Microwave Digestion and ICP-MS Determination of Major and Trace Elements in Waste Sm-Co Magnets
Takashi Okada
Round 1
Reviewer 1 Report
Dear Authors:
First, I would like to congratulate you on the excellent experimental work done during your research. You provide relevant information on the generation of analysis protocols for residual Sm-Co magnets.
Below, I provide you with observations and questions with the intention of contributing to your manuscript:
Page 1:
line 13 (and the rest of the manuscript): please write separately the units and the value of the physical quantity measured.
line 15: please subscript the 3 of HNO3.
line 36-39: What do you mean when the ICP method solves the problem of sample inhomogeneity? Wouldn't this aspect be affected by the collection and preparation of the sample? Do the solutions used for calibration have important differences from those used in other techniques such as atomic absorption?
Page 2:
line 62: please check the way you describe the concentration of the elements. It is not common to use this format. Also, the -1 should be an exponent.
Page 3:
How was the efficiency of the digestion process evaluated? From the extraction percentages evaluated later?
line 98: please check if 1 mass.% should be 1 wt.-%.
Page 4:
please check the comma in the value of the forward power.
Page 5:
line 115: what's the high purity value of the acids used?
line 136-138: please join both paragraphs.
Page 6:
lines 156 and 157: please check the possibility of removing "it" from the acid concentration range.
Fig 1.: The values of the degree of digestion by element were measured with respect to the composition of the solid or with respect to what was effectively dissolved, that is, from the solutions?
Page 7:
Lines 182-186: Was the hold time value used to evaluate the effect of the hold temperature on element recovery set before or after the hold time tests?
Page 9:
Figure 3. Please choose a single format to refer to nebuliser or nebulizer.
Lines 238-239: Please check the description of the relationship between the flow rate of the nebulizer and the Ii/I0 ratio.
Page 10:
Fig. 4: any explanation of why the ratio of the intensities is greater than 1? What might this imply in instrumental analysis?
Line 272: how can the negative influence of Sm on the determination of Eu, Gd, and Yb be checked with the information shown in Table 5 if these elements are not included in such a table?
Line 274: please check % wt. (wt. %)
Page 11:
please check the value of n in the caption of Table 6.
Again, I congratulate the authors for the evident effort you have put into this research.
Author Response
Point 1: P. 1, line 13: please write separately the units and the value of the physical quantity measured.
Response 1: We are grateful to the reviewer for the comments made. The sentence revised
Point 2: P. 1, line 13: please subscript the 3 of HNO3.
Response 2: The sentence revised
Point 3 P. 1, line 36-39: What do you mean when the ICP method solves the problem of sample inhomogeneity? Wouldn't this aspect be affected by the collection and preparation of the sample? Do the solutions used for calibration have important differences from those used in other techniques such as atomic absorption?
Response 3: Insufficient sample homogeneity can have a strong influence on the results of the analysis. To do this, the samples are subjected to additional operations of preparation, mixing, averaging. For solid-state methods, this problem may remain unresolved, especially if they are accompanied by small sample portions. But in the ICP methods we have portion of sample is 0.2 g, several replicates and sample dissolution, the necessary degree of homogeneity is ensured. This is very important when working with non-stereotypical and heterogeneous materials. This is what we wanted to say in Introduction. This is a clear advantage over solid-state methods.
To prepare calibration standards in this work, we used multi-element and single-element standards (High-Purity Standards, USA); these standards can be used in other techniques, working with solutions, including atomic absorption
Point 4: P. 2, line 62: please check the way you describe the concentration of the elements. It is not common to use this format. Also, the -1 should be an exponent
Response 4: The sentence on the page 62 revised. -1 set as an exponent
Point 5: P. 3: How was the efficiency of the digestion process evaluated? From the extraction percentages evaluated later?.
Response 5: We evaluated the efficiency of the digestion process with reference materials from the recovery after each digestion experiment
Point 6: P. 3, line 98: please check if 1 mass.% should be 1 wt.-%.
Response 6: Corrections have been made
Point 7: P. 4: please check the comma in the value of the forward power
Response 7: Corrections have been made
Point 8: P.5, line 115: what's the high purity value of the acids used?
Response 8: We clarified values of the acids used.
Point 9: P.5, line 136-138: please join both paragraph
Response 9: Corrections have been made
Point 10: P.6, line 156-157: please check the possibility of removing "it" from the acid concentration range
Response 10: Corrections have been made
Point 11: P.6. Fig 1.: The values of the degree of digestion by element were measured with respect to the composition of the solid or with respect to what was effectively dissolved, that is, from the solutions?
Response 11: The values of the degree of digestion by element were measured with respect to the composition of the solid.
Point 12: P.7, line 182-186: Was the hold time value used to evaluate the effect of the hold temperature on element recovery set before or after the hold time tests?
Response 12: The additional information is given on the page 7.
The hold time value used to evaluate the effect of the hold temperature set after the hold time tests.
Point 13: P. 9. Fig 3. Please choose a single format to refer to nebuliser or nebulizer.
Response 13: Corrections have been made.
Point 14: P. 9, lines 238-239: Please check the description of the relationship between the flow rate of the nebulizer and the Ii/I0 ratio
Response 14: Corrections have been made
Point 15: P. 10. Fig. 4: any explanation of why the ratio of the intensities is greater than 1? What might this imply in instrumental analysis?
Response 15: The matrix effect can be better explained by such processes as the collision of analyte ions with matrix ions in the supersonic expansion region (supersonic expansion will affect the reactions of the atom/molecule and the behavior of excited states in the supersonic expansion region), space charge effects in the supersonic expansion region. ion-optical system and shifts of ionization equilibrium in plasma. Changing the operating conditions of the mass spectrometer, in turn, affects the degree of influence of space charge effects on the signal intensity of the elements being determined. This is achieved by changing the number of positive ions in the beam of the ion-optical system. However, the overall sensitivity of the mass spectrometer may be affected. If we consider the change in the potential at the extractor lens, then a decrease in the potential leads to a decrease in the sensitivity of the mass-spectrometer. However, in this case, an increase in the ratio Ii/I0 is observed, which is in good agreement with the calculations of Tanner [Scott D. Tanne. Space charge in ICP-MS: calculation and implications. // Spectrochimica Acta. 1992. Vol 47 B. No. 6. P 809-823. https://0-doi-org.brum.beds.ac.uk/10.1016/0584-8547(92)80076-S], who showed back in 1992 that if mass-spectrometer not set to maximum sensitivity, it is possible to increase the signal from isotopes of impurity elements in the presence of a matrix element.
Point 16: P. 10, lines 272: how can the negative influence of Sm on the determination of Eu, Gd, and Yb be checked with the information shown in Table 5 if these elements are not included in such a table?
Response 16: The explanation is added to the text (Р. 10). Additional information was included in Table 5.
Point 17: P. 10, lines 274: please check % wt. (wt. %)
Response 17: Corrections have been made.
Point 18: P.11: please check the value of n in the caption of Table 6.
Response 18: Corrections have been made.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Analysis of multi-component waste material is important, and information of optimal conditions in combinations of sample dissolution and its analysis are useful. However, originality of the paper is not clear and I have several comments on this work. Before publication of the manuscript, major revision is needed.
1. In the introduction, originality of this study should be clearly described. The authors examine microwave digestion and optimal conditions of ICP-MS, and describe analytical results of ICP-OES, focusing on Sm-Co magnet waste analysis. Please clarify the originality of these work.
2. In page 2, the abbreviation “LOD” is used, but its definition (limit of detection) is not explained. Please provide the definition.
3. In Figure 2, maximum analytical signal of Ce is different from those of the other elements. Also, in Figure 3, the maximum signals varied depending on the element species. If possible, please explain their reasons.
4. In the manuscript, the LOQs of ICP-MS and ICP-OES shown in Table 6 are not compared in terms of analytical possibility. This is one of goal in the present study, but the explanation is “these two methods make it possible to determine macro and micro components in waste Sm-Co magnets in a wide range of concentrations”. Providing further explanation is better for the comparison of these two methods.
Author Response
Point 1: In the introduction, originality of this study should be clearly described. The authors examine microwave digestion and optimal conditions of ICP-MS, and describe analytical results of ICP-OES, focusing on Sm-Co magnet waste analysis. Please clarify the originality of these work
Response 1: We are grateful to the reviewer for the comments made.
The originality of this work consists in a combination of type of wastes that has not previously investigated as an object of analysis. Analytical methods have not been developed for it, and the basic requirements for monitoring the chemical composition have not been formulated. In this work, in addition to the new object, a rational method of sample decomposition in a microwave system and a combination of two analytical methods were used for the most reliable evaluation of this important waste.
Some explanations and corrections are included in the introduction
Point 2: In page 2, the abbreviation “LOD” is used, but its definition (limit of detection) is not explained. Please provide the definition.
Response 2: Corrections have been made.
Point 3: In Figure 2, maximum analytical signal of Ce is different from those of the other elements. Also, in Figure 3, the maximum signals varied depending on the element species. If possible, please explain their reasons.
Response 3: If we understood correctly, this comment concerns Figure 3.
The absence of a strict dependence of the signal suppression on the atomic mass for the isotope of the element is explained by the presence of other factors affecting the sensitivity, in addition to the dissipation of the ion beam on the space charge. For example, the difference in the first ionization potentials of different elements leads to a different effect on the temperature of the central channel of the plasma discharge of these elements. This, in turn, can lead to a noticeable difference in the suppression of the sensitivity of the mass-spectrometer, even if the matrix elements are contained in the analyzed solution in the same concentrations and are close in mass.
Thus, the optimal parameters of the nebulizer gas flow of a mass spectrometer for measuring solutions containing a matrix differ from those for solutions without a matrix. In other words, a setting that gives a low signal when measuring a solution that does not contain a matrix can give a significantly higher signal when measuring a solution with a matrix; in this case, the signal level depends on the concentration of the matrix. As follows from the experimental data, this dependence has a maximum, the position of which is determined by the specific settings of nebulizer gas flow and the ratio of the atomic masses of the measured isotope and the matrix element.
As can be seen from experimental data , for most elements, the maximum ratios Ii/I0 were obtained with nebulizer gas flow of 0.85 L/min, and for Ce - 0.90 L/min. Therefore, we corrected the text and recommend to use the nebulizer gas flow 0.85-0.90 L/min.
Point 4: In the manuscript, the LOQs of ICP-MS and ICP-OES shown in Table 6 are not compared in terms of analytical possibility. This is one of goal in the present study, but the explanation is “these two methods make it possible to determine macro and micro components in waste Sm-Co magnets in a wide range of concentrations”. Providing further explanation is better for the comparison of these two methods
Response 4: The explanation and the comparison of these two methods is added to the text (P. 11-12)
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper evaluated the applicability of ICP-MS and ICP-OES for elemental analysis in waste Sm-Co magnet, and the dissolution of samples was prepared using microwave digestion. The overall organization of the manuscript is relatively poor. Also, the discussion part lacks logic analysis and validity. My detailed comments are as following.
1. The authors gave a long introduction. However, it is kind of pile up in writing and lacks internal relevance. The introduction should be revised in a concise and informative way.
2. It seems that both ICP-OES and ICP-MS were applied for the elemental analysis of waste Sm-Co magnet according to the Experimental. However, only ICP-MS was mentioned in other parts, such as the title and line 77. The overall presentation is confusing.
3. What’s your purpose by giving lines 136-138?
4. Line 159: When even small amounts of 159 hydrofluoric acid (0.1 ml) are added, however, samarium precipitated.
So why do you still use hydrofluoric acid in the mixed acids?
5. Line 174-175: The hold time was varied within the range of 5 to 60 min; the interval step was 5-10 min;
What’s the detailed interval step?
6. For the Introduction (line 13) and Conclusions (line 294) parts, the authors claimed a temperature of 250℃ was adopted for microwave digestion. Why do you choose 170℃ for selection of ratio of acids (line 148)?
7. The term of Ii/I0 was adopted in Fig. 2 to refer to the magnitude of the matrix effect. However, the detailed explanation was given in line 228.
8. Line 238-239: As can be seen from Fig. 3, an increase in the nebuliser flow rate from 0.6 to 0.85 238 L/min led to a decrease in the Ii/I0 ratio.
According to Fig. 3, the increase in nebuliser flow rate from 0.6 to 0.85 238 L/min resulted in increase in Ii/I0 ratio.
9. Line 255-259: No data or detail results concerning the effects of sampling depth was given in the manuscript. So how did you choose 101? Please explain.
10. In Part 3.2, a summary of analysis results obtained by ICP-MS and ICP-OES was given. However, there was no discussion. So what is the relative advantages of ICP-MS and ICP-OES in this case?
11. The technical terms should be given in the same way:
(1) Sm-Co magnets or samarium-cobalt magnets?
(2) Microwave-assisted digestion (line 84) or microwave digestion?
Author Response
Response to Reviewer 3 Comments
We are grateful to the reviewer for the comments made.
We took into account all the comments on the manuscript. We agree with the comments and present the new version of the article, checked by a native English-speaking colleague.
Point 1: The authors gave a long introduction. However, it is kind of pile up in writing and lacks internal relevance. The introduction should be revised in a concise and informative way.
Response 1: The introduction was revised
Point 2: It seems that both ICP-OES and ICP-MS were applied for the elemental analysis of waste Sm-Co magnet according to the Experimental. However, only ICP-MS was mentioned in other parts, such as the title and line 77. The overall presentation is confusing.
Response 2: Actually two methods (ICP-MS and ICP-OES) were used in this work. In our previous work the possibilities of ICP-OES for the direct determination of trace and matrix elements in waste samarium cobalt magnets were investigated [Petrova KV, Baranovskaya VB, Korotkova NA. Direct inductively coupled plasma optical emission spectrometry for analysis of waste samarium-cobalt magnets. Arab J Chem. 2022;15:1-10. https://0-doi-org.brum.beds.ac.uk/10.1016/j.arabjc.2021.103501].
In this manuscript ICP-OES was used to verify the accuracy of the ICP-MS and to compare the analytical possibilities of these two technics for analysis of waste Sm-Co magnets.
Necessary additions and comments are added to the manuscript (P.10-11, line 273-285)
Point 3: What’s your purpose by giving lines 136-138?
Response 3: Corrections have been made. The lines 136-138 were deleted
Point 4: Line 159: When even small amounts of 159 hydrofluoric acid (0.1 ml) are added, however, samarium precipitated.
So why do you still use hydrofluoric acid in the mixed acids?
Response 4: We used hydrofluoric acid to dissolve refractory metals (Zr, Nb, Mo) and Si. Unfortunately, without the addition of this acid, complete dissolution of these elements does not occur. However, rare earth metals are sensitive to this acid, so we have to stabilize the dissolution
process. It was decided to conduct an experiment with the addition of hydrochloric and sulfuric acid to the solution. These acids affect the overall chemical process and the chemical equilibrium in solution. Experimentally, we selected the ratio of acids leading to the complete dissolution of all target analytes.
Point 5: Line 174-175: The hold time was varied within the range of 5 to 60 min; the interval step was 5-10 min;
What’s the detailed interval step?
Response 5: The interval step was 5 min for the range of the hold time 5-30 min. For the hold time 30-60 min the interval step was 10 min.
Point 6: For the Introduction (line 13) and Conclusions (line 294) parts, the authors claimed a temperature of 250℃ was adopted for microwave digestion. Why do you choose 170℃ for selection of ratio of acids (line 148)?
Response 6: We studied the influence of the ratio of acids on the degree of dissolution before studying the influence of hold time and hold temperature. These investigations were performed at the temperature 170 ℃ (the standard conditions, reccommended by the software of microwave systems MARS 6 laboratory system (CEM Corp., USA).
Point 7: The term of Ii/I0 was adopted in Fig. 2 to refer to the magnitude of the matrix effect. However, the detailed explanation was given in line 228
Response 7: Corrections have been made. The detailed explanation is given in line 216-225.
Point 8: Line 238-239: As can be seen from Fig. 3, an increase in the nebuliser flow rate from 0.6 to 0.85 238 L/min led to a decrease in the Ii/I0 ratio.
According to Fig. 3, the increase in nebuliser flow rate from 0.6 to 0.85 238 L/min resulted in increase in Ii/I0 ratio.
Response 8: Corrections have been made.
Point 9: Line 255-259: No data or detail results concerning the effects of sampling depth was given in the manuscript. So how did you choose 101? Please explain.
Response 9: The ratio Ii/I0 did not change significantly depending on the sampling depth. The intensity of the signals of analyte ions in the absence of a matrix element was maximum at a plasma sampling depth of 101. An increase in the distance between the torch and the sampler decreased the signal intensity of analyte ions both in the presence and absence of a matrix element. An increase in the plasma sampling depth up to 500 decreased the signal intensity of the determined elements by 70–80% for solutions without a matrix. Thus, further experiments were carried out at a plasma sampling depth of 101.
In order not to lengthen the article, we did not include this information in the text of the article.
Point 10: In Part 3.2, a summary of analysis results obtained by ICP-MS and ICP-OES was given. However, there was no discussion. So what is the relative advantages of ICP-MS and ICP-OES in this case?
Response 10: Corrections and detailed discussion of advantages of ICP-MS and ICP-OES are added to the manuscript (P. 10-11, Line 273-285)
Point 11: The technical terms should be given in the same way:
(1) Sm-Co magnets or samarium-cobalt magnets?
(2) Microwave-assisted digestion (line 84) or microwave digestion?
Response 11: Corrections have been made
Author Response File: Author Response.pdf
Reviewer 4 Report
The manuscript investigated the feasibility of an analytical method to determine metal contents in waste Sm-Co magnets using microwave digestion followed by ICP assays. The study appears to be necessary and of interest in the field of research, i.e., recycling of this particular waste. The authors may consider the below comments to improve the manuscript:
1. Line 14-15 of Abstract - use vol.% instead of ml when stating the amount of reagents used.
2. Table 4 - how the Recovery % was calculated? This can be given in Section 2 Method.
3. Caption of Figure 2 - Sm-Co 500 mg/L is confusing. 500 mg/L for Sm and Co, respectively, or the concentration of Sm+Co is 500 mg/L?
4. The information about the "reference material of waste Sm-Co magnets" may be given in Section 2.
Author Response
Point 1: Line 14-15 of Abstract - use vol.% instead of ml when stating the amount of reagents used.
Response 1: If this comment is not of a fundamental, but a recommendation, we would like to leave the units used. As practice shows, indicating the volume of the solution in ml is usually more understandable to the reader. We would like to keep the volume units in ml.
Point 2: Table 4 - how the Recovery % was calculated? This can be given in Section 2 Method
Response 2: The values of the degree of digestion by element were measured with respect to the composition of the solid.
Point 3: Caption of Figure 2 - Sm-Co 500 mg/L is confusing. 500 mg/L for Sm and Co, respectively, or the concentration of Sm+Co is 500 mg/L?
Response 3: Corrections have been made: (Sm-Co 500 mg/L: Sm 200 mg/L, Co 300 mg/L)
Point 4: The information about the "reference material of waste Sm-Co magnets" may be given in Section 2
Response 4: The additional information is given in Section 2. The reference material of waste magnets were developed at the Federal State Research and Development Institute of Rare Metal Industry “Giredmet” (Russia).
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The corrections have been made appropriately based on the reviewer's comments. The revised article is acceptable for publication.
Author Response
Point 1: The corrections have been made appropriately based on the reviewer's comments. The revised article is acceptable for publication.
Response 1: We are grateful to the reviewer for the comment made.
Author Response File: Author Response.pdf
Reviewer 3 Report
The revised manuscript showed some improvement in quality. However, only a revised version was given and there was no clean version of manuscript. I suggest the authors check the manuscript carefully again.
I noticed that the results given in Table 4 (The recovery of elements by hold temperature) have been changed. Why?
Author Response
Point 1: The revised manuscript showed some improvement in quality. However, only a revised version was given and there was no clean version of manuscript. I suggest the authors check the manuscript carefully again.
I noticed that the results given in Table 4 (The recovery of elements by hold temperature) have been changed. Why?
Response 1: We agree with the comments and submitted the clean version of the article. We checked the manuscript carefully again. Also the article was checked by a native English-speaking colleague.
During revision we added additional information to the Tables 5 and 6 (according to the comments of Reviewers 1-2, 4). In addition, we noticed inaccuracies when transferring data to the Table 4 and corrected the data.
Author Response File: Author Response.pdf
