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Recovery of Ge(IV) from synthetic leaching solution of secondary zinc oxide by solvent extraction using tertiary amine (N235) as extractant and trioctyl phosphate (TOP) as modifier
Minerals Engineering 136 (2019) 155–160
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Recovery of Ge(IV) from synthetic leaching solution of secondary zinc oxide by solvent extraction using tertiary amine (N235) as extractant and trioctyl phosphate (TOP) as modifier ⁎
Tao Zhang, Tao Jiang , Zhihong Liu
T
⁎
School of Metallurgy and Environment, Central South University, Changsha 410083, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Germanium extraction Tertiary amine N235 Trioctyl phosphate TOP Secondary zinc oxide
The N235–TOP–sulfonated kerosene system was applied for the recovery of Ge(IV) from a synthetic sulfuric acid leaching solution of secondary zinc oxide, containing Ge(IV), Zn(II), As(III), Fe(III), Si(IV), and Cd(II). The coextraction of As(III) with Ge(IV) is a challenging problem in the extraction of Ge(IV) from aqueous solution by N235. However, TOP, as a modifier, suppressed the co-extraction of As(III) significantly. The effects of factors including N235 concentration, extraction time, tartaric acid to Ge(IV) molar ratio and temperature on the extractions of Ge(IV) and As(III) were studied. At an organic/aqueous (O/A) phase ratio of 1/1, aqueous phase pH of 1.2, extraction time of 10.5 min, tartaric acid-to-Ge(IV) molar ratio of 5.0, and temperature of 25 °C, the extraction efficiency of Ge(IV) reached 93.9%, whereas only 1.3% of As(III) was extracted by the 25%(v/v) N235–10%(v/v) TOP–sulfonated kerosene system in a single stage extraction. The Ge(IV) in the raffinate was less than 1.5 mg/L when employing a five-stage continuous countercurrent extraction at an O/A ratio of 1/10. In addition, aqueous NaOH solution of 0.5 mol/L was an appropriate reagent for the stripping of both Ge(IV) and As (III) at an O/A ratio of 1/1 and 25 °C for 10.0 min.
1. Introduction Germanium is scattered metal, which is often applied in the fields of communication optical fibers, infrared optics, semiconductors and catalysis (Depuydt et al., 2006; Kuroiwa et al., 2014). Although there are some independent germanium minerals in the earth crust, such as stottite and schauerteite, these minerals lack commercial exploitation value because of their rarity (Moskalyk, 2004). Therefore, germanium is mainly extracted from nonferrous metal concentrates and coal fly ash, with which it is associated, or from secondary resources (Su, 1974; Zhang, 1986; Arroyo and Fernández-Pereira, 2008). Some lead–zinc deposits in south and southwest China are rich in germanium (Shen et al., 2015; Teng et al., 2016), and germanium bearing concentrates of lead and zinc are produced through the exploitation of these deposits and the beneficiation. In the smelting of these concentrates, germanium is further enriched in secondary zinc oxide, an intermediate product, which is produced by fuming the lead smelting slag and zinc leaching residue in a Waelz kiln(Yu, 1995; Li et al., 2017) or a fuming furnace (Su, 1974). The secondary zinc oxide generally contains hundreds of grams of germanium per ton (500–990 g/t) as shown in table 1, and is a main raw material for the production of germanium in China (Su, 1974;
⁎
Li and Peng, 1990; Li et al., 2017; Fu et al., 2018). In order to recover germanium from the secondary zinc oxide, hydrometallurgy is often used, inevitably using sulfuric acid leaching as the first step. During leaching, most of the zinc and germanium are dissolved, and as a result, an acidic zinc sulfate solution containing Ge(IV) and some impurity ions, such as, Fe(III), As(III), Si(IV) and Cd(II), is produced. Subsequently, the recovery of Ge(IV) from the leaching solution becomes another key step, and many methods can be utilized for this, such as the replacement of Ge(IV) by zinc (Teng et al., 2016; Liu et al., 2016) or iron (Zhou et al., 2013) powders, co-precipitation of Ge(IV) with Fe(III) hydroxides (He et al., 2003), Ge(IV) precipitation by tannins (Liang et al., 2008), solvent extraction (Virolainen et al., 2013; Haghighi et al., 2018) and ion exchange (Virolainen et al., 2013). Among these methods, only the tannins process has been put into commercial use on the large scale. However, the tannins process is not perfect, having shortcomings including the large consumption of tannins, high cost (Wang and Gu, 2016) and the adverse effects caused by residual tannins and their decomposition products in the solution on zinc electrowinning (Qiu, 2000). Therefore, intensive efforts have been made to find a method to replace the tannins process for the recovery of germanium in zinc hydrometallurgy. Solvent extraction is a promising method to solve
Corresponding authors. E-mail addresses: [email protected] (T. Jiang), [email protected] (Z. Liu).
https://doi.org/10.1016/j.mineng.2019.03.011 Received 24 December 2018; Received in revised form 18 March 2019; Accepted 18 March 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
Minerals Engineering 136 (2019) 155–160
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The synthetic leaching solution, the chemical composition of which was identical to the industrial leaching solution of the secondary zinc oxide in a lead and zinc smelter of China, was prepared by dissolving ZnSO4·7H2O, CdSO4·8/3H2O, Fe2(SO4)3·xH2O, As2O3, Na2SiO3·9H2O and GeO2 into an aqueous sulfuric acid solution individually in a certain amount according to the composition shown in Table 1. The pH of the solution was modified by adding sulfuric acid or sodium hydroxide solution.
Table 1 Ge(IV) contents in secondary zinc oxide from various sources. Ge(g/t)
Reference
500 990 640 770 590
Jian Su (1974) Zhexiong Li et al. (2017) Li and Peng (1990) Weiqing Fu et al. (2018) this study
2.2. Procedure and analyses this problem, and it has been widely used in the extraction of other scattered metals, such as indium and gallium (Nusen et al., 2016) from aqueous solutions because of its high selectivity and efficiency. Many studies related to the recovery of Ge(IV) from aqueous solutions by solvent extraction methods have been carried out but, to date, have not been successful commercially (Nusen et al., 2015). Tertiary amines are promising extractants for the anionic extraction of Ge(IV) from the secondary zinc oxide leaching solution. Tertiary amines have advantages such as the low acidity required for the aqueous phase, high selectivity in Ge(IV) extraction, and ease in stripping. The tertiary amine extraction of Ge(IV) from gasification coal fly ash (Haghighi et al., 2018), zinc refinery residues (Liu et al., 2017a), and waste optical fiber (Cheng et al., 2017) leachate has been found to be experimentally effective. In acidic and neutral sulfate solutions, Ge(IV) occurs as electrically neutral Ge(OH)4 (Virolainen et al., 2013). Thus, it cannot be extracted by an anionic extractant tertiary amine. To achieve extraction, a complexant must be added to the acidic leaching solution to transform Ge (OH)4 into an anionic complex (Haghighi et al., 2018). Many complexants, including tartaric acid, critic acid, oxalic acid and catechol, are capable of coordinating with Ge(OH)4 in aqueous solution (Pokrovski and Schott, 1998), and can be used as the complexant in the extraction of Ge(IV) with a tertiary amine. In the extraction of Ge(IV) from zinc sulfate solutions containing As (III) using tertiary amine N235 as an extractant, the co-extraction of As (III) is challenging, and this problem has not yet studied systematically or solved (Li and Peng, 1990; Li, 1996). In this work, the extractions of Ge(IV) from As(III) in a synthetic sulfuric acid leaching solution of the germanium bearing secondary zinc oxide was studied systematically under different conditions using N235, a tertiary amine, as an extractant, trioctyl phosphate (TOP) as a modifier, and tartaric acid as a complexant. On the basis of our experiments, the suppressive effect of TOP on the co-extraction of As(III) has been confirmed in the extraction process.
In all extraction tests, the synthetic leaching solution, into which tartaric acid was added as a complexant in a certain ratio, and the organic solution were placed in a 60-mL separating funnel; then, the funnel was placed in a thermostatic water bath at a certain temperature, oscillated, and, subsequently, kept static. After phase separation, 1 mL of the aqueous phase was sampled for analysis. The concentrations of ions in the aqueous phase were determined by a Thermo iCAP 7000 inductively coupled plasma optical emission spectrometer (ICP-OES), and the concentrations of ions in the loaded organic phase were obtained by mass balance calculations. The pH of the aqueous solutions was measured by an INESA PHS-3E digital pH meter. The ion extraction efficiency was calculated by Eq. (2), where E denotes the extraction efficiency (%) of the ion, V0 and V1 represent, respectively, the volume of the aqueous phase before and after extraction, and C0 and C1 are the concentrations of the ion in the aqueous phase before and after extraction, respectively. The ion stripping efficiency was calculated by Eq. (3), where S denotes the stripping efficiency of the ion, Vorg and V2 represent, respectively, the volumes of the loaded organic phase and strip liquor, and Corg and C2 are the ion concentrations of the loaded organic phase and strip liquor, respectively. βGe/As denotes the separation coefficient of Ge(IV) to As(III) in the extraction, which was calculated using Eq. (4). (2)
S = 100% × C2 V2/ Corg Vorg
(3)
EGe ⎤ ⎡ EAs ⎤ βGe/As = ⎡ / ⎥ ⎣ ⎢ 1 − EAs ⎦ ⎢ 1 − EGe ⎦ ⎥ ⎣
(4)
3. Results and discussion 3.1. Extraction pH isotherms of ions The extraction pH isotherms of the ions with 25% (v/v) N235 in sulfonated kerosene at an O/A ratio of 1/1, tartaric acid to Ge(IV) molar ratio of 5.0 and 25 °C for 10.5 min are shown in Fig. 1a. When the pH of the aqueous phase was 0.2, the extractions of Ge(IV) and As(III) reached 53.9% and 11.4% respectively, whereas the extractions of the other ions, such as Zn(II), Fe(III), and Cd(II), were all below 3.5%. With increase in pH from 0.2 to 1.2, the extraction of Ge(IV) increased rapidly from 53.9% to 93.5%. In contrast, the extractions of all other ions except Ge(IV) decreased gradually with increasing pH from 0.2 to 0.7, almost reaching 0.0% for Zn(II), Fe(III), and Cd(II) and 6.7% for As(III) at pH 0.7. Subsequently, further changes were not observed with increasing pH. In the Ge(IV) extraction process, the reaction for the coextraction of As(III) can be described by Eq. (5). Therefore, it is crucial to find a way to suppress the co-extraction of As(III) with Ge(IV) for the separation of Ge(IV) from other ions in the leaching solution when using N235 as an extractant.
2. Experimental 2.1. Reagents and synthetic secondary zinc oxide leaching solution Tri(octyl–decyl) amine N235 with a purity of 96% and sulfonated kerosene were provided by Shanghai Rare Earth Chemical Co., Ltd., China. The modifier trioctyl phosphate (TOP) of analytical purity was purchased from Aladdin Industrial Corporation. Analytical grade tributyl phosphate (TBP) purchased from Sinopharm Chemical Reagent Co., Ltd. was used as a contrast modifier. In addition, sulfuric acid of analytical purity was supplied by Kelong Chemical Corporation. The other reagents of analytical grade and GeO2 with a purity of 99.999% were obtained from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used in all experiments. The organic phase consisting of N235, TOP, or TBP and sulfonated kerosene was prepared, and acidified by an aqueous sulfuric acid solution of 70 g/L under an O/A ratio of 1/1 at 25 °C for 30 min before use. The acidification reaction of N235 is given in Eq. (1).
R3N(org) + H2 SO4 → R3NH·HSO4(org)
E = 100% × (C0 V0 − C1 V1)/ C0 V0
R3NH·HSO4(org) + H3 AsO3 = R3NH·H2 AsO3(org) + H2 SO4
(5)
To solve this problem, TBP was used as a modifier, and the extraction tests were carried out by using 25% (v/v) N235 and 10% (v/v) TBP in sulfonated kerosene as an organic phase under the conditions
(1) 156
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Fig. 1c. Extraction pH isotherms of ions with 25% (v/v) N235 and 10% (v/v) TOP in sulfonated kerosene.
Fig. 1a. Extraction pH isotherms of ions with 25% (v/v) N235 in sulfonated kerosene.
chain; thus, TOP occupies a larger space volume than TBP. When 10% (v/v) TOP was mixed into the organic phase, the combination of amine molecules with arsenic acid protons was hindered by steric effects, which increased the cavity energy of organic phase for As(III) extraction. Thus, the selectivity of organic system increased and the co-extraction of arsenic was inhibited.
3.2. Effects of multifactors on the extractions of Ge(IV) and As(III) In previous studies (Liu et al., 2017a; Liu et al., 2017b), Fe(III) has been shown to have a great influence on the extraction of Ge(IV) by solvent extraction, and Fe(III) was removed from 5.4 to 0.20 g/L before the solvent extraction process. However, as shown in Table 2, Fe(III) concentration in the synthetic leaching solution was only 0.23 g/L and too low to influence the extraction of Ge(IV) in the present work. Because the extractions of Fe(III), Zn(II), and Cd(II) were negligible, they were not studied in subsequent experiments. The extractions of Ge(IV) and As(III) from the synthetic secondary zinc oxide leaching solution using the N235–TOP–sulfonated kerosene system was studied through single factor experiments. The control conditions for the experiments were as follows: 25% (v/v) N235 and 10% (v/v) TOP in sulfonated kerosene, tartaric acid to Ge(IV) molar ratio of 5.0, temperature of 25 °C, O/A phase ratio of 1/1, aqueous phase pH of 1.2 and extraction time of 10.5 min.
Fig. 1b. Extraction pH isotherms of ions with 25% (v/v) N235 and 10% (v/v) TBP in sulfonated kerosene.
mentioned above. The obtained extraction pH isotherms of the ions, as shown in Fig. 1b, are roughly similar to those without the addition of TBP shown in Fig. 1a. The addition of 10% (v/v) TBP improved the performance of the phase separation and did not result in the co-extraction of Fe(III) because of the low concentration of Fe(III) in the aqueous phase; however, it increased the extractions of both Ge(IV) and As(III) simultaneously, which reduced their separation effect. Further extraction tests were conducted by using 25% (v/v) N235 and 10% (v/v) TOP in sulfonated kerosene as an organic phase under the previously mentioned conditions. The obtained extraction pH isotherms of the ions, as shown in Fig. 1c, were slightly different from those shown in Figs. 1a and 1b. Over the pH range from 0.2 to 1.2, the co-extraction of As(III) was inhibited; at pH 1.2, the extraction of Ge (IV) was as high as 93.9%, whereas the extraction of As(III) was only 1.3%. Furthermore, no other ions (aside from Ge(IV) and As(III)) were extracted. These results demonstrate the possibility of Ge(IV) separation from other ions in the leaching solution using N235 as the extractant. Compared with sulfonated kerosene, TBP is a polar substance. When the 10% TBP was added to the organic phase, the polarity of the organic phase was increased, thus improving the Ge(IV) and As(III) extraction efficiencies based on the rule of similarity. TOP has similar polarity to TBP. However, the addition of 10% TOP resulted in the opposite effects because the substituent in TOP has a longer and more branched carbon
3.2.1. N235 Concentration Fig. 2a illustrates the effects of N235 concentration (v/v) on the extraction efficiencies of Ge (IV) and As (III). With the N235 concentration increasing from 5%(v/v) to 25%(v/v), the extractions of Ge (IV) and As(III) increased from 84.0%, 0% to 93.9%, 1.3%, respectively; and after that, a further increase in the N235 concentration(v/v) caused a sharp rise in the extraction of As(III), therefore, in the case of that the extraction of Ge(IV) and the separation of Ge(IV) from As(III) were considered comprehensively, the optimal N235 concentration was determined to be 25%(v/v) appropriately. In addition, phase separation in the extraction was well and fast in the N235 concentration ranging from 5% (v/v) to 30% (v/v). Table 2 Chemical composition of the synthetic leaching solution.
157
Element
Ge(IV)
Zn(II)
As(III)
Fe(III)
Si(IV)
Cd(II)
H2SO4
Concentration (g/L)
0.055
41.50
0.84
0.23
0.11
0.22
10
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Fig. 2c. Effects of tartaric acid to Ge(IV) molar ratio (mol/mol) on the extraction efficiencies of Ge(IV), As(III) and the separation coefficient of Ge(IV) to As(III).
Fig. 2a. Effects of N235 concentration (v/v) on the extraction efficiencies of Ge (IV) and As(III).
molar ratio from 1.0 to 9.0, the extraction of Ge (IV) increased parabolically from 0.0% to 96.3%, whereas the extraction of As(III) decreased first and then increased, having the lowest value of 1.3% at a tartaric acid to Ge(IV) molar ratio of 5.0 where the separation coefficient of Ge(IV) to As(III) reached the highest value of 1144. There was sufficient tartaric acid for complexation with Ge(OH)4 at a tartaric acid to Ge(IV) molar ratio of 5.0, so the Ge(IV) extraction changed little on increasing the dosage of tartaric acid beyond this value. Nevertheless, when the tartaric acid to Ge(IV) molar ratio rose from 5.0 to 7.0, the extraction of As(III) increased from 1.3% to 4.9% markedly, which caused a sudden drop in the Ge(IV) to As(III) separation coefficient at the tartaric acid to Ge(IV) molar ratio of 7.0. The rapid increase in the Ge(IV) extraction on the addition of tartaric acid (H4T) can be attributed to the complexation of H3T− and/or H2T2− generated by the deprotonation of tartaric acid with Ge(IV) as shown in Eqs. (6) and (7). In these reactions, the Ge(OH)4 molecules are transformed into Ge (H3T/H2T)x(OH)4y− complex anions (Pokrovski and Schott, 1998). Because Ge(H3T/H2T)x(OH)4y− are anions and have a larger radius than Ge(OH)4, Ge(IV) could be extracted easily after adding tartaric acid to the leaching solution.
3.2.2. Extraction time The effects of extraction time on the extraction efficiencies of Ge (IV) and As (III), as well as the pH of the aqueous phase are shown in Fig. 2b. The extraction process of Ge (IV) was very fast and tended to equilibrium in about 4.0 min. The extraction of As(III) attained the maximum value, 4.5%, after 4.0 min; with further increase in the time from 4.0 to 10.5 min, the extraction of As(III) reduced to 1.3%. Therefore, 10.5 min was sufficient to ensure that Ge(IV) extraction was at equilibrium. As shown in Fig. 2b, as the extraction proceeded, the pH of the aqueous phase decreased from initial 1.2 to final 1.0, indicating that the extraction reaction of Ge(IV) was a process releasing H+ ions. The aqueous phase pH had a significant effect on the extraction of Ge (IV) and the increase of the pH in a certain range can improve the extraction efficiency of Ge(IV), as shown in Figs. 1a–1c and 2b. 3.2.3. Tartaric acid to Ge (IV) molar ratio Haghighi et al. (2018) pointed out that tartaric acid has a significant effect on the extraction of Ge(IV) when using amine as an extractant. Thus, we also chose tartaric acid as a complexing agent in our experiments. The effects of tartaric acid to Ge(IV) molar ratio on the extraction efficiencies of Ge(IV) and As(III), as well as the separation coefficient of Ge(IV) to As(III), are shown in Fig. 2c. Ge(IV) in aqueous solution at pH 1.2 exists in the form of Ge(OH)4 (Virolainen et al., 2013). However, N235 is an anionic extractant and Ge(IV) cannot be extracted without the assistance of complexants (Haghighi et al., 2018). As shown in Fig. 2c, with the increase of the tartaric acid to Ge (IV)
Ge(OH)4 + H3 T− = Ge(OH)2 (HT)− + 2H2 O
(6)
Ge(OH)4 + H2 T 2 − = Ge(OH)2 (T)2 − + 2H2 O
(7)
3.2.4. Temperature The effects of temperature on the extraction efficiencies of Ge(IV) and As(III), as well as the separation coefficient of Ge(IV) to As(III), are shown in Fig. 2d. It was found that the temperature had a great influence on the extraction of Ge(IV). As the temperature increased from 25 to 55 °C, the extraction of Ge(IV) decreased from 93.9% to 75.3% because of the higher solubility of the extracted complex in the aqueous phase at a high temperature. Fig. 2d also indicates that the extraction of As(III) increased from 1.3% to 6.7% with the temperature increase from 25 °C to 35 °C. Subsequent increases in the temperature had little influence on the extraction of As(III). Therefore, the optimal temperature of the extraction process was determined to be 25 °C, at which the separation coefficient of Ge(IV) to As(III) had the maximum value of 1144. 3.3. Extraction equilibrium isotherm of Ge(IV) The Ge(IV) extraction distribution isotherm was obtained with an organic phase consisting of 25% (v/v) N235, 10% (v/v) TOP and 65% (v/v) sulfonated kerosene and a sulfate leaching solution containing 0.055 g/L Ge(IV) , as shown in Fig. 3. It was found that the extraction
Fig. 2b. Effects of time on the extraction efficiencies of Ge (IV), As(III) and pH value of the aqueous solution. 158
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Table 3 Effects of different stripping agents on the stripping of Ge(IV) and As(III). Reagent
Concentration (mol/L)
Ge(IV) Stripping (%)
As(III) Stripping (%)
NaOH NH4Cl NH3·H2O
0.5 0.5 0.5
100.0 16.5 94.6
100.0 51.8 100.0
Fig. 2d. Effects of temperature on the extraction efficiencies of Ge(IV), As(III) and separation coefficient of Ge(IV) to As(III).
Fig. 4. Effects of NaOH concentration on the stripping of Ge(IV) and As(III).
was found that when using NaOH, NH4Cl, NH3·H2O individually as stripping agents, the stripping efficiencies reached 100.00%, 16.54%, and 94.58%, respectively, for Ge(IV) and 100.00%, 51.73%, and 100.00%, respectively, for As(III). These results indicate that NH4Cl is not a suitable agent for the stripping, and, comparing NH3·H2O with NaOH, NaOH is better in the stripping efficiency for Ge(IV). 3.4.2. Effects of NaOH concentration on the stripping Fig. 4 shows the effects of NaOH concentration on the stripping of Ge(IV) and As(III) with the loaded organic phase containing 0.546 g/L Ge(IV) and 0.400 g/L As(III). This process was conducted under the conditions of temperature of 25 °C, O/A ratio of 1/1, NaOH as stripping agents, and stripping time of 10 min. As the concentration of NaOH increased from 0.1 to 0.5 mol/L, the stripping increased from 1.2% to 100.0% for Ge(IV) and from 19.3% to 100.0% for As(III), respectively. A further increase in the NaOH concentration had no effect on the stripping of Ge(IV) and As(III), indicating that 0.5 mol/L NaOH was sufficient to strip the Ge(IV) and As(III) together from the loaded organic phase. The possible reactions in the stripping process are given by Eqs. (8) and (9) (Li, 1996), where G represents the extracted species of Ge(IV) and As(III) in the organic phase. The reaction in Eq. (8) results in disadvantageous depletion of NaOH, which led to cost increase in the stripping process. To prevent the reaction shown in Eq. (8), it is necessary to ensure the saturation of Ge(IV) for extraction.
Fig. 3. The extraction equilibrium isotherm of Ge(IV).
isotherm of Ge(IV) showed an upward sloping line and did not level off, even at an O/A phase ratio of 1/20, indicating that the loaded organic phase did not achieve saturation on the extraction of Ge(IV). Therefore, for industrial applications, a smaller O/A phase ratio is required to use the extractant N235 fully. McCabe–Thiele analysis predicted that the concentration of Ge(IV) in the raffinate could be reduced to less than 1.5 mg/L by adopting a five-stage continuous countercurrent extraction at an O/A ratio of 1/10. 3.4. Stripping As mentioned above, in the Ge (IV) extraction process, the co-extraction of As(III) can be suppressed by using TOP as a modifier, which results in less As(III) into the stripping stage. When Ge(IV) and As(III)bearing stripping solutions were treated by chlorination distillation using calcium salt (Li and Peng, 1990; Li, 1996), the separation of Ge (IV) from As(III), Ca(II) and other impurity could be achieved. For this reason, it is feasible to strip Ge(IV) and As(III) together by using some kind of stripping agent in the stripping process.
R3NH·HSO4(org) + 2NaOH = R3N(org) + 2H2 O + 2Na+ + SO24− (R3NH)a ·G + bNaOH = aR3N(org) + cH2 O +
bNa+
+
Ga −
(8) (9)
4. Conclusions (1). The co-extraction of As(III) was obvious in N235–sulfonated kerosene and N235–TBP-sulfonated kerosene systems at an aqueous phase pH ranging from 0.2 to 1.2. TOP could suppress the co-extraction of As(III) in the extraction of Ge(IV) from the leaching solution containing arsenic at a pH of 1.2 by using N235 as the extractant. Over 93.9% of Ge(IV) and 1.3% of As(III) were
3.4.1. Screening for the stripping agent Table 3 presents the effects of different stripping agents on the stripping of Ge(IV) and As(III) with the loaded organic phase containing 0.546 g/L Ge(IV) and 0.400 g/L As(III). The stripping was conducted under the conditions of temperature of 25 °C, O/A ratio of 1/1, stripping agent concentration of 0.5 mol/L, and stripping time of 10 min. It 159
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extracted, whereas the co-extraction of Zn(II), Fe(III), and Cd(II) was negligible with 25% (v/v) N235 and 10% (v/v) TOP in sulfonated kerosene at an O/A phase ratio of 1/1, tartaric acid to Ge (IV) molar ratio of 5.0, and 25 °C for 10.5 min. Compared with N235–sulfonated kerosene and N235–TBP–sulfonated kerosene systems, the N235–TOP-sulfonated kerosene system was more suitable for the extraction of germanium from the model sulfuric acid secondary zinc oxide leaching solution. (2). The McCabe–Thiele diagram showed that the concentration of Ge (IV) in the raffinate could be reduced to less than 1.5 mg/L by employing a five-stage continuous countercurrent extraction at an O/A ratio of 1/10. The generation of H+ by the extraction reaction inhibited the extraction of Ge(IV) and improved the co-extraction of impurity elements Zn(II), As(III), Fe(III), and Cd(II). Therefore, adjusting the pH of the aqueous phase to 1.2 was beneficial for the recovery of Ge(IV), and an appropriate dosage of tartaric acid would increase the extraction efficiency of Ge(IV) rapidly. (3). An aqueous NaOH solution was appropriate for Ge(III) and As(III) stripping. Almost 100.0% of both Ge(IV) and As(III) were stripped with 0.5 mol/L NaOH solution at an O/A ratio of 1/1 and 25 °C for 10.0 min.
Hydrometallurgy 175, 164–169. Kuroiwa, K., Ohura, S.I., Morisada, S., Ohto, K., Kawakita, H., Matsuo, Y., Fukuda, D., 2014. Recovery of germanium from waste solar panels using ion-exchange membrane and solvent extraction. Miner. Eng. 55, 181–185. Li, Q.X., Peng, R.Q., 1990. Study on the treatment technology of secondary zinc oxide with high arsenic. Extract. Metall. Part Nonferr. Metals China 06, 27–29. Li, S.P., 1996. The separation of Ge and zinc by N235-tartaric acid system. Chinese J. Rare Metals 05, 334–338. Liang, D.Q., Wang, J.K., Wang, Y.H., Wang, F., Jiang, J.B., 2008. Behavior of tannins in germanium recovery by tannin process. Hydrometallurgy 93, 140–142. Liu, F.P., Liu, Z.H., Li, Y.H., Liu, Z.Y., Li, Q.H., Wen, D.M., 2016. The sulfuric acid leaching process of zinc refinery residues. Chinese J. Nonferrous Metals 26, 908–918. Liu, F.P., Liu, Z.H., Li, Y.H., Wilson, B.P., Liu, Z.Y., Zeng, L., Lundström, M., 2017a. Recovery and separation of gallium(III) and germanium(IV) from zinc refinery residues : Part II: Solvent extraction. Hydrometallurgy 171, 149–156. Liu, F.P., Liu, Z.H., Li, Y.H., Wilson, B.P., Lundström, M., 2017b. Recovery and separation of gallium(III) and germanium(IV) from zinc refinery residues: Part I: Leaching and iron(III) removal. Hydrometallurgy 169, 564–570. Li, Z.X., Wang, Y.C., Yin, X.M., Gao, M.Z., 2017. The recovery of zinc and germanium from secondary zinc oxide. Extract. Metall. Part Nonferr. Metals China 09, 45–47. Moskalyk, R.R., 2004. Review of germanium processing worldwide. Miner. Eng. 17, 393–402. Nusen, S., Zhu, Z.W., Chairuangsri, T., Cheng, C.Y., 2015. Recovery of germanium from synthetic leach solution of zinc refinery residues by synergistic solvent extraction using LIX 63 and Ionquest 801. Hydrometallurgy 151, 122–132. Nusen, S., Chairuangsri, T., Zhu, Z.W., Cheng, C.Y., 2016. Recovery of indium and gallium from synthetic leach solution of zinc refinery residues using synergistic solvent extraction with LIX 63 and Versatic 10 acid. Hydrometallurgy 160, 137–146. Pokrovski, G.S., Schott, J., 1998. Experimental study of the complexation of silicon and germanium with aqueous organic species: Implications for germanium and silicon transport and Ge/Si ratio in natural waters. Geochim. Cosmochim. Acta. 62, 3413–3428. Qiu, G.W., 2000. Comprehensive recovery of germanium and zinc from secondary zinc oxide. YunNan Metall. China 29, 17–21. Su, J., 1974. The recovery of germanium from carbonate lead-zinc ores. Nonferrous Metals China 12, 6–10. Shen, Z.W., Cai, S.H., Zhao, J.W., 2015. Current situation of germanium resources exploitation in China and countermeasures for sustainable development in China. Min. Res. Devel. China 35, 108–112. Teng, N.G., Luo, Z.R., Xu, B., 2016. The improvement for Ge recovery by zinc oxygen pressure leaching. Technol. Econ. Guide China 18, 79. Virolainen, S., Heinonen, J., Paatero, E., 2013. Selective recovery of germanium with Nmethylglucamine functional resin from sulfate solutions. Sep. Purif. Technol. 104, 193–199. Wang, R.S., Gu, L.K., 2016. Experimental study on the factors affecting the tannic acid consumption ratio in process of germanium precipitate. YunNan Metall. China 45, 40–44. Yu, C.R., 1995. Comprehensive utilization of lead and zinc resources. China Nonferrous Metall. 03, 36–40. Zhang, X.H., 1986. The recovery of Germanium from lead-zinc smelting process in closed blast furnace. Rare Metals Cemented Carbides China 03, 33–37. Zhou, Z.A., Chu, G., Gan, H.X., Yang, T.Z., Chen, L., 2013. Ge and Cu recovery from precipitating vitriol supernatant in zinc plant. Trans. Nonferrous Met. Soc. China 23, 1506–1511.
Acknowledgements This work was funded by the National Natural Science Foundation of China (Grants: 51874364). References Arroyo, F., Fernández-Pereira, C., 2008. Hydrometallurgical recovery of germanium from coal gasification fly ash. Solvent extraction method. Ind. Eng. Chem. Res. 47, 3186–3191. Cheng, W.S., Chang, B.C., Chiu, K.L., 2017. Recovery of germanium from waste Optical Fibers by hydrometallurgical method. J. Environ. Chem. Eng. 5, 5215–5221. Depuydt, B., Theuwis, A., Romandic, I., 2006. Germanium: From the first application of Czochralski crystal growth to large diameter dislocation-free wafers. Mater. Sci. Semicond. Process. 9, 437–443. Fu, W.Q., Yang, D.J., Zou, W., Liu, J.C., Mou, X.B., Zhai, Z.B., 2018. The effective leaching of Ge from the secondary zinc oxide by combined process. Extract. Metall. Part Nonferr. Metals China 08, 48–51. He, J., Tang, M.T., Lu, J.L., Liu, Z.Q., Yang, S.H., Yao, W.Y., 2003. Concentrating Ge in zinc hydrometallurgical process with hot acid leaching-halotrichite method. J. Cent. South Univ. Technol. 10, 307–312. Haghighi, H.K., Irannajad, M., Fortuny, A., Sastre, A.M., 2018. Recovery of germanium from leach solutions of fly ash using solvent extraction with various extractants.
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Recovery of Ge(IV) from synthetic leaching solution of secondary zinc oxide by solvent extraction using tertiary amine (N235) as extractant and trioctyl phosphate (TOP) as modifier ⁎
Tao Zhang, Tao Jiang , Zhihong Liu
T
⁎
School of Metallurgy and Environment, Central South University, Changsha 410083, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Germanium extraction Tertiary amine N235 Trioctyl phosphate TOP Secondary zinc oxide
The N235–TOP–sulfonated kerosene system was applied for the recovery of Ge(IV) from a synthetic sulfuric acid leaching solution of secondary zinc oxide, containing Ge(IV), Zn(II), As(III), Fe(III), Si(IV), and Cd(II). The coextraction of As(III) with Ge(IV) is a challenging problem in the extraction of Ge(IV) from aqueous solution by N235. However, TOP, as a modifier, suppressed the co-extraction of As(III) significantly. The effects of factors including N235 concentration, extraction time, tartaric acid to Ge(IV) molar ratio and temperature on the extractions of Ge(IV) and As(III) were studied. At an organic/aqueous (O/A) phase ratio of 1/1, aqueous phase pH of 1.2, extraction time of 10.5 min, tartaric acid-to-Ge(IV) molar ratio of 5.0, and temperature of 25 °C, the extraction efficiency of Ge(IV) reached 93.9%, whereas only 1.3% of As(III) was extracted by the 25%(v/v) N235–10%(v/v) TOP–sulfonated kerosene system in a single stage extraction. The Ge(IV) in the raffinate was less than 1.5 mg/L when employing a five-stage continuous countercurrent extraction at an O/A ratio of 1/10. In addition, aqueous NaOH solution of 0.5 mol/L was an appropriate reagent for the stripping of both Ge(IV) and As (III) at an O/A ratio of 1/1 and 25 °C for 10.0 min.
1. Introduction Germanium is scattered metal, which is often applied in the fields of communication optical fibers, infrared optics, semiconductors and catalysis (Depuydt et al., 2006; Kuroiwa et al., 2014). Although there are some independent germanium minerals in the earth crust, such as stottite and schauerteite, these minerals lack commercial exploitation value because of their rarity (Moskalyk, 2004). Therefore, germanium is mainly extracted from nonferrous metal concentrates and coal fly ash, with which it is associated, or from secondary resources (Su, 1974; Zhang, 1986; Arroyo and Fernández-Pereira, 2008). Some lead–zinc deposits in south and southwest China are rich in germanium (Shen et al., 2015; Teng et al., 2016), and germanium bearing concentrates of lead and zinc are produced through the exploitation of these deposits and the beneficiation. In the smelting of these concentrates, germanium is further enriched in secondary zinc oxide, an intermediate product, which is produced by fuming the lead smelting slag and zinc leaching residue in a Waelz kiln(Yu, 1995; Li et al., 2017) or a fuming furnace (Su, 1974). The secondary zinc oxide generally contains hundreds of grams of germanium per ton (500–990 g/t) as shown in table 1, and is a main raw material for the production of germanium in China (Su, 1974;
⁎
Li and Peng, 1990; Li et al., 2017; Fu et al., 2018). In order to recover germanium from the secondary zinc oxide, hydrometallurgy is often used, inevitably using sulfuric acid leaching as the first step. During leaching, most of the zinc and germanium are dissolved, and as a result, an acidic zinc sulfate solution containing Ge(IV) and some impurity ions, such as, Fe(III), As(III), Si(IV) and Cd(II), is produced. Subsequently, the recovery of Ge(IV) from the leaching solution becomes another key step, and many methods can be utilized for this, such as the replacement of Ge(IV) by zinc (Teng et al., 2016; Liu et al., 2016) or iron (Zhou et al., 2013) powders, co-precipitation of Ge(IV) with Fe(III) hydroxides (He et al., 2003), Ge(IV) precipitation by tannins (Liang et al., 2008), solvent extraction (Virolainen et al., 2013; Haghighi et al., 2018) and ion exchange (Virolainen et al., 2013). Among these methods, only the tannins process has been put into commercial use on the large scale. However, the tannins process is not perfect, having shortcomings including the large consumption of tannins, high cost (Wang and Gu, 2016) and the adverse effects caused by residual tannins and their decomposition products in the solution on zinc electrowinning (Qiu, 2000). Therefore, intensive efforts have been made to find a method to replace the tannins process for the recovery of germanium in zinc hydrometallurgy. Solvent extraction is a promising method to solve
Corresponding authors. E-mail addresses: [email protected] (T. Jiang), [email protected] (Z. Liu).
https://doi.org/10.1016/j.mineng.2019.03.011 Received 24 December 2018; Received in revised form 18 March 2019; Accepted 18 March 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
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The synthetic leaching solution, the chemical composition of which was identical to the industrial leaching solution of the secondary zinc oxide in a lead and zinc smelter of China, was prepared by dissolving ZnSO4·7H2O, CdSO4·8/3H2O, Fe2(SO4)3·xH2O, As2O3, Na2SiO3·9H2O and GeO2 into an aqueous sulfuric acid solution individually in a certain amount according to the composition shown in Table 1. The pH of the solution was modified by adding sulfuric acid or sodium hydroxide solution.
Table 1 Ge(IV) contents in secondary zinc oxide from various sources. Ge(g/t)
Reference
500 990 640 770 590
Jian Su (1974) Zhexiong Li et al. (2017) Li and Peng (1990) Weiqing Fu et al. (2018) this study
2.2. Procedure and analyses this problem, and it has been widely used in the extraction of other scattered metals, such as indium and gallium (Nusen et al., 2016) from aqueous solutions because of its high selectivity and efficiency. Many studies related to the recovery of Ge(IV) from aqueous solutions by solvent extraction methods have been carried out but, to date, have not been successful commercially (Nusen et al., 2015). Tertiary amines are promising extractants for the anionic extraction of Ge(IV) from the secondary zinc oxide leaching solution. Tertiary amines have advantages such as the low acidity required for the aqueous phase, high selectivity in Ge(IV) extraction, and ease in stripping. The tertiary amine extraction of Ge(IV) from gasification coal fly ash (Haghighi et al., 2018), zinc refinery residues (Liu et al., 2017a), and waste optical fiber (Cheng et al., 2017) leachate has been found to be experimentally effective. In acidic and neutral sulfate solutions, Ge(IV) occurs as electrically neutral Ge(OH)4 (Virolainen et al., 2013). Thus, it cannot be extracted by an anionic extractant tertiary amine. To achieve extraction, a complexant must be added to the acidic leaching solution to transform Ge (OH)4 into an anionic complex (Haghighi et al., 2018). Many complexants, including tartaric acid, critic acid, oxalic acid and catechol, are capable of coordinating with Ge(OH)4 in aqueous solution (Pokrovski and Schott, 1998), and can be used as the complexant in the extraction of Ge(IV) with a tertiary amine. In the extraction of Ge(IV) from zinc sulfate solutions containing As (III) using tertiary amine N235 as an extractant, the co-extraction of As (III) is challenging, and this problem has not yet studied systematically or solved (Li and Peng, 1990; Li, 1996). In this work, the extractions of Ge(IV) from As(III) in a synthetic sulfuric acid leaching solution of the germanium bearing secondary zinc oxide was studied systematically under different conditions using N235, a tertiary amine, as an extractant, trioctyl phosphate (TOP) as a modifier, and tartaric acid as a complexant. On the basis of our experiments, the suppressive effect of TOP on the co-extraction of As(III) has been confirmed in the extraction process.
In all extraction tests, the synthetic leaching solution, into which tartaric acid was added as a complexant in a certain ratio, and the organic solution were placed in a 60-mL separating funnel; then, the funnel was placed in a thermostatic water bath at a certain temperature, oscillated, and, subsequently, kept static. After phase separation, 1 mL of the aqueous phase was sampled for analysis. The concentrations of ions in the aqueous phase were determined by a Thermo iCAP 7000 inductively coupled plasma optical emission spectrometer (ICP-OES), and the concentrations of ions in the loaded organic phase were obtained by mass balance calculations. The pH of the aqueous solutions was measured by an INESA PHS-3E digital pH meter. The ion extraction efficiency was calculated by Eq. (2), where E denotes the extraction efficiency (%) of the ion, V0 and V1 represent, respectively, the volume of the aqueous phase before and after extraction, and C0 and C1 are the concentrations of the ion in the aqueous phase before and after extraction, respectively. The ion stripping efficiency was calculated by Eq. (3), where S denotes the stripping efficiency of the ion, Vorg and V2 represent, respectively, the volumes of the loaded organic phase and strip liquor, and Corg and C2 are the ion concentrations of the loaded organic phase and strip liquor, respectively. βGe/As denotes the separation coefficient of Ge(IV) to As(III) in the extraction, which was calculated using Eq. (4). (2)
S = 100% × C2 V2/ Corg Vorg
(3)
EGe ⎤ ⎡ EAs ⎤ βGe/As = ⎡ / ⎥ ⎣ ⎢ 1 − EAs ⎦ ⎢ 1 − EGe ⎦ ⎥ ⎣
(4)
3. Results and discussion 3.1. Extraction pH isotherms of ions The extraction pH isotherms of the ions with 25% (v/v) N235 in sulfonated kerosene at an O/A ratio of 1/1, tartaric acid to Ge(IV) molar ratio of 5.0 and 25 °C for 10.5 min are shown in Fig. 1a. When the pH of the aqueous phase was 0.2, the extractions of Ge(IV) and As(III) reached 53.9% and 11.4% respectively, whereas the extractions of the other ions, such as Zn(II), Fe(III), and Cd(II), were all below 3.5%. With increase in pH from 0.2 to 1.2, the extraction of Ge(IV) increased rapidly from 53.9% to 93.5%. In contrast, the extractions of all other ions except Ge(IV) decreased gradually with increasing pH from 0.2 to 0.7, almost reaching 0.0% for Zn(II), Fe(III), and Cd(II) and 6.7% for As(III) at pH 0.7. Subsequently, further changes were not observed with increasing pH. In the Ge(IV) extraction process, the reaction for the coextraction of As(III) can be described by Eq. (5). Therefore, it is crucial to find a way to suppress the co-extraction of As(III) with Ge(IV) for the separation of Ge(IV) from other ions in the leaching solution when using N235 as an extractant.
2. Experimental 2.1. Reagents and synthetic secondary zinc oxide leaching solution Tri(octyl–decyl) amine N235 with a purity of 96% and sulfonated kerosene were provided by Shanghai Rare Earth Chemical Co., Ltd., China. The modifier trioctyl phosphate (TOP) of analytical purity was purchased from Aladdin Industrial Corporation. Analytical grade tributyl phosphate (TBP) purchased from Sinopharm Chemical Reagent Co., Ltd. was used as a contrast modifier. In addition, sulfuric acid of analytical purity was supplied by Kelong Chemical Corporation. The other reagents of analytical grade and GeO2 with a purity of 99.999% were obtained from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used in all experiments. The organic phase consisting of N235, TOP, or TBP and sulfonated kerosene was prepared, and acidified by an aqueous sulfuric acid solution of 70 g/L under an O/A ratio of 1/1 at 25 °C for 30 min before use. The acidification reaction of N235 is given in Eq. (1).
R3N(org) + H2 SO4 → R3NH·HSO4(org)
E = 100% × (C0 V0 − C1 V1)/ C0 V0
R3NH·HSO4(org) + H3 AsO3 = R3NH·H2 AsO3(org) + H2 SO4
(5)
To solve this problem, TBP was used as a modifier, and the extraction tests were carried out by using 25% (v/v) N235 and 10% (v/v) TBP in sulfonated kerosene as an organic phase under the conditions
(1) 156
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Fig. 1c. Extraction pH isotherms of ions with 25% (v/v) N235 and 10% (v/v) TOP in sulfonated kerosene.
Fig. 1a. Extraction pH isotherms of ions with 25% (v/v) N235 in sulfonated kerosene.
chain; thus, TOP occupies a larger space volume than TBP. When 10% (v/v) TOP was mixed into the organic phase, the combination of amine molecules with arsenic acid protons was hindered by steric effects, which increased the cavity energy of organic phase for As(III) extraction. Thus, the selectivity of organic system increased and the co-extraction of arsenic was inhibited.
3.2. Effects of multifactors on the extractions of Ge(IV) and As(III) In previous studies (Liu et al., 2017a; Liu et al., 2017b), Fe(III) has been shown to have a great influence on the extraction of Ge(IV) by solvent extraction, and Fe(III) was removed from 5.4 to 0.20 g/L before the solvent extraction process. However, as shown in Table 2, Fe(III) concentration in the synthetic leaching solution was only 0.23 g/L and too low to influence the extraction of Ge(IV) in the present work. Because the extractions of Fe(III), Zn(II), and Cd(II) were negligible, they were not studied in subsequent experiments. The extractions of Ge(IV) and As(III) from the synthetic secondary zinc oxide leaching solution using the N235–TOP–sulfonated kerosene system was studied through single factor experiments. The control conditions for the experiments were as follows: 25% (v/v) N235 and 10% (v/v) TOP in sulfonated kerosene, tartaric acid to Ge(IV) molar ratio of 5.0, temperature of 25 °C, O/A phase ratio of 1/1, aqueous phase pH of 1.2 and extraction time of 10.5 min.
Fig. 1b. Extraction pH isotherms of ions with 25% (v/v) N235 and 10% (v/v) TBP in sulfonated kerosene.
mentioned above. The obtained extraction pH isotherms of the ions, as shown in Fig. 1b, are roughly similar to those without the addition of TBP shown in Fig. 1a. The addition of 10% (v/v) TBP improved the performance of the phase separation and did not result in the co-extraction of Fe(III) because of the low concentration of Fe(III) in the aqueous phase; however, it increased the extractions of both Ge(IV) and As(III) simultaneously, which reduced their separation effect. Further extraction tests were conducted by using 25% (v/v) N235 and 10% (v/v) TOP in sulfonated kerosene as an organic phase under the previously mentioned conditions. The obtained extraction pH isotherms of the ions, as shown in Fig. 1c, were slightly different from those shown in Figs. 1a and 1b. Over the pH range from 0.2 to 1.2, the co-extraction of As(III) was inhibited; at pH 1.2, the extraction of Ge (IV) was as high as 93.9%, whereas the extraction of As(III) was only 1.3%. Furthermore, no other ions (aside from Ge(IV) and As(III)) were extracted. These results demonstrate the possibility of Ge(IV) separation from other ions in the leaching solution using N235 as the extractant. Compared with sulfonated kerosene, TBP is a polar substance. When the 10% TBP was added to the organic phase, the polarity of the organic phase was increased, thus improving the Ge(IV) and As(III) extraction efficiencies based on the rule of similarity. TOP has similar polarity to TBP. However, the addition of 10% TOP resulted in the opposite effects because the substituent in TOP has a longer and more branched carbon
3.2.1. N235 Concentration Fig. 2a illustrates the effects of N235 concentration (v/v) on the extraction efficiencies of Ge (IV) and As (III). With the N235 concentration increasing from 5%(v/v) to 25%(v/v), the extractions of Ge (IV) and As(III) increased from 84.0%, 0% to 93.9%, 1.3%, respectively; and after that, a further increase in the N235 concentration(v/v) caused a sharp rise in the extraction of As(III), therefore, in the case of that the extraction of Ge(IV) and the separation of Ge(IV) from As(III) were considered comprehensively, the optimal N235 concentration was determined to be 25%(v/v) appropriately. In addition, phase separation in the extraction was well and fast in the N235 concentration ranging from 5% (v/v) to 30% (v/v). Table 2 Chemical composition of the synthetic leaching solution.
157
Element
Ge(IV)
Zn(II)
As(III)
Fe(III)
Si(IV)
Cd(II)
H2SO4
Concentration (g/L)
0.055
41.50
0.84
0.23
0.11
0.22
10
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Fig. 2c. Effects of tartaric acid to Ge(IV) molar ratio (mol/mol) on the extraction efficiencies of Ge(IV), As(III) and the separation coefficient of Ge(IV) to As(III).
Fig. 2a. Effects of N235 concentration (v/v) on the extraction efficiencies of Ge (IV) and As(III).
molar ratio from 1.0 to 9.0, the extraction of Ge (IV) increased parabolically from 0.0% to 96.3%, whereas the extraction of As(III) decreased first and then increased, having the lowest value of 1.3% at a tartaric acid to Ge(IV) molar ratio of 5.0 where the separation coefficient of Ge(IV) to As(III) reached the highest value of 1144. There was sufficient tartaric acid for complexation with Ge(OH)4 at a tartaric acid to Ge(IV) molar ratio of 5.0, so the Ge(IV) extraction changed little on increasing the dosage of tartaric acid beyond this value. Nevertheless, when the tartaric acid to Ge(IV) molar ratio rose from 5.0 to 7.0, the extraction of As(III) increased from 1.3% to 4.9% markedly, which caused a sudden drop in the Ge(IV) to As(III) separation coefficient at the tartaric acid to Ge(IV) molar ratio of 7.0. The rapid increase in the Ge(IV) extraction on the addition of tartaric acid (H4T) can be attributed to the complexation of H3T− and/or H2T2− generated by the deprotonation of tartaric acid with Ge(IV) as shown in Eqs. (6) and (7). In these reactions, the Ge(OH)4 molecules are transformed into Ge (H3T/H2T)x(OH)4y− complex anions (Pokrovski and Schott, 1998). Because Ge(H3T/H2T)x(OH)4y− are anions and have a larger radius than Ge(OH)4, Ge(IV) could be extracted easily after adding tartaric acid to the leaching solution.
3.2.2. Extraction time The effects of extraction time on the extraction efficiencies of Ge (IV) and As (III), as well as the pH of the aqueous phase are shown in Fig. 2b. The extraction process of Ge (IV) was very fast and tended to equilibrium in about 4.0 min. The extraction of As(III) attained the maximum value, 4.5%, after 4.0 min; with further increase in the time from 4.0 to 10.5 min, the extraction of As(III) reduced to 1.3%. Therefore, 10.5 min was sufficient to ensure that Ge(IV) extraction was at equilibrium. As shown in Fig. 2b, as the extraction proceeded, the pH of the aqueous phase decreased from initial 1.2 to final 1.0, indicating that the extraction reaction of Ge(IV) was a process releasing H+ ions. The aqueous phase pH had a significant effect on the extraction of Ge (IV) and the increase of the pH in a certain range can improve the extraction efficiency of Ge(IV), as shown in Figs. 1a–1c and 2b. 3.2.3. Tartaric acid to Ge (IV) molar ratio Haghighi et al. (2018) pointed out that tartaric acid has a significant effect on the extraction of Ge(IV) when using amine as an extractant. Thus, we also chose tartaric acid as a complexing agent in our experiments. The effects of tartaric acid to Ge(IV) molar ratio on the extraction efficiencies of Ge(IV) and As(III), as well as the separation coefficient of Ge(IV) to As(III), are shown in Fig. 2c. Ge(IV) in aqueous solution at pH 1.2 exists in the form of Ge(OH)4 (Virolainen et al., 2013). However, N235 is an anionic extractant and Ge(IV) cannot be extracted without the assistance of complexants (Haghighi et al., 2018). As shown in Fig. 2c, with the increase of the tartaric acid to Ge (IV)
Ge(OH)4 + H3 T− = Ge(OH)2 (HT)− + 2H2 O
(6)
Ge(OH)4 + H2 T 2 − = Ge(OH)2 (T)2 − + 2H2 O
(7)
3.2.4. Temperature The effects of temperature on the extraction efficiencies of Ge(IV) and As(III), as well as the separation coefficient of Ge(IV) to As(III), are shown in Fig. 2d. It was found that the temperature had a great influence on the extraction of Ge(IV). As the temperature increased from 25 to 55 °C, the extraction of Ge(IV) decreased from 93.9% to 75.3% because of the higher solubility of the extracted complex in the aqueous phase at a high temperature. Fig. 2d also indicates that the extraction of As(III) increased from 1.3% to 6.7% with the temperature increase from 25 °C to 35 °C. Subsequent increases in the temperature had little influence on the extraction of As(III). Therefore, the optimal temperature of the extraction process was determined to be 25 °C, at which the separation coefficient of Ge(IV) to As(III) had the maximum value of 1144. 3.3. Extraction equilibrium isotherm of Ge(IV) The Ge(IV) extraction distribution isotherm was obtained with an organic phase consisting of 25% (v/v) N235, 10% (v/v) TOP and 65% (v/v) sulfonated kerosene and a sulfate leaching solution containing 0.055 g/L Ge(IV) , as shown in Fig. 3. It was found that the extraction
Fig. 2b. Effects of time on the extraction efficiencies of Ge (IV), As(III) and pH value of the aqueous solution. 158
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Table 3 Effects of different stripping agents on the stripping of Ge(IV) and As(III). Reagent
Concentration (mol/L)
Ge(IV) Stripping (%)
As(III) Stripping (%)
NaOH NH4Cl NH3·H2O
0.5 0.5 0.5
100.0 16.5 94.6
100.0 51.8 100.0
Fig. 2d. Effects of temperature on the extraction efficiencies of Ge(IV), As(III) and separation coefficient of Ge(IV) to As(III).
Fig. 4. Effects of NaOH concentration on the stripping of Ge(IV) and As(III).
was found that when using NaOH, NH4Cl, NH3·H2O individually as stripping agents, the stripping efficiencies reached 100.00%, 16.54%, and 94.58%, respectively, for Ge(IV) and 100.00%, 51.73%, and 100.00%, respectively, for As(III). These results indicate that NH4Cl is not a suitable agent for the stripping, and, comparing NH3·H2O with NaOH, NaOH is better in the stripping efficiency for Ge(IV). 3.4.2. Effects of NaOH concentration on the stripping Fig. 4 shows the effects of NaOH concentration on the stripping of Ge(IV) and As(III) with the loaded organic phase containing 0.546 g/L Ge(IV) and 0.400 g/L As(III). This process was conducted under the conditions of temperature of 25 °C, O/A ratio of 1/1, NaOH as stripping agents, and stripping time of 10 min. As the concentration of NaOH increased from 0.1 to 0.5 mol/L, the stripping increased from 1.2% to 100.0% for Ge(IV) and from 19.3% to 100.0% for As(III), respectively. A further increase in the NaOH concentration had no effect on the stripping of Ge(IV) and As(III), indicating that 0.5 mol/L NaOH was sufficient to strip the Ge(IV) and As(III) together from the loaded organic phase. The possible reactions in the stripping process are given by Eqs. (8) and (9) (Li, 1996), where G represents the extracted species of Ge(IV) and As(III) in the organic phase. The reaction in Eq. (8) results in disadvantageous depletion of NaOH, which led to cost increase in the stripping process. To prevent the reaction shown in Eq. (8), it is necessary to ensure the saturation of Ge(IV) for extraction.
Fig. 3. The extraction equilibrium isotherm of Ge(IV).
isotherm of Ge(IV) showed an upward sloping line and did not level off, even at an O/A phase ratio of 1/20, indicating that the loaded organic phase did not achieve saturation on the extraction of Ge(IV). Therefore, for industrial applications, a smaller O/A phase ratio is required to use the extractant N235 fully. McCabe–Thiele analysis predicted that the concentration of Ge(IV) in the raffinate could be reduced to less than 1.5 mg/L by adopting a five-stage continuous countercurrent extraction at an O/A ratio of 1/10. 3.4. Stripping As mentioned above, in the Ge (IV) extraction process, the co-extraction of As(III) can be suppressed by using TOP as a modifier, which results in less As(III) into the stripping stage. When Ge(IV) and As(III)bearing stripping solutions were treated by chlorination distillation using calcium salt (Li and Peng, 1990; Li, 1996), the separation of Ge (IV) from As(III), Ca(II) and other impurity could be achieved. For this reason, it is feasible to strip Ge(IV) and As(III) together by using some kind of stripping agent in the stripping process.
R3NH·HSO4(org) + 2NaOH = R3N(org) + 2H2 O + 2Na+ + SO24− (R3NH)a ·G + bNaOH = aR3N(org) + cH2 O +
bNa+
+
Ga −
(8) (9)
4. Conclusions (1). The co-extraction of As(III) was obvious in N235–sulfonated kerosene and N235–TBP-sulfonated kerosene systems at an aqueous phase pH ranging from 0.2 to 1.2. TOP could suppress the co-extraction of As(III) in the extraction of Ge(IV) from the leaching solution containing arsenic at a pH of 1.2 by using N235 as the extractant. Over 93.9% of Ge(IV) and 1.3% of As(III) were
3.4.1. Screening for the stripping agent Table 3 presents the effects of different stripping agents on the stripping of Ge(IV) and As(III) with the loaded organic phase containing 0.546 g/L Ge(IV) and 0.400 g/L As(III). The stripping was conducted under the conditions of temperature of 25 °C, O/A ratio of 1/1, stripping agent concentration of 0.5 mol/L, and stripping time of 10 min. It 159
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extracted, whereas the co-extraction of Zn(II), Fe(III), and Cd(II) was negligible with 25% (v/v) N235 and 10% (v/v) TOP in sulfonated kerosene at an O/A phase ratio of 1/1, tartaric acid to Ge (IV) molar ratio of 5.0, and 25 °C for 10.5 min. Compared with N235–sulfonated kerosene and N235–TBP–sulfonated kerosene systems, the N235–TOP-sulfonated kerosene system was more suitable for the extraction of germanium from the model sulfuric acid secondary zinc oxide leaching solution. (2). The McCabe–Thiele diagram showed that the concentration of Ge (IV) in the raffinate could be reduced to less than 1.5 mg/L by employing a five-stage continuous countercurrent extraction at an O/A ratio of 1/10. The generation of H+ by the extraction reaction inhibited the extraction of Ge(IV) and improved the co-extraction of impurity elements Zn(II), As(III), Fe(III), and Cd(II). Therefore, adjusting the pH of the aqueous phase to 1.2 was beneficial for the recovery of Ge(IV), and an appropriate dosage of tartaric acid would increase the extraction efficiency of Ge(IV) rapidly. (3). An aqueous NaOH solution was appropriate for Ge(III) and As(III) stripping. Almost 100.0% of both Ge(IV) and As(III) were stripped with 0.5 mol/L NaOH solution at an O/A ratio of 1/1 and 25 °C for 10.0 min.
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