MSWI Fly Ash Multiple Washing: Kinetics of Dissolution in Water, as Function of Time, Temperature and Dilution
, Enrico Destefanis 1, Linda Pastero 1, Davide Bernasconi 1, Costanza Bonadiman 2 and Alessandro Pavese 1
Round 1
Reviewer 1 Report
It is known that fly ash formed during the combustion of solid household waste is an additional material for the construction industry. To do this, it must be pretreated by removing such dangerous substances as chlorides, sulfates and heavy metals. The article investigates the multicycle treatment of fly ash with water. Special attention is paid to the process of dissolution of occurring mineral phases and interpretation of observations. The efficiency of the treatment is evaluated by leaching tests in accordance with the European Standard. It is established that the main parameter is the ratio of water to solid. The number of washing cycles to remove halite and sylvite is also important. Quartz, calcite, anhydrite and the amorphous phase remain in the solid residue. The sequential extraction method and the modeling of dissolution kinetics provide additional data for the interpretation of leaching processes. It is concluded that the dissolution occurs by a two-stage mechanism. Multi-stage flushing is effective in reducing pollutants to remove non-hazardous waste. However, sulfates and some heavy metals still remain in the residue. The results obtained are very important for further research in the field of industrial waste application for the synthesis of new useful materials.
Author Response
We thank the reviewer to have analyzed and commented positively the text.
Reviewer 2 Report
Manuscript ID: minerals-1706689
Title: MSWI fly ash multiple washing: kinetics of dissolution in water, as function of time, temperature and dilution.
Authors: Caterina Caviglia et al.
Introduction:
Line 49, 56, 60. It is necessary to use the third-person singular, instead of the first-person singular or plural.
Line 47-50. Authors must describe all these methods in detail. Please, write information about heavy metal's removal efficiency.
Line 57-59. Authors write: “At which extent and cost are we able to transform the original fly ash into a product that is a non-hazardous waste, and possibly reusable, exploiting a treatment that relies upon iterative water washings only and uses comparatively small liquid/solid ratios?”. I wasn't to find information about cost calculation. Please, answered to your own question.
Line 113. What is the solvent used to regenerated wastewater? Add this information to article, only name.
Results:
Figure 2b. Use the same Y-axes as in Figure 2a. These figures must be writing in one style.
Table S5. Bassanite mineral appears in sample LS 2, where did it come from? Does it Bassanite or it is Gypsum? Why is it not present in other samples and in the original ash? Why does gypsum appear in samples LS 5-50? Why is there no gypsum in Table S6? I believe that Table S5-S6 must be added to article text, because it is very important information.
- How does the particle size distribution of fly ash change after washing tests?
Figure 3. I think it is necessary to combine the Figures with the data for the major elements and the figures for heavy metals into one figure. There should be 2 figures left.
Figure S5. The fitting curves must be exited from the “0” point. Please, change the calculation on these figures. The use of two temperatures is very small to determine the activation energy of the process. Temperatures should be at least 3, and preferably 4-5. There are only 2 points on the Figure S6.
Conclusions. Write some sentences about kinetics analysis of MSWI-FA water washing process.
Technical errors:
Line 35. “5 g cm− 3” must be changed to “5 g/cm3”
Line 37. “(up to 50 wt%;” must be changed to “(up to 50 wt%)”
Line 39. “(~10 -12;” must be changed to “(~10 -12)”.
Author Response
Introduction:
Comment: Line 49, 56, 60. It is necessary to use the third-person singular, instead of the first-person singular or plural.
Reply: Sentences have been changed using the impersonal form.
Comment: Line 47-50. Authors must describe all these methods in detail. Please, write information about heavy metal's removal efficiency.
Reply: these methods have been described and information were added about heavy metal’s removal efficiency.
Comment: Line 57-59. Authors write: “At which extent and cost are we able to transform the original fly ash into a product that is a non-hazardous waste, and possibly reusable, exploiting a treatment that relies upon iterative water washings only and uses comparatively small liquid/solid ratios?”. I wasn't to find information about cost calculation. Please, answered to your own question.
Reply: We thank the reviewer for pointing out this unclear sentence. Rather than economical sense, cost is here intended in terms of experimental parameters such as amount of water (expressed by L/S) and treatment time, especially through the comparison between the two washing treatments here evaluated as explicated in the Conclusion section. To prevent other misunderstandings, the word “cost” has been removed.
Comment: Line 113. What is the solvent used to regenerated wastewater? Add this information to article, only name.
Reply: No solvents were used to regenerate wastewater; the same washing water was regenerate by filtration and then reused.
Comment: Figure 2b. Use the same Y-axes as in Figure 2a. These figures must be writing in one style.
Reply: Figure 2b has been changed, following the indication of the reviewer.
Table S5. Bassanite mineral appears in sample LS 2, where did it come from? Does it Bassanite or it is Gypsum? Why is it not present in other samples and in the original ash? Why does gypsum appear in samples LS 5-50? Why is there no gypsum in Table S6? I believe that Table S5-S6 must be added to article text, because it is very important information.
Reply: Bassanite (CaSO4· 0.5H2O) and gypsum (CaSO4· 2H2O) are both hydrate phases of anhydrite (CaSO4) and, as such, they derive from the water washing treatment of FA, which contains a fraction of anhydrite formed when the incineration plant flue gas rich in SOx passes through lime sorbent filter. At low L/S (L/S=2), we notice that the hydration process of anhydrate stabilizes the lower hydrate (bassanite, hemihydrate), while for L/S>2 the dihydrate phase (gypsum) is stable, probably due to the relatively higher amount of water. These phases are however not observed when the Falling Head Water Washing treatment is applied. In our opinion, this is due to the kinetically controlled mechanism of this treatment, where the rapid flux of water, together with the slow dissolution kinetics of anhydrite, doesn’t allow the precipitation of hydrate phases, while in the previous case the material is put in contact with water for much longer time (24 hours), thus approximating thermodynamic equilibrium conditions. Some explanation is added in the text (L339-344), together with the Table S5-S6.
Comment: How does the particle size distribution of fly ash change after washing tests?
Reply: The particle size distribution after washing becomes coarser, since the finer fraction is dissolved in water. (line 215-216)
Figure 3. I think it is necessary to combine the Figures with the data for the major elements and the figures for heavy metals into one figure. There should be 2 figures left.
Reply: The four Figure 3a-d have been condensed into two, Figure 3 top and bottom.
Figure S5. The fitting curves must be exited from the “0” point. Please, change the calculation on these figures. The use of two temperatures is very small to determine the activation energy of the process. Temperatures should be at least 3, and preferably 4-5. There are only 2 points on the Figure S6.
Reply: We thank the reviewer for the on-point suggestion. Indeed, a physical constrain is necessary for the model fitting in Figure S5, to account for the initial condition of the process (t=0, α=0), and therefore the model curves have been modified to intersect the origin point. Although this change has generally improved the correlation coefficient R2, the relatively high data scattering still suggests evaluating a different model, as described in the text.
We absolutely agree about the necessity of at least 3-4 isotherms to correctly derive Ea in a kinetics analysis. In this case, we still tried to use the Arrhenius equation for a (very) qualitative and approximated evaluation of Ea in terms of order of magnitude, but without any claim of precision. This point has been further emphasized in the text. (L309-311)
Conclusions. Write some sentences about kinetics analysis of MSWI-FA water washing process.
Reply: A couple of sentences have been added in the Conclusions section to describe the kinetics analysis results. (L391-394)
Technical errors:
Line 35. “5 g cm− 3” must be changed to “5 g/cm3”
Line 37. “(up to 50 wt%;” must be changed to “(up to 50 wt%)”
Line 39. “(~10 -12;” must be changed to “(~10 -12)”.
Reply: changes have been made
Reviewer 3 Report
The manuscript successfully has addressed the mentioned goals in the introduction. Also, the manuscript had a solid scientific background.
Author Response
We thank the reviewer to have analyzed and commented positively the text.Reviewer 4 Report
The study was interesting and supported by a good experimental design. I gave specific comments to improve the quality of the article below:
1. Please avoid the use of first-person pronouns (e.g. we) and write only in the third person (e.g. This study, The current research, It has been found) throughout the manuscript.
2. Only one reference is given in section 2.2. Please add references in this section to support your mathematical approach.
3. In the legend of Figure 3, further details should be added and the abbreviations should be explained (e.g. Ct, Nws).
4. In Figure 4, the term "pH" should be given in the vertical "y" axis and not as a graph title. Also, in the Figure 4 legend, further details should be added and the abbreviation Nws should be explained.
Author Response
The study was interesting and supported by a good experimental design. I gave specific comments to improve the quality of the article below:
- Please avoid the use of first-person pronouns (e.g. we) and write only in the third person (e.g. This study, The current research, It has been found) throughout the manuscript.
Reply: The personal form was changed into an impersonal form.
- Only one reference is given in section 2.2. Please add references in this section to support your mathematical approach.
Reply: References [35] and [36] were added to this section.
- In the legend of Figure 3, further details should be added and the abbreviations should be explained (e.g. Ct, Nws).
Reply: The caption of Figure 3 was changed to provide all the required details.
- In Figure 4, the term "pH" should be given in the vertical "y" axis and not as a graph title. Also, in the Figure 4 legend, further details should be added and the abbreviation Nws should be explained.
Reply: The figure 4 was changed and further details were given in the caption.
Round 2
Reviewer 2 Report
The authors answered all the questions in detail, the kinetics calculation section has become much better. Significant additions were made to all sections of the article, including the Introduction, Methods, Results, and Conclusions.
There is one remark that does not reduce the scientific significance of the article, it can be corrected in the process of final revision of the article for publication:
Figure 7. The authors must add the Y-axis, there are only percents without a vertical line.
