Uranium recovery from wet-process phosphoric acid by a commercial ceramic product

2.1 Materials and analytical procedure

The studied commercial ceramic sample was obtained from Alfa Ceramic Co. Cairo Egypt. Its average chemical composition is shown in Table 1.

A uraniferous phosphoric acid stock solution was prepared by adding a 100 ml portion of uranium solution assaying 1000 mg/L (by dissolving 0.1782 g of uranyl acetate [UO₂ (CH₃COO)₂·2H₂O] of BDH Chemicals Ltd. Poole, England in 100 ml distilled water) to a previously prepared phosphoric acid solution assaying 5% P₂O₅. The buffers of pH 4 and 7 were used for calibration of the pH meter, and HCl and NaOH solutions were added for pH adjustment.

Uranium was analyzed in the different working aqueous phases using the Arsenazo III complex method (Marczenko, 1976). Absorbance of the formed uranium Arsenazo III complex was measured at 650 nm against proper standard solutions using a Lambada UV/VIS spectrophotometer (Perkin-Elmer, USA).

2.2 Preparation of the ceramic sample for adsorption

The obtained ceramic sample was ground in jaw and roll crushers, homogenized, pulverized (in a ball mill) and sieved through a 120 mesh (0.149 mm) sized sieve. The pulverized ceramic was treated with a 0.5 M HNO₃ solution at room temperature. The treated sample was filtered and washed with distilled water and dried overnight at 50 °C in a laboratory oven.

2.3 Equilibrium studies (batch experiments)

In order to study the relevant factors affecting the adsorption operation, many series of adsorption experiments were performed using the synthetic uraniferous phosphoric acid stock solution. These factors involved contact time, initial uranium concentration, temperature, solution pH, the adsorbent dosage and phosphoric acid concentration. From the obtained results, the adsorption isotherms were resolved. The adsorption experiments were performed by shaking 0.5 g sample portions of the prepared sample of ceramic adsorbent with 20 ml of the synthetic uraniferous phosphoric acid solution (of 50 mg/L) initial uranium concentration) using a magnetic stirrer. The adsorbed amounts of uranium were calculated by the difference between its equilibrium and initial concentrations.

For eluting (or de-sorbing) the loaded uranium from the ceramic adsorbent, a number of eluting agents were tested, namely NaCl–H₂SO₄, Na₂CO₃–CaSO₄ and citric (C₆H₈O₆) acid.

2.4 Columnar procedure

In the present work the study of uranium recovery (removal) from the wet-process phosphoric acid in concern was carried out using a glass column (of 1 cm diameter) packed with 10 g of the prepared ceramic adsorbent. The ceramic adsorbent bed (6 cm) was initially wetted with dilute phosphoric acid. The wet-process phosphoric acid solution was percolated through the ceramic bed under a fixed flow-rate (of 2 ml/min).

Uranium elution (or de-sorption) from the loaded ceramic bed was performed by passing the eluent solution of choice through the latter under a fixed flow-rate of 1 ml/min.

3.1 Results of equilibrium studies

3.1.1

3.1.1 Effect of contact time

In order to study the effect of increasing contact time upon uranium adsorption on the ceramic adsorbent, a series of adsorption experiments were performed by contacting a fixed weight (0.5 g) with a fixed portion (20 ml) from the synthetic uraniferous phosphoric acid solution (of 50 mg U/L) adjusted at pH 1 at room temperature (≈25 °C). The studied time intervals ranged from 5 up to 180 min. The obtained results were plotted in Fig. 1. From this figure, the uranium adsorption efficiency attained about 20% at the first experiment (of 5 min). Uranium adsorption efficiencies steadily increased by increasing time till the 3rd experiment (of 60 min) to attain about 95%. Increasing the contact time above 60 min gave no improvement in the adsorption efficiency. Therefore, 60 min is the optimum contact time.

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Figure 1

Effect of contact time upon uranium adsorption efficiency on ceramic adsorbent.

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3.1.2

3.1.2 Effect of initial uranium concentration

For studying the effect of initial uranium concentration upon the adsorption efficiency on the ceramic adsorbent, a series of experiments were performed by contacting a fixed weight (0.5 g) for 60 min at room temperature (≈25 °C) and pH 1. The studied initial uranium concentrations ranged from 50 up to 200 mg/L. The obtained results were plotted in Fig. 2. From this figure, it is clearly obvious that uranium adsorption efficiency decreases with increasing its initial concentration. Therefore, the uraniferous solution of 50 mg U/L could be chosen as the optimum concentration. The experimental capacity of the ceramic adsorbent could be concluded from Fig. 2 (about 11 mg U/g ceramic).

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Figure 2

Effect of initial uranium concentrations upon uranium adsorption efficiency on the ceramic adsorbent.

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3.1.3

3.1.3 Adsorption isotherms

The adsorption isotherms for uranium adsorption upon ceramic adsorbent were obtained depending on Fig. 2. The equilibrium adsorption isotherms are analyzed to obtain the Langmuir, Freundlich and Dubinin–Radushkevich (D–R) constants.

3.1.3.1

3.1.3.1 Langmuir isotherm

Langmuir adsorption isotherm models the monolayer coverage of the adsorbent surfaces and assumes that sorption occurs on a structurally homogeneous adsorbent and all its sorption sites are energetically identical. The linearized form of the Longmuir equation is given by the following equation (Hussein, 2011),

(1)

$${\text{C}}_{\text{e}}/{\text{q}}_{\text{e}}=1/({\text{bQ}}_0)+{\text{C}}_{\text{e}}/{\text{Q}}_0$$ where q_e is the amount of solute adsorbed per unit weight of adsorbent (mg/g), C_e is the equilibrium concentration of the solute in the bulk solution (mg/L), Q_o is the monolayer adsorption capacity (mg/g) and b is the sorption equilibrium constant which is related to the free energy of adsorption (b = ae^−Δ g/RT)

The graphic representation of (C_e/q_e) versus C_e gives a straight line with a slope of (1/Q₀) and intercept of 1/(bQ₀) as seen in Fig. 3. The Langmuir parameters are given in Table 2.

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Figure 3

Langmuir isotherm plot for uranium adsorption on ceramic adsorbent.

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Table 2 Langmuir, Freundlich and Dubinin–Radushkevich (D–R) parameters for uranium adsorption on ceramic adsorbent.

Parameter	Value	R2
Langmuir isotherm Q0 (mg g−1)	11.428	0.931
b (L mg−1)	0.0569
Freundlich isotherm Kf (mg g−1)	1.8	0.785
n	1.3
Dubinin–Radushkevich D–R isotherm β (kJ2mol−2)	−0.0001	0.955
qm (mmol g−1)	8.36
Eads (kJ mol−1)	70.7

3.1.3.2

3.1.3.2 Dubinin–Radushkevich (D–R) Isotherm

The D–R isotherm is more general than the Langmuir isotherm, because it does not assume a homogeneous surface or constant sorption potential. The D–R isotherm is represented by the following equation,

(2)

$$\text{ln}({\text{q}}_{\text{e}})=\text{ln}({\text{q}}_{\text{m}})-\beta {\mathrm{\epsilon }}^2$$ Where q_e is the adsorbed amount of uranium at equilibrium, β is a constant related to the adsorption energy, q_m is the theoretical saturation capacity, and ε is the Polanyi potential; i.e.,

(3)

$$\mathrm{\epsilon }=\text{RT}\text{ln}(1+1/{\text{C}}_{\text{e}})$$ where R is the gas constant (8.314 kJ/mol⁻¹ K⁻¹) and T is the absolute temperature (K). The values of q_m and β could be deduced by plotting ln (q_e) versus ε² (Fig. 4) and the mean energy of adsorption (E_ads) was calculated from the following equation;

(4)

$$\text{Ea}=1/{(-2\beta )}^{-1/2}$$ D–R constants are given in Table 2; from which it is clearly obvious that the type of uranium adsorption on a ceramic adsorbent is an ion exchange process.

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Figure 4

Dubinin–Radushkevich (D–R) isotherm plot for uranium adsorption on ceramic adsorbent.

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3.1.3.3

3.1.3.3 Freundlich isotherm

The empirical model of the Freundlich isotherm was shown to be consistent with experimental distribution of active centers, characteristic of heterogeneous surfaces. The logarithmic (linear) form of the Freundlich equation is written as follows (Freundlich, 1932),

(5)

$$\text{Log}({\text{q}}_{\text{e}})=\text{log}({\text{K}}_{\text{F}})+1/\text{n}\text{log}({\text{C}}_{\text{e}})$$ Where K_f and n are the Freundlich constants which represent sorption capacity and sorption intensity, respectively. A plot of log(qe) versus log(C_e) would result in a straight line with a slope of (1/n) and intercept of log(K_f) as seen in Fig. 5. The Freundlich intensity constant (1/n) of a value less than unity indicates a concentration dependent sorption for uranium on ceramic (Fangli et al., 2011). Freundlich constants are given in Table 2. The experimental data shows the adsorption of uranium on ceramic adsorbent fitted with Langmuir isotherm.

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Figure 5

Freundlich isotherm plot for adsorption of uranium on ceramic adsorbent.

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3.1.4

3.1.4 Effect of adsorption temperature

To study the effect of temperature upon uranium adsorption on a ceramic adsorbent, a series of the adsorption experiments were performed using different temperatures ranging from 25 upto 60 °C. In this series of experiments the other parameters were kept constant, i.e., 12.5 g ceramic/L, initial uranium concentration of 50 mg/L, pH 1 and 60 min contact time. The obtained results were plotted in Fig. 6. From this figure, it is clearly obvious that uranium adsorption efficiency decreased with increasing temperature. This indicates that the adsorption reaction is an exothermic process.

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Figure 6

Effect of temperature upon uranium adsorption efficiency on ceramic adsorbent.

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3.1.5

3.1.5 Effect of pH

The pH of the aqueous solution is an important variable in the adsorption operation. Therefore, the effect of pH upon uranium adsorption by ceramic was studied in the pH range of 1–9. The experiments were performed under the constant initial uranium concentration of 12.5 g ceramic/L at room temperature (≈25 °C) for 60 min shaking time. The obtained results were plotted in Fig. 7. From this figure, one could observe that uranium adsorption efficiency sharply decreases with increasing pH values. The decrease of uranium adsorption efficiency by increasing the solution pH's (after pH 1) uranyl ions starts to hydrolyze (Misaelides et al., 1999) and after pH 4 precipitation stats. Also, the oxides present in the studied ceramic sample (i.e. SiO₂, Al₂O₃, Fe₂O₃, CaO and MgO) cane cause a pronounced adsorption competition between them and uranium. Thus, we can recommend the use of a solution having pH value of 1.

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Figure 7

Effect of solution pH upon uranium adsorption efficiency on ceramic adsorbent.

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3.1.6

3.1.6 Effect of increasing ceramic amount (dose)

In order to study the effect if increasing ceramic amounts upon uranium adsorption efficiency, a series of adsorption experiments were performed using different ceramic amounts (doses) ranging from 2.5 up to 15 g ceramic/L. the other conditions (parameters) were fixed at 50 mg U/L as initial uranium concentration, 1 pH value and 60 min contact time at room temperature. The obtained data are plotted in Fig. 8 in which one can observe that uranium adsorption efficiency increased proportionally with increasing ceramic amount (dose) till the 5th experiment (12.5 g ceramic/L). Increasing the amounts of ceramic adsorbent beyond the dose of 12.5 g/L gave no improvement in the adsorption efficiency, due to the increase of undesired active sites exceeding the fixed uranium amount (50 mg/L) in the fixed solution portions 20 ml).

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Figure 8

Effect of increasing ceramic adsorbent amount (dose) upon uranium adsorption efficiency on ceramic adsorbent.

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3.1.7

3.1.7 Effect of phosphoric acid concentration

For studying the effect of phosphoric acid concentration upon uranium adsorption efficiency, a series of adsorption experiments were performed using different phosphoric acid (P₂O₅) concentrations ranging from 5 up to 40%. The other conditions were fixed at 12.5 g ceramic/L 50 mg U/L as initial uranium concentration, 1 pH value and 60 min contact time at room temperature. Fig. 9 shows a plot of the obtained results. From this figure, it is clearly obvious that uranium adsorption efficiency decreased (sharply) with increasing P₂O₅ concentration. This observation could be explained by the fact that, increasing P₂O₅ concentration is associated with the increase of hydrogen protons in the solution which can compete uranium adsorption on the surfaces of ceramic particles.

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Figure 9

Effect of increasing P₂O₅ concentration upon uranium adsorption efficiency.

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3.2 Uranium elution (de-sorption)

The following eluting agents, i.e. 1 M NaCl + 0.1 M H₂SO₄, 1 M NaCl + 0.05 M H₂SO₄, 0.5 M Na₂CO₃ + 1.5 M CaSO₄ and 0.5 M citric acid were tested for uranium elution from the loaded ceramic. The elution experiments were carried out by three elution circuits by shaking the loaded ceramic sample (4 g) with three fresh eluent portions (20 ml each). A systematic calculation of the eluted uranium amounts was summarized in Table 3. From this Table, it is clearly obvious that the 1 M NaCl + 0.1 M H₂SO₄ elution solution is the best solution tested for uranium elution from the loaded ceramic adsorbent.

3.3 Uranium recovery from crude phosphoric acid

As previously mentioned, the prepared ceramic adsorbent has a satisfactory uranium adsorption capacity (about 11 mg U/g ceramic). In this work, the study of uranium recovery from crude phosphoric acid was carried out using a glass column (1 cm diameter) packed with 10 g of the prepared ceramic adsorbent. The ceramic bed (6 cm) was initially wetted with dilute phosphoric acid. The crude phosphoric acid solution was percolated through the ceramic bed for uranium adsorption using a fixed flow-rate of 2 ml/min. For uranium elution (or de-sorption) from the loaded ceramic bed, the choiced eluent solution was passed through the latter under a fixed flow-rate of 1 ml/min.

3.3.1

3.3.1 Uranium adsorption

Adsorption operation (loading) of uranium is the first step in the ion-exchange process. The obtained data of uranium adsorption efficiencies are plotted in Fig. 10 which is a plot of the collected effluent samples versus throughput volumes (adsorption or loading curve). Actual uranium breakthrough has been observed at the 16th sample fraction (throughput value of 320 ml) where uranium concentration in the effluent attains 1.17 mg/L (about 2% of that in the feed). On the other hand, an almost adsorbent (ceramic) saturation was seen at the 42nd sample fraction (throughput volume 840 ml). Systematic calculation of the loaded uranium contents from its analysis in the effluent sample portions revealed that only 33.64 mg of uranium has been adsorbed. Comparing this loading capacity with the theoretical capacity of ceramic (about 11 mg U/g ceramic) indicates that under working conditions about 41% of theoretical capacity was realized. The decrease in the ceramic capacity after contacting with the working sample (crude phosphoric acid) may be due to the competition between uranium and the present ions (especially iron).

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Figure 10

Uranium adsorption curve of crude phosphoric acid by ceramic adsorbent.

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3.3.2

3.3.2 Uranium elution

The solution of 1 M NaCl + 0.1 M H₂SO₄ is used as an eluent for uranium from the ceramic bed. The plotted curve (Fig. 11) exhibits the famous bell-shaped curve with a major peak at the 8th throughput sample. Systematic calculation of the eluted samples (20 ml each) for uranium analysis in the collected solution gave about 95% uranium elution efficiency (about 31.9 mg U were eluted).

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Figure 11

Uranium elution curve of ceramic adsorbent using 1 M NaCl + 0.1 M H₂SO₄ as eluent.

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