Selective and Efficient Electrochemical Recovery ... - ACS Publications

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Selective and efficient electrochemical recovery of dilute copper and tellurium from acidic chloride solutions Wei Jin, Meiqing Hu, and Jiugang Hu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03150 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 16, 2018

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ACS Sustainable Chemistry & Engineering

Selective and efficient electrochemical recovery of dilute copper and tellurium from acidic chloride solutions

Wei Jin a,c,*, Meiqing Hu a, Jiugang Hu b,*

a

Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School

of Chemical and Material Engineering, Jiangnan University, No. 1800 Lihu Avenue, Wuxi 214122, China b

College of Chemistry and Chemical Engineering, Central South University, No. 932th South Lushan Road, Changsha, Hunan 410083, China

c

Institute of Process Engineering, Chinese Academy of Sciences, 1th Ber-er-tiao Zhongguancun, Beijing 100190, People’s Republic of China

*Corresponding author: E-mail: [email protected] (Wei Jin); [email protected] (Jiugang Hu)

Abstract: Sustainable recovery of tellurium (Te) from large scale wastewater is of great importance to offer stable supply of this strategic element. However, the concentration of tellurium is very low, and there is usually coexisting metallic ion of copper. In this study, a novel selective and efficient electrochemical extraction of Te and Cu from HCl solutions was first reported with stainless steel electrode. The

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interfacial growth behavior of electrodeposition was measured by the emerging in situ synchrotron radiation dynamic X-ray images. A turbulent reactor was employed for the stepwise electrodeposition and nanometric powder was produced owing to the largely improved mass transport. High purity 93.2% tellurium and 98.3% copper were selectively and efficiently recovered with the current efficiency of ～85% and extraction ratio of ～95%. This mass transport assisted method may be explored as a promising solution to overcome the limitation of present metal recycling and water purification.

Key words: Selective, Hydrochloric acid, Electrodeposition, Mass transport, Metallic powder

Introduction Tellurium (Te) is a strategic metalloid in emerging electronic devices, solar cells and batteries, due to its unique semiconducting features with wide 0.34 eV energy gap.1-2 Nevertheless, tellurium is an important scattered element with trace 0.001-0.005 g/t of the earth`s crust, which is comparable to the corresponding 0.0031 g/t and 0.0037 g/t of precious gold and platinum, respectively.3 Recently, more than 90% Te production originates from anode slimes by-product generated during the copper electrorefinning, where the mass ratio of tellurium can achieve 1-10%.4 Substantial efforts have been carried out to recover and extract these resources from the slimes, including soda ash calcination-reduction, NaOH pressure leaching and wet


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chlorination methods. Hoffmann5 identified that the wet chlorination process has many advantages of fast reaction, simple control and readily turnover of valuable metals. Consequently, the key issue is to selective and efficient recovery of Te and co-existing elements, particularly Cu. After the decoppering step of leaching solution usually via solvent extraction or crystallization, the aqueous Te compounds can be reduced and extracted as TeO2 by toxic sulfur dioxide.6 However, this process is only technically efficient if the initial concentration of dissolved Te is much larger than 2 g/L. Recently, electrodeposition (electrowinning) is a facile, selective and energy-efficient technique to extract metal/metalloid compounds in their most valuable states.7-10 Usually, a large amount of critical metals, such as rare-earth, nonferrous and noble metals are simultaneously existed in the waste streams. Stepwise electrodeposition can be achieved with respect to the different standard redox reaction potentials of metal/metalloid ions, resulting selective recovery of different metals. By optimization of the applied overpotential, pH and electrolyte conditions, Jin8 and Doulakas9 successfully developed the selective metal electrodeposition form acidic Cu-Bi and Cu-Pb-Cd-Zn systems, respectively. Furthermore, owing to the highly excellent time and spatial resolution, the emerging synchrotron radiation dynamic X-ray imaging has been employed to illustrate the interfacial behavior of metal electro-deposition under real time environment.11-13 Therefore, it is desirable to investigate the electrochemical metal recovery and provide fundamental evidence for the sustainable chemical engineering with novel in-situ techniques.



Besides, it has been demonstrated that the electro-deposition kinetic rate and current efficiency is substantially frustrated by the low metal solubility in acidic solutions.14-16 And the anodic chlorine evolution is severe in HCl solutions, resulting in the re-dissolution of deposited product and further decrease of current efficiency. It has been widely known that the mass transfer condition plays an important role during electrodeposition process, such as the employment of electrode movements, high surface area cathode or fast flow rate of solution. Kekesi et al.17 reported a progressive stirring technique to improve the Cu electrodeposition in hydrochloric media. Walsh et al.18 reported a rotating cylinder electrode reactor for the dilute 0.5 g/L precious palladium recovery from acidic solutions, in which 99% recovery ratio was achieved within 30 min treatment. Most recently, the emew® system attracts increasing attention for the dilute metal electrodeposition, in which an IrO2-Ta2O5 decorated titanium anode in the center and low-cost stainless steel foil surrounding the tube.19-21 Therefore, the aim of this investigation is to develop a selective and efficient electrochemical extraction of dilute copper and tellurium from harsh hydrochloric acid solutions. The electrodeposition behavior in acidic solutions is investigated by real-time synchrotron radiation X-ray imaging technique. The tubular emew® cell was employed, and the recovery ratios are expected to be systematically optimized, resulting an alternative process for sustainable resource recycling, wastewater remediation and powder metallurgy.

Material and methods


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Material

The chemical reagents used during this investigation were all of analytical grade quality and employed as received from the company of Sigma Aldrich, including hydrochloric acid, CuCl2 and sodium tellurite. The solution was obtained by dissolving the required reagents into ultrapure H2O (18 MΩ · cm).

Electrochemical studies Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) was measured at 25oC using a CHI760E workstation. A concentional three-electrodes cell (100mL) was employed with 316L working electrode (active area of 1.5 cm2), high corrosion-resistant IrO2-Ta2O5 decorated Ti counter electrode and Ag/AgCl (0.22 V vs. SHE) reference electrode. Initially, the stainless steel electrode was thoroughly polished to obtain a smooth deposition surface, following by the H2O wash and sonication. The electrochemical investigations were carried out in N2-purged solution by potential sweep at different scan rates. Chronopotentiometry (CP) was performed at 100 and 400 A/m2 with various stirring rates. Besides, a tubular reactor (emew® system) with a large cathode area of 420 cm2was selected to further improve the mass transfer condition, in which 3 liters of solution was tested with pumping rate of 3-5 L/min.

Physical characterizations



The surface morphology and metallic composition of as-prepared powder were determined by the field-emission scanning electron microscopy (FE-SEM, FEI Helios-FIB) with energy dispersive X-ray spectroscopy (EDS), while the corresponding crystalline structure was characterized using a X-ray diffraction (XRD, Bruker AXS, Germany) with Cu Kα radiations. Real time synchrotron radiation X-ray imaging was measured with BL13W1 beamline at Shanghai Synchrotron Radiation Facility. A polystyrene reactor (1.0 cm liquid thickness) was employed, in which the operated X-ray light was vertically concentrated towards the glassy carbon tested electrode/electrolyte interface. Copper electrodeposition was carried out with the remote electrochemical workstation to avoid the hazardous high-energy irradiation. The in-situ images with a high resolution of 2.25 µm/pixel were recorded by an X-ray charged couple device camera at a frequency of 1 frame per second.

Results and discussion Interfacial behavior of electrodeposition in HCl and H2SO4 Cyclic voltammetry (CV) investigations were performed to study the electrochemical reduction of Cu(II) in hydrochloric acid and sulfuric acid media. As illustrated in Figure 1a, an obvious oxidation peak at -0.04 V was observed in the hydrochloric acid (blank) solution, which is corrsponding to the anodic corrosion of stainless steel electrodes. Besides, a well-defined electro-reduction peak occurs at -0.38 V with 12.5 mA·cm-2 peak current density in the presence of Cu(II), while no


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corresponding reduction peak was observed in the same potential region of blank solution, indicating this peak is related to Cu(II) reduction. As presented in Figure 1b, two obvious Cu(II) reduction peaks at around -0.4 V were observed in HCl and H2SO4 solutions. It can be seen that better cathodic electrodeposition (larger peak current and more positive peak potential) and weaker anodic re-dissolution are obtained in hydrochloric acid. In order to illustrate the difference of electrodeposition behavior in two typical acidic solutions, i.e. HCl and H2SO4, the interfacial growth phenomenon of copper products were measured by the real time dynamic X-ray images. It should be noted that the solubility of tellurium is limited in H2SO4 solutions as mentioned above, and thus the single copper electrodeposition was determined. The stable potential of -0.65 V was used and the in-situ images at the cross section view at electrode/solution interface were observed during 600 s operation. As illustrated in Fig. 2a, loose nucleation and growth of copper deposit is obtained in HCl solutions, and no side reaction product of hydrogen bubble is observed. Besides, there is continuous lateral growth of deposit as indicated from the yellow dashline, and only trace needle deposits are observed in the arrowhead sections, suggesting good electrodeposition behavior in this dilute system. However, obvious contrast difference is achieved at the initial deposited area in the corresponding H2SO4 solutions, and uneven dentritic deposits are formed in the deposited plates, suggesting frustrated electrodeposition and potential low-quality products. Clearly, HCl system is better for the electrodeposition of dilute copper ions.



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Electrochemical reduction of Cu (II) and Te (IV) in chloride media As shown in Figure 3a, an electro-reduction peak emerged at -0.26 V in the presence of 0.5 g/L Te4+, indicating the peak is attributed to the Te(IV) reduction of Equation (1). Clearly, there is electrodeposition of tellurium ions to metallic Te at low-cost stainless steel plates. However, the Cu and Te ions reduction potentials are relatively close (-0.26 V and -0.38 V, respectively), which may lead to the difficult separation of these two metals. Interestingly, a clear reduction peak at -0.34 V is observed with 1 g/L Cu2+ and 0.5 g/L Te4+. This peak is owing to the combination of Cu and Te reactions, suggesting the possible simultaneous electrodeposition in the conventional cells. HTeO2- + 3H+ + 4e- → Te + 2H2O

(1)

As shown in Figure 3b, the UV-visible signals at 820 nm monotonically increase with the increase of Cl- concentration, indicating the generation of metal-chloride complexes in this multi-component solution. As HCl concentration increases, both Cu(II) and Te(IV) reduction peak potential shift positively and peak current increase (Figure 3c), indicating the depolarization effect of HCl. It has been demonstrated that there are two parallel mechanisms for electrochemical metal reduction in chloride solutions7, i.e. a classical reduction and a chloride-mediated pathways. Clearly, the complicated metallic speciation in HCl system may lead to different electrodeposition behaviors of Cu and Te, overcoming the bottlenecks of close reduction potentials mentioned above and resulting in selective extraction of these two metals.


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Mass transfer effect To further characterize the electro-deposition behavior, potential cycling was performed in the range between 50 mV/s and 180 mV/s. As shown in Figure 4a, a negative change of the peak potential with the increasing scan rate is observed, relating to the feature of a qusai-reversible reaction.20 In addition, a good linear characteristic between the peak currents and the square root of scan rate is obtained, indicating the diffusion-govern kinetics. Thus, its peak current can be calculated by the following equation:21 Ip = 367n3/2AD1/2Cv1/2

(2)

in which Ip (A) is the electrodeposition peak current, n is the electron transfer number, A (cm2) is the working area of stainless steel, D (cm2·s-1) is the reactant concentration, and v (V·s-1) is the employed scan rate. Consequently, the diffusion coefficients of Cu(II) and Te(IV) were determined to be 6 × 10-5 cm2·s-1 and 1.2 × 10-5 cm2·s-1, which requires further mass transfer enhancement in the following sections. As shown in Figure 5a, the peak current density increases with the increase of shirring rate, suggesting the enhancement effect of mass transfer. Besides, as the stirring rate (i.e. mass transfer) increases, the obtained potential positvely shifts from -0.78 V to -0.68 V in Figure 5b of chronopotentiometric test, suggesting the elimination of side-reaction of hydrogen evolution. This is particularly beneficial towards electrochemical deposition, which may leads to improved current efficiency and better deposited products.



Electrolytic metal deposition and separation As suggested from the electrodeposition potential difference of Cu and Te, stepwise electrolytic metal extraction was measured using the concentional three-electrode cell and a cylinder turbulent reactor, respectively. Initially, the electrodeposition was carried out at 100 A/m2 for 20 min to obtain the main product of Te, then the reaction was performed at 400 A/m2 with new cathode for another 12 min to get the main Cu product. After the complete washing and drying, the as-obtained products were determined by SEM, EDS and XRD. Firstly, the conventional three electrodes system was measured (Table 1 test 1#), it can be seen that selective electrodeposition was obtained with dominated Te and Cu products in the two steps. However, the efficiency and recovery ratio are limited (

Selective and Efficient Electrochemical Recovery ... - ACS Publications

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