Processes for recovering uranium from acid leach liquors used in the United States include solvent extraction with organic amines, solvent extraction with organophosphorus compounds, and anion exchange. Amine extraction in the so-called Amex process is described in this section with specific reference to the Kerr-McGee тШ. Solvent extraction with organophosphorus compounds in the so-called Dapex process was used in several U. S. mills, but is being phased out. It will be discussed briefly at the end of this section. Uranium recovery by anion exchange is to be discussed in Sec. 8.7.

Three papers from Oak Ridge National Laboratory provide a comprehensive summary of developments in solvent extraction of uranium from leach liquors. Coleman et al. [C2] describe studies of a number of possible amine extractants. Blake et al. [B4] describe work with organophosphorus compounds. Brown et al. [B8] describe processes based on both types of extractants.

Amine extraction. As explained in Sec. 5.4, long-chain organic amines act as liquid anion — exchange media for the uranyl sulfate complex anion through a reversible reaction such as

2(R3NH)iS04(o) + U02(S04)34-(®7) * (R3NH)4 U02(S04)3(o) + 2S04J-(e?)

The first reported use of high-molecular-weight amines to extract anions from aqueous solution was by the British workers Smith and Page [S3], who in 1948 used this method to separate strong acids from weak and suggested its use for recovering metals that form anionic complexes from aqueous solutions. Starting in 1953, Oak Ridge workers investigated a number of amines capable of extracting uranium as an anionic complex. Amines acting as liquid anion exchangers were found to be more selective in extracting uranium than organophosphorus compounds, which act as liquid cation-exchangers, because fewer of the metals associated with uranium in leach liquors form extractable anions than form extractable cations.

Of the numerous amines investigated [C2, B8], the one now used industrially in the Amex process is a mixture of straight-chain, saturated trioctyl and tridecyl amines, sold by General Mills Chemicals, Inc., under the trade name Alamine-336 and by Archer Daniels Midland Company under the name Adogen-364. Choice of this amine mixture has resulted partly from its commercial availability at an acceptable price of around $1.00/lb and partly from its having the necessary physical properties of good chemical stability, low aqueous-solubility, high uranium distribution coefficient, and good selectivity for uranium.

To use this amine, it is dissolved in a high-boiling kerosene diluent, at a concentration of around 3 v/o, approximately 0.1 M. To this solution is added around 3 v/o (0.2 M) of a long-chain alcohol, such as isodecanol, to increase solubility in the kerosene diluent of the sulfate and acid sulfate salts of the amine and its complexes with compounds of uranium and molybdenum.

Extraction equilibria with amines. Figure 5.7 shows the distribution of uranium between aqueous and organic phases observed [C2] at conditions similar to those used in the Amex process. Reported distribution coefficients range from nearly 200 in very dilute solutions to 90 to 130 at organic uranium concentrations around 0.01 M, with still lower values at higher concentrations. The leveling off of organic uranium concentration as aqueous concentration increases is attributed to approaching saturation of the amine with uranium, which would occur at 0.025 M for a complex of four amine molecules per uranium atom, as in the foregoing reaction. Physicochemical studies [C2] suggest that the complex actually contains around 4.7 amine molecules per uranium atom.

Distribution coefficients for uranium and other metals in trioctylamine are compared in Table 5.20. Although the conditions are not exactly those used in the Amex process, they indicate that the only element normally present in leach liquors that extracts readily with uranium is molybdenum. Ferric iron, which is always present in leach liquors and extracts in the Dapex process, is not extracted in the Amex process. Vanadium, if pentavalent, can be extracted by raising the pH from 1 to 2.

Aqueous uranium concentration, moles per liter

Figure S.7 Uranium extraction by 0.1 M tri-n-octylamine in 98 percent kerosene-2 percent tridecanol. Aqueous phase: pH = 1; 0.5 M S042 "

The uranium distribution coefficient decreases with increasing S042′ concentration, as shown in Fig. 5.8, owing to reversal of the preceding equilibrium. This permits stripping uranium from the amine by aqueous sulfate solution, as practiced at the Exxon mill, Table 5.19. The Kerr-McGee mill strips with 1.5 M Nad solution by the reaction

(R3 NH)4 U02 (S04 )3 (o) + 4NaCl(a?) ^ 4R3NHCl(o) + 2Na2S04(a?) + U02S04(aq)

Because the chloride salt of the amine is more stable than the sulfate salt or the uranyl sulfate complex, quite high aqueous uranium concentrations can be obtained with chloride stripping. Molybdenum is not stripped by sodium chloride and, if present, must be stripped by other means to prevent precipitation when its solubility of around 0.03 g/liter is exceeded.

Amine extraction in the Kerr-McGee mill. As a practical example of the use of organic amines to extract uranium from leach liquors, a description will be given of the solvent extraction section of the Kerr-McGee uranium mill, whose leaching section was described in Sec. 8.5 of this chapter [М3, Н4]. The solvent extraction plant consists of two similar circuits; process conditions approximating those of one circuit are shown in Fig. 5.9.

Leach liquor containing about 1 g U308/liter at a pH around 1.0 is fed at the rate of 3800 liters/min to the first of four mixer-settler states in series, where the uranium is extracted by a solution containing 3 v/o Alamine-336 (mixed и-trioctyl — and n-tridecylamines) and 3 v/o isodecanol in a high-boiling kerosene diluent. These four stages reduce the uranium content of the aqueous stream from 1 g U3Og/liter to around 0.001, while increasing that of the solvent from 0.002 g/liter to 3.33.

Table S.20 Distribution coefficients between aqueous sulfate solution and triisooctyl — aminet

Metal	Valence	Distribution coefficient
Uranium	6	90
Uranium	4	<1
Molybdenum	6	150
Zirconium	4	200
Vanadium	5	<1
Vanadium	5	~20 (pH = 2)
Vanadium	4	<0.01
Titanium	4	<0.1
Iron	2, 3	<0.01
Magnesium, calcium, manganese, copper, zinc	2	<0.01

^ pH = 1; S04 2 ~ = 1 M; amine 0.1 M in aromatic hydrocarbon diluent; 1 g metal per liter in aqueous feed; organic/aqueous volume ratio, 1:1. Source: C. F. Coleman et al., “Amine Salts as Solvent Extraction Reagents for Uranium and Other Metals,” PICG(2) 28: 278 (1958).

Figure 5.8 Variation of uranium dis­tribution coefficient in 0.1 M tri-n — octylamine with aqueous sulfate con­centration. pH = 1; 0.01 mol uranium/ liter in solvent.

Each stage consists of a central steel mixer tank 8 ft (2.5 m) in diameter and 9.5 ft (2.9 m) deep set in the center of a wooden settler tank 40 ft (12 m) in diameter. Mixing is by a turbine-type impeller. Mixed aqueous and solvent phases from the central tank flow through holes in its lateral wall to the settler annulus, where the phases separate. Aqueous phase leaves through holes at the outside of the settler in the bottom and flows down to the next stage, which is set 1 ft (03 m) lower. Solvent phase is collected in a circular launder surrounding the top of the outside of the settler and is pumped up to the preceding stage. A portion of the solvent phase is recycled from the settler to the mixer to permit the latter to operate with solvent phase continuous, a condition that reduces solvent losses by entrainment in aqueous effluent.

Uranium in rich solvent leaving the extraction section is transferred to the uranium product solution by back extraction into 1.5 M sodium chloride solution flowing at the rate of 114 liters/min in four uranium-stripping mixer-settler stages. Each of these consists of a separate wood mixer tank 8 ft (2.4 m) in diameter by 9 ft (2.7 m) high and a wood settler tank 22 ft (6.7 m) in diameter by 8 ft (2.4 m) high. In this section, solvent phase flows up by gravity and aqueous phase is pumped at such a rate as to control the interphase level in each settler. Solvent recycle is unnecessary, because solvent is the continuous phase, owing to its flow rate being higher than the aqueous.

Leach liquor feed,
from Fig. 5.6
3800 .(/min
I g U308/i

Ftoffinate,
to wash failings,
Fig. 5.6

0.001 g U308/i

Figure 5.9 Amex process for recovering uranium from leach liquor. Conditions approximately those of one circuit of Kerr-McGee mill.

Product solutions leaving the two solvent extraction circuits at a concentration around 30 g U308/liter are combined and flow through four stirred precipitation tanks 8 ft (2.4 m) in diameter by 12 ft (3.7 m) high in series. Steam is added to the first tank to heat its contents to 60°C. A mixture of two to four volumes of air and one volume of ammonia is added to the last three tanks to raise the pH to 7.0. This precipitates uranium as mixed sodium and ammonium diuranate.

The diuranate precipitate is separated from the mixed NaCl and Na2 S04 salt solution by a system of thickeners and filters. Filter cake from the first filter is washed with water, reslurried with water, filtered a second time, and washed again to reduce its content of NaCl and Na2 S04. When it is necessary to reduce the amount of sodium diuranate, a third stage of filtration is used, and the filter cake is reslurried with ammonium sulfate instead of water to replace most of the sodium with ammonium ion. Washed filter cake is dried by heating to 160 to 180°C.

Salt solution leaving the filtration system contains about 0.01 g U308 and 15 to 30 g СГ/liter. Most of this is recycled to make up part of the stripping solution, but some is bled to tailings to keep sulfate ion from building up.

Molybdenum is not stripped from the amine solvent by sodium chloride. If not kept below around 0.03 g Mo/liter, it precipitates as a sludge and interferes with uranium extraction. To control molybdenum concentration, a portion of the solvent leaving the uranium stripping section is contacted in a single mixer-settler with an aqueous solution of Na2C03 and NH4OH. This converts the molybdenum to sodium molybdate, Na2Mo04, and transfers it to the aqueous phase, from which molybdenum is recovered as a by-product.

The solvent makeup requirement reported by Hazen [H4] was only 0.21 volumes per 1000 volumes leach liquor treated.

Solvent extraction of uranium with organophosphorus compounds. The first reported use of organophosphorus compounds for solvent extraction of uranium from minerals was recovery of uranium from commercial phosphoric acid using as extractant the reaction product of phosphorus pentoxide and octyl alcohol [L2]. This led to research on many organophosphorus compounds for extraction of uranium from sulfuric acid uranium leach liquors by Dow Chemical Company and Oak Ridge National Laboratory, among others. The numerous compounds investigated have been described by Merritt [М3], Blake et al. [B4], and Brown et al. [B8]. The compound selected for use in three U. S. uranium mills [М3] in the late 1960s was di(2-ethylhexyl) phosphoric acid (EHPA) dissolved in kerosene, in the Dapex process, so-named by its developers at Oak Ridge National Laboratory.

Because of the long hydrocarbon chain, EHPA and its salts are insoluble in water, but are soluble in hydrocarbons such as kerosene. The reaction by which EHPA reacts with the uranyl cation in the aqueous phase and transfers it to the organic phase may be represented by

О О

II, H „

2HOP(OR)2(o) + U022+(®7) — U02 [OP(OR)2 ] 2 (о) + 2Haq)

although the actual reaction is more complex [B4]. Thus, EHPA acts as a liquid cation — exchanger.

At the conditions typical of the Dapex process (0.1 M EHPA, 0.5 M S042′, pH = 1) distribution coefficients at very low concentration are [B4]

U022+ 200

Fe3+ 135

Al3+ 0.03

Th4+ 20,000

V4* (0.01 A0 1,000

To maintain solvent capacity for uranium and to prevent contamination of extracted uranium by iron, it is necessary to reduce iron to the unextractable ferrous condition before solvent extraction. This is done by contacting the leach liquor with scrap iron, S02, or sodium sulfide. Because the iron content of leach liquor is high, reduction is costly, and the Amex process, in which ferric iron does not extract, is preferred for sulfuric acid leach liquors. The high distribution coefficient of other polyvalent cations such as Th4+ and V4+ in EHPA makes the Dapex process less selective for uranium than the Amex process.

In the Dapex process, uranium in the organic phase is usually stripped by contact with an aqueous solution of sodium carbonate, through the reaction

О О

II II

U02 [OP(OR)2] 2(o) + 3Na2C03(a<7) ^ 2NaOP(OR2)(o) + Na4U02(C03)3(aq)

The sodium uranyl carbonate is very soluble in the aqueous phase, but the sodium salt of EHPA has only limited solubility in the organic phase and tends to form a third liquid phase containing the salt, some diluent, and water. To prevent this, the organic phase is also made 0.1 M in TBP, in which the sodium salt of EHPA is soluble.

Although the Dapex process is no longer being widely used to extract uranium from sulfuric acid leach liquors, organic phosphoric acids are favored for extracting by-product uranium from commercial phosphoric acid. Organic amines are impractical for this application because they are too fully saturated by the strong acid.