Hydrometallurgical Recovery and Process Optimization of Rare Earth Fluorides from Recycled Magnets
Round 1
Reviewer 1 Report
The approach and idea of the paper is interesting and could bring new results in this area, but there are quite a lot of mistakes/imperfections or missing scientific information concerning to such proposed complex REE fluoride recovery process from Nd magnets.
Row:
2: The use of expression „Production“ in the Paper title is not very suitable...: for example „Recovery“ would be more convenient (accoriding to the concept of the article).
9: Abstract needs to be rewritten:
- Missing description of the most important results achieved in this study... parameters, data, numbers, not only simple name, what procedures have been performed.
- Correct (refine) some expressions: ...by composite..; ...reacting the filtrate (authors means probably precipitate);.... additional filtering...; ....quantitative XRD was not performed...;
- Was it realized some analysis and confirmation of the suitability (applicability) of ammonium sulphate prepared by authors` experiments (in the harmony with environmental requirement? Fluoride are present in this solution); please specify in abstract what type of moddeling was used...;
- correct indexes in formulas, spaces,
21: In Keywords the word „ Modelling“ or similar is probably missing
23: In Introduction should be less attention paid to general Nd-Fe-B magnet physical properties specific composition etc... (authors does not investigate in their work a physical properties, solvent extraction, separation of individual REE or REF, do not analyse the content of Al, Cu, Co etc...). On the contrary there are missing information (review) about precipitation of Nd solutions with NH4OH, HF or eventually other reagents (for instance: LYMAN, J.W., PALMER, G.R.: Recycling of Neodymium Iron Boron Magnet Scrap; Report of investigations, 1993 ) and some information about jarosite precipitation for Fe removal...
45: It is not clear, what means: ...“separate THEM (what?) in which chemical separation by leaching:.... Specify for example what was separated from what by leaching. (literature 16 and 17 seems to be not relevant to the Nd, Dy, Pr, Fe issue.).
57: For ion exchange it is more common an expression „elution“ and for SX stripping.
61-62: Change „filtrate“ in the sentence ...“transforms the filtrate into rare...“ to a proper word „ precipitate“ or similar.
64: Concerning to presented hazardous character of HF, NH4.HF is a hazardous chemical, too, a toxic and corrosive similarly as HF.
74: Do not understand.../Blue M. box furnace???. The sample was introduced into the cold or hot furnace?
84: Crushing and pulverizing was done in one typ of mill? What kind of pulverizer it was (ball, disc, knife mill or? Authors mean that granulometry of material after crushing was in the range 3.1 mm- 0.3 mm? Quite big grain size of a particles for pulverizing (it seems to be rather crushing, milling processes starts from around 0.125 mm). On the other hand, Nd magnets are brittle and the size of particles should be also few microns.
88-100: There are missing or not fully clear some conditions and parameters of processes of the stage 1 and stage 2. For example what pH was adjusted after adding of NH4OH; at what temperature and stirring rate and time. How REE rich precipitate was dried. What was the efficiency of leaching and precipitation with NH4OH and exact chemical analysis of the precipitate. How authors can prove and confirm the repeatability of such proposed and designed processes?
98: The sentence should be corrected: „...analyzed for REF content...“ authors analyzed the REE content of the final product, but REF was then only calculated based on assumption and XRD analysis.
103: Probably more convenient different title of chapter: for example „Modelling of REF recovery etc....
104: ...“flowsheet in Fig.1“....
111: what means opts?
122: ...the amount of REF produced“...authors should unify and harmonize an expression (use different kind of expressions for one issue): „the amount of REF obtained“, Amount of REF Recovery(%) in the Tab.2, Residue, Filtrate (filtrate means usually solution, the solid is called „solid residue“ or in this case for example final REF product...etc..)
129: Materials characterization chapter should belong to chapter 2.2.
136: SEM-EDX was performed on compact material before crushing (one individual demagnetized sample) or on crushed sample? What means representative sample? How many HDD magnets have been used for the sample preparation and for this study?
145: Correct the sentence:....“to quantify the materials????
Correct „filtrate“: ....“and the filtrate after NH4F.HF....
151: It is not explained reason for realization of TGA-DSC measurement of ammonium jarosite? No mention in abstract nor introduction nor 3.4 chapter.
157: In chapter Results it should be distinguished between input sample characterization (Grain size analysis and SEM-EDX) and characterization of intermediate products and products of individual proposed processes. Anyway, there are missing some analysis and process parameters: an input chemical analysis of leaching liquor before precipitation and other parameters, SEM-EDX of final REF product etc...
158: Grain size analysis (granulometry)
165: SEM-EDX analysis.
167: in the demagnetized magnet - individual compact magnet or?
176: Fig.3: authors mean by „composition of the magnet“ the one individual compact magnet after demagnetization or powder sample (representative? Or what?) SEM morfology of particles is not clearly visible...
178: Unify: “...and the dry filtrate after NH4F.HF...“ with other expressions used for the same issue „Final product“, „Residue „(Fig.1)
183:...For 30 minutes....differs from value in the row 97.(45 min)
188: Mistake: Fig.4: ...“in the REF rich precipitate....“ (probably mean „REE rich precipitate“)
What about the chemical reactions concerning to REF preparation!
190: Mistake: Fig.5: ..“in the filtrate after 30 min. in NH4F.HF...“ Why the final REF product after reaction was heated up to 400 °C? No notice anywhere...
192: The analysis of the final product is after heating at 400°C? Authors should pay attention to more precisely desription of experiments as well as results! What about washing of the REF product after filtration and before heating?
197: Remove the sentence...this is not a reason...authors should manage another type of analysis to confirm the results and exact total chemical composition or presence of impurities (Ni, Al, Co, Cu, Si, SO4, NH4, F etc...???. This was indeed the major aim of this article!
199- 200: Why author think that in final product (REF) should be present also jarosite and ammonium sulphate ? Ammonium jarosite was prepared in individual arm of flowsheet in stage 3, where no fluorides were present. In REF preparation step, REE rich precipitate (the question is also how rich?) was introduced into the ammonia fluoride system which should not contain sulphate or minimum (or authors did not perform washing step before fluoridizing experiments? If there is fluoride in the system, standard ammonium jarosite or ammonium sulphate could be formed? How authors solve this...moreover when REF sample was heated up to 400 °C and if there anyway would be present ammonium jarosite and ammonium sulphate, crystalline water and NH3 is then removed and authors can not calculate with presence of ammonium jarosite and ammonium sulphate in heated REF product and calculate the composition of product.
201: According to the Fe content in REF, the jarosite content should be 3.44 %, not 2.27%
202: Correction: heated and dried at 400°C (remove „dried“)
Generally, there is missing basic theory (theoretical precausions, at least neccessary thermodynamics) to the proposed precipitation and recovery processes: chemical reactions, E-pH diagrams, describing behaviour of studied systems. Missing explanation for example why Fe remains in sulphate solution after NH4OH adding when it should probably precipitate as FeOOH or Fe(OH)3 depending on pH as well as on redox potential or temperature of the system?
205: Only TGA – DSC diagram is described by reactions, but to be precise they are not so important in this study (if yes, it should be written somewhere in the paper) and they are not fully correct and not all properly interpreted (how authors can confirm the presence of ammonium sulphate? When disputed this option why authors did not realize XRD measurement or other measurement of obtained jarosite? Ammonium sulphate should be visible at XRD. On Fig.6 are visible 3 peaks, see for example: Ray L. Frost et all: Thermal decomposition of ammonium jarosite (NH4)Fe3(SO4)2(OH)6, 2006, Journal of Thermal Analysis and Calorimetry 84(2), DOI: 10.1007/s10973-005-6953-8
208: TGA-DSC graphs (it is only one graph) of the final product...! again authors use the same words for different obtained products.... probably mean ammonium jarosite???? See row 192 in Tab.1 final product is REF and in row 208.
232: Correct or explain„...;2-5 g of NH4F.HF was added... but there are not given results at 5 g of NH4F.HF addition.
233: Tab.2: what is „Std“?
Degree of stirring – probably better to use time of stirring (authors sometimes used also this expression) Please again unify all specific expressions in the text and use only one in each case! It brings chaos.
Amount REF Recovery (%) – should be more proper „REF recovery“, not amount...or?
How exactly were measured and calculated % values of amount REF recovery (%) and Purity of REF?
It is not clear what values in Tab.2 are comming from? From Tab. 1 it is visible that according to REE content authors achieved the product containing together cca 88.2 % REF ( calculated), base on what the values ~ 45- 65 % are calculated in tab. 2 for Amount of REF REcovery (%) and also for Purity of REF ?
For example - Amount of recovered REF means that 50 % it was obtained and 50 % of REF remains in the solution???
276: probably wrong part of the sentence: , ...“ optimum REF recovery with reasonable high recovery???? Moreover discrepancy with discussion of results, for example in row 245.
284: References: correction (formal writing according to rules) in some references is probably needed:, 1., 2., 6 etc...check it.
Equation should be numbered differently to chemical reactions (depends on journal rules of coarse...
Author Response
Attached is the answer to the comments
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript deals with the selective recovery of rare earths in the form of fluorides by a hydrometallurgical route. The authors studied the leaching of magnets by sulfuric acid followed selective precipitation as ammonium double sulfate. Then the double sulfate was converted to rare earth fluoride by treating it with ammonium bifluoride. For optimization, the authors used the Response Surface Methodology. The authors may modify the manuscript based on the following suggestions.
- The references, especially from 38 to 48, are not related to the text provided in the introduction. They can be modified.
- REEs can be recovered by electrolysis of chlorides and oxides. Fluorides are generally used for metallothermic reduction. However, it was mentioned in the text that the rare earth fluoride could be used for electrolysis. The authors need to provide a proper reference for that.
- The fluorides produced in aqueous electrolysis contain chemically bound moisture. The authors need to discuss the removal of this moisture before used it for further process.
Author Response
Attached are answers to the reviewers comment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
1.) It would be probably more convenient to correct in Fig.1 "PrF3 and NdF3 (Residue)" to for example: "REF residue or product (NdF3, PrF3, DyF3)...
2.) row 262: to provide additional information in the sentence:
" ...quality of the REF residue based on ICP analysis" (represented by the sum of the analyzed Nd,Pr, Dy elements` content in REF product).....or similar. For me it is still not clearly presented, because REF purity I understand the REE in the fluoride form (desired final product is REF not REE, what means content of NdF3+PrF3+DyF3 in your case) and if authors mean only REE content in the fluoride it should be said exactly. But what is then the total possible content of REF or REE in the product to what I should realate to? What is the real efficiency of processes and how I can deside if your process was good enough to achieve maximum yield of your processes (REE as well as REF products?)
Author Response
Correction have been made. please see attached file.
Author Response File: Author Response.pdf
