Effective Gold Recovery from Near-Surface Oxide Zone Using Reductive Microwave Roasting and Magnetic Separation

Round 1

Reviewer 1 Report

Line 22 of the abstract. Can you please write in full first then bracket the abbreviations - DRX, FX, and MEB.  

Line 23, maybe change tenor to grade?

Line 29, you may have to add information here or rewrite the last sentence - "meets the requirements of the metallurgy", meaning what?

Line 55, sort out subscripts.

Line 62, physio-chemical...

Line 73, -sampling protocol, where is it?  There is no mention of how the samples were collected, just a generic plance with no hint of theory or how you worked out that 120kg was optimum.  Line 82 onwards needs more details.  How did you verify? What was the prelim analysis you undertook?  Please detail here.

Line 91 - again you need to write in ful XRD and XRF, then abbreviate.

Line 94 should be stereological microscopy.  I think you may need to highlight the fact that you have only qualitative observational analysis on your sample, not quantitative.  This is ok and it shows a quick 'look see'analysis that some people like as a first pass.  Be clear that it's qualitative. 

Line 139 - you need to use more common microscopy description.  çolour from the heart to the edge - "The alteration changes from the centre of the particle/grain to the edge, from hematite to limonite (black to yellow).  

Line 149 - Table 1.  Where is it mentioned in the text?  Also what does FX mean?  Assay?

 Line 150+  You need to work on the language here please.  XRay Diffraction was carried out to establish modal mineralogy, Figure 9 shows the .....  It also is used as a QAQC to the results from the metallographic microscopy study.

What are the mineralogical percentages? Was the XRD qualitative?

Line 180  XRF is the standard way to write  this method, not XF

Line 181 CRAPC?  That doesn't fit your centre you described.

Line 182 T for Table.

Line 185 "high rate"?  What does that mean?  High percentage of clays.  Usable for other fields?  Be specific, what do you mean here?

Line 196 T for Table.  

You need to state what Ip is in Table 5 and what do you mean by slices in line 197.  Do you mean size fractions as shown in column 1 of Table 5?

Line 206 F for Figure.

Figure 11 needs more explanation .    Explain alpha, gamma, beta please.  I know you have it in the conclusion...  Really, I think this Figure needs to go earlier in the paper. Show the intentions up front and really, this is more important than the characterisation process.

The intent of the paper is obvious and there are some nice pieces of analysis in here, but you need to elaborate more on them.  Be specific.  Write everything out in long form first.  Discuss the methods, why you chose them.  Why did you not use automated mineralogy to give you a more quantitative approach. Really this paper relies on assay along, with some qualitative XRD as a  QAQC exercise. The paper is novel in its approach to what you are doing for an industrial purpose, but is lacking depth in the descriptions and reasons behind the chosen methods.

What out for the language chosen.  Some terms are not really 'common' metallurgical speak, or universal.

Really, this paper could do with restructuring and rewriting, though not majorly. You need to better integrate your mineralogical data with metallurgical outcomes.  Really justify why you bothered with the microscopy, when really, assay data is enough. 

Include a lot of the mineralogical analysis in reasons why the upgrade did or didn't work.

Reviewer 2 Report

The authors have investigated the gold recovery from near-surface oxide zone using a reductive microwave roasting and a magnetic separation. Conventional cyanidation or floatation process cannot extract gold from the mineralized oxidized zone. The authors’ method is an employment of reductive microwave roasting, which can transform non-magnetic hematite in oxide minerals into magnetic materials such as magnetite and maghemite. This mineral change provides a successful magnetic separation with gold.

The recycle of gold is very important issue. The presented method is highly original and experiments are carefully conducted. The paper attracts much attention of many researches in the field of recycle. So I believe the manuscript meets all criteria necessary for Metals. But, before the acceptance, the authors should address the comment listed below.

(1) I recommend the authors to note the detailed experimental conditions for all data plotted in Figure 8.

Author Response

Response to Reviewer 2 comments. 

(1) I recommend the authors to note the detailed experimental conditions for all data plotted in Figure 8.

è  Thank you for your recommendation. All data plotted in Figure 8 was the result of the overall RMR-MS tests listed in Table 2. So we changed the caption of Figure 8 as following

“Figure 8. (a) Increases (Δ) of gold contents and (b) silver contents during the increase of iron grade in RMR-MS processed concentrates. All data obtained in the overall RMR-MS tests in this study was plotted.”

Reviewer 3 Report

Title: Effective gold recovery from near-surface oxide zone…

Manuscript ID: metals-384968-v1

Authors: Kim et al.

Dear Authors,

Thank you for providing me the opportunity to read your article. I found it is an interesting work on gold recovery. On the other hand, at this stage, description of the results and discussion are rather weak and must be further improved for a potential reader to attract his/her interest. I will provide you the major and minor concerns you can consider addressing in order to improve the quality and readability of your article. I strongly suggest this article go through the major revision for another reviewing process.

I hope my comments are helpful.

Good luck,

A reviewer

Major concerns:

1.      Discussion must bring additional scientific/engineering insight, not only describing what you have done. The current state of “4. Discussion” looks like Conclusion that I would suggest you rename. If you claim this paper is a regular “article”, I would recommend discuss more. Otherwise, the overall contents fit better with “communication” (after the revision anyway). You can use my comments as tools to facilitate more discussion.

2.      Results are only limited to Fe and Au. What about other elements (S, Si, Al, Ag)? They could be associated with residual Au, and you can discuss the potential improvements of your process for the further study. Also, only recovery and grade were informed. How about yield (and/or ratio of concentration), enrichment ratio? Only recovery and grade cannot capture the overall efficiency of the process.

3.      Justification of selection of experimental conditions and methods should be clearly mentioned. For example, Table 1 list different experimental conditions you tested. Please explain how and why you selected those specific conditions. Also, explain how you determine the mineral composition given in Table 2. Table 2 should include your target elements of interest, Au, Ag, and As. In addition, I think you miss sulphides (e.g. pyrite introduced in Fig. 3). I would suggest show elemental composition instead of mineral composition if the mineral composition is unsure since you are using elemental% to evaluate your process results (Figs. 5 and 6) anyway.

4.      Images in Figs 3 and 4 should be more reader friendly. Point out the specific minerals you want the reader’s attention with arrows and names (e.g. pyrite, hematite). Unless the reader is familiar with those mineralogy, s/he cannot easily find what you are talking about with those images.

5.      Add all the experimental conditions in all the Figure/Table legends/footnotes. Otherwise a reader cannot understand them. For example, lines 157-159, “…the RMS was conducted on 50 g of…for 10 min” should go to the footnote of Table 3.

Minor concerns:

-English can be polished in order to improve the readability of your article.

-Line 51 (and elsewhere): “But”->However (We cannot start a sentence with “but”.)

-Lines 52-53: “…a high percentage of Au in hematite cannot be recovered by cyanidation…”->Explain why

-Line 105: “preprocessing”->pre-processing

-Line 117: “grinded”->ground

-Line 126-127: “Vibration Sample magnetometer”->Explain why it is necessary to use and what kind of magnetic properties you can extract from the measurements.

-Line 127: “As a control”->For a comparison

-Line 168: “dramatic”->noticeable

-Table 3: all the content should stay in the same page (e.g. page 5).

-Fig.4: “…the change…”->Explain what change

-Line 191: “Fig.4”->Fig.5?

-Lines 191-192: “26.91 emu/g”->Compare with literature value if available.

-Line 204: “Hematite was not reduced…without activated carbon (data not shown)…”->Explain why. It would be useful to show the data to compare with the good/successful case in order to highlight the necessity of activated carbon.

-Line 209: “>1.5”->    <1.5?

-Line 231-232: “the principle of Baykova”->Cite the reference

-Lines 236-237: “This is because…only with iron minerals.”->It seems that this statement is against the correlation between Fe and Au recovery. Better find another explanation if possible?

-Line 251: “…increase”->an increase

-Fig.8: I am confused with the symbol “delta” given in x and y axes. If you meant to plot grade and recovery as stated in the Figure legend, “delta” is unnecessary and should be removed.

Author Response

Response to Reviewer 3 comments. 

Major concerns:

1.       Discussion must bring additional scientific/engineering insight, not only describing what you have done. The current state of “4. Discussion” looks like Conclusion that I would suggest you rename. If you claim this paper is a regular “article”, I would recommend discuss more. Otherwise, the overall contents fit better with “communication” (after the revision anyway). You can use my comments as tools to facilitate more discussion.

è  Thank you for your recommendation. The sections ‘Results’ and ‘Discussion’ were renamed with ‘Results and Discussion’ and ‘Conclusions’, respectively. And the results have discussed more based on your comments, as you can see in the following response to your valuable comments. We can follow the editor’s decision on the article type, ‘article’ or ‘communication’. Please reconsider the type of our revised manuscript.

2.       Results are only limited to Fe and Au. What about other elements (S, Si, Al, Ag)? They could be associated with residual Au, and you can discuss the potential improvements of your process for the further study. Also, only recovery and grade were informed. How about yield (and/or ratio of concentration), enrichment ratio? Only recovery and grade cannot capture the overall efficiency of the process.

è  We observed that the silver contents were increased in the RMR-MS processed sample with increasing iron grade. The silver association with iron are reported in Figure 8b, additionally. In our previous research, S in pyrite was completely removed by evaporation during microwave roasting. However, the fates of Si and Al could not be treated significantly in this research, because this study was just focused on the recovery of gold associated with iron oxide mineral. As your comment, further gold recovery from the quartz- and pyrite- associated fractions in the residual might improve the efficiency of the gold recovery process from the near surface oxide zone. Therefore, we agreed that the further study is needed on the recovery of gold associated with the residual minerals. 

è  As your comments, the evaluation of the overall efficiency and economic analysis of the process will be conducted in further study using these additional performance criteria, such as yield and enrichment ratio. In this study, the values of yield and the enrichment ratio were overestimated due to the underestimated gold contents in the raw ore sample, which might be due to the incomplete decomposition during the pre-treatment for AA analysis. The information ofthe residual after RMR-MS was either unsure due the massive use of the reducing agents. So the performance of the gold recovery process should be estimated just based on the information of the magnetic concentrates. In the next study, we will evaluate the performance of the process in various aspects according to your comments. Thanks again for your valuable advice. 

3.      Justification of selection of experimental conditions and methods should be clearly mentioned. For example, Table 1 list different experimental conditions you tested. Please explain how and why you selected those specific conditions. Also, explain how you determine the mineral composition given in Table 2. Table 2 should include your target elements of interest, Au, Ag, and As. In addition, I think you miss sulphides (e.g. pyrite introduced in Fig. 3). I would suggest show elemental composition instead of mineral composition if the mineral composition is unsure since you are using elemental% to evaluate your process results (Figs. 5 and 6) anyway.

è  Experimental conditions were selected based on the previous researches. The relevant previous researches are described as follows. 

“RMR processing was conducted as a pre-processing to separate the Fe-oxides from the auriferous ore in the near-surface oxide zone. The microwave roasting process, especially controlling irradiation time, could be used to control the mineral phase change of ore sample [29]. For the reduction of hematite, various reducing agent, such as activated carbon, sodium carbonate and charcoal, have been used in the reductive roasting process [30-32].”

è  The mineral compositions in Table 2 was determined by XRF analysis for the raw ore sample. The contents of Au, Ag, and As are added in Table 2 as your recommendation. XRF analysis is necessary to confirm the mineral phase of iron oxide in the raw sample. And the content of pyrite could not be shown in the result of XRF analysis. So we corrected the table and the caption as follows

Table 2. XRF and AAS analysis for the ore sample from near-surface oxide zone in Moisan Gold deposit (Suth Korea). The contents of the target element such as Au, Ag, As and total Fe were analyzed using AAS. The mineral compositions were determined XRF analysis.

amg/kg.

4.      Images in Figs 3 and 4 should be more reader friendly. Point out the specific minerals you want the reader’s attention with arrows and names (e.g. pyrite, hematite). Unless the reader is familiar with those mineralogy, s/he cannot easily find what you are talking about with those images.

è  Thanks for your comment. As your recommendation, we specified the minerals by arrows and names in Figure 3 and 4. 

5.      Add all the experimental conditions in all the Figure/Table legends/footnotes. Otherwise a reader cannot understand them. For example, lines 157-159, “…the RMS was conducted on 50 g of…for 10 min” should go to the footnote of Table 3.

è  We added the experimental conditions in the footnote of Table 3, and the captions of Figure 4 and 5. Thanks for your comment.

Minor concerns:

-English can be polished in order to improve the readability of your article. 

è  We reviewed and checked again the expressions in overall manuscript considering your comments. Thank you.  

-Line 51 (and elsewhere): “But”->However (We cannot start a sentence with “but”.)

è  We corrected it as your comment. Thank you. 

-Lines 52-53: “…a high percentage of Au in hematite cannot be recovered by cyanidation…”->Explain why

è  The passivation phenomenon reduces the solubility of the gold by cyanidation: the high pH of the cyanide increases the solubility of the iron oxide, which results strong adsorption of the dissolved iron oxide on the surface of the gold, and thus, the cyanide dissolution of the gold lowers. We explained this simply in Line 54-55 as follows

“Under high pH of cyanide, the dissolved iron oxide adsorbs on the gold surface, and inhibiting the cyanide dissolution of the gold (passivation) [16-18].”

-Line 105: “preprocessing”->pre-processing

è  We corrected it as your comment. Thank you. 

-Line 117: “grinded”->ground

è  We corrected it as your comment. Thank you. 

-Line 126-127: “Vibration Sample magnetometer”->Explain why it is necessary to use and what kind of magnetic properties you can extract from the measurements.

è  Thanks for you comment. We explained it in Line154-156 as follows 

“In order to study the effect of MS process on the magnetic property obtained by the transformation of hematite, the magnetic hysteresis and magnetic intensity were analyzed using Vibration Sample magnetometer (VSM, BHV-50HTI).”

-Line 127: “As a control”->For a comparison

è  We corrected it as your comment. Thank you. 

-Line 168: “dramatic”->noticeable

è  We corrected it as your comment. Thank you. 

-Table 3: all the content should stay in the same page (e.g. page 5).

è  We corrected it. Thank you.

-Fig.4: “…the change…”->Explain what change

è  Caption of Figure 4 was corrected as follows 

“Figure 4. (a) Reflected light microphotograph showing mineral phase change from hematite to magnetite and maghemite. (b) X-ray diffraction pattern of the magnetic fraction from the roasted sample. The sample was treated with 5kW of microwave power in Exps.C.” 

-Line 191: “Fig.4”->Fig.5?

è  There was a mistake. We corrected it as your comment. Thank you. 

-Lines 191-192: “26.91 emu/g”->Compare with literature value if available.

è  Relavant literature was added as follows 

“The reduction of hematite using electric furnaces reported  that the increase in saturation magnetization intensities (Ms) from 9 to 37 emu/g by the complete conversion of hematite to magnetite [34]. Good magnetic separation could be achieved after roasting since the residual magnetism (Mr) increased from 0.5 to 3.9 emu/g [34].”

-Line 204: “Hematite was not reduced…without activated carbon (data not shown)…”->Explain why. It would be useful to show the data to compare with the good/successful case in order to highlight the necessity of activated carbon.

è  We corrected the expression to ‘Hematite was not sufficientlyreduced through microwave irradiation without activated carbon,’ 

-Line 209: “>1.5”->    <1.5?

è  You are right. We corrected it as your comment. Thank you. 

-Line 231-232: “the principle of Baykova”->Cite the reference

è  The relevant reference was added as your comment. Thank you. 

“According to the principle of Baykova, reduction of Fe-oxides, such as hematite, takes place in three steps: Fe₂O₃→ Fe₃O₄→ FeO → Fe [43].”

-Lines 236-237: “This is because…only with iron minerals.”->It seems that this statement is against the correlation between Fe and Au recovery. Better find another explanation if possible?

è  Thank you. We removed the sentence. 

-Line 251: “…increase”->an increase

è  We corrected it as your comment. Thank you. 

-Fig.8: I am confused with the symbol “delta” given in x and y axes. If you meant to plot grade and recovery as stated in the Figure legend, “delta” is unnecessary and should be removed.

è  The previous caption was quite confusing. Here the “delta” indicated the increases of the grade and content. Since we intended to show the increase of the gold grade by following the increase of iron grade, ‘delta’ expression is necessary. So we corrected the caption of Figure 8. 

“ Figure 8. (a) Increases (D) of gold contents and (b) silver contents during the increase of iron grade in RMR-MS processed concentrates. All data obtained in the overall RMR-MS tests in this study was plotted. (Raw ore sample contained 6.4 and 35.6 g/t of gold and silver and 28.6% of iron).

Round 2

Reviewer 3 Report

Dear Authors,

Thank you for your effort to revise your manuscript taking into account our comments.

Now, I found that the manuscript is ready for publication.

Best regards,

A reviewer