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Sustainable extraction of lead and re-use of valuable metals from lead-rich secondary materials
Journal of Cleaner Production 219 (2019) 110e116
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Sustainable extraction of lead and re-use of valuable metals from leadrich secondary materials Bin Yang a, b, d, *, Guozheng Zha a, b, William Hartley c, Xiangfeng Kong a, b, **, Dachun Liu a, b, d, Baoqiang Xu a, b, d, Wenlong Jiang a, b, Xinyu Guo a, b a
National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan, 650093, PR China Faulty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, 68 Wenchang Road, Kunming, Yunnan, 650093, PR China Crop and Environment Sciences Department, Harper Adams University, Newport, Shropshire, TF10 8NB, United Kingdom d State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan, 650093, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2018 Received in revised form 22 January 2019 Accepted 2 February 2019 Available online 10 February 2019
Traditional lead metallurgy, a highly complicated lead extraction process for valuable metal recovery, has exhausted resources and produced a variety of hazardous wastes that are extremely harmful to the environment and unfavorable to the sustainable development of the lead industry. Vacuum metallurgy has become a promising emerging technology contributing to refining primary metals and the recovery of secondary nonferrous metal wastes. The technology is therefore relevant in the context of a transition to a more environmentally friendly circular economy. There is a growing interest in using vacuum distillation for extraction of Pb and valuable metals recovery from lead-rich secondary materials including crude lead, waste Pb-Sn alloy (WPSA) and lead anode slime (LAS). Results indicated that vacuum extracted lead (99.5% Pb) was obtained from crude lead (92.88%) using the two stage high-low temperature vacuum distillation process. Removals of 99.99% for Cu, 99.50% for Sn, and 98.00% for Ag, were achieved respectively. The valuable metals of Ag, Cu and Sn were recovered and concentrated in the final residue. Vacuum separation results of WPSA demonstrated that the purity of Pb reached 99.4% and recovery of Sn was above 86.0%. A novel integrated smelting-vacuum method is proposed here to reuse LAS. Copper and Ag were successfully enriched in the residues whilst Pb was collected in volatiles. The appropriate vacuum process effectively extracted Pb whilst recycling valuable metals, in a sustainable and environmentally friendly manner. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Lead-rich secondary materials Vacuum separation Cleaner extraction Sustainable recovery
1. Introduction Developing a sustainable, economic and novel strategy for Pb extraction and valuable metals recovery from lead-rich secondary materials, is a key objective of the lead industry. Crude lead is mainly produced by the smelting of lead concentrates in sinterblast furnaces and by direct smelting-reduction processes (Kong et al., 2014a,b; Sohn and Olivas-Martinez, 2014). Lead ores
* Corresponding author. National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan, 650093, PR China. ** Corresponding author. National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan, 650093, PR China. E-mail addresses: [email protected] (B. Yang), [email protected] (X. Kong). https://doi.org/10.1016/j.jclepro.2019.02.011 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
contain metals impurities including Cu, Sn, Ag, As, Sb, Bi and Zn (Ojebuoboh et al., 2003), and are typically extracted during the smelting process. Pyro-metallurgical and/or electrolytic processes are used to separate the impurities and obtain purified lead (Kong et al., 2014a,b; Zhang et al., 2018). The volume of refining lead reached 11 million tons in 2016 (Zhang and Zeng, 2018). Currently, pyro-metallurgical processes contribute to approximately 70% of worldwide production. This process commonly includes the following procedures: liquation of Cu and addition of S to remove Cu, addition of NaNO3 to remove As, Sb and Sn, addition of Zn to remove Ag, oxidation, chlorination or vacuum, to remove Zn, addition of Ca and Mg to remove Bi,
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addition of NaOH and NaNO3 to remove the remaining impurities (Peng, 2010; Song, 2011). The process produces residues containing elements such as arsenic (As) which are classified as hazardous wastes (according to the National Hazardous Wastes List) and are of environmental concern (National Hazardous Wastes List, 2016). For example, the addition of NaNO3 creates large volumes of arsenic-contaminated residues (Peng, 2010; Binz and Friedrich, 2015). Therefore, residues require frequent remediation to reduce their potential to pollute water and land. Additionally, freshly formed dust containing heavy metals is harmful to smelting plant operatives. Furthermore, pyro-metallurgical processes have disadvantages such as complex procedures, high-energy consumption and low economic profits (Zhang et al., 2012; Hong et al., 2017). With the rapid development of electronic manufacturing, large volumes of waste Pb-Sn alloys (WPSAs), including waste lead-acid batteries and lead-tin solders, have been abandoned (Ellis and Mirza, 2010; Chen et al., 2018a,b; Meng et al., 2018). WPSAs contain precious metals including Pb, Cu, Sn, Ag, Au and Sb with great economic value, but which can cause serious environmental impacts when incorrectly discarded (Cayumil et al., 2016). The reuse of WPSAs is an important consideration not only for recycling of precious metals, but also for protection of the environment in today's resource-saving society. Two techniques have been developed to separate Pb and Sn from WPSAs. The first method involves solvent extraction, electrolytic deposition, precipitation and cementation, to separate Pb and Sn from the corrosive nitric solutions, producing Pb of approximately 99% purity; tin is precipitated as tin hydroxide (Sn(OH)2) from the solution (Lee et al., 2003). The second method, producing 90% Sn, involves acid leaching (HCl and H2SO4) and alkali precipitation (NaOH) (Castro and Martins, 2009). These methods have advantages, but the leached liquor and residues result in environmental issues. Hence, it is particularly urgent to develop a cleaner technique to recycle Pb and Sn from WPSAs. Electrolytic refining of crude lead can produce lead anode slime (LAS), and the volume of LAS generated is typically 1e2% of the lead produced (Peng, 2010). Most of the metals in LAS exist as oxides and are transformed to metals by reduction smelting. The alloy produced from this process is noble lead. Besides Pb, the main chemical components are rare and precious metals (as metal and alloy phases), including Cu, Ag, Pt, Rh, Ru, etc. with high economic value (Havuz et al., 2010; Wang, 2011). LAS is the primary resource for the complete recovery of precious and rare metals. More importantly, LAS is clearly listed in the National Hazardous Wastes List implemented in 2016, and as yet there is no clean alternative (National Hazardous Wastes List, 2016). The treatment of LAS is usually carried out by pyro-metallurgical processes (fire smelting, pyro-refining and/or electrolytic refining) including reduction smelting, oxidative refining of silver, reduction smelting and refining of antimony, bismuth slag smelting and follow-up electrolytic refining (Wang, 2011; He et al., 2017). The above processes result in the generation of metal dusts and waste residues. The subsequent treatment of these residues is most critical, because they contain large volumes of heavy metals and toxic substances (Chen et al., 2018a,b). Even with a dust collection system, they will inevitably contaminate the local environment through rainfall (Havuz et al., 2010). This is especially of concern for As, as the content of As in residues can reach 5.14% (Lin and Qiu, 2012). Arsenic will accumulate in ecological systems through atmospheric activities, causing soil and water secondary pollution, further harming human health. Additionally, the whole treatment process requires large amounts of energy to liquefy the materials. Our previous work demonstrated that vacuum distillation has been successfully applied to separate metal resources from primary
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metals (crude nickel, crude zinc, crude indium, raw silicon) and secondary wastes (In-Sn alloy, copper anode slime) (Wei et al., 2011; Liu et al., 2012, 2017; Li et al., 2012a, 2012b; Jiang et al., 2013). Vacuum technology has been widely applied to separation, purification and recycling of metals from nonferrous alloys and waste residues, because of its advantages including short process times, high metal recovery, reduced pollution, and low energy consumption, hence eliminating the disadvantages of traditional technologies (Dai and Yang, 2009; Kong et al., 2012; Yang et al., 2015; Zhang et al., 2015). This study describes a novel process, which uses vacuum distillation to efficiently extract Pb and effectively recycle valuable metals from lead-rich secondary materials including crude lead, WPSA and LAS. Calculation and analysis of the separation/recovery effects at saturated vapor pressure, together with the vacuum process required to extract lead and effectively recycle precious metals are considered. Finally, the work attempts to understand elemental behavior following vacuum distillation and its influence on the environment. 2. Material and methods 2.1. Characterization of lead-rich materials Samples of crude lead and LAS were collected from the Yunnan Tin Group (Holding) Company Ltd, China. The samples were placed in wooden boxes and transported to the laboratory. Samples were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and chemical titration for their respective metal contents (Table 1). Samples were also polished to a flat surface, 20 20 mm, and analyzed by X-ray powder diffraction (XRD) using a RIGAKUD/ MAX-2400 with Cu Ka (40 kV) radiation. XRD patterns were collected from 5 to 90 with fine scanning (1 2q min1 scan rate and 0.02 2q step size). XRD phases were identified by means of the Research/Match analysis package (S/M focus on major or minor phases). The XRD results (Fig. 1) indicate that metallic lead from crude lead is the dominant material. Samples of WPSA were collected from various resources, including waste lead-acid batteries and lead-tin solders, in China. These wastes were stripped of any plastic and metalloid materials then crushed and subsequently centrifuged (4000 rpm (15 min)) to collect the heavy metallic substances that settled out. The moist metal samples obtained were oven-dried at 368 K for two days. Lead and Sn were determined by chemical titration. Other elements were analyzed by ICP-AES. The chemical composition of WPSA consists mainly of Pb and Sn (Table 1). 2.2. Vacuum separation experiment 2.2.1. Crude lead and WPSA Samples of homogeneous crude lead (1 kg) and WPSA powder (1 kg) were respectively introduced to graphite crucibles, and placed inside the bottom distillation area of a HZSL-18 vacuum furnace (Dai and Yang, 2009), then vacuumed to a dynamic
Table 1 Chemical compositions of lead-rich materials (mass fraction %). Lead-rich materials
Pb
Cu
Sn
Ag
Zn
As
Sb
Bi
Crude lead WPSAa Noble leadb
92.88 76.30 31.98
2.46 0.01 8.81
0.78 23.2 0.00
0.67 0.02 12.03
0.60 0.00 0.00
1.04 0.00 16.14
1.28 0.33 17.18
0.27 <0.01 7.61
a Ni, Al, Fe, etc. in WPSA were detected, which were not presented and discussed herein. b Noble lead is the direct product of fire reduction smelting of LAS.
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controlled at 973 K, to separate high-volatile substances (Zn, As, etc.). The one-step low temperature distillation process for WPSA was maintained at 1173 K to separate Pb and Sn. 2.2.2. LAS (noble lead) The experimental equipment used for industrial separation of noble lead adopted a continuous vacuum furnace (Fig. 3). The furnace uses ohmic heating, where the vertical graphite heating column is located in the center of the furnace body, and the multistage distillation tower is uniformly arranged on the heating column from top to bottom. During the process, noble lead is completely liquefied and siphoned to the vacuum furnace (Fig. 3). As the smelting material entered the multistage distillation tower from above, lead was distilled, whilst the vapor phase was liquefied in the condenser with Cu, Ag, and Au being continuously moved down to the confluence plate. The remaining residues (non-volatilized) were collected in the residue discharge pot (Fig. 3). 2.3. Method of analysis Fig. 1. XRD pattern collected from crude lead. Copper phase peaks are likely to be presented in crude lead, but were not observed, being affected by the large peaks of Pb. Peaks of other metal phases were also not observed due to their contents lowering the detection limit of XRD.
pressure of 5e15 Pa with the appropriate combination of mechanical and diffusion pumping. A schematic diagram is shown in Fig. 2. Heat was generated through a graphite heater and transferred to a crucible inside the heater by radiation. A graphite crucible, made up of high-density graphite in a cylindrical shape (9 cm OD 8 cm ID 12 cm Ht), was filled with either crude lead and WPSA. The condenser was made of stainless steel to cool the volatiles, and is located approximately 8 cm above the graphite crucible. Vacuum distillation of crude lead was performed in two steps. Firstly, crude lead was subjected to 1373 K to separate and enrich low-volatile metals (Cu, Sn, Ag, etc.). When the furnace temperature cooled completely, the volatiles (mainly Pb) collected in the condenser were then subjected to a low temperature step,
Fig. 2. Schematic diagram of HZSL-18 vacuum distillation furnace.
Volatilized and non-volatilized residues were collected at random at six locations. Samples were then mixed to form a representative concentration. High-content elements were determined by chemical titration. ICP-AES was used to determine lowconcentration elements. 3. Theory and calculation 3.1. Principle of separation Both solid and liquid states of any substance have a tendency to volatilize into a gaseous state, and the gaseous state also has a tendency to condense into a solid state or agglomerate into a corresponding liquid state. Vacuum distillation technology is a typical physical method that exploits this characteristic to separate and purify metals and/or alloys. Each substance has a specific vapor pressure at a specific temperature range, and the vapor pressure contributes significantly to the evaporated characteristics of the substance. Substances with higher vapor pressure generally have greater volatility and will be evaporated earlier as volatiles, whereas substances with lower vapor pressure have less volatility, and will not be volatilized, and remain in the residue to achieve separation of the different materials. This is fundamental for
Fig. 3. Diagram of vertical continuous vacuum furnace.
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separating lead-rich secondary materials including crude lead, WPSA and noble lead, in order to extract lead and recover precious metals with a vacuum environment. 3.2. Saturated vapor pressure determination The different vapor pressures of metal substances in lead-rich secondary materials were theoretically calculated to estimate the feasibility of separation (Dai and Yang, 2009). Results of the relationship between metal pressure and temperature (Fig. 4) indicate that Zn, Sb and As pressures are much higher than those of other elements at the same temperature range, 600e1800 K. Therefore, Zn, Sb and As are preferentially evaporated and separated from lead-rich secondary materials at a relatively low temperature (Fig. 4). In contrast, Ag, Cu and Sn, with low saturation vapor pressure, are more difficult to evaporate, and remain in the residues. It is easy to remove and enrich these precious metals from lead-rich secondary materials at a relatively high temperature (Fig. 4). Meanwhile, reasonable use of a combined two-stage vacuum distillation process can effectively separate and purify Pb from lead-rich secondary materials (Fig. 4). 4. Results and discussion 4.1. Extraction of Pb and recovery of Cu, Sn and Ag from crude lead Quantitative analysis (Table 2) reveals the apparent extraction of Pb from crude lead with various metal impurities (Zn, Sb, As, Ag, Cu, Sn) in the two step high-low temperature vacuum distillation process (the first fraction: high temperature vacuum distillation, HTVD; the second fraction: low temperature vacuum distillation, LTVD). With the exception of Bi, most of the metal impurities (Table 2) have been separated and removed at different stages. The low-volatile impurities of Ag, Cu and Sn separated, and remained in the residue. Copper and tin removal rates reached almost 100% at HTVD whilst silver removal rate reached up to 98% (Table 2). The HTVD process effectively extracted Pb and efficiently removed Ag, Cu and Sn from crude lead (Fig. 4). The removed Ag, Cu and Sn were completely concentrated in the residue, forming a Ag-Cu-Sn alloy at HTVD (Table 3); this can be separated further and recovered for potential reuse. Zinc and As were evaporated as volatiles, being removed from
Fig. 4. Separation predication diagram of the main elements in lead-rich secondary materials.
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the crude lead at LTVD (Zn and As volatilized along with Pb at HTVD). The removal rates of Zn and As were more than 99% (Table 2), owing to their volatile characteristics at the volatile separation area (Fig. 4). Zinc and As impurities decreased further, promoting the purification of Pb. In addition, a large proportion of Sb decreased (Table 2) in crude lead, which resulted in the further increase in lead concentration. The volatilized Zn, As and Sb were enriched forming alloys at LTVD (Table 3). Bismuth is barely separated from crude lead at either HTVD or LTVD. This separation phenomenon can be ascribed to the similar physicochemical characteristics of Bi and Pb. The bismuth vapor pressure is almost similar to that of Pb (Fig. 4). Furthermore, the activity coefficient of Bi in crude lead affects its volatilization and accounts for its behavior separation (Kong et al., 2014b). Following the two stages of HTVD and LTVD, the mass fraction of Cu, Sn and As in vacuum extracted lead was only 0.0026%, 0.0042% and 0.0011%, respectively (Table 2); these meet the requirements of the Pb (99.90) standard product. The mass fraction of Ag, Zn and Sb in vacuum extracted lead was 0.014%, 0.0064% and 0.184%, respectively (Table 2), being greater than that of the Pb standard product and therefore requiring further refining. All impurities in the vacuum extracted lead were in accordance with the requirements for lead anode plate production, however further purification by electrolysis will remove Ag, Zn and Sb. The entire vacuum process only produced two by-products (CuSn-Ag alloy and Zn-As-Sb alloy) that can be either sold and/or recovered. Vacuum distillation results (Tables 2 and 3) revealed that HTVD and LTVD provided an effective and clean method to extract lead from crude lead whilst recycling valuable metals, which can replace the heavily polluted, complex procedure, of traditional pyro-metallurgical processes (Table 3). 4.2. Extraction of Pb and recovery of Sn from WPSA The fluctuating curves indicate separation and extraction of Pb from WPSA with high impurity tin in the vacuum distillation process. The lead curve has a shallow slope, reducing from 99.66% to 99.03% at a distillation time of 20e60 min (Fig. 5 A). For the impurities in extracted lead, the curve has a steep slope, increasing from 0.34% to 0.97% at the same time range (Fig. 5 A). Therefore, a distillation time of 22e42 min may be considered as the optimum extraction period for lead. In this range, lead purity remains over 99.4%, and tin remains at 0.06%. Results from the extracted lead (Fig. 5 A) suggest that much of the Pb in WPSA has been volatilized with only a small volume of Sn. The curves of residual results (Fig. 5 B) also demonstrate this distillation behavior (Pb and Sn) at a distillation time of 22e42 min. Lead mass fraction in the residue decreased from 76.30% to less than 0.8%, and Sn was approximately 86.0% in this range. Comprehensive results of WPSA (Fig. 5A and B) revealed that almost all Sn remained in the final residue. Therefore, WPSA with high tin content can be treated in one step of the vacuum distillation process. Lead and Sn were both recycled. The final leadseparation product is greater than 99.4% extracted lead. The final tin residue can be purified further by additional vacuum distillation. As the main components in WPSA are Pb and Sn, the T-x vaporliquid equilibrium (VLE) phase diagram (Fig. 6) at 5 Pa and 15 Pa was calculated, being based on VLE theory (Zhang et al., 2015) and the molecular interaction volume model (Tao, 2000). This generated a good explanation of the equilibrium state achieved by WPSA distillation. Fig. 6 shows that the temperature range of vapor and liquid lines decreases with a decrease in system pressure, low pressure has a positive effect on the extraction of lead from WPSA by vacuum distillation. The Pb mass fraction in vapor phase is 99.95
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Table 2 Lead extraction from crude lead using a two-stage high-low temperature vacuum distillation process. First stage conditions: distillation temperature (1373 K) and distillation time (30 min) to separate Cu, Sn and Ag. Second stage conditions: distillation temperature (973 K) and distillation time (30 min) to separate Zn, As, and Sb. Elements
Cu Sn Ag Zn As Sb Bi Pb a b
Contents (mass fraction %)
Removal rate (%)
Crude lead
Vacuum extracted lead
2.46 0.78 0.67 0.60 1.04 1.28 0.27 92.88
0.0026 0.0042 0.0144 0.0064 0.0011 0.1840 0.2870 99.50
99.99 99.50 98.00 99.00 99.90 86.56 00.92
(1st stage) (1st stage) (1st stage) (2nd stage) (2nd stage) (2nd stage) (total)
Standard products (mass fraction %) Pb99.90a
Lead anode plateb
0.01 0.005 0.002 0.002 0.01 0.05 0.03 99.90
0.03e0.08 Less than 0.25 0.1e0.4 0.00 0.1e1 0.4e1 0.2e0.04 More than 98.00
Pb99.90, standard product of 99.9% purity lead, and impurities total less than 0.1% (Peng, 2010). Lead anode plate is obtained from the traditional pyro-metallurgical process of crude lead that meets the requirement of further electrolysis refining (Peng, 2010).
Table 3 Effect of vacuum process on the environment and recovery behavior of precious metals in crude lead. Elements
Cu Sn Ag Zn As Sb Bi a b
Vacuum process
Traditional process
Technology
Recovery behavior
Impacts
Technology
Residuea
Impactsb
HTVD HTVD HTVD LTVD LTVD LTVD No separation
Remained in Cu-Sn-Ag alloy
Recovery
Volatilized into Zn-As-Sb alloy
Recovery
Liquation, add Cu Oxidation by NaOH Add Zn Vacuum Oxidation by NaOH Oxidation by NaOH Add Ca and Mg
Cu dross As residue Ag-Zn crust No As residue As residue Bi residue
Hazardous Hazardous Recovery Recovery Hazardous Hazardous Hazardous
No change
Waste residues generated from each step of the traditional pyro-metallurgical process (Peng, 2010; Sohn and Olivas-Martinez, 2014). Impact of residues on the environment according to the National Hazardous Wastes List (National Hazardous Wastes List, 2016).
%e99.97% and in the liquid phase is 5.7 %e18.3% when distillation reaches equilibrium at 1173 K and 5e15 Pa. This theoretical value is in good agreement with the trend of the experimental values. Evaporation rate affects volatilization time and separation efficiency of Pb and Sn. When the WPSA is heated under vacuum, molecules of the metals generated leave the surface of the material and they may encounter other gas molecules. If no collision occurs until a molecule reaches the condenser, Langmuir (Dai and Yang, 2000) postulates that the maximum evaporation rate of the metal will be pachieved. It can be described as umax ¼ 2:624 ffiffiffiffiffiffiffiffiffiffi 102 ap*i M=T , where umax is the maximum evaporation rate, g$cm2$min1; a is the accommodation coefficient, the probability of a distilled atom leaving the surface of the melting material, which is taken as close to unity; pi*is the saturated pressure of the pure element, Pa; M is the molecular weight of the distilled element; T is the melting surface temperature in K. The results of evaporation rates of Pb and Sn in material at 1173 K are 0.4658 g cm2$min1 and 3 105 g cm2$min1, respectively. Therefore, the evaporation rate of Pb is much higher than that of Sn, and Pb will evaporate preferentially in a relatively short time, which is consistent with the calculation results of saturated vapor pressure. Based on the kinetic calculation of evaporation rate, we can conclude that with an increase in distillation time, Pb with a higher evaporation rate in WPSA, will gradually completely volatilize, while Sn will slowly volatilize, resulting in the increase in impurity content of the volatile matter. As shown in Fig. 5 A, lead recovery rate tends to reach a maximum when distillation time exceeds 42 min. Continuing to increase distillation time will only lead to Sn volatilization to form impurities and rapidly reduce the purity of Pb. Consequently, this process can replace the typical solvent extraction-electrowinning-precipitation-cementation and acid leaching-alkaline precipitation procedure (Lee et al., 2003; Castro and Martins, 2009). Recovery of WPSA has been simplified to one
phase by vacuum distillation without the generation of leached liquor and waste residues. Vacuum utilization of WPSA is an obvious improvement, not only for recycling of Pb and Sn, but also for protection of the environment. 4.3. Recovery of valuable metals from noble lead Industrial results (Table 4) reveal a clear enrichment of Cu, Ag, Pb and Bi from noble lead at one stage of the vacuum distillation process. The Cu mass fraction in the residue increased from 8.81% (raw material) to 38.92%, and was reduced to 0.03% in volatiles (Table 4). For Ag, the initial content recorded was 12.03% but was significantly increased to 31.99% (Table 4). The distillation results indicate that Cu and Ag, in noble lead, are basically retained in residues, further suggesting that the preliminary separation and enrichment of Cu and Ag from noble lead has been achieved. This behavior can be attributed to their difficult volatile characteristics at the residual enrichment stage (Fig. 4). Lead and bismuth mass fractions in residues were 0.02% and 0.01%, respectively (Table 4), revealing that large volumes of Pb and Bi have been volatilized. Their contents increased from 31.98% to 67.63% and from 7.61% to 14.65%, respectively (Table 4). Volatile results also verify this phenomenon, owing to their equally volatile characteristics at the volatile separation area (Fig. 4). Both residuals and the volatiles contain a large amount of Sb. Antimony in noble lead alloy formed a number of stable compounds with Ag and Cu which resulted in a decrease in antimony volatile properties (Wang, 2011). Sb is difficult to separate from Ag and Cu under vacuum conditions. Here, results describe a novel vacuum separation procedure for the treatment of noble lead. The conventionally pyro-metallurgical process involving high-energy consumption, heavy pollution and a longer process time may be replaced with the vacuum separation process (Havuz et al., 2010; Wang, 2011; He et al., 2017). Compared
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(A)
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Table 4 Compositions of valuable metals recovered from noble lead using industrial vacuum distillation (input quantity of approximately 9 thousand tons). Vacuum conditions: distillation temperature (1173 K), distillation time (7 h) and system pressure (40e60 Pa) remained to recycle Cu, Ag, Pb and Bi. Elements of As, Te and Se were not discussed herein. Materials
Pb
Cu
Ag
Sb
Bi
Noble lead Volatile Residual
31.98 67.63 0.02
8.81 0.03 38.92
12.03 0.74 31.99
17.18 12.26 25.65
7.61 14.65 0.01
suggesting that vacuum distillation can avoid the multiple reduction smelting processes and can significantly reduce the high consumption rates of production. Vacuum treatment of noble lead is a more suitable process for sustainable development of the industry.
(B)
Fig. 5. Effect of distillation time on lead extraction (A) and tin recovery (B) from WPSA under vacuum at 1173 K. Where present, impurity change (Sn) refers to the impurities in vacuum extracted lead, represented by that of Sn (A). Impurity change (Pb) refers to the impurities in residual tin, represented by that of Pb (B). The content of other impurities was less and has been omitted.
5. Conclusions This work presents a novel and clean strategy of vacuum technology for extraction of Pb and recycling of precious metals from lead-rich secondary materials including crude lead, WPSA and noble lead. The product of vacuum extracted lead, in accordance with standards for lead anode plate, was acquired from crude lead using a two-stage, high-low temperature vacuum distillation process. Removals of 99.99% for Cu, 99.50% for Sn, 98.00% for Ag were achieved respectively. The precious metals of Ag, Cu and Sn were concentrated in the final residue. Extraction purity of Pb reached 99.4% and recovery of Sn was more than 86.0% from WPSA by a onestep vacuum distillation process. Copper and Ag in noble lead were successfully enriched in residues, and Pb and Bi were collected in the volatile stage. These findings are significant in that they help to separate valuable metals from lead-rich secondary materials, and benefit decision-making processes for clean recovery of metals by reducing environmental impacts. Acknowledgements This project was funded by the Fund of National Natural Science Foundation of China (grant No. U1502271), the construction of high-level talents of Kunming University of Science and Technology (EmpNo. 20180050), the National Key Research and Development Program of China (grant No. 2016YFC0400404), the Leader in Science and Technology of Yunnan Province (grant No. 2014HA003), and the Program for Nonferrous Metals Vacuum Metallurgy Innovation Team of Ministry of Science and Technology (grant No. 2014RA4018). The authors sincerely acknowledge the anonymous reviewers for their insights and comments to further improve the quality of the manuscript. References
Fig. 6. VLE for the Pb-Sn system under 5 Pa and 15 Pa.
with the traditional procedure, vacuum distillation can shorten the production cycle whilst benefitting the recovery of valuable metals. Furthermore, the treatment process is efficient, simple and clean. The distilled products are alloys, without additional residues,
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Liu, Y., Xie, K.Q., Ma, W.H., Wei, K.X., Xiao, P., Yang, S.C., 2017. Experimental study of comprehensive recovery valuable metals from miscellaneous copper anode slime. J. Kunming Univ. Sci. Tech. 42, 8e15 (in Chinese). https://doi.org/10. 16112/j.cnki.53-1223/n.2017. 02. 002. Meng, L., Gao, J.T., Zhong, Y.W., Wang, Z., Chen, K.Y., Guo, Z.C., 2018. Supergravity separation for recovering Pb and Sn from electronic waste. Sep. Purif. Technol. 191, 375e383. https://doi.org/10.1016/j.seppur.2017.09.041. National Hazardous Wastes List, 2016. Ministry of Ecology and Environment of People's Republic of China (in Chinese). http://www.mee.gov.cn/gkml/hbb/bl/ 201606/t20160621_354852.htm. Ojebuoboh, F., Wang, S.J., Maccagni, M., 2003. Refining primary lead by granulation - leaching - electrowinning. Jom 55, 19e23. Peng, R.Q., 2010. Lead Metallurgy. Central South University Press, ChangSha (in Chinese). Sohn, H.Y., Olivas-Martinez, M., 2014. Lead and zinc production. Treat. Process Metall. 3, 671e693. https://doi.org/10.1016/B978-0-08-096988-6.0025-0. Song, X.C., 2011. Heavy Nonferrous Metals Metallurgy. Metallurgical Industry Press, Beijing (in Chinese). Tao, D.P., 2000. A new model of thermodynamics of liquid mixtures and its application to liquid alloys. Thermochim. Acta 363, 105e113. https://dx.doi.org/10. 1016/S0040-6031(00)00603-1. Wang, G.Z., 2011. Pro-reduction and Oxidation Refining of Lead Anode Slime Using Oxygen Enriched Bottom Blowing. Master Dissertation. Central South University (in Chinese). Wei, K.X., Ma, W.H., Yang, B., Liu, D.C., Dai, Y.N., Morita, K., 2011. Study on volatilization rate of silicon in multicrystalline silicon preparation from metallurgical grade silicon. Vacuum 85, 749e754. https://doi.org/10.1016/j.vacuum.2010.11. 010. Yang, H.W., Zhang, C., Yang, B., Xu, B.Q., Liu, D.C., 2015. Vapor-liquid phase diagrams of Pb-Sn and Pb-Ag alloys in vacuum distillation. Vacuum 119, 179e184. https:// doi.org/10.1016/j.vacuum. 2015.05.017. Zhang, C., Jiang, W.L., Yang, B., Liu, D.C., Xu, B.Q., Yang, H.W., 2015. Experimental investigation and calculation of vapor-liquid equilibria for Cu-Pb binary alloy in vacuum distillation. Fluid Phase Equilib. 405, 68e72. https://doi.org/10.1016/j. fluid.2015.07.043. Zhang, F., Zeng, K., 2018. Analysis of lead smelting industry situation and counter measures for prevention and treatment of heavy metal pollution. World Nonferr. Metals 9, 1e3 (in Chinese). https://doi.org/10.3969/j.issn.1002-5065.2018. 09.001. Zhang, X., Yang, L., Li, Y., Li, H., Wang, W., Ye, B., 2012. Impacts of lead/zinc mining and smelting on the environment and human health in China. Environ. Monit. Asses. 184, 2261e2273. https://doi.org/10.1007/s10661-011-2115-6. Zhang, Y.W., Deng, J.H., Jiang, W.L., Mei, Q.S., Liu, D.C., 2018. Application of vacuum distillation in refining crude lead. Vacuum 148, 140e148. https://doi.org/10. 1016/j.vacuum.2017.11.004.
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Sustainable extraction of lead and re-use of valuable metals from leadrich secondary materials Bin Yang a, b, d, *, Guozheng Zha a, b, William Hartley c, Xiangfeng Kong a, b, **, Dachun Liu a, b, d, Baoqiang Xu a, b, d, Wenlong Jiang a, b, Xinyu Guo a, b a
National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan, 650093, PR China Faulty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, 68 Wenchang Road, Kunming, Yunnan, 650093, PR China Crop and Environment Sciences Department, Harper Adams University, Newport, Shropshire, TF10 8NB, United Kingdom d State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan, 650093, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 October 2018 Received in revised form 22 January 2019 Accepted 2 February 2019 Available online 10 February 2019
Traditional lead metallurgy, a highly complicated lead extraction process for valuable metal recovery, has exhausted resources and produced a variety of hazardous wastes that are extremely harmful to the environment and unfavorable to the sustainable development of the lead industry. Vacuum metallurgy has become a promising emerging technology contributing to refining primary metals and the recovery of secondary nonferrous metal wastes. The technology is therefore relevant in the context of a transition to a more environmentally friendly circular economy. There is a growing interest in using vacuum distillation for extraction of Pb and valuable metals recovery from lead-rich secondary materials including crude lead, waste Pb-Sn alloy (WPSA) and lead anode slime (LAS). Results indicated that vacuum extracted lead (99.5% Pb) was obtained from crude lead (92.88%) using the two stage high-low temperature vacuum distillation process. Removals of 99.99% for Cu, 99.50% for Sn, and 98.00% for Ag, were achieved respectively. The valuable metals of Ag, Cu and Sn were recovered and concentrated in the final residue. Vacuum separation results of WPSA demonstrated that the purity of Pb reached 99.4% and recovery of Sn was above 86.0%. A novel integrated smelting-vacuum method is proposed here to reuse LAS. Copper and Ag were successfully enriched in the residues whilst Pb was collected in volatiles. The appropriate vacuum process effectively extracted Pb whilst recycling valuable metals, in a sustainable and environmentally friendly manner. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Lead-rich secondary materials Vacuum separation Cleaner extraction Sustainable recovery
1. Introduction Developing a sustainable, economic and novel strategy for Pb extraction and valuable metals recovery from lead-rich secondary materials, is a key objective of the lead industry. Crude lead is mainly produced by the smelting of lead concentrates in sinterblast furnaces and by direct smelting-reduction processes (Kong et al., 2014a,b; Sohn and Olivas-Martinez, 2014). Lead ores
* Corresponding author. National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan, 650093, PR China. ** Corresponding author. National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan, 650093, PR China. E-mail addresses: [email protected] (B. Yang), [email protected] (X. Kong). https://doi.org/10.1016/j.jclepro.2019.02.011 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
contain metals impurities including Cu, Sn, Ag, As, Sb, Bi and Zn (Ojebuoboh et al., 2003), and are typically extracted during the smelting process. Pyro-metallurgical and/or electrolytic processes are used to separate the impurities and obtain purified lead (Kong et al., 2014a,b; Zhang et al., 2018). The volume of refining lead reached 11 million tons in 2016 (Zhang and Zeng, 2018). Currently, pyro-metallurgical processes contribute to approximately 70% of worldwide production. This process commonly includes the following procedures: liquation of Cu and addition of S to remove Cu, addition of NaNO3 to remove As, Sb and Sn, addition of Zn to remove Ag, oxidation, chlorination or vacuum, to remove Zn, addition of Ca and Mg to remove Bi,
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addition of NaOH and NaNO3 to remove the remaining impurities (Peng, 2010; Song, 2011). The process produces residues containing elements such as arsenic (As) which are classified as hazardous wastes (according to the National Hazardous Wastes List) and are of environmental concern (National Hazardous Wastes List, 2016). For example, the addition of NaNO3 creates large volumes of arsenic-contaminated residues (Peng, 2010; Binz and Friedrich, 2015). Therefore, residues require frequent remediation to reduce their potential to pollute water and land. Additionally, freshly formed dust containing heavy metals is harmful to smelting plant operatives. Furthermore, pyro-metallurgical processes have disadvantages such as complex procedures, high-energy consumption and low economic profits (Zhang et al., 2012; Hong et al., 2017). With the rapid development of electronic manufacturing, large volumes of waste Pb-Sn alloys (WPSAs), including waste lead-acid batteries and lead-tin solders, have been abandoned (Ellis and Mirza, 2010; Chen et al., 2018a,b; Meng et al., 2018). WPSAs contain precious metals including Pb, Cu, Sn, Ag, Au and Sb with great economic value, but which can cause serious environmental impacts when incorrectly discarded (Cayumil et al., 2016). The reuse of WPSAs is an important consideration not only for recycling of precious metals, but also for protection of the environment in today's resource-saving society. Two techniques have been developed to separate Pb and Sn from WPSAs. The first method involves solvent extraction, electrolytic deposition, precipitation and cementation, to separate Pb and Sn from the corrosive nitric solutions, producing Pb of approximately 99% purity; tin is precipitated as tin hydroxide (Sn(OH)2) from the solution (Lee et al., 2003). The second method, producing 90% Sn, involves acid leaching (HCl and H2SO4) and alkali precipitation (NaOH) (Castro and Martins, 2009). These methods have advantages, but the leached liquor and residues result in environmental issues. Hence, it is particularly urgent to develop a cleaner technique to recycle Pb and Sn from WPSAs. Electrolytic refining of crude lead can produce lead anode slime (LAS), and the volume of LAS generated is typically 1e2% of the lead produced (Peng, 2010). Most of the metals in LAS exist as oxides and are transformed to metals by reduction smelting. The alloy produced from this process is noble lead. Besides Pb, the main chemical components are rare and precious metals (as metal and alloy phases), including Cu, Ag, Pt, Rh, Ru, etc. with high economic value (Havuz et al., 2010; Wang, 2011). LAS is the primary resource for the complete recovery of precious and rare metals. More importantly, LAS is clearly listed in the National Hazardous Wastes List implemented in 2016, and as yet there is no clean alternative (National Hazardous Wastes List, 2016). The treatment of LAS is usually carried out by pyro-metallurgical processes (fire smelting, pyro-refining and/or electrolytic refining) including reduction smelting, oxidative refining of silver, reduction smelting and refining of antimony, bismuth slag smelting and follow-up electrolytic refining (Wang, 2011; He et al., 2017). The above processes result in the generation of metal dusts and waste residues. The subsequent treatment of these residues is most critical, because they contain large volumes of heavy metals and toxic substances (Chen et al., 2018a,b). Even with a dust collection system, they will inevitably contaminate the local environment through rainfall (Havuz et al., 2010). This is especially of concern for As, as the content of As in residues can reach 5.14% (Lin and Qiu, 2012). Arsenic will accumulate in ecological systems through atmospheric activities, causing soil and water secondary pollution, further harming human health. Additionally, the whole treatment process requires large amounts of energy to liquefy the materials. Our previous work demonstrated that vacuum distillation has been successfully applied to separate metal resources from primary
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metals (crude nickel, crude zinc, crude indium, raw silicon) and secondary wastes (In-Sn alloy, copper anode slime) (Wei et al., 2011; Liu et al., 2012, 2017; Li et al., 2012a, 2012b; Jiang et al., 2013). Vacuum technology has been widely applied to separation, purification and recycling of metals from nonferrous alloys and waste residues, because of its advantages including short process times, high metal recovery, reduced pollution, and low energy consumption, hence eliminating the disadvantages of traditional technologies (Dai and Yang, 2009; Kong et al., 2012; Yang et al., 2015; Zhang et al., 2015). This study describes a novel process, which uses vacuum distillation to efficiently extract Pb and effectively recycle valuable metals from lead-rich secondary materials including crude lead, WPSA and LAS. Calculation and analysis of the separation/recovery effects at saturated vapor pressure, together with the vacuum process required to extract lead and effectively recycle precious metals are considered. Finally, the work attempts to understand elemental behavior following vacuum distillation and its influence on the environment. 2. Material and methods 2.1. Characterization of lead-rich materials Samples of crude lead and LAS were collected from the Yunnan Tin Group (Holding) Company Ltd, China. The samples were placed in wooden boxes and transported to the laboratory. Samples were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and chemical titration for their respective metal contents (Table 1). Samples were also polished to a flat surface, 20 20 mm, and analyzed by X-ray powder diffraction (XRD) using a RIGAKUD/ MAX-2400 with Cu Ka (40 kV) radiation. XRD patterns were collected from 5 to 90 with fine scanning (1 2q min1 scan rate and 0.02 2q step size). XRD phases were identified by means of the Research/Match analysis package (S/M focus on major or minor phases). The XRD results (Fig. 1) indicate that metallic lead from crude lead is the dominant material. Samples of WPSA were collected from various resources, including waste lead-acid batteries and lead-tin solders, in China. These wastes were stripped of any plastic and metalloid materials then crushed and subsequently centrifuged (4000 rpm (15 min)) to collect the heavy metallic substances that settled out. The moist metal samples obtained were oven-dried at 368 K for two days. Lead and Sn were determined by chemical titration. Other elements were analyzed by ICP-AES. The chemical composition of WPSA consists mainly of Pb and Sn (Table 1). 2.2. Vacuum separation experiment 2.2.1. Crude lead and WPSA Samples of homogeneous crude lead (1 kg) and WPSA powder (1 kg) were respectively introduced to graphite crucibles, and placed inside the bottom distillation area of a HZSL-18 vacuum furnace (Dai and Yang, 2009), then vacuumed to a dynamic
Table 1 Chemical compositions of lead-rich materials (mass fraction %). Lead-rich materials
Pb
Cu
Sn
Ag
Zn
As
Sb
Bi
Crude lead WPSAa Noble leadb
92.88 76.30 31.98
2.46 0.01 8.81
0.78 23.2 0.00
0.67 0.02 12.03
0.60 0.00 0.00
1.04 0.00 16.14
1.28 0.33 17.18
0.27 <0.01 7.61
a Ni, Al, Fe, etc. in WPSA were detected, which were not presented and discussed herein. b Noble lead is the direct product of fire reduction smelting of LAS.
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controlled at 973 K, to separate high-volatile substances (Zn, As, etc.). The one-step low temperature distillation process for WPSA was maintained at 1173 K to separate Pb and Sn. 2.2.2. LAS (noble lead) The experimental equipment used for industrial separation of noble lead adopted a continuous vacuum furnace (Fig. 3). The furnace uses ohmic heating, where the vertical graphite heating column is located in the center of the furnace body, and the multistage distillation tower is uniformly arranged on the heating column from top to bottom. During the process, noble lead is completely liquefied and siphoned to the vacuum furnace (Fig. 3). As the smelting material entered the multistage distillation tower from above, lead was distilled, whilst the vapor phase was liquefied in the condenser with Cu, Ag, and Au being continuously moved down to the confluence plate. The remaining residues (non-volatilized) were collected in the residue discharge pot (Fig. 3). 2.3. Method of analysis Fig. 1. XRD pattern collected from crude lead. Copper phase peaks are likely to be presented in crude lead, but were not observed, being affected by the large peaks of Pb. Peaks of other metal phases were also not observed due to their contents lowering the detection limit of XRD.
pressure of 5e15 Pa with the appropriate combination of mechanical and diffusion pumping. A schematic diagram is shown in Fig. 2. Heat was generated through a graphite heater and transferred to a crucible inside the heater by radiation. A graphite crucible, made up of high-density graphite in a cylindrical shape (9 cm OD 8 cm ID 12 cm Ht), was filled with either crude lead and WPSA. The condenser was made of stainless steel to cool the volatiles, and is located approximately 8 cm above the graphite crucible. Vacuum distillation of crude lead was performed in two steps. Firstly, crude lead was subjected to 1373 K to separate and enrich low-volatile metals (Cu, Sn, Ag, etc.). When the furnace temperature cooled completely, the volatiles (mainly Pb) collected in the condenser were then subjected to a low temperature step,
Fig. 2. Schematic diagram of HZSL-18 vacuum distillation furnace.
Volatilized and non-volatilized residues were collected at random at six locations. Samples were then mixed to form a representative concentration. High-content elements were determined by chemical titration. ICP-AES was used to determine lowconcentration elements. 3. Theory and calculation 3.1. Principle of separation Both solid and liquid states of any substance have a tendency to volatilize into a gaseous state, and the gaseous state also has a tendency to condense into a solid state or agglomerate into a corresponding liquid state. Vacuum distillation technology is a typical physical method that exploits this characteristic to separate and purify metals and/or alloys. Each substance has a specific vapor pressure at a specific temperature range, and the vapor pressure contributes significantly to the evaporated characteristics of the substance. Substances with higher vapor pressure generally have greater volatility and will be evaporated earlier as volatiles, whereas substances with lower vapor pressure have less volatility, and will not be volatilized, and remain in the residue to achieve separation of the different materials. This is fundamental for
Fig. 3. Diagram of vertical continuous vacuum furnace.
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separating lead-rich secondary materials including crude lead, WPSA and noble lead, in order to extract lead and recover precious metals with a vacuum environment. 3.2. Saturated vapor pressure determination The different vapor pressures of metal substances in lead-rich secondary materials were theoretically calculated to estimate the feasibility of separation (Dai and Yang, 2009). Results of the relationship between metal pressure and temperature (Fig. 4) indicate that Zn, Sb and As pressures are much higher than those of other elements at the same temperature range, 600e1800 K. Therefore, Zn, Sb and As are preferentially evaporated and separated from lead-rich secondary materials at a relatively low temperature (Fig. 4). In contrast, Ag, Cu and Sn, with low saturation vapor pressure, are more difficult to evaporate, and remain in the residues. It is easy to remove and enrich these precious metals from lead-rich secondary materials at a relatively high temperature (Fig. 4). Meanwhile, reasonable use of a combined two-stage vacuum distillation process can effectively separate and purify Pb from lead-rich secondary materials (Fig. 4). 4. Results and discussion 4.1. Extraction of Pb and recovery of Cu, Sn and Ag from crude lead Quantitative analysis (Table 2) reveals the apparent extraction of Pb from crude lead with various metal impurities (Zn, Sb, As, Ag, Cu, Sn) in the two step high-low temperature vacuum distillation process (the first fraction: high temperature vacuum distillation, HTVD; the second fraction: low temperature vacuum distillation, LTVD). With the exception of Bi, most of the metal impurities (Table 2) have been separated and removed at different stages. The low-volatile impurities of Ag, Cu and Sn separated, and remained in the residue. Copper and tin removal rates reached almost 100% at HTVD whilst silver removal rate reached up to 98% (Table 2). The HTVD process effectively extracted Pb and efficiently removed Ag, Cu and Sn from crude lead (Fig. 4). The removed Ag, Cu and Sn were completely concentrated in the residue, forming a Ag-Cu-Sn alloy at HTVD (Table 3); this can be separated further and recovered for potential reuse. Zinc and As were evaporated as volatiles, being removed from
Fig. 4. Separation predication diagram of the main elements in lead-rich secondary materials.
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the crude lead at LTVD (Zn and As volatilized along with Pb at HTVD). The removal rates of Zn and As were more than 99% (Table 2), owing to their volatile characteristics at the volatile separation area (Fig. 4). Zinc and As impurities decreased further, promoting the purification of Pb. In addition, a large proportion of Sb decreased (Table 2) in crude lead, which resulted in the further increase in lead concentration. The volatilized Zn, As and Sb were enriched forming alloys at LTVD (Table 3). Bismuth is barely separated from crude lead at either HTVD or LTVD. This separation phenomenon can be ascribed to the similar physicochemical characteristics of Bi and Pb. The bismuth vapor pressure is almost similar to that of Pb (Fig. 4). Furthermore, the activity coefficient of Bi in crude lead affects its volatilization and accounts for its behavior separation (Kong et al., 2014b). Following the two stages of HTVD and LTVD, the mass fraction of Cu, Sn and As in vacuum extracted lead was only 0.0026%, 0.0042% and 0.0011%, respectively (Table 2); these meet the requirements of the Pb (99.90) standard product. The mass fraction of Ag, Zn and Sb in vacuum extracted lead was 0.014%, 0.0064% and 0.184%, respectively (Table 2), being greater than that of the Pb standard product and therefore requiring further refining. All impurities in the vacuum extracted lead were in accordance with the requirements for lead anode plate production, however further purification by electrolysis will remove Ag, Zn and Sb. The entire vacuum process only produced two by-products (CuSn-Ag alloy and Zn-As-Sb alloy) that can be either sold and/or recovered. Vacuum distillation results (Tables 2 and 3) revealed that HTVD and LTVD provided an effective and clean method to extract lead from crude lead whilst recycling valuable metals, which can replace the heavily polluted, complex procedure, of traditional pyro-metallurgical processes (Table 3). 4.2. Extraction of Pb and recovery of Sn from WPSA The fluctuating curves indicate separation and extraction of Pb from WPSA with high impurity tin in the vacuum distillation process. The lead curve has a shallow slope, reducing from 99.66% to 99.03% at a distillation time of 20e60 min (Fig. 5 A). For the impurities in extracted lead, the curve has a steep slope, increasing from 0.34% to 0.97% at the same time range (Fig. 5 A). Therefore, a distillation time of 22e42 min may be considered as the optimum extraction period for lead. In this range, lead purity remains over 99.4%, and tin remains at 0.06%. Results from the extracted lead (Fig. 5 A) suggest that much of the Pb in WPSA has been volatilized with only a small volume of Sn. The curves of residual results (Fig. 5 B) also demonstrate this distillation behavior (Pb and Sn) at a distillation time of 22e42 min. Lead mass fraction in the residue decreased from 76.30% to less than 0.8%, and Sn was approximately 86.0% in this range. Comprehensive results of WPSA (Fig. 5A and B) revealed that almost all Sn remained in the final residue. Therefore, WPSA with high tin content can be treated in one step of the vacuum distillation process. Lead and Sn were both recycled. The final leadseparation product is greater than 99.4% extracted lead. The final tin residue can be purified further by additional vacuum distillation. As the main components in WPSA are Pb and Sn, the T-x vaporliquid equilibrium (VLE) phase diagram (Fig. 6) at 5 Pa and 15 Pa was calculated, being based on VLE theory (Zhang et al., 2015) and the molecular interaction volume model (Tao, 2000). This generated a good explanation of the equilibrium state achieved by WPSA distillation. Fig. 6 shows that the temperature range of vapor and liquid lines decreases with a decrease in system pressure, low pressure has a positive effect on the extraction of lead from WPSA by vacuum distillation. The Pb mass fraction in vapor phase is 99.95
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Table 2 Lead extraction from crude lead using a two-stage high-low temperature vacuum distillation process. First stage conditions: distillation temperature (1373 K) and distillation time (30 min) to separate Cu, Sn and Ag. Second stage conditions: distillation temperature (973 K) and distillation time (30 min) to separate Zn, As, and Sb. Elements
Cu Sn Ag Zn As Sb Bi Pb a b
Contents (mass fraction %)
Removal rate (%)
Crude lead
Vacuum extracted lead
2.46 0.78 0.67 0.60 1.04 1.28 0.27 92.88
0.0026 0.0042 0.0144 0.0064 0.0011 0.1840 0.2870 99.50
99.99 99.50 98.00 99.00 99.90 86.56 00.92
(1st stage) (1st stage) (1st stage) (2nd stage) (2nd stage) (2nd stage) (total)
Standard products (mass fraction %) Pb99.90a
Lead anode plateb
0.01 0.005 0.002 0.002 0.01 0.05 0.03 99.90
0.03e0.08 Less than 0.25 0.1e0.4 0.00 0.1e1 0.4e1 0.2e0.04 More than 98.00
Pb99.90, standard product of 99.9% purity lead, and impurities total less than 0.1% (Peng, 2010). Lead anode plate is obtained from the traditional pyro-metallurgical process of crude lead that meets the requirement of further electrolysis refining (Peng, 2010).
Table 3 Effect of vacuum process on the environment and recovery behavior of precious metals in crude lead. Elements
Cu Sn Ag Zn As Sb Bi a b
Vacuum process
Traditional process
Technology
Recovery behavior
Impacts
Technology
Residuea
Impactsb
HTVD HTVD HTVD LTVD LTVD LTVD No separation
Remained in Cu-Sn-Ag alloy
Recovery
Volatilized into Zn-As-Sb alloy
Recovery
Liquation, add Cu Oxidation by NaOH Add Zn Vacuum Oxidation by NaOH Oxidation by NaOH Add Ca and Mg
Cu dross As residue Ag-Zn crust No As residue As residue Bi residue
Hazardous Hazardous Recovery Recovery Hazardous Hazardous Hazardous
No change
Waste residues generated from each step of the traditional pyro-metallurgical process (Peng, 2010; Sohn and Olivas-Martinez, 2014). Impact of residues on the environment according to the National Hazardous Wastes List (National Hazardous Wastes List, 2016).
%e99.97% and in the liquid phase is 5.7 %e18.3% when distillation reaches equilibrium at 1173 K and 5e15 Pa. This theoretical value is in good agreement with the trend of the experimental values. Evaporation rate affects volatilization time and separation efficiency of Pb and Sn. When the WPSA is heated under vacuum, molecules of the metals generated leave the surface of the material and they may encounter other gas molecules. If no collision occurs until a molecule reaches the condenser, Langmuir (Dai and Yang, 2000) postulates that the maximum evaporation rate of the metal will be pachieved. It can be described as umax ¼ 2:624 ffiffiffiffiffiffiffiffiffiffi 102 ap*i M=T , where umax is the maximum evaporation rate, g$cm2$min1; a is the accommodation coefficient, the probability of a distilled atom leaving the surface of the melting material, which is taken as close to unity; pi*is the saturated pressure of the pure element, Pa; M is the molecular weight of the distilled element; T is the melting surface temperature in K. The results of evaporation rates of Pb and Sn in material at 1173 K are 0.4658 g cm2$min1 and 3 105 g cm2$min1, respectively. Therefore, the evaporation rate of Pb is much higher than that of Sn, and Pb will evaporate preferentially in a relatively short time, which is consistent with the calculation results of saturated vapor pressure. Based on the kinetic calculation of evaporation rate, we can conclude that with an increase in distillation time, Pb with a higher evaporation rate in WPSA, will gradually completely volatilize, while Sn will slowly volatilize, resulting in the increase in impurity content of the volatile matter. As shown in Fig. 5 A, lead recovery rate tends to reach a maximum when distillation time exceeds 42 min. Continuing to increase distillation time will only lead to Sn volatilization to form impurities and rapidly reduce the purity of Pb. Consequently, this process can replace the typical solvent extraction-electrowinning-precipitation-cementation and acid leaching-alkaline precipitation procedure (Lee et al., 2003; Castro and Martins, 2009). Recovery of WPSA has been simplified to one
phase by vacuum distillation without the generation of leached liquor and waste residues. Vacuum utilization of WPSA is an obvious improvement, not only for recycling of Pb and Sn, but also for protection of the environment. 4.3. Recovery of valuable metals from noble lead Industrial results (Table 4) reveal a clear enrichment of Cu, Ag, Pb and Bi from noble lead at one stage of the vacuum distillation process. The Cu mass fraction in the residue increased from 8.81% (raw material) to 38.92%, and was reduced to 0.03% in volatiles (Table 4). For Ag, the initial content recorded was 12.03% but was significantly increased to 31.99% (Table 4). The distillation results indicate that Cu and Ag, in noble lead, are basically retained in residues, further suggesting that the preliminary separation and enrichment of Cu and Ag from noble lead has been achieved. This behavior can be attributed to their difficult volatile characteristics at the residual enrichment stage (Fig. 4). Lead and bismuth mass fractions in residues were 0.02% and 0.01%, respectively (Table 4), revealing that large volumes of Pb and Bi have been volatilized. Their contents increased from 31.98% to 67.63% and from 7.61% to 14.65%, respectively (Table 4). Volatile results also verify this phenomenon, owing to their equally volatile characteristics at the volatile separation area (Fig. 4). Both residuals and the volatiles contain a large amount of Sb. Antimony in noble lead alloy formed a number of stable compounds with Ag and Cu which resulted in a decrease in antimony volatile properties (Wang, 2011). Sb is difficult to separate from Ag and Cu under vacuum conditions. Here, results describe a novel vacuum separation procedure for the treatment of noble lead. The conventionally pyro-metallurgical process involving high-energy consumption, heavy pollution and a longer process time may be replaced with the vacuum separation process (Havuz et al., 2010; Wang, 2011; He et al., 2017). Compared
B. Yang et al. / Journal of Cleaner Production 219 (2019) 110e116
(A)
115
Table 4 Compositions of valuable metals recovered from noble lead using industrial vacuum distillation (input quantity of approximately 9 thousand tons). Vacuum conditions: distillation temperature (1173 K), distillation time (7 h) and system pressure (40e60 Pa) remained to recycle Cu, Ag, Pb and Bi. Elements of As, Te and Se were not discussed herein. Materials
Pb
Cu
Ag
Sb
Bi
Noble lead Volatile Residual
31.98 67.63 0.02
8.81 0.03 38.92
12.03 0.74 31.99
17.18 12.26 25.65
7.61 14.65 0.01
suggesting that vacuum distillation can avoid the multiple reduction smelting processes and can significantly reduce the high consumption rates of production. Vacuum treatment of noble lead is a more suitable process for sustainable development of the industry.
(B)
Fig. 5. Effect of distillation time on lead extraction (A) and tin recovery (B) from WPSA under vacuum at 1173 K. Where present, impurity change (Sn) refers to the impurities in vacuum extracted lead, represented by that of Sn (A). Impurity change (Pb) refers to the impurities in residual tin, represented by that of Pb (B). The content of other impurities was less and has been omitted.
5. Conclusions This work presents a novel and clean strategy of vacuum technology for extraction of Pb and recycling of precious metals from lead-rich secondary materials including crude lead, WPSA and noble lead. The product of vacuum extracted lead, in accordance with standards for lead anode plate, was acquired from crude lead using a two-stage, high-low temperature vacuum distillation process. Removals of 99.99% for Cu, 99.50% for Sn, 98.00% for Ag were achieved respectively. The precious metals of Ag, Cu and Sn were concentrated in the final residue. Extraction purity of Pb reached 99.4% and recovery of Sn was more than 86.0% from WPSA by a onestep vacuum distillation process. Copper and Ag in noble lead were successfully enriched in residues, and Pb and Bi were collected in the volatile stage. These findings are significant in that they help to separate valuable metals from lead-rich secondary materials, and benefit decision-making processes for clean recovery of metals by reducing environmental impacts. Acknowledgements This project was funded by the Fund of National Natural Science Foundation of China (grant No. U1502271), the construction of high-level talents of Kunming University of Science and Technology (EmpNo. 20180050), the National Key Research and Development Program of China (grant No. 2016YFC0400404), the Leader in Science and Technology of Yunnan Province (grant No. 2014HA003), and the Program for Nonferrous Metals Vacuum Metallurgy Innovation Team of Ministry of Science and Technology (grant No. 2014RA4018). The authors sincerely acknowledge the anonymous reviewers for their insights and comments to further improve the quality of the manuscript. References
Fig. 6. VLE for the Pb-Sn system under 5 Pa and 15 Pa.
with the traditional procedure, vacuum distillation can shorten the production cycle whilst benefitting the recovery of valuable metals. Furthermore, the treatment process is efficient, simple and clean. The distilled products are alloys, without additional residues,
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