Recycling of Strontium Ferrite Waste in a Permanent Magnet

Research Article pubs.acs.org/journal/ascecg

Recycling of Strontium Ferrite Waste in a Permanent Magnet Manufacturing Plant Alberto Bollero,*,† Javier Rial,† Melek Villanueva,† Karol M. Golasinski,† Ana Seoane,‡ Judit Almunia,‡ and Ricardo Altimira‡ †

Division of Permanent Magnets and Applications, IMDEA Nanoscience, C/Faraday 9, 28049 Madrid, Spain Ingeniería Magnética Aplicada, IMA S.L., Av. de Rafael Casanova 114, 08100 Mollet del Valles, Barcelona, Spain

‡

ABSTRACT: Residues resulting from the manufacture of strontium ferrite magnets have been recycled for further use in magnet fabrication instead of disposal as waste. The quality of the recycled ferrite powder has been tested and compared to that of the new starting ferrite material. The magnetic properties of the recycled powder not only match those of the starting material acquired by the company for the production of magnets but exceed them. A coercivity value 3.5 times larger than that of the new starting ferrite powder, accompanied by a 25% increase in remanence, makes this material a new and improved ferrite product to re-enter the production chain in the factory with an extended applications range. This improvement is proven to be due to tuning of the morphology and microstructure through processing and subsequent heat treatment. The use of processing conditions in the same range as those typically used in the preparation of ferrite powders and magnets, in combination with the superior magnetic quality of the resulting powders, makes this method a suitable path to guarantee sustainability and an efficient use of resources in permanent magnet companies. KEYWORDS: Recycling, Recovery, Sustainability, Ferrites, Permanent magnets

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INTRODUCTION Ferrites are the most widely used permanent magnets throughout the world, accounting for about 90 wt % of all permanent magnets on a weight basis.1,2 The permanent magnet market size was valued above USD 15 billion in 2015.3 Ferrites are used in a multitude of technological applications that cover low-field and low-power applications, high-frequency systems, and biotechnology. On the basis of the high demand for this type of magnet, recycling of the residues generated in the manufacturing process is beneficial from an environmental point of view but also economically interesting to reduce costs at the permanent magnet company while additionally guaranteeing sustainability by closing the loop in the production line. The importance of ferrites is growing because of the rapid increase in demand for permanent magnets in current technological applications, which is strongly driven by the large industrial machinery, wind turbine, and electric vehicle markets.1,4 Requirements for some of these applications (wind turbines and current designs of motors in the automotive industry, in particular) make rare earth (RE)-based permanent magnets irreplaceable elements in the near future because of the strong magnetic flux provided in a reduced volume. However, partial substitution of these magnets in relevant market sectors such as industrial motors, electric bikes and motorbikes, home appliances, etc. would mean a change in the market share in benefit of improved ferrites while relegating RE-based magnets to those applications where they cannot be © XXXX American Chemical Society

substituted. Moreover, traditional applications of ferrites are foreseen to expand in a short time frame by progressive improvement of their magnetic properties through the application of phenomena at the nanoscale together with the latest advances in nanotechnology. Many applications nowadays traditionally use RE-based permanent magnets because no relevant problems were raised when they were implemented in terms of availability or high and unstable pricing. The situation has dramatically changed in recent years because of the increased monopoly resulting from the strategic geographical situation, in particular, of heavy rare-earths,5 which has made mandatory the search for alternatives to RE-based permanent magnets in as many applications as possible. Ferrites count with important advantages by comparison with other magnets in view of practical applications: (i) Their constituent elements are abundant and cheap. (ii) They have reduced environmental impact in the extraction and postprocessing routes in comparison with REs. (iii) They are electrical insulators, therefore avoiding eddy currents (important for dynamic operating conditions such as those integrated in electrical motors). Received: December 14, 2016 Revised: February 17, 2017 Published: March 8, 2017 A

DOI: 10.1021/acssuschemeng.6b03053 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

prohibitive in terms of implementation costs for a company. However, a recovery and recycling process fulfilling the following requirements will guarantee straightforward implementation in production with no economical losses but rather gains: • The recycling process makes use of the facilities already existing in the company, i.e., avoiding the acquisition of new equipment. • The parameters of the recycling process are comparable to those used in the processing of the strontium ferrite starting powders acquired by the company from an external supplier. • The quality of the recycled material is not inferior to that of the starting new material, i.e., allowing its use in same applications with no negative impact on the performance of the final product. This last point is usually one of the drawbacks pushing back implementation of a recycling process for whatever material, especially when looking at products with technological implementations, which require demanding quality control according to well-established standards. This study proves the development of a recycling process fulfilling all of these requirements. To ensure the validity of the process and the material, quality tests were carried out after each step of the process. These tests comprised composition analysis, study of particle size and shape, and measurement of magnetic properties to validate the potential of the recycled powder in view of re-entering the production process for the fabrication of new compacted magnets. The hysteresis loop measured for magnetic characterization represents M versus H, where M stands for the magnetization of the sample and H refers to the external magnetic field applied to measure the curve. In a few words, this curve provides the magnetic response of a material in the presence of an external magnetic field. This loop is broad for a good permanent magnet since it has to resist demagnetization by external fields in addition to the field created by its own magnetization.11 Significant parameters associated with this loop are the remanence, Mr (the value of the magnetization when H = 0), and the coercivity, Hc (the reverse field required to reduce the net magnetization to zero, i.e., to demagnetize the material).

(iv) They are chemically inert and have practically no corrosion problems, making possible air processing, i.e., largely reducing fabrication costs. As in most manufacturing processes, residues are generated in the fabrication of ferrite magnets. The volume of residues generated during the process scales up with the production volume. This is the result of the large market for ferrites and the necessity of customization of the sintered magnets produced by the companies in accordance to the specific final application. Despite the large amount of ferrite waste generated during manufacturing, there is a lack of studies dealing with the possibility of recycling this strontium ferrite waste. There have only been studies dealing with the possibility of recycling the elemental constituents from different sources,6,7 but not the strontium ferrite material after completion of magnet manufacturing. Liu et al.6 prepared strontium ferrite from strontium residue provided by a factory producing strontium carbonate (SrCO3). The method involved leaching the Sr waste residue with ammonium chloride followed by roasting a mixture of SrCO3 and FeCl3. Only susceptibility values were provided in this study, thus making it difficult to assess the quality of the resulting strontium ferrite powder as permanent magnet material. Pullar et al.7 prepared strontium ferrite powders starting from iron-rich industrial waste sludge, which resulted from steel-based wiredrawing, mixed with oxides and fired at 1050−1150 °C. The magnetic properties attained for the resulting powder did not grant its application as hard ferrite but rather as a soft magnetic material, mainly because of the low coercivity obtained. Strontium ferrite (SrFe12O19) is a ceramic compound prepared from SrCO3, which is obtained from celestite (SrSO4), the natural ore of strontium. The synthesis typically comprises a calcination process of the mixture of SrCO3 and Fe2O3 at a temperature between 1000 and 1200 °C.8,9 Compaction of powders into magnets can be done by wet or dry pressing,1,10 requiring a final sintering process (∼1000 °C) to provide mechanical stability to the compacted powder. Wet pressing improves the density of the final magnets, i.e., the magnetic properties, while dry pressing provides magnets with an improved tolerance at the expense of detrimental magnetic properties due to a poorer density. Independently of the pressing process used, magnets need to be machined to the final shape. Mechanical strength is low for ferrites, thus making necessary the application of a progressive cutting process instead of cutting complete pieces out of the as-sintered magnet to achieve the desired shape and size. An efficient recycling of the ferrite residues generated during manufacturing would reduce the costs related to acquisition of the starting strontium ferrite powders by those enterprises that do not prepare the compound themselves but acquire it in powder form from an external company. As for those companies covering the complete synthesis and processing chain, successful recycling would reduce the amount of starting material to be processed in the ore. The aim of this study is to guarantee sustainability by demonstrating a method that can be easily and cost-efficiently implemented in a permanent magnet manufacturing plant. The prices of both the constituent elements and the processing route are very low for ferrites by comparison with those of REbased magnets. On this basis, recovery and recycling of the residues generated during the manufacturing process might be interesting only from an environmental point of view but

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EXPERIMENTAL SECTION

Material. Residues were collected from a line of cutting machines used to shape ferrite magnets and connected by a canalization system that converged to a common deposit (see picture in Figure 1). Each cutting machine was equipped with a diamond powder grinding wheel cutter disc rotating at a speed of 2850 rpm. The residue consisted of moisture comprising strontium ferrite (SrFe12O19) powder, water, and coolant fluid (taladrine) used in the cutting process of ferrite magnets to shape them according to customers’ requirements. Morphological and magnetic characterization of the residue prior to application of any additional treatment made it necessary to heat the moisture at a moderate temperature of 250 °C for 1 h for removal of organic components. Calcination in air at 1000 °C was carried out straightforward on the wet residueswith no need for a preheating processin a tubular furnace and, for comparison, in an industrial muffle furnace to evaluate their properties after heat treatment. The heating was carried out with a heating ramp of 10 K/min, maintenance of a constant temperature of 1000 °C for 1 h, followed by cooling by turning the supply current off. Morphological and Microstructural Characterization. The morphology of the samples was determined by scanning electron microscopy (SEM) using a Zeiss-Evo scanning electron microscope in B


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powder grinding wheel cutter disc rotating at a speed of 2850 rpm in the presence of a coolant, which guarantees fast finishing of the magnets while avoiding any undesirable breakage and/or introduction of internal fractures. The residue resulting from this process leaves each machine through a tube connected to a disposal line, which ends in a common deposit for all of these cutting machines (see the picture in Figure 1). The waste consists of moisture containing ferrite powder, water, and coolant. This moisture is usually put into containers that are afterward disposed by an external company. Study of the Ferrite Residues. Figure 2 shows a comparison of the first and second quadrants of the hysteresis

Figure 1. Schematic illustration of the typical processing flow (blue arrows) at a permanent magnet manufacturing company. The residues generated during the manufacturing process are taken away by an external company. The scope of this study involves closing the loop at the company (green arrows) to guarantee sustainability through an environmentally friendly and cost-efficient process. the backscattering mode. Crystal structures were examined by X-ray diffraction (XRD) on an X’Pert PRO Theta/2Theta diffractometer (Panalytical) with Cu Kα1 radiation (λ = 0.1541 nm). The grain size was determined by the Scherrer method from the width of the intensity diffraction peaks in the XRD patterns.11,12 One of the major causes of peak broadening is finite particle size (in addition to microstrain). As the number of diffracting planes reduces, the width of the line increases. This is analogous to diffraction of light from a grating, where the line width is proportional to the number of diffracting grooves in the grating. Magnetic Characterization. Room-temperature hysteresis loops were measured using a Lakeshore 7400 series vibrating sample magnetometer (VSM) with a maximum applied field of 15 kOe. The VSM was calibrated with a pure nickel sphere prior to measurements. These measurements allowed determination of the magnetization measured at a maximum applied field of 15 kOe (M15 kOe), the remanence (Mr), and the coercivity (Hc). Ball Milling Processing. Brand new strontium ferrite powders, identical to those used to fabricate the magnets, were milled for the sake of comparison. A rotation speed of 900 rpm was used with tungsten carbide vials and balls, and the ball-to-powder mass ratio was 40:1.13,14 Tungsten carbide milling media (balls and vessel) have been used to enhance the impact energy, i.e., to accelerate an efficient milling process, because of the high density of this material (14.95 g/ cm3). The loading and sealing of the vials, as well as the complete milling procedure, were performed in air. Milling was done for 1.5 and 3 min. As-milled powders were calcinated in air at 1000 °C for 1 h following a sequence identical to that used with the ferrite waste.

Figure 2. First and second quadrants of the hysteresis loops measured for commercial ferrite powder (circles) and the waste generated in the magnet manufacturing process after removal of both water and coolant through heating at 250 °C (triangles). The insets show representative SEM images of both materials.

loops measured for the commercial powder and the residue after the drying process at 250 °C. Application of identical heat treatment to the starting material did not have any influence on the magnetic properties. An increase in coercivity (60% higher) and a slight improvement in remanence are observed by simply drying out the residue material, showing that the process of cutting the magnets has a beneficial effect on the magnetic properties of the resulting ferrite powder. It is interesting to remark that, according to this result, the first step of the recycling procedure is already taking place when the magnets are cut, leading to a residue that can be considered an added value instead of a simple waste to be removed. This is of economic relevance for the manufacturing process because it implies that a material with magnetic properties superior to those of the initial new powder could be further used in the fabrication of magnets through a simple drying treatment. This improvement can be explained as due to a microstructure refinement. The SEM images shown in Figure 2 prove the particle size refinement and microstructure homogenization induced by the cutting process. The initial commercial powder consists of particles with a broad size distribution going from 3 to above 15 μm, by comparison with aggregates of particles with sizes below 2 μm observed for the dried ferrite waste. XRD patterns measured for both materials (Figure 3) show in both cases intensity peaks of SrFe12O19 together with a minor content of α-Fe2O3. No additional phases can be found in the residue, but the intensity peaks are significantly broader (Figure 3b) because of the grain refinement resulting from fine cutting

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RESULTS AND DISCUSSION Origin of the Ferrite Waste. The manufacturing procedure typically followed by a permanent magnet manufacturer is schematically illustrated in Figure 1. The starting ferrite powder is acquired from an external supplier and used to prepare compacted magnets. The magnets need to be adapted to specific dimensions dictated by the customer and based on the final application of the magnets. Several cutting machines are placed in line and loaded with a complete batch of ferrite magnets to allow for a continuous and automatic process. The cutting process involves the use of a diamond C


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Figure 3. XRD patterns for (a) new commercial strontium ferrite powder and (b) the residue after application of a heating procedure at 250 °C for removal of water and organic residues used in the manufacturing process of magnets. All diffraction peaks correspond to SrFe12O19 with the exception of those intensity peaks indicated with triangles, which are due to α-Fe2O3.

Figure 4. Recycling process comprising (a) collection of the ferrite waste produced during manufacturing; (b) close detail of the moisture consisting of ferrite residues, water, and coolant; (c) recycled ferrite powder resulting from application of the calcination procedure. (d) Second quadrants of the hysteresis loops measured for the waste shown in (a) after application of a calcination process at 1000 °C (squares), and, for comparison, the hysteresis loop measured for the starting commercial powder (circles) and same powder after calcination at 1000 °C (triangles).

of the compacted magnets. The estimated mean grain size of 40 nm contrasts with the value of 95 nm determined for the starting powder and proves the grinding effect produced by the cutting. The slight decrease magnetization M15 kOe measured for the residue (Figure 2) is likely due to an increased content of lattice defects15 induced in the material during the cutting process. Study of the Recycled Ferrite Waste. A first premise in the search for a feasible recycling process is exploring methods that allow the use of existing equipment and procedures in the companies to avoid increased production costs that might play against viability in implementation of the process. This is especially remarkable in the case of aiming recycling of a cheap material, as is the case for ferrites, where an extra investment would make adopting any recycling alternative costly and inefficient by comparison with the price of acquiring new material. A calcination temperature of 1000 °C for 1 h was applied to the resulting waste (Figure 4a,b) with no need to apply a predrying method (like the one discussed in the previous section). This temperature is in the range typically used in the fabrication of ferrite magnets (1000−1200 °C).8,9 Figure 4d shows a comparison of the second quadrant of the hysteresis loops measured for the starting powder material used in the preparation of the magnets, the waste after application of a calcination process at 1000 °C (visual appearance shown in Figure 4c), and, for the aim of comparison, the starting powder after an identical heating procedure. A comparison of the remanence values measured for the treated residues shows a 19% increase by comparison with the commercial powder (to 36.8 from 30.0 emu/g, respectively). The maximum magnetization measured at 15 kOe is slightly decreased by about 4% (from 66.5 to 64.0 emu/g for the commercial powder and calcinated residue, respectively) due to the enhanced content of the antiferromagnetic α-Fe2O3 generated during the calcination process (Figure 5). The lower maximum magnetization values obtained by comparison with the theoretical value of the saturation magnetization for SrFe12O19 (72 emu/g)16 are due first to the presence of α-Fe2O3 but also to the maximum applied field (15 kOe) not being sufficient to fully saturate the samples under study. More striking is the improvement in coercivity of the calcinated waste, with an increase from 0.9

Figure 5. XRD patterns of (a) calcinated commercial powder and (b) calcinated waste. All of the diffraction peaks correspond to SrFe12O19 with the exception of those intensity peaks indicated with triangles, which are due to α-Fe2O3.

kOe for the starting powder to 3.3 kOe. The coercivity of the commercial powder is also increased through calcination in comparison with the commercial powder, but only to 1.8 kOe, i.e., approximately 46% lower than the value achieved for the residue after application of an identical procedure. This result proves the efficiency of the combination of cutting followed by application of the calcination process. The results were reproducible when going from a tubular furnace (restricted loading) to a high-capacity industrial muffle furnace. XRD results (Figure 5) show the same phases (SrFe12O19 and αFe2O3) for the starting commercial powder and the waste after calcination, but an increased content of α-Fe2O3 for the latter. The grain size, however, is affected by the calcination treatment in the case of the waste, with a mean size of 55 nm. No change D


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ACS Sustainable Chemistry & Engineering was observed in the mean grain size of the commercial powder after calcination. Comparison Study with Ball-Milled Commercial Ferrite. Ball milling is a well-known technique used in laboratories for nanostructuration of materials and in different companies (not only ones aimed at magnets) for mixing and microstructure refinement at the macro- and/or micrometer scale. Mechanical alloying has been previously used in the preparation of SrFe12O19 powder starting from SrCO3 and Fe2O3,17 while intensive milling has been used to improve the magnetic properties of SrFe12O19 alloy.15,18,19 In this study, we made use of a novel “ultrafast” milling procedure applied to the starting commercial SrFe12O19 to prove that the effectiveness of the recycling method here proposed is based on an efficient tuning of the microstructure and, consequently, of the magnetic properties. This “ultrafast” milling technique has previously been proven to be successful on a laboratory scale in achieving microstructure refinement and development of permanent magnetic properties in cobalt−ferrite powder13 and MnAl particles;14 this was managed by using milling times ranging from a few seconds to a few minutes by comparison with typical times of a few to tens of hours required for conventional ball milling methods. The additional advantage of using short milling times is the avoidance of high temperatures (hundreds of degrees) achieved during milling for long periods, which may result in undesired grain growth effects and compositional changes. This short milling route provides a good comparison to the grinding process that takes place when fine-cutting the magnets in the presence of a coolant. Taking this aim into consideration, milling of the starting commercial ferrite powder followed by calcination was carried out, and the results were compared to those for the recycled powder resulting from waste calcination. Figure 6 shows the second quadrant of the hysteresis loops for the commercial powder milled for 1.5 and 3 min followed by calcination and, for comparison, the curve obtained for the

calcinated waste. Milling for 1.5 min results in a coercivity of 2.7 kOe, which is well above the value of 0.9 kOe obtained for the commercial powder. This result shows the efficiency of the ultrafast milling procedure applied to strontium ferrite. Interestingly, an extension of the milling time up to 3 min results in an identical coercivity value as that obtained for the calcinated waste, while the remanence takes a very close value. A comparison of the XRD patterns for the milled and calcinated powders (Figure 7) with those measured for both

Figure 7. XRD patterns of (a) commercial powder after milling for 1.5 min followed by calcination and (b) commercial powder after milling for 3 min followed by the same calcination procedure. All of the diffraction peaks correspond to SrFe12O19 with the exception of those intensity peaks indicated with triangles, which are due to α-Fe2O3.

the waste and commercial powders after calcination (Figure 5) do not show any compositional changes, i.e., only the presence of SrFe12O19 and α-Fe2O3. SEM images (Figure 8) show that milling for 1.5 min followed by calcination has a straightforward effect in refining the microstructure of the starting commercial ferrite powder. The coarse polycrystalline particles with a broad particle size distribution (3−15 μm) observed for the latter (Figure 2 inset) are reduced in size (1.5−5 μm) and homogenized through application of this short-milling-time process (Figure 8a). An extended milling time of 3 min enhances both effects by providing a more uniform particle size distribution and practical disappearance of aggregates. The mean grain size values determined from the XRD patterns (Figure 7) are 60 and 50 nm for powders milled for 1.5 and 3 min after calcination, respectively. The microstructures of the powder resulting from 3 min of milling and the waste powder after calcination are comparable except for the formation of aggregates of particles for the latter. The mean grain size for the waste powder obtained after calcination is 55 nm, which is very much comparable to the values obtained for the milled and calcinated powders. The large morphological and microstructural similarities obtained for the ferrite waste and the milled new commercial powders after application of the calcination process show the important correlation between both properties and the resulting magnetic properties of the powders.19 Furthermore, this result proves that the enhanced magnetic properties obtained for the recycled waste are due to a combination of the microstructural refinement and homogenization effects taking place during magnet manufacturing (cutting process) and application of the subsequent calcination procedure. The finer granulometry of the recycled

Figure 6. Second quadrant of the hysteresis loop measured for the waste after the calcination process at 1000 °C (squares), with a real image of the resulting powder shown in the inset, compared with the hysteresis loops for the commercial powder milled for 1.5 and 3 min after application of the same calcination process (solid and open stars, respectively). The insets show schematically the milling process and the macroscopic effect (breakage) produced by the collisions of two balls when trapping the particles. E


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the powder through processing and subsequent heat treatment. A coercivity value 3.5 times larger than that of the new starting ferrite powder, accompanied by a 25% increase in remanence, makes this material a new and upgraded ferrite product to reenter the production chain. The attained superior properties result in an added value based on the possibility of extending the applications range of the material for technologies demanding a higher magnetic performance than that of the starting powder. The use of processing conditions in the same range as those typically used in the preparation of ferrite powders and magnets, the absence of any chemical use in the recycling process, and the improved magnetic quality of the resulting powders make this method an environmentally friendly and suitable path to guarantee sustainability in permanent magnet companies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alberto Bollero: 0000-0002-3282-0981 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by EU-FP7 NANOPYME Project (310516), Ministerio de Economiá y Competitividad (MINECO) through ENMA (MAT2014-56955-R), and Regional Government (Comunidad de Madrid): NANOFRONTMAG (ref. S2013/MIT-2850).

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REFERENCES

(1) Goldman, A. Modern Ferrite Technology, 2nd ed.; Springer: New York, 2006. (2) Constantinides, S. Market Outlook for Ferrite, Rare Earth and Other Permanent Magnets: 2015 to 2025. Presented at the Magnetics Conference 2016. (3) Lewis, L. H.; Jimenez-Villacorta, F. Perspectives on Permanent Magnetic Materials for Energy Conversion and Power Generation. Metall. Mater. Trans. A 2013, 44, 2−20. (4) Gutfleisch, O.; Willard, M. A.; Brück, E.; Chen, C. H.; Sankar, S. G.; Liu, J. P. Magnetic materials and devices for the 21st century: stronger, lighter, and more energy efficient. Adv. Mater. 2011, 23, 821−842. (5) Smith Stegen, K. Heavy rare earths, permanent magnets, and renewable energies: An imminent crisis. Energy Policy 2015, 79, 1−8. (6) Liu, C.; Xu, L.; Yang, X.; Peng, T.; Ren, J. Preparation of strontium ferrite from strontium residue. Chin. J. Geochem. 2012, 31, 74−77. (7) Pullar, R. C.; Hajjaji, W.; Amaral, J. S.; Seabra, M. P.; Labrincha, J. A. Magnetic Properties of Ferrite Ceramics Made from Wastes. Waste Biomass Valorization 2014, 5, 133−138. (8) Sharma, P.; Verma, A.; Sidhu, R. K.; Pandey, O. P. Process parameter selection for strontium ferrite sintered magnets using Taguchi L9 orthogonal design. J. Mater. Process. Technol. 2005, 168, 147−151. (9) Tiwary, R. K.; Narayan, S. P.; Pandey, O. P. J. Min. Metall., Sect. B 2008, 44, 91−100. (10) Ceramic Processing; Terpstra, R. A.; Pex, P. P. A. C.; de Vries, A. H., Eds.; Chapman & Hall: London, 1995.

Figure 8. SEM images of (a) the starting commercial powder after milling for 1.5 min and calcination, (b) the starting commercial powder after milling for 3 min and calcination, and (c) the calcinated ferrite residue from manufacturing.

powder will most likely require an improved tolerance of the compaction tools (molds) to accommodate the powder during the pressing process used in the preparation of fully dense magnets, without any additional changes in the equipment. No need to introduce technological modifications is foreseen in the case of fabrication of bonded magnets. This work is currently underway.

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CONCLUSION A recycling procedure for the strontium ferrite residues produced in the manufacturing process of ferrite magnets has been developed, resulting in a ferrite powder with coercivity and remanence values superior to those of the starting commercial material. No chemicals have been used in the process, but advantage of the manufacturing process itself has been taken in the complete recycling procedure. The improvement in magnetic properties has been proven to be the result of efficient nanostructuration and homogenization of F


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ACS Sustainable Chemistry & Engineering (11) Chikazumi, S. Physics of Ferromagnetism; Oxford Science Publications: Oxford, U.K., 1997. (12) Scherrer, P. Bestimmung der Größe und der inneren Struktur von Kolloidteilchen mittels Röntgenstrahlen. Math.-Phys. Klasse 1918, 2, 98−100. (13) Pedrosa, F. J.; Rial, J.; Golasinski, K. M.; Rodriguez-Osorio, M.; Salas, G.; Granados, D.; Camarero, J.; Bollero, A. Tunable nanocrystalline CoFe2O4 isotropic powders obtained by co-precipitation and ultrafast ball milling for permanent magnet applications. RSC Adv. 2016, 6, 87282−87287. (14) Rial, J.; Villanueva, M.; Céspedes, E.; López, N.; Camarero, J.; Marshall, L.; Lewis, L. H.; Bollero, A. Application of a novel flashmilling procedure for coercivity development in nanocrystalline MnAl permanent magnet powders. J. Phys. D: Appl. Phys. 2017, 50, 105004. (15) Ketov, S. V.; Yagodkin, Y. D.; Lebed, A. L.; Chernopyatova, Y. V.; Khlopkov, K. Effect of Milling in Various Media and Annealing on the Structure and Magnetic Properties of Strontium Hexaferrite Powder. J. Magn. Magn. Mater. 2006, 300, e479−e481. (16) Wohlfarth, E. P. Ferromagnetic Materials; North Holland Publishing Company: Amsterdam, 1982; p 441. (17) Jin, Z.; Tang, W.; Zhang, J.; Lin, H.; Du, Y. Magnetic properties of isotropic SrFe12O19 fine particles prepared by mechanical alloying. J. Magn. Magn. Mater. 1998, 182, 231−237. (18) Kaczmarek, W. A.; Idzikowski, B.; Müller, K.-H. XRD and VSM study of ball-milled SrFe12O19 powder. J. Magn. Magn. Mater. 1998, 177-181, 921−922. (19) Wu, E.; Campbell, S. J.; Kaczmarek, W. A. A Mössbauer effect study of ball-milled strontium ferrite. J. Magn. Magn. Mater. 1998, 177181, 255−256.

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Recycling of Strontium Ferrite Waste in a Permanent Magnet

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