The pyrometallurgical processes are able to separate and recover almost 99% of REEs from the waste NiMH batteries and NdFeB magnet scraps. Experimental ...
Recycling the Rare Earth Elements from Waste NiMH Batteries and Magnet Scraps by Pyrometallurgical Processes Kai Tang, Arjan Ciftja, Ana Maria Martinez, Casper van der Eijk SINTEF Materials and Chemistry, N-7465 Trondheim, Norway Yuyang Bian, Shouqiang Guo, Weizhong Ding Shanghai Key Laboratory of Modern Metallurgy and Materials Processing, Shanghai University, Shanghai 20072, China
Abstract: The rare earths (REs) are now considered as the most critical, with the highest supply risk raw materials in EU. Because there exist only few exploitable natural resources of rare earths in Europe, the EU will have to rely on recycling of REs from pre- and post-consumer scraps and especially end-of-life products. Nickelmetal hydride (NiMH) batteries and rare earth magnets are the typical urban mining resources. High temperature processes designed for recovery of rare earth elements from the waste nickel-metal hydride batteries and magnet scraps have been developed recently. The calcium silicate slags and FeO-B2O3 fluxes were chosen to recover the REEs from the waste flows. The key parameters for high temperature recycling of rare earth elements (REEs) have been extensively studied both theoretically and experimentally. The pyrometallurgical processes are able to separate and recover almost 99% of REEs from the waste NiMH batteries and NdFeB magnet scraps. Experimental results also show that the RE oxides obtained from the high temperature treatment can reach as purity as 96 wt%. Key words: Rare Earth Elements, Recycling, NiMH Battery, Magnet Scraps, Pyrometallurgical Process 1. Introduction The increasing popularities of electronic consumer goods, hybrid and electric cars, and wind turbines lead to an unprecedented increase in the demand of rare earth elements (REEs). The rare earths are now considered as the most critical, with the highest supply risk raw materials in EU. Because there exist only few exploitable natural resources of rare earths in Europe, the EU will mainly have to rely on recycling of REEs from pre- and post-consumer scraps and especially End-of-Life (EoL) products, known as “urban mining”. Nickel-metal hydride (NiMH) batteries are currently used in many mobile applications: hybrid and electric cars, laptops, and mobile phones etc. Because NiMH batteries have about twice the energy density of Ni-Cd batteries and a similar operating voltage as that of Ni-Cd batteries, they are expected to become a mainstay in the current rechargeable batteries. The most common rare earth magnets are based upon a neodymium-iron-boron alloy (Nd2Fe14B as matrix phase, surrounded by a Nd-rich grain boundary phase), with small additives of Pr, Tb, and especially Dy. The heavy rare earth elements (HREEs) are added to magnets to increase their temperature resistance. The NdFeB magnets contain more than 30 wt% of REEs. It is estimated that 20-30% NdFeB scraps will be produced in the manufacturing process[1]. Furthermore, NdFeB magnet is easily to be oxidized at higher temperatures[2]. The oxidized scraps are not able be reuse in the conventional magnet production. It is essential to recycle the magnets and particular to extract the rare earth elements from the oxidized magnet scraps. Sustainable industrial recycling processes for the EoL NiHM batteries and pre-/post-consumer NdFeB magnets are still under developing. Several hydrometallurgical recycling processes for the discarded NiMH batteries have been reported in the literature[3-7]. The conventional ways to extract REEs from permanent magnets were also based mainly on the hydrometallurgical treatment[8]. However, huge amount of water and chemicals have to be used in the hydrometallurgical treatments. These processes are no longer considered as sustainable in the future. The processes based primary on the high temperature pyrometallurgical processes are now attracted more attention for their less environmental impact[9-11]. The existing state-of-the-art technologies for recycling of precious and other metals from electronic waste are based on high temperature smelting processes using Pb, Cu and Ni as collectors for the valuable metals. However, the smelting flow sheets are not developed yet for the REEs recovery, as they revert to the oxide phase (slags or fluxes) in a diluted form. One of
the key issues for the pyrometallurgical REEs recycling processes is to concentrate the REEs in the products. High temperature pyrometallurgical processes designed for recovery of REEs from the waste NiMH batteries and magnet scraps have recent been developed using the fluxes as REEs oxidants and collectors. For the recycling of the EoL NiMH batteries, the calcium silicate slags were used. The FeO-B2O3 fluxes were used for the extraction of REEs from magnet scraps. In this paper, experimental results will be presented in detail. Effects of temperature, treatment time and initial charge mixture will be examined both experimentally and theoretically. 2. Experimental 2.1. Pretreatments of NiMH Batteries The main parts of a NiMH battery are cathode, anode, electrolyte, polymer separator and the steel case. The cathode is made of nickel coated with nickel hydroxide. The anode consists of a hydrogen storage alloy based on mischmetal (mainly cerium, lanthanum, praseodymium and neodymium) and nickel alloys. The NiMH batteries used in the present study were cylindrical type with dimensions of 32mm and 89mm in height, weight about 250 grams. After mechanical processing, chemical analysis of a typical NiMH battery material indicates that it consists of 45-50wt% Ni, 9-11wt% Co and 13-16wt% mischmetal. The battery modules were first frozen in liquid nitrogen for the deactivation and brittle fracture treatment. The broken steel scraps and plastics were then separated by the mechanical classification and magnetic separation. The remaining positive and negative electrodes, together with the polymer separator, were heated to 600-800oC in a muffle furnace in order to remove the organic components and further separate the Ni-based negative electrode. XRF analyses show that the pretreated battery materials consist mainly of nickel, rare earth and cobalt oxides, as shown in Fig. 1.
Fig. 1. XRF analysis of the pre-treated materials from NiMH batteries The calcium silicate slag was a binary mixture of equimolar CaO and SiO2. This mixture was obtained by smelting calculated quantities of CaCO3 and SiO2 reagents in a Pt crucible at 1550°C. The master slag was crushed before re-melting to improve homogeneity. 2.2. Pretreatments of NdFeB magnet scraps The commercial bulk NdFeB magnets without magnetization were used as raw materials. The main compositions of the magnet were 61.6 wt% Fe, 30.7 wt% Nd, 4.4 wt% Pr, 1.6 wt% La, 0.96 wt% B and 0.8 wt% Al. The bulk NdFeB magnets was pulverized mechanically to the particles with size 98 wt% and boric acid reagent were used to synthetize the FeOB2O3 flux. FeC2O4∙2H2O and H3BO3 were first mixed with molar ratio of 1:2. The mixture was preheated in an iron crucible at 400oC for 5 hours under the inert atmosphere. The preheated mixture was then melted at 1000 oC (about 180oC higher than its liquidus temperature) for 2 hours. The sample was then cooled down to room
temperature in argon atmosphere. The flux was then crushed to fine powder before re-melting to improve the homogeneity. 3. Results 3.1. Slagging treatment of the NiMH batteries The pretreated NiMH battery materials were further refined by the high temperature slagging treatment. The calcium silicate slag was used as REEs collector in the present investigation. The slagging tests were conducted in a high temperature vacuum furnace. Temperature of sample can be measured by a W/W-Re thermocouple placed inside the crucible, while the heat generating current supplied in the furnace is adjusted manually. The graphite crucible was first filled with the pretreated materials. The slag powder was then placed on top of the pretreated battery materials and mounted to the furnace. After a vacuum of 1x10-3 mbar was reached, the furnace chamber was filled with argon gas up to a pressure of 700 mbar. The power was then supplied. The slagging process kept at 1650ºC for 60 minutes. After the furnace was cooled down to room temperature under the inert atmosphere, the crucible was taken out for sampling. Both metal and slag samples from the experiments were analyzed by electron probe microanalysis (EPMA), scanning electron microscope (SEM) and X-ray fluorescence (XRF), respectively. Fig. 2 shows the SEM micrograph of the oxide phase. The EPMA mapping of the oxide phase is shown in Fig. 3. The RE oxides were collected in the slag. In the SEM micrographs they appear as bright particles precipitated in the slag matrix. By comparing with the EPMA mapping results it is concluded that they are the RE silicates (e.g. RExSiyOz). XRF analysis of the oxide phase indicates that the 46wt% of the slag in the first run of test consists of RE oxides. The concentration of CeO2 in the oxide phase is 24 wt%, which makes the second largest component after CaO in slag phase.
Fig. 2. SEM micrograph of the oxide phase In order to examine the optimal ratio of slag addition, two additional tests were conducted in the same furnace. Equilibrium calculations confirm that increasing battery/slag ratio from 1 to 3 results in increase of RE oxide in slag phase by about two times. The battery materials to slag ratio were selected at 1:1 and 3:1, respectively. After slagging treatment at 1650oC for 1 hour, metal and slag were successfully separated. The cross sections of the crucibles are shown in Fig. 4. For the sample with low battery/slag ratio, slag has a uniform appearance (Fig. 4). Increasing the battery/slag ratio to 3 results in more light precipitated in the oxide phase in the SEM micrographs. It is well known that the RE oxides generally possess high melting temperature. The calcium silicate addition ratio affects the metal/oxide separation due mainly to the increasing of viscosity of oxide phase.
Fig. 3. EPMA mapping area of the oxide phase
Fig. 4. The cross sections of the crucibles after the slagging treatments with different battery/slag ratios
3.2. Fluxing treatment of the NdFeB magnet scraps An electric furnace with MoSi2 heating elements was used for the recycling of magnet in the laboratory. Temperature was measured by a Pt-Rh 30%/Pt-Rh 6% thermocouple. For protecting the graphite crucible and accelerating diffusion of carbon in the fluxing treatment, pure graphite powder was placed at the bottom of the crucible (32 mm inner diameter and 50mm height). The magnet powder was then placed into the crucible. Finally, the FeO-B2O3 flux was put on the top of magnet powders. The crucible was heated up to the designed temperature (1300, 1400, 1500 and 1550 oC, respectively) with different reaction times (1, 4 or 9 hours) using Ar flow rate at 200ml/min. After the measurements, the samples were cooled down at 5oC/min rate to 700oC and further to room temperature naturally. All the samples were examined using optical microscope. The microstructures of the samples were further analyzed by backscattered-electron microscope (BSEM) and energy dispersive spectrometer (EDS). The RE oxides were also characterized by X-ray diffraction (XRD), in order to identify the phases. The compositions of Nd, Pr, La, Al, Fe and B in oxide phases were also determined by inductively coupled plasma atomic emission spectrometer (ICP-AES). The contents of C in metals were measured by Leco CS-600. The samples after the fluxing experiments are shown in Fig. 5. Fig. 5(a), (b) and (c) are the samples obtained at 1400, 1500 and 1550oC for 9 hours, respectively. Their cross section pictures are presented respectively in Fig.
5(d), (e) and (f).
Fig. 5. The pictures of samples obtained at (a)(d) 1400oC, (b)(e) 1500oC and (c)(f) 1550oC for 9 hours, respectively. In all samples, the Fe-rich metals and rare earth containing oxides were separated successfully. Most of the Fe remains in the bulk metal phase. Small metal droplets were also observed in the oxide phase. The oxides obtained from the present study exhibit the high liquidus temperatures. Table 1 lists the chemical analysis of the typical samples in the present investigation. Almost all the REEs have been extracted to the oxide phase. The extract ratios of all REEs are more than 99 wt%. The main impurities in the oxides are alumina and boron trioxide. The purity of the RE oxides are higher than 96 wt%. Table 1. Chemical concentration of oxide and metal in different experimental conditions T (oC) 1550 1550 1550 1500 1400
time (hr) 1 4 9 9 9
Nd2O3 79.63 80.03 81.55 81.13 80.08
Oxide Phase (wt%) Pr2O3 La2O3 Al2O3 11.45 5.25 1.76 11.32 5.22 1.98 11.24 4.87 1.82 11.45 4.87 1.61 11.23 5.09 1.89
B2O3 1.92 1.46 0.51 0.93 1.71
Nd 0.10 0.15 0.13 0.27 0.23
Metal Phase (wt%) Pr La Al B 0.01
