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# Extraction of uranium from heap leaching solutions by liquid surfactant membranes

Didier Hartmann, Dominique Pareau, André Chesné, Gérard Durand

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Didier Hartmann, Dominique Pareau, André Chesné, Gérard Durand. Extraction of uranium from heap leaching solutions by liquid surfactant membranes. ISEC 1990, International Solvent Extraction Conference, Jul 1990, Kyoto, Japan. hal-04927275

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Distributed under a Creative Commons Attribution 4.0 International License
INTERNATIONAL SOLVENT
EXTRACTION CONFERENCE

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Kyoto (JAPAN)

July 16-21, 1990

article subsequently published in
"Process Metallurgy, 1992, vol.78, 1567-1572
EXTRACTION OF URANIUM FROM HEAP LEACHING SOLUTIONS
BY LIQUID SURFACTANT MEMBRANES

D. HARTMANN*, D. PAREAU**, A. CHESNE**, G. DURAND**

* COGEMA/SEPA B.P. 71 87250 Bessines, FRANCE

* Ecole Centrale de Paris, LCNI, 92295 Chatenay-Malabry, FRANCE

ABSTRACT

A pilot scale liquid surfactant membrane process, designed to purify and concentrate uranium from heap leaching solutions, has been developed. The process is based on a carrier-mediated transport of uranium by a tertiary amine, and stripping by a mixture of sodium carbonate and sodium hydroxide.

During batch experiments, membrane composition had been previously adjusted in order to promote uranium transfer, using a volumetric ratio of external phase to internal phase of 40.

In a laboratory pulsed column (with flowrates up to 10 l/h), operating parameters have been optimized, through hold-up measurements, by establishing a relationship between the emulsion's residence time in the column and the depletion of uranium in the external phase.

Pilot scale testing of the complete process has been performed using a 10 cm diameter by 1 m height pulsed column. Treatment of a leach solution containing 240 mg/l of uranium with a flowrate of 116 l/h, has yielded an extraction recovery of 97 %.

Crud formation does not hinder uranium transfer. Small amounts of iron contaminating the internal phase are easily removed by filtration. A pure solution of uranyl carbonate containing over 5 g/l of uranium is recovered, which is ready for yellow-cake precipitation.

INTRODUCTION

Most of low-grade ores are treated by heap leaching for economical reasons. In the case of uranium, heap leaching has been employed for a long time. In France, it yields solutions containing usually around 200 mg/l of uranium. Uranium is then concentrated and purified by ion exchange or/and solvent extraction.

Liquid membranes represent an alternative to these techniques for the recovery of metals from dilute solutions. A number of applications, using either surfactant or supported liquid membranes, led to the build-up of pilot facilities, concerning metals such as : copper [Subramanian, 1980 ; Bart et al., 1986], chromium [Kitagawa et al., 1977], zinc [Melzner et al., 1984], indium [Guerriero et al., 1988] and uranium, from both phosphoric acid [Hayworth et al., 1983 ; Dickens et al., 1984] and sulfuric acid leaching solutions
[Babcock et al., 1983]. However, the recovery of zinc seems to be the only example for which an industrial facility was set up [Draxler et al., 1986].

We have designed a liquid surfactant membrane process to purify and concentrate uranium from sulfuric acid heap leaching liquors [Hartmann, 1989]. Chemical and physical parameters of the system were preliminary studied with the objective of optimizing uranium transfer from synthetic solutions.

The aim of this work was to bring into operation a continuous process, first by studying uranium transfer in a laboratory pulsed column, and then by testing the complete process at pilot scale.

# EXPERIMENTAL

## 1. Solutions

Previous experiments have led to the following conditions :

* external aqueous phase :

- synthetic sulfuric acid solution containing 200 mg/l U.

- or heap leaching solution (U, Fe, Al, SiO₂, ...).

* membrane :

- tridodecylamine (MERCK) or Hostarex A327 (HOECHST) as carrier

- ECA 4360 (EXXON) as surfactant

- dodecane (PROLABO) or Isopar L (EXXON) as diluent.

* internal aqueous phase : Na₂CO₃ 1 M - NaOH 0.1 M (pH = 12.6).

## 2. Liquid - liquid contactors

* Laboratory pulsed column (mechanical pulsation) :

- inner diameter : 2.5 cm

- active height : 1 m

- 40 plates (disks and crowns) with 25 % free area

- distance between plates : 2.4 cm.

* Pilot scale column (pneumatic pulsation) :

- inner diameter : 10 cm

- active height : 1 m

- 20 plates (disks and crowns) with 25 % free area

- distance between plates : 4.5 cm.

# RESULTS AND DISCUSSION

## I. LABORATORY STUDY
## I.1. Influence of the flowrates of the two phases

The flowrates of the continuous aqueous phase ($q_c$) and dispersed emulsion phase ($q_d$) were varied, with a constant $q_c/q_d$ ratio (= 10).

The intensity of pulsation was kept constant (110 cm/min). The aqueous feed solution is a synthetic one. The results are summarized in the following table.

<caption>TABLE 1</caption>

Values of hold-up ($\beta$), emulsion's residence time ($t_d$) and uranium concentration in the raffinate ($C_f$) at steady state for different total specific throughputs (TST); $q_c/q_d$ = 10 and a.f = 110 cm/min.

<table><thead><tr><th>TST (l h<sup>-1</sup> cm<sup>-2</sup>)</th><th>β (%)</th><th>t<sub>d</sub></th><th>C<sub>f</sub> (mg/l)</th><th>Symbol</th></tr></thead><tbody><tr><td>1.5</td><td>9.4</td><td>4 min 23 s</td><td>6.4</td><td>△</td></tr><tr><td>1.8</td><td>16.5</td><td>6 min 25 s</td><td>2.5</td><td>◆</td></tr><tr><td>2.0</td><td>20.3</td><td>7 min 7 s</td><td>2.1</td><td>▼</td></tr><tr><td>2.24</td><td>29.5</td><td>9 min 14 s</td><td>1.3</td><td>□</td></tr></tbody></table>

Assuming a plug flow of the continuous phase and taking an average value for E.a along the column length, a mass balance leads to the following equation:

$$ \underset{C_f}{\mathrm{Ln}} \frac{C_0}{C_f} = \underset{\beta}{\mathrm{Ea}} \frac{q_d}{q_c} \times \underset{t_d}{\mathrm{x}} \times \underset{t_d}{\mathrm{x}} \quad \begin{array}{l} \mathrm{E} : \text{mass transfer coefficient} \\ \mathrm{a} : \text{external interface area} \\ \mathrm{D} : \text{diameter of emulsion globules} \end{array} \quad \mathrm{a} = 6 \beta/D $$

The plot of this relationship gives a straight line (Figure 1). The ordinate for $t_d = 0$ is different from zero because the plug flow model cannot be applied when the column height approaches zero.

The best uranium transfer is obtained for $D \approx 2$ mm, which corresponds to $E = 1.8 \times 10^{-3}$ cm/s.

<figure><img src="image_4.png" /><figcaption>Fig. 1: Relationship between uranium concentration in the raffinate ($C_f$) and emulsion's residence time in the column ($t_d$) at steady state for different total specific throughputs.</figcaption></figure>
## I.2. Influence of flowrates' ratio

The $q_c/q_d$ ratio was varied beyond 10 in order to increase the uranium concentration factor, while maintaining an acceptable uranium extraction. Optimal conditions found with a flowrates' ratio of 10 were conserved, i.e. $TST = 2.24 \text{ l h}^{-1} \text{ cm}^{-2}$ and $a.f = 110 \text{ cm/min}$.

<caption>TABLE 2</caption>

Values of hold-up ($\beta$), emulsion's residence time ($t_d$) and uranium concentration in the raffinate ($C_f$) at steady state for different flowrates' ratios.

<table><thead><tr><th>q<sub>c</sub>/q<sub>d</sub></th><th>β (%)</th><th>t<sub>d</sub></th><th>C<sub>f</sub> (mg/l)</th><th>[U] in the Internal Phase (g/l)</th><th>Concentration<sup>*</sup> Factor</th></tr></thead><tbody><tr><td>10</td><td>29.5</td><td>9 min 14 s</td><td>1.3</td><td>6.44</td><td>32.2</td></tr><tr><td>12</td><td>23.2</td><td>8 min 35 s</td><td>1.8</td><td>7.10</td><td>35.5</td></tr><tr><td>15</td><td>11.5</td><td>5 min 14 s</td><td>12</td><td>8.54</td><td>42.7</td></tr></tbody></table>

* The concentration factor is defined as $[U]$ in the internal phase/$C_0$.

An increase of uranium concentration factor is observed when higher $q_c/q_d$ ratios are used. However, ratios exceeding 12 lead to incomplete uranium extraction. In France, the specification for industrial waste streams is 1.8 mg/l of uranium.

A better depletion of uranium in the external phase could probably be obtained with $q_c/q_d = 15$ by using a higher column.

## I.3. Influence of emulsion phase ratio (O/A)

A higher uranium concentration factor was tested by increasing the O/A ratio of the emulsion. The other operating parameters were:
$q_c/q_d = 10$, $TST = 2.24 \text{ l h}^{-1} \text{ cm}^{-2}$ and $a.f = 110 \text{ cm/min}$.

<caption>TABLE 3</caption>

Values of hold-up ($\beta$), emulsion's residence time ($t_d$) and uranium concentration in the raffinate ($C_f$) at steady state for different emulsion O/A ratios.

<table><thead><tr><th>O/A</th><th>β (%)</th><th>t<sub>d</sub></th><th>C<sub>f</sub> (mg/l)</th><th>[U] in the Internal Phase (g/l)</th><th>Concentration Factor</th></tr></thead><tbody><tr><td>3</td><td>29.5</td><td>9 min 14 s</td><td>1.3</td><td>6.44</td><td>32.2</td></tr><tr><td>4</td><td>10.5</td><td>3 min 18 s</td><td>13</td><td>7.83</td><td>39.1</td></tr><tr><td>5</td><td>9.8</td><td>5 min 3 s</td><td>15</td><td>10.1</td><td>50.5</td></tr></tbody></table>

The uranium concentration factor increases when using higher emulsion O/A ratios, especially between $O/A = 4$ and $O/A = 5$, but the
extraction of uranium falls for O/A ≥ 4. This is due to an important decrease of the hold-up resulting from a reduced emulsion density.

## II. PILOT OPERATION

### II.1. Experiments with an industrial solution

The composition of the feed solution is as follows :

$$ [U] = 240 \text{ mg/l} \quad [Fe] = 1.83 \text{ g/l} \quad [SiO}_2 \text{ = 89 mg/l} \quad \text{pH} = 1.60. $$

The solution was clarified over a sand column and filtered through a 2 µm cartridge for complete elimination of suspended solids. The total specific throughput was reduced to 1.5 $\text{lh}^{-1}\text{cm}^{-2}$ ($q_c = 116 \text{ l/h}$, $q_d = 11.6 \text{ l/h}$), hence preventing loss by entrainment of the smallest emulsion globules to the external phase output.

With a pulsation intensity of 117 cm/min, a uranium extraction recovery of 98 % was obtained at steady state from a synthetic solution ($C_f = 4.1 \text{ mg/l}$, $\beta = 11.4 \%$, $t_d = 5 \text{ min 50 s}$). These operating parameters were maintained for the next experiments.

The influence of the industrial solution pH on uranium transfer and importance of crud formation was studied. The results are summarized in the following table.

<caption>TABLE 4</caption>

Values of uranium concentration in the raffinate ($C_f$), uranium concentration in the internal phase and quantity of crud for different initial pH values of a heap leaching solution.

<table><thead><tr><th></th><th>1.60</th><th>1.50</th><th>1.30</th><th>1.10</th></tr></thead><tbody><tr><td>Feed Solution pH</td><td></td><td></td><td></td><td></td></tr><tr><td>C<sub>f</sub> at Steady State (mg/l)</td><td>24</td><td>8.1</td><td>12</td><td>27</td></tr><tr><td>Uranium Extraction Recovery (%)</td><td>90</td><td>97</td><td>95</td><td>89</td></tr><tr><td>[U] in the Internal Phase (g/l)</td><td>5.41</td><td>5.55</td><td>5.51</td><td>5.22</td></tr><tr><td>Amount of Dried Crud (g/m<sup>3</sup> of feed solution)</td><td>379</td><td>118</td><td>98</td><td>74</td></tr></tbody></table>

At pH 1.50, no significant loss in uranium recovery is observed with the industrial solution. The small difference is due to crud formation at the external area of emulsion globules during transfer. Though cruds occupy an important volume before settling because they are swollen by solvent, they do not affect uranium transfer significantly for pH < 1.50. Moreover, the crud solid content remains very
limited.

The amount of crud decreases when the feed solution is acidified; however below pH 1.3, uranium transfer is slower due to the formation of amine bisulfate in the membrane.

Cruds contain a lot of Fe, SO₄, PO₄ and small quantities of U, Ca and SiO₂. They are probably generated by the interaction between low solubles salts and the surfactant.

## II.2. Electrostatic coalescence

The apparatus, designed by COGEMA/SEPA, has flat, parallel and horizontal electrodes, according to recommendations found in the literature [Hsu et al., 1985]. The characteristics of the coalescer are: active volume = 1 l, distance between the electrodes = 2 cm, thickness of polycarbonate insulator = 1 mm.

A complete coalescence was obtained with the following conditions:

- electrical parameters: alternative current, 2 500 V, 50 Hz

- emulsion flowrate: 16 l/h, corresponding to 3 min 45 s of residence time in the coalescer.

At 50 Hz, the minimum voltage required for a good coalescence lies between 2 000 and 2 500 V. For different voltages above 2 500 V, we noticed a favorable influence of low frequencies in the 25 - 1 000 Hz range. Initial problems encountered with the adhesion of the insulator to the electrodes were successfully overcome.

## REFERENCES

- Babcock, W.C., Chidlaw, M.B., Kelly, D.J. and LaChapelle, E.D., 1983. US Bureau of Mines Report No. 203-83.

- Bart, H.J., Wachter, R. and Marr, R., 1986. Proc. Int. Solv. Extr. Conf., Munich, III: 901-905.

- Dickens, N., Rose, M. and Davies, G.A., 1984. In "I. Chem. E. Symposium Series No. 88": 69-81.

- Draxler, J. and Marr, R., 1986. Chem. Eng. Process., 20: 319-329.

- Guerriero, R. and Meregalli, L., 1988. Hydrometallurgy, 20: 109-120.

- Hartmann, D., 1989. Doctor Thesis, Ecole Centrale de Paris, France.

- Hayworth, H.C., Ho, W.S., Burns, W.A. and Li, N.N., 1983. Sep. Sci. Technol., 18 (6): 493-521.

- Hsu, E.C. and Li, N.N., 1985. Sep. Sci. Technol., 20 (2,3): 115-130.

- Kitagawa, T., Nishikawa, Y., Frankenfeld, J.W. and Li, N.N., 1977. Environ. Sci. Technol., 11 (6): 602-605.

- Melzner, D., Tilkowski, J., Mohrmann, A., Poppe, W., Halwachs, W. and Schugerl, K., 1984. Hydrometallurgy, 13: 105-123.

- Subramanian, K.N., 1980. Proc. of the Liquid Membrane Seminar at UMIST, Manchester.