Optimizing Conditions for Scandium Extraction from Bauxite Residue Using Taguchi Methodology
, Lamprini-Areti Tsakanika, Konstantinos Hatzilyberis and Maria Ochsenkuehn-PetropoulouRound 1
Reviewer 1 Report
The present work reports a well executed exercise in Sc/Fe leaching from bauxite residue.
The Taguchi and ANOVA methods have been employed effectively and have provided comprehensive and verifiable results.
However the authors provide no insight into the mechanisms that lead to these results and hence stay only on the level of presenting us results/numbers and their statistical manipulation.
In the introduction of the work, line 69 page 2, the authors define their objective as ",the optimization of scandium selective leaching process from BR with sulfuric acid is investigated using Taguchi methodology” and furthermore “Method selectivity was estimated in respect to iron content since it is the major element of BR” . (also mentioned in the abstract “ Method optimization targeted the highest concentration of scandium combined with the lowest concentration of iron without taking into account applications constraints”)
Yet in the conclusions we are offered no insight into ‘selective leaching’ or selectivity in respect to iron content. In contrast we are provided with an optimum for Sc leaching at 6M acid 85 C and an optimum for least Fe leaching at 2 M acid 25 C. From the data presented in table 3 it would seem that in the first case of optimum Sc leaching we have also close to maximum iron concentration and in the second case of optimum Fe non-leaching we have also the minimum Sc concentration. So where is exactly the optimum between these two adverse effects? (also the author do not provide us with data on leaching efficiencies of Fe and Sc so we can not be sure whether the concentration in PLS is the result of solid/liquid ratio or leaching efficiency)
In addition we are provided with no data on the other elements of BR and their behavior in this leaching experiments. If the authors have fully dissolved all the iron content of BR at 6M 85 C and possibly most of aluminium, titanium and soda content then this work strays from any meaning of selectivity in Sc extraction. The authors do declare in their opening statement that their work will not take into consideration any downstream processing of the PLS to extract and purify Sc and this is to their credit, as often similar works fail to even mention the existence of a downstream process. It is however obvious that if there is no downstream process that can isolate the 16 mg/lt of Sc from the hundreds gr/lt of iron and other metals then the entire work presented here is meaningless. Authors should elaborate more on this crucial point
In minor comments
One must note that the statement in page 2 line 70, ‘Sulfuric acid was preferred for its mild environmental impact’ is practically negated when using a 6 M sulfuric acid leach solution .
In line 57 page the statement: Among all, hydrometallurgical treatment is the most widely used technique for REEs and Sc ecovery due to its simplicity and low cost” is false, as the only known (to this reviewer) instance of actually applied Sc extraction from BR is that of RUSAL, which is indeed hydrometallurgical but not with acid as implied here, but with NaHCO3
Equation 4 in page 6 and 5 in page 7 have problems with the fonts used
In page 6 line 212 ‘This result is very reasonable as Fe is fully coupled with temperature meaning that increased temperatures favor Fe dissolution’ The authors here fail to explain why this result is very reasonable, i.e. why does iron oxide need a higher temperature to be dissolved. A reference at least is need to back up this claim. Otherwise this is finding from the experiments and not a ‘reasonable’ expected result that further verifies the experiments
The closing statement of the paper “ The optimal conditions derived in this study are indicative for the leaching behavior of similar bauxite residues” is misleading at best, as bauxite residues depend on the bauxite ore mixture used in each specific alumina refinery and the processing conditions of said refinery. Furthermore not all bauxites (and hence bauxite residues) are ‘rich’ in Scandium. This is typical mistake found in many recent BR literature on Sc extraction. In the same way the equations provided in this paper have very little reliability outside the min-max conditions defined, so do the conclusions beyond the specific BR tested.
Author Response
The present work reports a well executed exercise in Sc/Fe leaching from bauxite residue.
The Taguchi and ANOVA methods have been employed effectively and have provided comprehensive and verifiable results.
Reviewer’s Comment: However the authors provide no insight into the mechanisms that lead to these results and hence stay only on the level of presenting us results/numbers and their statistical manipulation.
Authors’ Response: The leaching mechanism has been analytically presented in our previous studies, namely:
37. Ochsenkühn-Petropoulou, M.; Hatzilyberis, K.S.; Mendrinos, L.N.; Salmas, C.E. Pilot-plant investigation of the leaching process for the recovery of scandium from red mud. Industrial & Engineering Chemistry Research 2002, 41, 5794-5801
59. Hatzilyberis, K.; Lymperopoulou, T.; Tsakanika, L.-A.; Ochsenkühn, K.-M.; Georgiou, P.; Defteraios, N.; Tsopelas, F.; Ochsenkühn-Petropoulou, M. Process design aspects for scandium-selective leaching of bauxite residue with sulfuric acid. Minerals, 2018, 8, 79
both are included in the references of the paper.
Reviewer’s Comment: In the introduction of the work, line 69 page 2, the authors define their objective as ",the optimization of scandium selective leaching process from BR with sulfuric acid is investigated using Taguchi methodology” and furthermore “Method selectivity was estimated in respect to iron content since it is the major element of BR” . (also mentioned in the abstract “Method optimization targeted the highest concentration of scandium combined with the lowest concentration of iron without taking into account applications constraints”)
Yet in the conclusions we are offered no insight into ‘selective leaching’ or selectivity in respect to iron content. In contrast we are provided with an optimum for Sc leaching at 6M acid 85 C and an optimum for least Fe leaching at 2 M acid 25 C. From the data presented in table 3 it would seem that in the first case of optimum Sc leaching we have also close to maximum iron concentration and in the second case of optimum Fe non-leaching we have also the minimum Sc concentration. So where is exactly the optimum between these two adverse effects? (also the author do not provide us with data on leaching efficiencies of Fe and Sc so we can not be sure whether the concentration in PLS is the result of solid/liquid ratio or leaching efficiency)
Authors’ Response: The conclusions section was improved concerning the optimum conditions resulted from this study, according to the reviewer’s comment.
In relation to leaching efficiency, it decreases at higher S/L ratio and therefore scandium concentration in the solution is not linearly correlated to S/L ratio as shown in our previous study in Metals [63].
Furthermore leaching efficiency is of minor practical use since the viability of the extraction process depends mainly on Sc concentration in the leachate.
The scope of this study was to evaluate the interaction of different parameters. Previous studies [63] estimated the effect of each parameter either on specific leaching condition or by interaction of maximum of 2 parameters. The key point of the present work is to estimate the synergy effect of all leaching variables which had not been previously presented (see the related changes made in Page 2, lines 87-91).
Reviewer’s Comment: In addition we are provided with no data on the other elements of BR and their behavior in this leaching experiments. If the authors have fully dissolved all the iron content of BR at 6M 85 C and possibly most of aluminium, titanium and soda content then this work strays from any meaning of selectivity in Sc extraction. The authors do declare in their opening statement that their work will not take into consideration any downstream processing of the PLS to extract and purify Sc and this is to their credit, as often similar works fail to even mention the existence of a downstream process. It is however obvious that if there is no downstream process that can isolate the 16 mg/lt of Sc from the hundreds gr/lt of iron and other metals then the entire work presented here is meaningless. Authors should elaborate more on this crucial point.
Authors’ Response: Monitoring of iron concentration was selected among the other main elements as it constitutes the main component and therefore its removal becomes difficult and problematic in subsequent processes of scandium isolation. Also, it is a common place in relevant studies, to choose the 2 most ‘important’ elements to be studied and presented. In addition, Ti and Al concentrations have been reported in our previous study in [63].
It is clear that the maximization of Sc concentration can be achieved with increased dissolution of iron at the expense of selectivity. This is in agreement with studies reporting that most of scandium content is trapped in the hematite phase :
76. Vind, J.; Malfliet, A.; Bonomi, C.; Paiste, P.; Sajó, I.E.; Blanpain, B.; Tkaczyk, A.H.; Vassiliadou, V.; Panias, D. Modes of occurrences of scandium in greek bauxite and bauxite residue. Minerals Engineering 2018, 123, 35-48
The scope of the current study is not to suggest a vertical integrated process for scandium recovery, but to provide a useful tool which can predict all data needed according to leaching requirements.
In minor comments
Reviewer’s Comment: One must note that the statement in page 2 line 70, ‘Sulfuric acid was preferred for its mild environmental impact’ is practically negated when using a 6 M sulfuric acid leach solution.
Authors’ Response: We should clarify that sulphuric acid has mild environmental impact compared to other mineral acids of the same molarity. Thus, the necessary change was made in the text.
Reviewer’s Comment: In line 57 page 1 the statement: Among all, hydrometallurgical treatment is the most widely used technique for REEs and Sc recovery due to its simplicity and low cost” is false, as the only known (to this reviewer) instance of actually applied Sc extraction from BR is that of RUSAL, which is indeed hydrometallurgical but not with acid as implied here, but with NaHCO3
Authors’ Response: According to the complementary remark of both reviewers; we made the necessary rephrasing in this section, as follows:
“Pyrometallurgy although energy consuming, may result to an overall cost reduction due to major removal of main elements, thus having an important impact on the cost of the subsequent Sc purification [54].”
“Hydrometallurgy technique is performed for Sc and REEs recovery since it is usually of low cost at ambient conditions [26].”
Reviewer’s Comment: Equation 4 in page 6 and 5 in page 7 have problems with the fonts used.
Authors’ Response: The problem with fonts that the reviewer indicates for these 2 equations, might depend on his/her system’s fonts configuration, as the same fonts are used for all the equations in the manuscript and also no font problem appears in authors computers.
Reviewer’s Comment: In page 6 line 212 ‘This result is very reasonable as Fe is fully coupled with temperature meaning that increased temperatures favor Fe dissolution’ The authors here fail to explain why this result is very reasonable, i.e. why does iron oxide need a higher temperature to be dissolved. A reference at least is need to back up this claim. Otherwise this is finding from the experiments and not a ‘reasonable’ expected result that further verifies the experiments.
Authors’ Response: Following reviewer’s suggestion we have added the necessary references:
74. EL Bar, D.; Barket, D. Leaching of metals from hydrometallurgical residue by sulfuric acid. Aspects in Mining & Mineral Science 2018, 1, 1-6.
75. Liu, Z.-R.; Zeng, K.; Zhao, W.; Li, Y. Effect of temperature on iron leaching from bauxite residue by sulfuric acid. Bulletin of Environmental Contamination and Toxicology 2009, 82, 55-58.
Reviewer’s Comment: The closing statement of the paper “The optimal conditions derived in this study are indicative for the leaching behavior of similar bauxite residues” is misleading at best, as bauxite residues depend on the bauxite ore mixture used in each specific alumina refinery and the processing conditions of said refinery. Furthermore not all bauxites (and hence bauxite residues) are ‘rich’ in Scandium. This is typical mistake found in many recent BR literature on Sc extraction. In the same way the equations provided in this paper have very little reliability outside the min-max conditions defined, so do the conclusions beyond the specific BR tested.
Authors’ Response: We fully agree with the reviewer’s remark, and this is why we originally had written that: “the results are indicative” but in order to make our statement more clear/precise we complemented by mentioning “for similar bauxite residues”, thus having made the following rephrasing: “The optimal conditions derived in this study are indicative for the leaching behavior for bauxite residues of similar mineralogical and chemical composition”.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript by Theopisti Lymperopoulou, Paraskevas Georgiou, Lamprini-Areti Tsakanika, Konstantinos Hatzilyberis, Maria Ochsenkuehn-Petropoulou from National Technical University of Athens provides a study about to the optimization of scandium extraction from bauxite residue with sulfuric acid is 20 investigated using Taguchi methodology.
The study carried out well creating an interesting approach on how to optimize leaching processes.
Overall, with major changes, I would recommend this manuscript for publication.
Specific comments:
Line 38: References 4-16 are not up to date. Please update.
Line 43: Reference is missing to this sentence.
Line 46: Why Sc is the most important REE? It is the most valuable in the BR.
Line 51: Give a number to show what low concentration means as a comparison to the content in BR.
LIne 54-58: I do not agree with the section. Hydrometallurgical treatment has also a high cost due to acid, energy consumption. Moreover, after leaching there is a need for a further purification step which is costly and extremely difficult regarding the high concentration of elements such as Fe and Al compared to th low concentration of sc in BR. Pyrometallurgy treatment can reduce this cost and help increase the selectivity as main elements can be removed before leaching. Please rewrite this section according to literature for example https://doi.org/10.1016/j.hydromet.2019.01.005
Results and discussion:
In general, the discussion is missing from this part like reported results on the field compared to the findings of this study. This is not the first time when the DOE method published for REE recovery from BR: https://doi.org/10.1002/jctb.5300
The reader does not get information about how 16.6 mg/L extracted Sc is compared to another studies. Is this Taguchi methodology really work better than simple DOE? Is it really needed? Is the extraction yield higher than in other studies? The authors followed only Fe next to Sc while there are other elements in extremely high concentrations compared tot he Sc e.g. Al, Ti, Si, Ca, Na...
Author Response
The manuscript by Theopisti Lymperopoulou, Paraskevas Georgiou, Lamprini-Areti Tsakanika, Konstantinos Hatzilyberis, Maria Ochsenkuehn-Petropoulou from National Technical University of Athens provides a study about to the optimization of scandium extraction from bauxite residue with sulfuric acid is 20 investigated using Taguchi methodology.
The study carried out well creating an interesting approach on how to optimize leaching processes.
Overall, with major changes, I would recommend this manuscript for publication.
Specific comments:
Reviewer’s Comment: Line 38: References 4-16 are not up to date. Please update.
Authors’ Response: According to the reviewer’s suggestion, we have updated the list of references with the BR uses. Some of the ‘old’ references were removed while new up to date studies were added, so the list of updated references is within [4-44].
Reviewer’s Comment: Line 43: Reference is missing to this sentence.
Authors’ Response: Missing references were added as [45-48], namely:
45. Lymperopoulou, T. Determination and recovery of rare earths from bauxites and red mud. Ph.D. Thesis, National Technical University of Athens, Athens, Greece, 1996.
46. Tsakanika, L.-A. Separation and recovery of lanthanides from red mud by use of selective extraction and chromatographic techniques. Ph.D. Thesis, National Technical University of Athens, Athens, Greece, 2013.
47. Ochsenkühn-Petropoulou, M.; Tsakanika, L.-A.; Lymperopoulou, T. In Process control of an innovative method for the recovery and separation of rare earths from red mud by different analytical techniques, 1st European Rare Earth Resources Conference (ERES 2014), Milos Island, Greece, 4–7 September 2014, 2014; Milos Island, Greece, pp 28-29.
48. Ochsenkühn-Petropulu, M.; Lyberopulu, T.; Parissakis, G. Direct determination of landthanides, yttrium and scandium in bauxites and red mud from alumina production. Analytica Chimica Acta 1994, 296, 305-313.
Reviewer’s Comment: Line 46: Why Sc is the most important REE? It is the most valuable in the BR.
Authors’ Response: According to reviewer’s comment, the phrasing was made more precise, as follows: “Sc is the most valuable of rare earth elements in BR” by providing also the corresponding reference:
47. Ochsenkühn-Petropoulou, M.; Tsakanika, L.-A.; Lymperopoulou, T. In Process control of an innovative method for the recovery and separation of rare earths from red mud by different analytical techniques, 1st European Rare Earth Resources Conference (ERES 2014), Milos Island, Greece, 4–7 September 2014, 2014; Milos Island, Greece, pp 28-29.
Reviewer’s Comment: Line 51: Give a number to show what low concentration means as a comparison to the content in BR.
Authors’ Response: After reviewer’s comment, the sentence was rephrased as follows: “Scandium extraction from its main minerals is difficult due to their rarity as well as its low concentration in natural deposits (1-800 mg/kg), in addition to the presence of radioactive elements”. The corresponding references were cited, namely:
51. Williams-Jones; A. E., Vasyukova; O. V., The Economic Geology of Scandium, the Runt of the Rare Earth Element Litter, Economic Geology, 113 (4), 973–988
52. Samson, I.M.; Chassé, M. Scandium. In Encyclopedia of geochemistry: A comprehensive reference source on the chemistry of the earth, White, W.M., Ed. Springer International Publishing: Cham, 2016; pp 1-5.
53. Krishnamurthy, N., & Gupta, C. K. 2015. Extractive Metallurgy of Rare Earths, Second Edition: Taylor & Francis.
Reviewer’s Comment: Line 54-58: I do not agree with the section. Hydrometallurgical treatment has also a high cost due to acid, energy consumption. Moreover, after leaching there is a need for a further purification step which is costly and extremely difficult regarding the high concentration of elements such as Fe and Al compared to the low concentration of sc in BR. Pyrometallurgy treatment can reduce this cost and help increase the selectivity as main elements can be removed before leaching. Please rewrite this section according to literature for example https://doi.org/10.1016/j.hydromet.2019.01.005
Authors’ Response: After reviewer’s comment, sentences were reordered, and the key part was rewritten as follows:
“Pyrometallurgy although energy consuming, may result to an overall cost reduction due to major removal of main elements, thus having an important impact on the cost of the subsequent Sc purification [54].”
“Hydrometallurgy technique is performed for Sc and REEs recovery since it is usually of low cost at ambient conditions [26]” by giving the corresponding citation:
[54]. Rodolfo Marin Rivera, Buhle Xakalashe, Ghania Ounoughene, Koen Binnemans, Bernd Friedrich, Tom Van Gerven, Selective rare earth element extraction using high-pressure acid leaching of slags arising from the smelting of bauxite residue Hydrometallurgy, Volume 184, 2019, Pages 162-174.
Results and discussion:
Reviewer’s Comment: In general, the discussion is missing from this part like reported results on the field compared to the findings of this study. This is not the first time when the DOE method published for REE recovery from BR: https://doi.org/10.1002/jctb.5300
Authors’ Response: We agree with the reviewer that there are quite enough studies using Design Of Experiments (DOE) methods dealing with different objectives (such as aluminum hydroxide extraction, neutralization BR with pickling waste liquor etc.) and also different outcome (dependent) parameters (variables) investigated (such as pH, dissolution rate etc.). For example, the study [Red mud as secondary source for critical raw materials – extraction study, Éva Ujaczki, Yannick S Zimmermann, Christoph A Gasser, Mónika Molnár, Viktória Feigl and Markus Lenza, J Chem Technol Biotechnol 2017; 92: 2835–2844, DOI 10.1002/jctb.5300], performs critical raw materials (CRM) extraction with HCl, uses a different experimental design analysis and furthermore, the response value to determine the optimal conditions is the economic potential. This multiplicity/variety of different approaches in the BR’s element(s) targeted, the DOE method adopted, the selection of control parameters and the output/performance indicators (measured parameters), does not favor comparisons and cross checking.
Reviewer’s Comment: The reader does not get information about how 16.6 mg/L extracted Sc is compared to another studies. Is this Taguchi methodology really work better than simple DOE? Is it really needed? Is the extraction yield higher than in other studies? The authors followed only Fe next to Sc while there are other elements in extremely high concentrations compared to the Sc e.g. Al, Ti, Si, Ca, Na...
Authors’ Response:
· As also mentioned above, comparison with other studies cannot be performed due to different raw materials, acid and leaching conditions used by several researchers. For this reason, the necessary justification was added in the text.
· At this point, we are not sure what the reviewer means/characterizes as a “simple DOE” option. Taguchi methodology is one of the available designs of experiment methods that can be applied such as Box-Behnken design etc. In fact, Taguchi design has the advantage to reduce the set of experiments need to be done especially when the number of factors is at least 4.
· Concerning reviewer’s last comment, the authors chose to compare the effects of 2 elements as it is a common place in most relevant studies. Indicatively, we present some of these studies:
Rai, S., et al. (2012). "Neutralization of red mud with pickling waste liquor using Taguchi's design of experimental methodology." Waste Management & Research 30(9): 922-930.
Mbuya, B. I., et al. (2017). "Comparative Study of Approaches based on the Taguchi and ANOVA for Optimising the Leaching of Copper–Cobalt Flotation Tailings." Chemical Engineering Communications 204(4): 512-521.
Reena Pundir, G.H.V.C. Chary, M.G. Dastidar (2018). “Application of Taguchi method for optimizing the process parameters for the removal of copper and nickel by growing Aspergillus sp.”, Water Resources and Industry, (20): 83-92
Monitoring of iron concentration was selected among the other main elements present in BR because it constitutes the major component and therefore its removal becomes difficult and problematic in subsequent processes of scandium isolation.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Overall, the authors made the suggested changes, the study carried out well.
