Recovery of Uranium from Wet-Process Phosphoric A-cid by Extraction with Octylphenylphosphoric Acid F. J. Hurst* and D. J. Crouse Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
Commercial wet-process phosphoric acid that is produced from Florida phosphate rock contains 0.10.2 g of uranium/l. and represents a potentially substantial source of uranium. A process for recovering the uranium by extraction with a mixture of mono- and dioctylphenylphosphoric acids in an aliphatic diluent has been developed and demonstrated successfully in bench-scale mixer-settler tests. The uranium is recovered from the solvent by contacting it with 10 M H3P04 containing sodium chlorate; the chlorate oxidizes the uranium to the less-extractable hexavalent state and effects its transfer to the aqueous phase. The strip solution can be loaded with uranium to 15-20 g/l., a factor of 100 or more richer in uranium than the original acid. These solutions are highly amenable to processing in a second cycle with the synergistic extractant combination of di (2-ethylhexyl)phosphoric acid plus trioctylphosphine oxide. The uranium is stripped from this solvent with ammonium carbonate solution and recovered as a high-grade (>98% u 3 0 8 ) product.
Wet-process phosphoric acid produced from Florida phosphate rock contains 0.1-0.2 g of uranium/l. and represents a potential source of about 2000 tons of Us08 per year. At the present time this valuable resource is lost when the acid is converted to fertilizers and dispersed to the soil. A two-cycle process for recovering uranium from the acid by extraction with di(2-ethylhexy1)phosphoric acid (DBEHPA) plus trioctylphosphine oxide (TOPO) in an aliphatic diluent was recently developed and demonstrated successfully in bench-scale, continuous mixer-settler tests at Oak Ridge National Laboratory (Hurst, e t al., 1972; Hurst and Crouse, 1973). Since then we have developed an alternative first-cycle process that uses a commercial mixture of mono- and dioctylphenylphosphoric acid as the extractant (Ferguson, 1973). This reagent is much less expensive and has a higher extraction power for uranium than the earlier extractant. In addition, it extracts uranium(IV), the prevailing oxidation state of uranium in wetprocess acid; this eliminates the liquor oxidation step required in the earlier process. The D2EHPA-TOP0 solvent is used in the second cycle, as before, to produce a highgrade U3O8 concentrate. The octylphenylphosphoric acid reagents have been studied previously by Peppard for extracting thorium, rare earths, and other elements (Peppard, e t al., 1958, 1960). More recently they were examined by Murthy for recovering uranium from phosphoric acid, but definition of a complete process flow sheet and preparation of a suitable uranium product were not demonstrated (Murthy, et al., 1970). Experimental Section Phosphoric Acid Samples. Samples of wet-process acid were obtained from four commercial phosphate plants (identified as Companies A to D) that process Florida phosphate rock, one from a plant (Company E ) that processes North Carolina rock, and one from a plant (Company F) that processes western rock (Table I). Four of the companies (C-F) calcine their rock prior to the dissolution step. Calcination removes organic matter (humus) present in the rock, and the so-called “green acid” produced by this procedure is much easier to process in a solvent extraction plant than “brown acid” which is produced from uncalcined rock. The humus, present in the “brown acid,” is a major cause of phase separation problems that can 286
Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 3, 1974
lead to excessive solvent losses. In addition, the humus is extracted to some extent and, under some circumstances, conceivably could concentrate in the solvent to a sufficient extent to decrease its uranium extraction power. The acid samples had phosphate concentrations ranging from 5 to 6 M and contained 0.07-0.19 g of uranium/l. It has been reported (Stoltz, 1958) that up to 30% of the uranium is undissolved and lost to the gypsum residues when calcined rock is digested; this could account for the lower concentration of uranium in the “green acids.” Also, the phosphate rock of Carolina and the western states has a lower concentration of uranium than does Florida rock. For maximum efficiency in extractions with the octylphenylphosphoric acid mixture (OPPA), all of the uranium should be in the tetravalent state. Tests have indicated that this condition is met if the ferrous iron concentration of the acid is 0.5 g/l. or higher. As shown in Table I, the Fe(I1) concentration of the acid samples varied over a rather wide range, from 0.2 to 3.5 g/l. Most of these samples were several weeks old when received, and some of the Fe(I1) initially present had oxidized to Fe(1II) as the acid aged. Indications are that the Fe(I1) concentration of fresh acid will normally be higher than 0.5 g/l. and that no reduction of the acid will be necessary. In the event that reduction is needed, it probably could be accomplished most easily by completely reducing a small volume of acid and blending the resulting product with the main acid stream. Process Flow Sheet. The proposed process consists of two cycles (Figure 1).In the first cycle the acid is cooled to 40-45”C, and the uranium is extracted with a 0.3-0.4 M mixture of mono- and dioctylphenylphosphoric acid in an aliphatic diluent. Uranium is recovered from the solvent by contacting it with 10 M H3P04 containing sodium chlorate; the chlorate oxidizes the uranium to the lessextractable hexavalent state and effects its transfer to the aqueous phase. A convenient source of strip solution is the 45-5570 PZOS product acid from the evaporators. The strip solution can be loaded with uranium to 15 to 20 g/l. (a factor of 100 or more richer in uranium than the original acid). These solutions, after dilution to 6 M H3P04, are highly amenable to treatment (as demonstrated previously) (Hurst, et al., 1972) in a second cycle using the D2EHPA-TOP0 solvent. In the second cycle, the uranium is extracted in three stages with 0.3 M D2EHPA-0.075 M TOPO in Amsco 450
Table I. Analytical Results for Wet-Process Phosphoric Acid Samples
Acid from company
Source of phosphate rock
A B C D E F
Florida Florida Florida Florida N. Carolina Western SOLVENT'
Concentration, g/l.
Type of acid Brown Brown Green Green Green Green
U
Fe(III
Total Fe
PO,
0.14-0.17 0.16-0.19 0.10-0.13 0.07-0.09 0.06 0.06
0.3-0.8 0.3-2.6 0.2-0.7 2.0-3.5 3.4 2.6
7-10 10-12 6-7 8-9 6.8 4.5
5.0-6.0 M 5.4-6.0 M 5.2-5.3 M 5.5-5.7 M 5.5 M 5.9 M
Table 11. Effect of Mole Ratio of Mono- and Di-OPPA on Uranium Extraction and Phase Separation
RECYCLE
WET-PROCESS HsP04 EXTRACTION Ic--l ( 4 STAGES, 40-45'C)
PRCWCTON
I ,
Organic: Varying ratios of mono- and di-OPPA in Amsco 450 Aqueous: Company A acid Procedure: Equal volumes of organic and aqueous mixed 5 min at 25OC OPPA concn, M Mono-
SOLVENT"'
DZEHPA-0075 TOPO--AMSO 450
I
STRIPPING
(3 STAGES, 25-3O'C)
1
;R~~IZER-J
** 03M
' OXIDATIVE
RECYCLE
M
Figure 1. Process flow sheet for recovery of uranium from wetprocess phosphoric acid. diluent. The extract is scrubbed in two stages with water and then stripped in two stages with an ammonium carbonate solution. Operation of the system with a relatively concentrated (2-3 M ) ammonium carbonate solution results in direct precipitation of ammonium uranyl tricarbonate (AUT) in the stripping system. The AUT settles rapidly in the aqueous phase and is continuously removed by filtration. The filtrate is recycled for further stripping. The AUT is calcined to U308. Solvent Preparation. The octylphenylphosphoric acid extractant is available commercially (Mobil Chemical Co.) as an approximately equimolar mixture of the monoand diacids. The nonhomogeneous-looking' dark brown to light tan solid mixture, as received, was heated to about 65°C in order to liquefy and homogenize it for sampling. Equal volumes of the sample and 6 M HC1 were then stirred under reflux at 60°C for 16 hr in order to hydrolyze any pyro acids that may have been present. Thorough elimination of the pyro acids would not be required prior to process use of the octylphenylphosphoric acid but was necessary for experimental studies since the pyro acids are extremely powerful, although unstable, uranium extractants and their presence could produce misleading extraction results. After hydrolysis, the viscous mixture was diluted with three volumes of Amsco-450 diluent to aid in separation of the phases and then the solvent phase was filtered. This filtrate, after titration with NaOH solution to determine the concentrations of the mono- and diacids, was used as a stock solution. In order to study the behavior of the individual components of the octylphenylphosphoric acid mixture, some of the hydrolyzed mixture was separated into the pure mono- and diacid fractions. The choice of diluent for the extractant can significantly affect extraction performance and phase separation characteristics. Amsco Odorless 450 (American Mineral
0.200 0.175 0.150 0.125 0.100 0.075 0.050 0.025 -
D i-
0.025 0.050 0.075
0.100 0.125 0.150 0.175 0.200
Uranium Concn of Phase extraction F e in separation coefficient, extract, time, sec EBQ a/l. 900 900 400 180 55 50
40 30 30
2.1 5.3 10
16 30 30 37 30 2.3
0.95 1.08 0.94 1.03 1.16 1.43 1.42 1.13 0.29
Spirits Co.), a refined high-boiling, high-flash-point aliphatic solvent, was selected as a suitable diluent for process use and was used in most of the test work. On the basis of results obtained in cursory tests, Napoleum 470 (Phillips Petroleum) appears equally suitable. Extraction of Uranium. In addition to the fact that the OPPA mixture is commercially available, it has advantages over either the mono- or di- component alone. First, the mixture is readily soluble in aliphatic diluents such as Amsco 450, whereas the pure mono-OPPA and di-OPPA are sparingly soluble. Also, the extraction power of the mixture is higher. For example, the uranium extraction coefficient was about 30 in extractions with an equimolar mixture of the mono- plus di-OPPA, us. about 2 in extractions with the pure mono- component. The coefficient remained approximately constant as the di-/mono- ratio was increased to 7 / 1 (Table II). Finally, the coefficient obtained with the pure di-OPPA was essentially the same (-2) as with the pure mono- component. The phase separation was extremely slow when the monoacid was present in excess of the diacid. In process use of the OPPA mixture, the monoacid is lost preferentially from the solvent; this automatically adjusts the solvent composition in the direction of improved extraction and physical performance. The data of Table I1 indicate that a di-/monoratio in the range of 3/1 to 7/1 may be optimum. Table 111 shows a comparison of the uranium extraction power of the OPPA mixture with that of the D2EHPAT O P 0 solvent. Extraction coefficients with 0.32 M OPPA -Amsco 450 were a factor of 3-4 higher than with 0.5 M D2EHPA-0.125 M TOPO-Amsco 450. Extraction of uranium and the most important contaminant, Fe(III), is rapid with the OPPA solvent. In batch shakeouts, equilibrium was reached within 1 min at 40°C. Effect of Process Variables on Uranium Extraction. Variables that have a major influence on solvent performance include the phosphoric acid concentration, the Ind. Eng. Chern., Process Des. Develop., Vol. 13, No. 3,1974 287
50m50
4n
20-
10-
52-0 1
II
20
5.8 6.2 H3P04 CONCENTRATION IN AQUEOUS (M) 5.0
5.4
0 0
w
g
Organic: 0.32 M OPPA in Amsco 450 or 0.5 M D2EHPA0.125 M T O P 0 in Amsco 450 Aqueous: Wet-process phosphoric acid samples described in Table I Conditions: Equal volumes of organic and aqueous phases mixed 5 min at 23°C
A B C D E F a
16
29 27
>30 >30
("C)
r
100-
@
70-
1
50-
P8 a I-
30-
5-
2a
3-
2
I-
a
7 5 8
107-
x
w
D2EHPA-TOPOb
8
12 10
Uranium present as U(1V). * Uranium present as U(V1).
temperature, and the ferric iron concentration of the acid liquor. As shown in Figure 2, the uranium extraction coefficient decreased by a factor of about 1.5 as the HsP04 concentration was increased from 5 to 6 M , the typical concentration range of wet-process acid. Without cooling, the wet-process acid feed to a solvent extraction plant would have a temperature of about 60°C. Cooling the acid to about 40°C, which phosphate producers say can be done economically, increases the uranium extraction coefficient by a factor of about 2 (Figure 3). Cooling below 40°C does not appear advantageous because of higher cooling costs plus the added disadvantage of poorer phase separation encountered a t lower temperatures. The optimum temperature for the process, therefore, appears to be about 40°C. Iron(1II) is strongly extracted by the OPPA solvent and has an adverse effect on uranium extraction. The extraction of iron, unlike uranium, increases as the temperature of the acid is increased. In typical wet-process acid, the concentration of Fe(1II) is usually in the range of 5-10 g/l. (Table I). As shown in Figure 4, uranium extraction coefficients can be increased by a factor of 2-3 by reducing most of the Fe(II1) to the less-extractable Fe(I1). This procedure does not appear attractive, however, because the ratio of ferric iron to uranium in the acid is high and consumption of iron reductant would be large (20-40 lb of iron metal/lb of U308). Usually a uranium/iron decontamination factor of about 50 is achieved in the first cycle. This is adequate since the D2EHPA-TOP0 solvent efficiently separates the uranium from iron in the second cycle. The uranium extraction coefficient is dependent on the 288
I 60
2
Uranium extraction coefficient, Eao 30
50
Figure 3. Effect of temperature on uranium extraction from Company B acid with 0.34 M OPPA-Amsco 450.
Table 111. Extraction of Uranium from Commercial Phosphoric Acid Solutions with OPPA and D2EHPA-TOP0
OPPAa
40
TEMPERATURE
Figure 2. Effect of HaPo4 concentration on uranium extraction from Company C acid at 24'C with 0.34 M OPPA-Amsco 450.
Acid from company
30
I nd. Eng. Chem.,Process Des. Develop.,Vol. 13, No. 3, 1974
Table IV. Effect of Sodium Chlorate on Stripping Uranium from OPPA with 10 M HaPo,
Organic: 0.32 M OPPA-Amsco 450 containing 1 g of uranium/l. Aqueous: 10 M H3POa,prepared by evaporating Company D acid, plus NaC103 as shown Procedure: Solutions mixed at an organic/aqueous ratio of 5/1 fo? 5 min at 4OoC Concn
Concn of uranium, g/l.
of NaCIOs,
g/l.
Organic ~~
0 1
2 4
Uranium stripped,
Aqueous
%
1.95 2.58 4.09 5.99
34 52 85
~
0.75 0.48 0.14 0.06
95
total extractant concentration. As shown in Figure 5 , the dependence for the commercial OPPA mixture is about 1.5 power. The extractant concentration can be adjusted to meet conditions encountered in process application, which can vary from plant to plant. Although higher extraction efficiency can be attained by increasing the extractant concentration, this is done a t the expense of higher solvent costs. In most cases, an OPPA concentration in the range of 0.3-0.4 M is adequate to give a uranium extraction coefficient of about 10, which is sufficient for good processing efficiency. Stripping of Uranium. The OPPA is such a powerful uranium extractant that stripping of the uranium is difficult. Oxidizing the uranium to the hexavalent state enhances the stripping efficiency since uranium(VI) is extracted much less strongly than uranium(1V). However, even with a change in valence, use of a stripping solution with strong complexing power is necessary to obtain favor-
Table V. Summary of Extraction Data Obtained in Continuous Countercurrent Tests Uranium concn, g/l. Acid feed rate, ml/min
Feed ratio, aqueous/organic
Extract
Uranium recovery, %
Stripped solvent
Raffinate
Exptl
Calcd a
0.017 0.041
87 83 68
90 77 74
0.0014 0.004 0.005 0.006
98 94 93 91
98 96 94 91
0.012 0.029 0.037 0.045
93 83 78 74
92 89 83 76
Company C Acid, 0.13 g of Uranium/l. 105 140 154
7 10
0.84 1.01 0.98
11
60 75
4 5 6 7
90
105 53 60 75 90
0.005
0.016 0.009 Company D Acid, 0.07 g of Uranium/l. 0.006 0.30 0.33 0.004 0.005 0.35 0.45 0.004 Company B Acid, 0.17 g of Uranium/l. 0.57 0.007 0.65 0.008 0.70 0.008 0.76 0.006
3.5 4 5 6
0.022
a Calculated using the Fenske equation (see text) and assuming 100% stage efficiency; equilibrium uranium extraction coefficients obtained in batch tests at 45OC were 9.5, 10.0, and 5.5 respectively, for acids from Companies C , D, and B.
r
-032
M
OPPA-AMSCO
4 5 0 i15ml/rninl
t E X T R A C T I O N (45.C)
S T R l P P i N G (3O’Cl
I
I
R A F F I N AT E ..
013gU/iiter 2 6 9 Feihlliter 5 0 9 Feliiilliter
= I W
-
I
I
‘OI
5
5
P
5 0.1
0.2 0.3
I
NaC103 001 m l / m i n 500 q l l i t e r
10 M H 3 P 0 4 0 5 rnl/min 10 q NaClOs/liter 11 g Feliiilllitar
Figure 6. Typical stage data with Company C acid. Numbers in blocks show concentration in grams per liter at steady state.
0.5 0.7
OPPA CONCENTRATION
(
I EXTRACTION (45%)
M)
STRIPPING (30°C) 7
Figure 5. Effect of OPPA concentration on uranium extraction from Company A acid (0.13 g of U(IV), 1.5 g of Fe(II), and 9 g of Fe(III)/l.) at 40°C. able distribution of uranium(VI) to the aqueous phase. Efficient stripping is obtained by using a n oxidant such as NaC103 in combination with wet-process acid that has been evaporated to a H3P04 concentration of -10 M . Table IV shows data for stripping 0.32 M OPPA-Amsco 450 solvent containing about 1 g of uraniumll. In a batch contact a t an organic/aqueous phase ratio of 511, about 95% of the uranium was stripped when the NaC103 concentration was 4 g/L, whereas only 34% was stripped in the absence of the oxidant. Process Demonstration. A continuous demonstration of the first-cycle flow sheet was made in bench-scale mixer-settler units with four extraction and three stripping stages. The 0.32 M OPPA-Amsco 450 solvent was subjected to about 80 complete extraction-stripping cycles. In order to simplify the initial operation, “pure” 6 M H3P04 (containing only Fez+ and U4+) was processed over the first 35 cycles. About 200 gal of “green acid,” obtained from Companies C and D, was then processed successfully. Finally, “brown acid” from Company B was processed during the final 20 cycles. Typical stage data obtained when processing acid from Companies C, D, and B, respectively, are shown in Figures 6, 7 , and 8. Data for additional runs, covering a range of aqueous/organic flow ratios, are presented in Table V. Uranium recoveries in the four-stage extractor a t 45°C when treating “green acid” (Companies C and D) were in
..
0 0 7 3 g U/liter 2 8 g Fe l i i l / l i t e r 4 9 g Fe ( i i i ) / l i t e r
Figure 7. Typical stage data with Company D acid. Numbers in blocks show concentration in grams per liter at steady state. - 0 3 2 M OPPA--AMSCO 4 5 0 (15mI/minl
t & G F 7 / 1 413Fe 3 5 8 F c
22ZFe
I RA F F I N AT E
Figure 8. Typical stage data with Company B acid. Numbers in blocks show concentration in grams per liter at steady state. the range of 87 to 91%, with an aqueous/organic flow ratio of 7/1. Decreasing this ratio to 411 increased the recovery to 98%. Company B (“brown”) acid was more difficult to treat than the “green” acid and, in order to recover about 909’0 of the uranium in four stages a t 45”C, it was necessary to decrease the aqueous/organic flow ratio to 3.5/1. Ind. Eng. Chern., Process Des. Develop., Vol. 13, No. 3, 1974
289
Table VI. Effect of Cycling on Solvent Composition and Uranium Extraction Power No. of
Type of
organic cycles Start 35 60
acid processed
Uranium extraction coefficient,a E go
OPPA concn, M Mono-
Di~
11.4 11.3 10.7
Pure Green Brown
80
10.5
Totalb
~
0.17
0.15
0.15
0.16
0.15 0.17
0.17 0.17
0.32 0.31 0.32 0.34
Solvent sample was subjected to a standard extraction test with wet-process phosphoric acid a t 25OC. * Sum of the monoand di- components. The original solvent and the solvent that was added to compensate for volume losses over the first 60 cycles analyzed 0.17 M mono- and 0.15 M di-OPPA; the replacement solvent over the last 20 cycles analyzed 0.19 M monoand 0.16 M di-OPPA. Since the extraction isotherm for this system is linear, the Fenske equation can be used to predict the uranium recovery obtainable with different flow ratios
where K is the uranium extraction coefficient (Eao),E is the product of K and the organicfaqueous flow ratio, CF is the concentration of uranium in the feed, C, is the concentration of uranium in the raffinate, Yo is concentration of uranium in the solvent feed to the extractor, and IZ is the number of ideal stages. The good agreement between the predicted and the experimental recovery values for the continuous runs (Table V) indicates that the stage efficiencies in the mixer-settler units were probably 95% or higher. In each run, 98% or more of the uranium was recovered from the solvent in the three stripping stages which were operated at 30°C. The uranium concentration in the strip product solutions was usually in the range of 20 g/l., or a factor of 100-200 higher than that of the original wet-process acid. The solvent extract typically contained 2.5-3 g of iron/ l., about half of which was removed in the stripping system. The uranium/iron decontamination factor across the extraction-stripping system ranged from 35 to 50. The major consumption of chlorate in the stripping operation apparently was not due to the oxidation of uranium but to a side reaction in which chlorate reacts with chloride in the acid to produce chlorine and/or chlorine dioxide (Latimer, 1952). Much more efficient utilization of the chlorate was achieved in our tests when these gaseous products were contained by sealing the stripping units from the atmosphere, by operating the units at 30”C, and by adding most of the chlorate, in the form of an almost saturated water solution, to the first stripping stage (the balance was added to the third stage). Even under these conditions, the chlorate consumption in the demonstration tests was about 0.7 lb/lb of Us08 recovered, a factor of 5-6 higher than the stoichiometric quantity indicated by eq 2.
+
+
-
+
3U4+ C1033H20 3UO,’+ C1- + 6H+ (2) Strip product solutions, such as those denoted in Figures 6-8, can be diluted with water to about 6 M H3P04 and fed directly to the second cycle for recovery and purification of the uranium with DBEHPA-TOP0 solvent. Second-cycle operation was not demonstrated on a continuous basis since this had been done previously (Hurst, et al., 1972) with feed solutions very similar in composition to those derived from the OPPA first cycle. Some of the latter solution, however, was batch-extracted with 290
Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 3, 1974
D2EHPA-TOP0 solvent and stripped with ammonium carbonate solution to form AUT. Calcination of the AUT yielded a product that analyzed >98% U308, 0.5% Fe, 0.02% PO4, 1 ppm Ti, 3 ppm V, 0.8 ppm Mo, and
