Recovery of Magnesium from Ferronickel Slag to Prepare Magnesium Oxide by Sulfuric Acid Leaching

Round 1

Reviewer 1 Report

A chemical method for recuperation of Mg form from ferronickel slag was presented. Structural analysis, chemical routes, composition, thermochemical and kinetic factors were discussed. Some comments:
1) the little expression should be changed by as;
2) the resolution of Fig.1 should be improved;
3) the 5 and 7 chemical equations should be written as thermochemistry equations and the equal symbol does not must be applied;
4) in the 5 chemical equations, was NH3 bubbled in the solution? What 5(NH?
5) in page 5, line 184, to correct CoK by CuK;
6) Fig. 5 are two graphical;
7) Tables 6 and 7 are should be one.

Author Response

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Reviewer 2 Report

Yang et al. manuscript on ‘’Recovery of magnesium from ferronickel slag to prepare magnesium oxide by sulfuric acid leaching’’ has an interesting perspective for metallurgical industry, considering the double benefit coming from both the recovery of Mg (given the critical global availability of this strategic commodity) and reducing the amount of secondary waste from steel industry. Particularly, the optimization of the leaching yields by surface response methodology. The presented results are interesting but some further revision is needed, covering the following points:

1. The writing should be improved, especially toward a more formal language, appropriate scientific terminology and consistency. A few examples will follow but the authors should not only limit to (as these are mainly from the abstract):

• ‘’unreacted-shrinkage-nuclear’’ or ‘’shrinkage-nuclear-reaction’’ were arbitrarily used. Please refer as shrinking core model or unreacted shrinking core model consistently.
• ‘’It is inferred that the leaching process of magnesium obey composite control’’ -> obeys
• ‘’A normal leaching rate’’ what is the meaning of it? An average(d) leaching rate???
• ‘’ Mg in FNS was mainly existed’’ please correct the verb tense, etc.

1. Provide more detail on how the chemical characterization was performed (e.g. digestion, metal analysis, etc.). In regard to line 177-178 ‘’ The content of Mg in the samples was determined by atomic absorption spectrophotometer (Z-2000, Japan)’’in which samples? solids, leachates? XRF techniques, especially on light elements, might lead to large errors and methods need to be validated (matrix dependency, etc.)
2. Fig 3, 8, 9: please report the graphs with the same Y axis scale (same start and end e.g. 0-100 for all) for clearer comparison.
3. Were the metal extractions evaluated for the washing stages with hot sulfuric acid and included in the calculations prior to quantify η 1 , η 2 and η (= Mg leaching efficiencies). Otherwise the Mg released during washing is not counted and the leaching efficiencies not thoroughly descriptive of the system.
4. Given the low calculated apparent activation energy 19.57 kJ·mol−1 and the best fit with 1-2α/3-(1-α) 2/3=k2 t, it doesn’t seem Mg leaching to be controlled by chemical reactions as in the statements in 383-387 but mostly by diffusion processes at the interphase. Revision can be needed for ‘’It is inferred that the leaching process of magnesium obey composite control[39].At low temperature, the acid leaching reaction rate is slower and mainly controlled by chemical reactions. When the temperature rises to a certain value, the chemical reaction rate increases resulting in an increase in the thickness of the product layer, the control step changes from chemical control to solid film diffusion control (mainly manifested as internal diffusion control).’’
5. The expenses coming from the large amount of chemicals (e.g. ammonia and costly/impacting ones e.g. H2O2) used in the purification, and from energy consumption in the thermal treatment/ calcination can be covered with the monetary revenue from MgO? How much ammonia solution was needed (normalized for instance by volume of leachate solution) to reach pH 7? Please also specify the source of ammonia salt for precipitation of impurities.
6. Large share of SOx and CO2 will be produced in the calcination stage. There are other options for Mg recovery from sulfate media and/or to reduce the environmental footprint (energy + emissions)? Furthermore, CO2 will be produced by the neutralization of acid bringing the pH to 7 during precipitation if ammonium carbonates are used. How this CO2 can be used more wisely and sustainably in the proposed process?
7. Please provide (i.e. as Supplementary material) the chemical composition of the leachate after metals purification, as well as the metal removal yields, % of impurity precipitation.
8. Fig. 8 and 9: how the content of Mg(OH)2 (Fig. 8 y axis left) and MgO content % (Fig. 9 y axis right)? These may only represent the magnesium that isn’t precipitate along with the impurities for Fig 8.? While for Fig. 9 see the next point. Explain better in the manuscript. Adjust figures accordingly.
9. Purity of solid products. It is not very clear how it is calculated, especially for the Mg carbonate hydroxide. Therefore, it is suggested to revise these calculations which should be more properly calculated, for instance, as the content of Mg found in the obtained precipitate vs the theoretical amount of Mg in 4MgCO3•Mg(OH)2•4H2O. Why ammonium carbonate has been selected as precipitating agent? Please include some indication about it.
10. Solid by-products e.g. leach residue and the precipitate have potential to be classified as non-hazardous material to be directly landfilled or back-filled? What is the content of harmful elements e.g. Cr, Cd, sulfidic phases etc. that might affect the environment and human health? What are the mitigation measures for those and what would be their fate?

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Dear Authors,

The topic of the manuscript is important for many reasons. Ferronickel slag is a major waste stream, lacking utilization possibilities. Magnesium is an important element and its scarcity and/or price increase may occur in the near future. This manuscript studies the whole treatment process to recover market grade MgO from FNS. I consider the work valuable and worth publishing.

General comments:

Generally English is good, but some polishing should be done. I consider the Minerals English check is enough for this manuscript.

The background work has been done properly, by having several references.

All the key parameters in leaching process have been studied. The research parameters have been selected well and they are industrially relevant. I highly appreciate this.

Line 212-214: Could you explain in numbers what is “reasonable final pH”. Many people do not have an access to this previous article, so please explain it well. It is very important, as avoiding silica problems in filtration is essential.

Line 223-225: “Mg leaching velocity” Should this be Mg leaching kinetics?

Line 245-249: Did you use baffles in the leaching reactor to prevent vortex? This could be mentioned clearly as it dramatically affects to the agitation efficiency versus speed.

Technical question from industrial perspective: if you change lab scale chemicals to industrial (not-so-pure) chemicals, is your product purity affected?

Question about economic viability: I consider that even with laboratory experiments it would be good to have some initial insights to the economic viability. For example, you could check that the value of produced MgO is clearly higher than the cost of used chemicals, as a first indicator. This calculation is rather easy: just convert your lab numbers to tons. (1) Calculate how much chemicals you need to treat one ton of FNS. (2) Calculate how much MgO you produce from one ton of FNS. (3) Estimate prices for chemicals and MgO from public sources, e.g. Alibaba if no better information available. (4) Do the comparison of costs and sales.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

A sufficient quality of the manuscript has been achieved for publication.