Liquid–liquid extraction of yttrium from the sulfate leach liquor of waste fluorescent lamp powder: Process parameters and analysis

Minerals Engineering 152 (2020) 106341

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Liquid–liquid extraction of yttrium from the sulfate leach liquor of waste fluorescent lamp powder: Process parameters and analysis Ganesh Dattatraya Saratalea, Hee-Young Kimb, Rijuta Ganesh Saratalec, Dong-Su Kimb,

T

⁎

a

Department of Food Science and Biotechnology, Dongguk University-Seoul, Dongguk-ro, Ilsandong-gu, Goyang-si, Gyeonggi-do 10326, Republic of Korea Department of Environmental Science and Engineering, Ewha Womans University, New 11-1, Daehyeon-dong, Seodaemun-gu, Seoul 120-160, Republic of Korea c Research Institute of Biotechnology and Medical Converged Science, Dongguk University-Seoul, Ilsandong-gu, Goyang-si, Gyeonggi-do 10326, Republic of Korea b

A R T I C LE I N FO

A B S T R A C T

Keywords: Liquid–liquid extraction Fluorescent waste lamp powder Yttrium Rare earth metals Hydrometallurgical process

Global demand for rare earth metals (REMs), including yttrium, has motivated the scientific community to focus on the recovery of such metals from electronic waste materials. Herein, a solvent extraction method was used to isolate and recover yttrium from the original leaching solution from the fluorescent lamp waste powder dissolved by sulfate. The operating parameters were systematically investigated, including pH, equilibrium time, concentration of extractants, and organic/aqueous ratio using Versatic Acid 10, TOPO, D2EHPA, and Alamine 336. The extracting capacities were in the order of D2EHPA > Versatic Acid 10 > TOPO > Alamine 336. The reaction mechanism of yttrium with each extractant demonstrated the formation of complex compounds with concentration ratios of 1:3, 1:1, and 1:2 with Versatic Acid 10, D2EHPA, and TOPO, respectively. On investigating the extraction mode for yttrium and impurities in the range of equilibrium pH (pHeq) values from 0.95 to 2.25 using D2EHPA, pHe 2.02 (initial pH 2.53) was found to be the most suitable for extraction. Fe in the original leaching solution could be utterly eradicated through the acidity control method. Upon calculating the theoretical number of mixer–settler plates, more than 99% of yttrium was extracted in solution with only two plates as the organic phase. Finally, the stripping test showed favorable stripping rates and followed the order HCl (78.12%) > H2SO4 (76.36%) > HNO3 (74.86%) within 10 min. This study is a first step toward developing large-scale operations for extracting REMs from fluorescent lamp waste powder.

1. Introduction Rare earth metals (REMs) have been used as the main raw materials in the high-tech industry for items such as optical glasses, electronics, metallic additives, and catalysts due to their unique characteristics of chemical stability and ability to conduct heat well; hence, demand for such metals has increased drastically in the last ten years (Hidayah and Abidin, 2018; Tyler, 2004; Resende and Morais, 2010). The term “rare” earth is a contradiction as they are moderately available and found concentrated rarely in the Earth’s crust but are discrete and thus utilizable as economical minerals (Feng et al., 2014). REMs are unevenly distributed all over the world; they are found mainly in China (55 million tons), USA (13 million tons), India (3.1 million tons), Australia (2.1 million tons), and Brazil (2.2 million tons) (Tunsu et al., 2015, 2016). Specifically, 97% of rare earth minerals are produced in China (Du and Graedel, 2011). Because of this, the price of REMs has fluctuated a lot based on the policies of the supplier countries, and supplier

prices have been increasing since the implementation of the export quota system under the policy on resources as weapons in China (USGS, 2015). As a result, the European Commission (EC) and UN Environment Program and UN University have decided to secure REMs stably as the core component of economic development and announced the importance of securing a seamless and stable supply of REMs (UN Environment Program and UN University, 2009; European Commission, 2010; Banda et al., 2019). Currently, attention is being paid worldwide to solve this problem by collecting REMs from urban mining. Together with industrial development in the world, the amount of daily waste or industrial waste such as electric/electronic products and automobiles has been exponentially increasing (Binnemans et al., 2013). However, waste such as electronic goods and fluorescent lamp powder contain vast amounts of expensive REMs; these waste products have attracted considerable attention as new energy sources in view of REM shortages and waste control (Binnemans and Jones, 2015; Sethurajan et al., 2019).

⁎ Corresponding author at: Department of Environmental Science and Engineering, Ewha Womans University, Daehyundong 11-1, Seodaemungu, Seoul 120-750, Republic of Korea. E-mail address: [email protected] (D.-S. Kim).

https://doi.org/10.1016/j.mineng.2020.106341 Received 5 October 2019; Received in revised form 1 March 2020; Accepted 11 March 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.

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Table 1 Properties and structure of extractants used in this study. Type

Structure

Properties Molecular weight Flash point (°C) Specific gravity (at 20 °C) Surface tension (dyn.cm at 25 Viscosity at 25 °C(C.P.) Solubility (g/100 g solvent) Molecular weight Flash point (°C) Specific gravity (at 25 °C) Surface tension (dyn.cm at 25 Viscosity at 25 °C(C.P.) Solubility (g/1000 g cm3) Molecular weight Flash point (°C) Specific gravity (at 20 °C) Surface tension (dyn.cm at 25 Viscosity at 25 °C(C.P.) Solubility (g/100 g solvent) Molecular weight Flash point (°C) Specific gravity (at 25 °C) Surface tension (dyn.cm at 25 Viscosity at 25 °C(C.P.) Solubility (g/1000 g cm3)

Versatic acid 10 (R: C9H19, C10H20O2)

D2EHPA (R: C4H9CH(C2H5)CH2, C16H35O4P)

TOPO (R: C8H17, C24H51OP)

Alamine 336 (R: C8H17, C24H51N)

°C)

°C)

°C)

°C)

172.26 129 0.92 30.7 7 negligible 322.43 233 0.970 g/cm3 20 0.42 0.012 386.63 259.78 0.875 30.99 – immiscible 353.67 226 0.8153 g/cm3 53 10.4 0.01

studies for REM extraction have been conducted with reported extractants (Feng et al., 2014). In this study, a solvent extraction process was developed using four extractants (Versatic Acid 10, TOPO, D2EHPA, and Alamine 336) for the selective extraction of yttrium utilizing a sulfate leach liquor of fluorescent lamp powder, and the extraction reaction mechanism using each extractant and yttrium was investigated. The extraction features were investigated by changing the experimental variables for the solvent extraction, including pH, equilibrium time, concentration of extractant, and organic/aqueous (O/A) ratio; the ideal number of plates was suggested by the preparation of a McCabe–Thiele diagram to apply to the mixer–settler based on the results. The purpose of the study was to remove metals other than the target ones from the solution to make high-purity concentrates. The elimination features of Fe, Mg, and Ca, which exist in large amounts in waste fluorescent lamp powder, were also investigated by pH control during the precipitation and equilibrium pH control during the extraction. Finally, the stripping mechanism of yttrium was investigated in terms of the type, concentration, and reaction time of the acids used as the hydraulic-phase solution in the case of stripping to achieve the optimum stripping rate.

Among REMs, yttrium (Y) is a heavy rare earth element that is used in a variety of fields, including as an additive to enhance the strength of an alloy (Fan et al., 2012), as a laser to cut metal (Xiao et al., 1999; Mishra et al., 2019), and in the productions of camera lenses and superconductors (Permyakov, 2009). A total of 76.7% of yttrium production goes toward making the main component of fluorescent lamps, phosphor (Tan et al., 2015; Tanvar and Dhawan, 2019). Because of socioeconomic development, the advanced lifestyles and mindset of people for convenience, and the single use of products, huge amounts of electronic waste are generated in South Korea. Korea is an electronics hub, and its REM market is about $29,658 million. For example, yttrium and europium are required for phosphor screens of cathode ray tubes in TVs (Lee and Kim, 2014). Utilization of electronic waste, particularly fluorescent lamp waste, which is composed of yttrium and europium, will not only address the need for REMs but also diminish the waste disposal problem. Considering these perspectives, Korea is devoting huge research efforts to develop technologies to assure viable resource recovery from waste resources. Various methods, such as supercritical CO2 extraction (Yang et al., 2016; Liu et al., 2009), ionic liquids (Larsson and Binnemans, 2015), and solvent extraction (Banda et al., 2019; Innocenzi et al., 2017), are employed to recover and separate REMs from waste. Among these methods, the most popular process for collecting high-purity REMs is solvent extraction. Solvent extraction is generally considered the most suitable and commercial technology for separating REMs because a large volume of dilute liquor can be easily processed and REMs can be extracted from different groups of leachate (Belova, 2017; Hidayah and Abidin, 2018). It has been observed that single or specific elements are very difficult to separate from the mixture because of their similar physical and structural functions. In such cases, solvent extraction methods have been found to be suitable owing to the ease in terms of the equipment and the conditions associated with them and their efficiency for obtaining high-purity compounds (Quinn et al., 2015). The commonly used extractants to extract REMs during solvent extraction are carboxylic acids, phosphorous acids, solvating extractants, and anion exchangers. These extractants make complex compounds with REMs through different extraction reaction mechanisms (Nguyen et al., 2017; Balaram, 2019). Hence, in order to effectively extract yttrium, the best extractant must be chosen by investigating the extraction mechanism of each extractant and REMs. However, few comparison

2. Materials and methods 2.1. Materials Versatic Acid 10, D2EHPA (di-2-ethylhexyl phosphoric acid), TOPO (tri-n-octylphosphine oxide), and Alamine 336 (Aldrich) were used as the reagents without purification. Table 1 shows the physicochemical properties of the extractants used. The raw materials used were from a compound solution with similar composition to the final solution after leaching with sulfate to the fluorescent waste lamp powder (Table 2). This compound solution was produced using sulfates, including MgSO4·7H2O (Aldrich, USA, ≥98%), FeSO4·7H2O (Aldrich, USA, ≥99%), and CaSO4·2H2O (Aldrich, USA, ≥99%), and their concentrations were Ca: 3.045 g/L, Mg: 7.134 g/L, Fe: 0.945 g/L, and Y: 80.00 g/L, respectively. The diluent was kerosene from Aldrich, which was used to dilute the extractant and control the organic phase. Hydraulic-phase acids for stripping were used with hydrochloric acid (Samchun Chemicals, South Korea), sulfuric acid (Samchun Chemicals, South Korea, 95%), and nitric acid (Samchun 2

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Table 2 Final metals composition of leach liquor after leaching process in optimum condition by ICP-AES. Components

Average metal content in leach liquor (wt.%)

Al Ca Fe Mg Mn Si Ba Na Sr P Nd Sb Mo S Y

0.020 0.305 0.095 0.713 0.003 < 1 ppm < 1 ppm 0.019 0.003 0.102 < 1 ppm 0.003 < 1 ppm 5.157 8.132

Chemicals, South Korea, 60%). Fig. 1. Schematic diagram of experimental procedure for solvent extraction.

2.2. Analysis experiment was conducted with 3 mL of the organic phase from the extraction process and 3 mL of 3 N HCl, H2SO4, and a HNO3 solution with an O/A ratio of 1:1, which was the most effective, which had been inserted into a 100 mL separating funnel. The pH of the hydraulic phase was maintained at 2.2; the separating funnel was agitated for 1, 3, 5, 10, 20, and 30 min to determine the equilibrium time; the aliquots were left and separated for more than 1 h. The stripping rate was analyzed by AAS after collecting the separated hydraulic-phase solution. Through the same method mentioned above, the effect of acid concentrations of 0.5, 1.5, 3.0, 4.5, 6.0, and 7.5 N on the stripping rate was investigated.

To investigate the components and their purities, inductively coupled plasma atomic emission spectroscopy (Jobin Yvon, JY Ultima2C, France) was used. The operating conditions of the equipment were as per the manufacturer’s protocol. The concentrations of the metals in the final solution were analyzed by an atomic absorption spectrometry (AAS, Perkin Elmer, Analyst 400, USA) to assess the associated extraction yield. 2.3. Experimental procedures 2.3.1. Extraction Extraction and stripping experiments were conducted with the various extractants. The solution used was the hydraulic phase of the yttrium synthesis solution, which had the same concentration as the hydraulic-phase solution under the optimum conditions [3 N H2SO4, 40% S/L (solids/liquid) ratio, 1 h, 60 °C, 150 rpm] during the leaching process. After adjusting the pH of the hydraulic-phase solution with NaOH, 3 mL of the solution was inserted into a 100 mL Teflon stop-cock separating funnel with 3 mL of the organic-phase liquid. The resulting solution was mixed with the extractant and kerosene to have a 16% E/D (extractant/diluent) ratio, and the experiment was performed at 25 ± 1 °C. Aliquots of the resulting solution were agitated for 1, 3, 5, 10, 20, and 30 min to determine the equilibrium time and then separated into hydraulic and organic phases after being left for more than 1 h. The pH of the separated hydraulic phase was measured, and the concentration of yttrium was analyzed by AAS, whereas the concentration of the organic phase was calculated by material resin. With the same conditions and methods, the effects of the various extractants (Versatic 10, D2EHPA, TOPO, Alamine 336), concentrations (5, 9, 13, 16, 12% E/D), and acidic conditions (pH 0.5, 1, 2, 3, 4, 5, 6, 7) were also investigated. An isothermal extraction curve was prepared using the outcomes by the different ratios of hydraulic/organic phase. To remove impurities such as Fe, Mg, and Ca, which were present in the solution during the leaching of the fluorescent waste lamp powder, the extraction features of Fe, Mg, Ca, and yttrium were investigated at the initial pH range of 0.98–5.34 with 16% D2EHPA, A/O = 1:1 after making a compound solution with the same concentration as the leaching solution. A schematic flowsheet describes the solvent extraction procedure for processing fluorescent waste lamp (Fig. 1).

3. Results and discussion 3.1. Extraction of yttrium with various extractants 3.1.1. Effects of extractants (Versatic acid 10, TOPO, Alamine 336 and D2EHPA) Generally, extractants of REMs are organic compounds with the ability to complex REM cations, to be dissolved in the organic phase, and to be extracted (Yoo and Shin, 2003). The representative extractants currently utilized to extract REMs include carboxylic acids, phosphorous acids, solvating extractants, and anion exchangers, and they show different chemical extraction reactions as well as different extraction rates, distribution ratios, and distribution coefficients. Therefore, the extraction features were investigated by reaction time, the concentrations of extractants, and pH using Versatic Acid 10, D2EHPA, Alamine 336, and TOPO, which have different REM extraction features. The reaction times used were 1, 3, 5, 10, 20, and 30 min and the pH values ranged from 1 to 7. Investigating the extraction rate by reaction time, Versatic Acid 10 and TOPO reached equilibrium in 3 min, whereas Alamine 336 and D2EHPA reached it in 5 min (data not shown). Experiments were also conducted to investigate the efficiencies of the extractants by pH (Fig. 2). Because all the extractants showed equilibrium times over 5 min in the previous work, the extracting experiment was conducted for 10 min to perform the perfect extraction. Fig. 2 demonstrates the increased extraction rate for Versatic Acid 10, TOPO, and D2EHPA with increasing pH, as well as the decreased extraction rate for Alamine 336 with increasing pH. The reason for this might be because Alamine 336 is an anion exchanger. The maximum extraction rates were 94% for Versatic Acid 10 at pH 6.03, 93.81% for TOPO at pH 5.0, 97.88% for D2EHPA at pH 3.11, and 91.61% for Alamine 336 at pH 1.08. D2EHPA and TOPO achieved equilibrium at the mentioned pH values. Hence, the extraction capacity was observed

2.3.2. Stripping To strip yttrium that had passed to the organic phase, a stripping 3

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3.1.2. Stoichiometry of the complexes depending on various extractants The typical extractants used for the solvent extraction of REMs are classified as cation exchangers, anion exchangers, solvating extractants, and chelating exchangers. Cation exchangers can be sub-classified as carboxylic acid and phosphorous acid. These extractants have different reaction mechanisms to combine with REMs according to the type of extraction. The present study aimed to understand the extracting reaction mechanisms of yttrium and the typical extractants that are used to collect REMs. The extraction experiments were conducted with carboxylic acid, phosphorous acid, amines, and phosphorous ester. They were Versatic Acid 10 as a carboxylic acid, D2EHPA as a phosphorous acid, Alamine 336 as an amine, and TOPO as a phosphorous ester. Versatic Acid 10 and D2EHPA are the extractants used to generate the extraction reaction of an acidic complex, and they perform the extraction by cation exchange and coordination. These extractants are cation exchangers to disseminate hydrogen ions, and the disseminated ions play a role in exchanging the cations of REMs from the extract in the hydraulic phase. In addition, the oxygen atoms of C]O or P]O in the extractants make coordinate covalent bonds with rare earth metal ions to simultaneously form various complexes. These extractants exist mainly as dimers in nonpolar organic solvents, with negligible solubility in water and no association, the general formula for the extraction reaction is as follows:

Fig. 2. Extraction profile of Versatic Acid 10, D2EHPA, TOPO, and Alamine 336 at different initial pH values [10 min, 16% E/D, kerosene, O:A = 1:1 (3 mL), 25 °C].

i [M 3 +]aq + ij [H2 A2 ]org ⇔ [MA3 (HA)2 j − 3]org + 3i [H+]aq

(1)

where i is the oligomerization extent of the extracted complex and j is the number of extractants on the extracted metal ion. The above reaction formula can be expressed with equilibrium constant K and distribution ratio D as follows:

K ex =

D=

[MA3 (HA)2j - 3 ]org [H+]3i aq [M3+]i aq [H2 A2]ij org

(2)

[MA3 (HA)2j - 3 ]org [M3+]i aq

(3)

Substituting the distribution ratio D into formula (2) and taking the logarithm of both sides with distribution ratio D and equilibrium constant K we obtain the following formula: Fig. 3. Effect of extractant concentration on yttrium extraction.

log D = log K ex + 3ipH + ij log[H2 A2 ]org

(4)

With the final formula (4), the distribution ratio D, which is the index of extraction capacity, was confirmed to be proportional to the equilibrium constant K, pH, and the concentrations of the extractants. The graph of log D vs pH using formula (4) shows the i value, and the graph of log D vs log [extractants] based on the results shows the j value. Thus, Figs. S1, S2, S3, and S4 show the graphs of log D vs pH for each extractant and log D vs log [extractants] to confirm the extraction mechanisms of Versatic Acid 10 and D2EHPA. In the case of Versatic Acid 10, as was shown in Fig. S1 for the graph of log D vs. pH, the i value was about 1.02 from the extraction system of yttrium with Versatic Acid 10 based on a 3i value of 2.936 in formula (4). This is consistent with the study results of Du Preez and Preston (1992); and Singh et al. (2006) on the extraction experiments of REMs and yttrium by Versatic Acid 10. Applying the i value to formula (4), we obtain the following:

to be in the order of D2EHPA > Versatic Acid 10 > TOPO > Alamine 336 with respect to the extraction of yttrium. This means that yttrium can be extracted with better efficiency using cation exchange extractants. To understand the effects of the concentrations of the extractants, the extractions were performed for 10 min at pH 3.11 after mixing the compounds with O:A = 1:1 ratio. The concentration range of the extractants was 5–20% E/D. As the concentrations of Versatic Acid 10, TOPO, and D2EHPA increased from 27.63% to 94.98%, from 10.81% to 93.95%, and from 66.06% to 97.94%, respectively, the extraction rates increased and reached equilibrium at 16% E/D, whereas the concentration of Alamine 336 was increased from 9.61% to 91.67% at 16% E/D and decreased slightly to 89.98% at 20% E/D, as seen in Fig. 3. The reasons for the decreased extraction rate in Alamine 336 despite the concentration increase were basically because of the high viscosities of the extractants used in the solvent extraction, as well as its inhibition from combining yttrium and the extractant. Although the viscosity of Alamine 336 was the highest out of the four extractants used in the present investigation (Table 1), it is considered that the inhibition level of the yttrium extraction increased compared to other extractants with increasing concentration of the extractants. In the case of Alamine 336, the impact of its viscosity on the extraction initiated to be lowered over 20% E/D for the concentration of extractant. Because the differences of extraction capacity between 16% and 20% E/D were not significant, the following experiments were conducted with the condition of 16% E/D because of economic considerations.

log D = log K ex + 3pH + j log[H2 A2 ]org

(5)

The graph of log D vs log [Versatic Acid 102] demonstrates the calculation of the j value using formula (5), as seen in Fig. S2. As confirmed from Fig. S2, the slope was 2.9955, which resulted in a j value of approximately 3. Summarizing the reaction formula of the extraction system for yttrium and Versatic Acid 10 results in formula (6).

[M 3 +]aq + 3[H2 A2 ]org ⇔ [M (HA2 )3]org + 3[H+]aq 4

(6)

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It was confirmed by formula (6) that yttrium and Versatic Acid 10 reacted with a concentration ratio of 1:3 and that they developed a complex compound in the form of M(HA2)3. Some investigators reported that similar types of complex compounds were formed during the extraction process with carboxylic acid and REMs (Du Preez and Preston, 1992; Singh et al., 2006). The analysis of a similar experiment with Versatic Acid 10 to confirm the extraction system of yttrium by D2EHPA is shown in Fig. S3; the 3i value was 2.0524 in formula (4), which meant that the i value was about 0.684 in the extraction system. Mohammadi et al. (2015) also demonstrated the range of the slope value to be 2.1–3.0 as the concentration of D2EHPA was increased. Considering an i value of 1, rounding it off and putting it into formula (4) generates the following formula:

log D = log K ex + 3pH + j log[H2 A2 ]org

(7)

Using formula (7), the graph was drawn and is shown in Fig. S4, with a slope value of 0.85 for log D vs log [D2EHPA2], so the j value could be about 1. This means that the dimer D2EHPA and yttrium ions are considered to be reacted in a ratio of 1:1. This result could be applied with R2 of 0.979 in the concentration range of extractant 0.145–0.377 M. Therefore, different features might be seen in the above concentrations. TOPO, a solvating extractant, is used to induce a neutral complex extraction. When a neutral extractant like phosphorus is coordinated with molecules that can extract neutrally to form a neutral complex in the hydraulic phase, the extraction of the neutral complex compound occurs in the organic phase (Yoo and Shin, 2003). Considering the hydraulic phase as the sulfuric acid in this reaction, the following reaction formula can be obtained:

2[M 3 +]aq + n [TOPO]org + 3[SO4 2 −] aq ⇔ [M2 (SO 4 )3·nTOPO]org

Fig. 4. Extraction of metals by D2EHPA at different equilibrium pH values.

complicated, and thus it is beyond the scope of this study. Referring to the study results of Feng et al. (2014) and the report by BASF Corporation (2013), the reaction formula of yttrium extraction in a sulfate solution was estimated using Alamine 336 as follows:

D=

[M2 (SO4 )3·nTOPO]org (10)

The relationship between the distribution ratio and the equilibrium coefficient is as follows:

D = K ·[TOPO]n org ·[SO4 2 −]3aq

(11)

Summarizing after taking the logarithm of both sides, we obtain the following formula:

log D = log K + n log[TOPO]org + 3 log[SO4 2 −]aq

(12)

It is estimated that as TOPO increased along with the concentration of sulfate, the extraction capacity of the TOPO extractant increased. To solve the value of n by formula (12), Fig. S5 shows the graph of log D vs. log [TOPO]. As a result, n was confirmed to be about 4, and yttrium and TOPO were developed by the complex compound with the reaction ratio 1:2 [formula (13)]. This has been consistently confirmed in previous studies, including Meri et al. (1996) and Reddy et al. (1998).

2[M 3 +]aq

+ 4[TOPO]org +

3 log[SO4 2 −]aq

⇔ [M2 (SO4 )3·nTOPO]org

(15)

3.1.3. Effect of equilibrium pH The extraction mechanisms of ferrous iron, calcium, magnesium, and yttrium by equilibrium pH were confirmed using D2EHPA, which was shown to be the most effective for the extraction of yttrium. The extraction experiments for yttrium and impurity metals in the compound solution were performed at equilibrium pH values 0.95 to 2.25. Fig. 4 and Table 3 show the extraction rates of yttrium and impurities on equilibrium pH. As shown in Table 3, the extraction rate of yttrium increased from 31.17% to 97.01% as pHe was increased from 0.95 to 2.13. The extraction rate remained at 97 ± 0.5% at higher pHe. values. This is consistent with the relationship between pH and extraction capacity described in formula (7) of Section 3.1.2. Fig. 4 shows that the maximum extraction rate of yttrium was achieved at pHe 2.13 (pH 3.05), whereas extraction rates of calcium and ferrous iron were 75.18% and 95.46%, respectively, confirming a significant amount of co-extraction at this pHe. In addition, magnesium was confirmed to be completely eliminated by controlling pHe in the extraction using D2EHPA. These findings are consistent with the results of previous studies on the collection of calcium, magnesium, ferrous iron, and

(9)

[M3+]2aq

2[M 3 +]aq + 6[H2 SO4 ]aq ⇔ 2[MSO4 )3 −3 ]aq + 12[H+]aq

(16)

[M2 (SO4 )3·nTOPO]org [M3+]2aq ·[TOPO]n org ·[SO4 2 - ]3aq

(14)

2[MSO4 )33 −]aq + 3[(R3 NH3 )2 SO4 ]org ⇔ 2[(R3 NH3 )3MSO4 )3 ]org + 3SO4 2 −

(8)

The equilibrium coefficient K and distribution ratio D can be described in the extraction system as follows:

k ex =

2[R3 NH2]org + [H2 SO4]aq ⇔ [(RNH3)2 SO4]org

Table 3 Extraction of metals at different pHe levels.

(13)

Amine extractants, which are anion exchangers, are extracted by the extraction chemistry of ionic bonds, and amine cations react with anions of complex compounds in the aqueous solution by mutual affinity of positive and negative electric charges for the development of a neutral compound, which is known as an ionic bond compound (Yoo and Shin, 2003). This chemical extraction is called ionic bond extraction. The mechanism of yttrium collection on amine extractants is very 5

Initial pH

Eq. pH

Fe %

Ca %

Mg %

Y%

0.98 1.44 2.03 2.53 3.05 3.41 4.05 4.9 5.34

0.95 1.21 1.79 2.02 2.13 2.16 2.22 2.23 2.25

11.77 37.809 79.7 94.934 95.463 97.361 97.823 97.128 98.128

0 3.12 15.006 34.096 75.178 88.02 95.769 95.966 97.491

0 0 0 0 0 11.901 24.018 31.905 38.262

31.17 49.68 60.12 96.54 97.01 97.54 97.69 97.88 97.91

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yttrium by pHe using D2EHPA as the extractant (Zhang and Gozzelino, 2003; Van de Voorde et al., 2005; Luo et al., 2011). As mentioned previously, extraction should be performed by considering the conditions not only with the highest extraction efficiency of yttrium but also with the lowest efficiency of impurity metals. Through investigating the optimum pHe to minimize the impurity extractions and to maximize yttrium extraction, it was confirmed that the extraction rate of yttrium decreased minimally by 0.5% pHe not to 2.13 (pH 3.05) but to 2.02 (pH 2.53), whereas the extraction rate of calcium decreased remarkably by 34.1% and no magnesium was extracted. Hence, it is determined that the extraction should be performed at pHe 2.02 (pH 2.53) for the selective extraction of yttrium. However, ferrous iron still existed in the solution with the extraction rate of 94.93%, even after controlling pHe. Thus, it is suggested that the additional ferrous iron should be eliminated prior to and after the solvent extraction processes. Considering this, an experiment to eliminate ferrous iron was performed using 10% lime slurry in the pH range of 1.99–5.00. The precipitation rate increased from 2.01% to 97.77% as the pH was increased from 1.99 to 5.00, and equilibrium was achieved at 97 ± 0.5% at pH values over 3.8. It was observed that no precipitation was formed with calcium and yttrium beside ferrous iron in the pH range used. The formation of hydroxide may occur at pH values over 8 for yttrium and over 11 for calcium according to the Eh–pH diagram of calcium and yttrium. Hence, it can be concluded that precipitating ferrous iron would be advantageous at pH 3.8 with equilibrium considering the economic aspect. Assuming the complete elimination of ferrous iron by this process, the following experiments were performed in the solution without ferrous.

3.1.4. Isothermal extraction of yttrium To calculate the theoretical number of mixer–settler plates, an isothermal extraction curve was prepared with the resulting values after the extraction experiments for 10 min with different O/A ratios at equilibrium pH 2.02 (initial pH 2.53) using 16% D2EHPA. McCabe–Thiele diagram is shown in Fig. 5 for the extraction of yttrium. As can be seen in the figure, more than 99% of yttrium ions were extracted in the solution with only two plates as the organic phase at the O/A ratio of 10:1. It is considered that approximately 0.38 g/L of yttrium would be left in the raffinate in the case of extraction under this operation condition. However, the McCabe–Thiele Diagram demonstrates the ideal value, and so it is considered to operate three phases at the actual extraction process using mixer–settler plates.

Fig. 6. The effect of (a) shaking time by different metal acids (3 N acid concentration, 25 °C, O:A = 1:1 (5 mL)] and (b) acid concentration [15 min, 25 °C, O:A = 1:1 (5 mL)] on the yttrium stripping efficiency.

3.2. Stripping of yttrium from loaded organic phase 3.2.1. Effects of type of metal acids and concentration The target metal in the organic phase cannot be used as is anywhere; therefore, a recovery process for the aquatic phase is again required for the target compound transferred to the organic phase, which is called stripping (Innocenzi et al., 2017). The stability of the extractant is determined by the concentration and type of stripping solutions, hence, additional studies are required on the features of yttrium stripping in terms of the equilibrium times of stripping rate, types of acids, and concentrations of acids, which would significantly affect the stripping rate. Therefore, the features of yttrium stripping from organic solvents were investigated by using different strong acids such as HCl, H2SO4, and HNO3 at a 1:1 volume ratio of aqueous to organic phase. The effects of the concentrations of acid were checked within the range of 0.5–7.5 N. Fig. 6a shows the results of the stripping rates of yttrium based on the acid type and time. It can be seen from the results that all types of acids achieved equilibrium in 10 min, with the following order of stripping rates: HCl (78.12%) > H2SO4 (76.36%) > HNO3 (74.86%). However, the differences in yttrium stripping rates were minimal using different types of acid, with less than 4% deviation. Hydrochloric acid (HCl) was found to be effective for stripping REMs. Few studies have been performed to determine the most suitable acid for stripping, but

Fig. 5. McCabe–Thiele diagram for extraction of yttrium. 6

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– 99

Konishi et al. (1998) Fu et al. (2011) Rout et al. (2013) Fu and Tanaka (2006) Shimizu et al. (2005) Juang and Huang (1997) Thakur (2000) Gupta et al. (2003) Li et al. (2007) This study – –

some studies concluded differently that sulfate (Fan et al., 2013), chlorate (Parhi et al., 2015), and nitrate (Desouky et al., 2009) were favorable for the stripping of REMs based on previous reports, so it is difficult to decide which acid is the most suitable for yttrium stripping. Fig. 6b exhibits the stripping rate by the type of acid; it increased up to 3 N and decreased slightly at 3 N, reaching equilibrium from 4.5 N. These findings are consistent and in agreement with the data reported by Desouky et al. (2009). The comparision of various solvent extraction studies for Yttrium with obtained results are presented in Table 4. Using HCl as a stripping agent on a large scale requires more research considering the environment and economic concerns, which would be future goal of the current research work. 3.3. Significance of research work and future perspectives

acid

acid

acid

acid

6 5.1–5.6 1.-4. – – – – – – 2.53

South Korea has a huge electronics market, and it needs large amounts of REMs, particularly yttrium. Thus, the isolation and purification of yttrium by solvent extraction using fluorescent lamp waste could be a sustainable option in both economic and environment aspects. The goal of this study was to develop a solvent extraction process for the selective isolation and purification of yttrium using fluorescent waste powder. To achieve better results, the study determined the specific extractant and optimized various operational conditions. The results obtained from this study are significant as compared to the literature and are expected to be used as meaningful data for the collection and processing of yttrium from a variety of products containing yttrium, including fluorescent waste lamp powder. In the solvent extraction process, the cost and technical conditions mainly rely on the selection of extractant, operational conditions, and its selectivity. Furthermore, the economics of the process are mainly based on the market cost of REMs. To achieve more a sustainable, economically viable process, there is a need to develop various modeling options and computational methodologies using Kremser and mass balance equations.

Oxalic acid Hydrochloric – Hydrochloric Nitric acid – Hydrochloric – Nitric acid Hydrochloric Hydrochloric acid Hydrochloric acid Nitric acid Nitric acid Nitric acid Lactic acid – – Nitric acid Sulfuric acid Exxsol D80, isoparaffin hydrocarbon Shellsol D70 (Shell hemicals) – Shellsol D70 – Xylene Kerosene Kerosene Heptane Kerosene Versatic 10, PC-88A P507 (Daihachi Chemicals) [A336] + [DEHP],[A336] + [DGA]PC-88A tri-n-butyl phosphate (TBP) TOA(Tri-n-octylamine), D2EHPA DEHPA, PC-88A Cyanex 923 CYANEX 925 (mixture of branched chain alkylated phosphine oxides) Versatic Acid 10, D2EHPA, TOPO

Stripping Acid Type Extraction Acid Type Organic Solvent Extracting reagent

Table 4 Comparison of solvent extraction researches for recovery of yttrium.

– 99 – 95

Source pH

Efficiency (%)

G.D. Saratale, et al.

4. Conclusions The aim of this study was to understand the extraction mechanism of yttrium by optimizing the operational conditions, including reaction pH, equilibrium time, concentration of extractants, and O/A ratio, through the selection of four extractants that have different chemical features on REM extraction. From the results of the reaction mechanisms with yttrium by each extractant, yttrium was confirmed to form complex compounds at concentration ratios of 1:3, 1:1, and 1:2 with Versatic Acid 10, D2EHPA, and TOPO, respectively. Further the optimum pHe 2.02 was investigated to minimize the impurity extractions and to maximize yttrium extraction. Upon calculating the theoretical number of mixer–settler plates, more than 99% of yttrium ions could be extracted and approximately 0.38 g/L of yttrium would be left in the raffinate in the case of extraction under this operation condition. In addition, as a result of the stripping experiment, all types of acids reached equilibrium after 10 min, with the order of favorable stripping rates being HCl (78.12%) > H2SO4 (76.36%) > HNO3 (74.86%). This research is at an initial stage; however, it is considered that the results from this study could facilitate REM extraction using fluorescent lamp waste on a large scale and have the dual benefits of waste management and a sustainable supply of REMs. However, further research is required to make this process more practically applicable, particularly in terms of utilization of information, mechanisms at different extraction stages, and designing of various modeling options. CRediT authorship contribution statement Ganesh Dattatraya Saratale: Methodology, Conceptualization, Validation, Writing - review & editing. Hee-Young Kim: Methodology, Investigation, Data curation, Writing - original draft, Visualization. 7

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