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Solvent extraction behavior of metal ions and selective separation Sc3+ in phosphoric acid medium using P204
Separation and Purification Technology 209 (2019) 175–181
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Solvent extraction behavior of metal ions and selective separation Sc3+ in phosphoric acid medium using P204 ⁎
T
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Qing Ye, Guanghui Li1, , Bona Deng, Jun Luo1, , Mingjun Rao, Zhiwei Peng, Yuanbo Zhang, Tao Jiang School of Minerals Processing & Bioengineering, Central South University, Changsha, Hunan 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Scandium Phosphoric acid Solvent extraction P204
Acid leaching-solvent extraction is an effective process to extract scandium from scandium-bearing resources. This study was aimed to investigate the extraction behavior of metal ions, including Sc3+, Fe3+, Al3+and Ca2+, in phosphoric acid medium using P204. More than 95% of scandium was selectively extracted under the conditions of pH value of 1.5–1.8, aqueous-organic ratio of 3 and oscillation for 15 min. The impurity elements like Fe3+, Al3+ and Ca2+, were separated via the solvent extraction process using P204 (e.g., separation coefficient of scandium to iron is 298). In stripping process, the majority of co-extracted metal ions can be removed from the organic phase with hydrochloric acid solution. The scandium-bearing organic phase was stripped with 4 mol/L NaOH, wherein the recovery of scandium attained about 95% while that of the co-extracted iron and aluminum were only 3.1% and 1.2%, respectively. It was also confirmed that both P204 and H3PO4 played roles in extraction reaction, and the desirable extraction of scandium in P204 was attributed to the ion exchange between hydrogen ion of ePO(OH) and Sc3+ on acidic condition (pH = 0.4–1.5). Impurity elements (Fe3+ and Al3+) also reacted with phosphate anion to form hydrophilic ions, and in turn result in selective extraction of scandium in phosphoric acid leachate using P204.
1. Introduction Scandium is one of rare earth elements and has widely application on engineering industrials. Scandium mainly exists in bauxite ores, phosphorite deposits and rare earth ores, which is associated with other metal elements [1]. Bauxite ore accounts for 54% of scandium-bearing ore [2]. After processing bauxite ore in the alumina refineries, scandium and other elements like Fe, Ca and Ti were enriched in bauxite ore residues. Generally, the bauxite ore residues from China, Greece, and India etc., contain 100–200 mg/kg of scandium, which makes these materials exploitable as a scandium-containing resource [3]. Extraction of scandium from scandium-bearing resources can be achieved by directly acid leaching using a variety of lixiviates [4]. From the leachate, scandium is extracted using solvent extraction technology. The major issue is that the concentration of impurities, especially iron, aluminum and calcium in scandium-bearing resources are often much higher than scandium content, exerting adverse effects on the acidic leaching and solvent extraction process of scandium. For the leaching process of hydrochloric acid at concentration of 8 mol/L, majority of
metal ions are dissolved into the leaching solution [5]. Metal ions like Fe3+, Al3+ and Ca2+ would be co-extracted during solvent extraction process. A recent study reported that the rare earth elements (incl. scandium) in rare earth ore can be completely extracted into solutions by roasting with concentrated sulphuric acid at 250–300 °C followed by water leaching [6]. The fractional extractions, seven or even nine stages, are also needed to separate impurity elements from leachate. Kim [7] and Klauber [8] investigated the performance of bauxite residue in sulfuric acid leaching medium. Sulfuric acid concentration of 6 mol/L or even 10 mol/L is needed and it is not suitable for high calcium-bearing resources. Calcium sulfate leads to the formation of emulsion and the third phase during solvent extraction process, resulting in inadequate separation and scandium loss. Hence, it is important to further investigate the extraction behavior of metal ions and selective separation Sc3+ in phosphoric acid medium. Extraction behavior of scandium in acid medium e.g. nitric acid, hydrochloric acid and sulfuric acid was investigated in previous study. Several acidic extractants including P204, P507 and P350 are generally used to extract metal ions in previous study. P204 was used to extract
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Corresponding authors at: Peace Building, RM 253, Central South University, Changsha, Hunan 410083, China (G. Li). Bio-building, RM 204, Central South University, Changsha, Hunan 410083, China (J. Luo). E-mail addresses: [email protected] (G. Li), [email protected] (J. Luo). 1 Both corresponding authors contributed to this work equally. https://doi.org/10.1016/j.seppur.2018.07.033 Received 13 May 2018; Received in revised form 13 July 2018; Accepted 13 July 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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emission spectrometer (ICP-AES, Thermo Fisher Scientific, Icap7400 Radial, USA). The loaded organic phase was analyzed using Fourier transform infrared spectroscopy (FTIR, Nicolet, NEXUS-670, USA). Infrared absorption spectra of samples were obtained from the KBr pellets by a Fourier transform infrared spectrometer (Nicolet-470, USA).
scandium in sulfuric acid leachate derived from nickeliferous laterite ore [9]. The extraction ratio of scandium was about 80% after scrubbing process. However, it also needed multi segmentally stripping process to separate other metal ions. Zhu et al. [10] and Chang et al. [11] investigated scandium extraction from red mud by solvent extraction using P507 and P204. The extraction ratio of scandium reached above 90% in hydrochloric acid medium, while the metal ions like Fe3+, Al3+ and Ca2+ were usually co-extracted. Liu et al. [12] focused on the extraction mechanism by using phosphorodiamidate (TBP and P204) in nitric acid medium. Their results revealed that the coordination reaction occurred between rare earth ions and P]O group. P204 has strong extraction ability, but it was difficult to separate Sc3+ from impurity elements in acid medium [11]. In our previous study [13], P204-H3PO4 solvent extraction system is proposed to selectively separate scandium from leachate. In this system, the separation coefficients between scandium and metal ions (e.g. βSc/Fe = 2703) in phosphoric acid medium are much higher than other kinds of acid like H2SO4, HNO3 and HCl via solvent extraction process. However, the mechanism of selective separation of metal ions on extraction process was not clear. This study was aimed to systematically investigate the extraction behaviors and mechanisms of metal ions, including Sc3+, Fe3+, Al3+ and Ca2+, in phosphoric acid medium using P204. The ion exchange between hydrogen ion of ePO(OH) and metal ions was studied for the selective separation from the phosphoric acid leaching solution of scandium-bearing resources. Finally, a conceptual process flow sheet for separation of metal ions including Sc3+, Fe3+, Al3+ and Ca2+, was proposed.
3. Results and discussion 3.1. Solvent extraction behaviors of metal ions using P204 As described in our previous work [14,15], the scandium-rich material was obtained from bauxite ore residue after reductive roasting followed by H3PO4 leaching and NaOH leaching. Iron oxide is changed into metallic iron by reductive roasting, and aluminum silicate is transformed to amorphous silica via H3PO4 leaching process. Alumina enriched in acid residue is further leached by sodium hydroxide solution. Along with the stepwise recovery of valuable elements, scandium and titanium are further enriched in the alkaline leaching residue. The scandium-rich residue was leached by H3PO4 with 8 mol/L at 120 °C for 60 min. As TiO2+ is the main form in phosphoric acid leachate, before solvent extraction process, it can be removed as metatitanic acid precipitate by adjusting pH value [13]. After adjusting the pH value of solution, the main chemical composition of leachate used for solvent extraction test is shown in Table 1. The content of scandium is enriched to 23.62 mg/L in H3PO4 leachate. The solvent extraction behavior and selective separation Sc3+ from Fe3+, Al3+ and Ca2+ was investigated in this study (see Fig. 1).
2. Methods 3.1.1. Effect of P204 concentration The effect of P204 concentration (the volume ratio of P204/organic phase) on extraction behavior was investigated under the conditions of the phosphoric acid leachate pH value of 0.5, A/O (the volume ratio of aqueous phase/organic phase) of 3, and oscillation time of 15 min. The results are presented in Fig. 2. The extraction ratio of scandium increased from 45% to 75% with increasing P204 concentration from 1% to 4%, and the extraction of iron and aluminum also increased. The extraction ratio of iron was nearly 30% with 4% P204, whereas it was only 5% with 1% P204. The extraction of aluminum had the similar trend with increasing P204 concentration. As the results shown in Fig. 2(b), the trend of separation coefficient (β(Sc / Fe) ) was similar to that of β(Sc / Al) , which increased firstly and then decreased. However, the β(Sc / Ca) remained stable with P204 concentration increasing. Therefore, the optimal P204 concentration was recommended as 2–3% for extraction of scandium.
2.1. Experimental procedure The pH value of acid leachate was adjusted by adding phosphoric acid prior to solvent extraction. Samples of P204 reagent have been mixed with the required volume of kerosene at room temperature. The volumes of both the contacted aqueous and organic phase were filled into separating funnel and oscillated for 15 min. After stratifying, the aqueous phase was withdrawn for analysis, while the loaded organic phase was washed by HCl solution with appropriate concentration. Finally, the scandium-bearing organic phases was stripped with NaOH solution. The elemental concentration in acid solution was analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES). Furthermore, after solvent extraction, the concentration of the elements in aqueous phase were also analyzed using ICP-AES. The extraction ratio (η), distribution ratio (D), and separation coefficient (β) were calculated by Eqs. (1)–(3), respectively.
C ·V η = 1− A A × 100% Ct ·Vt
3.1.2. Effect of initial solution pH The effect of solution pH value on extraction behavior was investigated in the aqueous pH value range of 0.1–1.8 under the conditions of 2% (V/V) P204, A/O of 3 and oscillation time of 15 min. The results in Fig. 3(a) showed that the extraction of scandium increased obviously from 30% to 97% when the initial pH range increased from 0.1 to 1.5, and then remained constant with the further increase of pH value to 1.8. And also, the extraction ratio of iron remarkably decreased from 62% to 18% at the pH range of 0.1–1.5. The extraction ratio of aluminum decreased from 22% to 10% at the pH range of 0.1–1.5. The extraction performance of calcium was similar to scandium, increased from 40% to over 60% with pH value increased. As the results shown in Fig. 3(b), the β(Sc / Fe) increased with the increasing
(1)
where η is the extraction ratio; CA is elements contents of aqueous phase obtained after the solvent extraction, ppm; VA is the volume of aqueous phase, mL.
D=
c0 C ·V −C ·V = t t A A cA CA·VO
(2)
where D is distribution ratio; c0 is the concentration of elements in oil phase, ppm; VO is the volume of oil phase, mL.
β(1/2) =
D1 D2
(3) Table 1 The concentration of elements in H3PO4 leachate solution.
where β(1/2) is separation coefficient between 1 and 2 partition. 2.2. Instrument techniques Metal ions content of solution samples were determined by plasma 176
Element
Sc
Fe
Al
Ca
Content (mg/L)
23.62
1440
825
6639
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Phosphoric acid leachate (Sc3+, Fe 3+, Al3+, Ca2+) Phosphoric acid
aluminum attained maximum of 40% and 43% at A/O of 8, respectively. As the results shown in Fig. 4(b), the separation coefficient obviously decreased with the increasing of A/O. The separation coefficient was lower than 20 at A/O over 4. Actually, the extraction ratio of metal ions including Fe, Al and Sc showed relatively smooth change at the A/O range of 1–4. And the consumption of extractant decreased with A/O ratio increased at the same volume of aqueous phase. Hence, A/O of 3–4 was suitable to extract scandium in phosphoric acid medium.
Adjusting pH value
Solvent extraction
Aqueous phase containing Fe3+,Al3+ and Ca2+
Organic phase 1 Washing
Acid leachate containing Fe3+,Al3+ and Ca2+
3.1.4. Effect of extraction time The effect of increasing extraction time from 5 to 20 min on scandium extraction was investigated under the conditions of P204 concentration of 2%, initial pH value of 1.5 and A/O of 3. The results are indicated in Fig. 5. The results showed that scandium was almost extracted completely under the above conditions. Further, some impurity ions, particularly calcium ion, were also extracted. The extraction ratio of calcium was about 60% and remained constant as the extraction time increased. The extraction ratios of iron and aluminum were decreased with increasing extraction time. The extraction ratio scandium exceeded 95% under the same condition and remained stalely with extraction time extended. As the results shown in Fig. 5(b), the separation coefficient increased with increasing extraction time. Overall, the optimal extraction equilibrium was 15–20 min to selectively separate scandium. Above all, more than 95% of scandium was selectively extracted under the conditions of pH value of 1.5–1.8, aqueous-organic ratio of 3 and oscillation for 15 min. The impurity elements like Fe3+, Al3+ were selectively separated through solvent extraction process using P204. The β(Sc / Fe) and β(Sc / Al) reached above 298 and 200, respectively.
HCl solution
Organic phase 2 Stripping
NaOH Solution
Aqueous phase containing Sc(OH)3
Organic phase 3
To produce Scandium oxide & Scandium, etc.
Fig. 1. Procedure of solvent extraction scandium in phosphoric acid leachate.
of pH value and attained about 300 at pH 1.5. However, the separation coefficient between scandium and calcium was stable as the pH increased. Therefore, the optimal pH value was recommended as 1.5–1.8 for selective extraction scandium in phosphoric acid medium.
3.1.5. Stripping loaded organic phase Hydrochloric acid is widely adopted for stripping process [16]. The effect of the hydrochloric acid concentration was investigated at A/O of 3 with oscillation time of 15 min, the results are shown in Fig. 6. The iron stripping peaked at 4 mol/L acid with almost 72% Fe being removed. The removal ratio of aluminum was lower than iron and had similar trend with iron as the increasing acid concentration, which reached about 20% at acid concentration of 2 mol/L. And also, calcium was seldom stripped by hydrochloric acid solution and the remove ratio of Ca2+ was about 25%. Therefore, the optimal hydrochloric acid concentration was determined to be 4 mol/L for stripping condition. And then, the saturated loaded organic phase was stripped under the condition of sodium hydroxide concentration of 4 mol/L, A/O of 3
3.1.3. Effect of A/O The effect of the A/O on extraction behavior was investigated under the conditions of the initial pH value of 1.5 and oscillation time of 15 min, the results are presented in Fig. 4. It can be observed that the extraction ratio of scandium was almost stable at the A/O range of 1–4 and had descendent tendency with the increasing of A/O. The extraction ratio of calcium was increased from 40% to above 60% at the A/O range of 1–6. Thereafter, the extraction ratio of calcium was stable with A/O increased. The extraction ratio of iron and aluminum had similar trend with the increasing of A/O. At the A/O range of 1 to 3, the extraction ratio was less than 10% and then increased obviously from 4 to 8. The extraction ratio of iron and 80
12 Sc Fe Al Ca
70
10
Separatipn coefficient
60
Extraction ratio (%)
ȕ(ScFe) ȕ(ScAl) ȕ(ScCa)
50 40 30 20
8
6
4
2
10
(b)
(a) 1
2
3
4
0
5
1
2
3
Concentration of P204 (%)
Concentration of P204 (%)
Fig. 2. Effect of P204 concentration on extraction ratio and separation coefficient. 177
4
5
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3000
100 Sc Fe Al Ca
2850
Separation coefficient
Extraction ratio (%)
80
ȕ(ScFe) ȕ(ScAl) ȕ(ScCa)
60
40
2700 2550 300 200
20 100
(a) 0 0.0
0.4
0.8
1.2
1.6
(b)
0 0.0
2.0
0.2
0.4
0.6
0.8
pH
1.0
1.2
1.4
1.6
1.8
2.0
pH
Fig. 3. Effect of pH value on the extraction ratio and separation coefficient.
where K is extraction equilibrium constant.
and oscillation for 10 min. After stripping process, the recovery ratio of scandium reached 94.7%, and the precipitate of Sc(OH)3 obtained. And also, the stripping ratio of iron and aluminum were 3.1% and 1.2%, respectively.
D=
[M (HL)(2n − z ) Lz ]o (M n +)
(6)
Combine (7) and (8): 3.2. Mechanisms on selective extraction of scandium
D= 3.2.1. Extraction mechanism of scandium in phosphoric acid medium P204 is a kind of acidic phospholipid extraction agent, which is usually present in the form of dimer (H2L2) in non-polar solvent [17]. The extraction mechanism of P204 in the acid environment was generally shown as Eq. (4). The extraction mechanism was proposed and verified through the configuration of pure reagent tests, which validate and calculate the corresponding coefficient. In addition, the concentration of scandium on the experiments was the same as phosphoric acid leachate obtained. If the value of n and z were 3, the reaction model was established as Eq. (9).
M n + + nH2 L2 = [M (HL)(2n − z ) Lz ]o + zH+
K=
(7)
log D = log K + n log(H2 L2)o + zpH
(8)
The content in the acid solution of scandium was kept at a certain value and phosphoric acid was used to adjust the acidity (pH). When the concentration of P204 was kept at 2%, the extraction results are presented in Fig. 7(a). By preparing different concentrations of P204 at the same acidity, the extraction results are shown in Fig. 7(b). From Fig. 7(a), the slope of the straight line was 2.91, so the value of z was close to 3. From Fig. 7(b), the slope of the straight line is 3.02, the value of n also approached 3. Therefore, the extraction reaction equation could be expressed as:
(4)
Sc 3 + + 3H2 L2 = [Sc (HL) L3 ]o + 3H+
[M (HL)(2n − z ) Lz ]o ·[H+]z (H2 L2 )no ·(M n +)
(5)
(9)
FT-IR analysis of organic phase after extraction process in
100
1200
Sc Fe Al Ca
ȕ(ScFe) ȕ(ScAl) ȕ(ScCa)
1000
Separation coefficient
80
Extraction ratio %
K ·(H2 L2)no [H+]z
60
40
20
800
600
400
200
(b)
(a) 0
1
2
3
4
5
6
7
8
9
0
10
0
2
4
6
Aqueousorganic ratio
Aqueousorganic ratio
Fig. 4. Effect of A/O on the extraction ratio and separation coefficient. 178
8
10
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100
350
Sc Fe Al Ca
250
Separation coefficient
Extraction ratio (%)
80
60
40
20
4
6
8
10
12
14
16
18
20
200 150 100 60 40 20
(a) 0
ȕ(ScFe) ȕ(ScAl) ȕ(ScCa)
300
0
22
(b) 4
6
8
10
Time (min)
12
14
16
18
20
22
Time (min)
Fig. 5. Effect of time on the extraction ratio and separation coefficient. 80 70
Removal ratio (%)
phosphoric acid medium at pH value range of 0–1.5 is shown in Fig. 8. The band at 1458 cm−1 is assigned to eCH3 stretching vibration and characteristic vibrational bands for P204 like P-O-C is identified at 1035 cm−1 [18]. Comparing with Fig. 8(b) and (c), the peak at 1230 cm−1, which is assigned to ePO(OH) stretching, has shifted to 1207 cm−1 and the intensity of band decreases with increasing the pH value of phosphoric acid leachate. This phenomenon confirms that the cationic exchange generated on ePO(OH) in phosphoric acid medium [19].
Fe Al Ca
60 50 40 30
3.2.2. Role of H3PO4 in selective separation of metal ions In order to investigate the extraction characteristic of P204 in phosphoric acid medium, FTIR analysis was applied to reveal the bands between P204 and H3PO4. The analysis results of organic phase before and after phosphoric acid extraction are shown in Fig. 9. The bands at 2956 cm−1, 2925 cm−1 and 2856 cm−1 are assigned to eCH3 stretching vibration [20], and the band at 1378 cm−1 is assigned to eOH stretching vibration [21]. The characteristic vibrational band of P]O is identified at 1229 cm−1 and the peak at 1032 cm−1, which is assigned to P-O-C in P204, has shifted to 1034 cm−1 in the H3PO4 system [22]. This phenomenon proved that P204 might react with H3PO4 to form a molecular association on P]O stretching, as the
20 10 1
2
3
4
5
6
Hydrochloric acid concentration (mol/L) Fig. 6. Effect of hydrochloric acid concentration on washing process.
3.5 2.5
3.0
2.0
2.5
logD Sc
logD (Sc)
1.5 1.0 0.5
2.0
1.5 0.0
1.0
-0.5 -1.0
(a) 0.4
0.6
0.8
1.0
1.2
1.4
(b) 1.6
pH
0.5 -2.1
-2.0
-1.9
-1.8
-1.7
-1.6
log(P204)
Fig. 7. Effect of equilibrium pH and organic concentration on the D value. 179
-1.5
-1.4
-1.3
-1.2
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3000
2500
2000
į 519.30
889.99 728.87 726.74
1229.59 1157.12
1500
Wavenumbers (cm-1)
1035.02
1000
500
721.68
1034.34
668.96
892.26
1032.56
1378.38 1378.07
1458.82
2924.64 2856.3
6
8
10
12
In this study, the feasibility and mechanism of metal ions extraction from phosphoric acid leachate were investigated. The experimental results claimed that the extraction ratio of scandium exceed 95% with limited co-extraction of impurity elements involving Fe3+ and Al3+ in phosphoric acid medium under the optimal condition of P204 concentration of 2%, pH value of 1.5 and A/O of 3 with oscillation 15 min. However, the separation coefficient of Sc and Ca (β(Sc / Ca) = 27.3) was much lower than β(Sc / Fe) and β(Sc / Al) . And then, the loaded organic phase was washed with 4 mol/L HCl, wherein about 72% of impurity elements (Fe and Al) were removed, and striped with 4 mol/L NaOH, which the recovery ratio of scandium reached about 95%. The desirable extraction of scandium in P204 was attributed to the ion exchange between hydrogen ion of ePO(OH) and Sc3+ under acidic condition (pH = 0.2–1.5). The reaction between P204 and H3PO4 with the formation of a molecular association increases the dissociation of P204 to hydrogen ions, improving the selective separation of scandium by cationic exchange. The results also confirmed that the hydrophilicity and hydrophobicity of coordination ions was the main reason for selective extraction on the solvent extraction process. Besides, impurity elements (Fe3+ and Al3+) would also act with phosphoric acid radical
P204
2925.26
2956.95
1229.39
1541.12 1456.57 2854.87
2956.5
Transmittance %
4
4. Conclusions P204+H3PO4
2800
2400
2000
1600
1200
800
400
Wavenumbers (cm ) Fig. 9. IR spectra of organic phase before and after phosphoric acid extraction.
OH
2
elements (Fe3+, Al3+ and Ca2+) would also react with H2PO42−, HPO42− and PO43− to form complex ions in leaching and extraction process [23]. The distribution of ions in phosphoric acid medium was investigated and the results are shown in Figs. 11–13. As shown in Fig. 11 that H3PO4 molecule was the main form of phosphoric acid at pH range of 0–1 and content of H2PO4− increased obviously as the pH of liquor increased. The impurity elements like Fe and Al reacted with H2PO4− to form hydrophilic ion. According to our previous work, the precipitation pH of FePO4 and AlPO4 are 4.14 and 4.69, respectively, in 8 mol/L H3PO4 solution, hence, there content of precipitation did not express in figures [13]. Figs. 12 and 13 showed that the FeH2PO42+ and AlH2PO42+ were the main ion forms, while Ca2+ is the main type of calcium at pH range of 1.5–1.8. These hydrophilic ions (e.g. FeH2PO42+ and AlH2PO42+) were difficult to be extracted by P204 in organic phase, while the dissociated Ca2+ was easily co-extracted in phosphoric acid medium [24]. This phenomenon indicated that scandium and impurity elements like Fe, Al, could be selectively separated at optimal pH value range. Based on this method, the separation coefficient of scandium and impurity elements in phosphoric acid medium (e.g. β(Sc / Fe) = 298, β(Sc / Al) =200), including Fe3+ and Al3+, are much higher than other kinds of inorganic acid on optimal extraction condition
-1
R'R''P
0
Fig. 11. Cumulative distribution coefficient of phosphoric acid ions.
Fig. 8. FT-IR spectra of organic phase after extraction process at different pH values.
O
HPO2-4
pH
1033.22
1458.16
2855.47
0.0
(a) pH=0; (b) pH=1.0; (c) pH=1.5
3200
H2PO-4
H3PO4
0.2
1377.71
1463.21 1378.24
2730.27
3500
0.6
0.4
2957.94 2927.10 2857.12 2956.52
(a)
1207.21 1159.46 1035.68 894.38 769.95 727.79
2856.63
2957.94
2925.67
(b)
PO34
0.8
2925.09
Transmittance (%)
1458.63 1377.99
(c)
528.38
1.0
O +
P HO OH OH
R'R''P
OH
O OH
O
P
OH
OH
Fig. 10. Reaction between P204 and H3PO4 in organic phase.
results shown in Fig. 10. Compared with the effect of P]O group in P204 alone on extraction reaction in other acid medium, including nitric acid [12], hydrochloric acid and sulfuric acid, etc., this reaction would increase the dissociation of P204 to hydrogen ions and improve extraction ability by cationic exchange. Due to the characteristic of step ionization of H3PO4, impurity 180
Separation and Purification Technology 209 (2019) 175–181
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1.0
1.0 3+
Fe FeH2PO2+ 4
0.8
+ 4
FeHPO
0.8
AlH2PO42+
AlHPO4+
į
į
(b)
0.6
FeHPO+4
0.4
0.4
0.2
0.2
Fe3+ 0.0
AlHPO+4
FeH2PO2+ 4
0.6
0.0
Al3+ AlH2PO2+ 4
(a)
Al3+
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
pH
0.0 0.0
0.5
1.0
1.5
2.0
pH
2.5
3.0
3.5
4.0
4.5
Fig. 12. The distribution of metal ions in phosphoric acid medium: (a) Iron ions; (b) aluminum ions.
10
96–102. [4] C.R. Borra, Y. Pontikes, K. Binnemans, T.V. Gerven, Leaching of rare earth from bauxite residue (red mud), Min. Eng. 76 (2015) 20–27. [5] S. Sanmal, A.K. Ray, A. Bandopadhyay, Proposal for resource, utilization and processes of red mud in India – a review, Int. J. Min. Process. 118 (2013) 43–55. [6] D.Q. Li, Y. Zuo, S.L. Meng, Separation of thorium(Ⅳ) and extracting rare earths from sulfuric and phosphoric solutions by solvent extraction method, J. Alloys Compd. 374 (2004) 431–433. [7] S.C. Kin, S.W. Nahm, Y. Park, Property and performance of red mud-based catalysts for the complete oxidation of volatile organic compounds, J. Hazard. Mater. 300 (2015) 104–113. [8] C. Klauber, M. Grafe, G. Power, Bauxite residue issue: Ⅱ. Options for residue utilization, Hydrometallurgy 108 (2011) 11–32. [9] N.N. Hidayah, S.Z. Abidin, The evolution of mineral in extraction of rare earth elements using solid-liquid extraction over liquid-liquid extraction: a review, Min. Eng. 112 (2017) 103–113. [10] X.B. Zhu, W. Li, S. Tang, M.J. Zeng, P.Y. Bai, L.J. Chen, Selective recovery of vanadium and scandium by ion exchange with D201 and solvent extraction using P507 from hydrochloric acid leaching solution of red mud, Chemophere 175 (2007) 365–372. [11] H.T. Chang, M. Li, S.G. Liu, Y.H. Hu, F.S. Zhang, Study on separation of rare earth elements in complex system, J. Rare Earths 28 (2010) 116–119. [12] Y.C. Lu, H.Q. Wei, Z.F. Zhang, Y.L. Li, G.L. Wu, W.P. Liao, Selective extraction and separation of thorium from rare earths by a phosphorodiamidate extractant, Hydrometallurgy 163 (2016) 192–197. [13] G.H. Li, Q. Ye, B.N. Deng, J. Luo, M.J. Rao, Z.W. Peng, T. Jiang, Extraction of scandium from scandium-rich material derived from bauxite ore residues, Hydrometallurgy 178 (2018) 62–68. [14] B.N. Deng, G.H. Li, J. Luo, Q. Ye, M.X. Liu, Z.W. Peng, T. Jiang, Enrichment of Sc2O3 and TiO2 from bauxite ore residues, J. Hazard. Mater. 331 (2017) 71–80. [15] B.N. Deng, T. Jiang, G.H. Li, Q. Ye, F.Q. Gu, M.J. Rao, Z.W. Peng, Effects of reductive roasting with sodium salts on leaching behavior of non- ferrous elements in bauxite ore residue, Light Met. (2018) 157–164. [16] X.X. Sun, Y.Z. Sun, J.G. Yu, Removal of ferric ions from aluminum solution by solvent extraction, part Ⅰ: iron removal, Sep. Purif. Technol. 159 (2016) 18–22. [17] F. Principe, G.P. Demopoulos, Comparative study of iron (III) separation from zinc sulphate-sulphuric acid solution using organophosphorus extractants, OPAP and D2EHPA partⅡ.stripping, Hydrometallurgy 79 (2005) 97–109. [18] G. Xu, Solvent Extraction of Rare Earths, Science Press, China, 1987, pp. 98–120. [19] R. Torkaman, M.A. Moosavian, J. Safdari, M. Torab-Mostaedi, Synergistic extraction of gadolinium from nitrate media by mixtures of bis (2, 4, 4-trimethylpentyl) dithiophosphinic acid and di-(2-ethylhexyl) phosphoric acid, Ann. Nucl. Energy 62 (2013) 284–290. [20] N.V. Nguyen, A. Lizuka, E. Shibata, T. Nakamura, Study of adsorption behavior of a new synthesized resin containing glycol amic acid group for separation of scandium from aqueous solutions, Hydrometallurgy 165 (2016) 51–56. [21] S.H. Yin, W.Y. Wu, X. Bian, F.Y. Zhang, Effect of complexing agent lactic acid on extraction and separation of Pr (III)/Ce (III) with di-(2-ethylhexyl) phosphoric acid, Hydrometallurgy 131–132 (2013) 133–137. [22] L. Wen, W.X. Liang, Z.G. Zhang, J.C. Huang, The Infrared Spectroscopy of Minerals, Chongqing University Press, Chongqing, 1989. [23] G.H. Li, Q. Zhou, Z.P. Zhu, J. Luo, M.J. Rao, Z.W. Peng, T. Jiang, Selective leaching of nickel and cobalt from limonitic laterite using phosphoric acid: an alternative for value-added processing of laterite, J. Clean. Prod. 189 (2018) 620–626. [24] H.L. Huang, Extraction Technology of Rare Earth Elements, Metallurgy Engineering Press, Beijing, 2006 (in Chinese).
8 2+
Ca
6 4 2
logC
0
CaH
-2
2
-4
PO
+
4
CaHPO4
-6
-
O4 CaP
-8 -10 -12 -14
0
1
2
3
4
5
6
7
8
9
10
pH Fig. 13. The effect of pH on the constituent of Ca2+ in H3PO4 medium.
anion to form hydrophilic ions like FeH2PO42+ and AlH2PO42+. These hydrophobic hydrophilic interactions promote the selective separation of Sc3+ with Fe3+ and Al3+. The dissolution ions of Ca2+ could be coextracted with scandium using P204 in phosphoric acid medium. References [1] L.C. Zhang, Q.L. Chen, C. Kang, X. Ma, Z.L. Yang, Rare earth extraction from wet process phosphoric acid by emulsion liquid membrane, J. Rare Earth 34 (2016) 717–723. [2] W.C. Liu, J.K. Yang, B. Xiao, Review on treatment and utilization of bauxite residues in China, Int. J. Min. Process. 93 (2009) 220–231. [3] W.W. Wang, Y. Pranolo, C.Y. Cheng, Recovery of scandium from synthetic red mud leach solutions by solvent extraction with D2EFA, Sep. Purif. Technol. 108 (2013)
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Solvent extraction behavior of metal ions and selective separation Sc3+ in phosphoric acid medium using P204 ⁎
T
⁎
Qing Ye, Guanghui Li1, , Bona Deng, Jun Luo1, , Mingjun Rao, Zhiwei Peng, Yuanbo Zhang, Tao Jiang School of Minerals Processing & Bioengineering, Central South University, Changsha, Hunan 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Scandium Phosphoric acid Solvent extraction P204
Acid leaching-solvent extraction is an effective process to extract scandium from scandium-bearing resources. This study was aimed to investigate the extraction behavior of metal ions, including Sc3+, Fe3+, Al3+and Ca2+, in phosphoric acid medium using P204. More than 95% of scandium was selectively extracted under the conditions of pH value of 1.5–1.8, aqueous-organic ratio of 3 and oscillation for 15 min. The impurity elements like Fe3+, Al3+ and Ca2+, were separated via the solvent extraction process using P204 (e.g., separation coefficient of scandium to iron is 298). In stripping process, the majority of co-extracted metal ions can be removed from the organic phase with hydrochloric acid solution. The scandium-bearing organic phase was stripped with 4 mol/L NaOH, wherein the recovery of scandium attained about 95% while that of the co-extracted iron and aluminum were only 3.1% and 1.2%, respectively. It was also confirmed that both P204 and H3PO4 played roles in extraction reaction, and the desirable extraction of scandium in P204 was attributed to the ion exchange between hydrogen ion of ePO(OH) and Sc3+ on acidic condition (pH = 0.4–1.5). Impurity elements (Fe3+ and Al3+) also reacted with phosphate anion to form hydrophilic ions, and in turn result in selective extraction of scandium in phosphoric acid leachate using P204.
1. Introduction Scandium is one of rare earth elements and has widely application on engineering industrials. Scandium mainly exists in bauxite ores, phosphorite deposits and rare earth ores, which is associated with other metal elements [1]. Bauxite ore accounts for 54% of scandium-bearing ore [2]. After processing bauxite ore in the alumina refineries, scandium and other elements like Fe, Ca and Ti were enriched in bauxite ore residues. Generally, the bauxite ore residues from China, Greece, and India etc., contain 100–200 mg/kg of scandium, which makes these materials exploitable as a scandium-containing resource [3]. Extraction of scandium from scandium-bearing resources can be achieved by directly acid leaching using a variety of lixiviates [4]. From the leachate, scandium is extracted using solvent extraction technology. The major issue is that the concentration of impurities, especially iron, aluminum and calcium in scandium-bearing resources are often much higher than scandium content, exerting adverse effects on the acidic leaching and solvent extraction process of scandium. For the leaching process of hydrochloric acid at concentration of 8 mol/L, majority of
metal ions are dissolved into the leaching solution [5]. Metal ions like Fe3+, Al3+ and Ca2+ would be co-extracted during solvent extraction process. A recent study reported that the rare earth elements (incl. scandium) in rare earth ore can be completely extracted into solutions by roasting with concentrated sulphuric acid at 250–300 °C followed by water leaching [6]. The fractional extractions, seven or even nine stages, are also needed to separate impurity elements from leachate. Kim [7] and Klauber [8] investigated the performance of bauxite residue in sulfuric acid leaching medium. Sulfuric acid concentration of 6 mol/L or even 10 mol/L is needed and it is not suitable for high calcium-bearing resources. Calcium sulfate leads to the formation of emulsion and the third phase during solvent extraction process, resulting in inadequate separation and scandium loss. Hence, it is important to further investigate the extraction behavior of metal ions and selective separation Sc3+ in phosphoric acid medium. Extraction behavior of scandium in acid medium e.g. nitric acid, hydrochloric acid and sulfuric acid was investigated in previous study. Several acidic extractants including P204, P507 and P350 are generally used to extract metal ions in previous study. P204 was used to extract
⁎
Corresponding authors at: Peace Building, RM 253, Central South University, Changsha, Hunan 410083, China (G. Li). Bio-building, RM 204, Central South University, Changsha, Hunan 410083, China (J. Luo). E-mail addresses: [email protected] (G. Li), [email protected] (J. Luo). 1 Both corresponding authors contributed to this work equally. https://doi.org/10.1016/j.seppur.2018.07.033 Received 13 May 2018; Received in revised form 13 July 2018; Accepted 13 July 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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emission spectrometer (ICP-AES, Thermo Fisher Scientific, Icap7400 Radial, USA). The loaded organic phase was analyzed using Fourier transform infrared spectroscopy (FTIR, Nicolet, NEXUS-670, USA). Infrared absorption spectra of samples were obtained from the KBr pellets by a Fourier transform infrared spectrometer (Nicolet-470, USA).
scandium in sulfuric acid leachate derived from nickeliferous laterite ore [9]. The extraction ratio of scandium was about 80% after scrubbing process. However, it also needed multi segmentally stripping process to separate other metal ions. Zhu et al. [10] and Chang et al. [11] investigated scandium extraction from red mud by solvent extraction using P507 and P204. The extraction ratio of scandium reached above 90% in hydrochloric acid medium, while the metal ions like Fe3+, Al3+ and Ca2+ were usually co-extracted. Liu et al. [12] focused on the extraction mechanism by using phosphorodiamidate (TBP and P204) in nitric acid medium. Their results revealed that the coordination reaction occurred between rare earth ions and P]O group. P204 has strong extraction ability, but it was difficult to separate Sc3+ from impurity elements in acid medium [11]. In our previous study [13], P204-H3PO4 solvent extraction system is proposed to selectively separate scandium from leachate. In this system, the separation coefficients between scandium and metal ions (e.g. βSc/Fe = 2703) in phosphoric acid medium are much higher than other kinds of acid like H2SO4, HNO3 and HCl via solvent extraction process. However, the mechanism of selective separation of metal ions on extraction process was not clear. This study was aimed to systematically investigate the extraction behaviors and mechanisms of metal ions, including Sc3+, Fe3+, Al3+ and Ca2+, in phosphoric acid medium using P204. The ion exchange between hydrogen ion of ePO(OH) and metal ions was studied for the selective separation from the phosphoric acid leaching solution of scandium-bearing resources. Finally, a conceptual process flow sheet for separation of metal ions including Sc3+, Fe3+, Al3+ and Ca2+, was proposed.
3. Results and discussion 3.1. Solvent extraction behaviors of metal ions using P204 As described in our previous work [14,15], the scandium-rich material was obtained from bauxite ore residue after reductive roasting followed by H3PO4 leaching and NaOH leaching. Iron oxide is changed into metallic iron by reductive roasting, and aluminum silicate is transformed to amorphous silica via H3PO4 leaching process. Alumina enriched in acid residue is further leached by sodium hydroxide solution. Along with the stepwise recovery of valuable elements, scandium and titanium are further enriched in the alkaline leaching residue. The scandium-rich residue was leached by H3PO4 with 8 mol/L at 120 °C for 60 min. As TiO2+ is the main form in phosphoric acid leachate, before solvent extraction process, it can be removed as metatitanic acid precipitate by adjusting pH value [13]. After adjusting the pH value of solution, the main chemical composition of leachate used for solvent extraction test is shown in Table 1. The content of scandium is enriched to 23.62 mg/L in H3PO4 leachate. The solvent extraction behavior and selective separation Sc3+ from Fe3+, Al3+ and Ca2+ was investigated in this study (see Fig. 1).
2. Methods 3.1.1. Effect of P204 concentration The effect of P204 concentration (the volume ratio of P204/organic phase) on extraction behavior was investigated under the conditions of the phosphoric acid leachate pH value of 0.5, A/O (the volume ratio of aqueous phase/organic phase) of 3, and oscillation time of 15 min. The results are presented in Fig. 2. The extraction ratio of scandium increased from 45% to 75% with increasing P204 concentration from 1% to 4%, and the extraction of iron and aluminum also increased. The extraction ratio of iron was nearly 30% with 4% P204, whereas it was only 5% with 1% P204. The extraction of aluminum had the similar trend with increasing P204 concentration. As the results shown in Fig. 2(b), the trend of separation coefficient (β(Sc / Fe) ) was similar to that of β(Sc / Al) , which increased firstly and then decreased. However, the β(Sc / Ca) remained stable with P204 concentration increasing. Therefore, the optimal P204 concentration was recommended as 2–3% for extraction of scandium.
2.1. Experimental procedure The pH value of acid leachate was adjusted by adding phosphoric acid prior to solvent extraction. Samples of P204 reagent have been mixed with the required volume of kerosene at room temperature. The volumes of both the contacted aqueous and organic phase were filled into separating funnel and oscillated for 15 min. After stratifying, the aqueous phase was withdrawn for analysis, while the loaded organic phase was washed by HCl solution with appropriate concentration. Finally, the scandium-bearing organic phases was stripped with NaOH solution. The elemental concentration in acid solution was analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES). Furthermore, after solvent extraction, the concentration of the elements in aqueous phase were also analyzed using ICP-AES. The extraction ratio (η), distribution ratio (D), and separation coefficient (β) were calculated by Eqs. (1)–(3), respectively.
C ·V η = 1− A A × 100% Ct ·Vt
3.1.2. Effect of initial solution pH The effect of solution pH value on extraction behavior was investigated in the aqueous pH value range of 0.1–1.8 under the conditions of 2% (V/V) P204, A/O of 3 and oscillation time of 15 min. The results in Fig. 3(a) showed that the extraction of scandium increased obviously from 30% to 97% when the initial pH range increased from 0.1 to 1.5, and then remained constant with the further increase of pH value to 1.8. And also, the extraction ratio of iron remarkably decreased from 62% to 18% at the pH range of 0.1–1.5. The extraction ratio of aluminum decreased from 22% to 10% at the pH range of 0.1–1.5. The extraction performance of calcium was similar to scandium, increased from 40% to over 60% with pH value increased. As the results shown in Fig. 3(b), the β(Sc / Fe) increased with the increasing
(1)
where η is the extraction ratio; CA is elements contents of aqueous phase obtained after the solvent extraction, ppm; VA is the volume of aqueous phase, mL.
D=
c0 C ·V −C ·V = t t A A cA CA·VO
(2)
where D is distribution ratio; c0 is the concentration of elements in oil phase, ppm; VO is the volume of oil phase, mL.
β(1/2) =
D1 D2
(3) Table 1 The concentration of elements in H3PO4 leachate solution.
where β(1/2) is separation coefficient between 1 and 2 partition. 2.2. Instrument techniques Metal ions content of solution samples were determined by plasma 176
Element
Sc
Fe
Al
Ca
Content (mg/L)
23.62
1440
825
6639
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Phosphoric acid leachate (Sc3+, Fe 3+, Al3+, Ca2+) Phosphoric acid
aluminum attained maximum of 40% and 43% at A/O of 8, respectively. As the results shown in Fig. 4(b), the separation coefficient obviously decreased with the increasing of A/O. The separation coefficient was lower than 20 at A/O over 4. Actually, the extraction ratio of metal ions including Fe, Al and Sc showed relatively smooth change at the A/O range of 1–4. And the consumption of extractant decreased with A/O ratio increased at the same volume of aqueous phase. Hence, A/O of 3–4 was suitable to extract scandium in phosphoric acid medium.
Adjusting pH value
Solvent extraction
Aqueous phase containing Fe3+,Al3+ and Ca2+
Organic phase 1 Washing
Acid leachate containing Fe3+,Al3+ and Ca2+
3.1.4. Effect of extraction time The effect of increasing extraction time from 5 to 20 min on scandium extraction was investigated under the conditions of P204 concentration of 2%, initial pH value of 1.5 and A/O of 3. The results are indicated in Fig. 5. The results showed that scandium was almost extracted completely under the above conditions. Further, some impurity ions, particularly calcium ion, were also extracted. The extraction ratio of calcium was about 60% and remained constant as the extraction time increased. The extraction ratios of iron and aluminum were decreased with increasing extraction time. The extraction ratio scandium exceeded 95% under the same condition and remained stalely with extraction time extended. As the results shown in Fig. 5(b), the separation coefficient increased with increasing extraction time. Overall, the optimal extraction equilibrium was 15–20 min to selectively separate scandium. Above all, more than 95% of scandium was selectively extracted under the conditions of pH value of 1.5–1.8, aqueous-organic ratio of 3 and oscillation for 15 min. The impurity elements like Fe3+, Al3+ were selectively separated through solvent extraction process using P204. The β(Sc / Fe) and β(Sc / Al) reached above 298 and 200, respectively.
HCl solution
Organic phase 2 Stripping
NaOH Solution
Aqueous phase containing Sc(OH)3
Organic phase 3
To produce Scandium oxide & Scandium, etc.
Fig. 1. Procedure of solvent extraction scandium in phosphoric acid leachate.
of pH value and attained about 300 at pH 1.5. However, the separation coefficient between scandium and calcium was stable as the pH increased. Therefore, the optimal pH value was recommended as 1.5–1.8 for selective extraction scandium in phosphoric acid medium.
3.1.5. Stripping loaded organic phase Hydrochloric acid is widely adopted for stripping process [16]. The effect of the hydrochloric acid concentration was investigated at A/O of 3 with oscillation time of 15 min, the results are shown in Fig. 6. The iron stripping peaked at 4 mol/L acid with almost 72% Fe being removed. The removal ratio of aluminum was lower than iron and had similar trend with iron as the increasing acid concentration, which reached about 20% at acid concentration of 2 mol/L. And also, calcium was seldom stripped by hydrochloric acid solution and the remove ratio of Ca2+ was about 25%. Therefore, the optimal hydrochloric acid concentration was determined to be 4 mol/L for stripping condition. And then, the saturated loaded organic phase was stripped under the condition of sodium hydroxide concentration of 4 mol/L, A/O of 3
3.1.3. Effect of A/O The effect of the A/O on extraction behavior was investigated under the conditions of the initial pH value of 1.5 and oscillation time of 15 min, the results are presented in Fig. 4. It can be observed that the extraction ratio of scandium was almost stable at the A/O range of 1–4 and had descendent tendency with the increasing of A/O. The extraction ratio of calcium was increased from 40% to above 60% at the A/O range of 1–6. Thereafter, the extraction ratio of calcium was stable with A/O increased. The extraction ratio of iron and aluminum had similar trend with the increasing of A/O. At the A/O range of 1 to 3, the extraction ratio was less than 10% and then increased obviously from 4 to 8. The extraction ratio of iron and 80
12 Sc Fe Al Ca
70
10
Separatipn coefficient
60
Extraction ratio (%)
ȕ(ScFe) ȕ(ScAl) ȕ(ScCa)
50 40 30 20
8
6
4
2
10
(b)
(a) 1
2
3
4
0
5
1
2
3
Concentration of P204 (%)
Concentration of P204 (%)
Fig. 2. Effect of P204 concentration on extraction ratio and separation coefficient. 177
4
5
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3000
100 Sc Fe Al Ca
2850
Separation coefficient
Extraction ratio (%)
80
ȕ(ScFe) ȕ(ScAl) ȕ(ScCa)
60
40
2700 2550 300 200
20 100
(a) 0 0.0
0.4
0.8
1.2
1.6
(b)
0 0.0
2.0
0.2
0.4
0.6
0.8
pH
1.0
1.2
1.4
1.6
1.8
2.0
pH
Fig. 3. Effect of pH value on the extraction ratio and separation coefficient.
where K is extraction equilibrium constant.
and oscillation for 10 min. After stripping process, the recovery ratio of scandium reached 94.7%, and the precipitate of Sc(OH)3 obtained. And also, the stripping ratio of iron and aluminum were 3.1% and 1.2%, respectively.
D=
[M (HL)(2n − z ) Lz ]o (M n +)
(6)
Combine (7) and (8): 3.2. Mechanisms on selective extraction of scandium
D= 3.2.1. Extraction mechanism of scandium in phosphoric acid medium P204 is a kind of acidic phospholipid extraction agent, which is usually present in the form of dimer (H2L2) in non-polar solvent [17]. The extraction mechanism of P204 in the acid environment was generally shown as Eq. (4). The extraction mechanism was proposed and verified through the configuration of pure reagent tests, which validate and calculate the corresponding coefficient. In addition, the concentration of scandium on the experiments was the same as phosphoric acid leachate obtained. If the value of n and z were 3, the reaction model was established as Eq. (9).
M n + + nH2 L2 = [M (HL)(2n − z ) Lz ]o + zH+
K=
(7)
log D = log K + n log(H2 L2)o + zpH
(8)
The content in the acid solution of scandium was kept at a certain value and phosphoric acid was used to adjust the acidity (pH). When the concentration of P204 was kept at 2%, the extraction results are presented in Fig. 7(a). By preparing different concentrations of P204 at the same acidity, the extraction results are shown in Fig. 7(b). From Fig. 7(a), the slope of the straight line was 2.91, so the value of z was close to 3. From Fig. 7(b), the slope of the straight line is 3.02, the value of n also approached 3. Therefore, the extraction reaction equation could be expressed as:
(4)
Sc 3 + + 3H2 L2 = [Sc (HL) L3 ]o + 3H+
[M (HL)(2n − z ) Lz ]o ·[H+]z (H2 L2 )no ·(M n +)
(5)
(9)
FT-IR analysis of organic phase after extraction process in
100
1200
Sc Fe Al Ca
ȕ(ScFe) ȕ(ScAl) ȕ(ScCa)
1000
Separation coefficient
80
Extraction ratio %
K ·(H2 L2)no [H+]z
60
40
20
800
600
400
200
(b)
(a) 0
1
2
3
4
5
6
7
8
9
0
10
0
2
4
6
Aqueousorganic ratio
Aqueousorganic ratio
Fig. 4. Effect of A/O on the extraction ratio and separation coefficient. 178
8
10
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100
350
Sc Fe Al Ca
250
Separation coefficient
Extraction ratio (%)
80
60
40
20
4
6
8
10
12
14
16
18
20
200 150 100 60 40 20
(a) 0
ȕ(ScFe) ȕ(ScAl) ȕ(ScCa)
300
0
22
(b) 4
6
8
10
Time (min)
12
14
16
18
20
22
Time (min)
Fig. 5. Effect of time on the extraction ratio and separation coefficient. 80 70
Removal ratio (%)
phosphoric acid medium at pH value range of 0–1.5 is shown in Fig. 8. The band at 1458 cm−1 is assigned to eCH3 stretching vibration and characteristic vibrational bands for P204 like P-O-C is identified at 1035 cm−1 [18]. Comparing with Fig. 8(b) and (c), the peak at 1230 cm−1, which is assigned to ePO(OH) stretching, has shifted to 1207 cm−1 and the intensity of band decreases with increasing the pH value of phosphoric acid leachate. This phenomenon confirms that the cationic exchange generated on ePO(OH) in phosphoric acid medium [19].
Fe Al Ca
60 50 40 30
3.2.2. Role of H3PO4 in selective separation of metal ions In order to investigate the extraction characteristic of P204 in phosphoric acid medium, FTIR analysis was applied to reveal the bands between P204 and H3PO4. The analysis results of organic phase before and after phosphoric acid extraction are shown in Fig. 9. The bands at 2956 cm−1, 2925 cm−1 and 2856 cm−1 are assigned to eCH3 stretching vibration [20], and the band at 1378 cm−1 is assigned to eOH stretching vibration [21]. The characteristic vibrational band of P]O is identified at 1229 cm−1 and the peak at 1032 cm−1, which is assigned to P-O-C in P204, has shifted to 1034 cm−1 in the H3PO4 system [22]. This phenomenon proved that P204 might react with H3PO4 to form a molecular association on P]O stretching, as the
20 10 1
2
3
4
5
6
Hydrochloric acid concentration (mol/L) Fig. 6. Effect of hydrochloric acid concentration on washing process.
3.5 2.5
3.0
2.0
2.5
logD Sc
logD (Sc)
1.5 1.0 0.5
2.0
1.5 0.0
1.0
-0.5 -1.0
(a) 0.4
0.6
0.8
1.0
1.2
1.4
(b) 1.6
pH
0.5 -2.1
-2.0
-1.9
-1.8
-1.7
-1.6
log(P204)
Fig. 7. Effect of equilibrium pH and organic concentration on the D value. 179
-1.5
-1.4
-1.3
-1.2
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3000
2500
2000
į 519.30
889.99 728.87 726.74
1229.59 1157.12
1500
Wavenumbers (cm-1)
1035.02
1000
500
721.68
1034.34
668.96
892.26
1032.56
1378.38 1378.07
1458.82
2924.64 2856.3
6
8
10
12
In this study, the feasibility and mechanism of metal ions extraction from phosphoric acid leachate were investigated. The experimental results claimed that the extraction ratio of scandium exceed 95% with limited co-extraction of impurity elements involving Fe3+ and Al3+ in phosphoric acid medium under the optimal condition of P204 concentration of 2%, pH value of 1.5 and A/O of 3 with oscillation 15 min. However, the separation coefficient of Sc and Ca (β(Sc / Ca) = 27.3) was much lower than β(Sc / Fe) and β(Sc / Al) . And then, the loaded organic phase was washed with 4 mol/L HCl, wherein about 72% of impurity elements (Fe and Al) were removed, and striped with 4 mol/L NaOH, which the recovery ratio of scandium reached about 95%. The desirable extraction of scandium in P204 was attributed to the ion exchange between hydrogen ion of ePO(OH) and Sc3+ under acidic condition (pH = 0.2–1.5). The reaction between P204 and H3PO4 with the formation of a molecular association increases the dissociation of P204 to hydrogen ions, improving the selective separation of scandium by cationic exchange. The results also confirmed that the hydrophilicity and hydrophobicity of coordination ions was the main reason for selective extraction on the solvent extraction process. Besides, impurity elements (Fe3+ and Al3+) would also act with phosphoric acid radical
P204
2925.26
2956.95
1229.39
1541.12 1456.57 2854.87
2956.5
Transmittance %
4
4. Conclusions P204+H3PO4
2800
2400
2000
1600
1200
800
400
Wavenumbers (cm ) Fig. 9. IR spectra of organic phase before and after phosphoric acid extraction.
OH
2
elements (Fe3+, Al3+ and Ca2+) would also react with H2PO42−, HPO42− and PO43− to form complex ions in leaching and extraction process [23]. The distribution of ions in phosphoric acid medium was investigated and the results are shown in Figs. 11–13. As shown in Fig. 11 that H3PO4 molecule was the main form of phosphoric acid at pH range of 0–1 and content of H2PO4− increased obviously as the pH of liquor increased. The impurity elements like Fe and Al reacted with H2PO4− to form hydrophilic ion. According to our previous work, the precipitation pH of FePO4 and AlPO4 are 4.14 and 4.69, respectively, in 8 mol/L H3PO4 solution, hence, there content of precipitation did not express in figures [13]. Figs. 12 and 13 showed that the FeH2PO42+ and AlH2PO42+ were the main ion forms, while Ca2+ is the main type of calcium at pH range of 1.5–1.8. These hydrophilic ions (e.g. FeH2PO42+ and AlH2PO42+) were difficult to be extracted by P204 in organic phase, while the dissociated Ca2+ was easily co-extracted in phosphoric acid medium [24]. This phenomenon indicated that scandium and impurity elements like Fe, Al, could be selectively separated at optimal pH value range. Based on this method, the separation coefficient of scandium and impurity elements in phosphoric acid medium (e.g. β(Sc / Fe) = 298, β(Sc / Al) =200), including Fe3+ and Al3+, are much higher than other kinds of inorganic acid on optimal extraction condition
-1
R'R''P
0
Fig. 11. Cumulative distribution coefficient of phosphoric acid ions.
Fig. 8. FT-IR spectra of organic phase after extraction process at different pH values.
O
HPO2-4
pH
1033.22
1458.16
2855.47
0.0
(a) pH=0; (b) pH=1.0; (c) pH=1.5
3200
H2PO-4
H3PO4
0.2
1377.71
1463.21 1378.24
2730.27
3500
0.6
0.4
2957.94 2927.10 2857.12 2956.52
(a)
1207.21 1159.46 1035.68 894.38 769.95 727.79
2856.63
2957.94
2925.67
(b)
PO34
0.8
2925.09
Transmittance (%)
1458.63 1377.99
(c)
528.38
1.0
O +
P HO OH OH
R'R''P
OH
O OH
O
P
OH
OH
Fig. 10. Reaction between P204 and H3PO4 in organic phase.
results shown in Fig. 10. Compared with the effect of P]O group in P204 alone on extraction reaction in other acid medium, including nitric acid [12], hydrochloric acid and sulfuric acid, etc., this reaction would increase the dissociation of P204 to hydrogen ions and improve extraction ability by cationic exchange. Due to the characteristic of step ionization of H3PO4, impurity 180
Separation and Purification Technology 209 (2019) 175–181
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1.0
1.0 3+
Fe FeH2PO2+ 4
0.8
+ 4
FeHPO
0.8
AlH2PO42+
AlHPO4+
į
į
(b)
0.6
FeHPO+4
0.4
0.4
0.2
0.2
Fe3+ 0.0
AlHPO+4
FeH2PO2+ 4
0.6
0.0
Al3+ AlH2PO2+ 4
(a)
Al3+
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
pH
0.0 0.0
0.5
1.0
1.5
2.0
pH
2.5
3.0
3.5
4.0
4.5
Fig. 12. The distribution of metal ions in phosphoric acid medium: (a) Iron ions; (b) aluminum ions.
10
96–102. [4] C.R. Borra, Y. Pontikes, K. Binnemans, T.V. Gerven, Leaching of rare earth from bauxite residue (red mud), Min. Eng. 76 (2015) 20–27. [5] S. Sanmal, A.K. Ray, A. Bandopadhyay, Proposal for resource, utilization and processes of red mud in India – a review, Int. J. Min. Process. 118 (2013) 43–55. [6] D.Q. Li, Y. Zuo, S.L. Meng, Separation of thorium(Ⅳ) and extracting rare earths from sulfuric and phosphoric solutions by solvent extraction method, J. Alloys Compd. 374 (2004) 431–433. [7] S.C. Kin, S.W. Nahm, Y. Park, Property and performance of red mud-based catalysts for the complete oxidation of volatile organic compounds, J. Hazard. Mater. 300 (2015) 104–113. [8] C. Klauber, M. Grafe, G. Power, Bauxite residue issue: Ⅱ. Options for residue utilization, Hydrometallurgy 108 (2011) 11–32. [9] N.N. Hidayah, S.Z. Abidin, The evolution of mineral in extraction of rare earth elements using solid-liquid extraction over liquid-liquid extraction: a review, Min. Eng. 112 (2017) 103–113. [10] X.B. Zhu, W. Li, S. Tang, M.J. Zeng, P.Y. Bai, L.J. Chen, Selective recovery of vanadium and scandium by ion exchange with D201 and solvent extraction using P507 from hydrochloric acid leaching solution of red mud, Chemophere 175 (2007) 365–372. [11] H.T. Chang, M. Li, S.G. Liu, Y.H. Hu, F.S. Zhang, Study on separation of rare earth elements in complex system, J. Rare Earths 28 (2010) 116–119. [12] Y.C. Lu, H.Q. Wei, Z.F. Zhang, Y.L. Li, G.L. Wu, W.P. Liao, Selective extraction and separation of thorium from rare earths by a phosphorodiamidate extractant, Hydrometallurgy 163 (2016) 192–197. [13] G.H. Li, Q. Ye, B.N. Deng, J. Luo, M.J. Rao, Z.W. Peng, T. Jiang, Extraction of scandium from scandium-rich material derived from bauxite ore residues, Hydrometallurgy 178 (2018) 62–68. [14] B.N. Deng, G.H. Li, J. Luo, Q. Ye, M.X. Liu, Z.W. Peng, T. Jiang, Enrichment of Sc2O3 and TiO2 from bauxite ore residues, J. Hazard. Mater. 331 (2017) 71–80. [15] B.N. Deng, T. Jiang, G.H. Li, Q. Ye, F.Q. Gu, M.J. Rao, Z.W. Peng, Effects of reductive roasting with sodium salts on leaching behavior of non- ferrous elements in bauxite ore residue, Light Met. (2018) 157–164. [16] X.X. Sun, Y.Z. Sun, J.G. Yu, Removal of ferric ions from aluminum solution by solvent extraction, part Ⅰ: iron removal, Sep. Purif. Technol. 159 (2016) 18–22. [17] F. Principe, G.P. Demopoulos, Comparative study of iron (III) separation from zinc sulphate-sulphuric acid solution using organophosphorus extractants, OPAP and D2EHPA partⅡ.stripping, Hydrometallurgy 79 (2005) 97–109. [18] G. Xu, Solvent Extraction of Rare Earths, Science Press, China, 1987, pp. 98–120. [19] R. Torkaman, M.A. Moosavian, J. Safdari, M. Torab-Mostaedi, Synergistic extraction of gadolinium from nitrate media by mixtures of bis (2, 4, 4-trimethylpentyl) dithiophosphinic acid and di-(2-ethylhexyl) phosphoric acid, Ann. Nucl. Energy 62 (2013) 284–290. [20] N.V. Nguyen, A. Lizuka, E. Shibata, T. Nakamura, Study of adsorption behavior of a new synthesized resin containing glycol amic acid group for separation of scandium from aqueous solutions, Hydrometallurgy 165 (2016) 51–56. [21] S.H. Yin, W.Y. Wu, X. Bian, F.Y. Zhang, Effect of complexing agent lactic acid on extraction and separation of Pr (III)/Ce (III) with di-(2-ethylhexyl) phosphoric acid, Hydrometallurgy 131–132 (2013) 133–137. [22] L. Wen, W.X. Liang, Z.G. Zhang, J.C. Huang, The Infrared Spectroscopy of Minerals, Chongqing University Press, Chongqing, 1989. [23] G.H. Li, Q. Zhou, Z.P. Zhu, J. Luo, M.J. Rao, Z.W. Peng, T. Jiang, Selective leaching of nickel and cobalt from limonitic laterite using phosphoric acid: an alternative for value-added processing of laterite, J. Clean. Prod. 189 (2018) 620–626. [24] H.L. Huang, Extraction Technology of Rare Earth Elements, Metallurgy Engineering Press, Beijing, 2006 (in Chinese).
8 2+
Ca
6 4 2
logC
0
CaH
-2
2
-4
PO
+
4
CaHPO4
-6
-
O4 CaP
-8 -10 -12 -14
0
1
2
3
4
5
6
7
8
9
10
pH Fig. 13. The effect of pH on the constituent of Ca2+ in H3PO4 medium.
anion to form hydrophilic ions like FeH2PO42+ and AlH2PO42+. These hydrophobic hydrophilic interactions promote the selective separation of Sc3+ with Fe3+ and Al3+. The dissolution ions of Ca2+ could be coextracted with scandium using P204 in phosphoric acid medium. References [1] L.C. Zhang, Q.L. Chen, C. Kang, X. Ma, Z.L. Yang, Rare earth extraction from wet process phosphoric acid by emulsion liquid membrane, J. Rare Earth 34 (2016) 717–723. [2] W.C. Liu, J.K. Yang, B. Xiao, Review on treatment and utilization of bauxite residues in China, Int. J. Min. Process. 93 (2009) 220–231. [3] W.W. Wang, Y. Pranolo, C.Y. Cheng, Recovery of scandium from synthetic red mud leach solutions by solvent extraction with D2EFA, Sep. Purif. Technol. 108 (2013)
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