I
Interested in any of the chemical aspects of nuclear technology? I/EC starts with the 46 pages following the first of a three-part group of articles. This first group deals with the production and processing of uranium ores and preparation and recycle of feed materials for nuclear reactors Watch for the January 1959 issue for aqueous methods of reprocessing -irradiated fuels; the chemistry of heavy elements February 1959: volatility meihods of reprocessing irradiated fuels; fuel preparation and recycle by pyrometallurgical methods; uses of radiation in industrial chemical reactions
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A. A. JONKE, N. M. LEVITZ, ALBIN LITTY, and STEPHEN LAWROSKI Argonne National Laboratory, Lemont, 111.
Fluidization Techniques in Producing Refined Uranium from Ore Concentrates The fluid-bed, single-stage reactor units successfully processed materials of widely varying properties to uranium hexafluoride
A for refining uranium, based on fractional distillation of uranium PROCESS
hexafluoride, offers several advantages over the solvent extraction method (7). However, direct fluorination of uranium ore concentrates to uranium hexafluoride is not considered economically feasible because gangue and other impurities consume large amounts of fluorine. Therefore, a new process was developed (Figure 1) where uranium ore concentrates are converted to impure uranium hexafluoride by hydrogen-reduction, hydrofluorination, and fluorination, and then purified by fractional distillation (2). The development of the reduction and hydrofluorination steps of the refining process is described herein ; the fluorination and disrillation steps have been studied by other workers (5,6). In developing the process, a variety of concentrates representative of those produced in various plants were studied-six samples were selected (Table I). Coarse materials were crushed and screened to about 30 to 200 mesh. Fine powders were agglomerated by two methods-pressing in a standard briquetting machine into almond-shaped briquets and extruding dampened powder in a standard pelleting machine-then crushed and screened to size.
Fluidized-bed reactors, both in this laboratory and in production plants, have proved superior, particularly for heat transfer and temperature control ( 3 ) . Continuous agitation provided by fluidization helps prevent caking in the bed. Bench-Scale Experiments Bench-scale reactors for preparation of the crude uranium tetrafluoride intermediate were constructed of 3- or 6-inch diameter pipe with an enlarged section containing filters at the top (Figure 2). The reactant gas after being preheated entered at the bottom and flowed upward through a porous metal gas distributor and through the bed of granular ore concentrate producing fluidization. En-
trained dust particles were removed from the off-gas by two porous metal filters, one being in use while the other was being cleaned by reverse gas flow. Heat was transferred to the bed by tubular electric heaters on the outer wall, controlled by variable transformers. The bed temperature was measured by thermocouples and the bed depth was determined by pressure drop measurements. For continuous operation, solids were metered by a feed screw into the top of the reactor, and product was periodically withdrawn from the bottom through a plug valve into a hopper. Two units of this type were available-one 3 inches in diameter made of Monel and nickel, and one 6 inches made of stainless steel.
1 Reprints of any one group of these articles may be purchased at $1 .OO for single copies or $0.75 each, in lots of ten or more. Address Special Issue Sales Departmsnt, American Chemical Society, 1155 16th St., N.W., Washington 6, D. C.
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\
Figure 1. New uranium ore refining process uses fluidization and fractional distillation techniques so successfully that a full-scale plant for the new process is now under construction VOL. 50, NO. 12
DECEMBER 1958
1739
The Monel unit showed appreciable corrosion during the hydrogen reduction step. Particles of scale in the product were spectrochemically identified as copper and nickel, which flaked off the ivall of the reactor. Impurities in the ore concentrates included sulfate and nitrate ions, both corrosive to Monel; therefore, the stainless steel reactor was used for the reduction step and the Monel reactor for the hydrofluorination step. Batch Runs. To determine whether adequate chemical conversion could be achieved with a variety of ore concentrates, batch experimenrs were performed in which six different ore concentrates were hydrogen-reduced for 4 hours and hydrofluorinated for another 4 hollrs (Table 11). The percentage reduction of the uranium from the sexivalent to the quadrivalent state ranged from 82 to 98%. This \vas satisfactory because complete reduction is not essential. The sexivalent uranium compounds (for example, uranyl fluoride) are readily fluorinated to uranium hexafluoride but consume more fluorine than does uranium tetrafluoride. However, small amounts of unreduced uranium do not materially affect fluorine economy. Batch hydrofluorination of the reduced ore concentrates was carried out using about twice the stoichiometric amount of hydrogen fluoride required to convert the uranium and impurities to fluorides (Table 11). The fluoride content of the hydrofluorinated concentrates ranged from 21 to 25%. The maximum possible fluoride content is dependent upon the impurity level, especially the sodium content. The theoretical fluoride for pure uranium tetrafluoride is 24.2% and for the compound 7NaF.6UF4 (produced from sodium diuranate) is 27.0. All the ore concentrates studied could be converted rapidly to crude uranium tetrafluoride without caking or other difficulties. One of the major impurities in many ore concentrates is silica. Because of its low atomic weight, small amounts of silicon provide a high atomic ratio of silicon to uranium-for example, an ore concentrate having 5y0 by weight of silica would have an atomic ratio of silicon to uranium of about 1 to 3. The hydrofluorination step in the refining process is needed to remove silica. which would consume large amounts of fluorine in any direct fluorination process. The removal of silica as silicon tetrafluoride was essentially quantitative in all hydrofluorination runs. Continuous Runs. Continuous runs were made with some ore concentrates in the same equipment. Results show an average residence time of at least 3.5 hours required for the solids in the singlestage reacror to achieve 90 to 99% reduction. Results obtained in continuous hydrofluorination runs showed that these ore concentrates were exceedingly re-
BLOWBAGK N2 OFF- GAS
-
g
, I/
I
/.--FEED
GAGE GLASS
PURGE
-NZ
POROUS MONEL FILTERS SOLIDS METER
FEED INLET BAFFLE PRESSURE TAP AND THERMOCOUPLE WEL CALROD HEATERS FLUIDIZED BED
GAS DISTRIBUTOR PLATE GAS PREHEATER
NP SIGHT GLASS
HZ
HF
Figure 2. Preliminary studies were done in bench-scale, fluid-bed reactors, which rapidly converted ore concentrates to crude uranium tetrafluoride without caking oxides where high initial reaction temperatures were detrimental to conversion owing to surface sintering of the solid particles. Sintering is believed to result frcm the heat evolved in the reaction. Sodium impurity aggravates heat damage due to its effect in lowering the melting point of uranium tetrafluoride. In some of the runs, the hydrogen
active toward hydrofluorination under the proper operating conditions. Average solids residence times of 2 to 4 hours were adequate for over 90y0 conversion at 450" C. A higher hydrofluorination temperature (550"C.) resulted in greatly reduced conversion. This temperature effect is consistent with experience in the hydrofluorination of refined uranium
Table I. Samde 1 2 3 4
5 6 5
1 740 INDUSTRIAL AND ENGINEERING CHEMISTRY
Source and Nature of Various Uranium Concentrates Studied for Process Demonstration NatiiIe of Ore Concentration Concentrates Process Source Acid leach, ion exSouth Africa UO?,U3Os Anaconda Acid Leach plant, Bluewater, N. M. Anaconda Carbonate Leach plant, Bluewater, N. M. Vanadium Corp. of America, Durango, Colo. Union Carbide Nuclear Co., Uravan, Colo. Union Carbide Nuclear Co., Rifle, Colo.
Precipitant, NHIOH.
changea Acid leach, ion exchange" Carbonate leachb Salt roast, carbonate leach, fusion Salt roast, acid leach, ion exchangea Salt roast, acid leach5
* Precipitant, NaOH.
u o s , U,Os NadJ~07 Black oxide, indefinite composition Na2U207, UdOs N ~ z U I OUSOS ~,
NUCLEAR TECHNOLOGY BLOWBPCK
fluoride excess was only 25 to 30% over the stoichiometric amount required for uranium and impurities. Results obtained with these small excesses indicate that it may be feasible to neutralize the off-gas and discard excess hydrogen fluoride rather than to attempt recovery. In both the reduction and the hydrofluorination runs, the bed remained uncaked and the product was free flowing. A slight reduction in particle size indicated that the amount of attrition was small.
w
4. 1.
Pilot-Plant Operations
STAINLESS STEEL
Further demonstration of the process was carried out on a pilot scale using equipment which was originally installed for work on refined uranium oxides. The pilot plant consisted of two multistage, fluidized-bed reactors together with auxiliaries such as feeders, filters, and condensers. The two reactors were of the same basic design but differed in diameter and materials of construction, the stainless steel reduction reactor being 5 inches in diameter with four stages and the Monel hydrofluorination reactor being 6 inches in diameter with five stages. A schematic diagram of the pilot plant is shown in Figure 3. Each stage contained a fluidized bed 10 inches deep supported by a perforated plate. A downcomer tube extended from the top of each bed to the bottom of the next lower stage. As shown in Figure 4, the support plate contained conical holes to eliminate flat surfaces on which stagnant solids might collect and sinter during the highly exothermic reactions. To prevent draining of solids through the perforations in the support plates during shutdown a very small flow of nitrogen gas was maintained through the reactor. Heat was supplied to each stage by three 1000-watt tubular heaters bonded to the reactor wall by sprayed copper metal. In addition, the top stage of each reactor was provided with a cooling coil,
Table II.
Sample No 1 6 5 2 3 4
uq
HOPPER
UF, PROWCT HDPPER I
Figure 3. Pilot plant used two multistage reactors in series, achieving greater conversion in a shorter residence time
also bonded by the sprayed copper metal, to remove excess heat of reaction. Thermocouples were located in each fluidized bed and at other points to measure heater and wall temperatures. Pressure taps connected to manometers indicated amount of material on each stage. I n operating the pilot planr, ore concentrate was screw-fed from a hopper to a gas lift conveyor line having an outside diameter of 3/* inch, and carried to the top stage of the reduction reactor. The
Batch Fluidized Reduction and Hydrofluorination Show Satisfactory Conversion for Various Uranium Concentrates U in Reduction Process, %* Hydroduorinationb Total Totd UnconIn In Product reduofluoride, verted U, feed Total Reducedd tion % %e 69 66 65 73 63 71
'
81 73 82 85 70 72
73 72 71 78 58 64
90 98 87 92 a2 88
23 23 25 21 25 24
1.4 7.0 8.1
2.3 1.8
..
Operating conditions: 50% HI plus 50% Ne at 0.4 t o 0.6 foot per second for 4 hours: bed temperature. 575O C.: bed depth (static), 6 to 21 inches: 6-inch single-stage reactor. Operating conditions: 70% HF plus 30% NI at 0.4 to 0.6 foot per second for 4 hours: bed temperature, 450° C.; bed depth (static), 12 to 19 inches; 3-inch Monel reactor. See Table I for sample source and type. Reduced U is quadrivalent. 0 Determined from uranium content of ammonium oxalate insoluble fraction.
solids flowed downward by overflow from each stage leaving a t the bottom through a sight glass. They were then moved by another screw, and gas lifted to the top stage of the hydrofluorination reactor. The final product, crude uranium tetrafluoride, was collected at the bottom of the hydrofluorinator. The gas flow was countercurrent to the solids flow. For reduction, a preheated hydrogen-nitrogen mixture flowed upward fluidizing each bed, and after leaving, the reactor passed to a chamber containing four sintered, stainless steel bayonet-type filter elements where entrained solids were removed. The gas passed through a condenser and air jet before going to the ventilation system. For hydrofluorination, liquid anhydrous hydrogen fluoride was cooled to about 5' C. in a refrigerated box and metered by a pump to a preheater where it was vaporized before entering the bottom of the reactor. The off-gas passed through porous carbon filter elements, then through a condenser and caustic scrubber. The rate of flow of solids was determined by volume rates measured in sight glasses at the base of the ore feed hopper and at the base of the reduction column as well as by weight rate of the green salt VOL. 50, NO. 12
DECEMBER 1958
1741
product. The rate was adjusted by varying the feed screw speed. Operating Conditions. Three ore concentrates were tested in the pilot plant, two of the acid-leach type and one of the carbonate-leach type ( 4 ) . In the reduction reactor, all stages except the top stage were maintained at a temperature of 575 ' C. The top reduction stage acted as a dryer and solids preheater, and because of the large amount of heat required for evaporation of water and other volatiles, the top stage temperature was generally lower than the other stages. The inlet gas had a hydrogen-nitrogen ratio near 3, simulating cracked ammonia, which is used as a source of hydrogen in most uranium refineries. The solids feed rate ranged from 11 to 35 pounds per hour with corresponding average residence times ranging from 0.9 to 7 hours. In the hydrofluorination reactor, the bed temperatures were graded from 350' C. at the top to 600' C. a t the bottom stage. A low initial reaction temperature was used to avoid sintering of the solids and to decrease the effect of reverse reaction of water vapor with uranium tetrafluoride. Inlet gas velocities were similar to those in the reduction column (0.3 to 0.6 foot per second). The hydrogen fluoride flow rate ranged from 1.4 to 2.7 times the stoichiometric requirement for uranium and impurities. As the columns were in series and operated simultaneously the feed rate for the reduction column determined the feed rate to the hydrofluorinator. The average residence time of the solids in this column ranged from 2.3 to 10 hours. Chemical Conversion. The results of the reduction and hydrofluorination runs on the three types of ore concentrates are summarized in Table 111. In all of the runs, high conversion to crude uranium tetrafluoride was achieved even with solids residence time as low as 3 hours for both steps. At the highest feed rate employed, the throughput amounts to 80 pounds per hour per cubic foot of reduction bed volume and 42 pounds of hydrofluorination bed volume.
The analyses of stage samples taken at the end of these runs showed that the hydrofluorination was essentially completed in the first two stages of the reactor (Table IV). This was also true for the reduction reactor in most cases. These data confirmed the conclusions reached in the bench-scale experiments, that the ore concentrates are highly reactive. Particle Size Effects. The change in particle size of the ore concentrates was studied to determine the effect of fluidization or chemical reaction on particle decrepitation. As some of the ores had been agglomerated and then recrushed, it was important to know whether the particles had sufficient strength to maintain their size for the duration of the processing period. Excessive production of fines was undesirable because they reduce the quality of fluidization and/or were entrained over into the filter section, requiring extra handling. Initially some difficulty was encountered with carbonate leach ore concentrate due to rapid degradation of this material to fines after entering the reduction reactor. This was attributed to the flash drying of the residual moisture added in the pelleting process. This difficulty with carbonate leach material was corrected by predrying and screening to remove the -200-mesh fraction before the material was fed to the column. The sieve analyses of the material ar. various stages in the process (Table V) showed only a slight over-all decrease in size in going from the ore concentrate to the crude tetrafluoride. With some materials, the upper stages contained a larger percentage of fines than did the lower stages. This indicated a tendency for the fines to be entrained by the gas and accumulate in the upper stages of the
Table 111.
Sample NO.^ 2 2 3 1 1
reactor. However, only a moderate amount of such accumulation occurred. Corrosion Testing. To obtain preliminary corrosion data on materials of construction for the processing of uranium ore concentrates to crude green salts, metal coupons were placed in each bed of each reactor. Those tested in the reduction column were 304, 309, and 347 stainless steel. In the hydrofluorinator, Monel, Inconel, and Hastalloy B were used. After about 80 hours of exposure under process conditions, one complete set of samples was removed and examined. Corrosion of the stainless steels tested was low and about equal for all types, the highest being 0.053 mil for the 304 coupon in stage 2, and the majority being below 0.030 mil for the exposure period of 80 hours. The only gross physical change was a darker appearance for all the coupons in stage 1. The coupons in the hydrofluorinator definitely showed greater corrosion in the upper stages than in the lower. This may be due to the presence of higher water concentration and/or higher gaseous impurity concentrations in these stages. Corrosion rates were low (about 0.05 mil) for all three metals in the two lowest stages-where there was little or no reaction-even though the temperatures were the highest. In the top stage, Inconel and Hastalloy B were definitely superior to Monel. Operability of Fluid-Bed Reactors. The degree of trouble-free performance achieved in the fluidized-bed reactors varied with rhe type of equipment used and the type of ore concentrate processed. Acid-leached ore concentrates, which had been size-pretreated by pelleting, crushing, and screening, were processed without operating difficulties. With
Continuous Pilot-Plant Runs Show Ore ConcentratesAre Easily Converted to Crude Uranium Tetrafluoride" Product Analyses, % Solids Av. Res. Time, Hr. Feed Rate, Lb./Hr. 11.0 15.5 12.6 35.3 24.0
Hydrofluorination
Reduction 7
WaterTotal Reduced soluble U 72 72 65 71 72
10
5 6 0.9 1.3
8
7 2.3 3.7
U 70 70 56 70 71
uc
0.9 0.8 9.5 2.0
1.5
UOn 1.0 0.3 2.6 2.5 1.5
Total fluoride 25 25 24 25 25
See Table I for a Reduction temp. 5 7 5 O C., hydrofluorination temp. 350' to 600' C. sample source and type. 0 Water soluble uranium is a measure of the U02F2 content.
Table IV. Analyses Show Hydrofluorination Is Complete in First Two Stages of Five-Stage Reactor Sample
Figure 4. Bed-support plate and downcomer-pipe Conical holes improve distribution of fluidizing
NO.^ 2 3 1
15.5 12.6 35.3
Av. Solids Res. Time,
Hr.
8
7 2.3
See Table I for sample source and type. oxalate-insoluble fraction. a
gas
1742
Feed Rate. Lb./Hr.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Stage 1
0.3 13 3.7
Unconverted UOt, %& Stage Stage Stage 2 0.2 5.4 2.4
3 0.2 5.2 1.4
4
0.3 4.2
..
Stage 5
0.3 3.9 3.5
Determined from uranium content of ammonium
NUCLEAR TECHNOLOGY carbonate-leached ore concentrates, which have a high sodium content, it was necessary to use care in start-up to avoid rapid initial reaction. When the heat of reaction was not dissipated from the bed, caking resulted because of the relatively low sintering temperature of this material. However, caking was encountered in the fluid-bed reactors only when hydrogen fluoride was fed too rapidly into a fresh bed of reduced ore concentrate. During continuous operation there was no caking or sintering in either reduction or hydrofluorination reactors. Some of the ore concentrates, particularly the South African material, had very low bulk density and poor flow properties. This material was dificult to feed and convey at uniform flow rates, and occasional interruptions of the runs were necessary to unblock the feed system. This difficulty was encountered only with concentrates which were received from the mill in pellet form and reduced to size by crushing. A more satisfactory feed could be prepared by crushing these pellets then repelleting as was done with the other concentrates. The multistage reactors operated well on feeds which had only a small percent-
Table V.
I n Table V I are given the chemical analyses of the ore concentrates and crude uranium tetrafluoride for certain impurities which are affected by the reduction and hydrofluorination treatment.
mesh
Sieve Analyses, % 60 to 200 mesh
Ore concentrate Reduced product Hydrofluorinated product
46 44 32
32 33 52
22 23
Ore concentrate Reduced product Hydrofluorinated product
32 21
42 52
60
24 27 31
Ore concentrate Reduced product Hydrofluorinated product
25 33 14
69 56 66
6 11 19
Processing Status
1
3
Purification Effected by Reduction and Hydrofluorination
Particle Decrepitation during Processing Is Slight
Sample No."
2
age of fines initially, and which did not break u p into fines during processing. The multistage reactors reduced bypassing of incompletely converted solids, and therefore achieved greater conversion in a shorter residence time. However, as the residence time required for conversion of the ore concentrates was short, even in the single-stage units, the additional complexity of the four and five stages was hardly warranted. Nevertheless the use of two- or thrqe-stage reactors appears worthy of consideration. In general, the performance of the fluid-bed reactors was highly acceptable, and considerably superior to other types of gas-solid reactors in general use. The flexibility of the fluid-bed units for processing materials of widely varying properties was an important advantage.
.
zk 60
a
-200 mesh
Acknowledgment
Analyses were performed by the analytical section under R. P. Larsen and L. E. Ross. Evaluaton of corrosion samples was done by W. B. Seefeldt.
16
literature Cited
See Table I for sample source and type.
Table
VI.
Sample N0.b 1 2 3 4 5 6
Impurity Removal Effected by Reduction and HydrofluorinationVolatile Compounds Are Separated from Uranium' Impurity Concentration, P.P.M.
Status Feed Product Feed Product Feed Product Feed Product Feed Product Feed Product
S
B
aoo
10
280 7,800 70 600 35 7,500 600 64,000 600 24,000 2,000
