Microwave Digestion and ICP-MS Determination of Major and Trace Elements in Waste Sm-Co Magnets

Abstract

In this article, inductively coupled plasma mass-spectrometry (ICP-MS) and inductively coupled plasma optical-emission spectrometry (ICP-OES) were used for the development of an analytical procedure for analysis of the waste of Sm-Co magnets. Experimental parameters related to microwave digestion processes and acid concentrations were optimized. Microwave digestion was carried out in mixtures of HF, HCl, HNO₃ and H₂SO₄. The complete dissolution of the samples occurred in the system: 10 mL H₂O, 2 mL HNO₃, 10 mL HCl and 1 mL H₂SO₄. The dependence of the matrix effect on the ICP-MS analysis of waste Sm-Co magnets was studied and optimal instrumental parameters were investigated (nebulizer gas flow, sampling depth and potential at the extractor lens). The optimal conditions were a nebulizer gas flow of 0.85–0.90 L/min, a sampling depth of 101, potential at the extractor lens of −400 V and a sample flow rate of 50 rpm. A recovery test and inter-method experiments were performed to verify the accuracy of the proposed method.

1. Introduction

Samarium cobalt (Sm-Co) permanent magnets with excellent magnetic performance have wide applicability in the field of electric vehicles, microwave communications, instrumentation and other energy applications. During the manufacturing process of Sm-Co magnets, considerable amounts of scrap and residue are generated [1,2]. These magnetic waste materials and spent magnets can be considered as potential sources of rare, non-ferrous and refractory metals.

A variety of spectroscopic techniques have been employed in the quantification of elements in various rare earth magnets [3,4,5,6]. Among the analytical techniques, methods based on inductively coupled plasma (ICP), ICP-MS and ICP-OES are widely accepted due to their multi-element capability, high sensitivity, wide dynamic range and rapidity.

Despite the wide range of application of these methods for the analysis of magnetic materials, only a few articles on ICP-MS and ICP-OES analysis of waste Sm-Co magnets have been published [6,7,8,9,10,11]. Most of these articles present the results of the determination of major components (Sm, Co, Fe, Zr, etc.); however, the determination of impurities is also relevant to the choice of a recycling process for waste Sm-Co magnets. This means that the development of an approach to the analysis of such materials is of current interest.

The combination of these two techniques enables the determination of a very wide range of metal compound concentrations. ICP-OES is preferred in the context of major elements (Sm, Co, Fe, Si, etc.), as well as for some impurities, whereas ICP-MS is particularly suitable for trace metals analysis. Furthermore, these two instrumental techniques are complementary in that they are based on different physical principles; as such, they serve as independent quality control tools. This aspect is important, given that reference materials (RM) for waste Sm-Co magnets, with certified concentrations of elements in principle, are not available.

Several procedures have been described in the literature for sample preparation in relation to RE magnets [5,12,13,14,15]. Most of these are primarily based on decomposition using mixtures of acids in open vessels. The possible losses or contamination that are typical of open systems, however, give rise to the need to use closed vessels, which ensure a fairly fast flow of processes; closed vessels include autoclaves with resistive heating or microwave systems [16,17,18,19,20].

Microwave acid digestion has proven to be the most suitable method for the digestion of complex matrices such as soils, environmental samples and heat-resistant alloys [21,22]. This procedure allows for shorter digestion times and good recovery. In addition, it reduces the risk of external contamination and requires smaller quantities of acids. This improves detection thresholds and the overall accuracy of the analytical method and prioritizes the principles of green chemistry [23].

The present article is devoted to research of the microwave digestion method for the analysis of waste Sm-Co magnets by ICP-OES and ICP-MS methods. Some parameters of the microwave digestion method, such as acids concentrations, the hold temperature and hold time, were evaluated to assure the quantitative recoveries of the elements. The other goal of this work was to study the capabilities of the ICP-MS analysis of waste Sm-Co magnets, to choose optimal instrumental parameters (ICP power, nebulizer flow rates, sampling depth and potential at the extractor lens), the concentration of the matrix element and to compare the analytical possibilities of ICP-MS and ICP-OES methods.

2. Materials and Methods

2.1. Equipment

2.1.1. Microwave Digestion Systems

The microwave digestion of samples was carried out using a MARS 6 laboratory system (CEM Corp., Matthews, NC, USA). Easy-Prep iWave vessels were used for the experiment. This type of vessel allowed the experiments to be carried out with a wider range of acid ratios and temperatures. During the experiments, several parameters of microwave digestion were varied, namely acid concentration, hold time and hold temperature. In all the experiments, the volume of water was 10 mL, ramp time was 10 min, output power was 800 W and cooling time was 15 min. After each digestion experiment, the concentration of the elements in the solutions was determined using ICP-OES. The recovery (%) was calculated with respect to the composition of the RM for waste Sm-Co magnets.

The microwave parameters are listed in

Table 1. Operating parameters, temperature programs and digestion reagents for MARS 6 microwave systems (CEM Corp., Matthews, NC, USA).

Power/W	800
Vessels	EasyPrep iWave
No. of vessels	12
Vessel volume/mL	30
Sensor control	P
Ramp time/min	10
Hold time/min	5–60
Hold temperature/°C	150–250
Cooling time/min	15
Mass/mg	200
Volume of H2O/mL	10
Volume of HNO3/mL	0–5
Volume of HCl/mL	0–15
Volume of HF/mL	0–1
Volume of H2SO4/mL	0–1

.

2.1.2. Inductively Coupled Plasma Optical-Emission Spectrometry

A Thermo Scientific iCAP PRO XP ICP-OES with a vertical torch, a purged Echelle polychromator and a Charge Injection Device array detector were used to determine the elements with a concentration above 1 wt. %. A standard sample introduction kit suitable for aqueous samples, consisting of a glass cyclonic spray chamber, a SeaSpray glass nebulizer, a quartz glass duo torch and other components, was also used. Operating parameters, elements and their spectral lines are summarized in

Table 2. Instrumental operating conditions of the optical-emission spectrometer.

Forward power/W	1400
Wavelength range	167 to 852 nm
Coolant gas flow/L·min−1	15
Auxiliary gas flow/L·min−1	0.35
Nebulizer gas flow/L·min−1	0.5
Sample flow rate/rpm	60
Pump tube/mm	0.64
Radial viewing height/mm	10
Injector diameter/mm	2
Pneumatic nebulizer	SeaSpray Nebulizer, Glass Expansion
Spray chamber	Cyclonic Spray Chamber, Glass Expansion
Wavelengths of determined elements/nm	Al 167.079, Mg 285.213, Ti 368.520, Cr 205.560, Cr 425.435, Mn 257.610, Nb 295.088, La 333.749, La 626.230, Nd 430.358, Nd 445.157, Eu 281.394, Gd 301.013, Tb 332.440, Tb 387.417, Dy 400.045, Dy 369.481, Tm 534.649, Tm 286.923, Yb 328.937, Lu 291.139, Hf 232.247, Ni 227.021, Ni 230.300, Cu 224.700, Fe 259.940, Zr 257.139, Mo 202.030, Si 212.412, Sm 359.260 and Co 235.342

.

2.1.3. Inductively Coupled Mass-Spectrometry

ICP-MS analysis was performed using an XSeries II quadruple mass-spectrometer (Thermo Scientific, Waltham, MA, USA) with a conical spray chamber cooled to 3 °C, and a SeaSpray glass nebulizer. Instrumental operating conditions are summarized in

Table 3. Instrumental operating conditions of the mass-spectrometer.

Forward power/W	1300
Coolant gas flow/L·min−1	0.8
Auxiliary gas flow/L·min−1	13
Nebulizer gas flow/L·min−1	0.6–1.0
Sample flow rate/rpm	20–100
Sampling depth/relative units	100–500
Potential at the extractor lens/V	−100–−500
Spray booth temperature/°C	3
Level of oxide ions/%	<2
Level of doubly charged ions/%	<1.5
Measurement mode	Peak hopping
Pneumatic nebulizer	SeaSpray Nebulizer, Glass Expansion
Spray chamber	Quartz conical, Peltier cooled
Isotopes of elements to be determined/m/z	25Mg, 27Al, 45Sc, 47Ti, 52Cr, 55Mn, 57Fe, 62Ni, 63Cu, 89Y, 91Zr, 139La, 140Ce, 141Pr, 145Nd, 151Eu, 158Gd, 159Tb, 161Dy, 165Ho, 167Er, 169Tm, 174Yb, 175Lu, 177Hf
Internal standard	115In

.

2.2. Reagents and Solutions

The microwave digestion of the samples used the following concentrated acids: high purity nitric acid HNO₃ (nitric acid—70%), high purity hydrochloric acid HCl (hydrogen chloride—35–38%), high purity sulfuric acid H₂SO₄ (sulfuric acid—93.5–95.6%) and high purity hydrofluoric acid HF (hydrofluoric acid—46–49%). Deionized water with a resistivity of 18.2 MΩ cm at 25 °C was used for all the dissolutions and dilutions.

Aqueous calibration solutions were prepared from multi-element and single-element standards (High-Purity Standards, Charleston, SC, USA) by serial dilution to different volumes with a 2% HNO₃/HCl/HF/H₂SO₄ mixture. Calibration solutions for ICP-OES were prepared in a concentration range: 0.1–10 mg/L for Mg, Al, Ti, Cr, Mn, Nb, La, Nd, Eu, Gd, Tb, Dy, Tm, Yb, Lu and Hf; 0.1–300 mg/L for Ni, Cu, Fe, Zr, Nb and Mo; and 50–1500 mg/L for Sm and Co.

The RM for waste Sm-Co magnets was developed at the Federal State Research and Development Institute of Rare Metal Industry “Giredmet” (Moscow, Russia).

Calibration solutions for ICP-MS were prepared in a concentration range: 1.0, 10, 50 µg/L for Mg, Al, Sc, Ti, Cr, Mn, Fe, Ni, Cu, Y, Zr, La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Hf. To improve the accuracy of the results, an internal standard indium was used, which compensated for the drift of the spectrometer, partially due to matrix effects, as well as changes in the size of aerosol droplets and the rate of its supply to the torch; these, in turn, were caused by different viscosities of the analyzed solutions and calibration solutions. Before measurements were made, all the solutions were diluted 10 times for ICP-OES and 20 times for ICP-MS, and an internal standard was introduced—15 µg/L of In. Acidity in the final sample solutions (1–3%) was maintained at almost the same level as in the standard solutions used for the calibration. Corresponding process blanks, without the addition of a sample, were prepared in the same way, i.e., they were taken through the complete sample preparation (microwave digestion).

3. Results and Discussion

3.1. Optimization of Microwave Digestion Conditions

To dissolve the samples of the waste Sm-Co magnets, the selection of the mixture of acids for the dissolution was carried out. A mixture of mineral acids was used, in which the individual elements that make up the waste of Sm-Co magnets dissolve well: Al, Mg, Mn, Ni, Co, Sm and other rare earth elements (REE) dissolve in dilute nitric acid; in hydrochloric acid—Al, Fe, Cr, Co and REE; in the presence of hydrofluoric acid—refractory elements Si, Ti, Zr, Nb, Mo and Hf; and in sulfuric acid—Mn, Al and Ti [24]. Experiments on the selection of the ratio of acids were carried out at a hold temperature of 170 °C and a hold time of 60 min, and using a standard sample of a waste Sm-Co magnet containing Sm (33.85 wt. %) and Co (58.05 wt. %), Mo (4.60 wt. %), Nb (1.95 wt. %) and Si (1.45 wt. %). The completeness of the digestion was established by measuring the contents of the matrix and macro components using the ICP-OES method under the conditions shown in Table 2.

To establish the influence of the ratio of acids on the recovery of the elements, samples of 0.2 g were dissolved in a mixture of nitric, hydrochloric, hydrofluoric and sulfuric acids, in various ratios and 10 mL of water. The volume of nitric acid varied in the range of 0–5 mL; hydrochloric acid—0–15 mL; hydrofluoric acid—0–1 mL and sulfuric acid—0–1 mL. The experiment showed that 2 mL of nitric acid is sufficient for complete dissolution of Sm and Co, and 0.25 mL of hydrofluoric acid is sufficient for complete dissolution of Mo, Nb and Si. When even small amounts of hydrofluoric acid (0.1 mL) were added, however, samarium precipitated. To stabilize the dissolution process, it was decided to conduct an experiment with the addition of hydrochloric and sulfuric acid to the solution. During the study, it was found that the complete dissolution of the samples occurred in the system: 10 mL H₂O, 2 mL HNO₃, 10 mL HCl and 1 mL H₂SO₄.

Reagent blanks were also prepared with a concentrated HNO₃/HCl/HF/H₂SO₄ mixture, according to the proportions shown in Table 1. The vessels were capped and placed in the microwave system and digested at a temperature of 170 °C and a hold time of 60 min. At the end of the program, the vessels were cooled to room temperature in a fume hood and the pressure inside the vessels was slowly released. After cooling, the digested samples were transferred to graduated polypropylene tubes and brought up to 50 mL with deionized water.

In addition to the ratio of acids, the microwave process was influenced by the hold temperature and hold time. Therefore, the influence of these parameters on recovery was studied. The hold time was varied within the range of 5 to 60 min—the interval step was 5 to 10 min—and the hold temperature was 170 °C (Figure 1). The experiments showed that with a decrease in hold time below 60 min, recovery decreased. With a hold time of 50 min, the loss of the major elements was 0.1 wt. %; with a hold time of 5 min, the loss was 10–15 wt. %, depending on the element determined. As is shown in Figure 1, the complete recovery of the elements was achieved at the hold time 60 min and the hold temperature 170 °C.

In order to reduce the dissolution time of the sample, the hold temperature was varied within the range of 150 °C to 250 °C; the hold time was 30 min. The results are shown in

Table 4. The recovery of the elements, by hold temperature.

Hold Temperature, °C	Recovery, %
	Sm	Co	Mo	Nb	Si
150	74.3	72.1	72.7	77.8	72.4
170	90.4	90.6	92.3	90.2	81.2
200	95.1	94.9	95.5	95.3	95.6
220	95.2	95.5	96.1	95.4	95.9
250	100.1	99.3	100.2	100.0	99.5

. When the hold temperature decreased, recovery decreased. For example, at a hold temperature of 150 °C, the recovery of the elements was 72–78 wt. %. The complete recovery of the elements was achieved at the hold time of 30 min and the hold temperature of 250 °C.

3.2. ICP-MS and ICP-OES Determination of Elements

In addition to the digestion procedure itself, the method used for analyzing the resulting solutions had an important effect on both the list of elements to be determined, and the limits of determination achieved. ICP-MS, like any other instrumental method, has a number of limitations, the most important being the matrix effect and spectral interferences [25,26,27,28].

The matrix effect is expressed in the suppression of the signal of the ions of the element being determined with an increase in the concentration of the matrix element [25,26,27,28]. Figure 2 shows the dependence of the intensity of the signals of the elements Be, Mg, Cu, In, Ce, Tl, Pb and Th on the concentration of the matrix elements Sm and Co in solution under standard mass-spectrometer operating conditions. The selected elements characterize the entire mass scale.

As can be seen from the data obtained, an increase in the concentration of Sm-Co in the solution led to a decrease in the signal intensity of the analytes. For solutions with a Sm-Co concentration of 500 mg/L, the decrease in signal intensity for some elements was 11–30%. Such a noticeable change in signal intensity is explained by a number of processes occurring in the plasma, such as the collision of analyte-ions with matrix ions in the supersonic expansion region, shifts in ionization equilibrium in the plasma and space-charge effects in the ion optical system [25,26,27,28].

There are several ways to eliminate the matrix effect. The simplest way uses calibration solutions with a matrix concentration similar to the matrix of the samples analyzed. This technique is inconvenient for the routine analysis of multicomponent samples like waste Sm-Co magnets.

It is known that the choice of so-called ‘robust conditions’ (i.e., the nebulizer gas flow, the sampling depth and the potential at the extractor lens) makes it possible to significantly reduce the matrix effect [29,30]. In the current work, the dependence of the matrix effect on the ICP-MS analysis of waste Sm-Co magnets was studied and optimal instrumental parameters were investigated.

Experiments to study operating parameters were carried out using solutions containing 10 μg/L Li, In, Ba, Ce and U; and 500 mg/L Sm-Co (200 mg/L Sm and 300 mg/L Co). The concentrations of Sm and Co were set so that they were identical to the concentrations of the elements in waste Sm-Co magnets used in the present study (35–40 wt. % Sm and 57–62 wt. % Co). The signal from the solutions was compared with the signal from a pure 2% HNO₃ solution containing 10 μg/L of Li, In, Ba, Ce and U. The magnitude of the matrix effect was calculated as I_i/I₀, where I_i is the signal intensity of the isotope in a solution with matrix elements (500 mg/L Sm-Co) and I₀ is the intensity of the isotope signal in a pure nitric acid solution without matrix elements. The plasma power in all the experiments was 1300 W.

The dependence of the ratio I_i/I₀ on the nebulizer gas flow is shown in Figure 3. The nebulizer gas flow varied in the range of 0.6–1.0 L·min⁻¹.

As can be seen from Figure 3, an increase in the nebulizer flow rate from 0.6 to 0.85 L/min led to an increase in the I_i/I₀ ratio. With a further increase in nebulizer gas flow, the I_i/I₀ value decreased, while the level of oxide ions BaO⁺/Ba⁺ increased, as did the level of doubly charged ions Ba⁺⁺/Ba⁺. The minimal matrix effect and the maximum analytical signals for most elements were obtained with a nebulizer flow rate of 0.85–0.90 L/min.

The effect of the sample flow rate into the nebulizer was also investigated. The experiment was carried out in the range of 20–100 rpm. The ratio I_i/I₀ did not change much with the change in pump speed and was in the range of 0.7–0.8 relative units. Therefore, further work was carried out at a standard value of 50 rpm.

The dependence of the ratio I_i/I₀ on the potential at the extractor lens is shown in Figure 4. The potential at the extractor lens varied within the range −100–−500 V. As can be seen from Figure 4, the I_i/I₀ ratios were close to 1 at −400 V.

The sampling depth was investigated in the range of 100–500 relative units. As the sampling depth increased, the ratio of I_i/I₀ increased; however, a significant decrease in the signal intensity was observed both in the presence and in the absence of Sm-Co in the solution. Based on this, it was decided to carry out further research at the sampling depth of 101 relative units.

To obtain correct results in the ICP-MS analysis of waste Sm-Co magnets, it is necessary to take into account the influence of polyatomic ions from macro components. Experiments for the study and the quantitative characterization of the effects of polyatomic ions were performed using sets of standard solutions containing 500 mg/L Sm, Co, Mo, Nb, Fe or Ni in 2% HCl/HNO₃/HF/H₂SO₄ mixture. The signals from these solutions were compared with the signals from pure solutions containing 1 and 10 μg/L of Mg, Al, Sc, Ti, Mn, Fe, Cu, Y, Zr, La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Hf. The experiments showed that the influences of Co, Mo, Nb, Fe and Ni on the featured elements can be ignored. The greatest influence in ICP-MS have the oxide and hydroxide polyatomic interferences from samarium (

Table 5. Samarium spectral interferences for the featured elements.

Element	Isotope	Polyatomic Ion	Apparent Concentration of the Element, µg/g
Eu	151Eu	150Sm1H+	0.06
	153Eu	152Sm1H+	0.35
Gd	155Gd	154Sm1H+	0.70
	156Gd	144Sm12C+	0.20
	157Gd	144Sm13C+	0.10
	158Gd	144Sm14N+	0.10
Dy	161Dy	144Sm16O1H+	2.0
	162Dy	147Sm15N+, 148Sm14N+, 149Sm13C+, 150Sm12C+	3.5
	163Dy	147Sm16O+	50.5
	164Dy	147Sm16O1H+, 148Sm16O+	18.0
Ho	165Ho	148Sm16O1H+, 149Sm16O+	5.0
Er	166Er	149Sm16O1H+, 150Sm16O+	115.0
	167Er	150Sm16O1H+	4.0
	168Er	152Sm16O+	110.0
	170Er	154Sm16O+	150.0
Tm	169Tm	152Sm16O1H+	2.0
Yb	171Yb	154Sm16O1H+	15.0

). However, although polyatomic interferences from Sm had a negative influence on the determination of a number of elements (e.g., Eu, Gd, Yb), they can be taken into account by isotope selection: ¹⁵¹Eu, ¹⁵⁸Gd, ¹⁷⁴Yb. In the case of ¹⁶⁵Ho, ¹⁶¹Dy, ¹⁶⁷Er and ¹⁶⁹Tm, there was a slight increase in the limits of quantifications (LOQs) of these elements (up to n·10⁻⁴ wt. %). Nevertheless, these LOQs are sufficient for the analysis of waste Sm-Co magnets.

The sample of waste Sm-Co magnets was analyzed using microwave decomposition (hold temperature 250 °C and hold time 30 min) and further ICP-MS and ICP-OES analyses of the solutions.

Table 6. Analysis of the reference sample of waste Sm-Co magnets (n = 3, p = 0.95).

Element	LOQs * of ICP-MS, wt. %	LOQs ** of ICP-OES, wt. %	Content, wt. % ($\overline{\mathit{x}}$ ± 1.96·Sr, (n = 3, p = 0.95))
			ICP-MS, wt. %	ICP-OES, wt. %
Mg	3.0 × 10−5	5.0 × 10−4	(6.51 ± 0.09) × 10−3	(6.78 ± 0.17) × 10−3
Al	1.0 × 10−5	2.0 × 10−4	(2.61 ± 0.05) × 10−1	(2.57 ± 0.05) × 10−1
Si	2.5 × 10−3	1.0 × 10−4	ND ***	1.45 ± 0.02
Sc	1.0 × 10−5	3.0 × 10−4	<1.0 × 10−5	<5.0 × 10−5
Ti	1.0 × 10−5	5.0 × 10−4	(3.28 ± 0.03) × 10−2	(3.37 ± 0.07) × 10−2
Cr	3.6 × 10−4	2.0 × 10−4	(2.65 ± 0.04) ∙10−2	(2.63 ± 0.05) × 10−2
Mn	1.0 × 10−5	1.0 × 10−4	(5.15 ± 0.07) × 10−2	(5.09 ± 0.10) × 10−2
Fe	8.0 × 10−4	4.0 × 10−4	(2.35 ± 0.04) × 10−1	(2.29 ± 0.05) × 10−1
Co	ND	ND	ND	58.1 ± 0.9
Ni	6.0 × 10−6	5.0 × 10−4	(8.61 ± 0.12) × 10−2	(8.63 ± 0.18) × 10−2
Cu	2.0 × 10−5	1.0 × 10−3	(2.11 ± 0.05) × 10−2	(1.99 ± 0.08) × 10−2
Y	3.0 × 10−6	1.0 × 10−4	(2.14 ± 0.04) × 10−3	(2.18 ± 0.07) × 10−3
Zr	6.0 × 10−6	1.0 × 10−4	(1.21 ± 0.01) × 10−3	(1.19 ± 0.02) × 10−3
Nb	1.0 × 10−4	5.0 × 10−4	ND	1.95 ± 0.03
Mo	1.0 × 10−4	1.0 × 10−3	ND	4.60 ± 0.08
La	2.0 × 10−6	5.0 × 10−4	(1.06 ± 0.13) × 10−3	(9.86 ± 0.53) × 10−4
Ce	2.0 × 10−6	5.0 × 10−4	(1.32 ± 0.02) × 10−2	(1.36 ± 0.02) × 10−2
Pr	4.0 × 10−6	1.0 × 10−3	(7.41 ± 0.15) × 10−4	(8.05 ± 0.75) × 10−4
Nd	5.0 × 10−6	6.0 × 10−4	(1.78 ± 0.05) × 10−2	(1.61 ± 0.15) × 10−2
Sm	ND	ND	ND	33.8 ± 0.5
Eu	6.0 × 10−6	3.0 × 10−4	(3.53 ± 0.05) × 10−3	(2.85 ± 0.28) × 10−3
Gd	1.0 × 10−5	3.0 × 10−4	(6.82 ± 0.13) × 10−3	(6.58 ± 0.22) × 10−3
Tb	1.0 × 10−5	5.0 × 10−4	(1.55 ± 0.04) × 10−3	(1.64 ± 0.05) × 10−3
Dy	2.0 × 10−4	5.0 × 10−4	(6.95 ± 0.13) × 10−3	(7.28 ± 0.23) × 10−3
Ho	5.0 × 10−4	1.0 × 10−3	(3.35 ± 0.09) × 10−3	(3.12 ± 0.11) × 10−3
Er	4.0 × 10−4	1.0 × 10−3	(4.52 ± 0.11) × 10−3	(5.16 ± 0.25) × 10−3
Tm	2.0 × 10−4	3.0 × 10−4	(2.76 ± 0.09) × 10−3	(3.06 ± 0.21) × 10−3
Yb	5.0 × 10−6	1.0 × 10−4	(1.23 ± 0.12) × 10−3	(9.98 ± 0.39) × 10−4
Lu	3.0 × 10−6	5.0 × 10−4	(7.20 ± 0.07) × 10−5	<2.0 × 10−4
Hf	1.0 × 10−5	4.0 × 10−4	(1.21 ± 0.05) × 10−4	<2.0 × 10−4

* LOQs were determined using 3s-criteria of ten measurements of 300 mg/L Co and 200 mg/L Sm solution in a 2% HCl/HNO₃/HF/H₂SO₄ mixture obtained from High-Purity Standards (USA). ** LOQs were determined using 3s-criteria of ten measurements of 1200 mg/L Co and 800 mg/L Sm solution in a 2% HCl/HNO₃/HF/H₂SO₄ mixture obtained from High-Purity Standards (USA). *** ND—not determined.

shows the results of the analysis and LOQs. LOQs of ICP-MS analysis ranged from n·10⁻⁶ wt.% to n·10⁻⁵ wt.% for most elements. LOQs of ICP-OES ranged from n·10⁻⁵ wt.% to n·10⁻³ wt.%. There were, however, some empirical exceptions. For example, the limits of determination for Fe using the ICP-MS method were higher than in ICP-OES, due to the specific polyatomic interferences of argon and oxygen. For other impurities at a low level, however, the ICP-MS method was appropriate. For macro components, the ICP-OES method was generally suitable. For a number of impurities, it can implement internal accuracy control of the obtained results. As can be seen from Table 6, the combination of these two methods made it possible to determine macro and micro components in waste Sm-Co magnets in a wide range of concentrations.

4. Conclusions

A one-stage microwave oven digestion procedure was developed to process and analyze waste Sm-Co magnet samples using ICP-MS and ICP-OES. Microwave digestion was carried out in a mixture of HCl/HNO₃/HF/H₂SO₄. Experimental parameters related to the microwave decomposition processes were optimized for achieving the quantitative recovery of elements (>95%) through the analysis of a reference material of waste Sm-Co magnets. The analytical capabilities of ICP-MS analysis of waste Sm-Co magnets were studied. The effect of the operational parameters of an ICP mass-spectrometer on the matrix effect was investigated and ‘robust conditions’ were established. Under ‘robust conditions’ (nebulizer gas flow 0.85–0.90 L/min; sampling depth 101; potential at the extractor lens −400 V; and sample flow rate 50 rpm), the I_i/I₀ ratios for most elements varied from 0.89–1.05.

After analysis using a recovery test and inter-method experiments, it was concluded that the method developed was accurate and reproducible. Recovery was 99–100% and RSD was below 5%. The LOQs of the ICP-MS analysis of waste Sm-Co magnets were n·10⁻⁶–n·10⁻⁵ wt. %; the LOQs of ICP-OES ranged from n·10⁻⁵ wt. % to n·10⁻³ wt. %.