Facile method for the selective recover of Gd and Pr from LCD screen wastes using ultrasound-assisted leaching

Rare earth elements (REE) are essential for the production of technological devices. However, their high demand and low availability, together with an increase in electronic waste generation, compel the development of ecient, economic and green methods for recovering these elements from electronic waste. In this work, a facile method for selective recovering of REE from Liquid Crystal Display (LCD) screen wastes, employing ultrasound assisted leaching is presented. The screen wastes were milled and sieved to pass through a -325 mesh sieve. The milled powder was subjected to ultrasound-assisted leaching in an aqueous medium, at room temperature (25 °C) and pH 6 for 60 minutes. Subsequently, a magnetic separation was applied to the leach residue. ICP was employed to quantitatively analyze the composition of the LCD powders and determine the effectiveness of the extraction process. SEM-EDS allowed qualitative chemical analysis of the solid materials. The results shown that the LCD screen wastes are formed, mainly, by amorphous oxides of Si, Fe, In, Sn and REE. The amount of Gadolinium (Gd) and Praseodymium (Pr) in the wastes were 93 mg kg -1 and 24 mg kg -1 , respectively, which justies their recovery. X-ray diffraction analysis of the magnetic portion of the leach residue, conrmed the presence of an amorphous phase together with crystalline metallic iron alloy. The magnetic behavior, obtained by Vibration Sample Magnetometry, helped to understand the nature of the residues. The formation of this metallic alloy is attributed to the effect of high power ultrasonic during the leach. It was conrmed that the magnetic residues concentrates and recovers 87 wt. % of Gd and 85 wt. % of Pr contained in the original material. Therefore, ultrasound-assisted leaching is a selective and facile method for recovering Gd and Pr from waste LCD.


Introduction
Currently, electronic waste has become one of the major contributors to environmental pollution, mainly due to the politics of programmed obsolescence of many electronic devices over the past 20 years [1]. Now-a-days, the average life of a computer or a mobile phone is between 4 and 5 years [2], considerably increasing the amount of electronic waste generated [3]. Since the end of the 20th century, LCD (Liquid Crystal Display) screens have been used in various electronic products due to the advantages of light quality, small volume and low energy consumption; therefore, more than 700 million LCD panels have been produced worldwide in recent years. Considering the average lifespan of 3−8 years of these panels, large quantities of LCD panels will be reaching their end-of-life in the coming years, tremendously contributing to the generation of electronic waste [4]. Despite the many disadvantages, these wastes contain important amounts of different elements with high commercial value [5,6]. Among these, the rare earth elements (REE) [7] are considered critical raw materials [8], due to the di culty of their separation, acquisition and the uncertainty of their disposition [9]. Recent studies have shown the quantities of REE in LCD screens are economically attractive for recovery, however, the proposed methods have focused mainly on the backlight or LED´s of the LCD screens [10,11]. In contrast, this investigation studies the recovery of Gadolinium (Gd) and Praseodymium (Pr), among others, contained in the LCD panels, a subject that has not been reported. Furthermore, the importance of the recovery these REE is implicit not only in their use in new technologies, but also in their current high prices, which are among the REE with the greatest economic value [12].
The development of REE recovery strategies has been trending in recent years; most of the proposed processes employ inorganic acids such as HCL, H 2 SO 4 or HNO 3 for leaching [13][14][15], which can damage the environment and human health, if they are not adequately controlled. Additionally, more environmentally friendly reagents, such as sulfate-roasting followed by water leaching [16] and ionic liquids [17] have also been studied, to leach the REE. However, the chemicals and conditions employed in both proposals are costly and involve complicated downstream separation processes.
In the present investigation, the pyrophosphate ion (PPi) is used as a less hazardous alternative to inorganic acids since it has been employed as a selective ligand for the REE ions in other studies [18]. It is also employed in the preparation of medicines and the preservation of different foods, proving to be safe for the environment and for humans [19,20]. A thermodynamic study, in the form of species distribution diagrams using the Hydra-Medusa software suite [21] was performed to determine the appropriate leaching conditions. In addition, ultrasound was employed as an enhancement method during the leach, since recent studies have shown that the use of ultrasound accelerates the dissolution of metals [22] and rare earth elements, achieving nearly 100% in 1 to 3 hours [23][24][25]. In sonochemistry, molecules undergo chemical reactions promoted by the application of ultrasound radiation (20 kHz -10 MHz) in solidliquid systems; ultrasound enhances the diffusion of soluble species in the liquid phase and increases the rate of penetration into the solid principally by the cavitation effect, which leads to the creation of many microcracks on the solid surface. Furthermore, if the raw material is a powder, ultrasound energy can cause particle rupture, with a consequent increase in surface area available for reaction [26].
Hence, the present study explores the recovery of the REE compounds (Pr and Gd) from waste LCD screens by ultrasound-assisted leaching, using pyrophosphate ion. This proposal introduces a quick, easy and inexpensive method to recover these valuable elements from wastes.

Materials And Methods
The LCD screens were collected from electronic wastes (televisions, cell phones, electronic tablets, laptops and cameras). The total weight of a waste LCD screen was not determined, since it varies according to the type of electronic device. The LCD screens were cleaned by separating components that were not required, such as polarized, adhesives, connectors, diffusive sheets, re ective sheet and, the plastic frame. Subsequently, the LCD were subjected to the leaching assisted by ultrasound process or sono-leaching, followed by a magnetic separation shown in Figure 1.
First, 500 g of LCD screen wastes were milled for 30 minutes, using an automatic mortar grinder. The milled powder was sieved to pass through -325 mesh sieve, since the largest quantity of RE elements is recovered by leaching at this particle size [27]. The milling process to obtain the ne particle sizes consumes extra time and energy in the process; however, the use of inexpensive and safe reagents, compensate this expenditure, rendering the process economically feasible.
Subsequently, representative samples of 3 g were taken from the milled and screened powder, using the quartering method. The powder obtained was characterized by X-ray diffraction, XRD, (Equinox 2000 diffractometer), scanning electron microscopy with Energy-dispersive X-ray spectroscopy (SEM-EDS-Jeol, model IT 300) and, by vibration sample magnetometry, VSM (MicroSense EV7) at room temperature (20°C ) and a maximum magnetic eld of ± 18 kOe.
In order to quantify the chemical composition of the raw materials, 3g samples of LCD powders were digested in aqua regia (HCL:HNO 3 , 3:1) at 90 °C during 2 h. The obtained liquor was ltered. Aliquot of 1.5 ml of the liquor were diluted to 30 ml with deionized water, for their analysis by inductively coupled plasma (ICP-OES, Perkin Elmer, Optima 3000 XL). All tests were performed in triplicate in order to assure the repetitively of the analysis.
For the leaching tests, 3 g of the milled and sieved powder were immersed in a beaker with 150 mL of the solution of PPi (pyrophosphate ion, P 2 O 7 4-) 0.05 M PPi as leaching agent, using a liquid-to-solid ratio of 20 gL -1 . The solution pH was adjusted and maintained at 6, using sulfuric acid 1 M. These experimental conditions were selected in accordance with the results of previous studies [18]. The leaching solution was sonicated during the leaching time, using an Ultrasonic Homogenizer 300VT, equipped with a piezoelectric transducer at a frequency of 90 kHz and a solid titanium tip of 9.5 mm. The experiments were performed at a sonication output power of 120 W. The beaker was placed into an ice bucket, to guarantee that the temperature of the solution in all cases did not exceed room temperature (25°C). The ultrasound was used to promote the exposure of the rare elements to the leaching agent, through the rupture of the particles, and to increase the rate of penetration into the solid by the cavitation effect [26]. The leaching solution was ltered, obtaining a liquor and a solid phase (leach residue). The leach liquor was characterized by ICP. The solid residue was magnetically separated, into non-magnetic and magnetic powders, after having been air-dried at 80°C for 15 min. The magnetic and non-magnetic residues were characterized by XRD, SEM-EDS, VSM and ICP. The digestion procedures of the solid residues, for ICP analysis, is the same as that previously described for LCD powders.

Results And Discussion
In Figure 2, the XRD pattern of the milled LCD screen powder is presented. As may be observed, no speci c diffraction peaks are exhibited, indicating an amorphous material. This result may be expected, since the main component of the LCD screens is silicon [28], combined with small amounts of different metallic oxides, such as indium, REE and tin (not detectable by XRD due to the detection limit of the diffractometer).
To con rm the presence of rare earth elements (REE) in the milled and sieved LCD powder, SEM-EDS qualitative elemental analyses were carried out. The results are shown in Figure 3, where the qualitative chemical distributions of different elements are shown; silicon, aluminum, some REE, indium, tin and iron may be observed. In addition, all the RE elements are uniformly distributed. As can be appreciated, the elements with the highest concentrations are silicon, aluminum and oxygen, probably as oxides compounds (SiO 2 and Al 2 O 3 ), whereas the REE are concentrated in the smallest particles. Moreover, the ne particle size ensures the homogeneity of the sample and the percentage of the rare earth elements that can be recovered [29]. For this reason, the powder sieved at -325 mesh (44 µm) was selected for the leaching study.
The results of the chemical analysis obtained by Inductively Coupled Plasma (ICP-OES) show the presence of rare earths, such as Pr (24 mg kg -1 ), Gd (93 mg kg -1 ), Er (477 mg kg -1 ) and others elements, such as In (2422 mg kg -1 ), Sn (835 mg kg -1 ), Fe (2827 mg kg -1 ) and Zn (9 mg kg -1 ). According to the structural and chemical characterization (SEM-EDS and ICP), the LCD screen waste is composed of a mixture of oxides of Si, Al, Fe, and small amounts of oxides of Gd, In, Pr and Er. It is important to note that these materials are in su cient quantities to justify their separation [30].
To select the adequate experimental conditions for selectively separating Gd and Pr from the other elements, a thermodynamic analysis using the Hydra-Medusa software [21] was performed; this analysis shows that Gd(III) forms a soluble species Gd 2 (P 2 O 7 ) 2+ with the pyrophosphate ion (P 2 0 7 ) 4up to pH 8, In addition, the interaction between PPi ion and Fe(III) was considered, since plays an important role in the proposed method, which is based on a nal magnetic separation of the leaching residue. The Hydro-Medusa analysis con rms that Fe(III) forms a stable complex, Fe 2 (P 2 O 7 )up to pH 8, with a log k of 22.2.
Therefore, based on theoretical analyses, under the experimental conditions selected, the PPi forms stable complexed with Gd(III), Pr(III) and Fe(III). Furthermore, these ions precipitate as hydroxides in alkaline solutions (above pH 8). Species distribution diagrams are constructed from the logarithm of the reaction equilibrium constant (k) of the reagents [31]. The speciation diagram for the Gd is shown in Figure 4. This analysis helped to establish the adequate leaching conditions: pH values between 4 and 6, room temperature (25 °C), assisted by ultrasound to improve the dissolution process [25,33].
After performing the ultrasonic-assisted leaching process for 60 minutes at room temperature, a solid residue was obtained (leach residue), which was analysed by XRD ( Figure 5). As can be appreciated in Figure 5a, the leach residue consisted of an amorphous material, together with small amount of crystalline Fe, which is identi ed as a peak near to 2-theta of 44 °. Due to the presence of metallic iron, the residue was subjected to a magnetic separation, obtaining a magnetic and a non-magnetic solid. Both solids were independently analyzed by XRD (Figures 5b and 5c). As can be observed, the non-magnetic residue (Figure 5b) shows an XRD pattern typical of an amorphous material, attributed to silica base material, which was not affected by the leach. In contrast, the magnetic residue (Figure 5c) is a crystalline iron matrix, probably with small amounts of other metals (gadolinium, praseodymium or similar elements), since a slight displacement of the diffraction peak is detected from its theoretical position at 2-theta of 44 °. The three residues (combined leach, magnetic and non-magnetic), were qualitatively characterized by SEM, using back-scattered electrons (BSE). As can be observed, the powders are composed by irregular and polygonal particles. In addition, there are no differences in contrast in each residue, which indicates that the residues contain a homogenous distribution of atoms along the particles. However, comparing the different residues, the magnetic residue (Figure 5c) appears brighter, which may be ascribed to the presence of compounds that contain atoms with greater atomic number, such as REE.
To characterize their magnetic behavior, the hysteresis loops of each residue were acquired and are presented in Figure 6. In this gure, it can be observed that the magnetic residue presents a saturation magnetization of 120 emu g -1 , attributed to the presence of an iron alloy with unde ned composition, in good agreement with the XRD pattern show in Figure 5(c). It is known that, pure iron shows a speci c saturation magnetization near to 217 emu g -1 , therefore, the reduced magnetization value corresponds to iron, containing very low concentrations of materials that possesses slight magnetization, in accordance with the XRD patterns, since no other phases were detected.
The non-magnetic residue shows ferrimagnetic behavior, with a very low speci c saturation magnetization of approximately 0.08 emu g -1 , attributed to the presence of small amounts of ferrimagnetic materials as oxides, although these was not observed in XRD pattern due to the detection limit of the analysis equipment.
In addition, the magnetic hysteresis loop of the combined leach residue shows ferrimagnetic behavior, with a speci c saturation magnetization around 0.19 emu g -1 . This con rms mostly amorphous silica and aluminum oxides, together with small quantities of ferrimagnetic materials, as iron and RE metals and/or oxides.
The chemical composition of the leach liquor and the solid residues (magnetic and non-magnetic) were quanti ed by ICP. The results are shown in Table 1 as the percentages of the total element in the initial LCD powder in each of the following states: present in the leach liquor, remaining in the LCD powder (nonmagnetic residue) or recovered in the magnetic residue. According to these results, the magnetic material (0.3 g) is composed mainly of Fe, Pr and Gd, corresponding to 94.5%, 86.8% and 85.4%, respectively, of the total amount of each element contained in the LCD screens; this represents an important concentration of these elements, with a higher recovery compared to conventional leaching [34]. On the other hand, 98.6% of the In, 73.9% of the Sn and 84.34% of the Er remained in the non-magnetic solid (2.58 g). As for the leach liquor, it contained appreciable percentages of Er (12.0%), Sn (24.6%) and Zn (91.2%).
It is worth mentioning that when the leaching process is carried out without PPi, the separation of Gd and Pr was not achieved nor were these elements leached. on the other hand, when the leaching is performed without ultrasound, a magnetic residue is not produced; therefore, the ultrasound radiation promotes the selective separation of Gd and Pr from other REE, as magnetic materials and the pyrophosphate ion maintains the solubility of the REE.
As the magnetic residue shown a selective separation of Gd and Pr, together with iron, an elemental mapping was performed by SEM-EDS analysis, which is shown in Figure 7. In this gure, the presence of a homogeneous distribution of Fe, Gd and Pr can be observed, con rming the concentration of these elements into the magnetic residue.
The formation of an iron base alloy containing rare earth elements, as Gd and Pr, is an interesting result in itself, and it can be ascribed to the ultrasound effect during the leaching process. It is well-known that the ultrasound produces mechanical effects, such as micro jets and shock waves, which cause microscopic turbulence in the solution and high-speed collisions between the solids [35]. These effects are di cult to achieve with conventional mechanical agitation [26]. According to some authors [35,36], sonochemistry or ultrasonic irradiation of water produces the free radicals H· and OH· that can combine to produce H 2 O 2 . In ultrasonic leaching, the formation of these agents promote an iron ion reduction from Fe 3+ and/or Fe 2+ to Fe 0 , as shown in Eq. (5), which could incorporate Gd(III) and Pr(III) into its crystal structure or they could be also reduced to metallic phases as an alloy. These solid products can be recovered by applying a magnetic eld, obtaining a concentrated magnetic residue composed mainly of Fe, Gd and Pr, as was demonstrated previously. Therefore, the magnetic separation of the residue formed after the ultrasonicassisted leach, followed by a magnetic separation, is a facile and economic method for concentrating Gd and Pr elements from LCD wastes.

Conclusions
LCD screen wastes were found to contain 93 mg kg -1 and 24 mg kg -1 of Gd and Pr, respectively. To retrieve these REE, a facile method for selective concentrating of some REE is proposed. In particular, Gd and Pr from LCD screen wastes can be effectively recovered by ultrasonic-assisted leaching, using pyrophosphate ion as complexing ligand. A retrieval of 85 wt. % and 87 wt. % of Gd and Pr, respectively, was achieved, using an ultrasound-assisted leaching for 60 minutes at room temperature. The combination of ultrasound and leaching at room temperature showed positive impacts on enhancing the separation of REEs; substantial physicochemical changes occurred during the leach assisted with ultrasound, including structural transformations, chemical radical formation, chemical reduction, and even, compound decomposition. Structural analysis and chemical decomposition, promoted by the formation of water radicals, could explain the effectiveness of the ultrasound leaching in improving the recovery of Gd and Pr from LCD screen waste. However, other valuable REE, such as In and Er, remain in the non-magnetic solid residue.

Declarations
Availability of data and materials All data generated or analyzed during this study will be made available on request.

Competing interests
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.

Funding
The authors received no speci c funding for this work.