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Recycling rare earth elements from waste cathode ray tube phosphors: Experimental study and mechanism analysis
Accepted Manuscript Recycling rare earth elements from waste cathode ray tube phosphors: Experimental study and mechanism analysis Xiaofei Yin, Xiangmiao Tian, Yufeng Wu, Qijun Zhang, Wei Wang, Bin Li, Yu Gong, Tieyong Zuo PII:
S0959-6526(18)32769-0
DOI:
10.1016/j.jclepro.2018.09.055
Reference:
JCLP 14190
To appear in:
Journal of Cleaner Production
Received Date: 20 December 2017 Revised Date:
23 July 2018
Accepted Date: 7 September 2018
Please cite this article as: Yin X, Tian X, Wu Y, Zhang Q, Wang W, Li B, Gong Y, Zuo T, Recycling rare earth elements from waste cathode ray tube phosphors: Experimental study and mechanism analysis, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.09.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Recycling rare earth elements from waste cathode ray tube phosphors: experimental study and mechanism analysis Xiaofei Yin a,b,#, Xiangmiao Tian c,#, Yufeng Wu a,b,*, Qijun Zhang a, Wei Wang a, Bin Li a,
a
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Yu Gong a, Tieyong Zuo a,b
Institute of Circular Economy, Beijing University of Technology, Beijing, 100124, P. R.
China.
Institute of Beijing-Tianjin-Hebei Green Development, Beijing University of Technology,
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b
Beijing, 100124, P. R. China.
School of Environment, Tsinghua University, Beijing, 100084, P. R. China.
#
These authors contributed equally to this work.
*
Corresponding Author: E-mail: [email protected]; Fax: +86-10-67396234.
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Graphical Abstract
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An efficient recovery process was designed for waste CRT phosphors.
ACCEPTED MANUSCRIPT Recycling rare earth elements from waste cathode ray tube phosphors: experimental study and mechanism analysis Abstract
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In this work, we developed a clean and efficient process for recycling rare earth elements from waste CRT phosphors. A mixture of sulfuric acid and hydrogen peroxide was used for the oxidative leaching of the rare earth elements, and the leaching efficiencies of both Y and Eu reached 99% under the optimal leaching conditions: 3 M H2SO4, 4 vol.% H2O2, 55 °C, and
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1 h. The S2- was oxidized to S0, which efficiently avoided the sulfur pollution produced by existing technologies. The green solvent ionic liquid [OMIm][PF6] and the extractant
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Cyanex272 were employed for the separation of the rare earths. The optimal extraction parameters were determined to be 0.2 mol/L H2SO4, 0.4 vol.% of Cyanex272 in the organic phase, a 10:1 A/O ratio, and an extraction time of 20 min at room temperature, under which, the individual extraction efficiencies of Y, Eu, Zn, and Al were 99%, 87, 8%, and 0%, respectively, and the separation factor of rare earth to Zn reached 593. Additionally, the
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extraction system could be recycled and reused by stripping. The mechanisms of the leaching and extraction process were also analyzed. Based on a kinetics study, the leaching process was found to follow the model of diffusion-control via the product layer, and the apparent activation energies for Y and Eu were calculated to be 75.86 kJ/mol and 77.06 kJ/mol,
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respectively. Meanwhile, a simulated leaching model based on diffusion through the solid S0
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layer was established. The synergistic extraction mechanism was also discussed, and a cation exchange reaction was speculated to occur between the rare earth elements and Cyanex272. The findings in this study are expected to provide an effective and practical method for the recovery and reutilization of waste CRT phosphors. Keywords: waste CRT phosphor, rare earth, recycling, oxidative leaching, ionic liquid based extraction, mechanism analysis.
1. Introduction Cathode ray tubes (CRTs) were one of the most significant components of appliances such as 1
ACCEPTED MANUSCRIPT computer monitors and televisions (TVs) (Singh et al., 2016). However, with the rapid progress in electronics manufacturing technologies, CRTs have been replaced by LCDs and LEDs, which has resulted in the generation of a large number of obsolete CRT computer monitors and TVs every year (Ling and Poon, 2012; Petridis et al., 2016; Xing et al., 2017).
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For instance, approximately 30 million units of scrap CRT television monitors have been produced annually from 2013 to 2016 in China, according to the “White Paper on Waste Electrical and Electronic Equipment (WEEE) Recycling and Comprehensive Utilization Industry in China (2016)”. Additionally, waste CRT appliances represent the main component
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of WEEE, and a large number have been transported to developing countries where labor is relatively cheap, such as India, Pakistan, and China (Lodhia et al., 2017; Vidyadhar, 2016).
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Improper conventional treatments, such as disassembly in small workshops, incineration, and landfilling, have resulted in severe health and environmental consequences (Lecler et al., 2015; Nnorom et al., 2011; Song and Li, 2014, 2015). In the context of developing a circular economy worldwide, it is essential to develop a recycling industry for waste CRTs to avoid these problems. CRT devices are composed of three parts: the neck (5%), funnel (30%), and
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panel (65%); approximately 1-7 g of rare earth (RE) phosphors are coated on the panel. Previously, the collection and recycling of waste CRTs usually focused on dismantling and the reutilization of lead contained in the funnel and panel glass (Lee and Hsi, 2002; Liu et al.,
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2018). However, the waste CRT phosphors were not recycled, but instead discarded, landfilled, or stockpiled as hazardous waste, creating a serious waste of resources and
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environmental pollution (Oguchi et al., 2013; Ruan et al., 2016). Considering the availability of waste CRT phosphors and the instability in the supply of the critical RE raw materials, waste CRT phosphors have been proposed as a potential urban mining resource for RE recovery (Yin et al., 2016; Yin et al., 2018), and exploration of practical recycling technology for waste CRT phosphors cannot be delayed any longer (Dupont and Binnemans, 2015; Wu et al., 2014). Few previous studies have focused on recycling REs from waste CRT phosphors (Innocenzi et al., 2014). Concentrated sulfuric acid treatment followed by a water leaching process has been used to leach REs from waste computer monitor phosphors, but toxic H2S and SO2 were released directly into the air (Resende and Morais, 2010, 2015). Calcination followed by acid 2
ACCEPTED MANUSCRIPT leaching can improve the leaching efficiency of REs, but the energy consumption of this process is high and SO2 pollution cannot be avoided (Innocenzi et al., 2016; Ippolito et al., 2017; Wang et al., 2012; Zhang et al., 2013). REs can be recycled using an oxalate precipitation method, but the purity of the RE products is compromised by the presence of
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impurities such as Zn, Al, Si, Pb, and Ca. Different methods have been used to attempt to solve the problems of impurities. Sodium sulfide has been employed for the selective precipitation of Zn2+, but the co-precipitation of REs was as high as 15-20% (Innocenzi et al., 2013). Pretreatment with dilute acid before RE leaching removed 26.5% of the impurities, but
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this process was complicated and resulted in the loss of about 6% of the REs (Wang et al., 2014). Thus, all the existing methods present drawbacks such as secondary pollution, low
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efficiency, and high energy consumption. Solvent extraction is one of the most popular methods for the separation of rare earth elements (Yang et al., 2013). However, many of the volatile organic solvents used in conventional solvent extraction (for example, sulfonated kerosene) are problematic due to their high volatility, toxicity, and potential for environmental pollution. In contrast, ionic liquids (ILs) are a developing class of alternative green solvents
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with excellent characteristics such as high heat capacity, low vapor pressure, and stabile properties (Baba et al., 2011; Sun et al., 2008).
The process described in this article aims to recycle REs from waste CRT phosphors by
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employing environmentally friendly and circular economy principles. Firstly, the REs were oxidatively leached. Then, an ionic-liquid-based extraction system was employed to purify
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the RE lixivium. During the experiments, we investigated the effects of the main experimental parameters on the leaching and extraction efficiencies of REs. Additionally, the leaching kinetics and reaction mechanisms were explored systematically. The results of this study are expected to be useful to industry and governments to develop cleaner processes for recovering waste CRT phosphors.
2. Materials and Methods 2.1 Materials All the reagents, such as sulfuric acid (≥ 95.0-98.0%) and hydrogen peroxide (30%), used in this study were of the highest available purity, and were purchased from the Chemical 3
ACCEPTED MANUSCRIPT Reagent Company of Beijing. The IL used in this study was supplied by Shanghai chengjie chemical co., Ltd (Shanghai, China). The extraction agent Cyanex272 was purchased from Luoyang Aoda Chemical Co., Ltd (Luoyang, China). Deionized water (18.2 MΩ·cm) obtained from a Direct-Pure Up 10 water system (RephiLe Bioscience, Ltd.) was used
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throughout the experiments whenever needed. The waste CRT phosphors were provided by Green Eco-Manufacture Co., Ltd (GEM, China).
2.2 Equipment
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The powder composition was analyzed using X-ray fluorescence (XRF, PW2403, PANalytical, Holland). The concentrations of the rare earth elements and other metal ions in the liquid were
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determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8000, Perkin Elmer, Shelton, CT 06484, USA). Mineralogical analysis of the samples was carried out by X-ray diffractometry (XRD) with a Bruker AXS D8 Advance instrument using Cu Kα radiation (Bruker, Germany). The morphology of the samples was observed using scanning electron microscopy (SEM, SU-8020, Hitachi, Japan). The separation of the
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aqueous and organic phase was conducted using a centrifuge (H-2050R, Xiang Yi, Hunan, China). The magnetic stirrer used for stirring was purchased from Changzhou Guohua Electric Appliance Co., Ltd. (HJ-4A, Guohua, Changzhou, China).
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2.3 Experimental
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2.3.1 Oxidative Leaching
In most leaching experiments, tapered glass vials (50 mL) were used as the reaction vessels, and 1.0 g of waste CRT phosphors was used in each leaching experiment. The samples were mixed with the desired reagents and stirred at the target temperature with a solid/liquid ratio of 50 g/L for 1 h using the magnetic stirrer. The leachate was filtered into volumetric flasks to determine the RE concentrations in the solution using ICP-OES. All experiments were carried out three times to obtain the average values, and the relative standard deviations were found to be within ± 0.8%.
2.3.2 IL based Extraction 4
ACCEPTED MANUSCRIPT The feed solutions were prepared by diluting the RE leaching solutions to 100 mL, and adjusting their acidity with NaOH and sulfuric acid. The concentrations of metal ions in the feed solutions at different pH values were determined by ICP-OES. Isotope bottles (20 mL) were filled with 5 mL of the feed solution, and a fixed amount of the IL-based extraction
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agent with a certain volume ratio of IL to organic solvent was added. A magnetic rotor was then added to each of the bottles, and the mixtures were stirred at room temperature for 10 min on the magnetic stirrer. The extraction system was then poured into a centrifuge tube and placed in a centrifuge (5000 rpm, 10 min) to separate the aqueous phase and organic phase,
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after which the two phases were separated using a separating funnel. The metal ion concentrations in the raffinate were determined by ICP-OES, and the extraction efficiency
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was calculated by dividing the final concentration by the initial concentration in the feed solution. Then, the organic phase containing the REs was poured into another isotope bottle, and a certain volume of dilute sulfuric acid was added. The experimental operation of stripping process was the same as the above extraction process. The IL-based extraction system was reused for further extraction after stripping. The extraction efficiency (E%),
equations:
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distribution ratio (D), and separation factor (β) can be expressed using the following
E% = [M]
[M]i [M]f
× 100
[M]i [M]f
aq
[M]f
V
× Vaq
(1) (2)
IL
D
β = D1
(3)
2
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D = [M]IL =
[M]i
Where [M]i and [M]f (mol/L) represent the metal concentration in the aqueous phase before and after extraction. Vaq and VIL (mL) are the volumes of the aqueous phase and ionic liquid phase, respectively. D1 and D2 represent the distribution of metal 1 and 2.
3. Results and Discussion 3.1 Synergistic Oxidative Leaching 3.1.1 Leaching of Rare Earths The waste phosphors (consisting of Y2O2S:Eu and ZnS) were collected by the vacuum suction 5
ACCEPTED MANUSCRIPT method; some impurities, such as glass cullet (SiO2, PbO) and aluminum foils (Al), were also present. The powder was screened using a 200 mesh sieve in order to remove these large impurities. Table 1 shows the composition of the waste CRT phosphors before and after screening as determined by X-ray fluorescence (XRF). The phosphor content was enriched to
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over 90%, and the impurities were efficiently removed. Additionally, almost no rare earth compounds were lost. Subsequently, a mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) was employed for the leaching of REs; this method has already been demonstrated in a previous report of ours (Tian et al., 2016). H2O2 is a strong oxidant and an
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environmentally safe reagent (Antonijević et al., 2004), and can effectively improve the leaching efficiency and avoid the generation of the poisonous gas H2S by transforming
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negative bivalent sulfur (S2-) into an elemental sulfur (S0) precipitate (which will be discussed in the next section). Under the optimal leaching conditions (3 M H2SO4, 4 vol.% H2O2, 55 °C, 90 min), the leaching efficiency of both Y and Eu reached 99%. The leaching efficiency of the major impurities, Zn and Al, were 45% and 100%, respectively. Table 1. Composition of the waste CRT phosphors (wt%) Y2O2S:Eu
ZnS
SiO2
PbO
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Component
Al
BaO
K 2O
others
24.1
48.0
7.68
4.86
10.5
0.95
1.04
2.87
After Screening
30.2
60.4
4.3
0.98
2.63
0.325
0.552
0.613
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Before Screening
Temperature was the parameter that had the greatest influence on the leaching of rare earth
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compounds from the waste CRT phosphors. Fig. 1 shows the effect of different temperatures on the leaching efficiency of the rare earth compounds. As can be seen, the leaching efficiencies and leaching rates of Y and Eu increased obviously with increasing temperature, and both Y and Eu showed the same leaching trends. When the temperature was raised to 55 °C, the leaching efficiency of Y and Eu reached 94.4% and 93.7% after 60 min, respectively, and then plateaued. Further increasing the temperature to 60 °C resulted in only a small increase in the leaching efficiency of the rare earth elements after 60 min, although the leaching rate was higher than at 50 °C at times of less than 20 min. On the other hand, higher temperature is known to promote the decomposition of H2O2 (Dimitrijevic et al., 1996), 6
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and also increased the energy consumption.
Fig. 1. Effect of temperature on the leaching of rare earth elements from waste CRT phosphors: (a)
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Leaching efficiency of Y and (b) leaching efficiency of Eu.
3.1.2 Leaching Kinetics Analysis
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The dissolution of the waste CRT phosphors by sulfuric acid and hydrogen peroxide is a typical liquid-solid reaction, and the leaching efficiency of the REs was analyzed using a
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shrinking core model for kinetic analysis (Innocenzi et al., 2017). In the present work, solid elemental sulfur was generated during the RE leaching process; this reaction can be expressed as follows:
Afluid + Bparticle → P1, solid +P2, aqueous
(4)
Generally, the following three kinetic equations can be applied for different rate-controlling steps (Randhawa et al., 2015; Wadsworth and Miller, 1979; Wu et al., 2015). If the leaching process is controlled by diffusion through a liquid boundary layer, its integrated rate equation will follow Equation (5). If the leaching process is controlled by a chemical reaction, its integrated rate equation will follow Equation (6). If the leaching process is controlled by 7
ACCEPTED MANUSCRIPT diffusion through the product layer, its integrated rate equation will follow Equation (7). (a) Process controlled by diffusion through a liquid boundary layer:
x = kl t
(5)
(b) Process controlled by a chemical reaction: 1
1 1 x3 = kc t (c) Process controlled by diffusion through the product layer: 2
2 3
1 x 1 x3 = kd t
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(6)
(7)
calculated from Equations (5)-(7), respectively.
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where x is the leaching efficiency of the REs, and kl, kc, and kd are the apparent rate constants
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The leaching efficiencies of Y and Eu in the temperature range from 30 to 60 °C were plotted against the reaction time according to Equations (5)-(7), respectively. As shown in Fig. 2, the correlation coefficient (R2) indicated the good linear fit of the data to the model involving diffusion through the product layer for both Y and Eu, indicating that Equation (7) was most suitable for representing the kinetics of the RE leaching process using H2SO4 in the presence
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of H2O2.
Fig. 2. RE dissolution ratio versus time at various temperatures, fitted by the equations corresponding to reactions controlled by (a) diffusion through a liquid boundary layer for Y, (b) chemical reaction for Y, and (c) diffusion through a product layer for Y; (d) diffusion through a liquid boundary layer for Eu, (e) chemical reaction for Eu, and (f) diffusion through a product layer for Eu.
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Ea RT
(8)
gas constant, and T is the absolute temperature.
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where Ea is the apparent activation energy, A0 is the pre-exponential factor, R is the universal
The natural logarithm of the reaction rate constant (ln k) at different temperatures calculated using the model of diffusion through a product layer was plotted against the reciprocal of
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absolute temperature (1/T) (Fig. 3), and the apparent activation energies Ea for the leaching reactions of Y and Eu were calculated to be 75.86 kJ/mol and 77.06 kJ/mol, respectively,
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based on Equation (8). Actually, both Y and Eu were contained in the same crystal structure, Y2O2S:Eu (Thirumalai et al., 2007), so they followed the same leaching dynamics model and their apparent activation energies were very similar. The values of 75.86 kJ/mol and 77.06 kJ/mol were unusual, as they were significantly higher than the maximum theoretical value for a reaction controlled by diffusion through the product layer (Sun et al., 2008). This may be
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because the produced sulfur layer was very dense, and the complete reaction system contained ZnS and other impurities as well as Y2O2S:Eu; both these factors could lead to a high activation energy. Many other researchers also have reported high activation energies for
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processes controlled by diffusion through a product layer. For example, Wadswork found that the reaction of ferric sulfate with chalcopyrite followed the model of a process controlled by
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diffusion through a product layer control, and had an activation energy as high as 83.72 kJ/mol (Wadsworth and Miller, 1979). Other high energy values, such as 48.9 kJ/mol and 53.7 kJ/mol (Tsuchida et al., 1982), 62 kJ/mol to 79 kJ/mol (Paspaliaris and Tsolakis, 1987), 67.1 kJ/mol and 62.4 kJ/mol (Olanipekun, 1999), 67.1 kJ/mol and 50 kJ/mol (Gbor et al., 2000), and 46.32 kJ/mol (Martínez-Luévanos et al., 2011) have also been found for various diffusion-controlled processes. Thus, it was speculated that the rate control model was better predicted by the final correlation coefficient of the reaction equations or by models of complexed heterogeneous dissolution reactions rather than the value of the apparent activation energy. 9
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Fig. 3. Arrhenius plots for the leaching of Y and Eu.
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3.1.3 Reaction Mechanism Discussion
Fig. 4 (a) and (b) show the SEM images of waste CRT phosphors before and after synergistic oxidative leaching. It is obvious that the bulky particles shrank, and their surfaces became
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coarse and seemed to be covered with a thin film. XRD analysis (Fig. S1) illustrated that solid elemental sulfur was generated during the leaching process, and the reaction can be expressed
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by Equation (9).
Y, Eu2 O2 S + 6H+ + H2 O2 → 2Y3+ Eu3+ + 4H2 O + S↓
(9)
Based on the experimental results and the kinetics analysis, a simulated reaction model was established according to the classical theory of the shrinking core model (Wadsworth and Miller, 1979) as shown in Fig. 4 (c-e). Firstly, the Y2O2S:Eu phosphor particles were considered to be spherical and compact as shown in Fig. 4 (c). When the particles came into contact with the lixivium containing H2SO4 and H2O2, the redox reaction took place and solid sulfur began be generated. As the reaction continued, an increasing amount of Y2O2S:Eu was dissolved into the liquid phase, and the particle size was reduced. Fig. 4 (d) shows the 10
ACCEPTED MANUSCRIPT intermediate stage of the leaching process, in which a solid layer of the sulfur product has been generated (layer B); the outer layer is usually referred to as the boundary layer (layer C), in which the concentration of the ions is different from the liquid environment. In our work, stirring was employed during all the leaching experiments, which enhanced mass transfer
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between the boundary layer and the liquid environment. Thus, the diffusion of H+ and H2O2 into the boundary layer was not considered to be the main factor controlling the leaching process, in agreement with the results shown in Fig. 2 (a) and (d). The sulfur product layer shown in Fig. 4 (d) was not very permeable and covered the surface of the unreacted core
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(layer A). Thus, the inner diffusion of H+ and H2O2 through the product layer of sulfur into the unreacted core became the determining factor influencing the reaction rate, in agreement with
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the results shown in Fig. 2 (c) and (f). Finally, the inner core of Y2O2S:Eu was completely
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transformed into RE ions and solid sulfur (Fig. 4e).
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Fig. 4. Reaction mechanism analysis. (a) SEM image of the waste CRT phosphor and (b) SEM image of the waste CRT phosphor after leaching. (c-e) Simulated reaction model: (c) early stage, (d) intermediate stage, and (e) end stage.
3.2 IL based Extraction
After oxidative leaching, impurities such as Pb, Ba, and Ca were separated by precipitation in an aqueous solution containing a high concentration of sulfate radicals. Zn2+ and Al3+ are considered the most problematic elements in terms of purity in RE recycling, because their precipitation behavior is similar to that of REs. In this work, the feed solution for extraction was composed of Y3+ (25.2 mmol/L), Eu3+ (1.0 mmol/L), Zn2+ (28.5 mmol/L), Al3+ (5.6 11
ACCEPTED MANUSCRIPT mmol/L) as well as other inessential ions such as K+. The purpose of the work described in this section was to separate Y and Eu from the waste CRT phosphor powder leachate using an IL-based extraction system containing the ionic liquid [OMIm][PF6] and the extractant
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Cyanex 272 (Fig. 5).
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Fig. 5. Molecular structures of the extractant and ionic liquid.
3.2.1 Optimization of the Extraction Parameters Fig. 6 (a) shows the effect of the concentration of the acid H2SO4 on the extraction efficiency of the different metals. The acidity clearly had a significant influence on the separation of REs.
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The extraction agent Cyanex272 is a weak organic acid (HA), and the metal ions (M+)
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displaced the hydrogen ions (H+) and were extracted by Cyanex272, which will be discussed in the following mechanism analysis section. Thus, high acidity was unfavorable for highly efficient extraction due to its effects on the reaction equilibrium. As the concentration of the acid H2SO4 was increased from 0.1 mol/L to 0.2 mol/L, the extraction efficiencies of Y and Eu decreased slightly from 99% and 95% to 96% and 92%, respectively. In contrast, the extraction efficiency of Zn decreased substantially from 72% to 37%, and the extraction of Al was still nearly zero. The extraction efficiency of the REs decreased rapidly as the acidity was further increased. Thus, the optimal acid concentration for highly efficient extraction of the rare earth elements and the separation of impurities was determined to be 0.2 mol/L, and this concentration was used in subsequent experiments. 12
ACCEPTED MANUSCRIPT The effect of the volume ratio (vol.%) of Cyanex272 in the organic phase on the extraction was investigated at an acid concentration of 0.2 mol/L. As shown in Fig. 6 (b), the extraction efficiencies of Y, Eu, and Zn increased with an increasing ratio of Cyanex272, while Al showed the opposite trend. When the vol.% of Cyanex272 was increased from 0 to 0.4, the
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extraction efficiency of Y, Eu, and Zn reached 95%, 92%, and 21%, respectively, while the Al efficiency dropped to less than 1%. Further increasing the volume of Cyanex272 increased the extraction efficiency of Y and Eu to 99.5% and 95%, but the extraction of Zn also increased to approximately 40%, which was unfavorable to the separation. To achieve good extraction
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of Y and Eu while minimizing the content of Zn and Al, a ratio of 0.4 vol.% Cyanex272 was selected.
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Another important factor to be optimized was the volume ratio of the aqueous phase to organic phase (A/O). Fig. 6 (c) shows the effects of the A/O on the extraction efficiencies of the different metals. At a phase ratio of 1:1, the extraction ratios of Y, Eu, and Zn were 99.9%, 97% and 65%, respectively. When the A/O was increased to 10:1, a slight decline in the extraction efficiency of Y and Eu occurred (to 94.5% and 86.5%), while the extraction
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efficiency of Zn fell sharply from 65% to around 10%. Thus, optimizing the phase ratio could enhanced the separation of the REs from Zn. However, increasing the phase ratio further resulted in a great reduction of the extraction efficiency for Eu. Thus, 10:1 was chosen as the
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optimal A/O phase ratio.
Fig. 6 (d) illustrates the effect of the extraction temperature on the extraction. Interestingly,
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temperature did not have a significant influence on the extraction efficiencies of Y and Eu, with only a 3% increase being observed when the temperature was increased from 25 °C to 55 °C. In contrast, the extraction efficiency of Zn increased approximately 27%. Thus, to minimize impurities and reduce energy consumption, the extraction temperature was set as 25 °C (room temperature). Finally, we investigated the effect of extraction time on the extraction efficiency of Y, Eu, Zn, and Al. As shown in Fig. 6 (e), the extraction efficiency of Y and Eu increased from 83% and 56% to 99% and 87% respectively when the extraction time was extended from 1 min to 20 min. Zn showed the opposite trend, with the extraction efficiency dropping from 13% to 8%, and the extraction of Al was negligible in this system. After 20 min, the extraction system 13
ACCEPTED MANUSCRIPT tended to reach equilibrium. Fig. 6 (f) summarizes the separation factors of the REs to zinc during the optimization of the extraction conditions corresponding to Fig. 6 (a-e). As can be seen, the separation factors for both Y and Eu to Zn increased as parameters were optimized. Although the extraction
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efficiency of Eu decreased from 92% to 87% during the optimization process, the separation factor showed an obvious increase from 19 to 76. The separation factor of total REs to zinc increased to 593 using the optimal extraction parameters. Using these optimized parameters, the individual extraction efficiencies of Y, Eu, Zn, and Al were 99%, 87, 8% and 0%,
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respectively. Thus, the impurities were efficiently removed by the above extraction system.
Fig. 6. Optimization of the IL-based extraction process. (a) Effect of the acid (H2SO4) concentration; (b)
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effect of the volume ratio of Cyanex272 in organic phase; (c) effect of the volume ratio of the aqueous phase to the organic phase; (d) effect of the extraction temperature; (e) effect of the extraction time; and (f) separation factors of the rare earths to zinc during the optimization of the extraction conditions.
3.2.2 Discussion of the Synergistic Extraction Mechanism Cyanex272 was proven to play a critical role in the extraction of the REs, as both Y and Eu were barely extracted by the IL alone (Fig. 6b). Thus, we speculated that in this extraction system, a chemical reaction took place between the RE ions and Cyanex272, and that [OMIm][PF6] acted as a diluent for promoting the dissolution of the extraction reaction products. To further confirm the extraction mechanism, the common slope analysis method 14
ACCEPTED MANUSCRIPT and the constant molar method were employed. Firstly, a Cyanex272-only system was tested in order to determine the equation of the extraction reaction. Fig. 7 (a) shows the relationship between the logarithm of the distribution ratio (Log D) and the logarithm of acidity (-Log [H+]) in the aqueous phase. Straight lines with slopes of 2.96 and 3.07 were obtained from the
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plots, which indicated that three moles of H+ were released to extract a mole of Y3+ or Eu3+. The plots of Log (D) vs. log ([Cyanex272]) also showed straight lines with a slope of 3 for both Y and Eu, as shown in Fig. 7 (b). Meanwhile, the effect of SO42- was tested by adding different concentrations of Na2SO4, and the results indicated that SO42- had no influence on
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the extraction. Thus, the extraction equation for Y and Eu can be expressed as Equation 10, where HA represents Cyanex272 and RE represents Y or Eu. Other researchers also found
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similar results for the mechanism of RE extraction by Cyanex272 (Jiao et al., 2009; Liao et al., 2010). Fig. 7 (c) shows plots of Log (D) vs. -Log ([H+]) in the [OMIm][PF6] + Cyanex272 system. The slope of 3 for Y and Eu indicated that the reaction mechanism was unchanged in the mixed system, and the ionic liquid [OMIm][PF6] acted as a diluent. The extraction mechanism proposed here still requires deeper study in our future works. (10)
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RE3+ + 3HA → REA3 + 3H+
Fig. 7. Analysis of the extraction mechanism. (a) Log (D) vs. -Log ([H+]) in the Cyanex272-only extraction system; (b) Log (D) vs. Log ([Cyanex272]) in the Cyanex272-only extraction system; and (c) Log (D) vs. Log ([H+]) in the [OMIm][PF6] + Cyanex272 extraction system.
3.2.3 Stripping and Recycling We compared the stripping efficiencies of different acids (HCl, H2SO4, and HNO3), and concluded that H2SO4 was the most efficient in stripping Y and Eu. A concentration of 3 mol/L H2SO4 was employed in the experiments. Fig. 8 (a) shows the effect of different 15
ACCEPTED MANUSCRIPT volume ratios of the organic phase and aqueous phase (O/A) on the RE stripping efficiency. As the O/A was increased, the Y and Eu stripping efficiencies both obviously increased, and the individual Y and Eu stripping efficiencies reached 93.4% and 96.7, respectively, at an O/A of 1:10. As the proportion of the aqueous phase was further increased, the stripping efficiency
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remained stable. However, use of large volumes of stripping agent would dilute the target metals and require the use of greater quantities of the chemicals. Thus, an O/A ratio of 1:5, at which 83% and 90% of Y and Eu can be stripped out, was determined to be optimal from economic and environmental viewpoints. Finally, the REs can be recycled by simple
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precipitation. Additionally, after three cycles of stripping, the IL-based extraction system can be reused in further extractions. As shown in Fig. 8 (b), the [OMIm][PF6] + Cyanex272
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extraction system retained a relatively high extraction ability after five cycles, and little
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decline in the extraction efficiency was observed.
Fig. 8. Stripping of REs and recyclability of the IL-based extraction system. (a) Effect of the O/A ratio on the stripping efficiency of the REs; and (b) RE extraction efficiency of the extraction system during repeated cycles.
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4. Conclusion In this research, an efficient combined process consisting of oxidative leaching and IL-based
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extraction was investigated for the recycling of REs from waste CRT phosphors. The recycling parameters were optimized, and the reaction mechanisms of the process were explored systematically. The conclusions can be summarized as follows:
(1) A mixed lixiviate containing H2SO4 and H2O2 was used in RE leaching for its synergistic
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effects. Temperature was shown to significantly influence the leaching. The leaching efficiencies of both Y and Eu reached 99% under the optimal leaching conditions: 3 M
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H2SO4, 4 vol.% H2O2, 55 °C, and 1 h. The S2- was oxidized to S0, which efficiently avoided the sulfur pollution produced by existing technologies.
(2) According to a kinetics study using the shrinking core model, the leaching followed the model of a process controlled by diffusion through the product layer. The apparent activation energies were calculated to be 75.86 kJ/mol and 77.06 kJ/mol for Y and Eu,
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respectively. The leaching reaction equation was determined, and RE3+ and S0 were found to be the main reaction products. Additionally, a simulated reaction model based on diffusion through the S0 layer was established.
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(3) The green solvent ionic liquid [OMIm][PF6] and the extractant Cyanex272 were employed for the separation of the REs from the lixivium. The optimal extraction
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parameters were determined to be 0.2 mol/L H2SO4, 0.4 vol.% of Cyanex272 in the organic phase, a 10:1 A/O ratio, and an extraction time of 20 min at room temperature. Under these conditions, the individual extraction efficiencies of Y, Eu, Zn, and Al were 99%, 87, 8%, and 0%, respectively, and the rare earth to Zn separation factor reached 593. The REs could then be stripped using 3 mol/L H2SO4 at an O/A ratio of 1:5. Additionally,
reuse of the extraction system was considered to be possible. (4) The synergistic extraction mechanism was discussed, and a chemical reaction was speculated to take place between the RE ions and Cyanex272, with [OMIm][PF6] acting as a diluent. The reaction between the REs and Cyanex272 was a cation exchange. However, the extraction mechanism requires further investigation. 17
ACCEPTED MANUSCRIPT Acknowledgements This research was supported by Beijing Natural Science Foundation (2182009, 2174065, 2174067), Beijing Social Science (No. 17YJA001), National Key R&D Plan — Comprehensive Evaluation on Environmental and Economic Performance of Building
Academician Workstation in Yunnan Province.
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Materials Made from Industrial and Urban Solid Wastes (2017YFC0703206-03), and the
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Antonijević, M.M., Janković, Z.D., Dimitrijević, M.D., 2004, Kinetics of Chalcopyrite Dissolution by Hydrogen Peroxide in Sulphuric Acid. Hydrometallurgy. 71, 329-334. Baba, Y., Kubota, F., Kamiya, N., Goto, M., 2011, Recent Advances in Extraction and Separation of Rare-Earth Metals Using Ionic Liquids. J. Chem. Eng. Jpn. 44, 679-685. Dimitrijevic, M., Antonijevic, M.M., Jankovic, Z., 1996, Kinetics of Pyrite Dissolution by Hydrogen Peroxide in Perchloric Acid. Hydrometallurgy. 42, 377-386. Dupont, D., Binnemans, K., 2015, Recycling of Rare Earths from NdFeB Magnets Using a Combined Leaching/Extraction System Based on the Acidity and Thermomorphism of the ionic liquid [Hbet][Tf2N]. Green Chem. 17, 2150-2163. Gbor, P.K., Ahmed, I.B., Jia, C.Q., 2000, Behaviour of Co and Ni During Aqueous Sulphur Dioxide Leaching of Nickel Smelter Slag. Hydrometallurgy. 57, 13-22. Innocenzi, V., De Michelis, I., Ferella, F., Vegliò, F., 2017, Leaching of Yttrium from Cathode Ray Tube Fluorescent Powder: Kinetic Study and Empirical Models. Int. J. Miner. Process. 168, 76-86. Innocenzi, V., De Michelis, I., Ferella, F., Beolchini, F., Kopacek, B., Vegliò, F., 2013, Recovery of Yttrium from Fluorescent Powder of Cathode Ray Tube, CRT: Zn Removal by Sulphide Precipitation. Waste Manage. 33, 2364-2371. Innocenzi, V., De Michelis, I., Kopacek, B., Vegliò, F., 2014, Yttrium Recovery from Primary and Secondary Sources: A Review of Main Hydrometallurgical Processes. Waste Manage. 34, 1237-1250. Innocenzi, V., Ippolito, N.M., De Michelis, I., Medici, F., Vegliò, F., 2016, A Hydrometallurgical Process for the Recovery of Terbium from Fluorescent Lamps: Experimental Design, Optimization of Acid Leaching Process and Process Analysis. J. Environ. Manage. 184, 552-559. Ippolito, N.M., Innocenzi, V., De Michelis, I., Medici, F., Vegliò, F., 2017, Rare Earth Elements Recovery from Fluorescent Lamps: A New Thermal Pretreatment to Improve the Efficiency of the Hydrometallurgical Process. J. Clean. Prod. 153, 287-298. Jiao, Y., Liao, C., Nie, H., Jiang, P., Liang, Y., 2009, Adsorption Mechanism of Heavy Rare Earth by Cyanex272 Impregnated Resin. Chin. J. Proc. Eng. (Chinese), 1099-1102. Lecler, M., Zimmermann, F., Silvente, E., Clerc, F., Chollot, A., Grosjean, J., 2015, Exposure to Hazardous Substances in Cathode Ray Tube (CRT) Recycling Sites in France. Waste Manage. 39, 226-235. 18
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Highlights REs were recycled efficiently by oxidative leaching and IL based extraction.
•
The leaching kinetics was investigated systematically.
•
The reaction mechanisms of the leaching and extraction process were
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•
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analyzed.
S0959-6526(18)32769-0
DOI:
10.1016/j.jclepro.2018.09.055
Reference:
JCLP 14190
To appear in:
Journal of Cleaner Production
Received Date: 20 December 2017 Revised Date:
23 July 2018
Accepted Date: 7 September 2018
Please cite this article as: Yin X, Tian X, Wu Y, Zhang Q, Wang W, Li B, Gong Y, Zuo T, Recycling rare earth elements from waste cathode ray tube phosphors: Experimental study and mechanism analysis, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.09.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Recycling rare earth elements from waste cathode ray tube phosphors: experimental study and mechanism analysis Xiaofei Yin a,b,#, Xiangmiao Tian c,#, Yufeng Wu a,b,*, Qijun Zhang a, Wei Wang a, Bin Li a,
a
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Yu Gong a, Tieyong Zuo a,b
Institute of Circular Economy, Beijing University of Technology, Beijing, 100124, P. R.
China.
Institute of Beijing-Tianjin-Hebei Green Development, Beijing University of Technology,
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b
Beijing, 100124, P. R. China.
School of Environment, Tsinghua University, Beijing, 100084, P. R. China.
#
These authors contributed equally to this work.
*
Corresponding Author: E-mail: [email protected]; Fax: +86-10-67396234.
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Graphical Abstract
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An efficient recovery process was designed for waste CRT phosphors.
ACCEPTED MANUSCRIPT Recycling rare earth elements from waste cathode ray tube phosphors: experimental study and mechanism analysis Abstract
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In this work, we developed a clean and efficient process for recycling rare earth elements from waste CRT phosphors. A mixture of sulfuric acid and hydrogen peroxide was used for the oxidative leaching of the rare earth elements, and the leaching efficiencies of both Y and Eu reached 99% under the optimal leaching conditions: 3 M H2SO4, 4 vol.% H2O2, 55 °C, and
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1 h. The S2- was oxidized to S0, which efficiently avoided the sulfur pollution produced by existing technologies. The green solvent ionic liquid [OMIm][PF6] and the extractant
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Cyanex272 were employed for the separation of the rare earths. The optimal extraction parameters were determined to be 0.2 mol/L H2SO4, 0.4 vol.% of Cyanex272 in the organic phase, a 10:1 A/O ratio, and an extraction time of 20 min at room temperature, under which, the individual extraction efficiencies of Y, Eu, Zn, and Al were 99%, 87, 8%, and 0%, respectively, and the separation factor of rare earth to Zn reached 593. Additionally, the
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extraction system could be recycled and reused by stripping. The mechanisms of the leaching and extraction process were also analyzed. Based on a kinetics study, the leaching process was found to follow the model of diffusion-control via the product layer, and the apparent activation energies for Y and Eu were calculated to be 75.86 kJ/mol and 77.06 kJ/mol,
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respectively. Meanwhile, a simulated leaching model based on diffusion through the solid S0
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layer was established. The synergistic extraction mechanism was also discussed, and a cation exchange reaction was speculated to occur between the rare earth elements and Cyanex272. The findings in this study are expected to provide an effective and practical method for the recovery and reutilization of waste CRT phosphors. Keywords: waste CRT phosphor, rare earth, recycling, oxidative leaching, ionic liquid based extraction, mechanism analysis.
1. Introduction Cathode ray tubes (CRTs) were one of the most significant components of appliances such as 1
ACCEPTED MANUSCRIPT computer monitors and televisions (TVs) (Singh et al., 2016). However, with the rapid progress in electronics manufacturing technologies, CRTs have been replaced by LCDs and LEDs, which has resulted in the generation of a large number of obsolete CRT computer monitors and TVs every year (Ling and Poon, 2012; Petridis et al., 2016; Xing et al., 2017).
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For instance, approximately 30 million units of scrap CRT television monitors have been produced annually from 2013 to 2016 in China, according to the “White Paper on Waste Electrical and Electronic Equipment (WEEE) Recycling and Comprehensive Utilization Industry in China (2016)”. Additionally, waste CRT appliances represent the main component
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of WEEE, and a large number have been transported to developing countries where labor is relatively cheap, such as India, Pakistan, and China (Lodhia et al., 2017; Vidyadhar, 2016).
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Improper conventional treatments, such as disassembly in small workshops, incineration, and landfilling, have resulted in severe health and environmental consequences (Lecler et al., 2015; Nnorom et al., 2011; Song and Li, 2014, 2015). In the context of developing a circular economy worldwide, it is essential to develop a recycling industry for waste CRTs to avoid these problems. CRT devices are composed of three parts: the neck (5%), funnel (30%), and
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panel (65%); approximately 1-7 g of rare earth (RE) phosphors are coated on the panel. Previously, the collection and recycling of waste CRTs usually focused on dismantling and the reutilization of lead contained in the funnel and panel glass (Lee and Hsi, 2002; Liu et al.,
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2018). However, the waste CRT phosphors were not recycled, but instead discarded, landfilled, or stockpiled as hazardous waste, creating a serious waste of resources and
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environmental pollution (Oguchi et al., 2013; Ruan et al., 2016). Considering the availability of waste CRT phosphors and the instability in the supply of the critical RE raw materials, waste CRT phosphors have been proposed as a potential urban mining resource for RE recovery (Yin et al., 2016; Yin et al., 2018), and exploration of practical recycling technology for waste CRT phosphors cannot be delayed any longer (Dupont and Binnemans, 2015; Wu et al., 2014). Few previous studies have focused on recycling REs from waste CRT phosphors (Innocenzi et al., 2014). Concentrated sulfuric acid treatment followed by a water leaching process has been used to leach REs from waste computer monitor phosphors, but toxic H2S and SO2 were released directly into the air (Resende and Morais, 2010, 2015). Calcination followed by acid 2
ACCEPTED MANUSCRIPT leaching can improve the leaching efficiency of REs, but the energy consumption of this process is high and SO2 pollution cannot be avoided (Innocenzi et al., 2016; Ippolito et al., 2017; Wang et al., 2012; Zhang et al., 2013). REs can be recycled using an oxalate precipitation method, but the purity of the RE products is compromised by the presence of
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impurities such as Zn, Al, Si, Pb, and Ca. Different methods have been used to attempt to solve the problems of impurities. Sodium sulfide has been employed for the selective precipitation of Zn2+, but the co-precipitation of REs was as high as 15-20% (Innocenzi et al., 2013). Pretreatment with dilute acid before RE leaching removed 26.5% of the impurities, but
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this process was complicated and resulted in the loss of about 6% of the REs (Wang et al., 2014). Thus, all the existing methods present drawbacks such as secondary pollution, low
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efficiency, and high energy consumption. Solvent extraction is one of the most popular methods for the separation of rare earth elements (Yang et al., 2013). However, many of the volatile organic solvents used in conventional solvent extraction (for example, sulfonated kerosene) are problematic due to their high volatility, toxicity, and potential for environmental pollution. In contrast, ionic liquids (ILs) are a developing class of alternative green solvents
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with excellent characteristics such as high heat capacity, low vapor pressure, and stabile properties (Baba et al., 2011; Sun et al., 2008).
The process described in this article aims to recycle REs from waste CRT phosphors by
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employing environmentally friendly and circular economy principles. Firstly, the REs were oxidatively leached. Then, an ionic-liquid-based extraction system was employed to purify
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the RE lixivium. During the experiments, we investigated the effects of the main experimental parameters on the leaching and extraction efficiencies of REs. Additionally, the leaching kinetics and reaction mechanisms were explored systematically. The results of this study are expected to be useful to industry and governments to develop cleaner processes for recovering waste CRT phosphors.
2. Materials and Methods 2.1 Materials All the reagents, such as sulfuric acid (≥ 95.0-98.0%) and hydrogen peroxide (30%), used in this study were of the highest available purity, and were purchased from the Chemical 3
ACCEPTED MANUSCRIPT Reagent Company of Beijing. The IL used in this study was supplied by Shanghai chengjie chemical co., Ltd (Shanghai, China). The extraction agent Cyanex272 was purchased from Luoyang Aoda Chemical Co., Ltd (Luoyang, China). Deionized water (18.2 MΩ·cm) obtained from a Direct-Pure Up 10 water system (RephiLe Bioscience, Ltd.) was used
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throughout the experiments whenever needed. The waste CRT phosphors were provided by Green Eco-Manufacture Co., Ltd (GEM, China).
2.2 Equipment
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The powder composition was analyzed using X-ray fluorescence (XRF, PW2403, PANalytical, Holland). The concentrations of the rare earth elements and other metal ions in the liquid were
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determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8000, Perkin Elmer, Shelton, CT 06484, USA). Mineralogical analysis of the samples was carried out by X-ray diffractometry (XRD) with a Bruker AXS D8 Advance instrument using Cu Kα radiation (Bruker, Germany). The morphology of the samples was observed using scanning electron microscopy (SEM, SU-8020, Hitachi, Japan). The separation of the
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aqueous and organic phase was conducted using a centrifuge (H-2050R, Xiang Yi, Hunan, China). The magnetic stirrer used for stirring was purchased from Changzhou Guohua Electric Appliance Co., Ltd. (HJ-4A, Guohua, Changzhou, China).
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2.3 Experimental
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2.3.1 Oxidative Leaching
In most leaching experiments, tapered glass vials (50 mL) were used as the reaction vessels, and 1.0 g of waste CRT phosphors was used in each leaching experiment. The samples were mixed with the desired reagents and stirred at the target temperature with a solid/liquid ratio of 50 g/L for 1 h using the magnetic stirrer. The leachate was filtered into volumetric flasks to determine the RE concentrations in the solution using ICP-OES. All experiments were carried out three times to obtain the average values, and the relative standard deviations were found to be within ± 0.8%.
2.3.2 IL based Extraction 4
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agent with a certain volume ratio of IL to organic solvent was added. A magnetic rotor was then added to each of the bottles, and the mixtures were stirred at room temperature for 10 min on the magnetic stirrer. The extraction system was then poured into a centrifuge tube and placed in a centrifuge (5000 rpm, 10 min) to separate the aqueous phase and organic phase,
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after which the two phases were separated using a separating funnel. The metal ion concentrations in the raffinate were determined by ICP-OES, and the extraction efficiency
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was calculated by dividing the final concentration by the initial concentration in the feed solution. Then, the organic phase containing the REs was poured into another isotope bottle, and a certain volume of dilute sulfuric acid was added. The experimental operation of stripping process was the same as the above extraction process. The IL-based extraction system was reused for further extraction after stripping. The extraction efficiency (E%),
equations:
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distribution ratio (D), and separation factor (β) can be expressed using the following
E% = [M]
[M]i [M]f
× 100
[M]i [M]f
aq
[M]f
V
× Vaq
(1) (2)
IL
D
β = D1
(3)
2
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D = [M]IL =
[M]i
Where [M]i and [M]f (mol/L) represent the metal concentration in the aqueous phase before and after extraction. Vaq and VIL (mL) are the volumes of the aqueous phase and ionic liquid phase, respectively. D1 and D2 represent the distribution of metal 1 and 2.
3. Results and Discussion 3.1 Synergistic Oxidative Leaching 3.1.1 Leaching of Rare Earths The waste phosphors (consisting of Y2O2S:Eu and ZnS) were collected by the vacuum suction 5
ACCEPTED MANUSCRIPT method; some impurities, such as glass cullet (SiO2, PbO) and aluminum foils (Al), were also present. The powder was screened using a 200 mesh sieve in order to remove these large impurities. Table 1 shows the composition of the waste CRT phosphors before and after screening as determined by X-ray fluorescence (XRF). The phosphor content was enriched to
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over 90%, and the impurities were efficiently removed. Additionally, almost no rare earth compounds were lost. Subsequently, a mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) was employed for the leaching of REs; this method has already been demonstrated in a previous report of ours (Tian et al., 2016). H2O2 is a strong oxidant and an
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environmentally safe reagent (Antonijević et al., 2004), and can effectively improve the leaching efficiency and avoid the generation of the poisonous gas H2S by transforming
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negative bivalent sulfur (S2-) into an elemental sulfur (S0) precipitate (which will be discussed in the next section). Under the optimal leaching conditions (3 M H2SO4, 4 vol.% H2O2, 55 °C, 90 min), the leaching efficiency of both Y and Eu reached 99%. The leaching efficiency of the major impurities, Zn and Al, were 45% and 100%, respectively. Table 1. Composition of the waste CRT phosphors (wt%) Y2O2S:Eu
ZnS
SiO2
PbO
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Component
Al
BaO
K 2O
others
24.1
48.0
7.68
4.86
10.5
0.95
1.04
2.87
After Screening
30.2
60.4
4.3
0.98
2.63
0.325
0.552
0.613
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Before Screening
Temperature was the parameter that had the greatest influence on the leaching of rare earth
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compounds from the waste CRT phosphors. Fig. 1 shows the effect of different temperatures on the leaching efficiency of the rare earth compounds. As can be seen, the leaching efficiencies and leaching rates of Y and Eu increased obviously with increasing temperature, and both Y and Eu showed the same leaching trends. When the temperature was raised to 55 °C, the leaching efficiency of Y and Eu reached 94.4% and 93.7% after 60 min, respectively, and then plateaued. Further increasing the temperature to 60 °C resulted in only a small increase in the leaching efficiency of the rare earth elements after 60 min, although the leaching rate was higher than at 50 °C at times of less than 20 min. On the other hand, higher temperature is known to promote the decomposition of H2O2 (Dimitrijevic et al., 1996), 6
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and also increased the energy consumption.
Fig. 1. Effect of temperature on the leaching of rare earth elements from waste CRT phosphors: (a)
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Leaching efficiency of Y and (b) leaching efficiency of Eu.
3.1.2 Leaching Kinetics Analysis
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The dissolution of the waste CRT phosphors by sulfuric acid and hydrogen peroxide is a typical liquid-solid reaction, and the leaching efficiency of the REs was analyzed using a
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shrinking core model for kinetic analysis (Innocenzi et al., 2017). In the present work, solid elemental sulfur was generated during the RE leaching process; this reaction can be expressed as follows:
Afluid + Bparticle → P1, solid +P2, aqueous
(4)
Generally, the following three kinetic equations can be applied for different rate-controlling steps (Randhawa et al., 2015; Wadsworth and Miller, 1979; Wu et al., 2015). If the leaching process is controlled by diffusion through a liquid boundary layer, its integrated rate equation will follow Equation (5). If the leaching process is controlled by a chemical reaction, its integrated rate equation will follow Equation (6). If the leaching process is controlled by 7
ACCEPTED MANUSCRIPT diffusion through the product layer, its integrated rate equation will follow Equation (7). (a) Process controlled by diffusion through a liquid boundary layer:
x = kl t
(5)
(b) Process controlled by a chemical reaction: 1
1 1 x3 = kc t (c) Process controlled by diffusion through the product layer: 2
2 3
1 x 1 x3 = kd t
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(6)
(7)
calculated from Equations (5)-(7), respectively.
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where x is the leaching efficiency of the REs, and kl, kc, and kd are the apparent rate constants
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The leaching efficiencies of Y and Eu in the temperature range from 30 to 60 °C were plotted against the reaction time according to Equations (5)-(7), respectively. As shown in Fig. 2, the correlation coefficient (R2) indicated the good linear fit of the data to the model involving diffusion through the product layer for both Y and Eu, indicating that Equation (7) was most suitable for representing the kinetics of the RE leaching process using H2SO4 in the presence
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of H2O2.
Fig. 2. RE dissolution ratio versus time at various temperatures, fitted by the equations corresponding to reactions controlled by (a) diffusion through a liquid boundary layer for Y, (b) chemical reaction for Y, and (c) diffusion through a product layer for Y; (d) diffusion through a liquid boundary layer for Eu, (e) chemical reaction for Eu, and (f) diffusion through a product layer for Eu.
8
ACCEPTED MANUSCRIPT The rate constants (k) for the leaching of Y and Eu were calculated from the slopes of the straight lines shown in Fig. 2 (c) and (f). The apparent activation energy of the reaction is strongly dependent on temperature, according to the Arrhenius equation: lnk = lnA0
Ea RT
(8)
gas constant, and T is the absolute temperature.
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where Ea is the apparent activation energy, A0 is the pre-exponential factor, R is the universal
The natural logarithm of the reaction rate constant (ln k) at different temperatures calculated using the model of diffusion through a product layer was plotted against the reciprocal of
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absolute temperature (1/T) (Fig. 3), and the apparent activation energies Ea for the leaching reactions of Y and Eu were calculated to be 75.86 kJ/mol and 77.06 kJ/mol, respectively,
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based on Equation (8). Actually, both Y and Eu were contained in the same crystal structure, Y2O2S:Eu (Thirumalai et al., 2007), so they followed the same leaching dynamics model and their apparent activation energies were very similar. The values of 75.86 kJ/mol and 77.06 kJ/mol were unusual, as they were significantly higher than the maximum theoretical value for a reaction controlled by diffusion through the product layer (Sun et al., 2008). This may be
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because the produced sulfur layer was very dense, and the complete reaction system contained ZnS and other impurities as well as Y2O2S:Eu; both these factors could lead to a high activation energy. Many other researchers also have reported high activation energies for
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processes controlled by diffusion through a product layer. For example, Wadswork found that the reaction of ferric sulfate with chalcopyrite followed the model of a process controlled by
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diffusion through a product layer control, and had an activation energy as high as 83.72 kJ/mol (Wadsworth and Miller, 1979). Other high energy values, such as 48.9 kJ/mol and 53.7 kJ/mol (Tsuchida et al., 1982), 62 kJ/mol to 79 kJ/mol (Paspaliaris and Tsolakis, 1987), 67.1 kJ/mol and 62.4 kJ/mol (Olanipekun, 1999), 67.1 kJ/mol and 50 kJ/mol (Gbor et al., 2000), and 46.32 kJ/mol (Martínez-Luévanos et al., 2011) have also been found for various diffusion-controlled processes. Thus, it was speculated that the rate control model was better predicted by the final correlation coefficient of the reaction equations or by models of complexed heterogeneous dissolution reactions rather than the value of the apparent activation energy. 9
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Fig. 3. Arrhenius plots for the leaching of Y and Eu.
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3.1.3 Reaction Mechanism Discussion
Fig. 4 (a) and (b) show the SEM images of waste CRT phosphors before and after synergistic oxidative leaching. It is obvious that the bulky particles shrank, and their surfaces became
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coarse and seemed to be covered with a thin film. XRD analysis (Fig. S1) illustrated that solid elemental sulfur was generated during the leaching process, and the reaction can be expressed
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by Equation (9).
Y, Eu2 O2 S + 6H+ + H2 O2 → 2Y3+ Eu3+ + 4H2 O + S↓
(9)
Based on the experimental results and the kinetics analysis, a simulated reaction model was established according to the classical theory of the shrinking core model (Wadsworth and Miller, 1979) as shown in Fig. 4 (c-e). Firstly, the Y2O2S:Eu phosphor particles were considered to be spherical and compact as shown in Fig. 4 (c). When the particles came into contact with the lixivium containing H2SO4 and H2O2, the redox reaction took place and solid sulfur began be generated. As the reaction continued, an increasing amount of Y2O2S:Eu was dissolved into the liquid phase, and the particle size was reduced. Fig. 4 (d) shows the 10
ACCEPTED MANUSCRIPT intermediate stage of the leaching process, in which a solid layer of the sulfur product has been generated (layer B); the outer layer is usually referred to as the boundary layer (layer C), in which the concentration of the ions is different from the liquid environment. In our work, stirring was employed during all the leaching experiments, which enhanced mass transfer
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between the boundary layer and the liquid environment. Thus, the diffusion of H+ and H2O2 into the boundary layer was not considered to be the main factor controlling the leaching process, in agreement with the results shown in Fig. 2 (a) and (d). The sulfur product layer shown in Fig. 4 (d) was not very permeable and covered the surface of the unreacted core
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(layer A). Thus, the inner diffusion of H+ and H2O2 through the product layer of sulfur into the unreacted core became the determining factor influencing the reaction rate, in agreement with
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the results shown in Fig. 2 (c) and (f). Finally, the inner core of Y2O2S:Eu was completely
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transformed into RE ions and solid sulfur (Fig. 4e).
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Fig. 4. Reaction mechanism analysis. (a) SEM image of the waste CRT phosphor and (b) SEM image of the waste CRT phosphor after leaching. (c-e) Simulated reaction model: (c) early stage, (d) intermediate stage, and (e) end stage.
3.2 IL based Extraction
After oxidative leaching, impurities such as Pb, Ba, and Ca were separated by precipitation in an aqueous solution containing a high concentration of sulfate radicals. Zn2+ and Al3+ are considered the most problematic elements in terms of purity in RE recycling, because their precipitation behavior is similar to that of REs. In this work, the feed solution for extraction was composed of Y3+ (25.2 mmol/L), Eu3+ (1.0 mmol/L), Zn2+ (28.5 mmol/L), Al3+ (5.6 11
ACCEPTED MANUSCRIPT mmol/L) as well as other inessential ions such as K+. The purpose of the work described in this section was to separate Y and Eu from the waste CRT phosphor powder leachate using an IL-based extraction system containing the ionic liquid [OMIm][PF6] and the extractant
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Cyanex 272 (Fig. 5).
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Fig. 5. Molecular structures of the extractant and ionic liquid.
3.2.1 Optimization of the Extraction Parameters Fig. 6 (a) shows the effect of the concentration of the acid H2SO4 on the extraction efficiency of the different metals. The acidity clearly had a significant influence on the separation of REs.
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The extraction agent Cyanex272 is a weak organic acid (HA), and the metal ions (M+)
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displaced the hydrogen ions (H+) and were extracted by Cyanex272, which will be discussed in the following mechanism analysis section. Thus, high acidity was unfavorable for highly efficient extraction due to its effects on the reaction equilibrium. As the concentration of the acid H2SO4 was increased from 0.1 mol/L to 0.2 mol/L, the extraction efficiencies of Y and Eu decreased slightly from 99% and 95% to 96% and 92%, respectively. In contrast, the extraction efficiency of Zn decreased substantially from 72% to 37%, and the extraction of Al was still nearly zero. The extraction efficiency of the REs decreased rapidly as the acidity was further increased. Thus, the optimal acid concentration for highly efficient extraction of the rare earth elements and the separation of impurities was determined to be 0.2 mol/L, and this concentration was used in subsequent experiments. 12
ACCEPTED MANUSCRIPT The effect of the volume ratio (vol.%) of Cyanex272 in the organic phase on the extraction was investigated at an acid concentration of 0.2 mol/L. As shown in Fig. 6 (b), the extraction efficiencies of Y, Eu, and Zn increased with an increasing ratio of Cyanex272, while Al showed the opposite trend. When the vol.% of Cyanex272 was increased from 0 to 0.4, the
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extraction efficiency of Y, Eu, and Zn reached 95%, 92%, and 21%, respectively, while the Al efficiency dropped to less than 1%. Further increasing the volume of Cyanex272 increased the extraction efficiency of Y and Eu to 99.5% and 95%, but the extraction of Zn also increased to approximately 40%, which was unfavorable to the separation. To achieve good extraction
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of Y and Eu while minimizing the content of Zn and Al, a ratio of 0.4 vol.% Cyanex272 was selected.
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Another important factor to be optimized was the volume ratio of the aqueous phase to organic phase (A/O). Fig. 6 (c) shows the effects of the A/O on the extraction efficiencies of the different metals. At a phase ratio of 1:1, the extraction ratios of Y, Eu, and Zn were 99.9%, 97% and 65%, respectively. When the A/O was increased to 10:1, a slight decline in the extraction efficiency of Y and Eu occurred (to 94.5% and 86.5%), while the extraction
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efficiency of Zn fell sharply from 65% to around 10%. Thus, optimizing the phase ratio could enhanced the separation of the REs from Zn. However, increasing the phase ratio further resulted in a great reduction of the extraction efficiency for Eu. Thus, 10:1 was chosen as the
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optimal A/O phase ratio.
Fig. 6 (d) illustrates the effect of the extraction temperature on the extraction. Interestingly,
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temperature did not have a significant influence on the extraction efficiencies of Y and Eu, with only a 3% increase being observed when the temperature was increased from 25 °C to 55 °C. In contrast, the extraction efficiency of Zn increased approximately 27%. Thus, to minimize impurities and reduce energy consumption, the extraction temperature was set as 25 °C (room temperature). Finally, we investigated the effect of extraction time on the extraction efficiency of Y, Eu, Zn, and Al. As shown in Fig. 6 (e), the extraction efficiency of Y and Eu increased from 83% and 56% to 99% and 87% respectively when the extraction time was extended from 1 min to 20 min. Zn showed the opposite trend, with the extraction efficiency dropping from 13% to 8%, and the extraction of Al was negligible in this system. After 20 min, the extraction system 13
ACCEPTED MANUSCRIPT tended to reach equilibrium. Fig. 6 (f) summarizes the separation factors of the REs to zinc during the optimization of the extraction conditions corresponding to Fig. 6 (a-e). As can be seen, the separation factors for both Y and Eu to Zn increased as parameters were optimized. Although the extraction
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efficiency of Eu decreased from 92% to 87% during the optimization process, the separation factor showed an obvious increase from 19 to 76. The separation factor of total REs to zinc increased to 593 using the optimal extraction parameters. Using these optimized parameters, the individual extraction efficiencies of Y, Eu, Zn, and Al were 99%, 87, 8% and 0%,
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respectively. Thus, the impurities were efficiently removed by the above extraction system.
Fig. 6. Optimization of the IL-based extraction process. (a) Effect of the acid (H2SO4) concentration; (b)
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effect of the volume ratio of Cyanex272 in organic phase; (c) effect of the volume ratio of the aqueous phase to the organic phase; (d) effect of the extraction temperature; (e) effect of the extraction time; and (f) separation factors of the rare earths to zinc during the optimization of the extraction conditions.
3.2.2 Discussion of the Synergistic Extraction Mechanism Cyanex272 was proven to play a critical role in the extraction of the REs, as both Y and Eu were barely extracted by the IL alone (Fig. 6b). Thus, we speculated that in this extraction system, a chemical reaction took place between the RE ions and Cyanex272, and that [OMIm][PF6] acted as a diluent for promoting the dissolution of the extraction reaction products. To further confirm the extraction mechanism, the common slope analysis method 14
ACCEPTED MANUSCRIPT and the constant molar method were employed. Firstly, a Cyanex272-only system was tested in order to determine the equation of the extraction reaction. Fig. 7 (a) shows the relationship between the logarithm of the distribution ratio (Log D) and the logarithm of acidity (-Log [H+]) in the aqueous phase. Straight lines with slopes of 2.96 and 3.07 were obtained from the
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plots, which indicated that three moles of H+ were released to extract a mole of Y3+ or Eu3+. The plots of Log (D) vs. log ([Cyanex272]) also showed straight lines with a slope of 3 for both Y and Eu, as shown in Fig. 7 (b). Meanwhile, the effect of SO42- was tested by adding different concentrations of Na2SO4, and the results indicated that SO42- had no influence on
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the extraction. Thus, the extraction equation for Y and Eu can be expressed as Equation 10, where HA represents Cyanex272 and RE represents Y or Eu. Other researchers also found
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similar results for the mechanism of RE extraction by Cyanex272 (Jiao et al., 2009; Liao et al., 2010). Fig. 7 (c) shows plots of Log (D) vs. -Log ([H+]) in the [OMIm][PF6] + Cyanex272 system. The slope of 3 for Y and Eu indicated that the reaction mechanism was unchanged in the mixed system, and the ionic liquid [OMIm][PF6] acted as a diluent. The extraction mechanism proposed here still requires deeper study in our future works. (10)
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RE3+ + 3HA → REA3 + 3H+
Fig. 7. Analysis of the extraction mechanism. (a) Log (D) vs. -Log ([H+]) in the Cyanex272-only extraction system; (b) Log (D) vs. Log ([Cyanex272]) in the Cyanex272-only extraction system; and (c) Log (D) vs. Log ([H+]) in the [OMIm][PF6] + Cyanex272 extraction system.
3.2.3 Stripping and Recycling We compared the stripping efficiencies of different acids (HCl, H2SO4, and HNO3), and concluded that H2SO4 was the most efficient in stripping Y and Eu. A concentration of 3 mol/L H2SO4 was employed in the experiments. Fig. 8 (a) shows the effect of different 15
ACCEPTED MANUSCRIPT volume ratios of the organic phase and aqueous phase (O/A) on the RE stripping efficiency. As the O/A was increased, the Y and Eu stripping efficiencies both obviously increased, and the individual Y and Eu stripping efficiencies reached 93.4% and 96.7, respectively, at an O/A of 1:10. As the proportion of the aqueous phase was further increased, the stripping efficiency
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remained stable. However, use of large volumes of stripping agent would dilute the target metals and require the use of greater quantities of the chemicals. Thus, an O/A ratio of 1:5, at which 83% and 90% of Y and Eu can be stripped out, was determined to be optimal from economic and environmental viewpoints. Finally, the REs can be recycled by simple
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precipitation. Additionally, after three cycles of stripping, the IL-based extraction system can be reused in further extractions. As shown in Fig. 8 (b), the [OMIm][PF6] + Cyanex272
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extraction system retained a relatively high extraction ability after five cycles, and little
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decline in the extraction efficiency was observed.
Fig. 8. Stripping of REs and recyclability of the IL-based extraction system. (a) Effect of the O/A ratio on the stripping efficiency of the REs; and (b) RE extraction efficiency of the extraction system during repeated cycles.
16
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4. Conclusion In this research, an efficient combined process consisting of oxidative leaching and IL-based
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extraction was investigated for the recycling of REs from waste CRT phosphors. The recycling parameters were optimized, and the reaction mechanisms of the process were explored systematically. The conclusions can be summarized as follows:
(1) A mixed lixiviate containing H2SO4 and H2O2 was used in RE leaching for its synergistic
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effects. Temperature was shown to significantly influence the leaching. The leaching efficiencies of both Y and Eu reached 99% under the optimal leaching conditions: 3 M
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H2SO4, 4 vol.% H2O2, 55 °C, and 1 h. The S2- was oxidized to S0, which efficiently avoided the sulfur pollution produced by existing technologies.
(2) According to a kinetics study using the shrinking core model, the leaching followed the model of a process controlled by diffusion through the product layer. The apparent activation energies were calculated to be 75.86 kJ/mol and 77.06 kJ/mol for Y and Eu,
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respectively. The leaching reaction equation was determined, and RE3+ and S0 were found to be the main reaction products. Additionally, a simulated reaction model based on diffusion through the S0 layer was established.
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(3) The green solvent ionic liquid [OMIm][PF6] and the extractant Cyanex272 were employed for the separation of the REs from the lixivium. The optimal extraction
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parameters were determined to be 0.2 mol/L H2SO4, 0.4 vol.% of Cyanex272 in the organic phase, a 10:1 A/O ratio, and an extraction time of 20 min at room temperature. Under these conditions, the individual extraction efficiencies of Y, Eu, Zn, and Al were 99%, 87, 8%, and 0%, respectively, and the rare earth to Zn separation factor reached 593. The REs could then be stripped using 3 mol/L H2SO4 at an O/A ratio of 1:5. Additionally,
reuse of the extraction system was considered to be possible. (4) The synergistic extraction mechanism was discussed, and a chemical reaction was speculated to take place between the RE ions and Cyanex272, with [OMIm][PF6] acting as a diluent. The reaction between the REs and Cyanex272 was a cation exchange. However, the extraction mechanism requires further investigation. 17
ACCEPTED MANUSCRIPT Acknowledgements This research was supported by Beijing Natural Science Foundation (2182009, 2174065, 2174067), Beijing Social Science (No. 17YJA001), National Key R&D Plan — Comprehensive Evaluation on Environmental and Economic Performance of Building
Academician Workstation in Yunnan Province.
References:
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Materials Made from Industrial and Urban Solid Wastes (2017YFC0703206-03), and the
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Highlights REs were recycled efficiently by oxidative leaching and IL based extraction.
•
The leaching kinetics was investigated systematically.
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The reaction mechanisms of the leaching and extraction process were
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analyzed.