Selective recovery of Sb and Te from the sodium sulfide leach solution of Te-bearing alkaline skimming slag by drop-wise H2O2 addition followed by Na2S–Na2SO3 precipitation

Journal Pre-proof Selective recovery of Sb and Te from the sodium sulfide leach solution of Te-bearing alkaline skimming slag by drop-wise H2O2 addition followed by Na2S–Na2SO3 precipitation

Zhipeng Xu, Xueyi Guo, Dong Li, Qinghua Tian, Liu Zhu PII:

S0304-386X(19)30411-6

DOI:

https://doi.org/10.1016/j.hydromet.2019.105219

Reference:

HYDROM 105219

To appear in:

Hydrometallurgy

Received date:

6 May 2019

Revised date:

23 October 2019

Accepted date:

27 November 2019

Please cite this article as: Z. Xu, X. Guo, D. Li, et al., Selective recovery of Sb and Te from the sodium sulfide leach solution of Te-bearing alkaline skimming slag by dropwise H2O2 addition followed by Na2S–Na2SO3 precipitation, Hydrometallurgy(2019), https://doi.org/10.1016/j.hydromet.2019.105219

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Journal Pre-proof

Selective recovery of Sb and Te from the sodium sulfide leach solution of Te-bearing alkaline skimming slag by drop-wise H2O2 addition followed by Na2S–Na2SO3 precipitation Zhipeng Xu a b

a,b,c,d

, Xueyi Guo

a,b,d, *

, Dong Li a,b,d, Qinghua Tian

a,b,d

, Liu Zhu

c

School of Metallurgy and Environment, Central South University, Changsha, 410083, China

National & Regional Joint Engineering Research Center of Nonferrous Metal Resource Recycling, Changsha, 410083, China

511517, China

Hunan Key Laboratory of Nonferrous Metal Resources Recycling, Changsha, 410083, China

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d

f

National Engineering and Technology Research Center of Scattered Metals, Vital Materials Co., Ltd., Qingyuan,

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c

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* Corresponding author. E-mail address: [email protected] (X. Guo).

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Abstract: Sb and Te in Te-bearing alkaline skimming slag were extracted into a leach solution using a two-stage sodium sulfide leaching process. In this study, a possible method for selective recovery of Sb and Te from the sodium sulfide leach solution by drop-wise H2 O2 addition

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followed by Na2 S–Na2 SO3 precipitation was developed. 99% of Sb was precipitated selectively at 50 °C for 100 min with addition of 90 mL H2 O2 while Te precipitation was near-zero. Te in the

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oxidized solution was converted to the form of TeS4 2– with Na2 S, and recovered by Na2 SO3

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reduction. The results indicated that 95% of Te was precipitated under the optimum conditions: 1.0 of Na2 S excess coefficient, 1.75 of Na2 SO3 excess coefficient, 50 °C of temperature and 30 min of time. XRD pattern and chemical analysis indicated that the H2 O2 oxidation precipitate was NaSb(OH) 6 with Sb content of 46.92% and the Na2 S–Na2 SO3 precipitate was elemental Te with a purity of 98.61%. Keywords: Antimony; Tellurium; Tellurium-bearing alkaline skimming slag; Hydrogen peroxide; Sodium sulfide; Sodium sulfite.

1. Introduction Tellurium is a p-type semiconductor with a narrow band gap energy of 0.34 eV (Wu et al., 2017; Abad et al., 2015; Huang et al., 2016). Owing to its highly anisotropic crystal structure, Te exhibits numerous unique properties including photoconduction, piezoelectricity and thermo-electric effect (Woodhouse et al., 2013;

Journal Pre-proof Liu et al., 2010; Wang et al., 2006; Lee et al., 2013; Meroz et al., 2016; Wu et al., 2017). Additionally, Te can also be used in stainless steel, copper and lead alloys in small amounts to improve its machinability (Yang et al., 2018; Huang et al., 2018; Guo et al., 2009). However, Te is a typical scattered element, the natural abundance of which in the earth’s crust is only 6×10–6 (Kabata-Pendias and Pendias, 2000; Belzile and Chen, 2015; Mandal et al., 2004; Kavlak and Graedel, 2013). Currently, most of Te is produced from copper anode slime (Hoffmann, 1989; Wang, 2011). During oxidation refining of crude dore bullion from copper anode slime treatment, Te accumulates in the alkaline skimming slag, in which the mass fraction of Te can reach

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10–30% (Wang, 2011; Fan et al., 2013; Shibasaki et al., 1992). The Te-bearing

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alkaline skimming slag is an important raw material for Te recycling (Makuei and Senanayake, 2018).

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Numerous viable approaches have been developed to treat Te-bearing alkaline

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skimming slag including ball milling- water leaching, acid leaching-reduction, alkali leaching- neutralization and chlorination leaching (Hoffmann, 1989; Wang, 2011;

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Zhang et al., 2017; Rhee et al., 1999; Fan et al., 2013). Among these approaches, ball milling-water leaching and acid leaching- reduction have been widely used on an industrial scale. Ball milling-water leaching is available worldwide because of its

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simplicity and its attractive costs compared with other processes while the Te

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recovery is relatively low (40%–50%) (Wang, 2011). Acid leaching-reduction shows the advantage of short reaction time and easy operation, and generally results in high 2017).

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Te extraction. However, the reagent consumption is relatively high (Zhang et al., Sulfide leaching, a high-effective metallurgical process, was developed to the treatment of antimony (Sb) -bearing copper concentrates, arsenic bearing dust and gold-bearing ores (Awe et al., 2013; Guo et al., 2016; Celep et al., 2011). Recently, a two-stage sodium sulfide leaching process was proposed for step extraction of Te and Sb from Te-bearing alkaline skimming slag (Xu et al., 2018). 88% of Te was extracted selectively in the first-stage sodium sulfide leaching, while 95% of Te and 96% of Sb was leached efficiently in the second-stage sodium sulfide leaching (Xu et al., 2018; Guo et al., 2018). The second-stage sodium sulfide leach solution, containing approximately 20 g·L–1 Sb and 2 g·L–1 Te, deserved separation and recovery. Herein, we proposed a flowsheet (Fig. 1) for comprehensive recovering metal

Journal Pre-proof values from Te-bearing alkaline skimming slag by two-stage sodium sulfide leaching combined with downstream treatments. A leach solution produced in the second-stage sodium sulfide leaching was processed by drop-wise H2 O2 addition followed by Na2 S–Na2 SO3 precipitation for selective recovery of Sb and Te. The effects of H2 O2 dosage, oxidation temperature and time on Sb and Te precipitation in H2 O 2 oxidation precipitation were studied. Moreover, the effects of Na 2 S and Na2 SO3 excess coefficient, reduction temperature and time on Te precipitation in Na2 S–Na2 SO3 reduction precipitation were explored.

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2. Experimental 2.1. Materials

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The Te-bearing alkaline skimming slag used in the experiments was obtained from oxidation refining of crude dore bullion at nonferrous metals smelters in Chenzhou

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province, China. Table 1 shows the chemical composition of Te-bearing alkaline skimming slag. The XRD analysis of Te-bearing alkaline skimming slag revealed that

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antimony was predominantly as sodium antimony hydroxide (NaSb(OH) 6 ) (Fig. 2a). Other mineral phases present were lead bismuth oxide (Pb 5 Bi8 O17 ), bismuth (Bi),

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bismuth Iron Oxide (BiFeO 3 ), silicon oxide (SiO 2 ) and gahnite (Zn8 Al16 O32 ). However, the mineral phase of tellurium has not been detected by XRD measurement,

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which may be due to the poor crystallization of its compounds (Benmadani et al.,

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2013; Zhang et al., 2011). Therefore, X-ray photon spectroscopy (XPS) analysis of Te-bearing alkaline skimming slag was carried out in order to clarify the mineral composition of Te. XPS spectra for Te 3d2/5 illustrated a peak at 576.83 eV binding energy value, which was assigned to Na2 TeO 4 (576.83 eV) (Fig. 2b) (Wagner, 1975). This indicates the mineral phase of Te presented in the Te-bearing alkaline skimming slag is Na2 TeO 4 . The sodium sulfide leach solution of Te-bearing alkaline skimming slag was prepared according to the leaching procedure which was previously described (Xu et al., 2018). Chemical analysis of the leach solution indicates that the content of Sb and Te is 22.18 g·L–1 and 2.45 g·L–1 , respectively, which is predominantly present as Na3 SbS4 and Na2 TeS4 . The concentration of Na2 S is 117.47 g·L–1 . The pH value of the leach solution is 13.76. All other chemical reagents used in the experiments were of analytical grade supplied by Sinopharm Chemical Reagent Co., Ltd., and were used without further

Journal Pre-proof purification. The water used in all experimental work was ultrapure water (conductivity 0.2 μS·cm–1 ) unless otherwise specified. 2.2. Experimental procedures 2.2.1. H2 O2 oxidation precipitation The H2 O2 oxidation precipitation experiments were carried out in a 500 mL round-bottomed glass reactor equipped with a condenser to prevent evaporation losses. 200 mL of the sodium sulfide leach solution was first added to the reactor and heated in a temperature controlled water bath. when the desired temperature was reached, a

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certain amount of H2 O2 (30 wt%) was added at a rate of 0.9 mL·min–1 using a

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peristaltic pump, whilst stirring at 300 rpm. When the H2 O2 oxidation precipitation experiment was completed, the precipitate was then collected, filtered off, and washed

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with ultrapure water to remove residual ions before it was finally dried overnight at

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80 °C and weighted. 2.2.2. Na2 S–Na2 SO 3 reduction precipitation

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The Na2 S–Na2 SO3 reduction precipitation experiments were performed in a 500 mL four-necked round bottomed flask equipped with a condenser to prevent evaporation losses. 200 mL of the oxidized solution obtained by H2 O2 oxidation

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precipitation process was transferred into the flask. Once the temperature set-point

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was reached, certain amounts of Na2 S and Na2 SO3 were added. The solution was continuously stirred at a constant stirring rate of 300 rpm and the flask was heated in a

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temperature controlled water bath. When the Na2 S–Na2 SO3 reduction precipitation experiment was completed, the precipitates were filtered, washed with ultrapure water and dried in an oven at about 80 °C for 12 h. 2.3. Characterization and analyses Chemical analysis for aqueous samples, Te-bearing alkaline skimming slag and precipitates was performed using Inductively Coupled Plasma Atomic Emission Spectrometry (PS-6, Baird Corp, USA). The pH value of the sodium sulfide leach solution was measured by potential-pH meter (PHS-3C, INESA, China). X-ray diffraction (XRD) patterns of oxidation precipitate and reduction precipitate were collected using a Bruker D8-Discover diffractometer in the range of 10–80° 2θ at a scanning rate of 1° per min with Cu Kα radiation. XRD patterns analysis of

Journal Pre-proof Te-bearing alkaline skimming slag was conducted in the range of 5–90° 2θ at a scanning rate of 0.5° per min with Cu Kα radiation. XPS analysis was used for an elemental analysis of Te-bearing alkaline skimming slag (ESCALAB 250Xi, Thermo Fisher, USA). After acquiring XPS spectra, XPSPEAK41 software was used for fitting peaks. The surface morphologies of the oxidation precipitate and reduction precipitate were determined using SEM equipment (JSM-6360LV, Rigaku, Japan). 3. Results and discussion

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3.1. H2 O 2 oxidation precipitation

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3.1.1. Thermodynamic analyses

H2 O2 is a quite unique oxidant (Strukul, 2013). It has a high active oxygen content

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and its reduction product is water (Strukul, 2013; Shu and Shi, 2001). When adding H2 O 2 into the sodium sulfide leach solution, SbS4 3– and TeS4 2– are oxidized to

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NaSb(OH)6 and Na2 TeO 4 , respectively, while S2– may be transformed to S, SO32– , S2 O32– , SO 42– . The primary reactions that occur during H2 O2 oxidation are given in

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Eqs. (1) – (6) (Wikedzi et al., 2016).

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Na3 SbS4 + 8H2 O2 + 2NaOH = NaSb(OH)6 ↓ + 2Na2 S2 O3 + 6H2 O Na2 TeS4 + 8H2 O2 + 4NaOH= Na2 TeO 4 + 2Na2 S2 O3 + 10H2 O

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Na2 S + H2 O 2 = S + 2NaOH

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2Na2 S + 4H2 O2 = Na2 S2 O3 + 2NaOH + 3H2 O

(1) (2) (3) (4)

Na2 S +3H2 O2 = Na2 SO 3 + 3H2 O

(5)

Na2 S +4H2 O2 = Na2 SO 4 + 4H2 O

(5)

In order to predict the thermodynamic trend of reactions in the H2 O2 oxidation process, the Gibbs Free Energy changes at 25 °C for Eqs. (1) – (6) were considered. The related thermodynamic data, free energy of formation (ΔfGmθ/kJ·mol–1 ) at 25 °C are listed in Table 2 (Dean, 1992). The Gibbs Free Energy changes at 25 °C for Eqs. (1) – (6) were calculated to be -2378.80 kJ·mol–1 , -2234.50 kJ·mol–1 , -279.88 kJ·mol–1 , -1237.90 kJ·mol–1 , -922.50 kJ·mol–1 , -1297.20 kJ·mol–1 , respectively. The obtained Gibbs free energy changes for Eqs. (1) – (2) are extremely negative, indicating that the thermodynamic trend of SbS4 3– and TeS4 2– transformed to NaSb(OH)6 and Na2 TeO 4 are very strong. And the Gibbs free energy changes for Eqs. (3) – (6) are relatively negative, suggesting that S2– can spontaneously be converted to S, SO3 2– ,

Journal Pre-proof S2 O3 2– , SO 4 2–, thermodynamically. 3.1.2. Effect of H2 O2 dosage The oxidation precipitation of Sb and Te was performed by drop-wise addition of H2 O 2 instead of immediate oxidation, the latter resulted in generating a large amount of heat instantly and causing the solution to splash (Multani et al., 2017; Wikedzi et al., 2016). Slow drop-wise addition of H2 O2 has proven to be effective in the oxidation and removal of arsenic because it greatly avoids the loss of the reagent which undergoes rapid decomposition at elevated temperatures (Lin et al., 1991;

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Dabekaussen et al., 2001; Wang et al., 2018; Multani et al., 2017). Fig. 3 presents Sb

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and Te precipitation efficiency at 50 °C for 150 min with H2 O2 dosage of 36 mL, 48 mL, 60 mL, 72 mL, 90 mL and 120 mL. The dosage of H2 O2 added to the reactor was

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based on the stoichiometric requirement for the complete oxidation of the total amounts of both SbS4 3– and TeS4 2– presented in the solution according to Eqs. (1) –

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(2).

As shown in Fig. 3, it can be observed that about 47% of Sb was precipitated with

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36 mL H2 O2 added and when the H2 O2 dosage increased to 72 mL, 99% of Sb was precipitated. Any further increase in H2 O 2 dosage resulted in marginal increase in Sb

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precipitation. Surprisingly, there was none of Te precipitated within the range of H2 O2 added. This may be attributed to the low concentration of Na 2 TeO 4 formed by TeS4 2–

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since Na2 TeO 4 is insoluble in the alkaline solution. Therefore, the dissolving behavior

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of Na2 TeO 4 in alkaline solution deserves specific consideration. Fig. 4 displays the equilibrium concentrations of Na 2 TeO 4 in the clear solutions with different molar concentration of NaOH (0.25, 0.50, 0.75, 1.00, 2.00, 4.00, 8.00, and 12.00 mol·L–1 ) and temperatures (20, 40 and 60 °C). It can be seen that the change of Na2 TeO 4 equilibrium concentration was similar to that of temperature. The equilibrium concentration of Na2 TeO 4 increased with an increase in temperature. This may be due to the increase in temperature to accelerate the ionization of Na2 TeO 4 , which is a strong electrolyte (Mönnighoff et al., 2017). The ionization of Na2 TeO 4 is expressed as Eq. (7). Na2 TeO 4 → 2Na+ + TeO 4 2–

(7)

The generated TeO 4 2– would hydrolyzed to produce OH– , HTeO4 – and H2 TeO 4 , which are given as Eqs. (8) – (9).

Journal Pre-proof TeO 4 2– + H2 O →HTeO4 – + OH–

(8)

HTeO4 – + H2 O→ H2 TeO 4 + OH–

(9)

It was also observed that the equilibrium concentration of Na2 TeO 4 firstly decreased and then increased with increasing concentration of NaOH (Fig. 4). At low NaOH concentration, increasing concentration of NaOH reduced the equilibrium concentration of Na2 TeO 4 , this may be attributed to ion effect of Na + and OH– (Chen et al., 2013), which was produced from Na2 TeO 4 dissolving in alkaline solution according to Eqs. (7) – (9). However, the reason for the increase in the equilibrium

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concentration of Na2 TeO 4 at high NaOH concentration is not known.

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Furthermore, in order to determine the appropriate amount of H2 O 2 added during the oxidation precipitation process, it was necessary to investigate the effect of H2 O2

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dosage on the recovery of Te in the oxidized solution. 200 mL of the oxidized solution obtained by H2 O2 addition of 60 mL, 72 mL and 90 mL was added with 1.26

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g Na2 S and 3.80 g of Na2 SO3 at 50 °C for 60 min. Table 3 depicts the Te recovery at different H2 O2 addition. It can be seen that none of Te was precipitated with H2 O2

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addition of 60 mL and 72 mL, but about 83% of Te was precipitated with H2 O2 addition of 90 mL. This may be attributed to the fact that the S2– oxidation are not

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complete with H2 O2 addition of 60 mL and 72 mL, and the residual S2– would dissolve the produced Te precipitate. Based on the stoichiometric requirement for the

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complete oxidation of SbS4 3– , TeS42– and S2– presented in the solution according to

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Eqs. (1) – (2) and (4), the total amount of H2 O2 is 90 mL. The digital photos of the oxidized solution at different H2 O 2 dosage had good agreement with it (Fig. 5). With addition of H2 O2 , the color of the oxidized solution gradually changed from yellow to white and completely turned white and transparent under 90 mL of H2 O 2 addition. Therefore, a moderate H2 O 2 dosage of 90 mL was selected for subsequent experiments. 3.1.3. Effect of oxidation temperature Fig. 6 illustrates the effect of oxidation temperature on Sb and Te precipitation efficiency. As shown in the Fig. 6, Sb precipitation was almost independent of the reaction temperature and showed a precipitation efficiency of more than 99% at 30 °C. It was also observed that addition of H2 O 2 increased the temperature of the system beyond the set value due to its exothermic reaction (Eq. (10)) (Wikedzi et al., 2016; Strukul, 2013; Shu and Shi, 2001). The actual temperatures of the system after H2 O2

Journal Pre-proof addition are shown in Table 4. This might probably explain the reason for little influence on Sb precipitation efficiency with increasing temperature. And Te precipitation decreased from 35% to approximately zero by increasing the reaction temperature from 30 °C to 50 °C after which this remained almost steady up to 70 °C. This is due to the fact that the equilibrium concentration of Na2 TeO 4 increases with an increase in temperature (Fig. 4). Consequently, an optimal reaction temperature of 50 °C was chosen in the current study. 2H2 O2 → 2H2 O + O2 + 98 kJ/g-mol H2 O 2

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3.1.4. Effect of oxidation time

(10)

The effect of oxidation time on Sb and Te precipitation efficiency is shown in Fig.

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7. It can be seen that the Sb oxidation and precipitation was almost complete within 100 min. By further prolonging the oxidation time, there was no obvious

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improvement on the precipitation of Sb. Furthermore, there was almost no precipitation of Te within the range of oxidation time applied. Thus, 100 min of

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oxidation time was determined optimal.

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3.1.5. Oxidation precipitate characterization

Based on results above, the optimum H2 O2 oxidation precipitation conditions were

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determined as follows: 90 mL of H2 O2 dosage, 50 °C of reaction temperature and 100 min of reaction time. Under the optimum conditions, comprehensive experiment was

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carried out and 99% of Sb was precipitated, while the Te precipitation was closed to zero. The XRD, SEM and chemical composition analys is of the precipitate were shown in Figs. 8–9 and Table 5. As can be seen from Fig. 8, there was good correspondence of the precipitate diffraction peaks to those of NaSb(OH) 6 's reference pattern. The morphology of the precipitate was regular cuboid with an average particle size 50–100 μm (Fig. 9). From Table 5, it can be known that the mass ratio of Sb and Na was 46.92% and 9.70%, respectively, which was slightly lower than the theoretical value of NaSb(OH)6 . And the main impurities were Bi, Ca, Sn, Al, Fe and Mg. 3.2. Na2 S–Na2 SO 3 reduction precipitation Table 6 displays the chemical composition of the oxidized solution, and the results showed that the oxidized solution contained 1.45 g·L–1 Te and small amount of Sb, Bi,

Journal Pre-proof Fe, Pb and Zn. The decrease in Te concentration from the initial 2.45 g·L–1 to 1.45 g·L–1 in the oxidized solution is as a result of wash-water dilution during the Sb precipitation process. Te in the oxidized solution mainly existed in the form of TeO 4 2– . In the traditional treatment of TeO 4 2– in the alkaline medium, it is usually first acidified with sulfuric acid and then reduced with sulfur dioxide to recover Te, which has high alkali consumption and environmental problems (Wang, 2011; Mokmeli, et al., 2014, 2015, 2016). In the present study, based on the fact that TeS4 2– in the solution could be

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decomposed by SO 3 2– with precipitation of element Te (Guo et al., 2017; Feigl, 1949;

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Mellor, 1931; Hageman, 1919), S2– ion was introduced into the oxidized solution to transform TeO 4 2– to TeS4 2– , then adding Na2 SO3 to recover Te. The primary reactions

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that occur during Na2 S–Na2 SO3 reduction precipitation are expressed as Eqs. (11) –

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(12).

(11)

TeS4 2– + 3SO 3 2– = 3S2 O3 2–+ S2– + Te↓

(12)

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TeO 4 2– + 4S2– + 4H2 O = TeS4 2– +8OH–

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3.2.1. Effect of Na2 S excess coefficient

Series experiments were carried out under the following conditions (Na2 SO3 excess

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coefficient of 1.5 (according to Eq. (12)), 50 °C of temperature and 30 min of time) in order to study the effect of Na2 S excess coefficient (according to Eq. (11)) on Te

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precipitation. As shown in Fig. 10, Na2 S excess coefficient was observed to have a significant effect on the Te precipitation. Te precipitation decreased fastly from 90.88% to 4.10% by increasing Na2 S excess coefficient from 1.0 to 3.0. This may be attributed to the fact that the produced Te precipitate would dissolve in the solution when Na2 S was excessive. The primary reaction is expressed as Eq. (13). Te +3Na2 S + O2 + 2H2 O = Na2 TeS3 + 4NaOH

(13)

Thus, 1.0 of Na2 S excess coefficient was selected. 3.2.2. Effect of Na2 SO3 excess coefficient Fig. 11 illustrates the effect of Na2 SO3 excess coefficient on Te precipitation. The results indicate that the precipitation of Te is dependent on the Na2 SO3 excess coefficient. Te precipitation increased gradually from 86.70% to 93.28% as the

Journal Pre-proof Na2 SO 3 excess coefficient increased from 1.0 to 1.75. This result is consistent with the previous study by Zheng and Sun (2010) when Te was recovered from copper sulphate mother solution containing Te. Consequently, 1.75 was chosen as the optimal Na2 SO3 excess coefficient. 3.2.3. Effect of reduction temperature The effect of reduction temperature on Te precipitation is shown in Fig. 12. It can be observed that the reduction temperature has a significant effect on the Te precipitation. Te precipitation increased from ~73% to ~94% with the reduction

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temperature rising from 30 °C to 60 °C, after which it remained almost steady up to

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70 °C. Similar behavior of Te precipitation has been reported by our previous study

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(Guo et al., 2017). Thus, 60 °C was selected as the optimum reduction temperature.

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3.2.4. Effect of reduction time

Fig. 13 presents the effect of reduction time on Te precipitation. It was observed

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that the precipitation of Te is efficient and fast. Approximately 83% of Te was precipitated within 5 min. When the reduction time increased to 30 min, Te in the oxidized solution was precipitated almost completely. In contrast, it takes more than

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48 hours to precipitation of Te as Cu2-x Te from decopperizing leach solution with

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copper powder at 85 °C (Guo et al., 2017; Shibasaki et al., 1992). Consequently, an

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optimal reduction time of 30 min was selected. 3.2.5. Reduction precipitate characterization Therefore, the optimal conditions for Na2 S–Na2 SO 3 reduction precipitation were determined as follows: 1.0 of Na2 S excess coefficient, 1.75 of Na2 SO 3 excess coefficient, 50 °C of temperature and 30 min of time. Under the optimal conditions, 95% of Te was precipitated meanwhile Te concentration decreased to 0.07 g·L–1 in the reduced solution. After filtration, the dried precipitate was ground and analyzed by XRD and SEM, Fig. 14 and Fig. 15 display the XRD patterns and SEM image of the precipitate, respectively. As can be seen from Fig. 14, XRD peaks attributed to elemental Te can clearly be observed. And the morphology of the precipitate was spindle with an average particle size of 5–10 μm (Fig. 15). The chemical composition of the precipitate is listed in Table 7. It was found that the impurity content of crude Te is relatively low and its purity reaches 98.61%.

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4. Conclusions 1) Drop-wise H2 O2 addition followed by Na2 S–Na2 SO 3 precipitation was developed to selectively recover Sb and Te from the sodium sulfide leach solution of Te-bearing alkaline skimming slag. 2) 99% of Sb was precipitated selectively in the H2 O2 oxidation precipitation process while Te precipitation was closed to zero. The optimum conditions were determined as 90 mL of H2 O2 addition, 50 °C of reaction temperature and 100 min of

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reaction time. XRD pattern and chemical analysis indicated that the precipitate was NaSb(OH)6 with Sb content of 46.92%.

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3) The TeO 4 2– in the oxidized solution was transformed to TeS4 2– with S2– ion, and recovered with Na2 SO3 addition. Optimum conditions were found to be 1.0 of Na 2 S

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excess coefficient, 1.75 of Na2 SO 3 excess coefficient, 50 °C of temperature and 30 min of time. Under the optimum conditions, 95% of Te was precipitated and

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elemental tellurium with a purity of 98.61% was obtained.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51604303). References Abad, B., Rull-Bravo, M., Hodson, S.L., Xu, X., Martin-Gonzalez, M., 2015. Thermoelectric properties of electrodeposited tellurium films and the sodium lignosulfonate effect. Electrochim. Acta 169, 37–45. Awe, S.A., Sundkvist, J.E., Bolin, N.J., Sandström, Å., 2013. Process flowsheet development for recovering antimony from Sb-bearing copper concentrates. Miner. Eng. 49, 45–53. Belzile, N., Chen, Y.W., 2015. Tellurium in the environment: A critical review focused on

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conversion of arsenic trioxide to crystalline scorodite. CIM Bull. 94 (1051), 116–122.

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Dean, J.A., 1992. Lange’s Handbook of Chemistry, 14th ed., McGraw -Hill, Inc., New York. Fan, Y., Yang, Y., Xiao, Y., Zhao, Z., Lei, Y., 2013. Recovery of tellurium from high tellurium

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bearing materials by alkaline pressure leaching process: thermodynamic evaluation and

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Guo, X.Y., Xu, Z.P., Li, D., Tian, Q.H., Xu, R.Z., Zhang, Z., 2017. Recovery of tellurium from high tellurium-bearing materials by alkaline sulfide leaching followed by sodium sulfite precipitation. Hydrometallurgy 171, 355–361.

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Guo, X.Y., Xu, Z.P., Li, D., Tian, Q.H., Zhang, Z., 2018. Novel process for selectively

1008–1015 (in chinese).

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(6), 2109–2114 (in Chinese).

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Fig. 1. The proposed flowsheet for recovery of Te-bearing alkaline skimming slag.

Fig. 2. XRD patterns and XPS spectrum (Te 3d) of Te-bearing alkaline skimming slag.

Fig. 3. Effect of H2 O2 dosage on Sb and Te precipitation efficiency (oxidation temperature of

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50 °C and time of 60 min).

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Fig. 4. The equilibrium concentrations of Na2 TeO4 in the NaOH solution.

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Fig. 5. Photos of the oxidized solution at different H2 O2 dosage (1# : 36 mL; 2# : 48 mL; 3# : 60 mL; 4# : 72 mL; 5# : 90 mL; 6# : 120 mL).

Fig. 6. Effect of oxidation temperature on Sb and Te precipitation efficiency (H2 O2 dosage of 90 mL and time of 60 min).

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Fig. 7. Effect of oxidation time on Sb and Te precipitation efficiency (H2 O2 dosage of 90 mL and temperature of 50 °C).

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Fig. 8. XRD patterns of the precipitate obtained by H2 O2 oxidation precipitation.

Fig. 9. SEM image of the precipitate obtained by H2 O2 oxidation precipitation.

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Fig. 10. Effect of Na2 S excess coefficient on Te precipitation efficiency (Na2 SO3 excess coefficient of 1.5, reduction temperature of 50 °C and time of 30 min).

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Fig. 11. Effect of Na2 SO3 excess coefficient on Te precipitation efficiency (Na2 S excess coefficient of 1.0, reduction temperature of 50 °C and time of 30 min).

Fig. 12. Effect of reduction temperature on Te precipitation efficiency (Na2 S excess coefficient of 1.0, 1.75 of Na2 SO3 excess coefficient and time of 30 min).

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Fig. 13. Effect of reduction time on Te precipitation efficiency (Na2 S excess coefficient of 1.0, 1.75 of Na2 SO3 excess coefficient and temperature of 50 °C).

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Fig. 14. XRD patterns of the precipitate obtained by Na2 S–Na2 SO3 reduction.

Fig. 15. SEM image of the precipitate obtained by Na2 S–Na2 SO3 reduction.

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Element wt (%)

Table 1 Chemical composition of Te-bearing alkaline skimming slag. Sb Te Pb Na Bi Zn Si Fe 23.60 11.60 11.50 9.74 5.21 1.71 2.29 2.16

Al 1.49

Table 2 Standard free energy of formation of species in the H2 O2 oxidation process at 25 °C. Compound

Na3 SbS4

H2 O2

NaOH

NaSb(OH)6

Na2 SO4

H2 O

Δ f G m (kJ·mol )

-821.83

-120.42

-419.20

-1487.51

-1268.40

-237.14

Compound

Na2 TeO3

Na2 TeO4

Na2 S

Na2 TeS3

Na2 TeS4

NaOH

-916.75

-980.85

-438.10

-634.67

-569.63

-419.20

-1

θ

-1

θ

Δ f G m (kJ·mol )

60

Te recovery (%)

0

72

90

0

83.31

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H2 O2 addition (mL)

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Table 3 The Te recovery at different H 2 O2 addition.

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Table 4 Change in precipitation temperature after H 2 O2 addition. 30

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Initial temperature (℃)

Temperature change after H2 O2 injection (℃)

52

40

50

60

70

54

62

68

78

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Table 5 Chemical composition of the precipitate obtained by H 2 O2 oxidation precipitation. Sb

Na

O

wt (%)

46.92

9.70

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39.69

Bi

Ca

Sn

Al

Fe

Mg

0.045

0.041

0.093

0.005

0.003

0.002

Table 6 Chemical composition of the oxidized solution.

*

Element

Te

Sb

Bi*

Fe*

Pb*

Zn*

Concentration (g·L-1 )

1.45

0.11

7.45

1.15

0.90

0.08

mg·L-1

Table 7 Chemical composition of the precipitate obtained by Na 2 S-Na2 SO3 reduction precipitation. Element

Te

Bi

Na

Cu

Ca

Pb

Zn

wt (%)

98.61

0.85

0.25

0.098

0.072

0.027

0.025

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Highlights

 A possible process for selective recovery of Sb and Te from the sodium sulfide leach solution of Te-bearing alkaline skimming slag was proposed.  SbS3-4 was precipitated selectively as NaSb(OH)6 while Te precipitation was near-zero in H2 O2 oxidation. TeS2-4

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 TeO2-4 in the oxidized solution was converted to

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decomposed with precipitation of elemental Te by SO32- .

with S2-, and