I
PAUL NOBLE, Jr., W. 1. WATSON, 1. M. WHITTEMORE, R. A. CARLSON, J. C. HUGGINS, and L. A. McCLAlNE Western Division, Arthur D. Little, Inc., San Francisco, Calif,
Uranium Adsorption from Leach Slurries This new process for recoverin uranium from acid-leached ore ly economically competitive with other methslurries is not ods but also tentially useful for recovering other metals
IN
MILLING URANIUM from the Colorado Plateau, thickening and clarification are extremely difficult. Such difficulties, experienced in the gold industry, have been solved by adding to the slurry of ore and cyanide leach solution a char adsorbent which is then screened from ore slurry (7). After gold is recovered, the char is recycled and the slurry discarded. This process has been used by the Golden Cycle Corp. Such a process can be useful for recovering uranium and an adsorbent can be produced by placing on the surface of a porous char a substance such as di-2-ethylhexyl pyrophosphate which will bond with uranium ( 2 ) .
for strong adsorption to a suitable surface. The organophosphorus acids generally fulfilled these requirements and di-2-ethylhexyl pyrophosphate (OPPA) was selected for this study. The char can be modified by agitation with this chelating agent, water emulsion or in soluti organic solvent such as et ( 3 ) . The water emulsion, more feasible economically, yielded modified chars having activity comparable to that of the solution and was adsorption of uranium Concentration. Fifty grams of char were placed in continuous contact with 18 liters of solution containing 2.0
Experimental
grams of uranium oxide per liter and 0.5M sulfuric acid. The char was sampled periodically and analyzed for uranium. The rate of adsorption from pulp was determined by agitating several samples of char mixed with pulp for various times. The pulp was prepared with Lukachukai ore at 30% solids, and wet ground to -65 mesh. T o 69 grams of ore in a 500-ml. bottle were added 6.0 ml. of concentrated sulfuric acid and 150 ml. of water. After an overnight leach, the appropriate amount of modified char (18 or 36 grams) was added. The bottle was agitated for the required time, the char screened from the pulp, the pulp filtered, and the effluent analyzed for uranium. For the continuous countercurrent
Procedure. Nut shell and fruit pit chars satisfied the practical requirements for uranium ore slurries-Le., the particle size is large enough to permit char separation from the ore pulp by screening, and hardness is sufficient for minimum abrasion losses. In this work, steam-activated peach pit char (R. T. Collier Corp.) was used.
Properties of Peach Pit Char Gradea SP Moisture, % 3.0 Ash, % 6 Apparent densitr, g./ml. Surface area, sq. m./g. Adsorption Butane, ml./g. CCla, %
0.31 950-1000
0.35 64
Based on CC14 vapor adsorption.
Previous investigations (2, 3) indicated the chelating agent must be a strong acid, form a strong chelate with uranium, be water-insoluble, and meet molecular size and spatial requirements
Solution Concentration, 8
u308
/I.
1, Adsorption of uranium from Lukachukai pulp on treated char. uranium adsorbed depends on contact time
Quan-
Leached at 30% solids and 3 15 pounds of sulfuric acid per ton of ore VOL. 50, NO. 10
OCTOBER 1958
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1050 Ib. Char-Yro
A-
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I Figure 2.
3 4 Confact Time, Hours
leached at 30% solids with 315 pounds of sulfuric acid per
adsorption rum, 400-gram batches of Lukachukai ore were wet ground to - 65 mesh at 67% solids. This pulp was placed in a 2.5-liter bottle, diluted to 35.270 solids with 75 grams concentrated sulfuric acid and 459 grams of water; then char from the second stage was added. The bottle was agitated for the required time; and then the char was removed by a 48-mesh screen and washed
Activity of Remodified Life Test Chars
(Total cumulative time: in leach sol., 2400 hours; in desorbing solution, 850 hours) Activ-
Original SP grade char Char at end of life test 6 M H 8 O a leached 6 M HB04 leached, remodified 2 M HF leached 2 M H F leached, remodilied 111.1 NaSCOa leached, remodified 1M Na~C03leached (70' C.), remodified 0 . 5 M NaOH leached, remodified
24
7 7
4-5 2 2
8
4.5
12
2.2
19
3.8
20
7.7
19
5.4
19
6.5
rlmount adsorbed a t a solution concentration of 1 g. u304/1, pH 1, and 100 g. a
sod1. 1 5 14
6
Adsorption rate for uranium increases as the ratio of char increases
Lukachukai pulp with 10% OPPA. ton of ore
Table 1.
5
with 200 grams of water which when added to the pulp resulted in a dilution to 30% solids for subsequent stages. Char, screened from the pulp in the later stages, was washed with supernatant prior to advance. The pulp was retained and subsequently agitated with char from succeeding stages until it reached the last stage where 90 to 125 grams of fresh or desorbed char was added. Char, removed from the first stage, desorbed countercurrently in three stages using a 10% sodium carbonate solution, was then washed with water and returned to the adsorption circuit.
Results and Discussion Adsorption. The capacity of modified char to adsorb uranium from leach slurries (Figures 1 and 2) is less than that for pure uranyl sulfate solutions (Z), because of the competitive adsorption of other ions. The competition, primarily from vanadium and iron, is minimized at 10% pH values. I n process operation, a pH of 1 to 2 should be maintained. Rate data show that retention times of several hours per stage would be required to attain equilibrium in each stage. The number of stages required can be minimized by attaining equilibrium in each stage and by using the maximum practicable char-to-ore ratio. However, the maximum rate of transfer
INDUSTRIAL AND ENGINEERING CHEMSTRY
occurs when the system is far from equilibrium. Thus, to minimize the total retention time, the maximum practical char-to-ore ratio should be balanced with a short retention time per stage and sufficient stages to provide the required recovery. The optimum situation from an economic standpoint is probably some intermediate position between a large number of stages and minimum contact time. Desorption of Uranium. A sodium carbonate solution appeared to be the most practical of several types of desorption solutions investigated. Desorption equilibrium of the modified chars was attained with a solution containing 1070 sodium carbonate and 2.5% sodium bicarbonate in 6 to 8 hours. A three-stage countercurrent desorption circuit as satisfactory for removing all of the readily desorbable uranium. Residual uranium, which required weeks to desorb, accumulated in the first five cycles to a constant value of 0.3y0 uranium oxide. Its presence has no apparent effect on subsequent adsorption. The maximum concentration of uranium in the carbonate stripping solution is limited principally by the concentration of dissolved salts. In the continuous tests, concentrations of about 6 grams of uranium oxide per liter were realized. Life of Modified Carbons. The loss in activity in the initial cyclic test studies was greater than that expected from the gradual decrease (Figure 3) in modifier concentration. Spectrographic and quantitative analyses of these chars indicated that an increase in the amount of silica on the cycled chars was the only significant change following prolonged use. Microscopic examination of small char particles which were carefully ashed indicated precipitation cf silica within the pores. Therefore, it appeared that gradual dissolution of siliceous material-e.g., clay-in the acid leach circuit, followed by a slow deposition of silica in the char pores and on the surface, was taking place. This deposit could be removed by leaching, using dilute solutions of hydrofluoric acid, sodium hydroxide, and hot sodium carbonate. Removal of this
Table II. Analysis of Lukachukai Ore and Leach Solution Constituent u308
V20E Fen03 A1203
coz
CaO MgO
Leach Soln. Concn., G./L.
1.3 0.9 2.5 0.2
...
...
...
Ore Concn., %
0.33 .93 1.15 4.08
4.16 5.18 0.73
U R A N I U M RECOVERY silica increases the activity for uranium and, furthermore, permits remodification to approximately the original activity (Table I). Thus, occlusion of the char pores is a principal factor regulating activity. This necessitates close observation of ash build-up and of the constituents of this ash in considering process applications to different ores. Use of 10% sodium carbonate desorb solutions is sufficient to inhibit silica build-up on the recycled char. Thus,, the activity in process use is limited only by the slow loss of modifier (Figure 3). Countercurrent Multistage Adsorption. T o establish the over-all feasibility of the present adsorption process, a completely cyclic operation was studied, using Lukachukai ore from Monticello stockpile 16 (Table 11). Effects of the number of stages and over-all retention time were the principal factors investigated (Table 111). The over-all recovery for 10 stages at 1 hour per stage (total retention time 10 hours) is 95% with soluble losses of about 0.05 to 0.06 gram of uranium oxide per liter. A decrease to 7 stages for a total retention time of 7 hours reduces the recovery to 92% with soluble losses of 0.09 gram of uranium oxide per liter. However, reducing the over-all retention time to 5 hours while using 10 stages ( I / i hour per stage) increases the recovery to 94% with soluble losses of 0.066 gram. When compared to the original char with respect to adsorption of uranium from synthetic solution, the desorbed char loses approximately 25% of its activity. T o maintain activity, adsorbed silica must be controlled and a fraction of the recycled char must be continuously modified.
Process Description
Ore Leaching. Leaching and adsorption can be achieved concurrently. For Lukachukai ore, contact times of 1 hour were adequate, because more than 9!iy0 of the uranium was leached with the first half hour, The leaching conditions were 375 pounds of sulfuric acid per ton, solution to solid ratio of 2 to 1, and no oxidants. Adsorption Circuit. The adsorption circuit for 95y0 uranium recovery requires a countercurrent system of 10 stages at 1 hour per stage, 625 pounds of char per ton of ore, and a solution of p H 1 to 2. The slurry containing the dispersed char is transferred from the adsorption agitators to vibrating screens by the use of air lifts. The pulp then passes through the screen and flows by gravity to the next adsorption stage. The char, retained by the screen, passes to the
8" M* U (D
Adsorpi ion Contad, Hours Figure 3. In initial cyclic tests, activity loss for the char was greater than that e6pected from the gradual decrease in modifier concentration
preceding adsorption stage. A representative analysis of the loaded char is: Constituent
Concentration, %
Po4
5 1.3 0.1
Us08
vzos
Losses would approximate 0.1 pound of uranium oxide per ton of solution as soluble losses, plus 0.07 pound per ton of ore as insoluble losses. A 10-minute, single-stage water wash of the loaded char leaving the adsorption circuit is stipulated to minimize acid carry-over into the desorption circuit. Desorption Circuit. The desorption circuit consists of a countercurrent flow of char and carbonate solution in three
Table 111,
Average Equilibrium Conditions" Total Retention, Hr.
Effluent Charb
G./L.
uao&%
Over-allc Recovery, %
0,036 0.045 0.054 0.066 0.089 0.084 0.062
1.1 1.1 1.1 1.0 1.2 1.4 1.1
97.6 96.8 95.0 94.1 92.5 93.0 94.5
Cycles
Stages
5-10 17-25 28-35 40-46 51-70 74-82 86-96
10 10 10 10
20 15 10
7 7
7 14 10
10
stages of 1 hour per stage. Vibrating screexis together with air lifts are again used to transfer and separate the char and desorb solution between stages. The char-to-solution ratio in the desorbers is l to 6, and the solution is at ambient temperature. Acid carried over from the adsorption circuit and unchelated modifier on the char gradually neutralize the sodium carbonate. The char leaving the desorption ciTcuit is given a singlestage, 10-minute water wash prior to recycling to the adsorption circuit to minimize the sodium carbonate carryover. The amount of uranium desorbed is essentially that being adsorbed because the undesorbed uranium on the char remains practically constant.
5
uses,
Includes residual uranium 4 625 pounds of char and 375 pounds of H2SOd per ton of ore. Feed 0.33% Usb-ore residues < O . 003% UaOs. (undesorbed) of 0.3% Uaos.
VOL. 50, NO. 10
OCTOBER 1958
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Table IV. Total Capital Requirement and Operating Costs (200 tons ore per day; as of 1955)
Capital Requirements Process plant Auxiliaries Engineering and construction Direct plant cost Indirect plant cost Total construction cost Start-up expense, working capital
$
473,349 303,100 76,650 ZGi99 187,050 1,040,149
693,000 $1,733,149 Per Ton
A
5,4T/D
Total direct Indirect
Total operating cost $10.94 Amortization 3.78 3.53 Return on investment Ore cost (0.3% 0 3 0 8 , 0 lime) 25.50 Cost of product $43.75 Cost of product, corrected to zero lime content in ore $ 7.67 /lb. u806
15 1 6
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Cha r 1 , ----------
Modified T/l Storage
-----
R emodif ication
Drain S c r e e n
Leaching and adsorption can b e achieved concurrently
aluminum oxides. Uranium is precipitated by neutralization with sulfuric acid and air blowing to drive out carbon dioxide. Char Regeneration and Remodification Circuit. Activity of char may decrease because of silica pickup in the adsorption circuit, but sodium carbonate is sufficiently alkaline to remove any such silica coating. Therefore, a sodium carbonate feed to the desorption circuit is stipulated. Thus, the char in each cycle passes through the third desorber containing the strong carbonate feed solution and can be maintained in a regenerated condition. All of the OPPA will be lost in 150 adsorption-desorption cycles. I t is assumed that 1% of the total OPPA is lost per char cycle. T o maintain activity uniformly high, 5% of the char should be removed per cycle to pass
Representative Analysis of Desorption Circuit
Constituent Feed solution Pregnant solution
Desorbed char
INDUSTRIAL AND ENGlNEERlNO CHEMISTRY
-1
,‘4 P r o d u ~ f ~ i 2 1 7 1 b . ~ 0 8 / D
$ 1.70
5.58 0.34 $7.62 3.32 -
dl CUI
Ore Labor Supplies Utilities
I
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A
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The proposed process.
Uranium Precipitation Circuit. The sodium carbonate desorbing solution will be utilized until only 1% bicarbonate remains in the desorbing solution. At this time, the pregnant solution is clarified to remove suspended iron and
4
5
Modified L,,,,,,C h a r
200 T/D o id s 400T/D Solution Figure 4.
P r e c i p ita t i o n
i wa;e;. I
Countercurrent Adsorption IO Sfages yS04 37.5T/D
Filter
Wash Tank
(0
v)
H2s04
- Na2C03, 5.75 T/D
Gount e rcu went D e s o r lion 3 Stales
a01
I
at e r ,362.5 T/D Tails
Na2C03 NaHC03 UaO8 Salts
VZOS UaOz Na+
Concn., % 10 1 1
7 0.1 0.3 2.5
through a remodification operation. + The modification of make-up char or the remodification of cycled char is obtained by agitating the char with a water emulsion of OPPA. -4char loss of 1 pound per ton of ore treated is assumed. Economic Data
Although a reliable estimate of all economic factors involved in such a process cannot be made without pilotplant data, expected costs can be reasonably approximated. Of course, a pilotplant study would probably necessitate changes in the process which would alter cost estimates to some extent. I n Table IV, the operating cost, based on an ore of zero lime content, may be corrected for any given ore merely by adding the cost of acid necessary to neutralize the lime.
literature Cited (1) Krebs, R. W., U. S. Patent 2,476,420 (July 19, 1949). (2) McClaine, L. A., Noble, P., Jr., Bullwinkel, E. P., J . Phys. Chem. 62, 299 (1958). (3) Noble, P., Jr., Whittemore, I. M., Carlson. R. A,. Watson. W. I.. “Development of a Char-In-Pulp Process for the Recovery of Uranium,” RMO2616, October 1954. RECEIVED for review November 6, 1957 ACCEPTED May 5, 1958
