I
F. Z. POLLARA, N. LEVINE, J. R. KILLELEA, R. C. MUSA, and M. D, HASSlALlS Mineral Beneficiation Laboratory, Columbia University, New York 27, N. Y.
Recovery of Uranium from Chattanooga Shale Two processes are available for extracting uranium from Chattanooga shale, should the need arise. Choice of process would depend on relative costs of sulfur for acid production vs. power for oxygen production
PRIOR
to the recent, extensive uranium discoveries, the U. S. Atomic Energy Commission provided a program for recovering uranium from Chattanooga shale. This formation, which extends through Tennessee and its border states, is one of the world's largest uranium reserves. The portion of this structure of interest is the Gassaway member, which contains about 63 p.p.m. of uranium in an undetermined form. It consists of three conformal horizontal beds, which in ascending order are designated as Middle Black, Upper Silt (Upper Gray), and Top Black. Each bed consists of a compact series of fine bedding planes. A clay-organic complex matrix binds the inorganic granular material, which is principally feldspar, quartz, and pyrite (7). Other beds existing above and below are too low in uranium content to be of interest. Batch leaching of l o p Black Shale
Of the lixiviants used, sulfuric acid proved most worthy of process consideration. Retorting markedly reduced subsequent uranium extraction, and roasting, a costly and sensitive operation, did not yield higher subsequent extractions than those obtained with raw shale. Uranium extractions a t two levels of pulp density and various acid concentrations are given in Figure 1. The poor extraction with the thicker slurry is attributed to the higher concentration of contaminants associated with the thick slurry, as detailed studies have shown that the grind and time differences indicated have a very small effect. Thus, to develop an effective leaching process, high acid concentration with low contaminant level must be obtained; the high acid feed required by batch leaching must be reduced; and the liquid-solids feed ratio must be minimized to maximize uranium concentration in solution.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Top Black shale (0.0079% U)
Time (hrs.) Temoerature
0
4
12
8
16
4 3 reflux
20
24
SULFURIC ACID CONCENTRATION-
*/e
Figure 1. Uranium extraction is a function of acid concentration High acid concentration with low contaminant level is needed
1000
-
000
-
00
e
Q
P
-
1.
1
I
i
l
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l
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I
1 1 1 0 9 8 7 6 5 4 3 2 1 STAGE NUMBER Figure 2.
I
I
Ion levels in countercurrent decantation study
2*6k\
Countercurrent leaching
-Water Feed
2.4 2.2
Solids Liquids r"
1.61
0.8 0.6 0.4'' I ' I ' I I I " I 1 1 1 0 9 8 7 6 5 4 3 2 1 STAGE NUMBER Figure 3.
I
'
pH in countercurrent decantation study
7 2 0 : l L/S
90
s I
Q W
batch leaching
L
L
a
countercurrent I eaching a t 211 LIS flow r a t i o
40 =)
IO
"'//
acid concentration expressed as weight of acid fed t o weight f proquct splutioq , , I , Q
Because a major portion of the contaminants are leached out immediately upon acid-shale cdntact, the contaminant concentration in the later stages of a countercurrent leaching system can be reduced to any desired level, depending upon the number of stages and underflow-overflow ratio. The acid in such a system is strongest in the later stages, where its action is most required-where the more refractory uranium is being solubilized. These effects are demonstrated in Figures 2 and 3, wherein the steady-state results for a countercurrent leach of Top Black shale employing seven leaching and four washing stages are given. Dry shale is added to the first stage, acid to the seventh, and wash water to the eleventh in this systemProduct solution is taken from the first stage and thickened slurry is discharged from the eleventh stage. With such a system, in the final leaching stages the solution conditions obtained in batch leaching a t very low pulp densities are reproduced. > As the number of countercurrent leaching stages employed was increased, the extraction efficiency approached that of the batch system employing the more dilute pulp (Figure 4). Thus the soughtfor effectiveness of dilute batch leaching was obtained in the countercurrent system at very much lower liquid-solid feed ratios. For a given acid-shale feed ratio countercurrent leaching is more effective than cocurrent or batch leaching. The product solution is discharged at higher p H ; this eliminates an acid recovery operation, and simplifies subsequent uranium recovery. Countercurrent leaching of the Gassaway composite was also investigated (Table I). Batch leaching at the same
Table 1. Seven-Stage Countercurrent Leaching Gave Higher Extractions Than Batch Leaching at Same Acid Feed
I " " " " " " I
(Steady-state values. Gassaway raw shale, 0.00635% U. Particle size, -65 mesh. Leach temp., reflux. Leach time, 71/2-hr. reflux periods. HnSOJshale ratio, 0.071 by wt.)
0
40 80
120
160 200 240
TIME, MINUTES Figure 5. 'For oxygen overpressures above 75 p.s.i. approximately all pyrite is consumed in 50 minutes
mow ratio (solution to solids), ml./g. 2.05 Uranium extracted, % 73 Proddct solution Uranium level, mg./l. 24.1 Volume, ml. 205 PH 1.40 Weight loss on leachin+ % 5.5 Residue analysis, % U 0.0018 Acid consumed, g. Per g. of shale fed 0.061 Per g. of acid fed 0.85 Soluble uranium losses, % 0.24
VOL. 50, NO. 12
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DECEMBER 1958
1.00 73 47.5 100 1.34 5.3 0.0018 0.064 0.91 0.4
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Eci 801
d
0
40
80
I20
160 200 240
280
TIME, MINUTES
TIME- MINUTES Figure 6.
Acid concentration doubles with recycling
acid feed results in extractions of 54 and 63% for liquid-solid ratios of 1 to 1 and 2 to 1 ml. per gram, respectively.
Pressure leaching of Gassaway Shale Sulfide minerals can be decomposed by aqueous oxidation. The acid produced can be applied to extract uranium from pyritic pitchblende ores (2). Two major reactions are postulated :
+ FeS04 + H2S04 + 3HzSOa 3UOz(SOa) 4- 3HzO
FeS2 f 7/2 0% HzO cao8
.
+ 1/2
0 2
-+
-+
As shale is about 10% pyrite, work was undertaken to test such a system. Batch Pressure Leaching. Batch pressure leaching was studied on slurries containing minus 65-mesh shale at 33y0 solids heated to 175" C. in an agitated, 1-gallon, 3 16-stainless steel autoclave (Figures 5 to 8). Except where specified, recycle liquor, the filtrate from a previous run made at comparable conditions, was used. Oxygen overpressure was maintained by intermittently bleeding out the freeboard gases and admitting oxygen to re-establish the fixed total system pressure. For oxygen overpressures above 75 p s i . approximately all of the pyrite is consumed within 50 minutes (Figure 5). Figure 6 shows sulfuric acid produced from the decomposed pyrite with and without recycle of the leach liquor, and with and without bleeding of the freeboard gases. Initial oxygen overpressure was 150 p.s.i. in all cases. Acid strength doubles with recycling and achieves a level of about 1N. The continued increase in acid strength with time, for the case in which both recycle and bleeding were employed, is attributed to the hydrolysis of ferric ion continually produced from ferrous ion by bleeding oxygen to the system. Without bleed, the oxygen overpressure falls with its con-
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With Bleed Initial oxygen overpressure- 150 psi
70
Figure 7. Under favorable conditions retention time is about 70 minutes Continuous Pressure Leaching. A 10-gallon, 316 stainless steel autoclave was fitted with a variable-speed impeller, an internal cooling coil to remove excess reaction heat when required, ports for convenience in attaching the various accessory lines, and a removable septum to allow subdivision into two cells when desired. External heating was supplied, when required, by a heating mantle. Residence time was controlled by a cycle timer which set the frequency of measured discharges. The level of slurry in the autoclave was held constant by a capacitance level-probe which caused a piston pump to feed slurry when the level in the autoclave fell because of discharging. The feed slurry was made up in a 5-gallon stainless steel, agitated prepulper from shale fed by a metering feeder and metered recycle liquor (and/ or water). The level of the slurry in the prepulper was maintained by conductance level-probes which operated the solids feeder and recycle pumps (and/or water solenoids) as required.
sumption and the acid concentration falls off as it is continually consumed by the shale. Recycling has a major effect upon extraction due to the increase in acid strength, whereas bleeding has a minor but definite effect (Figure 7). Under the more favorable cpditions, a retention time of about 70 minutes is indicated. The rate of pyrite oxidation was found to increase with temperature up to 175' C. Above this temperature corrosion of the stainless steel was noted. The rate of oxygen consumption with and without bleed is given in Figure 8. The over-all oxygen consumption by the shale is equivalent to 13% of the shale's weight. Because these and other batch experiments indicated that reasonable uranium extractions could be obtained with the concentration of acid produced by aqueous oxidation of pyrite, the process was then carried out on a continuous basis.
INDUSTRIAL AND ENGINEERING CHEMISTRY
W -I
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t
-18 16
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14 -I2
I 1 I 0 20
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[
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60 80 40 TIME MINUTES
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100
z
8
I v)
LL
0
g za v)
LL
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0 8 0 6 \ 4 3 v) 2 a
'0
0 U (3
5
$
3 (3
Figure 8. Over-all oxygen consumption by shale i s a function of time
NUCLEAR TECHNOLOGY
FROM COMPRESSOR \
SLOW B L E E D
FAST B L E E D
L E V E L PROBE+
PRESS. CONTROL 1
A U T 0 C L AVE DISCHARGE Figure 9. pressure
Slurry was discharged to a receiver a t atmospheric
To discharge the slurry, the volume of each slug was controlled and discharged to a receiver a t atmospheric pressure without undue erosion of the discharge valve system. The discharge lock (Figure 9) was first pressurized with compressed air to a pressure equal to that in the autoclave. Slurry was then admitted to the discharge lock when the cycle timer simultaneously opened the lock intake (autoclave outlet) valve, 3, and the slow bleed
Table II. Results of Continuous Pressure Leaching Were in Agreement with Prediction
Conditions. Gassaway shale Mesh size. -65 Temperature. 175" C. Pressure, p.s.i. Total 280 Water 130 Slurry. 29% solidsa Retention time. 1.4 hours Results Uranium extraction,% b 73 Solution Acid concn., N 0.8
sod, &/I.
Fe(III), g./l. Fe(II), g,/L Residue, FeS2, yo Oxygen consumption, Ib./lb. shale
73 7 4
0.3 0.13
One half of product solution returned as recycle, t o yield over-all liquid-solid flow ratio of approximately 1. Estimated from batch results 71.6 to 73.1%. a
valve, 2. During this operation, the velocity through thrr lock intake valve was reasonably small. When the slurry reached the level probe in the discharge lock, the lock intake valve and the slow air bleed valve were closed by the probe relay. The slurry was held in the lock as long as the chosen discharge cycle permitted to allow for as much cooling as possible by the water jacket before the fast air bleed valve, 4,dropped the pressure to atmospheric. The timer then opened the lock drain valve, 5 , and the cool pulp was pumped to a solid-liquid separation unit. The discharge system and the autoclave were provided with safety interlocks, rupture disks, and blow-off valves, so that any situation could be handled. The results obtained and the conditions used for a steady-state run are given in Table 11. The result, 73% uranium extraction, is in fair agreement with that predicted from the batch-leaching studies.
Ion exchange studies indicated a concentration factor'of a t best 20 with the most amenable solutions; solvent extraction gave factors of greater than 300. Of all the complexants used in solvent extraction, 6-benzylamino-3,9-diethyltridecane (NBA), suggested by K. B. Brown of Oak Ridge National Laboratory, has proved most applicable to shale leach liquors. This amine, in addition to very high extraction coefficients and excellent selectivity for uranium, exhibits rapid phase break, gerod solubility of its sulfate salts in the organic phase, and low loss to the aqueous phase. All other amines investigated were inferior in extraction coefficients or lacked one or more of the other required properties. Carbonate was used for stripping, because the sodium uranyl carbonate formed is highly stable and because it regenerates the, amine sulfates produced in extraction directly to the free amine, which can then be recycled. The results of a countercurrent solvent extraction run made with NBA in kerosine, in which sodium carbonate was employed for stripping, are given in Table 111.
Table 111. Solvent Extraction of Uranium from Pressure Leach Liquor
Organic concentration (kBA in kerosine), M Extraction flow ratio (A/O)" No. of extraction stages Aqueous-feed U, mg./l.
0.05 30
5 53 80 0.6 13.4 4.6 0.03 3
sod, g./1.
PH Fe (total), g/.L Fe(II), gJ1. Ra5natebU,mg./l. No! of stripping stages Uranium loading in 1st stage,
18
g./1.
a Aqueousorganic. 3rd stage. Three extraction stages would suffice.
The final uranium concentration of
18 grams per liter approximates a 340fold increase and substantiates effectiveness of the NBA.
the
Literature Cited Recovery of Uranium from Leach Liquors
The uranium concentration in liquors derived from countercurrent or from pressure leaching is 40 to 50 mg. per liter. To increase this to about 15 grams per liter for precipitation purposes requires a Concentration factor of about 300. Of the methods for recovering the uranium from the leach liquor. ion exchange and solvent extraction were investigated in most detail.
(1) Bates, T. F., Shrahl, E. O., Bull. Geol. SOC. Am, 68, 1305-14 (1957). (2) Forward, F. A., Halpern, J., J. Metals
7, A.I.M.E. Trans. 203,463-6 (1955).
RECEIVED for review April 7, 1958 ACCEPTED August 29, 1958. Division of Industrial and Engineering Chemistry, Symposium on Extraction of Uranium and Thorium from Ores, 133rd Meeting, ACS, San Francisco, Calif., Auril 1958. Work Derformed for Atomic EAer y Commission' under contract AT(49-17-621. VOL. 50, NO. 12
DECEMBER 1958
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