Review Reports - A Quantitative Relationship between Oxidation Index and Chalcopyrite Flotation Recovery

Round 1

Reviewer 1 Report

The manuscript (minerals-1800951-peer-review-v1) mainly investigates the effect of hydrogen peroxide on oxidation and flotation properties of chalcopyrite mineral. The study is in the scope of “Minerals” journal and can be published after some revisions.

1. The effect of oxidation on the flotation sulfide minerals is studied well in flotation (Kuopanportti et al., 1997). As known, sulfide minerals do not float in the absence of oxygen, which is very oxidative for minerals. However, they can be easily floated in the presence of oxygen by using short chain sulfhydryl collectors (e.g. xanthates). Slight oxidation of surfaces enhances flotation of sulfide minerals, on the other hand excessive oxidation disturb their hydrophobicity. Therefore, I suggest using “slight oxidation” instead of “mild oxidation” which is preferred in the text.

It should be recalled that H2O2 could only increased flotation recovery from 93.1% to 94.8% at most.

2. Some claims are not proved well by the authors. For example;

a) Microthermokinetic results lead authors to infer that “…. when the H2O2 concentration was further increased to 0.14 vol. %, the mechanism of the reaction was changed from mild oxidation to significant oxidation” (Table 3 and Figure 6). In fact, Microthermokinetic results revealed the nature of the reaction between particles and H2O2, but did not provide any insight into the key relation. Decreasing activation energy to 162.87 kJ/mol at 0.14 % H2O2 concentration refer to ease of reaction between the chalcopyrite and H2O2, but not to the degree of surface oxidation (“mild” or “significant”).

b) The authors states that “The increased Fe-O/OH contents during low-concentration H2O2 oxidation treatment is probably due to the adsorption of iron hydroxide on chalcopyrite surface, as shown in Eq. 7.

Please, indicate (the mechanism) how iron hydroxide can adsorb on chalcopyrite surfaces as claimed.

3. Some parts of the manuscript is not well written and some sentences are not clear how they are related to the topic. See the attached pdf file (the reviewed manuscript) and revise the manuscript accordingly.

My final decision is MEDIUM revision

Reference: Kuopanportti et al., (1997). Effects of oxygen on kinetics of conditioning in sulphide ore flotation, Minerals Engineering Volume 10, Issue 11, November 1997, Pages 1193-1205

Comments for author File: Comments.pdf

Author Response

Reply to Reviewers’ Comments

Reviewer #1:

The effect of oxidation on the flotation sulfide minerals is studied well in flotation (Kuopanportti et al., 1997). As known, sulfide minerals do not float in the absence of oxygen, which is very oxidative for minerals. However, they can be easily floated in the presence of oxygen by using short chain sulfhydryl collectors (e.g. xanthates). Slight oxidation of surfaces enhances flotation of sulfide minerals, on the other hand excessive oxidation disturb their hydrophobicity. Therefore, I suggest using “slight oxidation” instead of “mild oxidation” which is preferred in the text.

Reply: Yes, H₂O₂ could only increase flotation recovery from 93.1% to 94.8%. Therefore, the positive role due to the addition of H₂O₂ is limited for the chalcopyrite sample used in this study. However, significantly negative roles to chalcopyrite flotation were observed due to greater H₂O₂ concentration.

As suggested by this reviewer, "mild oxidation" was revised to "slight oxidation" throughout this manuscript.

Some claims are not proved well by the authors. For example;
a) Microthermokinetic results lead authors to infer that “…. when the H₂O₂ concentration was further increased to 0.14 vol. %, the mechanism of the reaction was changed from mild oxidation to significant oxidation” (Table 3 and Figure 6). In fact, Microthermokinetic results revealed the nature of the reaction between particles and H₂O₂, but did not provide any insight into the key relation. Decreasing activation energy to 162.87 kJ/mol at 0.14 % H₂O₂ concentration refer to ease of reaction between the chalcopyrite and H₂O₂, but not to the degree of surface oxidation (“mild” or “significant”).
b) The authors states that “The increased Fe-O/OH contents during low-concentration H₂O₂ oxidation treatment is probably due to the adsorption of iron hydroxide on chalcopyrite surface, as shown in Eq. 7. Please, indicate (the mechanism) how iron hydroxide can adsorb on chalcopyrite surfaces as claimed.

Reply: a) The conclusion "the mechanism of the reaction was changed from mild oxidation to significant oxidation" was derived from the change in the E_a of chalcopyrite reacted with 0.06%, 0.1% and 0.14% H₂O₂, i.e., 172.07, 173.27 kJ/mol and 162.87 kJ/mol, respectively. In order to clarify this, we have revised the following contents:

“When chalcopyrite was treated with 0.14 vol.% H₂O₂, peak roseintensity increased significantly and shifted to the left position, at 30℃ (Fig. 5a), indicating that a higher concentration of H₂O₂ promoted facilitated the oxidation between chalcopyrite and H₂O₂and increased reaction rate, , i.e., an easier oxidation may occur on chalcopyrite surface.”

"It is therefore reasonable to infer according to the changes in E_a that when the H₂O₂ concentration was further increased to 0.14 vol.%, the mechanism of the reaction was changed from mildslight oxidation to significant oxidation, easier oxidation between chalcopyrite and H₂O₂ can occur."

b) The iron ions can be dissolved from the chalcopyrite surface during chalcopyrite flotation process. These dissolved ions hydrolyzed to form positively charged ferric hydroxide species that tending to adsorb onto the negatively charged sulfur sites via electrostatic action. In order to clarify this mechanism, we have modified the sentence in Line 255:

"The increased Fe-O/OH contents during low-concentration H₂O₂ oxidation treatment is probably due to the electrostatic adsorption of positively charged iron hydroxide on the negatively charged chalcopyrite surface, as shown in Eq. 73."

Some parts of the manuscript is not well written and some sentences are not clear how they are related to the topic. See the attached pdf file (the reviewed manuscript) and revise the manuscript accordingly.

Reply: Thanks for your suggestion. We have revised the manuscript to more clearly state the topic. Please refer to the modifications in the revised manuscript.

My final decision is MEDIUM revision

Reference: Kuopanportti et al., (1997). Effects of oxygen on kinetics of conditioning in sulphide ore flotation, Minerals Engineering Volume 10, Issue 11, November 1997, Pages 1193-1205

Author Response File: Author Response.pdf

Reviewer 2 Report

I recommend reviewing the English in the document

2.1. Materials and reagents

What is the chalcopyrite law? That had no impurities?

What concentration process was previously carried out on the sample to have such purity?

In table 3, in editing, avoid cutting between two pages

Author Response

Reply to Reviewers’ Comments

Reviewer #2:

I recommend reviewing the English in the document. 2.1. Materials and reagents. What is the chalcopyrite law? That had no impurities? What concentration process was previously carried out on the sample to have such purity?

Reply: We were very sorry that we made typing error on sulfur content of 35.2% in the original version. The S content of this chalcopyrite sample is 33.2%, not 35.2%. We are very sorry for this typing error.

In order to clarify the purity of chalcopyrite sample used in this study, we made the following revision.

"The chalcopyrite crystal sample was supplied from GEO discoveries, Australia. The macroscopic impurity mineral particles were removed manually after crushing, to obtain a high purity chalcopyrite sample, with 34.3% Cu, 30.4% Fe, and 353.2% S, and an impurity of 2.1%."

In table 3, in editing, avoid cutting between two pages?

Reply: Table 3 has been shifted in one page in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear authors,

Thank you very much for the submission of your article to MDPI minerals. The article deals with the characterization of the oxidation state of chalcopyrite and its correlation with its floatability using butylxanthate as collector. The degree of oxidation is related to the surface chemistry of chalcopyrite and an equation for the relation of the degree of oxidation and the floatability of chalcopyrite was established. In general, the article is well written and structured. After a thorough reading of the article, the following questions are still open and I would like to kindly ask the authors to answer them.

1. The introduction states several works form different researchers. When the authors state some major facts, could also some numbers be added to the text to clarify exactly what was done in previous work, e.g., line 37-38 the concentration of H₂O₂ used by Suyantra, line 54-55 under which conditions were copper and iron dissolved? This would allow the reader also to understand why the authors have used the ranges of parameters in their study (e.g., H₂O₂concentration).

2. Eq. (3) and (4) have equal statements on each site. What do the authors want to show there? The same is true for Eq. (7).

3. In line 97, the authors mention that pH 8 was used to avoid precipitation, but could the authors please add what would precipitate at other pH values?

4. Line 98, the weight of the concentrates were determined after 1, 3, 5, etc. min, but the rate with which the froth was removed from the slurry had another rate I assume?

5. Line 146-147, the authors claim that a steric hindrance hinders the adsorption of SBX on the chalcopyrite surface. I would argue that this is wrong and would kindly ask the authors to elaborate this further for readers to understand what was meant here.

6. For the presentation of contact angles I would delete all the digits after the point (e.g., use only 80° or 93°)

7. The definition of hydrophobicity and hydrophilicity throughout the manuscript is confusing and has to be stated more clearly. For instance, the flotation recovery was determined in the presence of SBX and not with a pristine chalcopyrite surface. Therefore, one could argue that it would be scientifically more sound to describe in the article the conditions under which an optimum SBX adsorption on the chalcopyrite surface takes place, rather than to argue under which conditions the chalcopyrite surface itself is hydrophobic or hydrophilic. Therefore, it would be more conclusive to relate the degree of oxidation to the adsorption density of SBX on chalcopyrite rather than with the floatability. The measurement of the adsorption density is, however, out of scope of this article, but the discussion should be adapted accordingly.

8. The last two paragraphs of section 4.3 require adaption according to the previous comment. To distinguish the surface species of chalcopyrite as “hydrophilic” or “hydrophobic” is not meaningful. I would suggest to argue that SBX adsorption is favored by the presence of certain surface species on the mineral surface.

Author Response

Reply to Reviewers’ Comments

Reviewer #3:

The introduction states several works from different researchers. When the authors state some major facts, could also some numbers be added to the text to clarify exactly what was done in previous work, e.g., line 37-38 the concentration of H₂O₂ used by Suyantra, line 54-55 under which conditions were copper and iron dissolved? This would allow the reader also to understand why the authors have used the ranges of parameters in their study (e.g., H₂O₂ concentration).

Reply: Thanks for your suggestion, we have made the following revisions.

Line 40:

"H₂O₂ promoted the adsorption of PAX onto chalcopyrite surface at a low concentration of H₂O₂ (0.1 mM)."

Line 60:

"the dissolution of copper and iron ions from chalcopyrite surface, leaving a metal-deficient, sulfur-rich surface in alkaline solution, at pH 10."

Eq. (3) and (4) have equal statements on each site. What do the authors want to show there? The same is true for Eq. (7).

Reply: Eq. (3) shows the bonding between to form while Eq. (4) indicates the bonding between to form . The left side and right side are different. In order to clarify this, we added some sentences at the end of this paragraph.

“The -S^- stands for S atoms with broken bonds on the chalcopyrite surface. The dissolution of Fe and Cu may result in the formation of and in the solution. The formation of -S^--Fe(OH)_n^(3-n)+ and -S^--Cu(OH)_n^(2-n)+ stands for the bonding between the iron and copper hydroxides and S atoms on the chalcopyrite surface.”

Eq. (3) and Eq. (7) are repeated. Therefore, Eq. (7) was deleted in the revised manuscript.

In line 97, the authors mention that pH 8 was used to avoid precipitation, but could the authors please add what would precipitate at other pH values?

Reply: Thanks for your suggestion. The inevitable ions such as Mg²⁺ and Ca²⁺ in solution or the dissolution from the gangue minerals form Mg(OH)₂ and Ca(OH)₂ precipitates. These precipitates can be adsorbed on chalcopyrite surface to seriously reduce chalcopyrite recovery. However, the Ca²⁺ and Mg²⁺ ions in the solution will not precipitate at pH 8. Therefore, pH 8 was selected for the experiments. In order to clarify this, we have made the following revisions.

"It is worth noting that pH 8 was selected in this study to avoid the formation of precipitation (e.g., Mg(OH)₂, Ca(OH)₂) under high alkali environment (pH > 10) in the presence of inevitable ions Ca²⁺ and Mg²⁺ in solution."

Line 98, the weight of the concentrates were determined after 1, 3, 5, etc. min, but the rate with which the froth was removed from the slurry had another rate I assume?

Reply: Fig. 1 shows chalcopyrite recovery at different flotation time. It can be found that the recovery changed insignificantly at 10 min. Therefore, the flotation time was set up to 10 min, i.e., 1, 3, 5, 7 and 10 min was selected for experiment. For instance, the weight of concentrate at 1 min means that we collected all the floatable chalcopyrite in the froth within 1 min, while that at 3 min means all the floatable chalcopyrite in the froth was collected. The collection of froth was in a fixed rate within 10 min, for instance, one scrape per 5 s. This operation action during flotation process. However, only the concentrate cumulatively collected at 1, 3, 5, 7 and 10 min was shown and discussed in this manuscript, which is also the common practice in the flotation research.

Fig. 1 Relationship between chalcopyrite recovery and time with different H₂O₂

In order to clarify this, we modify the sentence in Line 116:

"Flotation was operated at an air flow rate of 0.1 L/min to collect the floatable chalcopyrite in the froth within the related flotation time (i.e., 1, 3, 5, 7 and 10 min), with the cumulatively collected froth products being filtrated, dried and weighted to calculate chalcopyrite recovery. "

Line 146-147, the authors claim that a steric hindrance hinders the adsorption of SBX on the chalcopyrite surface. I would argue that this is wrong and would kindly ask the authors to elaborate this further for readers to understand what was meant here.

Reply: We are very sorry that this is a typing error. We aimed to express that the formed hydroxides occupied the adsorption sites of SBX on the chalcopyrite surface, thereby hindering the adsorption of SBX. The use of the word "sterically" made the sentence ambiguous, so we removed it in the revised manuscript.

"In addition, these hydrophilic species sterically hinder the adsorption of collector."

For the presentation of contact angles I would delete all the digits after the point (e.g., use only 80° or 93°)

Reply: As suggested by this reviewer, we have made the revision in the revised manuscript, including Fig. 3 and the related illustration.

Figure 3. Contact angles of (a) untreated chalcopyrite and treated chalcopyrite with (b) 0.06% H₂O₂, (c) 0.08% H₂O₂, (d) 0.1% H₂O₂, (e) 0.12% H₂O₂ and (f) 0.14% H₂O₂ in pure water at pH 8, with 200 g/t butyl xanthate.

The definition of hydrophobicity and hydrophilicity throughout the manuscript is confusing and has to be stated more clearly. For instance, the flotation recovery was determined in the presence of SBX and not with a pristine chalcopyrite surface. Therefore, one could argue that it would be scientifically more sound to describe in the article the conditions under which an optimum SBX adsorption on the chalcopyrite surface takes place, rather than to argue under which conditions the chalcopyrite surface itself is hydrophobic or hydrophilic. Therefore, it would be more conclusive to relate the degree of oxidation to the adsorption density of SBX on chalcopyrite rather than with the floatability. The measurement of the adsorption density is, however, out of scope of this article, but the discussion should be adapted accordingly.

Reply: The hydrophilic species such as Fe-O/OH/SO, SO2- 3 and SO2- 4 are unfavorable to chalcopyrite flotation. In contrast, hydrophobic species such as S2- 2 and S2- n/S⁰ are beneficial to flotation. The aim of this study is to identify the optimum oxidation state for chalcopyrite flotation. This manuscript proposed a good fit of the curve (y = -298.81x + 213.05) to evaluate the relationship between chalcopyrite flotation recovery and the oxidation degree, which is very innovative. We believe this quantitative equation is valuable for other researchers and can be widely applied for other conditions, as discussed in Section 4.3. The establishment of this quantitative equation is of significant importance for the studies on the flotation of sulfide minerals due to oxidation.

As suggested by this reviewer, chalcopyrite flotation recovery can be related to the adsorption density of SBX. Therefore, the optimum chalcopyrite flotation condition can be regarded as the highest adsorption density of SBX on chalcopyrite surface. However, as the concentration of SBX was not changed under different flotation processes, the adsorption density of SBX on chalcopyrite surface wat not investigated. In order to clarify this, we have made the following revision at the end of Section 4.2.

“It should also be noted that chalcopyrite flotation recovery is positively related to the adsorption density of SBX on chalcopyrite surface. In other words, the highest chalcopyrite flotation recovery (the optimal oxidation points ranging from 0.378 to 0.388) can be regarded as the highest SBX adsorption on chalcopyrite surface, although the actual adsorption density is not measured herein. However, further oxidation on chalcopyrite due to higher H₂O₂ concentration is negative to the adsorption of SBX on chalcopyrite, thereby decreasing chalcopyrite flotation.”

The last two paragraphs of Section 4.3 require adaption according to the previous comment. To distinguish the surface species of chalcopyrite as “hydrophilic” or “hydrophobic” is not meaningful. I would suggest to argue that SBX adsorption is favored by the presence of certain surface species on the mineral surface.

Reply: Section 4.3 aims to verify the applicability of the quantitative equation proposed herein. The data collected from the previously published papers focuses on the flotation recovery and the surface species (hydrophilic or hydrophobic), which can be related to the data for the oxidation degree calculation. In addition, as the scientific contribution of this manuscript is to identify the quantitative relationship between oxidation index and chalcopyrite flotation recovery, the oxidized species need to be carefully identified. In fact, significant oxidation of chalcopyrite normally produces hydrophilic species while the slight oxidation normally produces hydrophobic species, the oxidation degree needs to be investigated based on the ratio of hydrophilic species to hydrophobic species. This is the reason why we identify hydrophilic and hydrophobic species for chalcopyrite flotation in the presence of H₂O₂ oxidation.

As indicated by this reviewer, the adsorption of SBX on chalcopyrite surface should also be considered. Therefore, we have made the following revision at the end of Section 4.2.

In the Conclusion section:

“In addition, the optimal oxidation index of chalcopyrite was identified as between 0.378 and 0.388, which is related to the highest adsorption density of SBX on chalcopyrite surface.”

In Abstract:

“…thereby giving a guideline to better manage chalcopyrite flotation by controlling its surface oxidation and SBX adsorption on chalcopyrite surface.”

Author Response File: Author Response.pdf