Metal oxide varistors (MOVs) are a type of resistor with significantly nonlinear current-voltage characteristics commonly used in power lines to protect against overvoltages. If a proper recycling plan is developed MOVs can be an excellent source of secondary zinc because they contain over 90 weight percent zinc oxide. The oxides of antimony, bismuth, and to a lesser degree cobalt, manganese, and nickel are also present in varistors. Characterization of the MOV showed that cobalt, nickel, and manganese were not present in the varistor material at concentrations greater than one weight percent. This investigation determined whether a pH selective dissolution (leaching) process can be utilized as a starting point for hydrometallurgical recycling of the zinc in MOVs. This investigation showed it was possible to selectively leach zinc from the MOV without coleaching of bismuth and antimony by selecting a suitable pH, mainly higher than 3 for acids investigated. It was not possible to leach zinc without coleaching of manganese, cobalt, and nickel. It can be concluded from results obtained with the acids used, acetic, hydrochloric, nitric, and sulfuric, that sulfate leaching produced the most desirable results with respect to zinc leaching and it is also used extensively in industrial zinc production.
It is the vision for Europe to have market and policy incentives in place by 2020 that will stimulate new innovations in resource efficient production methods with all companies being able to measure their lifecycle resource efficiency [
In Sweden, there is an initiative to recycle MOV as opposed to landfilling due to environmental concerns, rising costs of landfilling, awareness of the potential value of the material in the MOV, and the quantity of material available for recycling. In Sweden from 2009 to 2013 over 500 tons of MOV was available for recycling [
MOVs are made by combining powdered metal oxides of zinc, antimony, bismuth, manganese, nickel, and cobalt. The metal oxide powder is sintered in a process during which three main microstructural phases form: ZnO grains, an antimony-rich phase, and a bismuth-rich intergranular phase [
It has been reported in literature that metal oxides such as MnO2, NiO, and Co2O3 and other minor metal oxides may be present in the MOV added to enhance the characteristics of the MOV [
This work investigates the feasibility of selectively leaching zinc from the MOV at a certain pH as an initial step for recovery of secondary zinc. Optimal zinc leaching would avoid coleaching of antimony, bismuth, and other minor metals present in the MOV making the leachate easy to integrate into industrial zinc electrowinning solutions. Industrially sulfuric acid is used in zinc production and it was therefore investigated in this work as well as other acids including acetic acid, which is a weak monoprotic organic acid, nitric acid, and hydrochloric acid.
In general there are two routes available for industrial zinc purification and production. First, a high temperature pyrometallurgical process where activated charcoal is added to the zinc oxide containing material and heated to temperatures above 1000°C at which point zinc is vaporized. The zinc vapor is condensed and collected either as ZnO or impure zinc which is further refined electrolytically [
The second route is hydrometallurgical purification of ZnO feed material which produces around 80% of the world’s zinc [
Identification and composition of the additives in the specific type of MOV investigated needed to be determined as only the major metal oxides: zinc, bismuth, and antimony were known. Additives or impurities (any metal other than zinc or ZnO) in the MOV sample may have an impact on zinc leaching and the eventual electrolytic process. New MOVs approximately 70 mm in diameter and weighing 1000 g were broken up into pieces approximately 2 cm in diameter. An impact mill was used for further particle size reduction. The crushed MOV was mechanically sieved. In leaching experiments material having a particle size smaller than 63
The appearance of the ground and sieved MOV was analyzed with a scanning electron microscope (SEM) with energy dispersive X-ray (EDX) spectroscopic element detection (Hitachi TM 3000 with EDX, Quantax 70) to obtain qualitative data about the elements present and to determine the occurrence and distribution of the components. X-ray powder diffractometry (XRD) (Bruker 2D Phaser) equipped with a characteristic Cu radiation source and a scintillation detector was used to identify crystalline compounds present in the MOV powder. Compound identification was made by comparisons with standards in the Joint Committee of Powder Diffraction Standards database [
To determine the metal content in the MOV, complete dissolution of the MOV powder was performed in triplicate using concentrated hydrochloric acid at an elevated temperature. The MOV material (approximately 2.5 g) was heated with 50 mL concentrated hydrochloric acid (37%) at
Leaching experiments were started by mixing 0.5 g of powdered MOV and 50 mL of milli-Q water in a straight wall, 150 mL capacity, titration vessel. The vessel was equipped with a pH electrode, a stir bar, and a dosing device connected to a Metrohm 905 Titrando titrator connected to a computer for monitoring and controlling the acid addition. Acid was titrated into the MOV-water mixture resulting in a leachate with a specified pH.
Small aliquots of the leachate were taken at times 0, 2, 10, 30, 60, 120, and 240 minutes in each leaching experiment. The pH was controlled using a silver/silver chloride (Ag/AgCl) glass electrode. Calibration of the pH electrode was done weekly using Metrohmn Ion analysis buffer solutions of pH 4, 7, and 9 while the measured pH value was not corrected to compensate for changes in the ionic strength as the ionic strength of this solution is lower than 1. The temperature of the system was maintained at 25°C ± 1.
In total four acid solutions were used for the leaching studies: acetic acid (≥99.7%, Sigma Aldrich), hydrochloric acid (37%, Sigma Aldrich), nitric acid (65%, Suprapur, Merck), and sulfuric acid (95.0–98.0%). Leaching experiments were carried out at constant pH of 1, 3, and 5 for each acid solution with the exception of acetic acid in which leaching experiments were carried out having final pH 2, 3, and 5. The acid leaching solutions were not initially prepared to the desired pH. Rather the desired pH was entered into the titration program and a more concentrated acid solution was added to the water-MOV system until the desired pH of the system was reached. The system was stirred so the stagnant layer around the solid particles could be perturbed ensuring mass transport from the liquid in the pores to the outer leachate where the pH and metal concentrations were measured.
In order to determine the concentration of the leached metals as a function of time an aliquot taken at each point of time was centrifuged and diluted with 1 M
Because each acid has the ability to form complexes with metal ions the speciation of zinc was also considered in each acid solution. The software used for speciation of metal ions in the leachates, PHREEQC [
The MOV used in this study was purchased from a commercial varistor production company. The assumption is made that the composition of the varistor does not change over its useful life, at least on the macroscopic scale. On a microscopic (monolayer) scale it has been shown by Stucki et al., 1987 [
Literature suggests that varistors may contain metal additives (in the oxide form) such as cobalt, chromium, copper, magnesium, manganese, nickel, sulfur, antimony, titanium, tungsten, and yttrium [
Chemical composition of MOV.
Metal oxide | mol % | wt % |
---|---|---|
Bi2O3 | 2.34 ± 0.06 | 5.1 ± 0.1 |
Co2O3 | 1.16 ± 0.03 | 0.94 ± 0.02 |
MnO2 | 0.76 ± 0.02 | 0.52 ± 0.01 |
NiO | 0.89 ± 0.02 | 0.79 ± 0.02 |
Sb2O3 | 3.21 ± 0.08 | 4.4 ± 0.1 |
ZnO | 91.6 ± 3.3 | 88.2 ± 3.1 |
A SEM micrograph of the pulverized (particle size less than 63
(a) SEM micrograph of pulverized MOV prior to leaching depicting three phases present: (I) ZnO grains, (II) Sb-rich phase, and (III) Bi-rich phase. (b) EDX map of varistor material seen in Figure
The microstructure of the MOV is polycrystalline making it somewhat complicated to analyze the composition, each phase having different dopants, dopant concentrations, shape, and size. Separation by recycling of the individual metals from the Sb-rich phases may be more complex than leaching of metal ions from the metal oxides. Eventually this may lead to reduced yield and slower kinetics during leaching compared to whether only pure metal oxides had been present. However, from Figure
The result from qualitative mineralogical analysis of the MOV using XRD was a spectrum as shown in Figure
XRD spectra of the MOV showing peaks for ZnO (●), Bi2O3 (◆), Zn2.33Sb0.67O4 (
By leaching the MOV in oxidizing acids (nitric and sulfuric acids), a nonoxidizing acid (hydrochloric acid), and a weak acid (acetic acid) it was expected that a clearer picture of the leaching behavior of zinc, bismuth, and antimony would be determined.
Acetic acid (HAc) was very effective for the leaching of zinc from MOV, as shown in Figure
The leached fraction as given by the left ordinate for zinc (
pH 2
pH 3
pH 5
Speciation of zinc, regardless of the pH in the range used here, was approximately 44%
As for the other metals present in the MOV, over 90% of the cobalt was leached in the pH 2 solution with the amount of cobalt leached decreasing with increasing pH. Nickel and manganese were both leached around 40% in pH 2 solutions and showed the same trend as cobalt, of decreased leaching with increasing pH.
Leaching with pH 1 hydrochloric acid (HCl) solution yielded
The leached fraction as given by the left ordinate for zinc (
pH 1
pH 3
pH 5
In HCl solutions with pH greater than 1,
The speciation of zinc in hydrochloric acid solutions as calculated by PHREEQC indicates that
Not only were hydrochloric acid solutions efficient for zinc leaching, they also worked relatively well for the leaching of manganese, nickel, and especially cobalt. In pH 1 hydrochloric acid solution the percent of cobalt leached was 86% whereas close to 70% and 62% of nickel and manganese, respectively, were leached. Thus, HCl leaching did not give a selective leaching of zinc.
Leaching of MOV in pH 1, 3, and 5 nitric acid (HNO3) solutions yielded results as shown in Figure
The leached fraction as given by the left ordinate for zinc (
pH 1
pH 3
pH 5
Less than 35% of the manganese content was leached from the MOV in the pH 1 nitric acid solution, while 50 and 76% of the nickel and cobalt, respectively, were leached at the same pH. Lower amounts of all these metals, manganese, nickel, and cobalt, were leached at lower concentrations of nitric acid, that is, pH 3 and 5.
Leaching of MOV in sulfuric acid solutions with pH 1, 3, and 5 gave results as shown in Figures
The leached fraction as given by the left ordinate for zinc (
pH 1
pH 3
pH 5
PHREEQC calculations showed that approximately 65% of the zinc in the pH 1 leachate occurred as
Impurities in the zinc leachate include cobalt of which approximately 65% was leached in all solutions investigated. Manganese and nickel were approximately 25% leached in pH 1 solution, 17% at pH 3 solution, and 27% at pH 5. It is not known what causes a lower leaching fraction in pH 3 solution but it could be due to a change in speciation or precipitation of the metals to secondary compounds.
As shown it was possible to selectively leach Zn from the MOV without significant coleaching of bismuth and antimony by selecting a suitable pH, mainly higher than 3 in all acids investigated here. It was not possible to leach zinc without coleaching of manganese, cobalt, and nickel. However, such minor contaminations can be removed before electrowinning of zinc by cementation.
It was concluded that sulfate leaching produced the most desirable results with respect to zinc leaching and coleaching of other metals ions as well as its extensive use in industrial zinc production. It was also important to determine if zinc leaching was due to bulk leaching of the ZnO grain or if the zinc within the pyrochlore and spinel phases was also leached thus destroying the spinel phase and liberating antimony. The insoluble residue remaining after leaching of the MOV in a pH 1 sulfuric acid solution for 240 minutes is shown in Figure
SEM micrograph of pulverized MOV after leaching in pH 1 sulfuric acid solution for 240 minutes. The Sb-rich phase remains along with some undissolved Bi-rich phase.
XRD analysis results for the pH 1 sulfuric acid leaching residue (Figure
XRD spectrum (—) of leaching residue (pH 1, sulfuric acid) compared to XRD spectrum of nonleached starting material (- - -). Chemical compounds are represented as follows: Bi2O3 (◆), Zn2.33Sb0.67O4 (□), and Zn7Sb2O12 (■).
Also present in the MOV are Zn2Bi3Sb3O14 (★) and Bi2O3 (◆) both having identical peaks. It is most logical based on characterization and literature data that pyrochlore (Zn2Bi3Sb3O14) and spinel both cubic (Zn2.33Sb0.67O4) and orthorhombic (Zn7Sb2O12) as well as the as well as the residual Bi2O3 are present residual Bi2O3 are present in the sample. It is also probable to have the presence of cobalt, nickel, and manganese in the sample; however the chemical form of those metals is not known. The presence of minor metal oxides is typical of sintered material. The spectrum for the starting material contained prominent peaks for ZnO whereas the appearance of ZnO peaks in the leaching residue was nonexistent. The XRD result also shows that it will be difficult to solubilize the zinc that is present in the combined zinc-antimony oxides.
To summarize, in total four acids were investigated each at three different pH levels. Typically pH 1, pH 3, and pH 5 were used except in the case of acetic acid where pH 1 was difficult to obtain and pH 2 was used instead. Acetic acid leaching results show that selective leaching of zinc from the MOV with respect to bismuth and antimony can be achieved using leaching solution with a pH 5. However in pH 5 acetic acid solutions some bismuth (1.3% ± 0.1) was leached. In hydrochloric acid solutions zinc can be successfully selectively leached from bismuth and antimony in pH 5 solutions. Similar results for selective leaching of zinc occur in nitric acid solutions with no bismuth or antimony detected in pH 5 solutions. With acetic, hydrochloric, and nitric acid the percent of zinc leached decreased with increasing pH.
For acetic acid nearly 90% of the zinc was leached at pH 2, 3, and 5 while all zinc could be leached using hydrochloric, nitric, and sulfuric acid at pH 1. Increase in pH 5 in hydrochloric and nitric acid solutions resulted in lower zinc leaching with approximately 82% and 78% zinc leached, respectively. Minor metal coleaching at pH 5 is summarized in Table
Percentage of minor metals coleached with zinc in pH 5 leaching solutions.
Acid | Co | Mn | Ni |
---|---|---|---|
|
74.5 ± 0.4 | 23.4 ± 0.1 | 19.6 ± 0.3 |
HCl | 72.6 ± 5.5 | 23.1 ± 2.9 | 18.5 ± 0.9 |
HNO3 | 63.3 ± 0.7 | 19.7 ± 0.3 | 17.0 ± 0.2 |
H2SO4 | 66.4 ± 3.5 | 27.1 ± 5.0 | 24.8 ± 4.6 |
Finally, leaching in sulfuric acid solutions was highly effective for zinc leaching at each of the three pH investigated. Leaching at pH 3 resulted in a leachate pure of antimony and bismuth; however the minor metals given in Table
Regardless of the leachate used acetic, hydrochloric, nitric, or sulfuric acid further purification of the leachate is required if it is to be used in the zinc electrowinning process. A purification method such as cementation would be effective for removing antimony, bismuth, nickel, and cobalt from the leaching solution. Antimony has the added benefit of being an activator in cementation by increasing the kinetics of the cementation reaction [
Speciation modeling using PHREEQC of the zinc in each acid demonstrated that the most prominent form of Zn is Zn2+. This is important because the state of the zinc and knowledge of its complexes can affect further zinc recycling steps such as cementation or electrowinning.
This work set out to determine whether it is possible to separate metal components of the MOV via pH selective leaching in acetic, nitric, hydrochloric, and sulfuric acidic solutions having pH 1, 3, and 5. Initially the composition of the MOV was determined in order to quantify the metals within the MOV. The MOV contains simple oxides such as zinc oxide but it also contains more complex oxides such as Zn7Sb2O12 as shown using the SEM and XRD.
Experimental data showed that lower pH acid solutions gave higher percent of zinc leaching except for the case where H2SO4 was used and zinc was shown to be fully leached at pH 5 and below. It was possible to selectively leach Zn from the MOV without significant coleaching of bismuth and antimony by selecting a suitable pH, mainly higher than 3 in all acids investigated here. It was not possible to leach zinc without coleaching of manganese, cobalt, and nickel. Even though these metals are present in small amounts in the leachate production of pure zinc metal will require their removal. Sulfuric acid leaching is also preferred because nearly 80% of zinc is produced by electrowinning in sulfate solutions.
This investigation concludes that either acetic, nitric, hydrochloric, or sulfuric acid solutions at pH 5 can be used to selectively leach zinc from the MOV without significant coleaching of antimony or bismuth. However, the efficiency of zinc leached decreases with increasing leaching pH except in the case of sulfate solution. Regardless of the pH in sulfate leaching 100% of the zinc in the MOV was leached making this the ideal selective leaching solution for leaching zinc from MOV. Selective zinc leaching with respect to minor metals such as cobalt, nickel, and manganese could not be successfully done with the acids and pH range under investigation in this study.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors of this work would like to thank fellow colleagues Marcus Hedberg and Mikael Karlsson for their contributions to this work. Funding was provided by Chalmers Area of Advance Production which is gratefully acknowledged.