Preliminary study on separation of rare earth metals from leach solution of discarded fluorescent powder by solvent extraction - Nguyen Duc Quang

Vietnam Journal of Science and Technology 56 (2C) (2018) 133-139 PRELIMINARY STUDY ON SEPARATION OF RARE EARTH METALS FROM LEACH SOLUTION OF DISCARDED FLUORESCENT POWDER BY SOLVENT EXTRACTION Nguyen Duc Quang, Ha Vinh Hung*, Vu Minh Trang, Le Huy Viet, Huynh Trung Hai School of Environmental Science and Technology, Hanoi University of Science and Technology, 1 Dai Co Viet, Ha Noi *Email: hung.havinh@hust.edu.vn Received: 20 May 2018; Accepted for publication: 22 August 2018 ABSTRACT Discarded fluorescent lamps were considered as hazardous waste in Vietnam (code is 160106). However, its composition contained valuable rare earth metals, which can be recycled and recovered by leaching and solvent extraction processes. The present study sought to define the conditions of separation for rare earth elementals (REEs) from acid leach solution by solvent extraction using PC88A. The acid leach solution was obtained from leaching of fluorescent powder. Efficiencies on REEs extractions as functions of pH levels, extractant concentration, O/A ratio, metal concentration were investigated. For pH values greater than 0.0 and less than 1.0, the orders of the yields extraction were determined: yttrium > terbium > europium > aluminum and calcium. The best separation circuit included four-stage counter current extraction for yttrium separation with 20 %v/v PC88A in kerosene (O/A = 1/1, room temperature, 20 min of contact, pH of 0.38) and stripping step with 3 M HCl acid (O/A = 1/1, room temperature, 30 min of contact). After yttrium separation, the leach solution was extracted at pH of 0.98 in the same other conditions with two-stage counter current extraction to recover terbium and europium group. The final recovery from leach liquor was higher than 98 % and the grade of the final product was 94.2 % (for yttrium) and 93.8 % (for terbium and europium group). Keywords: rare earth, recovery, fluorescent lamp, solvent extraction, PC88A. 1. INTRODUCTION Legally, the discarded fluorescent lamps are considered as hazardous waste (code 160106 as classified in Circular 36/TT-BTNMT on management of hazardous wastes), due to the Hg vapor and fluorescent powder composition. Nevertheless, it also contain Rare Earth Elements (REEs), which are used as optical composition of fluorescent powder. Modern fluorescent lamps use a mix of red (Y2O3: Eu3+), green (CeMgAl10O17: Tb3+) and blue (BaMgAl10O17: Eu2+) phosphors to generate white light [1]. Therefore, light color of the lamp depends on the REEs presented in the fluorescent powder. In red light lamp, Y2O3 is taken 85.3-90.0 % of fluorescent Ha Vinh Hung, et.al. 134 powder and Eu2O3 is about 6.8-7.6 % [2]. In green light lamp, CeO2 is 11.5-15 % and Tb4O7 is 6.2-7.4 %. The number is 2.0-2.2 % for Eu2O3 in blue light lamp. The weight of Y2O3 in a lamp ranges from 0.005 g/unit (in case of light emitting diodes) to 0.70-0.77 g/unit (in case of compact fluorescent lamp) and even up to 1.1-1.2 g/unit in case of linear fluorescent lamp [3]. The phosphors mixture accounts for 2 wt% of a typical 40 W fluorescent lamp (4–6 g powder) [1]. Hydrometallurgical methods have been studied for the recovery of REEs in fluorescent lamps. Leaching, precipitation and solvent extraction were investigated completely. Major solvents are researched including HCl [1, 4, 5], H2SO4 [4-6], HNO3 [7-8]. The extraction solvents were mostly including Tributyl phosphate, 2- ethylhexyl phosphoric acid, mono 2- ethylhexyl ester and Di (2-ethylhexyl) phosphoric acid [9, 10]. For the precipitation, oxalic acid and ammonium bicarbonate are widely used [6]. The common efficiency of these researches is more than 95 %. In Vietnam, the fluorescent lamps are now only treated as a hazardous waste in 24 licensed facilities with the average capacity of 0.2 ton/day. All of the treatment lines are focused on treatment/recovery Hg vapor, aluminum part and glass, but not recycling of fluorescent powder. Nevertheless, the research on recovery REEs from fluorescent powder is rare. In our previous research, HCl has been used to leach Y and Eu from the fluorescent powder with 100 % of Y yield and 99 % of Eu [11]. This research focuses on the extraction of REEs using PC88A in kerosene from acid leach solution obtained from leaching of fluorescent for the efficient recycling of fluorescent lamps in Viet Nam. 2. MATERIALS AND METHODS 2.1. Materials and reagents The experiments were conducted by using the acid leach solution from discarded fluorescent powder. The composition of REEs and others have been determined by ICP-MS and shown in Table 1. The PC88A in kerosene was used as extractant. NaOH solution (1M) was added to adjust the pH values of the leach solution as requested by the experimental tests. Table 1. Chemical composition of the leach solution. Element Y Eu Tb Al Ca Concentration (mg/L) 511.90 55.01 32.60 153.00 29770 2.2. Solvent extraction tests Loading of metals from REE-containing leachate was conducted using PC88A in kerosene. The solutions were mixed by separatory funnel shaker (Lab companion RS-1, Korea) having six separating funnels. The investigated factors were pH values, extractant concentration, O/A ratio, metal concentration in order to define the optimum conditions. Each test was performed by shaking with speed of 250 rpm (vertical reciprocating) for 20 min in separatory funnel, the 100 ml volume of aqueous phase and organic phase at ambient temperature. After contacting time, the funnel-containing samples were rested on the rack for separation. Stripping with 2 M hydrochloric acid solutions was investigated under similar conditions. The equilibrium pH of Preliminary study on separation of rare earth metals from leach liquor of discarded fluorescent 135 aqueous phase was determined by titration of H+ ion concentration. The amounts of metals extracted/stripped were calculated using mass balance, by measuring their concentration in the aqueous phases before and after phase contact by ICP-MS [11] (Elan 9000 – Perkin Elmer). 3. RESULTS AND DISCUSSIONS 3.1. Effect of equilibrium pH on extraction efficiency The investigation results on the effect of pH on the extraction efficiency are shown in Fig. 1. It is found that, for each metal ion the extraction efficiency increases as the pH increases and there is a sudden change within a narrow pH range: Y3+ ion from (-0.18) to 0.56; Tb3+ ion from 0.16 to 0.86; and Eu3+ ion from 0.57 to 1.14. It can be explained that the extraction reaction is the reciprocal reaction and following the Le Chatelier’s principle, when the pH of the solution increases, the concentration of H+ should be decreased and the equilibrium moves to generate RE(HX2)3or by the extraction reaction: RE3+w + 3H2X2or ↔ RE(HX2)3or + 3H+w (1) In addition, it is found that, at pH ≤ 1.09 almost impurities (Al3+ and Ca2+ ions) are not extracted, the extraction efficiency is about 11% and <1.6%, respectively. The coefficient of thermal equilibrium dynamic of extraction reaction is calculated as: Keq = = D* (2) where: RE3+ denotes rare-earth element; H2X2 is PC88A solvent; w denotes water phase; or denotes organic phase and D is distribution coefficient, D=[RE(HX)3or]/[RE3+w]. Depending on the properties of the metal, the dependence of the distribution coefficient D on the pH of each metal ion is not the same. This behavior can be attributed to the difference of stability constants of metal complexes with PC88A. Thus, pH control is the most basic procedure for separating metals. From this result in Fig. 1 also showed that at pH of 0.38 the extraction efficiency of Y, Tb, Eu, Al and Ca is about 79 %, 35 %, 11 %, 1.6 % and <1.6 %, respectively, thus it can be selected the pH of 0.38 for Y separation in first stage. Similar, after separation of Y, it can be selected the pH of 0.98 for separation of Tb and Eu group in second stage due to the extraction efficiency of Tb, Eu, Al and Ca is about 96 %, 87 %, 11 % and <1.6 %, respectively. 3.2. Effect of extractant ratio on extraction efficiency The investigating result on extractant ratio Ror (ml/ml) on the extraction efficiency is shown in Fig. 2. It is found that at the beginning of reaction, when Ror increases, the efficiency increases rapidly. But as the percentage of solvent in the solvent increases, the efficiency of the process has slowly increased up to balance point. At the initial time, the concentration of REEs exceeds the extraction capacity of the extractant. It is clear that, PC88A ratio was increased synonymous with the increasing of [HX2]- radical, lead to the number of [HX2]- – metal ion pairs increased so the efficiency was increased. However, the metal ion concentration was fixed so that the efficiency of the process has slowly increased up to the balance point. Ha Vinh Hung, et.al. 136 Figure 1. Effect of equilibrium pH on extraction efficiency. Figure 2. Effect of PC88A ratio on extraction efficiency. 3.3. Effect of REEs concentration in the acid leach solution Figure 3. Effect of extractant concentration on extraction efficiency of Y. Figure 4. Effect of extractant concentration on extraction efficiency of Eu and Tb. Figure 5. Effect of organic/acid leach solution phase ratio on extraction efficiency. Figure 6. Effect of rate of organic/ acid leach solution phase ratio on organic phase after extraction. Preliminary study on separation of rare earth metals from leach liquor of discarded fluorescent 137 Figure 7. Determination of number of Y extraction and stripping step. Figure 8. Determination of number of Eu and Tb extraction and stripping step. Figure 3 and Fig. 4 show that the extraction efficiency slightly decreases when metal concentration in the acid leach solution increases. It is due to (1) the limited concentration of extractant; (2), the forming of H+ ion is increased when metal concentration in the acid leach solution increases, lead to the decreasing of efficiency; and (3), the increasing of metal concentration will cause the decreasing of activity degree of REEs ion, which cause the decreasing of efficiency. 3.4. Effect of organic/acid leach solution ratio The increase of organic/acid leach solution (O/A) rate (i.e. increase of [H2X2]) will increase the extraction efficiency as follow the Le Chatelier’s principle in Equation 1, as shown in Fig. 5. Nevertheless, it leads to the decrease of REEs concentration in the organic phase (Fig. 6), and thus, cause the decrease of recovery REEs. This behavior is similar to the case in subsection 3.2. 3.5. Recovery REEs from acid leach solution The determination of extraction and stripping step for REEs recovery is shown in Fig. 7 and Fig. 8, based on the investigation results. 3.5.1. Yttrium extraction Table 2. REEs and metal concentrations in stripping solution of yttrium. Element Y Eu Tb Al Ca Concentration in stripping solution (mg/L) 511.43 5.88 25.19 <0.06 0.558 Purity (% of cations) 94.2 1.1 4.6 0 0.1 Based on the investigation results, the proper separation circuit is selected included four- stage counter current extraction for yttrium separation with 20 %v/v PC88A in kerosene (O/A = 1/1, room temperature, 20 min of contact, pH of 0.38). The extract after extraction was stripped a stage with 3 M HCl acid, which equals with pH -0.477 (O/A = 1/1, room temperature, 30 min of contact). The composition of yttrium stripping is shown in Table 2. Ha Vinh Hung, et.al. 138 3.5.2. Europium and terbium extraction After yttrium extraction, the raffinate was extracted at pH of 0.98 in the same other conditions with two-stage counter current extraction to recover terbium and europium group. The extract phase was stripped a stage with 3 M HCl acid, the composition of europium and terbium group stripping is shown in Table 3. Table 3. REEs and metal concentrations in stripping solution of terbium and europium group. Element Y Eu Tb Al Ca Concentration in stripping solution (ppm) 0.32 49.00 6.11 0.41 2.92 Purity (% of cations) 0.5 83.4 10.4 0.7 5.0 4. CONCLUSION In this research, the efficiencies on rare-earth elements extractions as functions of pH levels, extractant concentration, O/A ratio, metal concentration were investigated. Based on that, the proper conditions of REEs recovery were set up to investigate the efficiency of Y, Eu and Tb recovery from acid leach solution that was leached from fluorescent powder of discarded fluorescent lamps in hydrochloric acid. The grade of the final product was 94.2 % (for yttrium) and 93.8 % (for terbium and europium group). Acknowledgement. The authors wish to acknowledge Ministry of Education and Training of Vietnam for financially supporting under project entitled "Studying on Yttrium and Europium recovery technology in waste fluorescent lamps – Code: B2017-BKA-43". REFERENCES 1. Cristian Tunsu, Martina Petranikova, Christian Ekberg, Teodora Retegan - A hydrometallurgical process for the recovery of rare earth elements from fluorescent lamp waste fractions. Separation and Purification Technology 161 (2016) 172–186. 2. Yufeng Wu, Xiaofei Yin, QSijun Zhang, Wei Wang, Xianzhong Mu - The recycling of rare earths from waste tricolor phosphors in fluorescent lamps: A review of processes and technologies. Resources, Conservation and Recycling 88 (2014) 21–31. 3. Erika Machaceka, Jessika Luth Richterc, Komal Habibd, Polina Klosseke - Recycling of rare earths from fluorescent lamps: Value analysis of closing-the-loop under demand and supply uncertainties. Resources, Conservation & Recycling 104 (2015) 76-93. 4. Otto R., Woojtalewicz-kasprazac A. - Method for recovery of rare earths from fluorescent lamps, US Patent 0027651 A1, 2012. 5. Yufeng Wu, Xiaofei Yin, QSijun Zhang, Wei Wang, Xianzhong Mu - The recycling of rare earths from waste tricolor phosphors in fluorescent lamps: A review of processes and technologies. Resources, Conservation and Recycling 88 (2014) 21–31. 6. Zhang S. G., Yang M., Liu H., Pan D. A., Tian J. J. - Recovery of waster a reearth fluorescent powders by two steps acid leaching, Rare Metals 32 (2011) 609–615. Preliminary study on separation of rare earth metals from leach liquor of discarded fluorescent 139 7. Nicolo Maria Ippolito, Valentina Innocenzi, Ida De Michelis, Franco Medici and Francesco Veglio - Rare earth elements recovery from fluorescent lamps: A new thermal pretreatment to improve the efficiency of the hydrometallurgical process, Journal of Cleaner Production 153 (2017) 287-298. 8. Sherrington L. - Commercial processes for rare earths and thorium. In:Lo T. C., Baird M. H. I., Hanson, C.(Eds.), Handbook of Solvent Extraction, Wiley Interscience, New York, 1983, pp.712–723. 9. Preston J. S. - The recovery of rare earth oxides from a phosphoricacid by product. Part4. The preparation of magnet grade neodymium oxide from the light rare earth fraction, Hydrometallurgy 42 (2) (1996) 151–167. 10. Vinh Hung Ha, Trung Hai Huynh, Minh Trang Vu, Huy Viet Le, Duc Quang Nguyen, Nhat Quang Nguyen - Leaching kinetics of rare earth elements from wasted fluorescent powder, Book of extended abstract of the 10th regional conference on environmental engineering 2017 (RC EnvE 2017), Hanoi University of Science and Technology, Hanoi, Vietnam, 2017, pp. 117-119, 11. Innocenzi V., Ippolito N. M., Pietrelli L., Centofanti M., Piga L., Veglio F. - Application of solvent extraction operation to recover rare earths from fluorescent lamps, Journal of Cleaner Production 172 (2018) 2840–2852.