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Article

Separation of Cobalt, Samarium, Iron, and Copper in the Leaching Solution of Scrap Magnets

by
Jian-Zhi Wang
*,
Yi-Hsun Hsieh
,
Yi-Chin Tang
and
Yun-Hwei Shen
Department of Resources Engineering, National Cheng Kung University, Tainan 70101, Taiwan
*
Author to whom correspondence should be addressed.
Metals 2023, 13(1), 90; https://doi.org/10.3390/met13010090
Submission received: 26 November 2022 / Revised: 22 December 2022 / Accepted: 24 December 2022 / Published: 31 December 2022
(This article belongs to the Special Issue EARTH 2022-Green Technologies for Resources Processing and Circulation)
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Abstract

:
With the growing awareness of protecting the urban environment and the increasing demand for strategic materials, recycling of SmCo magnets has become imperative. This paper provides a series of methods regarding the available hydrometallurgical technologies for recycling scrap magnets. This study aimed to recover samarium (Sm), cobalt (Co), copper (Cu), and iron (Fe) from acid leachate of SmCo scrap by using precipitation and ion exchange. IRC748 showed a good adsorption capacity for Fe and Cu. Elution tests were conducted using sulfuric acid at the concentration of 2N as eluents. Precipitation was performed first using a selective chemical precipitation method, and the Sm was first precipitated as a sodium samarium sulfate powder. Then, the samarium-deprived solution was placed in the beaker, and the addition of oxalic acid promoted cobalt oxalate precipitation. Furthermore, the leachate, which is rich in Cu and Fe, was mixed with oxalic acid to obtain the copper oxalate precipitation. This study successfully recovered SmCo magnets through ion exchange and precipitants.

1. Introduction

Rare-earth elements are a group of seventeen chemical elements in the periodic table. Currently, these metals have become very critical to several modern technologies, ranging from televisions, displays, cell phones, LED light bulbs, rare-earth magnets, and automobile catalysts [1]. Rare-earth resources in China, which holds the largest rare-earth resources in the world, account for about 48.4% of the world supply (55 million tons) [2]. In the 2020 EU review of the list of critical raw materials report, rare-earth elements have been regarded as critical raw materials [3]. Therefore, the recovery of rare earth from various resources will become an important issue in the future [4].
Neodymium (Nd) and Sm are often used to make rare-earth magnets [5]. Nd2Fe14B magnets are used in electric vehicles, generators, and power storage devices [6]. SmCo5 magnets are used in military fields, aircraft, and national defense [7]. Nd2Fe14B magnets have the highest energy product (BHmax) of all permanent magnets (47 to 51MGOe). SmCo5 magnets have 28–30 MGOe energy product (BHmax). Compared with NdFeB magnets, SmCo5 magnets reveal their unique performance, such as higher coercivity and better temperature resistance (350 °C). Thus, SmCo5 magnets are suitable for some special applications in military technologies, such as precision-guided missiles [7].
Compared with SmCo5 magnets, the recovery of rare earth from Nd2Fe14B magnets is more widely studied [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. There are only a few recovery methods for SmCo5 magnets reported in the literature. Swain et al. [25] suggested a solvent extraction process with 1.5 mol/L Aliquat 336 (quaternary ammonium cation) from 5 mol/L NaNO3 at pH 3 containing SmCo solution. Three-stage acid extraction and stripping are used to complete the extraction of Sm in the presence of Co. The paper by Swain et al. [26] provided extensive discussion of the 2-stage extraction of 0.4 mol/L TOPO (organophosphorus compound) from 3 mol/L NaNO3 at pH 3 containing SmCo simulation solution. Two-stage extraction and stripping are used to complete the extraction of Sm in the presence of Co. Sinha et al. [7] proposed a solvent extraction method using 1M Cyanex572 (organophosphorus compound) to extract Sm from spent SmCo5 magnet leachate, with O/A = 2:1 and two-stage extraction to completely extract Sm in the presence of Co. Orefice et al. [27] proposed a solvent extraction step, using 50 wt% Aliquat336 (extractant) diluted in toluene saturated with 37 wt% hydrochloric acid, and successfully separated Co, Fe, and Cu from the leaching solution containing Sm, Co, Fe, and Cu. The final extraction of Sm was achieved by 20 vol.% Cyanex 272 (organophosphorus compound) in dodecane.
All the literature indicated that the solvent extraction method is an effective process to separate valuable metals in SmCo5 magnets. However, most of the literature did not consider how to effectively separate metals under the condition of four metals. When dealing with waste magnets, the composition is usually more complex. This study proposes for the first time the use of IRC748 resin to adsorb comparatively worthless metallic Fe and Cu. After separating the high-value Sm and Co and the low-value Fe and Cu, an effective precipitation separation method is proposed. In addition, the influence of different molar ratios, temperature, and time on the precipitation effect was studied.

2. Materials and Methods

2.1. Materials

The SmCo5 magnet scrap was supplied by Tai Chuan metal Co., Ltd.(Taipei, Taiwan). A chelating cation exchange resin IRC748 obtained from Rohm and Haas was used in this study. The resin was soaked three times in sulfuric acid and sodium hydroxide. Chemicals used in the present investigation were of analytical grades and obtained from Shin-Shin Chemical Co. (Tainan, Taiwan). The contents of Sm, Co, Fe, and Cu in the magnet scrap are displayed in Table 1. The contents of the leachate obtained by leaching the scrap with 2N sulfuric acid (add 5 vol.% of concentrated H2O2 (28%)) for 240 min at 353 K and 50 g/L pulp density (5 g SmCo scrap was mixed with 2N H2SO4 (100 mL) in beaker with stirrer) is given in Table 2.

2.2. Resin and Column Experiments

We examined several commercial resins [28,29] for their suitability for heavy metal separation and a commercial weak acid-chelating resin, IRC748, produced by Rohm and Haas, appeared to be very promising for this purpose. The adsorption rates of IRC748 on Cu, Fe, and Co are 50, 50, and 5%, respectively [29]. Compared with M4195, IRC748 has a better desorption effect when the resin is rich in Cu and Fe [29]. Ultimately, IRC748 resin was chosen for the separation experiments. The recovery of Fe and Cu from the leachate using ion-exchange was carried out via the column operation. A 15 cm-long and 1.5 cm-diameter column was filled with 12 mL of the resin. The loaded column was then eluted by passing sulfuric acid (3 of bed volumes) through the column at a flow rate of 0.25 mL/min.

2.3. Precipitation Separation Experiment

In the Sm separation experiment, we refer to the research of Zhou et al. [30] and take a temperature of 80 °C, Na2SO4 to Sm molar ratio of 4:1, and precipitation time of 90 min as the initial conditions. By adjusting the temperature, molar ratio, and time, the best parameters are obtained.
For Co precipitation, we refer to the research of Sinha et al. [7] and Swain et al. [26] and use oxalic acid as the precipitating agent for Co. We choose a molar ratio of 1, a temperature of 60 °C, and a time of 60 min as initial parameters. Firstly, the effect of the molar ratio on the precipitation efficiency was discussed, because Co could not be effectively precipitated if insufficient oxalic acid was added. Furthermore, the effect of temperature on the precipitation efficiency was explored, because temperature affects the solubility of particles in the liquid. Finally, the influence of time on the precipitation efficiency was discussed.
In terms of separating Cu and Fe, we refer to the studies of Soare et al. [31] and Makarova et al. [15] and found that oxalic acid has selective precipitation for Cu. The initial parameters are the molar ratio of 1 (theoretical ratio), normal temperature, a time of 30 min, and the pH value of the original elution. In terms of the selection of the experimental sequence, we choose to discuss the molar ratio first and then discuss the temperature, time, and pH. The reason is the same as that of Co precipitation.

2.4. Analytical Methods

The concentration of metal ions in the solutions obtained after the leaching, ion exchange, and chemical precipitation experiments were determined by using Atomic Absorption Spectroscopy (AAS, PinAAcle 900F AA Spectrometer, PerkinElmer Inc., Waltham, MA, USA). The pH of the solution was monitored by a pH meter (PL-700PVS, Dogger Science, Taipei, Taiwan). To control the precipitation temperature and heating rate, a thermostatic bath with magnetic stirring (Shin-Kwang Precision Industry Ltd., New Taipei, Taiwan) was employed. The crystal structure of the precipitation was analyzed by X-ray diffractometry (XRD, DX-2600, Dandong, China), with Cu Kα radiation ranging from 2θ = 10–80°, to identify the phases present. The contents of Sm, Co, and Cu in the precipitation were analyzed by energy-dispersive spectroscopy (EDS, HITACHI SU8000, Bruker, Billerica, MA, USA).

2.5. Separation Process

First, IRC748 resin adsorbs Fe and Cu. Sulfuric acid will elute Cu and Fe to obtain a solution containing both. Second, sodium sulfate is used to separate the Sm and Co present in the effluent. Third, oxalic acid is used for cobalt oxalate precipitation. Furthermore, oxalic acid is used to separate the Cu and Fe. The precipitation efficiency is mainly affected by the molar ratio, temperature, and time. Consequently, the precipitation experiments are all used to explore their influence on the precipitation efficiency. The flowchart is shown in Figure 1.

3. Results and Discussion

3.1. Ion-Exchange

The leachate obtained by leaching the scrap with 2 N sulfuric acid for 240 min at a condition of 353 K and 50 g/L pulp density was used in the ion exchange experiments.
Due to the high metal concentration, the collected leachate with a 1/2 dilution ratio and pH values adjusted to 0.6, 1.0, and 2.0 were passed through the column packed with the IRC748 resin at a constant flow rate. The breakthrough curves were plotted as a dimensionless concentration factor C/C0 versus a dimensionless effluent volume, bed volume, BV. Figure 2a–c show the breakthrough curves for ion exchange at pH of 0.6, 1.0, and 2.0, respectively. At pH = 0.6, the carboxylic and amine groups in IRC748 resin are fully pronated, and all four metal ions cannot form complexes with IRC748 resin. Thus, the breakthrough curves at pH 0.6 show the fast breakthrough for all four metal ions. At pH = 1.0, the carboxylic and amine groups in IRC748 resin are partially pronated, and the selectivity of the resin upon all four metal ions starts emerging, as demonstrated. The affinity order of the metals for the IRC748 resin is Fe > Cu > Co = Sm. At pH = 2.0, the carboxylic and amine groups in the IRC748 resin are well pronated, and the selectivity of the resin upon all four metal ions is fully displayed. The apparent sequence of the affinity of the metals in the IRC748 resin is Fe = Cu > Co = Sm. The Fe and Cu breakthrough capacity reaches 6 BV, in comparison to 2BV for Co and Sm. This suggests that the IRC748 resin can successively separate Fe, Cu and Co, Sm at pH =2.0. Further increasing the pH causes the precipitation of Fe, and pH 2.0 was the optimum value for this separation. The obtained effluent containing Sm and Co was used for selective precipitation experiments.
In the elution experiments, the elution was conducted immediately after the completion of the loading experiment. Figure 3a–c plot the elution curves for metal elution from the loaded column using 1.0 N, 2.0 N, and 3.0N H2SO4, respectively. Eluting with H2SO4 successfully displaced Fe and Cu from the resin. The Cu desorption increased from 47.4% to 99.1% and Fe desorption increased from 93.0% to 99.7% with the sulfuric acid concentration increased from 1N to 2N. Thus, 2.0 N H2SO4 proves to be an efficient choice when compared to other acid concentrations. The amount of metal that was desorbed from the resin was calculated by the elution formula. The obtained elution effluent containing Fe and Cu was used for selective precipitation experiments.
Elution formula:
Elution weight/Adsorption weight= Elution rate

3.2. Selective Precipitation of Sm by Na2SO4

This part discusses the influence of the molar ratio, temperature, and time on the precipitation efficiency of the Sm.
The results are shown in Figure 4a. As the molar ratio increases, the Sm precipitation efficiency increases significantly. The co-precipitation rate of the Co is always below 5%, which is beneficial to the separation of Sm and Co. When the molar ratio is 4, the precipitation rate of the Sm reaches 98.4%. If the molar ratio is further increased, the Sm precipitation efficiency cannot be effectively improved again. Compared with the molar ratio, the temperature plays a more important role, as shown in Figure 4b. When the temperature is too low, the solubility of NaSm(SO4)2 is higher, resulting in a significant reduction in precipitation efficiency. Therefore, a higher temperature is the key to the precipitation separation of Sm and Co. The results for time, Figure 4c, show that the Sm precipitation reaction rate is slow, and it takes 90 min to effectively precipitate Sm.
According to the above results, the optimum conditions for selective precipitation of Sm were proposed as follows: reaction temperature at 80 °C, molar ratio of Na2SO4 to Sm of 4:1, and reaction time of 90 min, which results in a Sm precipitation efficiency of 98.7%.
The precipitate obtained was analyzed by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). Figure 5 displays the results of the XRD analysis, and the major peaks of NaSm (SO4)2·H2O at 29.766°, 31.703° confirms the presence of crystalline NaSm(SO4)2·H2O as a dominate phase. There is only one sampling point in the EDS analysis, and the sample is a compacted powder. The results of the EDS analysis were shown in Table 3. The EDS analysis result showed that the molar contents of the Sm and Na in the precipitate were 13.23% and 12.08% respectively, which agrees with the theoretical values of 1:1.

3.3. Selective Precipitation of Co by Oxalate

The effects of the oxalate to Co molar ratio, reaction temperature, and reaction time on the recovery of Co were investigated in this section.
The results of the cobalt oxalate precipitation are shown in Figure 6a–c. The molar ratio and temperature did not play a significant role in the Co precipitation efficiency. When oxalic acid is added to the solution, no matter what the molar ratio or temperature is, more than 95% of Co can be precipitated. This result shows that oxalic acid is a very good precipitant for Co. In terms of time, the precipitation efficiency of oxalic acid is relatively slow, and it takes 60 min to precipitate more than 95% of the cobalt oxalate.
According to the above results, the optimum conditions for selective precipitation of Co were proposed as follows: reaction temperature of 60 °C, molar ratio of oxalate to Co of 1:1, and reaction time of 60 min, which results in a Co precipitation efficiency of 96.2%.
The precipitate obtained was analyzed by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). Figure 7 displays the results of the XRD analysis, and the major peaks of CoC2O4 at 18.745°, 34.98° confirm the presence of crystalline CoC2O4 as a dominate phase. There is only one sampling point in the EDS analysis, and the sample is a compacted powder. The results of the EDS analysis were shown in Table 4. The EDS analysis result showed that the molar contents of cobalt, carbon, and oxygen in the precipitate were 12.88%, 28.90%, and 57.55%., respectively, which agrees with the theoretical values of 1:2:4.

3.4. Selective Precipitation of Cu by Oxalate

This part discusses the effect of the molar ratio, temperature, time, and pH value on copper precipitation separation.
The results of the molar ratio and Cu precipitation efficiency are shown in Figure 8a. Because of the elution process, the pH of the solution is lower. Therefore, oxalic acid is difficult to dissolve in the solution, and an excessive amount of oxalic acid needs to be added to precipitate Cu. Compared with the molar ratio, the temperature, time, and pH value have little effect on Cu precipitation, as shown in Figure 8b–d. Regardless of how much the molar ratio is, the temperature, time, and pH value changed, and the co-precipitation efficiency of Fe was always less than 1%. This means that oxalic acid has good separation efficiency for Cu and Fe.
According to the above results, the optimum conditions for selective precipitation of Cu were proposed as follows: reaction temperature of 20 °C, molar ratio of oxalate to Cu of 10:1, pH value of 0.24, and time of 30 min, which results in a Cu precipitation efficiency of 97.7%.
The precipitate obtained was analyzed by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). Figure 9 displays the results of XRD analysis and the major peaks of CuC2O4·H2O at 22.902°, and 36.251°, confirming the presence of crystalline CuC2O4·H2O as a dominate phase. There is only one sampling point in the EDS analysis, and the sample is a compacted powder. The results of the EDS analysis were shown in Table 5. The EDS analysis result showed that the molar contents of copper, carbon, and oxygen in the precipitate were 14.4%, 28.10%, and 51.50%, respectively, which agrees with the theoretical values of 1:2:4.

4. Conclusions

In this study, a novel process for the recovery of valuable metals from acid leachate of SmCo5 scrap was proposed. IRC748 resin successfully adsorbs Cu and Fe from solutions containing Sm, Co, Fe, and Cu, allowing the separation of higher-value Sm and Co.
In the Sm precipitation part, temperature is particularly important. Higher temperature can effectively improve the precipitation efficiency of Sm. Under the conditions of a temperature of 80 °C, a molar ratio of 4, and a time of 90 min, the Sm precipitation rate reached 98.7%.
In the Co precipitation part, oxalic acid is a very good precipitant. When the temperature is 60 °C, the time is 60 min, and the molar ratio is 1, the Co precipitation efficiency rate is 96.2%.
Finally, it was found in the Cu precipitation experiments that the molar ratio is quite significant for Cu precipitation. When the molar ratio is 10, the temperature is 20 °C, the time is 30 min, and the pH value is 0.24, the Cu precipitation rate reaches 97.7%. The precipitation rate of iron is less than 1%.
In this study, the ion exchange method was successfully used to separate precious metals and cheap metals from four metals. Furthermore, we explored the effect of individual parameters on metal precipitation.

Author Contributions

Conceptualization, J.-Z.W., Y.-H.H. and Y.-C.T.; methodology, J.-Z.W., Y.-H.H. and Y.-C.T.; validation, J.-Z.W., Y.-H.H. and Y.-C.T.; investigation, J.-Z.W., Y.-H.H. and Y.-C.T.; resources, J.-Z.W., Y.-H.H. and Y.-C.T.; data curation, J.-Z.W., Y.-H.H. and Y.-C.T.; writing—original draft preparation, J.-Z.W. and Y.-C.T.; writing—review and editing, J.-Z.W. and Y.-C.T.; supervision, Y.-H.S.; project administration, Y.-H.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

The authors wish to acknowledge the support of National Cheng Kung University’s Department of Resources Engineering and the Ministry of Science and Technology, R.O.C.

Conflicts of Interest

The authors declare no conflict of interest.

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Metals 13 00090 g001 550
Figure 1. Proposed flowsheet for recovering Sm, Co, Fe, and Cu from SmCo5 scrap magnets.
Figure 1. Proposed flowsheet for recovering Sm, Co, Fe, and Cu from SmCo5 scrap magnets.
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Figure 2. Adsorption curve at (a) pH 0.6, (b) pH 1, and (c) pH 2.
Figure 2. Adsorption curve at (a) pH 0.6, (b) pH 1, and (c) pH 2.
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Figure 3. Desorption of Cu, Fe, Co, and Sm at different pH-values ((a) 1N, (b) 2N, (c) 3N) using H2SO4 as eluting agent.
Figure 3. Desorption of Cu, Fe, Co, and Sm at different pH-values ((a) 1N, (b) 2N, (c) 3N) using H2SO4 as eluting agent.
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Figure 4. Effects of (a) Na2SO4 to Sm molar ratio, (b) temperature, and (c) time for the Sm precipitation.
Figure 4. Effects of (a) Na2SO4 to Sm molar ratio, (b) temperature, and (c) time for the Sm precipitation.
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Figure 5. XRD analysis of NaSm(SO4)2.
Figure 5. XRD analysis of NaSm(SO4)2.
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Figure 6. Effects of (a) oxalate to Co molar ratio, (b) temperature, and (c) time on the Co precipitation.
Figure 6. Effects of (a) oxalate to Co molar ratio, (b) temperature, and (c) time on the Co precipitation.
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Figure 7. XRD analysis of CoC2O4.
Figure 7. XRD analysis of CoC2O4.
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Figure 8. Effects of (a) oxalate to Cu molar ratio, (b) temperature, (c) time, and (d) pH on the Cu precipitation.
Figure 8. Effects of (a) oxalate to Cu molar ratio, (b) temperature, (c) time, and (d) pH on the Cu precipitation.
Metals 13 00090 g008aMetals 13 00090 g008b
Metals 13 00090 g009 550
Figure 9. XRD analysis of CuC2O4.
Figure 9. XRD analysis of CuC2O4.
Metals 13 00090 g009
Table
Table 1. Metal content in SmCo5 magnet material.
Table 1. Metal content in SmCo5 magnet material.
ElementSmCoFeCu
wt %21.9448.0416.964.63
Table
Table 2. Metal composition of leachate.
Table 2. Metal composition of leachate.
ElementSmCoFeCu
Concentration (ppm)13,61826,76275842695
Table
Table 3. EDS analysis of NaSm(SO4)2.
Table 3. EDS analysis of NaSm(SO4)2.
SampleNaSmSO
NaSm(SO4)212.08%13.23%21.57%53.12%
Table
Table 4. EDS analysis of CoC2O4.
Table 4. EDS analysis of CoC2O4.
SampleCoOC
CoC2O412.88%57.55%28.90%
Table
Table 5. EDS analysis of CuC2O4.
Table 5. EDS analysis of CuC2O4.
SampleCuCO
CuC2O414.40%28.10%51.50%
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Wang, J.-Z.; Hsieh, Y.-H.; Tang, Y.-C.; Shen, Y.-H. Separation of Cobalt, Samarium, Iron, and Copper in the Leaching Solution of Scrap Magnets. Metals 2023, 13, 90. https://doi.org/10.3390/met13010090

AMA Style

Wang J-Z, Hsieh Y-H, Tang Y-C, Shen Y-H. Separation of Cobalt, Samarium, Iron, and Copper in the Leaching Solution of Scrap Magnets. Metals. 2023; 13(1):90. https://doi.org/10.3390/met13010090

Chicago/Turabian Style

Wang, Jian-Zhi, Yi-Hsun Hsieh, Yi-Chin Tang, and Yun-Hwei Shen. 2023. "Separation of Cobalt, Samarium, Iron, and Copper in the Leaching Solution of Scrap Magnets" Metals 13, no. 1: 90. https://doi.org/10.3390/met13010090

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