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    Hydrometallurgical recycling of waste NdFeB magnets: design of experiment, optimisation of low concentrations of sulphuric acid leaching and process analysis
    (Taylor & Francis, 2022) Uysal, Emircan; Al, Serhat; Kaya, Elif Emil; Stopic, Srecko; Gürmen, Sebahattin; Friedrich, Bernd
    Efficient recovery of rare earth elements (REEs) from scrap NdFeB magnets is a significant requirement for a circular economy. Therefore, the development of a cost-effective and environmentally friendly leaching process for the dissolution of iron and REEs has received extensive interest. In this study, the dissolution of NdFeB magnet powders in sulphuric acid solution was investigated in detail. Taguchi orthogonal array was employed for the first time to define and optimise the influence of sulphuric acid concentration, solid-to-liquid ratio and stirring speed on the extraction of iron and rare earth elements (REEs). The acid concentration of sulphuric acid was determined as a key factor for iron and REEs dissolution, while solid-to-liquid ratio and stirring speed slightly affected the dissolution of REEs and iron. Maximum iron and REEs extraction were achieved under optimal conditions as the sulphuric acid concentration of 1 M, solid-to-liquid ratio of 1/15 and stirring speed of 350 rev min(-1). Consequently, after the validation experiment, it was proved that the design of experiments based on Taguchi orthogonal array is an efficient way for the optimisation of process parameters.
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    NdFeB Magnets recycling process: an alternative method to produce mixed rare earth oxide from Scrap NdFeB Magnets
    (MDPI, 2021) Kaya, Elif Emil; Kaya, Ozan; Stopic, Srecko; Gürmen, Sebahattin; Friedrich, Bernd
    Neodymium iron boron magnets (NdFeB) play a critical role in various technological applications due to their outstanding magnetic properties, such as high maximum energy product, high remanence and high coercivity. Production of NdFeB is expected to rise significantly in the coming years, for this reason, demand for the rare earth elements (REE) will not only remain high but it also will increase even more. The recovery of rare earth elements has become essential to satisfy this demand in recent years. In the present study rare earth elements recovery from NdFeB magnets as new promising process flowsheet is proposed as follows; (1) acid baking process is performed to decompose the NdFeB magnet to increase in the extraction efficiency for Nd, Pr, and Dy. (2) Iron was removed from the leach liquor during hydrolysis. (3) The production of REE-oxide from leach liquor using ultrasonic spray pyrolysis method. Recovery of mixed REE-oxide from NdFeB magnets via ultrasonic spray pyrolysis method between 700 degrees C and 1000 degrees C is a new innovative step in comparison to traditional combination of precipitation with sodium carbonate and thermal decomposition of rare earth carbonate at 850 degrees C. The synthesized mixed REE- oxide powders were characterized by X-ray diffraction analysis (XRD). Morphological properties and phase content of mixed REE- oxide were revealed by scanning electron microscopy (SEM) and Energy-dispersive X-ray (EDX) analysis. To obtain the size and particle size distribution of REE-oxide, a search algorithm based on an image-processing technique was executed in MATLAB. The obtained particles are spherical with sizes between 362 and 540 nm. The experimental values of the particle sizes of REE- oxide were compared with theoretically predicted ones.
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    New proposal for size and size-distribution evaluation of nanoparticles synthesized via ultrasonic spray pyrolysis using search algorithm based on image-processing technique
    (Mdpi, 2020) Emil Kaya, Elif; Kaya, Ozan; Alkan, Gözde; Gürmen, Sebahattin; Stopic, Srecko; Friedrich, Bernd
    Nanoparticle properties are correlated to their size, size distribution, and shape; it is essential to accurately measure these features in the field of nanoscience. In this study, silver nanoparticles (AgNPs) were synthesized with the ultrasonic-spray-pyrolysis (USP) method from a water solution of silver nitrate. The synthesized AgNPs were characterized by Dynamic Light Scattering (DLS) analysis and Scanning Electron Microscopy (SEM) to reveal their size and size distribution. A search algorithm based on an image-processing technique to obtain particle size and particle-size distribution from SEM micrographs is proposed. In order to obtain more quantitative information and data with respect to the morphology of particles synthesized under different process parameters, SEM micrographs with a nonhomogeneous background contrast were examined via image-processing techniques in MATLAB. Due to the inhomogeneous contrast of SEM micrographs, defining an overall threshold value was insufficient in the detection of whole nanoparticles. Thus, subimages were directly created according to the maximum and minimum particle size specified by the user to determine local threshold values. The obtained results were automatically combined to represent both particle dimension and location in the SEM micrographs. We confirmed that the results of our DLS analysis, theoretical calculation, and image-processing technique were correlated with our expected results.
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    Production of rare earth element oxide powders by solution combustion: a new approach for recycling of NdFeB magnets
    (Royal Society of Chemistry, 2022) Kaya, Elif Emil; Stopic, Srecko; Gürmen, Sebahattin; Friedrich, Bernd
    NdFeB magnets are employed in various technological applications due to their outstanding magnetic properties, such as high maximum energy product, high remanence and high coercivity. Production of NdFeB has gathered more interest, therefore the demand for rare earth elements (REEs) has continuously increased. The recovery of REEs has become essential to satisfy this demand in recent years. In the present study, a promising flowsheet is proposed for REEs recovery from NdFeB magnets, as follows: (1) acid baking, (2) employment of ultrasound-assisted water leaching, (3) the production of rare earth oxides (RE oxides) by a solution combustion method, and (4) a calcination process. There are several problems in conventional precipitation such as loss of a high amount of metal during precipitation and use of a high amount of precipitation agents. It is worth mentioning that the consumed precipitation agents in the solution are not easily recyclable. This study aims especially to investigate the production of RE oxides from recycled NdFeB magnets by solution combustion as an alternative to conventional precipitation methods. In this way, impurities that may have come to the system from the precipitation agents were prevented. Moreover, in the production of RE oxides via the above-mentioned method, precipitation agents and filtration steps were not necessary.
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    Recovery of rare earth elements from spent NdFeB-magnets: separation of iron through reductive smelting of the oxidized material (second part)
    (MDPI-Multidisciplinary Digital Publishing Institute, 2022) Chung, Hanwen; Stopic, Srecko; Kaya, Elif Emil; Gürmen, Sebahattin; Friedrich, Bernd
    This paper proposes a pyrometallurgical recycling method for end-of-life NdFeB magnets by oxidizing them in air and subsequently smelting them. The smelting process enabled the recovery of rare earth elements (REEs), producing a new reach concentrate separating the iron as a metallic phase. From the products of smelting, the metallic phase showed a maximum Fe content of 92.3 wt.%, while the slag phase showed a maximum total REE (Nd, Pr, and Dy) content of 47.47 wt.%, both at a smelting temperature of 1500 degrees C. ICE-OES and XRD analysis were conducted on both phases, and results showed that the metal phase consists mainly of Fe and Fe3C while the slag phase consists of the RE-oxides, leftover Fe2O3, and a mixture of Fe6Nd4. The obtained slag concentrate based on the oxides of rare earth elements is suitable for further pyrometallurgical or hydrometallurgical treatment in order to obtain rare earth elements.
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    Recovery of rare earth elements through spent NdFeB magnet oxidation (first part)
    (MDPI-Multidisciplinary Digital Publishing Institute, 2022) Stopic, Srecko; Polat, Buse; Chung, Hanwen; Kaya, Elif Emil; Smiljanic, Slavko; Gürmen, Sebahattin; Friedrich, Bernd
    Due to their remarkable magnetic properties, such as a high maximum energy product, high remanence, and high coercivity, NdFeB magnets are used in a variety of technological applications. Because of their very limited recycling, high numbers of spent NdFeB magnets are widely available in the market. In addition to China's monopoly on the supply of most rare earth elements, there is a need for the recovery of these critical metals, as their high import price poses an economic and environmental challenge for manufacturers. This paper proposes a pyrometallurgical recycling method for end-of-life NdFeB magnets by oxidizing them in air as first required step. The main goal of this method is to oxidize rare earth elements from NdFeB magnets in order to prepare them for the carbothermic reduction. The experimental conditions, such as the oxidation temperature and time, were studied in order to establish the phase transformation during oxidation using the Factsage Database and experimental conditions. Our thermogravimetric analysis TGA analysis revealed an increased sample mass by 35% between room temperature and 1100 degrees C, which is very close to the total calculated theoretical value of oxygen (36.8% for all elements, and only 3.6% for rare earth elements REE), confirming the complete oxidation of the material. The obtained quantitative analysis of the oxidation product, in (%), demonstrated values of 53.41 Fe2O3, 10.37 Fe3O4; 16.45 NdFeO3; 0.45 Nd2O3, 1.28 Dy2O3, 1.07 Pr2O3, and 5.22 alpha-Fe.
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    Recycling of NdFeB magnets employing oxidation, selective leaching, and iron precipitation in an autoclave
    (Royal Soc Chemistry, 2023) Emil-Kaya, Elif; Polat, Buse; Stopic, Srecko; Guermen, Sebahattin; Friedrich, Bernd
    The increasing production of neodymium-iron-boron (NdFeB) magnets for technological applications results in disposal problems. NdFeB magnets contain a significant quantity of rare earth elements (REEs). China is the largest REEs producer, but it applies quotas and increases the export prices of REEs. To address this issue, this study aims at investigating the recovery process of REEs from scrap NdFeB magnets. After oxidation of NdFeB magnet powders, selective leaching with nitric acid was carried out to achieve high-purity REE-rich leaching liquor. First, the oxidation kinetics of NdFeB powders was studied in detail to determine the oxidation temperature and duration. Afterwards, the effects of selective leaching parameters, including acid concentration, leaching temperature, stirring speed and solid/liquid ratio, were examined by analysis of variance (ANOVA) analysis based on Taguchi method. The most substantial parameters were assigned to be the temperature and solid/liquid ratio. Eventually, the dissolution kinetics were studied to propose a model for REEs. Several universal equations for dissolution kinetics were tested, and (1 - (1 - x) = k x tn) gives the best results for REEs. The findings show that the leaching process follows the shrinking core model. Activation energy was calculated to be 40.375 kJ mol(-1) for REEs. As the last step, the iron dissolved during leaching was precipitated as hematite in the autoclave. The hematite precipitation experiments were performed based on the Box-Behnken design. The effect of precipitation parameters was investigated by ANOVA analysis, and the precipitation process was optimized using response surface methodology (RSM), which resulted in the minimum iron and maximum REEs content in the leach liquor.