or stainless steel, with UCl4 introduced regularly, generally in liquid or gaseous form, at a temperature and under conditions such that UCl4 reacts in the gaseous state with the reducing agent, that the resulting chloride is liquid and that the uranium produced remains solid.
• This is usually carried out between about 600 ° C and 1100 ° C and preferably between about 800 ° and 1000 ° C, under a reducing or inert atmosphere (H2, He, Ar ...), in a reactor generally made of steel which can be heated from the outside, possibly in several zones regulated at different temperatures.
• a charge of reducing metal is introduced into the crucible in solid or liquid form, it is closed with a cover, the air is purged by evacuation and / or sweeping with a reducing or neutral gas, it is heated so as to bring the enclosure to the chosen reaction temperature, and bring or maintain the reducing metal in liquid form.
• UCl4 is then introduced, for example in gaseous form, which reacts with the molten reducing agent, U collects at the bottom of the crucible and / or along the walls in more or less agglomerated solid form; the liquid chloride of the reducing metal and the unreacted liquid reducing metal float above the uranium in two successive layers classified in order of their density; usually the layer of reducing agent is above and the liquid salt in contact with uranium. It is advantageous to regularly withdraw said liquid chloride, to increase the processing capacity of the crucible. At the end of the reaction, therefore, a more or less compact mass of uranium is obtained which is contaminated by inclusions of reducing metal and of salt (chloride) formed.
• To purify the U obtained from these inclusions it is possible either to heat the crucible under vacuum to distil the reducing metal, or to wash the uranium-bearing mass with acidulated water, after having extracted it from the reactor and possibly crushed, to remove the inclusions from the salt formed. It is also possible to effect a fusion, a decantation and a pouring of the uranium, previously extracted from the crucible, either before or preferably after distillation of the excess reducing agent.
• This fusion is carried out according to techniques known to those skilled in the art: for example induction furnace with electronic bombardment, crucible graphite coated with a refractory inert towards uranium, cold crucible ..., and the casting can be done in the form of ingot, wire, ribbon etc ... using all known methods of one skilled in the art.
• the reducing metal chloride by-product is preferably subjected to electrolysis to recover the chlorine and the reducing metal, respectively recycled in the first and in the second stage, according to methods well known to those skilled in the art.
• the process according to the invention therefore avoids the production of annoying by-products or effluents to be treated and eliminated, is economical and makes it possible to obtain a metal of a purity at least sufficient to be used in particular in an enrichment process isotopic by laser; If one starts from an oxidized compound of nuclear pure uranium, such as that obtained in conventional conversion processes, the quality obtained according to the invention is as follows: C ⁇ 50 ppm O ⁇ 200 ppm ⁇ Fe and transition metals ⁇ 250 ppm Cl ⁇ 20 ppm expressed by weight with respect to U the contents of the other impurities being lower than those present in the starting product.
• the reduction is carried out by electrolysis (first variant), it is possible to add to it a simultaneous electroraffinage by introducing into the bath at least one complementary electrode, cathodically polarized with respect to the main cathode where the crude uranium is deposited.
• This example illustrates the implementation of the invention according to the first variant, that is to say that after having transformed UO3 into UCl4, the metal is then obtained by electrolysis.
• the operation is carried out in a vertical pilot reactor made of silica glass with a diameter of 50 mm, a height of 800 mm, its outlet is equipped with a silica cloth filter, followed by a condenser by quenching on a wall cooled with water.
• the sort of nuclear nucleus pure uranium oxide is introduced, at a rate of 600 g / h, with the carbon in approximately stoichiometric quantity, in the form of a mixture of powders.
• the chlorine gas flow rate is 335 g / h.
• the temperature in the reaction zone is 980-1000 ° C, and the pressure slightly higher by a few millimeters of mercury at atmospheric pressure, filtration is carried out at 800 ° C.
• UCl4 is obtained at the rate of 789 g / h containing less than 2.5% by weight of UCl4 and UCl6.
• the residual gases, CO2, CO, Cl in excess are removed.
• - second step obtaining U metal by igneous electrolysis. .
• the bath temperature is 725-750 ° C and the cathode current density of 0.18 A / cm2.
• the electrolysis is carried out at 200 A and UCl4 is added continuously at the rate of 400 gU / h.
• the deposit is washed with acidulated water and then with pure water, and 8 kg of a metallic uranium powder are thus recovered, of which: 7.2 kg grain size> 0.85 mm 0.8 kg grain size ⁇ 0.85 mm
• the FARADAY cathodic yield is approximately 90%.
• the quality of the particle size fraction> 0.85 mm is as follows: C ⁇ 10 ppm WHERE 120 to 170 ppm Fe ⁇ 20 ppm Cr ⁇ 10 ppm Ni ⁇ 10 ppm other metals ⁇ 150 ppm Cl ⁇ 20 ppm
• This example illustrates the implementation of the invention according to the second variant, that is to say that after the transformation of UO3 into UCl4 the latter is reduced by metallothermy.
• - first step obtaining UCl4 It is carried out in an identical manner to that of Example 1.
• - second step the operation is carried out in a pilot reactor made up of an AISI 304 steel tube with a diameter of 150 mm and a useful height of 250 mm, supplied by a powder UCl4 distributor. This reactor can be put under vacuum for the purification operation, it is placed in a thermostatically controlled enclosure.

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• 1989
  • 1989-07-06 FR FR8909454A patent/FR2649417B1/fr not_active Expired - Fee Related
• 1990
  • 1990-07-03 DD DD90342472A patent/DD298001A5/de not_active IP Right Cessation
  • 1990-07-03 US US07/547,186 patent/US5164050A/en not_active Expired - Fee Related
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