A Study on the Applicability of Agitated Cyanide Leaching and Thiosulphate Leaching for Gold Extraction in Artisanal and Small-Scale Gold Mining

Round 1

Reviewer 1 Report

In this paper, cyanide gold extraction and thiosulfate gold extraction are used, and the two methods are very mature and basically uninnovative

Author Response

Response to Reviewer 1:

• Cyanide and thiosulphate are used, and the two methods are very mature and basically uninnovative.

Response: The following paragraph was added in the introduction to clarify this very important point which explains the value or contribution of this study to the ASGM research field. The other reviewers pointed this out as well:

'It is a fact that cyanide and thiosulphate leaching are mature technologies for gold extraction. However, in the context of ASGM, their applicability has not been widely demonstrated experimentally. Cyanidation in ASGM has mostly been applied only in vat leaching of amalgamation tailings which further pollutes the environment via the dumping of tailings (Hg and CN^- contaminated) in rivers. In addition, given the increasingly complex nature of the ores currently processed in ASGM, it is crucial that technological investigations are conducted using materials sourced from ASGM areas. This study’s contribution to the ASGM field consists firstly of demonstrating that the direct agitated leaching of actual ores found in ASGM areas with cyanide and thiosulphate, at ambient conditions, can compete with and even outperform the Hg amalgamation process. Ambient conditions are advantageous given the rudimentary nature of the equipment typically used in the sector. Secondly, ores found in ASM are often not characterised thoroughly enough, leading to poor implementation of theoretically successful technologies. Therefore, in this study, a thorough mineralogical characterisation of the samples used is done, using XRF, XRD, QEMSCAN, SEM-EDS and a diagnostic leach sequence.'

Reviewer 2 Report

The article is of potential interest to readers. However, there are some issues that need to be resolved.

1. The title of the article does not reflect its content. It is necessary to remove "amalgamation" from the title. Cyanide and thiosulfate leaching of gold has been known for a long time, so the title should reflect the specificity of the article.

2. What is the novelty of the work? What is fundamentally new introduced by the authors? This needs to be reflected in the introduction.

3. Fig.5: what explains the decrease in gold recovery after 24 hours of leaching compared to 7 hours of leaching? How does gold go back into the solid phase?

4. The manuscript is more like a technical report than an article. A discussion of the results in terms of novelty should be given in Results and Discussion. What was found for the first time?

5. How economically feasible is it to use agitation leaching for the processing of poor raw materials?

I recommend a major revision of the manuscript.

Author Response

Response to Reviewer 2:

• Title of article does not reflect content. It is necessary to remove “amalgamation” from the tittle. Cyanide and thiosulphate leaching of gold has been known for a long time, so the tittle should reflect the specificity of the article.

Response: The title was changed to: A study on the applicability of agitated cyanide leaching and thiosulphate leaching for gold extraction in artisanal and small-scale gold mining

• What is the novelty of the work?

Response: It is a fact that cyanide and thiosulphate leaching are mature technologies for gold extraction. However, in the context of ASGM, their applicability has not been widely demonstrated experimentally. Cyanidation in ASGM has mostly been applied only in vat leaching of amalgamation tailings which further pollutes the environment via the dumping of tailings (Hg and CN^- contaminated) in rivers. In addition, given the increasingly complex nature of the ores currently processed in ASGM, it is crucial that technological investigations are conducted using materials sourced from ASGM areas. This study’s contribution to the ASGM field consists firstly of demonstrating that the direct agitated leaching of actual ores found in ASGM areas with cyanide and thiosulphate, at ambient conditions, can compete with and even outperform the Hg amalgamation process. Ambient conditions are advantageous given the rudimentary nature of the equipment typically used in the sector. Secondly, ores found in ASM are often not characterised thoroughly enough, leading to poor implementation of theoretically successful technologies. Therefore, in this study, a thorough mineralogical characterisation of the samples used is done, using XRF, XRD, QEMSCAN, SEM-EDS and a diagnostic leach sequence. This was added to the introduction to emphasize the motivation for the study and the conclusion was modified slightly to bring out the key findings that are valuable for ASGM.

• Fig 5. What explains the decrease in gold recovery after 24h of leaching compared to 7 h of leaching?

Response: The XRF data on the 3 ores revealed quite high LOI% of 5.4% (Sample 1), 10.8 % (sample 2) and 7.6% (Sample 3). As much as LOI % predominantly accounts for water in the ore, it also indicates the organic matter content of a sample. A high carbon content in an ore can indeed cause preg-robbing, which reabsorbs Au, therefore it could be a reason for the drop in recovery. However, such interpretations must be made with caution since LOI% accounts mostly for water loss and does not tell exactly how much of it is carbonaceous matter. This has been added in the mineralogy section and in explanation of fig 5 in the results section.

• How economically feasible is it to use agitation leaching for the processing of poor raw materials?

Response: If poor raw materials imply low grade gold ores or highly refractory ores, then agitated leaching would definitely not be feasible, but then that would be the case for Hg amalgamation as well because even on high Au grade materials, it only achieves 30-50% Au extraction. However, agitated leaching was shown in the study to work significantly better than mercury amalgamation on ores originating form ASM areas (Au grades of 6 g/t or higher). If the question is on the economic feasibility of achieving agitation in ASGM, then it would indeed be difficult to achieve by artisanal miners but for small-scale miners who have shown to possess basic mechanised operations, agitation can be implemented if it is aligned with profit from the Au recovered, this was highlighted in the conclusions.

Reviewer 3 Report

Suggestions and questions about the article are listed below.

·          Gold extraction made on the streets and in small-scale facilities cause very big environmental problems but is still preferred due to cheap human labor. Adopting modern leaching techniques can also make a significant change in the economy. The authors have done a lot of work on leaching using CN and thiosulphate, but are not able to present them as they deserve. There are deficiencies in the interpretation of data and the representation of figures. Although the article is still understandable in its current form, it did not comply with the technical article writing rules in most places. Even if it is not suitable in its current form and it is really hard to follow it must definitely be given a chance. My current personal opinion on this manner is a major revision, however, I am expecting an acceptance after the revision.

·         Some grammatical errors were corrected on the uploaded pdf. I also added some recommendations from the literature. Please read and discuss where it is necessary.

·         Figures 1 and 3 are not clear. What is “B” in Figure 2?

·         Figure 5A: The gold dissolution rate of Samples 1 and 2 started to increase after 24 hours and Sample 3 remains constant. Please explain this situation.

·         Instead of mg/L, you may use dissolution efficiency in %. You can show the dissolution efficiency of each step as a total of 100%. For example, 1 step (Gravity) 40%, 45%, 50% gold recovery for Sample 1, Sample 2, and Sample 3, 2 step CN leaching 25%, 20%, 15%, 3 step HCl leaching 15%, 20%, 25%, and so on. Totally, there will be 100% recovery for all three ores after the 9^th stage. So, you may make comparisons easily.

·         Show Figure 6 as the total Au recovery rate for each step.

·         In Figure 8, the 24^th hour data of Sample 1 is not entered.

·         In Figure 9, the 3^rd hour data of Sample 1 falls outside the curve.

·         How can one explain the lower gold dissolution efficiencies at 3 g/L CN concentration? When the cyanide concentration increased, the dissolution efficiency increased again. Why?

·         In Figure 10, does mg Au/kg ore mean Au content of the remaining cake after leaching? As much as I understand you leached the ore using 5 g/L CN and the cake (tailings) contained more Au than diagnostic leaching. As I commented previously if you use Au recovery rate as percentages it will be clearer.

·         To reveal the effect of grain size you should have done the experiments with samples that were completely reduced below a certain size, -300, -200, -100, etc. Because each fraction can have a different structure and content. This makes it difficult to comment.

·         Were thiosulfate experiments performed on the original material that had not been treated before?

·         The conclusion section is too long. Here, the innovation, results, and suggestions of the study should be mentioned.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 3:

• Some grammatical errors were corrected, and literature recommendations given, to use where necessary

Response: Thank you for the careful read-through of our manuscript. The grammatical changes have been implemented and the recommended literature was included where suggested.

• Figures 1 and 3 are not clear. What is B in figure 2?

Response: The 'B' in figure 2 does not belong there and was removed. Figures 1 and 3: the images unfortunately did not get clearer after zooming in during the manual search of the gold particles using SEM. In figure 2, the gold is clear and very visible, which unfortunately was not the case for the other two particles found. Since the manual search of gold particles was very time consuming and the equipment was available for a limited time, these pictures were the only ones available.

• Figure 5A: The gold dissolution rate of Samples 1 and 2 started to increase after 24 h and Sample 3 remains constant. Explain this further

Response: The 3 ores have different grades, compositions and mineralogies; it is therefore not expected that they would exhibit the same extraction characteristics although similar general trends are expected.

• Instead of mg/L, use dissolution efficiency in %

Response: After panning in stage 1 of the diagnostic leach, the concentrate obtained was amalgamated using Hg. However, the amalgamation experiment was unsuccessful since not enough concentrate was generated to form an amalgam that could yield a gold sponge. This meant that the gold removed in stage 1 was lost and could not be included in calculations to make a total from which dissolution efficiencies in % could be shown. It is for that reason that the results are presented as concentrations (mg/L)

• Show figure 6 as total Au recovery rate for each step

Response: Since the gold extracted in stage 1 was lost, it was decided that it would be more pertinent to show the cumulative total of Au extraction for each sample in each stage (stage 2 onwards) for which Au was quantified in a mg Au per kg of ore basis.

• In fig 8, the 24 h data of Sample 1 is not entered

Response: It was highlighted (paragraph below figure 9) that for Sample 1, at 3 g/L NaCN, gold extraction data was plotted only up to 12 h of leach time as the leach sample at the 24 h mark was lost.

• In fig 9, the 3h data of sample 1 falls outside the curve

Response: This is in fact true. This could be indicative of a slight re-absorption of Au after 3h of leaching. This could be caused by the presence of carbonaceous mater causing preg robbing. A discussion of this this was included in the revisions of the paper.

• How can you explain the lower gold dissolution efficiencies at 3 g/L CN concentrations?

Response: Increasing NaCN concentration from 1 g/l to 3 g/l did not seem to have a significant impact on extraction as well as the initial kinetics for the 3 ores while increasing it to 5 g/l improved leaching. This is likely due to the presence of cyanide consuming minerals such as pyrite and arsenopyrite, shown to be present in the ore. Other metals like Cu consume cyanide too, causing the leaching to be more effective only at much higher cyanide concentrations.

• In fig 10, does mg Au/kg ore mean Au content of the remaining cake after leaching?

Response: mg Au/kg ore means that for a given amount of ore, X mg of Au were recovered from it by the diagnostic leach sequence and Y mg by cyanide leach, with the total x + y being the head grade, or total content, of gold in the ore.

• Were thiosulphate experiments performed on the original material that had not been treated before?

Response: Yes, the thiosulphate leaching experiments were performed on fresh untreated ore samples. A sentence was added in the methodology to emphasize this.

• The conclusion is long

Response: The conclusion section is indeed a bit lengthy; it has been shortened where possible. However, we felt that there was a need to emphasize certain points specifically for the benefit of ASGM specific research for which there is not a lot of literature available. Further ASGM in constant flux (socio-economic, political, legislative, educational, etc.) when it comes to acceptance of new technologies. It is important to acknowledge, as scientists and engineers, that successful implementation of technologies developed for the benefit of ASGM, however efficient, depends greatly on the realities and specific context of each artisanal mine.

Reviewer 4 Report

This article investigates the possibility of changing Hg amalgamation to cyanidation or thiosulphate leaching for gold recovery in artisanal and small-scale gold mining. The topic of the investigation is relevant at the present time and has practical importance.

There are the following questions and comments to the authors of the article:

1. The "Introduction" section seems quite lengthy, the authors could consider shortening the overview slightly.

2. Line 186. What is the reasoning for the choice of sample size for the leaching experiments? In the source [12] there are also the following data "grinding in a Chilean mill to 80% passing 0.150 mm, a sluice box was used to pre-concentrate the gold producing a concentrate with grade of 17.3 g Au/tonne". Is this size not the reason for the low gold leaching recoveries? The authors could explain the choice of this size for leaching experiments. 

3. The article contains errors "Error! Reference source not found.", e.g. line 216 etc. Which makes it difficult to evaluate the literature sources.

4. Mineralogical investigation of gold forms is a determining factor in the choice of gold recovery technology. For example, for ores in which gold is associated with pyrite and arsenopyrite, the authors have suggested this (line.497) was the reason for low leaching results. Generally, for refractory ores of this type, applications such as pressure oxidation (POX) of sulphides are necessary, otherwise recovery will be low. The reference on line 254 "The XRD and QEMSCAN bulk minerology data is shown in "Error! Reference source not found." just does not allow the mineralogical features of the ore samples under investigation to be investigated. It must also be explained if the ores contain carbonaceous material, for example, which may sorb gold from solutions and result in low gold recoveries. 

5. Authors have conducted researches on leaching of gold at 1,3 and 5 g/l NaCN and the analysis of results shown in figures 7, 8 and 9 shows that with increase of cyanide concentration gold recovery increases. At the same time, using a concentration of 20 g/l did not yield high values. The authors need to explain why these values of NaCN concentration were investigated? 

6. Figure 9 shows data on gold recovery when using 5 g/l NaCN.  At the same time, as in Figure 5 B for the sample 1 is characterized by a decrease in gold recovery after 3 hours of exposure. Do the authors have any idea what this is due to? Which brings back to question 4 about the presence of substances sorption-active towards gold in the ore, which can adsorb gold on themselves from the solution.

Author Response

Response to Reviewer 4:

• Introduction too long

Response: The introduction was shortened slightly as suggested.

• What is the reasoning to the choice of sample size for the leaching experiments?

Response: In the same paper by Viega et al. (2009), the authors suggest (page 1378) that 80 % below 0.25 mm is considered reasonably fine grain for artisanal gold processing. And based on experience from co-author/co-supervisor, average PSD used in Zimbabwe by artisanal miners is ~250-300 microns. It is based on the cited paper and the field knowledge that the decision was made work in the size selected. Additionally, in ASM, most miners apply gravity separation and therefore tend to stay away from very fine particles to avoid losing high amounts of Au, even before Hg amalgamation. To run our experiment at conditions similar to ASM, the PSD was selected in that range. However , it was shown in the current paper (fig 11), in accordance with your feedback, that a finer particle size leads to better Au extraction.

• Article contains some ‘Error! Reference source not found”

Response: The errors have been rectified accordingly.

• Mineralogical investigations of gold forms is a determining factor in the choice of gold recovery technology…..It must also be explained if the ores contain carbonaceous material, for example, which may absorb gold from solution and result in low gold recoveries

Response:  The XRF data on the 3 ores revealed quite high LOI% of 5.4% (Sample 1), 10.8 % (sample 2) and 7.6% (Sample 3). As much as LOI % predominantly accounts for water in the ore, it also indicates the organic matter content of a sample. A high carbon content in an ore can indeed cause preg-robbing, which re-absorbs Au and leads to low extractions, as highlighted by the reviewer. In fact, Sample 1 which had the lowest LOI% achieved the highest extraction at the 3 cyanide concentrations. However, since the LOI% percentages include water loss and we were not able to know exactly how much of those percentages accounted for carbonaceous material to then link to the low Au extractions, it was decided to not emphasize those links out as such interpretations would be a bit of a stretch especially since the ores have different mineralogies, as also recommended by one of the thesis examiners.

• Results show that increasing cyanide concentration increases gold recovery. At the same time, using a concentration of 20 g/L did not yield high values. Explain

Response: That is because the cyanide leaching at the 3 cyanide concentrations (1 g/L, 3 g/L and 5 g/L) was done on fresh ores, while the 20 g/L NaCN leach was conducted at stage 2 of the diagnostic leach sequence which followed stage 1 (gold gravity separation) which had removed significant amounts of gold already.

• Fig 9 shows data on gold recovery when using 5 g/L NaCN. At the same time, as in fig 5B for the sample 1 is characterised by a decrease in gold recovery after 3 h of exposure. Do the authors have any idea what this is due to?

Response:  In fig 9, the leaching is done at 5 g/L NaCN on a fresh ore sample that is untreated. While fig 5B shows stage 4 of the diagnostic leach sequence which is the second cyanide leach of the sequence, at this stage most of the gold has already been removed from the material while for the direct leach at 5 g/L, the sample was fresh and untreated.

Round 2

Reviewer 2 Report

Taking into account the significant revisions to the article made by the authors, I recommend accepting the manuscript.

Reviewer 3 Report

Congratulations. Good luck with your future works.