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 HNO
3/HCl/HF/H
2SO
4 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. 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 Ii/I0, where Ii 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
Ii/
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
Ii/
I0 ratio. With a further increase in nebulizer gas flow, the
Ii/
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 Ii/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
Ii/
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
Ii/
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 Ii/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/HNO
3/HF/H
2SO
4 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). 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 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.