Review on the Comparison of the Chemical Reactivity of Cyanex 272, Cyanex 301 and Cyanex 302 for Their Application to Metal Separation from Acid Media
Round 1
Reviewer 1 Report
This is a good review comparing the chemical reactivities of Cyanex 272, 301, and 302 and is expected to help the researchers and engineers of metal solvent extraction. The description is accurate and sufficient number of related papers is cited. I think that this review is publishable after correcting few minor points as follows:
- L351: The word, ‘function’, should be ‘functional’.
- L353: The meaning of the sentence, ‘Also, the existence of sulphur atoms …. ‘ is the same as that in the former sentence (L351-352). Thus, the sentence on L353 is not necessary. It should be deleted.
- L355: According to Table 4, the extraction order of Cu(II) is in the order: 301 >> 302 > 272.
- L356: As far as seen from the value of log K, 302 cannot extract Cu(II) completely at low pH.
- L374-375: Although the authors describe the selectivity of Fe(III) with 272, there is no description on those with 301 and 302. The purpose of this review is comparison among 272, 301, and 302. Please mention the selectivity of Fe(III) with 301 and 302.
Author Response
The authors would like to thank the reviewer for the comments.
1) L351: The word, ‘function’, should be ‘functional’.
The manuscript has been revised according to the comments.
Modification page 8
L286-287: In addition, the replacement of sulphur in functional group increases the acidity of Cyanex 301/302 and leads to the more efficient extraction of Cu(II), Co(II), and Ni(II).
2) L353: The meaning of the sentence, ‘Also, the existence of sulphur atoms …. ‘ is the same as that in the former sentence (L351-352). Thus, the sentence on L353 is not necessary. It should be deleted.
The manuscript has been revised according to the comments.
‘Also, the existence of sulphur atoms …. ‘ has been deleted.
3) L355: According to Table 4, the extraction order of Cu(II) is in the order: 301 >> 302 > 272.
The manuscript has been revised according to the comments.
Modification page 8
L289-290: As expected, the extraction order of Cu(II) follows Cyanex 301 >> Cyanex 302 > Cyanex 272 and complete extraction of Cu(II) by Cyanex 301 and 302 was obtained at low pH.
4) L356: As far as seen from the value of log K, 302 cannot extract Cu(II) completely at low pH.
Ans. We agree with the comment that Cyanex 302 cannot extract Cu(II) completely at low pH on the basis on the value of equilibrium constant. However, Cu(II) extraction by Cyanex 302 is also influenced by the affinity between metal ions and the extractant on the basis of HSAB principle. Since the replacement of oxygen with sulfur in the organophosphorus acids such as Cyanex 301 and Cyanex 302 increases their soft character, Cyanex 302 interacts with a soft acid Cu(II), resulting in the quantitative extraction.
5) L374-375: Although the authors describe the selectivity of Fe(III) with 272, there is no description on those with 301 and 302. The purpose of this review is comparison among 272, 301, and 302. Please mention the selectivity of Fe(III) with 301 and 302.
The manuscript has been revised according to the comments.
Modification page 9
L308-311: Cyanex 272 can selectively extract Fe(III) from sulfate solution over Cr(III) and Zn(II) or Co(II), Ni(II), Mn(II) in the solution pH range from 1.8 to 2.5, while Cyanex 302 could only separate Fe(III) over Co(II) and Ni(II) [6,44,45]. In contrast, the metal ions such as Cu(II), Zn(II), Fe(III), Co(II) and Ni(II) can be extracted from sulfate solution in the pH range of from zero to 2.5 by Cyanex 301 [6].
Reviewer 2 Report
This paper offers in a well-structured text a fairly complete review concerning the chemical reactivity of the three extractants Cyanex 272, Cyanex 301 and Cyanex 302. However, there are missing or insufficiently addressed parts :
- the high viscosity of these extractants requires their dilution in a diluent and there is very little mention of the concentration in extractant generally used (0.2 M Cyanex 272 or 0.08M Cyanex 301 at lines 238-239 of the document version v2). Table 4 should explicitly mention the concentrations of extractant used and if possible the diluent.
- The loading capacity in the presence of a diluent is never reported. It is therefore an essential element for the choice of a process
- The risk of third phase occurrence is not analyzed in this review. This should be the subject of a specific paragraph.
- The solubility in aqueous phase of Cyanex 272 is just mentioned in line 415-416 without indication of its value. A review of the solubility of the three extractants would be desirable.
It is generally observed that the viscosity of the organic phase increases with the metal concentration. This is not reported in this paper nor discussed.
Paragraph 4.4 is entitled "regeneration of the extractants" but in fact it is only about recycling because no process of regeneration of the extractants by specific treatments is proposed.
Author Response
Thank you for your comments to our manuscript.
The manuscript has been revised and the replies to the comments have been
uploaded.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
The information provided in this manuscript can maybe be of use for people who are planning a new solvent extraction process. I am very doubtful as to if it is valuable as a starting point for development of new extractants, however. The statement that this is the case is too strong and should be removed.
The structure of the manuscript needs to be improved in order to show the useful information better. In the rewriting process it is recommended to remove repetitions and other not useful text parts. The structure of the paper should be more stringent and better organized so the reader can get the chemical information needed.
The part describing stripping problems should be more developed since that is a very important part of an SX process.
There is also another aspect of the use of these extractants - the possibility to reuse the extractant several times and the stability of the extractant in such reuse cycles. This aspect is totally omitted in the present manuscript and that is a serious weakness of the manuscript. It should include information from real life industrial processes if possible - if it is not possible to get that type of information - results from multiple stage extractions in lab can be used.
Author Response
The authors would like to thank the reviewer for the comments.
1) The information provided in this manuscript can maybe be of use for people who are planning a new solvent extraction process. I am very doubtful as to if it is valuable as a starting point for development of new extractants, however. The statement that this is the case is too strong and should be removed.
The manuscript has been revised according to the comments.
Modification page 2
L52-53: This can provide important imformation on the chemical reactivity and extraction characteristics of the above mentioned Cyanex extractants which are employed in hydrometallurgy for the extraction and separation of metal ions.
2) The structure of the manuscript needs to be improved in order to show the useful information better. In the rewriting process it is recommended to remove repetitions and other not useful text parts. The structure of the paper should be more stringent and better organized so the reader can get the chemical information needed.
The manuscript has been revised according to the comments.
The structure of the manuscript has been rearranged.
Detail: Modification pages 13-15
3) The part describing stripping problems should be more developed since that is a very important part of an SX process.
More contents have been added to the parts on stripping as follows.
Modification pages 12, 13
L351-355: Beside these inorganic solutions, the mixture of ammonia and ammonium chloride was employed to strip Ni(II) from the loaded Cyanex 301 by utilizing the complex formation tendency between Ni(II) and ammonia [49]. The addition of ammonium chloride to ammonia could enhance phase transfer rate and lead to good separation of the two phases. The optimum composition for stripping of Ni(II) was reported to be the mixture of 5% NH4Cl and 75% ammonia [49].
L382-387: In order to prevent the oxidation of the extracted Co(II) to Co(III) as well as oxidation of Cyanex 301 to the disulfide by oxygen, the extraction was performed in an inert gas stream (nitrogen or carbon dioxide) [40]. This route can result in efficient stripping of Co(II) and prevent the oxidation of Cyanex 301. However, the use of concentrated hydrochloric acid for the stripping is a disadvantage of this process [40].
L394-398: In the case of Cu(II) stripping from the loaded phase of Cyanex 301, an aqueous solution of thiourea, hydrazine, and sodium hydroxide was used, resulting in a quantitative stripping as well as the regeneration of the extractant. In this mixture, thiourea made a role as the stripping agent, while hydrazine lessened the formation of disulfide. However, the high cost of thiourea and the toxicity of hydrazine limited the commercial application of this method [40].
4) There is also another aspect of the use of these extractants - the possibility to reuse the extractant several times and the stability of the extractant in such reuse cycles. This aspect is totally omitted in the present manuscript and that is a serious weakness of the manuscript. It should include information from real life industrial processes if possible - if it is not possible to get that type of information - results from multiple stage extractions in lab can be used.
The regeneration of the Cyanex extractants has been added to the revised manuscript.
Modification pages 15
L464-490: 4.4 Regeneration of the extractants
The regeneration of Cyanex 272/301/302 is necessary for their economic utilization in the solvent extraction process. In order to investigate the regeneration ability of these extractants, consecutive extraction-stripping cycles were employed [49, 63-70]. The regeneration ability of Cyanex extractants strongly depends on their hydrolytic stability [63]. From the UV–VIS spectra data of the organic phase (before and after extraction as well as after striping), it was reported that the organic solution of Cyanex 272 could be regenerated for Co(II) extraction [64]. The regeneration of Cyanex 272 was confirmed in the closed-loop solvent extraction process for the recovery of Co(II) from sulfate leachate of NiMH battery, indicating that the regenerated Cyanex 272 can be efficiently reused for several cycles [65]. In addition, the regeneration and recycling ability of Cyanex 272 was also studied for Zr(IV) extraction from acidic chloride solution. The results showed that Zr(IV) extraction and stripping performance were kept (over 99.8% for extraction and over 80% for stripping) up to ten cycles of extraction [66]. Similarly, the investigation on the regeneration ability of Cyanex 301/302 for the extraction of Hf(IV) from 0.1 M HCl solution indicated their high stability during extraction and stripping. The organic phase after stripping with 8 M H2SO4 was washed with distilled water and then used for further extraction. The results revealed no significant difference in the extraction and stripping efficiency of the extractant within 10 cycles operation [67,68]. Gupta et al., reported that Cyanex 301 exhibited good hydrolytic stability, regeneration power, and reasonable loading capacity in extraction and separations of Ga(III)/V(IV) from different mineral acid media [69]. The hydrolytic stability and regeneration ability of Cyanex 301 were investigated by contacting Cyanex 301 in toluene with 5 M HCl/ 3% H2O2 solution for 50 days. The results showed that there was no change in the extraction/stripping of Ga(III) and V(IV) up to 10 extraction and stripping cycles [69]. In the same way, another study on the successive extraction (from 8 M HCl) by Cyanex 301 and stripping of Ge(IV) by 0.5 M HCl revealed negligible difference (2–3%) in extraction/stripping percentage of the metal ion up to 15 cycles [70]. Besides, it has been reported that the decrease in the extractability of Cyanex 301 for Ni(II) from 5 M H2SO4 was insignificant within 3 days, while after 3 days 15% decline was observed [49].
Round 2
Reviewer 3 Report
The additions have made the paper much more valuable. Publication is recommended.
Author Response
Thank you for your comments.
