Four-step eco-friendly energy efficient recycling of contaminated Nd2Fe14B sludge and coercivity enhancement by reducing oxygen content

Complete recycling of Nd2Fe14B sludge by chemical methods has gained significance in recent years, however, it is not easy to recycle highly contaminant sludge and obtain product with good magnetic properties. Herein we report a simple four-step process to recycle the Nd2Fe14B sludge containing ~ 10% of contaminants. Sludge was leached in H2SO4 and selectively co-precipitated in two steps. In the first co-precipitation, Al3+ and Cu2+ were removed at pH 6. Thereafter, in the second co-precipitation Fe2+ and RE3+ sulfates were converted to the Fe and RE hydroxides. By annealing at 800 °C RE and Fe hydroxides precipitates were converted to the oxides and residual carbon was oxidized to CO2. After the addition of boric acid, Fe and RE oxides were reduced and diffused to the (Nd-RE)2Fe14B by calciothermic reduction diffusion. Removal of CaO by washing with D.I. water in glove box reduced the oxygen content (~ 0.7%), improved crystallinity and enhanced the magnetic properties significantly. Coercivity increased more than three times (from 242.71 to 800.55 kA/m) and Mr value was also enhanced up to more than 20% (from 0.481 to 0.605 T). In this green process Na2SO4 and Ca(OH)2 were produced as by-product those are non-hazardous and were removed conveniently.

. They have drawn attention due to their applications in modern appliances which lead to large market demand and a rapid increase in their production [6][7][8][9][10][11] . A huge amount (21%) of rare earth elements (RE) are being consumed for the synthesis of permanent magnets. RE resources are depleting and the cost of RE extraction from the ores is continuously soaring, hence recycling of the Nd 2 Fe 14 B sludge is becoming an important area of modern research.
A large quantity (~ 30%) of the Nd 2 Fe 14 B sludge is produced in the cutting and grinding process and more than 95 wt% of it, is recyclable 12 . However, because of costly physical recycling processes and high level of contamination, usually, recycling magnet sludge is not economically feasible in most part of the world. Nd 2 Fe 14 B sludge mainly consists of oxidized particles of Nd 2 Fe 14 B with different RE, C, Al, and d-block transition metals (e.g. Cu, Co, Zn, Mn, Cr, Ni). Al, Zn, Mn, Cr, and Ni come from the protective coatings, those are applied to avoid the corrosion of bulk magnet surface. Cu is added to the sintered Nd 2 Fe 14 B magnets to decouple the magnetic grains to stop the fast flip-over of the magnetic domains and enhance the coercivity 11 . Co addition to enhances the M r value and curie temperature. Sludge can have a high quantity of carbon because of the mixing of machine oil and lubricant during the cutting process. All these contaminants reduce the value of sludge.
Commonly used physical method for recycling of Nd 2 Fe 14 B sludge is a multi-step process which requires a huge amount of chemicals, energy, and produces hazardous wastes as byproduct 13 . These wastes include oxides (of carbon, sulfur, and nitrogen), dangerous metals (e.g. As), organic solvents, RE vapors and electrolytes. Several chemical methods have been introduced for the recovery of RE from Nd 2 Fe 14 B magnets scrap/sludge 12-26 but complete recycling of Nd 2 Fe 14 B, via chemical route is relatively new field. Haider et al. 12 and Yin et al. 13 have recently introduced chemical methods. These methods are very innovative and useful, but they deal with sludge www.nature.com/scientificreports/ with a low level of contamination. It is very difficult to control the contamination in the sludge, hence it was required to introduce a new method to recycle this kind of sludge with good magnetic properties.
Herein we propose a time and energy efficient-recycling method for recycling of Nd 2 Fe 14 B sludge which is equally useful for sludge with high contamination and variable composition. Na 2 SO 4 and Ca(OH) 2 are produced as byproducts, those are non-hazardous and easy to remove. Magnetic properties are further enhanced by the removal of contamination and reduction of oxygen content. A comparison of between our method and the common physico-chemical 12 method used for the recycling of Nd 2 Fe 14 B sludge is given in the Fig. 1a, b. Specimens for TEM were prepared by focused ion beam (FIB-NX2000, Hitachi) using the lift-out technique. For TEM measurement, the sample was treated as the same process reported by Kim et al. 27 and orientation of the sample along the required zone axis was confirmed by using electron backscatter diffraction (EBSD) by TEAM™ Pegasus, Ametek Co. Ltd. USA.

Results and discussion
Magnet sludge produced during the cutting of Nd 2 Fe 14 B was dissolved in the H 2 SO 4 . Overall composition of the sludge before and after the precipitation is provided in Fig. 1d. Sludge mainly consisted of RE (Nd, Pr, Tb, Dy) Fe, Cu, Al, and C. Traces of other elements (e.g. Ho, Zr, Ga, Ni, Co) were also detected in the ICP analysis but their total concentration was ~ 0.4%. Effect of these small impurities on the magnetic properties of the final product was studied and provided in the supporting information. Cu, Al and C were ~ 10% those could significantly reduce the magnetic properties of the (Nd-RE) 2 Fe 14 B produced from them. After leaching of sludge in the H 2 SO 4 , next step was the selective precipitation to remove Al 3+ and Cu 2+ . Precipitation of multivalent (e.g. Al 3+ and Cu 2+ ) ions from the solution depends on many factors e.g. oxidation state, Ksp value, the concentration of other ions, precipitating agent, and temperature. Precipitation of Fe 3+ , Al 3+ , and Cu 2+ from their chloride solution occurred at pH values of 3.5, 5.0, and 6.0 28 . However, different results were observed, when the solution of acid mine drainage containing Fe 3+ , Al 3+ , and Cu 2+ was co-precipitated 28 . In the acid mine drainage experiment, Fe 3+ , Al 3+ , and Cu 2+ chlorides were precipitated at pH 3.5, 4.5, and 5.5, respectively 28 . Precipitation of Al 3+ and Cu 2+ at pH value of 5.5 was also observed 29 . Different pH of the precipitation for Al 3+ and Cu 2+ in the previous studies indicated is a complicated process. It is commonly observed that Fe 3+ precipitates completely at pH value ~ 3, however, Fe 2+ precipitates at pH value of ~ 7 30 . In our study, when pH approached 4, Al (Ksp constant = 1.9 × 10 −33 ) started to precipitate.
Ksp constants of Cu(OH) 2 is 1.6 × 10 −19 and it was next to be precipitated hence precipitated out between the pH values of 5-6. Almost ~ 90% hydroxides of Al and Cu were separated in the form of precipitates at pH 6, they were removed from the leachate solution by centrifugation. Separated Al and Cu hydroxides were analyzed, and analysis details are provided in the supporting information (Figs. S3, S4).
Fe in the leachate exists as Fe 2+ which does not precipitate below pH 6 but slightly (~ 2%) precipitates at pH 6 ( Fig. S5). However, when pH value exceeded 6, Fe 2+ (Ksp constant = 7.9 × 10 −15 ) started to precipitate as Fe(OH) 2 . Soe et al. 31 reported that Nd 3+ , Pr 3+ , Dy 3+ , and Tb 3+ also start to precipitate at pH value of ~ 7 and similar was observed in our study. www.nature.com/scientificreports/ Co-precipitation was stopped at pH values of 10, 11, 12 and 13 in four different experiments. The maximum percentage yield was obtained at pH value of 13 (Fig. S6). At pH 13, a mixture of hydroxides of RE, and Fe, was obtained with traces of some impurities e.g. Cu, Al, Ho, Zr, Ga, Ni, Co (less than 1%). In a separate experiment, selective precipitation was not performed and all the elements in the leachate solution were co-precipitated, hence, a mixture of RE, Fe, Al, and Cu hydroxide was obtained.
Hydroxides obtained by co-precipitation were aqua complexes of RE and other metals. Being amorphous, these hydroxides could not be detected by XRD (Fig. 2e) analysis. SEM image of the hydroxide precipitates produced by regular and selective co-precipitation (Fig. 2a, b) confirmed that hydroxide particles were of irregular morphology and size (Fig. 2c, d). TEM analysis revealed that the average size of the hydroxide precipitates was ~ 25 nm. TEM-EDS images (Figs. S7, S8) confirmed that oxides of all the metals are homogeneously mixed.
Most of the carbon was removed during the precipitation and centrifugation but still, noticeable quantity was detected in the hydroxide precipitates (Fig. 1d). Leftover carbon was removed by the annealing in air at 800 °C. Oxidation at 800 °C converted all the C to CO 2 . Meanwhile, annealing also converted hydroxide precipitates to the oxides. SEM analysis revealed that the average particle size of the oxide particles was ~ 150 nm (Fig. 3a, b).
XRD confirmed the presence of Fe 2 O 3 and REFeO 3 phases in the oxide mixture (Fig. 3m). The mechanism of formation of these oxides is given in Fig. 1c. Peaks of Cu and Al oxides were also observed in the XRD patterns of oxides produced from Nd, RE, Fe, Al, Cu hydroxide precipitates. TEM-EDS images reveal that all RE, Fe, Cu, and Al are distributed evenly throughout the oxide intermediates (Fig. 3c-l). Co-precipitation brought the Fe 2 O 3 and REFeO 3 particles very close. This homogeneous distribution is very effective for the efficient reduction diffusion process. Oxides produced from both the selective and regular co-precipitation were mixed with the boric acid and CaH 2 in two separate experiments. These mixtures were reduced and diffused at 1000 °C to obtain (Nd-RE) 2   www.nature.com/scientificreports/ facet of (Nd-RE) 2 Fe 14 B was detected. Any peak of Al or Al alloy was not detected in the XRD, however, in TEM-EDS image, Al was detectable ( Fig. 4i). This may refer to that Al or Al alloy was not crystallized well during reduction-diffusion or maybe oxidized during the washing with water and became amorphous. However, [121] facet of NdCu was identified in the HRTEM with the d-spacing value of 0.256 nm (Fig. 4d). www.nature.com/scientificreports/ SEM images in Fig. 4a-c revealed that the magnetic particles had irregular morphology and the size varied from 0.3 to 10 μm. It was determined that the average particle size of all three particles was ~ 1.8 µm (Fig. 4a-c). Different studies have revealed that Dy, Pr Tb, and other heavy RE are substituted inside the Nd 2 Fe 14 B crystal lattice [32][33][34][35][36][37][38][39][40][41][42][43] . In our study HRTEM and SEM-EDS images (Fig. 4l-n) also confirmed it. HRTEM shows that the crystal lattice d-spacing at [214] facet is 0.239 nm, which is slightly smaller (241 nm) than NdFe 14 B [214] facet. Slight reduction in the d-spacing may also indicate the substitution of Pr, Tb, or Dy in the crystal lattice. TEM-EDS images of (Nd-RE) 2 Fe 14 (AlCu)B confirmed the homogeneous mixing of all RE, indicate the substitution of Dy, Pr, and Tb in the Nd 2 Fe 14 B crystal lattice (Fig. 4f-n). TEM and TEM-EDS images of (Nd-RE) 2 Fe 14 B and (Nd-RE) 2 Fe 14 B (low oxygen) are provided in supporting information as Figs. S9, S10 and S11.
Oxygen content in the commercial (Nd-RE) 2 Fe 14 B powders is ~ 0.4%. This one of the reasons that commercial (Nd-RE) 2 Fe 14 B powders exhibit excellent magnetic properties. In the rare earth based magnetic particles produced by the R-D, washing with the water is employed to remove the CaO, byproduct of the R-D process 44 . On average, (Nd-RE) 2 Fe 14 B magnetic particles produced by the reduction diffusion process contains ~ 2% of oxygen. It was concluded that oxygen content reduces the crystallinity Nd 2 Fe 14 B because no oxide is detected in the XRD patterns. To solve the oxidation problem, (Nd-RE) 2 Fe 14 B was washed in the glove box. Before washing the (Nd-RE) 2 Fe 14 B with the water in the glove box, nitrogen gas was blown in the water, which further removed the dissolved oxygen in the water. N 2 was simply purged into the water at the rate of 25 ml/s for 40 min. This condition was taken from the work by Butler et al. 45 as they have reported that more than 60% of the oxygen can be removed at this optimum condition. The glove box was filled with the Ar, with the oxygen level reduced to the ~ 50 ppm. By taking these preventive measures, the oxygen content of (Nd-RE) 2 Fe 14 B was reduced to ~ 0.7%. Figure 4e shows that the Nd peak is absent when the washed (Nd-RE) 2 Fe 14 B was washed in air. This further confirms that washing with the water reduces the crystallinity of the Nd too. It is well-known fact that the addition of the Nd phase (up to a certain limit) enhances the magnetic properties especially, coercivity 12 .
LAADF-STEM images (Fig. 5b-d) were zoomed in to investigate the micro-structure of the magnetic particles. Line EDS mapping from the HAADF-STEM image was taken and studies to determine the degree of oxidation near the grain boundary (Fig. 5e, f). Pr, Dy, and Tb are substituted inside the (Nd-RE) 2 Fe 14 B, hence their EDS mapping was not studied to avoid the complexity of data. It was found that oxygen content suddenly increased near the grain boundary because these parts of the grain are directly exposed to the water during the washing process. Pb and Sn detected in TEM-EDS come from the solder.
It is evident from the XRD patterns (Fig. 4e) that Cu and Al reduce the crystallinity of the final product, as well as few crystal facets (e.g. [216],[324]), are also missing. The exact mechanism that how Cu and Al affects the (Nd-RE) 2 Fe 14 B crystal is yet unknown but most probably Cu and Al interfere during the diffusion of RE, Fe, and B. Poor crystallization after reduction-diffusion is the major factor that reduced the magnetic properties of (Nd-RE) 2 2 Fe 14 B that refers to the enhanced Mr value of (Nd-RE) 2 Fe 14 B-(low oxygen), which slightly affected the Ms. However, this enhancement in the Ms value is much higher as compared to the Ms value, which is also evident in the squareness ratio graph (Fig. 5a) showing the enhanced squareness ratio of (Nd-RE) 2 Fe 14 B-(low oxygen).
(Nd-RE) 2 Fe 14 (AlCu)B exhibit the highest magnetic moment (21.6 μB) and M s (1.057 T) value among all three products. Higher magnetic moment and low M r value of (Nd-RE) 2 Fe 14 (AlCu)B refer to the low anisotropic field or presence of amorphous soft magnetic phase (e.g. Fe). Higher M r value of (Nd-RE) 2 Fe 14 B-(low oxygen) contributes to its highest squareness ratio among all three products (Fig. 5a). Individual values of magnetic moments of (Nd-RE) 2 (Fig. 5a). Complete hysteresis loops with applied magnetic field range of − 9.5 to 9.5 Tesla are provided in supporting information as Fig. S12.
Reduction in magnetic moment enhanced the coercivity. From the hysteresis loops, coercivity values of (Nd-RE) 2

Conclusion
Contaminated Nd 2 Fe 14 B sludge was recycled by four-step chemical process. The process consisted of leaching of sludge in H 2 SO 4 , removal of impurities by selective co-precipitation, annealing, and calciothermic reduction diffusion. Al 3+ and Cu 3+ were removed by co-precipitation at pH 6 and residual carbon was removed by annealing at 800 °C. CaO byproduct was separated by washing in the glove box, in presence of very low level of oxygen that reduced the oxidation of (Nd-RE) 2 Fe 14 B produced. Removal of impurities and low oxygen content (~ 50 ppm) www.nature.com/scientificreports/