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Preparation of zinc nano structured particles from spent zinc manganese batteries by vacuum separation and inert gas condensation
Separation and Purification Technology 142 (2015) 227–233
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Preparation of zinc nano structured particles from spent zinc manganese batteries by vacuum separation and inert gas condensation Xishu Xiang a, Fafa Xia a, Lu Zhan a,b,⇑, Bing Xie a a Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of Ecological and Environmental Science, East China Normal University, 500 Dong chuan Road, Shanghai, PR China b Shanghai Cooperative Centre for WEEE Recycling, Shanghai Second Polytechnic University, Shanghai 201209, PR China
a r t i c l e
i n f o
Article history: Received 15 October 2014 Received in revised form 22 December 2014 Accepted 10 January 2015 Available online 17 January 2015 Keywords: Spent zinc manganese batteries Vacuum separation Inert gas condensation Zinc nano-particles
a b s t r a c t Vacuum separation and inert gas condensation were adopted to recycle zinc and prepare zinc nanoparticles from spent zinc manganese batteries in this paper. As a result of the relatively high vapor pressure, zinc can be easily separated from the other metals in zinc manganese batteries by vacuum evaporation with a high separation efficiency. Simultaneously, morphology controlled zinc nano-particles were prepared by inert gas condensation. The results showed that inert gas pressure, heating and condensing temperature, condensation distance away from the evaporation source, and condensation substrate were the significant factors on nano-particles preparation. Higher inert gas pressure was beneficial for preparing inerratic nano-particles because of the diffusing effect. At higher heating temperature and lower condensing temperature, more inerratic nano-particles could be prepared. With more collisions and cementations, the particles became irregular at the places far away from the heater. It was also observed that nano structured particles with different morphologies including hexagonal prisms, fibriform and sheet shapes were prepared on different substrates. Uniform zinc nano hexagonal prisms with the diameter of 100–300 nm and the purity of more than 99 wt.% were successfully prepared on the quartz substrate under the optimized conditions as follows: 1000 Pa inert pressure, 1073 K heating temperature, lower than 473 K condensing temperature and 10–30 cm condensation distance. The zinc separation efficiency can attain to 99.68%. This work provided an efficient and promising way for recycling zinc with high purity and high added values from spent zinc manganese batteries. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The tremendous consumption of portable batteries has sharply increased in recent decades for their portabilities, cheap prices and heavy demands by toys, remote controls, watches, radios, cameras and other devices requiring small quantities of power [1]. It is estimated that about 60 billion of alkaline manganese based batteries are produced annually [2]. More than 15 billion of acidic or alkaline zinc manganese (Zn–Mn) batteries have been produced annually after 2002 in China [3]. Guo et al. reported that more than 300,000 tons of zinc were used in zinc based batteries manufacturing in 2003 and the number increased to 535,000 tons in 2005 [4]. Due to the low prices and portabilities, Zn–Mn batteries occupy over 90% of the total annual sales of portable batteries in the developing countries like China [5]. Usually, they are run out rapidly and ⇑ Corresponding author at: Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of Ecological and Environmental Science, East China Normal University, 500 Dong chuan Road, Shanghai, PR China. Tel./fax: +86 21 54341064. E-mail address: [email protected] (L. Zhan). http://dx.doi.org/10.1016/j.seppur.2015.01.014 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
thrown away [6]. Most of the used batteries are treated as domestic wastes, and some of them are discarded directly and landfilled. As a result, metallic ions and electrolytes are carried to the surface water body or groundwater through different routes because of inner or outer chemical reaction. High concentration intake of zinc may lead to toxic effects in humans and also animals. Muyssen et al. reported that high concentration zinc exposure can inhibit Ca intake and eventually result in reducing total Ca body contents [7]. The Occupational Safety & Health Administration recognizes zinc as an irritant to the eyes, nose, throat, and skin and can potentially cause acute lung damage [8]. Therefore, recycling Zn from spent Zn–Mn batteries is not only beneficial for realizing sustainable utilization of resources, but can also reduce the possible heavy metal contaminations. At present, hydrometallurgy and pyrometallurgy are the two main methods for recovering zinc from spent Zn–Mn batteries [9]. In the case of hydrometallurgy process, different acidic solutions were used to dissolve metals, then the leaching solutions were electrodeposited or precipitated to recover different metals
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[9–12]. Inevitably, various chemicals are heavily consumed and wastewater discharged may cause secondary pollution. Some researchers focused on bio-hydrometallurgy (bioleaching) which requires no industrial acids in leaching process [5]. However, the bioleaching processes are time consuming. Some traditional pyrometallurgy methods under atmospheric are well industrialized for recycling of exhausted batteries, such as BATREC [13], SNAM– SAVAN [14], and SAB–NIFE [15]. All of these processes possess the characteristics like strict equipment requirements, high energy consumption and emissions of dust or gases requiring further treatment [1]. Recently, vacuum metallurgy recycling is attracting more and more attentions because of its environmentally friendly benefits [16–21]. Processing in the sealed system without impurities, products with high purity can be obtained. As a result of lower boiling points of metals under vacuum, recycling of metals by vacuum metallurgy requires less energy consumption compared with traditional pyrometallurgy methods [22,23]. Recycling of metals with added values from wastes is a promising direction. The preparation of high value-added products from spent zinc manganese or zinc carbon batteries have been carried out, such as Zn–Mn ferrite particles and so on [3,10,24]. Ultrafine zinc powders, another kind of high value-added product, are widely used in batteries, catalysts, lubricating materials, anti-corrosion paint, inorganic antibacterial materials and composite material additives for their unique physical and chemical effect [25,26]. Zhang reported that the anodes made of ultrafine zinc powders can improve the discharging capacity greatly [27]. However, there is little information on preparing zinc nano-particles from spent Zn–Mn batteries. In this paper, spent Zn–Mn batteries were used as research subjects. Zinc hexagonal prisms, nano fiber and flaky nano-particles were prepared by vacuum evaporation and inert gas condensation. Relevant influencing factors of zinc nano-particles preparation like inert gas pressure, heating and condensing temperature, condensation distance from the heater, and condensing substrate were investigated in details. Additionally, the growth mechanisms of the prepared nano-particles with different morphologies were also explored. It is expected that this study provides theoretical foundations for recycling Zn and preparing nano-particles from spent Zn– Mn batteries.
furnace included three heating chambers with the total length of 1.2 m. Each chamber deployed a thermal couple to detect the heating temperature. The vacuum pump team was consisted of a mechanical pump and an oil diffusion pump. A water cooling jacket was designed at the end of the quartz tube to realize complete condensation of the metal vapor. The corundum crucible was placed in the heating chamber which can be heated up to 1273 K by the resistance furnace. A thermal couple which can be adjusted flexibly was inserted into the quartz tube to detect the condensation temperatures of different points. 2.2. Materials In this work, AA/R6c zinc manganese batteries (Guangzhou Tianqiu Enterprise Co., Ltd) were discharged and dismantled firstly. The schematic diagram of a dismantled spent Zn–Mn battery is shown in Fig. 2. Waste zinc cathodes were chosen as materials for experiments in this study. The plastic can be collected by pneumatic separation and sent to the plastic plant. Carbon rod and copper cap can be screened out and reused in battery manufacturing. Alternatively, the carbon rod can be used as reduction materials [9]. For electrolyte, it can be treated by leaching or electrolysis to recycle the manganese [2,11]. 2.3. Methods The zinc cathodes loaded with a corundum crucible was placed in the heating chamber. A 200 mesh stainless steel net was covered on the crucible to avoid spreading of residues. The system was evacuated to 1 Pa firstly, then nitrogen gas was flowed into the chamber. After that, the furnace was heated to the preset temperature with the heating rate of 10 K/min. When the furnace cooled down, the phase of the prepared products were characterized by X-ray Diffraction (XRD-6100, SHIMADZU, Japan) with Cu KR radiation, operated at 40 kV and 25 mA. The purity was analyzed by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP, ICAP6300, THERMO ELECTRO, America). The morphology were examined by Scanning Electron Microscope (SEM, S-4800, HITACHI, Japan). The residues were also examined.
2. Materials and methods 3. Results and discussion 2.1. Apparatus 3.1. Separation of zinc and dispersing effect of nitrogen gas Fig. 1 shows a schematic illustration of the experimental system which was mainly consisted of three sections: nitrogen cylinder, resistance heating tube furnace and vacuum pump team. The
According to Clausius–Clapeyron equation [28], the saturated vapor pressures of metals at different temperatures are shown in
Fig. 1. Schematic illustration of the experimental system.
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Fig. 2. Schematic diagram of a dismantled spent Zn–Mn battery.
Fig. 3. It can provide the theoretical foundation for separating zinc from other metals in the zinc cathodes by vacuum evaporation. In this section, 8 g of dismantled zinc manganese battery cathodes loaded with a corundum crucible was put into the heating chamber. When the vacuum pressure was evacuated to about 1 Pa, nitrogen gas (N2, 99.99%) of different pressures (1, 100, 1000, 10,000 Pa) were flowed into the system. The inert gas flowed into the system might inhibit the evaporation and separation of zinc. Therefore, the zinc vapor pressure should be higher than the inert gas pressure when choosing the heating temperature. According to Fig. 3, the vapor pressure of zinc is about 4.56 104 Pa at 1073 K, which is much higher than the pressures of nitrogen gas in these experiments. Additionally, the vapor pressure of zinc is about 5 103 times higher than that of lead at 1073 K. Therefore, zinc in the dismantled cathodes can be easily evaporated and separated from other impurities (Pb, Mn, etc). Results showed that the separation efficiencies of the experiments in this section were all above 99%. In order to avoid the spreading of residues, a 200 mesh stainless steel net was covered on the crucible. Different product under different nitrogen gas pressures was obtained as shown in Fig. 4. These products ranged from big zinc membrane to nano hexagonal prisms. When no nitrogen gas was flowed into the system, the evaporated zinc vapor condensed near the evaporation source where the temperature was relatively high. Without quenching and dispersing effect of carrier gas, zinc atoms collided with each other and condensed on the inner wall of the quartz tube continually. A big sheet of zinc membrane was obtained (Fig. 4a). The nitrogen gas of 100 Pa had effect of quenching and blowing off but not that obvious. Small pieces of zinc mem-
Fig. 3. The relationship between temperature and saturated vapor pressure of Zn, Pb and Mn.
branes were obtained as shown in Fig. 4b. When the system nitrogen gas pressure increased to 1000 Pa and 10,000 Pa respectively, the condensation chamber was covered with dark powders. Fig. 4c indicates that most of the prepared nano sized flakiness are hexagonal or nearly hexagonal shape under 1000 Pa nitrogen gas, but there are still some big columnar particles existing. Homogeneous zinc nano hexagonal prisms are prepared under 10000 Pa as shown in Fig. 4d. There are two main stages for the nano-particles formation which are nucleation and growth respectively. Firstly the zinc was vaporized into the gas phase at a high temperature. Then the carrier nitrogen gas swept the zinc vapor to the cold region in a short time, and the zinc vapor became supersaturated there. During this stage, these zinc atoms will collide with cold nitrogen atoms fiercely to transfer their energy to the nitrogen gas. Nucleation will occur when the temperature of these zinc atoms decreased to a certain value and small zinc droplets emerged. The growth stage always involves in coalescence and agglomeration because these zinc droplets will collide with each other randomly. To avoid this phenomenon and get highly dispersed uniform nano-particles, these zinc droplets should be swept to the cold region and solidified as quickly as possible. Therefore, the quantity of nitrogen atoms is a crucial factor of influencing the morphology of nano-particles. When increasing the carrier gas pressure, zinc atoms in per unit volume of nitrogen gas became fewer, which was beneficial for preparing nano sized particles without disturbance and collisions.
3.2. The effect of heating and condensation temperature As is shown in the triple phase graph of zinc (Fig. 5), the zinc vapor of point M is unsaturated. It will be swept left to district L by the carrier gas and become supersaturated to generate zinc droplets. Then, these zinc droplets will pass through district L and be solidified. The supersaturated vapor obtained could be nucleated and formed as the fine nano-particles only if the sharp temperature drop is achieved [29]. Higher heating temperature and lower condensation temperature can lead to high temperature gradient, which will result in larger supersaturation degree. Therefore, the sharp temperature gradient is necessary to produce uniform zinc nano-particles. The experimental results showed that the high condensation temperature (more than 523 K) would result in the generation of zinc membrane just like the morphology of Fig. 4a. In order to get sharp temperature gradient, two plugs made of Al2O3 were placed at each side of the heating chamber to prevent heat diffusion. With a hole in the middle of the plug, zinc vapor and nitrogen gas could pass through it into the condensation chamber. The temperature distribution of the furnace with two plugs is shown in Fig. 6 which shows that the two plugs have an excellent heat
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Fig. 4. Pictures and SEM images of products prepared under different nitrogen pressures, (a) 1 Pa, (b) 100 Pa, (c) 1000 Pa, (d) 10,000 Pa.
Fig. 5. The triple phase equilibrium graph of zinc. Fig. 6. Temperature distribution of the furnace.
insulation effect. The condensation temperature was controlled below 473 K at the place 10 cm from the plug where the dark powders began to emerge and sharply decreased to 350 K at 30 cm. Fig. 6 also indicates that the temperatures of the condensation chamber under different heating temperatures are almost the same due to the heat insulation effect of the plugs. Fig. 7 shows the SEM images of the prepared zinc nano-particles under different heating temperatures with the carrier gas pressure of 10,000 Pa. Compared with Fig. 7c and d, particles prepared at lower temperature show irregular morphology as shown in Fig. 7a and b. When increasing the heating temperature, the temperature gradient becomes larger because the condensation temperature remains the same as shown in Fig. 6. As a result, the zinc vapor with larger supersaturated degree will be generated, which is beneficial for the formation of the nucleus [30]. Simchi also reported that higher heating temperature and lower condensation temperature would decrease the nucleation energy barrier and can lead to the formation of large number of fine particles
[31]. Therefore, increasing the heating temperature and reducing the condensation temperature, the prepared particles are more inerratic and uniform. In addition, compare Fig. 7c with Fig. 7d, hexagonal prisms prepared under 1073 K is much thicker than that of 1023 K, it is speculated that these hexagonal prisms are inclined to grow longer instead of bigger. The separation efficiencies of zinc at different heating temperatures (923 K, 973 K, 1023 K, 1073 K) were obtained as shown in Fig. 8. When temperature rose from 923 K to 1073 K, the separation efficiency increased from 98.74% to its maximum of 99.68%. It indicates that the zinc contained in the waste cathodes can be evaporated and separated almost completely. 3.3. The conglutinating effect The growth stage proceeded in the condensation chamber till these zinc droplets solidified and condensed on the wall of the
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Fig. 7. SEM images of the prepared zinc nano-particles under different heating temperature, (a) 923 K, (b) 973 K, (c) 1023 K, and (d) 1073 K.
bigger particles are more likely to generate by colliding and inserting with each other (Fig. 9c). A complete process for preparation of zinc nano hexagonal prisms from spent Zn–Mn batteries are carried out as shown in Fig. 10. XRD was conducted to analyze the prepared powders under optimal conditions. By comparing the diffraction peaks with ICDD card No. 04-0831, it indicated that the obtained powders were pure zinc. The purity of the prepared powders was 99.23 wt.% by ICP examination. Residues analysis indicated that heavy metals like Pb, Mn were concentrated in the residues. The metals left in the residues are indeed important from the view of either resources recovery or environmental pollutions. The residues can be sent to lead smelter to further extract Pb or batch lead alloys.
Fig. 8. Separation efficiency of zinc from waste zinc-manganese battery cathodes at different heating temperature.
quartz tube. When the zinc droplets carried by the nitrogen gas passed through the condensation chamber, the longer path they went through, the more coalescences and agglomerations occurred through collisions. Finally, agglomerates of zinc nano-particles might emerge. It is analyzed that the bonds between the nuclei that comprise an agglomerate were depending on the collisions the nucleus had experienced with other nucleus due to Brownian motion [32]. Fig. 9 shows SEM images of the nano-particles prepared at different condensation distances under the followed conditions (10,000 Pa carrier gas pressure, 1073 K heating temperature). It is obvious that the prepared zinc nano-particles become irregular when the condensation distance increases. There exists few hexagonal prisms. Lower temperature of the places far away from the evaporation source is in favor of condensation and nucleation. However, longer condensation distance means more collision chances during the growth stage. Nonuniform and
3.4. Morphologies of nano zinc particles on different condensation substrates The foregoing nano structured particles were all spontaneously nucleated and condensed on the inner wall of the quartz tube, and the morphology was hexagonal. In fact, the morphology of hexagonal prism is determined by the hexagonal crystal system of zinc itself. Other morphologies of zinc nano-particles were observed on different substrates (Fig. 11). Nano fibers were prepared on the 200 mesh stainless steel net as shown in Fig. 11a and b. The curved nano fibers can be explained by oriented growth on the stainless steel net [33]. However, some hexagonal prisms are also found in Fig. 11b, which may results from the turbulent flow of nitrogen gas when passing through the porous stainless steel net. The turbulent flow may cause changes of the saturation degree of zinc vapor at different positions. As shown in Fig. 11c, a length of fiber from the refractory fiber felt is completely covered with flaky zinc nano-particles. It can be inferred that these nano-particles were prepared when the zinc vapor took the asperities of the refractory fiber felt as the nucleation center. However, the growth mechanism of nano-particles prepared on different substrates needs further investigation in the future.
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Fig. 9. SEM images of prepared products at different condensation distance, (a) 10–30 cm, (b) 30–60 cm, and (c) 60–90 cm.
Fig. 10. Zinc nano-particles preparation flowchart from spent Zn–Mn batteries.
Fig. 11. Images of prepared products on different substrates, (a) micrograph of 200 mesh net with zinc nano fibers, (b) SEM image of zinc nano fibers grown on 200 mesh stainless steel net, (c) SEM image of a length of fiber with zinc nano particles, and (d) high magnification SEM image of (c).
X. Xiang et al. / Separation and Purification Technology 142 (2015) 227–233
4. Conclusions Due to the different vapor pressures of metals, zinc can be separated from spent zinc manganese battery cathodes by vacuum evaporation. High separation efficiency of 99.68% was achieved when heating temperature rose to 1073 K at 10,000 Pa nitrogen gas pressure. Simultaneously, morphology controlled zinc nanoparticles were prepared and operation conditions were optimized. As a result of the diffusing effect, higher inert gas pressure was of assistance for preparing inerratic nano-particles. Supersaturated vapor could be generated at higher heating temperature and lower condensing temperature, and the generated supersaturated vapor was beneficial for preparing more inerratic nano-particles. The particles became irregularly shaped when collected at the places far away from the heater because of more collisions. In addition, the condensation substrate could play an important role in preparing zinc nano particles with different morphologies. Under the optimized conditions which were the insert pressure of 10,000 Pa, the heating temperature of 1073 K, the condensing temperature of lower than 473 K and the condensation distance of 10–30 cm, uniform zinc nano hexagonal prisms with the diameter of 100–300 nm and the purity of more than 99 wt.% were successfully prepared on the quartz substrate. This study could provide theoretical foundations for recycling zinc with high added values from spent zinc manganese batteries and other Zn contained wastes.
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Preparation of zinc nano structured particles from spent zinc manganese batteries by vacuum separation and inert gas condensation Xishu Xiang a, Fafa Xia a, Lu Zhan a,b,⇑, Bing Xie a a Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of Ecological and Environmental Science, East China Normal University, 500 Dong chuan Road, Shanghai, PR China b Shanghai Cooperative Centre for WEEE Recycling, Shanghai Second Polytechnic University, Shanghai 201209, PR China
a r t i c l e
i n f o
Article history: Received 15 October 2014 Received in revised form 22 December 2014 Accepted 10 January 2015 Available online 17 January 2015 Keywords: Spent zinc manganese batteries Vacuum separation Inert gas condensation Zinc nano-particles
a b s t r a c t Vacuum separation and inert gas condensation were adopted to recycle zinc and prepare zinc nanoparticles from spent zinc manganese batteries in this paper. As a result of the relatively high vapor pressure, zinc can be easily separated from the other metals in zinc manganese batteries by vacuum evaporation with a high separation efficiency. Simultaneously, morphology controlled zinc nano-particles were prepared by inert gas condensation. The results showed that inert gas pressure, heating and condensing temperature, condensation distance away from the evaporation source, and condensation substrate were the significant factors on nano-particles preparation. Higher inert gas pressure was beneficial for preparing inerratic nano-particles because of the diffusing effect. At higher heating temperature and lower condensing temperature, more inerratic nano-particles could be prepared. With more collisions and cementations, the particles became irregular at the places far away from the heater. It was also observed that nano structured particles with different morphologies including hexagonal prisms, fibriform and sheet shapes were prepared on different substrates. Uniform zinc nano hexagonal prisms with the diameter of 100–300 nm and the purity of more than 99 wt.% were successfully prepared on the quartz substrate under the optimized conditions as follows: 1000 Pa inert pressure, 1073 K heating temperature, lower than 473 K condensing temperature and 10–30 cm condensation distance. The zinc separation efficiency can attain to 99.68%. This work provided an efficient and promising way for recycling zinc with high purity and high added values from spent zinc manganese batteries. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The tremendous consumption of portable batteries has sharply increased in recent decades for their portabilities, cheap prices and heavy demands by toys, remote controls, watches, radios, cameras and other devices requiring small quantities of power [1]. It is estimated that about 60 billion of alkaline manganese based batteries are produced annually [2]. More than 15 billion of acidic or alkaline zinc manganese (Zn–Mn) batteries have been produced annually after 2002 in China [3]. Guo et al. reported that more than 300,000 tons of zinc were used in zinc based batteries manufacturing in 2003 and the number increased to 535,000 tons in 2005 [4]. Due to the low prices and portabilities, Zn–Mn batteries occupy over 90% of the total annual sales of portable batteries in the developing countries like China [5]. Usually, they are run out rapidly and ⇑ Corresponding author at: Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of Ecological and Environmental Science, East China Normal University, 500 Dong chuan Road, Shanghai, PR China. Tel./fax: +86 21 54341064. E-mail address: [email protected] (L. Zhan). http://dx.doi.org/10.1016/j.seppur.2015.01.014 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
thrown away [6]. Most of the used batteries are treated as domestic wastes, and some of them are discarded directly and landfilled. As a result, metallic ions and electrolytes are carried to the surface water body or groundwater through different routes because of inner or outer chemical reaction. High concentration intake of zinc may lead to toxic effects in humans and also animals. Muyssen et al. reported that high concentration zinc exposure can inhibit Ca intake and eventually result in reducing total Ca body contents [7]. The Occupational Safety & Health Administration recognizes zinc as an irritant to the eyes, nose, throat, and skin and can potentially cause acute lung damage [8]. Therefore, recycling Zn from spent Zn–Mn batteries is not only beneficial for realizing sustainable utilization of resources, but can also reduce the possible heavy metal contaminations. At present, hydrometallurgy and pyrometallurgy are the two main methods for recovering zinc from spent Zn–Mn batteries [9]. In the case of hydrometallurgy process, different acidic solutions were used to dissolve metals, then the leaching solutions were electrodeposited or precipitated to recover different metals
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[9–12]. Inevitably, various chemicals are heavily consumed and wastewater discharged may cause secondary pollution. Some researchers focused on bio-hydrometallurgy (bioleaching) which requires no industrial acids in leaching process [5]. However, the bioleaching processes are time consuming. Some traditional pyrometallurgy methods under atmospheric are well industrialized for recycling of exhausted batteries, such as BATREC [13], SNAM– SAVAN [14], and SAB–NIFE [15]. All of these processes possess the characteristics like strict equipment requirements, high energy consumption and emissions of dust or gases requiring further treatment [1]. Recently, vacuum metallurgy recycling is attracting more and more attentions because of its environmentally friendly benefits [16–21]. Processing in the sealed system without impurities, products with high purity can be obtained. As a result of lower boiling points of metals under vacuum, recycling of metals by vacuum metallurgy requires less energy consumption compared with traditional pyrometallurgy methods [22,23]. Recycling of metals with added values from wastes is a promising direction. The preparation of high value-added products from spent zinc manganese or zinc carbon batteries have been carried out, such as Zn–Mn ferrite particles and so on [3,10,24]. Ultrafine zinc powders, another kind of high value-added product, are widely used in batteries, catalysts, lubricating materials, anti-corrosion paint, inorganic antibacterial materials and composite material additives for their unique physical and chemical effect [25,26]. Zhang reported that the anodes made of ultrafine zinc powders can improve the discharging capacity greatly [27]. However, there is little information on preparing zinc nano-particles from spent Zn–Mn batteries. In this paper, spent Zn–Mn batteries were used as research subjects. Zinc hexagonal prisms, nano fiber and flaky nano-particles were prepared by vacuum evaporation and inert gas condensation. Relevant influencing factors of zinc nano-particles preparation like inert gas pressure, heating and condensing temperature, condensation distance from the heater, and condensing substrate were investigated in details. Additionally, the growth mechanisms of the prepared nano-particles with different morphologies were also explored. It is expected that this study provides theoretical foundations for recycling Zn and preparing nano-particles from spent Zn– Mn batteries.
furnace included three heating chambers with the total length of 1.2 m. Each chamber deployed a thermal couple to detect the heating temperature. The vacuum pump team was consisted of a mechanical pump and an oil diffusion pump. A water cooling jacket was designed at the end of the quartz tube to realize complete condensation of the metal vapor. The corundum crucible was placed in the heating chamber which can be heated up to 1273 K by the resistance furnace. A thermal couple which can be adjusted flexibly was inserted into the quartz tube to detect the condensation temperatures of different points. 2.2. Materials In this work, AA/R6c zinc manganese batteries (Guangzhou Tianqiu Enterprise Co., Ltd) were discharged and dismantled firstly. The schematic diagram of a dismantled spent Zn–Mn battery is shown in Fig. 2. Waste zinc cathodes were chosen as materials for experiments in this study. The plastic can be collected by pneumatic separation and sent to the plastic plant. Carbon rod and copper cap can be screened out and reused in battery manufacturing. Alternatively, the carbon rod can be used as reduction materials [9]. For electrolyte, it can be treated by leaching or electrolysis to recycle the manganese [2,11]. 2.3. Methods The zinc cathodes loaded with a corundum crucible was placed in the heating chamber. A 200 mesh stainless steel net was covered on the crucible to avoid spreading of residues. The system was evacuated to 1 Pa firstly, then nitrogen gas was flowed into the chamber. After that, the furnace was heated to the preset temperature with the heating rate of 10 K/min. When the furnace cooled down, the phase of the prepared products were characterized by X-ray Diffraction (XRD-6100, SHIMADZU, Japan) with Cu KR radiation, operated at 40 kV and 25 mA. The purity was analyzed by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP, ICAP6300, THERMO ELECTRO, America). The morphology were examined by Scanning Electron Microscope (SEM, S-4800, HITACHI, Japan). The residues were also examined.
2. Materials and methods 3. Results and discussion 2.1. Apparatus 3.1. Separation of zinc and dispersing effect of nitrogen gas Fig. 1 shows a schematic illustration of the experimental system which was mainly consisted of three sections: nitrogen cylinder, resistance heating tube furnace and vacuum pump team. The
According to Clausius–Clapeyron equation [28], the saturated vapor pressures of metals at different temperatures are shown in
Fig. 1. Schematic illustration of the experimental system.
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Fig. 2. Schematic diagram of a dismantled spent Zn–Mn battery.
Fig. 3. It can provide the theoretical foundation for separating zinc from other metals in the zinc cathodes by vacuum evaporation. In this section, 8 g of dismantled zinc manganese battery cathodes loaded with a corundum crucible was put into the heating chamber. When the vacuum pressure was evacuated to about 1 Pa, nitrogen gas (N2, 99.99%) of different pressures (1, 100, 1000, 10,000 Pa) were flowed into the system. The inert gas flowed into the system might inhibit the evaporation and separation of zinc. Therefore, the zinc vapor pressure should be higher than the inert gas pressure when choosing the heating temperature. According to Fig. 3, the vapor pressure of zinc is about 4.56 104 Pa at 1073 K, which is much higher than the pressures of nitrogen gas in these experiments. Additionally, the vapor pressure of zinc is about 5 103 times higher than that of lead at 1073 K. Therefore, zinc in the dismantled cathodes can be easily evaporated and separated from other impurities (Pb, Mn, etc). Results showed that the separation efficiencies of the experiments in this section were all above 99%. In order to avoid the spreading of residues, a 200 mesh stainless steel net was covered on the crucible. Different product under different nitrogen gas pressures was obtained as shown in Fig. 4. These products ranged from big zinc membrane to nano hexagonal prisms. When no nitrogen gas was flowed into the system, the evaporated zinc vapor condensed near the evaporation source where the temperature was relatively high. Without quenching and dispersing effect of carrier gas, zinc atoms collided with each other and condensed on the inner wall of the quartz tube continually. A big sheet of zinc membrane was obtained (Fig. 4a). The nitrogen gas of 100 Pa had effect of quenching and blowing off but not that obvious. Small pieces of zinc mem-
Fig. 3. The relationship between temperature and saturated vapor pressure of Zn, Pb and Mn.
branes were obtained as shown in Fig. 4b. When the system nitrogen gas pressure increased to 1000 Pa and 10,000 Pa respectively, the condensation chamber was covered with dark powders. Fig. 4c indicates that most of the prepared nano sized flakiness are hexagonal or nearly hexagonal shape under 1000 Pa nitrogen gas, but there are still some big columnar particles existing. Homogeneous zinc nano hexagonal prisms are prepared under 10000 Pa as shown in Fig. 4d. There are two main stages for the nano-particles formation which are nucleation and growth respectively. Firstly the zinc was vaporized into the gas phase at a high temperature. Then the carrier nitrogen gas swept the zinc vapor to the cold region in a short time, and the zinc vapor became supersaturated there. During this stage, these zinc atoms will collide with cold nitrogen atoms fiercely to transfer their energy to the nitrogen gas. Nucleation will occur when the temperature of these zinc atoms decreased to a certain value and small zinc droplets emerged. The growth stage always involves in coalescence and agglomeration because these zinc droplets will collide with each other randomly. To avoid this phenomenon and get highly dispersed uniform nano-particles, these zinc droplets should be swept to the cold region and solidified as quickly as possible. Therefore, the quantity of nitrogen atoms is a crucial factor of influencing the morphology of nano-particles. When increasing the carrier gas pressure, zinc atoms in per unit volume of nitrogen gas became fewer, which was beneficial for preparing nano sized particles without disturbance and collisions.
3.2. The effect of heating and condensation temperature As is shown in the triple phase graph of zinc (Fig. 5), the zinc vapor of point M is unsaturated. It will be swept left to district L by the carrier gas and become supersaturated to generate zinc droplets. Then, these zinc droplets will pass through district L and be solidified. The supersaturated vapor obtained could be nucleated and formed as the fine nano-particles only if the sharp temperature drop is achieved [29]. Higher heating temperature and lower condensation temperature can lead to high temperature gradient, which will result in larger supersaturation degree. Therefore, the sharp temperature gradient is necessary to produce uniform zinc nano-particles. The experimental results showed that the high condensation temperature (more than 523 K) would result in the generation of zinc membrane just like the morphology of Fig. 4a. In order to get sharp temperature gradient, two plugs made of Al2O3 were placed at each side of the heating chamber to prevent heat diffusion. With a hole in the middle of the plug, zinc vapor and nitrogen gas could pass through it into the condensation chamber. The temperature distribution of the furnace with two plugs is shown in Fig. 6 which shows that the two plugs have an excellent heat
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Fig. 4. Pictures and SEM images of products prepared under different nitrogen pressures, (a) 1 Pa, (b) 100 Pa, (c) 1000 Pa, (d) 10,000 Pa.
Fig. 5. The triple phase equilibrium graph of zinc. Fig. 6. Temperature distribution of the furnace.
insulation effect. The condensation temperature was controlled below 473 K at the place 10 cm from the plug where the dark powders began to emerge and sharply decreased to 350 K at 30 cm. Fig. 6 also indicates that the temperatures of the condensation chamber under different heating temperatures are almost the same due to the heat insulation effect of the plugs. Fig. 7 shows the SEM images of the prepared zinc nano-particles under different heating temperatures with the carrier gas pressure of 10,000 Pa. Compared with Fig. 7c and d, particles prepared at lower temperature show irregular morphology as shown in Fig. 7a and b. When increasing the heating temperature, the temperature gradient becomes larger because the condensation temperature remains the same as shown in Fig. 6. As a result, the zinc vapor with larger supersaturated degree will be generated, which is beneficial for the formation of the nucleus [30]. Simchi also reported that higher heating temperature and lower condensation temperature would decrease the nucleation energy barrier and can lead to the formation of large number of fine particles
[31]. Therefore, increasing the heating temperature and reducing the condensation temperature, the prepared particles are more inerratic and uniform. In addition, compare Fig. 7c with Fig. 7d, hexagonal prisms prepared under 1073 K is much thicker than that of 1023 K, it is speculated that these hexagonal prisms are inclined to grow longer instead of bigger. The separation efficiencies of zinc at different heating temperatures (923 K, 973 K, 1023 K, 1073 K) were obtained as shown in Fig. 8. When temperature rose from 923 K to 1073 K, the separation efficiency increased from 98.74% to its maximum of 99.68%. It indicates that the zinc contained in the waste cathodes can be evaporated and separated almost completely. 3.3. The conglutinating effect The growth stage proceeded in the condensation chamber till these zinc droplets solidified and condensed on the wall of the
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Fig. 7. SEM images of the prepared zinc nano-particles under different heating temperature, (a) 923 K, (b) 973 K, (c) 1023 K, and (d) 1073 K.
bigger particles are more likely to generate by colliding and inserting with each other (Fig. 9c). A complete process for preparation of zinc nano hexagonal prisms from spent Zn–Mn batteries are carried out as shown in Fig. 10. XRD was conducted to analyze the prepared powders under optimal conditions. By comparing the diffraction peaks with ICDD card No. 04-0831, it indicated that the obtained powders were pure zinc. The purity of the prepared powders was 99.23 wt.% by ICP examination. Residues analysis indicated that heavy metals like Pb, Mn were concentrated in the residues. The metals left in the residues are indeed important from the view of either resources recovery or environmental pollutions. The residues can be sent to lead smelter to further extract Pb or batch lead alloys.
Fig. 8. Separation efficiency of zinc from waste zinc-manganese battery cathodes at different heating temperature.
quartz tube. When the zinc droplets carried by the nitrogen gas passed through the condensation chamber, the longer path they went through, the more coalescences and agglomerations occurred through collisions. Finally, agglomerates of zinc nano-particles might emerge. It is analyzed that the bonds between the nuclei that comprise an agglomerate were depending on the collisions the nucleus had experienced with other nucleus due to Brownian motion [32]. Fig. 9 shows SEM images of the nano-particles prepared at different condensation distances under the followed conditions (10,000 Pa carrier gas pressure, 1073 K heating temperature). It is obvious that the prepared zinc nano-particles become irregular when the condensation distance increases. There exists few hexagonal prisms. Lower temperature of the places far away from the evaporation source is in favor of condensation and nucleation. However, longer condensation distance means more collision chances during the growth stage. Nonuniform and
3.4. Morphologies of nano zinc particles on different condensation substrates The foregoing nano structured particles were all spontaneously nucleated and condensed on the inner wall of the quartz tube, and the morphology was hexagonal. In fact, the morphology of hexagonal prism is determined by the hexagonal crystal system of zinc itself. Other morphologies of zinc nano-particles were observed on different substrates (Fig. 11). Nano fibers were prepared on the 200 mesh stainless steel net as shown in Fig. 11a and b. The curved nano fibers can be explained by oriented growth on the stainless steel net [33]. However, some hexagonal prisms are also found in Fig. 11b, which may results from the turbulent flow of nitrogen gas when passing through the porous stainless steel net. The turbulent flow may cause changes of the saturation degree of zinc vapor at different positions. As shown in Fig. 11c, a length of fiber from the refractory fiber felt is completely covered with flaky zinc nano-particles. It can be inferred that these nano-particles were prepared when the zinc vapor took the asperities of the refractory fiber felt as the nucleation center. However, the growth mechanism of nano-particles prepared on different substrates needs further investigation in the future.
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Fig. 9. SEM images of prepared products at different condensation distance, (a) 10–30 cm, (b) 30–60 cm, and (c) 60–90 cm.
Fig. 10. Zinc nano-particles preparation flowchart from spent Zn–Mn batteries.
Fig. 11. Images of prepared products on different substrates, (a) micrograph of 200 mesh net with zinc nano fibers, (b) SEM image of zinc nano fibers grown on 200 mesh stainless steel net, (c) SEM image of a length of fiber with zinc nano particles, and (d) high magnification SEM image of (c).
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4. Conclusions Due to the different vapor pressures of metals, zinc can be separated from spent zinc manganese battery cathodes by vacuum evaporation. High separation efficiency of 99.68% was achieved when heating temperature rose to 1073 K at 10,000 Pa nitrogen gas pressure. Simultaneously, morphology controlled zinc nanoparticles were prepared and operation conditions were optimized. As a result of the diffusing effect, higher inert gas pressure was of assistance for preparing inerratic nano-particles. Supersaturated vapor could be generated at higher heating temperature and lower condensing temperature, and the generated supersaturated vapor was beneficial for preparing more inerratic nano-particles. The particles became irregularly shaped when collected at the places far away from the heater because of more collisions. In addition, the condensation substrate could play an important role in preparing zinc nano particles with different morphologies. Under the optimized conditions which were the insert pressure of 10,000 Pa, the heating temperature of 1073 K, the condensing temperature of lower than 473 K and the condensation distance of 10–30 cm, uniform zinc nano hexagonal prisms with the diameter of 100–300 nm and the purity of more than 99 wt.% were successfully prepared on the quartz substrate. This study could provide theoretical foundations for recycling zinc with high added values from spent zinc manganese batteries and other Zn contained wastes.
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