Patent GB-812794-A (GB)
Title: Method of recovering uranium
IPC: C01G43/08
Filed: 1945-06-27
Abstract: Uranium is recovered from a uranium containing deposit on a metallic iron, chromium, nickel or copper surface by bringing the uranium values into solution, preferably with acid and thereafter precipitating the uranium <PICT:0812794/III/1> <PICT:0812794/III/2> as uranous oxalate at a hydrogen ion concentration equivalent to 1-3 normal.  The invention is particularly applicable to the removal of uranium and its compounds from parts of a calutron described below where a portion of the uranium has been enriched in U235.  The uranous oxalate so precipitated may be converted to the oxide by heating and if desired recycled to the calutron as the tetrachloride. The calutron consists of a means for vaporizing a quantity of an uranium compound for example uranium tetrachloride and subjecting such vapour to ionization whereby ions of the several isotopes of uranium are produced, which ions are segregated from the unionized vapour, accelerated at high velocities and deflected by an electromagnetic field along a curved path, the radius of curvature being proportional to the square root of the masses of the ions, whereby the ions are concentrated in accordance with their masses. Such a device is described in U.S.A.  Specification 2,709,222.  As shown in Fig. 1, a calutron comprises upper and lower polepieces 11 and 12 which produce a uniform and strong magnetic field therebetween. Between the polepieces is placed a D-shaped tank 13 having end walls 18 and 19 and <PICT:0812794/III/3> <PICT:0812794/III/4> <PICT:0812794/III/5> <PICT:0812794/III/6> containing a spaced liner 41.  The tank 13 is evacuated to a pressure of 10-5 to 10-4 mm. of mercury.  The removable end wall 18 carries an insulator 20 supporting a charge block 21 which is provided with a cavity 22 for receiving the charge of material to be treated.  The charge block 21, heated by embedded heating elements 23 has attached thereto a tubular member 24 supporting a carbon or graphite arc block 25.  The arc block 25 contains a vertical slot 26 in the wall thereof removed from the charge block 21 and has a centra arc cavity 27, communicating through the tube 24 with the cavity in the charge block 21.  A filamentary cathode 30 is aligned with respect to the upper end of the cavity 27 in the arc block 25 and similarly an anode 31 is arranged below.  The end wall 18 also carries a pair of upstanding plates 32 on an insulating support, which act as an ion accelerating structure when a voltage is applied between the arc block 25 and the plates 32.  In operation a charge of uranium tetrachloride is placed in the charge pocket 22 whereby it is heated and vaporized, the vapour passing through the tube 24 to the cavity 27 of the arc block 25.  The cathode is rendered electron emissive; the circuit between the filamentary cathode 30 and the anode 31 is then closed whereby an arc discharge is struck between them, electrons proceeding from the filamentary cathode 30 to the anode 31.  These electrons break up the molecular form of the uranium tetrachloride producing positive uranium ions.  The electric circuit between the arc block 25 and the ion accelerating structure made up of plates 32 is then completed, plates 32 being at a high negative potential with respect to the arc block 25, whereby positive ions are attracted and accelerated by the impressed voltage, such ions proceeding from the interior of the cavity 27 through the block 26 and through the slit 33 formed between the plates 32 and into the interior of the D-shaped liner 41 in tank 13, where they are subjected to the strong applied magnetic field.  A collection device 35 supported by the other end wall 19 consists of a block of stainless steel containing two spaced cavities 36 and 37 with aligned slots 38 and 39.  After a suitable number of charges have been vaporized, deposits of the uranium isotopes are formed in pockets 36 and 37 in the collector block and these deposits are treated according to the invention.  The uranium metal deposited in pocket 36 is enriched with U235 whilst U238 is collected in pocket 37.  The uranium in pocket 36 is also considerably enriched in U234. The majority of the uranium tetrachloride vapour is however unionized in the cavity 27 and is largely condensed upon part of the calutron near the source region, particularly on the interior of the lining 41 and the exterior surfaces of the arc block and charge block.  In addition a minor fraction of the uranium tetrachloride is ionized to form positive atomic ions such as U+, U++, Cl+ and Cl++; and positive molecular ions including Cl2+, Cl2++, UCl4+, UCl4++, UCl3+, UCl3++, UCl2+, UCl2++, UCl+, UCl++.  Of these only the singly ionized U+ ions are focused through the slot into the pockets 36 and 37 of the collector block, the other ions being deflected in arcs of shorter or greater radius to form deposits on various parts of the inside of the lining 41.  The calutrons are suitably employed in series whereby a first stage calutron gives an enrichment factor of 20 in respect of U235 and a second stage calutron is fed with the enriched product from the first calutron whereby an enrichment factor of 20 gives a final enrichment factor of about 400.  Thus the U235 collected in pocket 36 is recovered and after conversion to the tetrachloride is treated in a second stage calutron whereby a deposit of uranium in pocket 36 of the second calutron is doubly enriched in U235.  The metallic uranium deposited in the collectors of the calutrons is recovered by washing with hydrochloric acid containing hydrogen peroxide, the wash waters from each of the collectors being kept separate; they then may be separatedly concentrated by evaporation if desired, as illustrated in Fig. 3, the water vapour driven off by evaporation being condensed and employed with further acid for the dissolution of subsequent deposits.  Concentrated solutions containing uranyl ion together with iron, chromium and nickel in the higher valent state result from this treatment.  At the same time the parts of the calutron near the source region thereof are scrubbed and washed with hot water thereby taking up the uranium tetrachloride and the copper, iron, chromium and nickel impurities.  These wash solutions from the first and second calutron are kept separate and after filtering, are each treated with an oxidizing agent such as hydrogen peroxide, filtered and evaporated.  The evaporated water is condensed and re-employed to scrub parts of the calutron in subsequent operations (see Fig. 2).  A concentrated solution containing uranyl ions and the impurities in the higher valent state results.  The various concentrated wash solutions are then mixed for convenience of treatment.  Thus the wash water from the source region of the first calutron and the acid wash from the second pocket of the collector of the second stage calutron are mixed to produce a first composite solution which has a natural or normal content of U235. Also the wash water from the source region of the second stage calutron and the acid wash from the first pocket of the collector of the first stage calutron are mixed to produce a second composite solution which is singly enriched with U235.  Finally, the acid wash solution derived from the first pocket of the collector of the second stage calutron constitutes a third solution which is doubly enriched in U235.  These composite solutions are individually treated as illustrated in Fig. 4.  They are first acidified if necessary with hydrochloric acid to render them 1 to 3 normal in hydrogen ion concentration and then reduced to convert the uranyl iron to the uranous ion and the ferric ion to the ferrous ion respectively. Such reduction may be effected in an electrolytic cell as described in U.S.A.  Specification 2,743,228, where the acidified solution is treated in an electrolytic cell having an amalgamated nickel cathode, a graphite anode and a porous alundum barrier separating the two. The reduced solution is then treated with excess of oxalic acid thereby selectively precipitating uranous oxalate, the acid solution being 1 to 3 normal in hydrogen ion during the precipitation.  The uranous oxalate is then calcined to convert it to UO2.  Three batches of UO2 will result from the three composite wash solutions, the first being of normal U235 content, the second being singly enriched in U235 and the third batch being doubly enriched in U235.  The latter batch is available for commercial use whilst the first and second batches of UO2 are converted back to UCl4 for retreatment in the first and second stage calutrons respectively, such conversion being illustrated in Fig. 5, where the UO2 is reacted at about 450 DEG  C. with carbon tetrachloride vapour, and the crude UCl4 produced is sublimed at 600 DEG  C. to produce pure UCl4 which is recycled to the appropriate first or second stage calutron.  In order to conserve any uranium which may be lost in any of the precipitates and filtrates produced in the above described process, the precipitate or filtrate is dissolved in or acidified by hydrochloric acid to produce an acidified salvage solution, the various salvage solutions produced being maintained separate unless their content of U235 is approximately the same. They are then treated as shown in Fig. 6. Each of the salvage solutions is first reduced, for example electrolytically as above described, and thereafter a quantity of water soluble alkaline earth metal salts such as calcium chloride is added whereupon on the addition of excess oxalic acid the uranium oxalate is precipitated in the presence of a calcium oxalate carrier. Again the hydrogen ion content is about 1 to 3 normal during and after the precipitation.  The composite oxalate precipitate is then calcined at about 500 DEG  C. in the non-oxidizing atmosphere to produce CaO and UO2, the oxides then can be dissolved in 6 normal hydrochloric acid.  The solution is then prec