Influence of Lignosulfonate on the Hydrothermal Interaction Between Pyrite and Cu(II) Ions in Sulfuric Acid Media

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

• Introduction is too lengthy and this section should be more concise with imitations of the reported methods and novelties of this work.
• Author must mention about the purity of “natural sulfide mineral pyrite” in sec 2.2. Is the collected raw sample treated before the XRD analysis?
• Author must check the section numbering.
• 1, Generally, raw samples might have co-elements such as inorganics, organics, etc. How did you get clean XRD pattern?
• Sec: “Experimental Equipment and Procedures”, this heading needs to be more clear and detailed experimental conditions such as time, volume of the solvents, concentration of the reagents, quantity of materials, etc.
• Why did you choose this temperature at which atmospheric conditions (O2/Ar/N2)?
• How did you derive the chemical equations 4-9. Provide the evidence.
• Page no 6, para 3-4, reviewer could not find the instrumental analytical/measurement evidences.
• Error bar is missing in all plots.
• Sec 3.1., How did you quantify these presented data.
• Sec 3.3., authors must specify the pH and chemical reaction and composition of the precipitations. Why did not use other acids or buffer?
• Fig 5B & D, why the patterns are different as well as % extraction of Fe and Cu?
• Sec 3.4., discuss more in detail with the complete reaction mechanism.
• Fig 8 -11 don’t have any scale, unit, resolution etc.
• Discussion on Fig 8 -11 is not clear and ideal. Authors must incorporate more instrumental data analysis from various techniques to derive the conclusions such as more XRD, XPS, TGA pattern, elemental analysis, Solid state UV, etc
• Sec 3.5, doesn’t discuss the mechanism of chemical reaction.
• Conclusion must be concise not like a summary.

Author Response

Dear Reviewer,

We sincerely thank you for your careful and constructive review of our manuscript. Your insightful comments and recommendations were highly valuable and helped us to improve the clarity, methodological transparency, and overall scientific quality of the paper. We have carefully considered each point raised and revised the manuscript accordingly. We believe that the changes implemented in response to your feedback have significantly strengthened the presentation and interpretation of our results, thereby bringing the revised work to a higher publication standard.

Comment  1. Introduction is too lengthy and this section should be more concise with imitations of the reported methods and novelties of this work.

Answer 1. We appreciate the reviewer's comment. We agree that the introduction should be more concise and focused. In the revised version, we have shortened the overview sections, leaving only those statements that directly justify the problem statement and the choice of experimental approach. A short paragraph will be added at the end of the introduction, stating the objective, key methods, and scientific novelty.

Comment 2. Author must mention about the purity of “natural sulfide mineral pyrite” in sec 2.2. Is the collected raw sample treated before the XRD analysis?

Author must check the section numbering.

Answer 2. Thank you for clarifying. The current version does indeed require a more explicit description of the purity of the source pyrite and the preparation of the sample for XRD. The manuscript already provides the elemental composition of pyrite (Fe 44.1 wt.%, S 50.8 wt.%, others 5.1 wt. %), but we will additionally clarify that the sample was used as a natural mineral after standard mechanical preparation (crushing–sieving–grinding to d80 = 40 μm), and that this averaged ground sample without chemical treatment was used before XRD to avoid changes in surface and phase compositions. We also agree that the numbering of the sections needs to be aligned.

Comment 3. Generally, raw samples might have co-elements such as inorganics, organics, etc. How did you get clean XRD pattern?

Answer 3. Thank you for your insightful comment. We will clarify in the text that the purity of the XRD pattern refers to the detectable phase composition within the sensitivity of the method. Since the source pyrite is a natural mineral and contains other components, trace impurities below the XRD detection limit or in an amorphous state are possible. We will add an explanation in section 2.2 that the X-ray diffraction pattern (Figure 1) shows the dominance of pyrite, the absence of pronounced lines of impurity crystalline phases within the sensitivity of the instrument, and that “other 5.1 %” mainly reflects chemical impurities that do not necessarily form separate crystalline phases.

Comment 4. Sec: “Experimental Equipment and Procedures”, this heading needs to be more clear and detailed experimental conditions such as time, volume of the solvents, concentration of the reagents, quantity of materials, etc.

Answer 4. Agreed. Although some of the parameters have already been provided, the description needs to be structured and all key conditions (time, concentration ranges, S:L, etc.) need to be clearly specified.

Comment 5. Why did you choose this temperature at which atmospheric conditions (O2/Ar/N2)?

Answer 5. Thank you for your comment. The manuscript already states that further experiments were performed at 220 °C, as this temperature ensures maximum process intensity. We will strengthen the justification for the selected temperature range of hydrothermal treatment of 180–220 °C, which allows us to observe phase transformations. The temperature of 220 °C was chosen as the upper limit at which the effect of the parameters and the influence of surfactants are most clearly manifested. The text will clarify that no gas phase is expected during hydrothermal treatment.

Comment 6. How did you derive the chemical equations 4-9. Provide the evidence.

Answer 6. Accepted. Equations (4–9) in the current version are presented without explanation of their origin. In the revised version, we will add a brief explanation that the reactions are stoichiometric schemes describing the observed transformations of FeS₂ and Cu (II) in a sulfate environment with the formation of secondary Cu–S phases and S⁰, as well as an indication that their relevance is confirmed by XRD analysis (the presence of CuS and Cu_1.8S) . Additionally, we will provide references to the thermodynamic logic obtained using the HSC Chemistry 9.5 database.

Comment 7. Page no 6, para 3-4, reviewer could not find the instrumental analytical/measurement evidences.

Answer 7. Thank you for your feedback. We will strengthen the link between “indicator → measurement method → calculation.” In particular, the indicators “iron leaching” and “copper precipitation” will continue to be used as quantitative metrics of efficiency, but we will add a clear indication on the page with the formulation (and next to the equations). The iron concentration in the solution was determined by ICP-MS, the phase composition of solid products was determined using XRD, and the morphology and distribution of elements were determined based on SEM-EDX results (the list of instruments has already been provided).

Comment 8. Error bar is missing in all plots.

Answer 8. It is accepted. We will add error-bars to all graphs and calculate them using standard deviation.

Comment 9. Sec 3.1., How did you quantify these presented data.

Answer 9. Thank you. The text already states that efficiency was assessed based on the degree of iron transfer into the solution and the amount of copper precipitation, but the calculation formulas do indeed need to be explicitly stated. We will add definitions of these formulas to the text.

Comment 10. Sec 3.3., authors must specify the pH and chemical reaction and composition of the precipitations. Why did not use other acids or buffer?

Answer 10. Thank you. In Section 3.3, the initial concentration of H₂SO₄ varies (10–30 g/dm³), but the pH is not given. We will make the necessary clarifications. The pH values were not measured because at 220 °C the formal pH of aqueous solutions and dissociation constants change significantly, so for reproducibility it is more correct to specify the acidity via [H₂SO₄]₀ and the sulfate concentration. H₂SO₄ was chosen because the study focuses on the sulfate hydrothermal system and the FeS₂–Cu(II) redox exchange processes (reactions 4–9). The introduction of other acids (especially chlorine-containing ones) changes the complex formation and mechanism, complicating the interpretation. Buffer systems at 220 °C are generally chemically unstable, as will be briefly explained. We will make a more direct reference to the composition of the precipitates through XRD (CuS, Cu_1.8S) and SEM-EDX mapping (Cu/S/Fe distribution) in the text.

Comment 11. Fig 5B & D, why the patterns are different as well as % extraction of Fe and Cu?

Answer 11. Thank you for your comment. There may be some ambiguity in the interpretation of the figures. The caption for Figure 5 indicates that (a,c) is without lignosulfonate, and (b,d) is with lignosulfonate. Accordingly, the difference in trends between 5b/5d and 5a/5c reflects a change in the mechanism when lignosulfonate is added. The text already notes that lignosulfonate exhibits dispersing and adsorption properties and reduces the passivation of elemental sulfur S⁰ and secondary Cu–S phases.

Comment 12. Sec 3.4., discuss more in detail with the complete reaction mechanism.

Answer 12. Accepted. We will expand the discussion of the mechanism in 3.4, linking the observed dependencies on [Cu]₀ with the sequence of phase formation CuS → Cu_1.8S  and the diffusion influence of the product layer, which is already discussed in section 3.5.

Comment 13. Fig 8 -11 don’t have any scale, unit, resolution etc.

Answer 13. It is accepted. The captions already indicate the scales (500 µm, 10 µm), but the micrographs themselves do not have a scale bar or imaging parameters. We will add them directly to the images and expand the captions with standard SEM parameters to meet reproducibility requirements.

Comment 14. Discussion on Fig 8 -11 is not clear and ideal. Authors must incorporate more instrumental data analysis from various techniques to derive the conclusions such as more XRD, XPS, TGA pattern, elemental analysis, Solid state UV, etc

Answer 14. Thank you for your recommendation. We agree that an expanded set of methods, such as XPS and TGA, could deepen the interpretation of surface states and sulfur forms. However, the key conclusions of this work are based on a consistent set of data from ICP-MS (solutions), XRD (phase composition), and SEM-EDX (morphology and element mapping, which directly confirm the formation of secondary Cu-S phases and the nature of Cu/Fe/S distribution across the surface). In the revision, we will make the discussion more method-oriented and refine the wording where higher surface resolution methods are required. We will add elemental analysis of the obtained cake to confirm the formation of secondary copper particles, and add references to XPS/TGA as promising future research in the “Conclusion” section.

Comment 15. Sec 3.5, doesn’t discuss the mechanism of chemical reaction.

Answer 15. Accepted. In its current form, 3.5 indeed describes the XRD results and the general role of the product layer rather than formalizing the mechanism. We will add a mechanistic fragment combining reactions (4–9), the sequence of CuS→Cu1.8S phase transformations, and the role of SLS in reducing local passivation and changing the nature of Cu-S deposition (as seen in the mapping).

Comment 16. Conclusion must be concise not like a summary.

Answer 16. Agreed. The conclusion will be reduced to 5–7 sentences, including only the main established effect of SLS, accepted conditions, confirmed phases by XRD, practical conclusion, and 1–2 sentences about the prospects for research.

Respectfully,

Dr. M. Tretiak, on behalf of all authors.

Reviewer 2 Report

Comments and Suggestions for Authors

Influence of Lignosulfonate on the Hydrothermal Interaction Between Pyrite and Cu(II) Ions in Sulfuric Acid Media is very interesting paper in copper hydrometallurgy. Hydrometallurgical pretreatment of pyrite-bearing concentrates and tailings by hydro- thermal interaction with Cu(II) solutions is a promising route for chemical beneficiation and mitigation of acid mine drainage but is limited by passivation caused by elemental sulfur and secondary copper sulfides. Minor revisions are required. Using of lignosulfonate is a new idea for dissolution process.

Line 18: Process efficiency was evaluated by Fe extraction into solution (at room temperature?)

Line 48: The presence of pyrite in concentrates and tailings is one of the key technological problems (in selective dissolution of copper)

Line 62: This process significantly increases the mobility of heavy metals (such as…)

Line 72: The method involves the interaction of undesirable pyrite (FeS2) with copper  ions in aqueous solutions at elevated temperatures (in what temperature range?)

Line 187: Upon reaching the specified temperature (in what range?)

Line 196: The reactions describing the autoclave processing of the pyrite are presented. Unfortunately, the formation of H2S is not included in this consideration! Why?

Line 254: Thus, the addition of SLS intensifies the sulfide matrix destruction and iron release into solution (in what time?)

Line 317, 318: In the early stages of transformation or at low temperatures, the presence of CuS could be detected in the solid product, whereas at elevated temperatures or longer reaction times, the formation of Cu1·8S phase is observed. Is the presence of the elemental sulfur detected?)

Line 378, 379: Micrographs and EDS mapping of a pyrite gas-extracted solid waste cake sample in the presence of SLS are shown in Figure 10. (Please can you add some scale or magnificence on your figures in order to understand the size of the shown particles)

Line 411: leading to the formation of a developed fine fraction (what is particle size of the fine fraction?)

Conclusion:

Line 430, 431: As can be seen from the presented images (Figure 11), fine particles are formed pri- marily as a result of the interaction of copper ions with elemental sulfur, fine pyrite particles, and the subsequent separation of the formed copper sulfides (what are reaction parameters for this reaction?)

Line 441, 442: The complex effect of SLS is expressed in the acceleration of redox exchange stages and increased mobility of the surface layers of the solid phase what are values of redox potential and pH-Values?)

Author Response

Dear Reviewer,

We greatly appreciate the time and effort you devoted to evaluating our manuscript. Your detailed and thoughtful observations provided an excellent opportunity to refine the paper, improve the structure and readability, and better substantiate the key conclusions. We have addressed each of your comments point by point and implemented corresponding revisions throughout the manuscript. We are confident that your feedback has helped us elevate the work and present it in a clearer and more rigorous form.

Comment 1. Line 18: Process efficiency was evaluated by Fe extraction into solution (at room temperature?)

Answer 1. Yes, the Fe extraction rate was calculated based on chemical analysis of the liquid phase after cooling (ex-situ). This is not explicitly stated in the current version of the Abstract, so the relevant clarification will be added.

Comment 2. Line 48: The presence of pyrite in concentrates and tailings is one of the key technological problems (in selective dissolution of copper)

Answer 2. Comment accepted. This passage refers not to the selective hydrometallurgical solubility of copper, but to the technological problem of pyrite in enrichment processes and subsequent tailings management. Activation of pyrite by copper ions reduces selectivity, and the presence of pyrite in tailings increases environmental risks. We will reword the sentence to eliminate the ambiguity.

Comment 3. Line 62: This process significantly increases the mobility of heavy metals (such as…)

Answer 3. Agreed. The text already partially lists impurities (Zn, Pb, As), but for the sake of accuracy, we will add an expanded list (e.g., Cd, Ni) of typical pollutants whose migration is possible when tailings are deposited.

Comment 4. Line 72: The method involves the interaction of undesirable pyrite (FeS₂) with copper  ions in aqueous solutions at elevated temperatures (in what temperature range?)

Answer 4. Agreed. The study actually examined the range of 180–220 °C, and this should be clearly stated in the introduction.

Comment 5. Line 187: Upon reaching the specified temperature (in what range?)

Answer 5. Comment accepted. In the methodology section, we will explicitly state that the specified temperature corresponded to the set temperature of the experiment in the range of 180–220 °C.

Comment 6. Line 196: The reactions describing the autoclave processing of the pyrite are presented. Unfortunately, the formation of H₂S is not included in this consideration! Why?

Answer 6. We agree that this is an important question and will add an explanation. In the system under consideration, Cu(II) acts as an oxidizing agent, and the transformations of sulfur in the proposed stoichiometries lead to the elemental form S0 and secondary Cu sulfides, which corresponds to the oxidation mechanism of pyrite. The formation of H₂S requires the reduction of sulfur (S⁻→S²⁻) and the presence of additional reducing catalysts. Under conditions where the electron balance is determined by the reduction of Cu(II) → Cu(I) and the precipitation of Cu(I) as Cu_xS, the formation of H₂S is not the dominant pathway. In addition, at low pH and in the presence of Cu(II), any trace amounts of H₂S/HS⁻ will quickly bind to Cu and convert to CuS without accumulating in the gas phase, which was not analyzed in this study.

Comment 7. Line 254: Thus, the addition of SLS intensifies the sulfide matrix destruction and iron release into solution (in what time?)

Answer 7. This statement refers to the end point of 120 minutes, which is further quantified below. For clarity, we will make the connection explicit.

Comment 8. Line 317, 318: In the early stages of transformation or at low temperatures, the presence of CuS could be detected in the solid product, whereas at elevated temperatures or longer reaction times, the formation of Cu1·8S phase is observed. Is the presence of the elemental sulfur detected?)

Answer 8. In the XRD data presented, we identified copper sulfides (CuS and Cu_1.8S) and did not observe any distinct responses from elemental sulfur S⁰. However, the formation of S⁰ is inherent in the stoichiometry of reactions (4)–(9) and corresponds to the generally accepted mechanism of passivation during the hydrothermal interaction of pyrite with Cu(II). We will add a clear clarification to the text that crystalline elemental sulfur has not been confirmed by XRD and its presence is considered probable (including in thin-film or amorphous form or below the XRD detection limit).

Comment 9. Line 378, 379: Micrographs and EDS mapping of a pyrite gas-extracted solid waste cake sample in the presence of SLS are shown in Figure 10. (Please can you add some scale or magnificence on your figures in order to understand the size of the shown particles)

Answer 9. Agreed. In the current version, the scale is indicated in the caption (e.g., 500 µm / 10 µm), but we will add the scale directly to the micrograph.

Comment 10. Line 411: leading to the formation of a developed fine fraction (what is particle size of the fine fraction?)

Answer 10. Accepted. We will specify the size range of the fraction based on the granulometry data.

Conclusion:

Comment 11. Line 430, 431: As can be seen from the presented images (Figure 11), fine particles are formed pri- marily as a result of the interaction of copper ions with elemental sulfur, fine pyrite particles, and the subsequent separation of the formed copper sulfides (what are reaction parameters for this reaction?)

Answer 11. Agreed. The conclusions should indeed mention that the interpretation in Figure 11 refers to specific experimental conditions (H2SO4, Cu(II), SLS, T). We will add these parameters directly to the final thesis.

Comment 12. Line 441, 442: The complex effect of SLS is expressed in the acceleration of redox exchange stages and increased mobility of the surface layers of the solid phase what are values of redox potential and pH-Values?)

Answer 12. Comment accepted. For pH, we will add a range corresponding to the initial concentrations of H₂SO₄ (10–30 g/dm³) and emphasize that reactions (5)–(9) are accompanied by the generation of H₂SO₄, i.e., the medium remains strongly acidic. For Eh, we will clarify that in-situ measurements of Eh at 180–220 °C in an autoclave were not performed (design limitations, high temperature), and the redox conditions are determined by the internal redox buffering capacity of Cu(II)/Cu(I) and Fe(III)/Fe(II) pairs, which change during the reduction of Cu(II) and precipitation of Cu(I) in the form of Cu_xS. We will add the pH range for the initial solutions and an explanation of Eh.

Sincerely,

Dr. M. Tretiak, on behalf of all authors.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authors are responded to all my queries.