Recovery of Antimony: A Laboratory Study on the Thermal Decomposition and Carbothermal Reduction of Sb(III), Bi(III), Zn(II) Oxides, and Antimony Compounds from Metal Oxide Varistors

As antimony is typically present in industrial and commercial products only in small amounts, the concentration of antimony in waste types is low and a limited amount of antimony is currently recycled. One product relatively rich in antimony is the metal oxide varistor (MOV) used for overvoltage protection in electric circuits. To increase the antimony concentration, the MOV was pulverized (< 65 μm) and leached, resulting in an insoluble MOV residue containing 186 ± 2 mg/g of antimony. This work investigates the thermal decomposition and carbothermal reduction of pure metal oxides (Sb2O3, Bi2O3, and ZnO) and MOV residue. Thermogravimetric (TG) analysis was used in order to propose a temperature range in which it is possible to separate antimony oxide from the MOV residue. TG results indicate that during thermal decomposition of pure metal oxides, sublimated antimony oxide can be recovered at 650 °C, leaving Bi2O3 and ZnO unreacted. The addition of carbon caused mainly volatilization, with some reduction, of Sb2O3 and reduction of Bi2O3 to occur at nearly the same temperature, approximately 600 °C. However, volatilization of Bi was not troublesome below 800 °C due to slow kinetics. Thermal decomposition of antimony from the MOV residue was not possible in the temperature range studied (< 1000 °C), while carbothermal reduction to the MOV residue revealed antimony volatilization occurred near 800 °C.


Introduction
Within the European Union (EU), the major use of antimony is in the form of antimony oxide (Sb 2 O 3 ) as a flame retardant in plastics and textiles [1]. Worldwide, metallic antimony is used in lead-acid batteries and in ammunition. Other uses are as a vulcanizing agent in rubber production, as a polymerization catalyst in the production of plastics and as a pigment in ceramic and glass [2].
The EU is dependent entirely on imports of antimony [1], and in 2015 the United States (US) imported 23,600 metric tons of antimony for industrial consumption [3]. The antimony used in the US was imported mainly from the People's Republic of China (63%), Bolivia (8%), Belgium (7%), and Thailand (6%). The People's Republic of China is currently the leading producer of antimony with its government classifying antimony as a strategic material and controlling annual production quotas [3]. Currently, there are known to be 1.5 million tons of antimony reserves worldwide. At current mining rates, known antimony reserves will be depleted by 2027 [2].
From a recycling perspective, hardly any antimony is recycled. This is mostly because its applications are highly dispersive and collection is difficult [4]. Conventional recycling routes are typically not applicable to metals such as antimony [5] if no pretreatment step(s) to create concentrates are taken. Antimony is used extensively as a flame retardant, thus it is found in very low concentrations in plastics and textiles, making it impossible to design a The contributing editor for this article was Bernd Friedrich.
& Toni Karlsson tonig@chalmers.se recycling route from this waste stream [5] unless pretreatment steps are taken. There are several waste streams from which antimony can be recovered; one success story is recycling antimony from lead-acid batteries. In the lead-acid battery, antimony is used in lead-antimony alloy plates in order to make this alloy harder. The concentration of antimony in lead-acid batteries has been declining due to advances in battery technology; however, the use of lead-acid batteries continues to increase. Nearly 85% of lead-acid batteries are recycled in the United States due to implementation of a policy restricting battery disposal in landfills and the availability of collection sites. Lead-acid batteries are the main source of secondary antimony. Antimony is recovered from the secondary lead smelter as an antimonial lead mixture and put back into production of new batteries [2]. In 2015, the US recycled 3850 metric tons of antimony [3].
Antimony recovery from municipal solid waste (MSW) incineration ash is another possible source of secondary antimony. MSW contains items having antimony thinly dispersed within them. Incineration can be thought of as a type of pretreatment step. Household wastes such as textiles contain 25.9 mg/kg antimony while plastics and rubber contain 12.9 and 8.4 mg/kg, respectively [6]. Items such as curtains contain larger concentrations of antimony and things like wood contain much less antimony. It is estimated that the average antimony concentration in MSW is around 10-60 mg/kg [7,8]. After incineration, the concentration of antimony in fly ash varies depending on the starting material and incineration conditions, but it has been shown to be less than \ 2000 mg Sb/kg dry ash [9][10][11].
In ammunition, lead is alloyed with antimony (2-5 wt%) [12]; therefore, the accumulation of ammunition fired at shooting ranges could be a potential source of secondary antimony. In a study by Johnson et al. [12], the antimony concentration in shooting range soil samples was shown to be 35-17,500 mg/kg.
In some cases, antimony is added to ceramics as a pigment and it can also be used to enhance specific characteristics. Antimony oxide is used in a type of ceramic known as the metal oxide varistor (MOV), which is a zinc oxide-based ceramic semiconducting device with a high degree of non-linear current-voltage characteristics. The main application of zinc oxide varistors is in circuits for overvoltage protection. An example of its use is in surge arrestors or lightening protectors on power lines [13]. Zinc oxide varistors are commonly referred to as MOVs due to the presence of zinc, bismuth, antimony, cobalt, manganese, and nickel oxides in addition to other metal oxides used in low amounts. It is not known how much antimony is available from varistors worldwide. However, used varistors could be a good source of recovered antimony since their Sb concentration is relatively high. In Sweden, it has been estimated that from 2009 to 2013 just over 520 metric tons of varistors were produced [14]. If secondary antimony oxide could be recovered, 23 metric tons of antimony would be available for reuse.
It has previously been shown by Gutknecht et al. [15] that a pretreatment step such as acid leaching of the pulverized MOV material solubilizes the zinc oxide phase while leaving the antimony and bismuth-rich phases concentrated in the insoluble residue. Before leaching, the amount of antimony in the MOV was 35 mg per gram MOV; after leaching there were 186 mg of antimony per gram residue, i.e., leaching as a pretreatment step increased the antimony concentration over 5 times.
Antimony is typically classified as a volatile heavy metal, and low-grade ores are commonly produced by pyrometallurgical methods [20]. Industrially, reduction of antimony oxide to metallic antimony is achieved by heating and the addition of carbon in a reverberatory furnace. Volatilization of antimony can be reduced by the addition of an alkaline flux but it cannot be eliminated [20] and volatilized antimony oxide is recovered in flues, condensing pipes, bag houses, precipitators, or a combination [21]. The volatile nature of antimony oxide can be utilized in designing a high-temperature method for separating gaseous antimony oxide from the matrix of the MOV leaching residue. Since the MOV residue is composed mainly of compounds containing zinc-antimony-oxygen, it was thought that the addition of carbon would help lower the Sb 2 O 3 volatilization temperature by decomposition of the spinel/pyrochlore compounds.

Materials and Methods
This work used thermogravimetric analysis (TGA) to investigate the amount and rate of weight change of simple metal oxides and MOV residue as a function of temperature in inert and reducing conditions. Simple metal oxides were used as a reference to help predict the behavior of sintered metal oxide compounds (Zn 7 Sb 2 O 12 , Zn 2.33 Sb 0.67 O 4 , and Zn 2 Bi 3 Sb 3 O 14 ) present in the MOV residue. Carbothermal reduction experiments were done to observe if the volatilization temperature can be lowered compared to thermal decomposition from heat treatment. From TG results, a suitable high-temperature separation process can be proposed for the recovery of volatilized antimony oxide from the MOV residue.

MOV Pretreatment
A pulverized (crushed to a particle size \ 65 lm) MOV was leached at pH 4 using H 2 SO 4 . Sulfuric acid was used because the leachate, once separated from the insoluble residue, could be sold to a zinc electrowinning facility for the production of secondary metallic zinc. It was demonstrated that the leachate contained low concentrations of cobalt and nickel, which can be removed by cementation [16], as well as some manganese. In previous work, the concentration of zinc in the leachate was shown to be 1 mol/l but can be increased to suit the needs of the electrowinning facility [16]. Other mineral acids as well as organic acids could also have been used for zinc leaching [15], but from those leachates, zinc or zinc oxide would have been recovered by precipitation or solvent extraction. The process for pretreatment of the MOV is described in detail in previous work by Gutknecht et al. [16].
In order to determine the composition of the insoluble MOV residue, a sample was dissolved in concentrated hydrochloric acid (37%) for 24 h while being continuously stirred using a magnetic stir bar. The solution was bluegreen but transparent after dissolution. HCl was used due to the reported solubility of antimony compounds in concentrated hydrochloric media [22]. Elemental analysis using inductive coupled plasma with optical emission spectrometric detection (ICP-OES) (iCAP 6500, Thermo Fischer) yielded the results shown Table 1. The concentrations shown in Table 1 are given as milligrams of metal per gram of MOV residue. Metals account for approximately 85% of the MOV residue while it is assumed oxygen accounts for the remaining 15%. It was reasonable to assume oxygen makes up the remaining 15% due to X-ray analysis and compound identification.
Starting materials were analyzed for purity and identification of crystalline compounds present in the samples using an X-ray diffractometer (Bruker 2D Phaser) equipped with a characteristic Cu radiation source and a scintillation detector. Compound identification was made by comparisons with standards in the Joint Committee of Powder Diffraction Standards database [23]. The appearance and composition of the dried residue was analyzed with a SEM: A FEI Quanta 200 environmental SEM equipped with Oxford Inca energy dispersive X-ray detector (EDX) system. Imaging was done with accelerating voltages between 10 and 20 kV to obtain qualitative data about the elements present and to determine the occurrence and distribution of the components.

Materials Used
Pure samples of Sb 2 O 3 (Sigma Aldrich, 99.9%), Bi 2 O 3 (Sigma Aldrich, 99.9%), and ZnO (Sigma Aldrich, C 99.9%) were used to determine the temperature range needed to thermally decompose or reduce the pure oxides. Because X-ray powder diffraction patterns showed that the metal oxides supplied were pure, the oxides were used without any further purification.
The pure metal oxides were a less complex system than the MOV residue and they provided a good fundamental understanding of the behavior of the metal oxides prior to analysis of the more complex MOV residue system. Pure metal oxides were also used as quasi-method checks, since their thermal properties are well documented in literature. The MOV residue was prepared as previously described having the metal concentration as shown in Table 1. The microstructure of the MOV residue is shown in Fig. 1. There are two main phases in the MOV residue: the Bi 2 O 3 phase and the antimony-rich phase containing compounds such as spinel (a-Zn 7 Sb 2 O 12 , b-Zn 2.33 Sb 0.67 O 4 ) and pyrochlore (Zn 2 Bi 3 Sb 3 O 14 ).
In the MOV residue, it was seen that Bi 2 O 3 encompassed the Sb-rich material as shown in Fig. 1. Cobalt, manganese, and nickel are also present in the MOV in much lower concentrations, most likely substituted into the spinel lattice. XRD analysis of the MOV residue supports this conclusion where peaks were seen for Zn-Co-Sb-O and Zn-Ni-Sb-O compounds. XRD also showed the presence of Bi 2 O 3 , Zn 7 Sb 2 O 12 , and Zn 2 Bi 3 Sb 3 O 14 .
Thermogravimetric Experiments HSC Chemistry 9 software was used for thermodynamic calculations regarding the equilibrium composition and Ellingham diagram reactions over a temperature range  Table 2. Thermolysis curves are displayed as % mass loss (m) versus temperature or when needed the derivative (Dm/Dt) showing the rate of weight loss as a function of temperature. The TG was calibrated with five high-purity standards using their Curie temperature and peak inflection point as the calibration temperature. Two general types of experiments were performed: (1) thermal decomposition where the metal oxides as well as MOV residue were heated in a nitrogen atmosphere and (2) carbothermal reduction where charcoal was added to the metal oxides and MOV residue. It has been proven to work in zinc production [24] but the effect on the oxides of antimony and bismuth as well as the MOV residue is not well known.
For carbothermal reduction experiments, activated charcoal (DARCO Ò , -100 mesh) was mixed with the metal oxides and the MOV residue at multiple C/O molar ratios. Homogenization was done using a mortar and pestle. The C/O ratio for the MOV residue was determined by assuming the MOV residue to be composed of 85% metals (determined by ICP-OES) with the remaining 15% being oxygen (Table 3).

Thermodynamic Predictions
Thermodynamic models based on the calculation of the variation of Gibbs free energy (DG°) can point out which reactions are more likely to take place under the conditions in a thermal decomposition or carbothermal reduction. Thermodynamic calculations do not model exactly the real processes, since simplified assumptions have to be made (ideal thermodynamic conditions, homogeneous distribution of the elements, chemical equilibrium, etc.), which can be far from reality in some systems Nevertheless, thermodynamic models can be powerful tools for understanding chemical behavior during thermal and reducing conditions. Thermodynamic analysis was done over a temperature range between 100 and 1000°C. The possible reactions occurring during thermal treatment of Sb 2 O 3 , Bi 2 O 3 , and ZnO are shown as a function of equilibrium composition in Fig. 2a. Sb 2 O 3 (s) starts to equilibrate with Sb 2 O 3 (l) near 630°C, the predicted melting temperature of Sb 2 O 3 . It is also seen that Sb 4 O 6 (g) forms around the same temperature. Bi 2 O 3 is stable through the entire temperature range.    [25]. ZnO is stable in the temperature range shown in Fig. 2a.
An Ellingham diagram as shown in Fig. 2b was used to determine the variation of DG°(compound stability) with temperature for the oxidation of a series of metals. The lower the position of a metal's line in the Ellingham diagram, the greater the stability of its oxide. For instance, ZnO is more stable than Bi 2 O 3 . In general, the stability of metal oxides decreases with temperature. A reduced metal, whose Gibbs free energy of formation is lower on the diagram at a given temperature, will reduce an oxide whose free energy of formation is higher on the diagram. For example, metallic zinc can reduce Sb 2 O 3 to metallic antimony, the zinc itself being oxidized to ZnO. Because the distance is greater between Bi 2 O 3 and ZnO as compared to Sb 2 O 3 and ZnO, Zn will be more effective as a reducing agent for Bi 2 O 3 .
The DG°of carbon dioxide (CO 2 ) is almost independent of temperature, while that of carbon monoxide (CO) has negative slope and intersects the CO 2 line near 700°C. According to the Boudouard reaction (R1), carbon monoxide is the dominant oxide of carbon at higher temperatures (above 700°C), and the higher the temperature the more effective CO is as a reducing agent. The effect of mass loss as a function of heating rate on Sb 2 O 3 is shown in Fig. 3a and the rate of mass loss is given in the derivative thermogravimetric (DTG) curve shown in Fig. 3b. It has been reported that the first thermal effect to occur upon heating senarmontite is the onset of sublimation occurring at approximately 500°C [30,31]. This is shown in Fig. 3a. From the DTG curve, the peak volatilization temperature was determined to be 644°C at a heating rate of 20°C/min and decreased with slower heating rates. The temperature range in which Sb 2 O 3 was sublimated was 500-650°C. Thermal decomposition curves indicate that Sb 2 O 3 sublimated in one step from Sb 2 O 3 (s) to Sb 4 O 6 (g); this is seen in Fig. 3a where mass loss curves are smooth and continuous. Similar results were reported by Orman et al. [32] for cubic Sb 2 O 3 decomposition. Generally, the oxides of more noble metals can be converted directly by thermal decomposition to metal from their oxides in the presence of a reducing agent, such as carbon. Antimony oxides can be reduced with charcoal in a reverberatory furnace at temperatures near 1200°C. The reaction proceeds according to reaction R2-R3 [20].
The thermograms for carbothermal reduction of antimony oxide are shown in Fig. 4 Fig. 4 it can be see that the rate of mass loss changes in the presence of carbon around 600°C. The slower reaction kinetics could be due to the formation and then diffusion of Sb 4 O 6 gas molecules through the sample. It could also be caused by carbothermal reduction of liquid or gaseous Sb 2 O 3 which is predicted to occur near 430°C from the Ellingham diagram shown in Fig. 2b. Antimony is less volatile than Sb 2 O 3 and would therefore lower the rate of mass loss when compared to the pure Sb 2 O 3 sample [22,33]. However, no further analysis was performed on the remaining residue and therefore the presence of antimony metal could not be determined.

Bi 2 O 3
Bi 2 O 3 has a melting point of 820°C [29] and is volatile at higher temperatures. As shown in Fig. 5, the black curve shows no mass loss of Bi 2 O 3 below 980°C and therefore no thermal decomposition occurred. The addition of carbon to bismuth oxide results in carbothermal reduction of Bi 2 O 3 into a metallic Bi product [34]. Carbothermal reduction reactions depicted in Fig. 5   Inspection of the sample after TG experiments indicated that carbon was added in excess and parts of it remained unreacted in the crucible. Mass balance calculations indicate that 50% of the carbon in the C/O = 2 sample and 25% in the C/O = 4 sample was used. Metallic pieces of bismuth metal were also seen in the crucible.
Complete carbothermal reduction and Bi volatilization would yield a mass loss of 93% and 82% for C/O = 2 and 4, respectively. In these experiments, the samples were heated to 980°C and held isothermally for 30 min. The TG curves indicate an 80% mass loss when the C/O = 2 and 65% mass loss for C/O = 4. The difference between theoretical and experimental results is explained by the kinetics of bismuth volatilization. It is possible that not all molten bismuth was volatilized before the end of the heating segment.

ZnO
It is well known that in high-temperature zinc production carbon is added to zinc oxide causing carbothermal reduction, yielding zinc. Carbothermal reduction of zinc oxide occurs according to reactions R4-R5 [34]. For ZnO to be continuously reduced, the equilibria in R4 and R5 must be satisfied [35]. Reduction with carbon is governed by the chemical equilibria and reduction kinetics.
ZnO þ CO ! ZnðgÞ þ CO 2 ðR4Þ For zinc oxide, it is well documented that the melting point is 1975°C and it does not thermally decompose below this temperature [22]. Carbothermal reduction of ZnO producing Zn(g) and CO 2 (g) has been shown to occur at 907°C [36,37]. TG curves of mass loss as a function of C/O ratio are shown in Fig. 6. In the absence of carbon, zinc oxide showed no weight loss up to 950°C. The addition of carbon with a C/O molar ratio of 2 and 4 resulted in carbothermal reduction of zinc starting near 800°C. The boiling point of Zn is 907°C under a pressure of 1 bar and evaporation starts when the partial pressure of vapor of the zinc is lower than the equilibrium vapor pressure [22,38]. As the temperature was increased, the rate of zinc reduction increased which has been seen in other reported in literature [37]. Volatilization of Sb 2 O 3 occurred from 400 to 650°C with the peak volatilization temperature of 644°C at a heating rate of 20°C/min. The addition of carbon had no effect on the Sb 2 O 3 volatilization temperature because of the low temperature at which sublimation occurred. However, the volatilization rate was affected due to carbothermal reduction of a portion of the Sb 2 O 3 and subsequent volatilization of antimony. For Bi 2 O 3 , the addition of carbon resulted in a 10-15% mass loss near 600°C. The reaction thought to occur at this temperature is transition of Bi 2 O 3 to molten Bi. The melting temperature of Bi metal is 217°C [29]. Another mass loss with onset near 875°C is due to volatilization of Bi. It is known that bismuth metal is volatile at high temperatures. However, when in the presence of other metals, bismuth will not volatilize at high temperatures; rather it will alloy with the metals. Carbothermal reduction of ZnO to Zn(g) occurred just above 901°C in these experiments. This is a very well-known reaction and the results obtained here correlate well with literature. Thermal treatment of the simple metal oxides suggests that sublimated antimony oxide could be separated from a mixture of Sb 2 O 3 , Bi 2 O 3 , and ZnO by heating to 650°C and collected by distillation. The addition of carbon causes the volatilization of Sb 2 O 3 and the reduction of Bi 2 O 3 to occur at nearly the same temperature. If separation of bismuth and zinc was of interest, then after antimony volatilization, carbon could be added to lower the reduction temperature for their recovery.

MOV and MOV Residue
Prior to pretreatment, the MOV consisted of three main phases: firstly, the bulk zinc oxide phase, which accounted for nearly 90% of the MOV; secondly, the bismuth oxide phase, found in the region between the zinc oxide grains; and lastly the phase containing antimony compounds such as spinel (Zn 7 Sb 2 O 12 ) and pyrochlore (Zn 2 Bi 3 Sb 3 O 14 ). The antimony compounds are found within the bismuth oxide phase. Cobalt, manganese, and nickel are also present in the MOV most likely substituted into the spinel lattice. Work by Wong [39] determined that the octahedral crystal of the Zn 7 Sb 2 O 12 spinel was doped with Mn and Co when Mn and Co were present during sintering. XRD analysis of the residue supports this conclusion where peaks were seen for Zn-Co-Sb-O and Zn-Ni-Sb-O compounds.
It was shown in previous research that leaching-pulverized MOVs with a pH 3 sulfuric acid solution resulted in selective leaching of the zinc oxide phase, leaving the bismuth-and antimony-containing phases in the MOV residue [15]. Since antimony is not found as Sb 2 O 3 but within the spinel and pyrochlore compounds, the fate of antimony during thermal treatment of the MOV residue is not clear. The spinel compound has been reported to be stable up to 1350°C [40]. Filipek Thermal decomposition of the MOV residue in the absence of carbon resulted in less than 5% mass loss. The addition of carbon to the MOV residue had a large effect on the mass loss as seen in Fig. 7. For all samples containing carbon, there are four inflections indicating several stages of decomposition occurring in the MOV residue-carbon samples. Mass loss increased with temperature due to thermal decomposition, representing the mass of volatile matter removed from the sample. The decomposition regions are labeled in Fig. 7 The first reaction is the loss of oxygen forming liquid Bi (R7) occurring at 600°C. In the presence of carbon, R8 has The experimental mass loss between 700 and 980°C was 71.5%, indicating that the decomposition of spinel occurs in this range. In addition it is possible that a portion of the Bi was also volatilized in this range explaining why the experimental mass loss is higher than theoretical mass loss.
Since it is known that ZnO decomposed according to R9 near 900°C, the reaction near 800°C is assumed to be volatilization of Sb 4 O 6 . The theoretical weight loss is 22.8%, while the experimental weight loss for Sb 4 O 6 and CO was 17.3%. The difference in values is expected due to multiple reactions occurring near 900°C. This is depicted in Fig. 7b, where the derivative curve does not return to baseline before the ZnO reduction reaction (R9) begins.
The calculated and experimentally determined weight losses for reactions R7-R9 are given in Table 4. Theoretical weight loss calculations were made using the following assumptions and the composition of the MOV residue given in Table 1: 1. All Bi in the sample is in the form of Bi 2 O 3 . 2. All Sb in the sample is in the form Zn 7 Sb 2 O 12 . This also accounts for 97% of the Zn in the sample. 3. Liquid Bi, Co, Ni, Mn remain as a solid in the residue. 4. The C used is calculated from reactions R7-R9.
There are small deviations when comparing theoretical and experimental data. From the derivative curves shown in Fig. 7b, it is seen that decomposition R8 is not complete before decomposition R9 begins. Multiple reactions occur simultaneously; therefore the experimental mass loss from an explicit decomposition reaction should only be viewed as an estimate. Simultaneous reactions will cause deviation between theoretical and experimental data. Also, the volatilization of Bi could not be accounted for in the theoretical calculations and therefore the mass loss will always be slightly lower than the experimentally obtained data.
Metals such as cobalt, nickel, and manganese make up less than 5% of the MOV residue and are not accounted for in the mass balance calculations. Since they are substituted into the spinel lattice they are believed to remain after decomposition and volatilization of zinc and antimony. As shown in Table 4, Subtotal refers to total mass loss from reaction R7-R9 while Total accounts for the other metals (Co, Ni, Mn), liquid bismuth, and unreacted carbon. The Bi, unreacted carbon, and other metal remains are assumed to remain in the solid phase and do not contribute to mass loss.
It has been reported once that molten bismuth will volatilize at high temperatures. This is seen in Fig. 5 when reducing pure Bi 2 O 3 . The weight loss curve above 700°C is smooth with no sharp inflection points, indicating the slow kinetics of bismuth volatilization. In the presence of other metals, bismuth tends to be less volatile. It can be concluded that some of the bismuth remained in the sample and some was volatilized. This would also explain the deviation between theoretical and experimental mass loss.

Conclusions
The MOV under investigation in this study was composed of sintered metal oxides of zinc, antimony (3.2 mol%), bismuth (2.3 mol%), cobalt (\ 1 mol%), nickel (\ 1 mol%), and manganese (\ 1 mol%) [15,16]. The microstructure of the MOV contains three phases: the most dominant phase is that of the zinc oxide grains; the bismuth-rich phase encompasses the zinc oxide grains; and the antimony-rich phase is located within the bismuth oxide phase.
When recycling low concentration metals from waste, pretreatment is often needed. Pulverized (\ 65 lm) MOV was leached at pH 4 using H 2 SO 4 yielding a pure zinc sulfate solution and an antimony-rich residue. XRD analysis showed antimony compounds such as Zn 7   In this work, thermogravimetry was used to investigate the weight loss and rate of weight change of simple metal oxides (Sb 2 O 3 , Bi 2 O 3 , ZnO) as well as those of the MOV leaching residue as a function of temperature in inert and reducing conditions. Simple metal oxides were used as a reference to help predict the behavior of the sintered metal oxide compounds (Zn 7 Sb 2 O 12 , Zn 2.33 Sb 0.67 O 4 , and Zn 2 Bi 3 Sb 3 O 14 ) present in the MOV residue.
The thermal treatment of the simple metal oxides suggested that sublimated antimony oxide could be separated from a mixture of Sb 2 O 3 , Bi 2 O 3 , and ZnO by heating to 650°C and collected by distillation. The addition of carbon causes volatilization and reduction of Sb 2 O 3 followed by subsequent Sb volatilization at a slower rate than for Sb 2 O 3 . Carbothermal reduction of Bi 2 O 3 and subsequent volatilization of Bi(l) occurred at nearly the same temperature. For the separation of bismuth and zinc, once antimony was volatilized, carbon could be added to lower the reduction temperature for their recovery.
It was not known whether the spinel Zn 7 Sb 2 O 12 is found in the cubic, orthorhombic, or a mixture of both forms in the MOV residue. Also, some antimony may be found in the pyrochlore Zn 2 Sb 3 Bi 3 O 14 phase. For this work, all mass balance calculations were made assuming antimony was in the Zn 7 Sb 2 O 12 compound. This could give rise to small deviations in mass balance calculations. The major part of theoretical deviations are most likely due to Bi vaporization.
Others have suggested thermal decomposition of spinel occurs near 1350°C yielding ZnO and Sb 4 O 6 . Therefore, spinel decomposition was not expected to occur below 1350°C without the addition of carbon. It was seen that decomposition of the MOV residue in the absence of carbon resulted in less than 5% mass loss. The addition of carbon to the MOV residue had a large effect on the mass loss. The first reaction, occurring around 600°C, was the reduction of Bi 2 O 3 from the MOV, while near 800°C volatilization of Sb 4 O 6 occurred. The theoretical weight loss was 22.8%, while the experimental weight loss for Sb 4 O 6 and CO 2 was 17.3%. At temperatures above 900°C, zinc was volatilized and reduced.
Thermal treatment and carbothermal reduction of Bi 2 O 3 showed that once reduced, Bi metal was slowly volatilized. This is also believed to occur during carbothermal reduction of the MOV residue. Therefore, antimony can be collected from the vapor phase at 800°C with small amount of bismuth present. Further separation of antimony and bismuth was not investigated here.