RU2158973C2 - Method for recovery of uranium-containing compounds - Google Patents

Abstract

FIELD: recovery of radioactive wastes. SUBSTANCE: method is used for recovery of uranium-aluminum compounds to extract commercial uranium monoxide-oxide and refined metal aluminum from compounds. Uranium-aluminum compound is brought in contact with carbon component; stepped heating is conducted in vacuum including isothermal seasoning at melting point of intermetallic components; vacuum distillation of aluminum is made; uranium-containing component is transformed to high- melting carbide compounds; aluminum carbide is subjected to disintegration in the course of isothermal seasoning at its dissociation temperature; uranium-containing carbide compounds are cooled down, crushed, subjected to oxidizing roasting until carbon is fully extracted and uranium monoxide- oxide is produced and subjected to hydrometallurgic cleaning. EFFECT: improved efficiency and environmental safety.

Description

 The invention relates to methods for processing uranium-containing materials, namely to the processing of uranium-aluminum compositions.

These compositions at an aluminum concentration of 72 to 86 wt.% Consist of metallic aluminum and a number of intermetallic phases UAl ₂ , UAl ₃ , UAl ₄ . In addition, the compositions may contain a significant amount (up to several percent) of other aluminum compounds, for example, AlSi ₃ , Al ₂ Zr, Al ₂ O ₃ , due to impurities introduced into the composition by the starting reagents (uranium and aluminum) and technological components at the stage manufacturing compact products (N ₂ , O ₂ , Si, Fe, Zr, etc.).

 The task of processing uranium-aluminum compositions in order to extract uranium oxide-uranium from them is an urgent problem of developing an environmentally friendly and relatively inexpensive technological process for the extraction of components. Its solution will significantly reduce the amount of radiation and chemically hazardous uranium-aluminum waste to be disposed of.

 The chemical activity of aluminum does not allow using known methods for processing uranium-containing compositions to ensure its extraction in the form of pure metal without additional recovery and purification operations.

Known methods for hydrometallurgical processing of uranium-aluminum alloys are based on the dissolution of compositions in nitric acid and alkalis using mercury as a catalyst; the process of extraction or re-extraction using organic extractants; subsequent refinement of uranium from impurities using oxalate or peroxide refining; precipitation of uranium and getting oxide-oxide or dioxide as a finished product. (See "Nuclear fuel reprocessing" edited by S. Stolyar and R. Richard, Moscow, Atomizdat, 1964, pp. 63-67, 76-86; "Fuel reprocessing of power reactors" Collection of articles, Moscow, Atomizdat, 1972 g.)
A disadvantage of the known methods is that aluminum does not separate from uranium before extraction, which requires the preparation of large volumes of solutions (≈7000 l, per 1 kg of uranium) to ensure the required purity of uranium oxide-alumina in aluminum and other impurities. In addition, the waste from processing contains a lot of mercury, which causes corrosion and complicates the process of their disposal and leads to significant losses of uranium and the release of toxic substances into the environment, which determines the environmental hazard of production. There is a potential explosion hazard due to the presence of hydrogen and oxygen in the exhaust gas during processing. The chemical methods used do not allow the direct production of aluminum metal.

Closest to the claimed method according to the technical problem to be solved - the prototype - is a method of processing uranium-beryllium compositions, including heating until molten; vacuum distillation and condensation of beryllium; crystallization of the non-volatile residue and its calcination in an air atmosphere; processing of uranium oxide in nitric acid with heating to a boil; filtration of uranyl nitrate and its double peroxide precipitation at a pH of 1.5-2.0; moreover, after each deposition, the peroxide is fired in an atmosphere of air at a temperature of 750-800 ^o C for 2 hours, and the beryllium condensate is distilled in vacuum at a pressure of not higher than 1 • 10 ^-5 torr and a temperature of not higher than 1400-1500 ^o C (see patent Russia N2106029, MK G 21 C 19/44, C 01 G 43/01, C 22 B 35/00, dated 02.27.1998).

 The disadvantage of this method is that at the stage of the pyrometallurgical process technological impurities are not removed and up to 5 wt.% Beryllium remains, which are converted into oxides during the burning of the uranium-containing component, which leads to peroxide deposition for the final purification of uranium oxide. In addition, the uranium content to 0.01 wt.% Requires vacuum distillation to reduce the uranium content in the resulting beryllium to an acceptable level.

 The authors' task is to develop a technological process for the processing of uranium-aluminum compositions, which ensures the achievement of the purpose of the invention - effective and environmentally safe extraction of commercial uranium oxide and refined metal aluminum from the compositions.

 This goal is achieved in contrast to the known method in that the uranium-aluminum composition is brought into contact with the carbon component; conduct stepwise heating in vacuum with isothermal extracts at the melting points of the intermetallic compounds; conduct vacuum distillation of aluminum; the uranium-containing component is converted to refractory carbide compounds; carry out the decomposition of aluminum carbide during isothermal exposure at the temperature of its dissociation; uranium carbide compounds are cooled; crushed; conduct oxidative roasting until carbon is completely removed and uranium oxide is formed; and non-evaporated process impurities are removed by hydrometallurgical treatment of the resulting product.

 The essence of the proposed method is as follows.

The contact of uranium-aluminum alloys with a carbon component (carbon black, graphite, carbon fiber) is necessary in order to bind uranium into a carbide compound with a melting point 1700-1800 ^o C higher than the melting point of aluminum already at the stage of melting of the intermetallic compound.

 At the stage of pyrometallurgical processing (distillation of aluminum in vacuum), efficient separation of uranium-aluminum alloys into components that differ in vapor elasticity by 10-12 orders of magnitude is ensured, which makes it possible to selectively select metallic aluminum, which is in the form of intermetallic compounds and metal.

The process of formation of uranium-containing carbides is accompanied by an exothermic effect, which can spontaneously increase the process temperature by 1000-1200 ^o C per mole of the initial uranium-aluminum composition. Such an increase in temperature can lead to the destruction of equipment and the evaporation of uranium not bound in a carbide compound. To eliminate this effect, the process is carried out stepwise with isothermal holdings at the melting temperatures of uranium intermetallic compounds, during which the melt is carbidized without an avalanche-like development of the process.

The first isothermal exposure is carried out at a melting point UAl ₄ (≈750 ^o C), the second at melting UAl ₃ (≈1350-1400 ^o C) and the third at a melting point UAl ₂ (≈1600 ^o C). It was experimentally established that holding for 5-10 minutes at the melting points of the intermetallic compounds is sufficient to complete the carbidization process and that the temperature rises by no more than 50-100 ^o C from the melting point of the corresponding intermetallic compound. The heat released from the formation of carbides is effectively spent on the evaporation of aluminum. During the rise in temperature and isothermal holdings, part of the aluminum manages to form carbides of variable composition (Al ₃ C ₄ , AlC ₂ ), the decomposition temperature of which lies in the range of 2150-2200 ^o C. Therefore, further heating after decomposition of uranium intermetallic compounds is necessary to clean the formed uranium-containing carbides from aluminum carbides.

It was experimentally established that the aluminum content in uranium-containing carbides does not exceed 3 • 10 ^-4 wt.%. The concentration of uranium in aluminum condensate is not more than 2 • 10 ^-3 wt.%.

Uranium carbides (UC ₂ , UC) are usually in the form of sintered conglomerates and contain (up to several percent) free carbon. To remove carbon and transfer uranium to oxide compounds, oxidative roasting of carbides pre-crushed to a particle size of ≈ 100-200 μm is carried out. Firing is carried out in air at a temperature of active carbon oxidation in carbide compounds (600-800 ^o C) until complete removal of carbon. The process is completed after the cessation of CO emission determined by the chromatographic method.

 Hydrometallurgical purification of the obtained uranium oxide is carried out to remove impurities remaining in the material from additives in the initial uranium-aluminum composition of elements and their carbides having low vapor pressure, which cannot be removed in the temperature range of the pyrometallurgical process.

 An example implementation of the method.

 1. Pyrometallurgical processing of uranium-aluminum composition.

 Download from a uranium-aluminum composition containing 13.2 wt.% Uranium, 86.0 wt. % aluminum, 0.4 wt.% zirconium, 0.4 wt.% silicon were placed on a graphite fabric and placed in a graphite crucible, which was installed in a vacuum furnace equipped with an ampoule device and a vapor condenser, which prevented the evaporation of the evaporated components from entering the internal volume of the furnace.

Heating of the load was started when the vacuum in the furnace volume reached at least 1 • 10 ^-4 Torr and was maintained during the whole process. The rate of temperature rise during the entire process was maintained as high as possible for the design of the furnace, which was regulated only by the inertia of the load and amounted to ≈ 2000 deg./h.

The first exothermic effect, a spontaneous rise in temperature by 100-150 ^o C, occurred in the temperature range of 700-800 ^o C and disappeared when the furnace was kept at constant power after 5 minutes. The second exothermic effect was observed in the range of 1300-1400 ^o C. The temperature spontaneously rose to 200-250 ^o C and recovered after 7 minutes of exposure. The third exothermic peak was manifested in the range of 1500-1600 ^o C with a spontaneous rise in temperature by 250-300 ^o C. When holding for 10 minutes, the set process temperature was set. With a subsequent rise in temperature to 2150-2200 ^o C, exothermic effects were not observed, since the uranium-containing component turned into carbide compounds with a melting point of 2450-2500 ^o C.

In the process of raising the temperature to the decomposition point of aluminum carbide, a vacuum drop to 1 • 10 ^-2 Torr was observed. Upon reaching the decomposition temperature of aluminum carbide, the vacuum was restored to 1 • 10 ^-4 Torr after holding for 1.5-2.0 hours.

The sintered uranium-containing conglomerate and aluminum condensate obtained as a result of thermal annealing were analyzed by spectral and X-ray diffraction methods. The conglomerate consisted of carbides UC ₂ , UC, (Zr _0.7 U _0.3 ) C and free carbon. The aluminum content was 1.5 • 10 ^-4 wt. %, silicon - 3 • 10 ^-4 wt.%. The precipitate in the condenser consisted of refined aluminum metal with a content in the surface layer (≈ 40-50 microns) of 2 • 10 ^-3 wt.% uranium, and X-ray measurements showed the values found in the limits of the natural background.

 2. Processing of uranium carbide conglomerate.

The sintered conglomerate of uranium-containing carbides was crushed on a hydraulic press in trichlorethylene medium to a particle size of 100-200 microns. The powder was loaded into a rotating quartz tube and heated in air to the temperature of active carbon oxidation in carbide compounds (≈700 ^o C). The resulting gas (CO) was removed from the interaction zone through a filter system. The process was interrupted after completion of the CO. The duration of the process is regulated by the amount of oxidizable material and did not exceed 3-4 hours.

The material obtained as a result of oxidative annealing consisted of uranium oxide (U ₃ O ₈ ) ≈90.8 wt.% And ≈9.2 wt.% Of uranium-zirconium spinel composition (U _0.3 Zr _0.7 ) O ₂ . The zirconium content was 3.5 wt%. Purification of uranium oxide by the well-known hydrometallurgical method used in production using nitric, sulfuric acids and an aqueous solution of ammonia with a total consumption of 8.7 L per kilogram of uranium made it possible to obtain uranium dioxide with a total impurity content of 0.083 wt.%, Including the number of 0.005 wt.% zirconium.

 Practical implementation of the proposed method shows that the proposed solution allows you to efficiently process uranium-aluminum compositions into commercial oxide-uranium oxide and refined metal aluminum, which cannot be obtained by known methods. The inventive method is carried out on standard technological equipment with safety measures for staff and the environment, is easily controlled, does not require distillation of aluminum condensate, and the use of chemicals is 2-3 orders of magnitude less than with the hydrometallurgical method.

Claims (1)

 A method of processing uranium-containing compositions, including heating before melting, vacuum distillation and subsequent hydrometallurgical treatment, characterized in that when processing uranium-aluminum compositions they are brought into contact with the carbon component, stepwise heating in vacuum with isothermal holdings at the melting temperatures of uranium-containing intermetallic compounds is carried out, vacuum distillation of aluminum, transfer the uranium-containing component to a refractory carbide compound, carry out the decomposition of aluminum carbide During isothermal holding at a temperature of dissociation, uranium-containing carbide compound is cooled, pulverized, oxidizing roasting is carried out until complete removal of carbon and the formation of the mixed oxide of uranium and carried hydrometallurgical treatment.