The Removal of Silicate(IV) by Adsorption onto Hydrocalumite from the Sodium Hydroxide Leaching Solution of Black Dross
Round 1
Reviewer 1 Report
This manuscript presents an interesting approach for the recycling of two types of waste. The text is easy to understand. I have only minor comments: 1) Page 4 (lines 6-7): The analyses were done in triplicates. Is it possible to give the standard deviation values for the concentrations of metals ? 2) The authors give a mass balance about the metal recovery. That is a good thing. Is it possible to also present the mass balance of the reactants require to operate the process (NaOH, H2O2, ethanol, e.g.) ? This is important to evaluate the operating costs associated to this process.
Author Response
Ques.1. Page 4 (lines 6-7): The analyses were done in triplicates. Is it possible to give the standard deviation values for the concentrations of metals ?
Ans. 1) Thank you for the comments. In each set of the experiment, two replicate experiments were carried out and the errors associated with the adsorption percentages of silicate(IV) were varied within ±4%. Besides that, we have added error bar in representing the data related to concentration of Al(III) and Si(IV).
Ques.2. The authors give a mass balance about the metal recovery. That is a good thing. Is it possible to also present the mass balance of the reactants require to operate the process (NaOH, H2O2, ethanol, e.g.) ? This is important to evaluate the operating costs associated to this process.
Ans. 2). We are doing the whole experiments to recover alumina from the black dross. Once we get all the data, then we can represent the mass balance for the whole process. At this time, it is difficult to represent the mass balance. The manuscript has been revised according to comment.
Modification
Page 10, line 217: The change in the concentration of Al(III) and Si(IV) before and after adsorption together with the purity of Al(III) are shown in Table 1. The data in Table 1 was obtained from the following conditions: 30 g/L hydrocalumite, 50oC, 2hs, 400rpm.
Table 1. Change in the concentration of Al(III) and Si(IV) in the solution and the corresponding purity of Al(III) before and after adsorption by hydrocalumite.
|
Composition |
Al(III) |
Si(IV) |
Purity of Al(III) (%) |
|
Before adsorption (ppm) |
14954.8 |
150 |
99 |
|
After adsorption (ppm) |
15028.0 |
0.6 |
99.99 |
Reviewer 2 Report
Line 32: Silica has higher solubility in alkaline than neutal and acidic solution. Metal oxides of Mn, Fe, Cu has higher solubility in acidic solution than weak alkaline solution, but with increasing solubility as the solution becomes strongly alkaline. There are many references to this. The classic wouod be Stumm and Morgan 1996, Aquatic Chemistry. The statement should be corrected /elaborated on.
Line 77: it would be usefull to use mole concentration not mg/L for Al and Si
Line 91: add comma and respectfully.
Result section : include a table with the solution Si, Al, pH, Ca, Cl, analysis for the variable tests.
Include uncertainties in the graphs plotted.
The new phase is not hydrocalumate but….. Could the phase change have been due to drying of the mineral, before analysis. Under what conditions did the drying take place
This process is possibly taking place but you have only 0.0016 mol Si in solution, while using 30 gram hydrocalumate, this amounts to 0.1 mol. You can then only convert about 1.5 mol % of the hydrocalumate. This does not make sense relative to the XRD in figure 6. Hydrocalumate should still be your main phase. SiO2 would also be a minor phase since it is also formed from the added silica of 150 mg/L.
Your equation 2 suggests a phase change, while the figure is only showing the ion exchange. Write the ion exchange equation as well as indicated by the figure 5.
Line 158: Need SEM photo to show the cubic crystals! What is the reason that you have quartz precipitation? Is there a drop in pH?
Line 182 and line 211: There could have been more Al removed than silica, but the accuracy of the analysis is not good, sentence needs to be changed to reflect this more properly. Common uncertainty is abut 5-10wt.% 5 percent would amount to 750 mg/L that could have been sorbed without you noticing it.
Line 217: this is not applicable when you have ion exchange or especially phase change. You need to have your processes understood as sorption before using this.
Line 256-258. Remove last sentence unless better justification in the discussion.
Author Response
Ques.1. Line 32: Silica has higher solubility in alkaline than neutal and acidic solution. Metal oxides of Mn, Fe, Cu has higher solubility in acidic solution than weak alkaline solution, but with increasing solubility as the solution becomes strongly alkaline. There are many references to this. The classic would be Stumm and Morgan 1996, Aquatic Chemistry. The statement should be corrected /elaborated on.
Ans. 1) Thank you for the comments. The manuscript has been revised according to the comments. Reference has been added to manuscript.
Modification
Page 1, line 31: In contrast, an alkaline leaching solution has some selectivity for alumina because the oxides of Ca, Fe, and Mg would not dissolve in this medium, resulting in pure aluminate(III) solution with a small amount of silicate(IV) [3].
Reference: Stumm, W.; Morgan, J. J. Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters; Wiley, 1996; ISBN 0471511846.
Ques.2. Line 77: it would be useful to use mole concentration not mg/L for Al and Si
Ans. 2). The manuscript has been revised according to the comments.
Modification
Page 77: The concentration of Al(III) and Si(IV) in 5 M NaOH solution was fixed at 0.4815 M and 0.0054 M, respectively.
Ques.3: Line 91: add comma and respectfully.
Ans. 3). The manuscript has been revised according to the comments.
Modification
Page 95: where C0 and Cf represent the concentration (ppm) of silicate(IV) and aluminate(III) ion in the solution before and after the experiment, respectively.
Ques.4. Result section: include a table with the solution Si, Al, pH, Ca, Cl, analysis for the variable tests.
Ans. 4). It might be possible that pH affects the removal behavior of silicate from the aluminate solution. In previous work, we reported the optimum leaching conditions to obtain pure aluminate solution(Nguyen et al. 2018). According to these works, 5 M NaOH concentration was the optimum leaching condition and thus we fixed the concentration of NaOH in this work. Therefore, we have not investigated the effect of pH on the removal of silicate in this work.
In the case of Ca, there is no Ca(II) present in the sodium aluminate solution (analysis by ICP) after silicate (IV) removal in this work. This is the reason why the value of Ca was not mentioned in the results.
In this work, AlCl3 was used in preparing the synthetic leaching solution. Therefore, it is difficult to determine Cl in the solution which exists from synthetic leaching solution or from hydrocalumite. However, Cl can be removed during aluminum hydroxide formation by precipitation in the next work.
The concentration of Al(III) and Si(IV) before and after adsorption have been added to the manuscript.
Modification
Page 10, line 217: The change in the concentration of Al(III) and Si(IV) before and after adsorption together with the purity of Al(III) are shown in Table 1. The data in Table 1 was obtained from the following conditions: 30 g/L hydrocalumite, 50oC, 2hs, 400rpm.
Table 1. Change in the concentration of Al(III) and Si(IV) in the solution and the corresponding purity of Al(III) before and after adsorption by hydrocalumite.
|
Composition |
Al(III) |
Si(IV) |
Purity of Al(III) (%) |
|
Before adsorption (ppm) |
14954.8 |
150 |
99 |
|
After adsorption (ppm) |
15028.0 |
0.6 |
99.99 |
Ques.5. Include uncertainties in the graphs plotted.
Ans. 5). The figures have been revised according to the comments.
Ques.6. The new phase is not hydrocalumate but….. Could the phase change have been due to drying of the mineral, before analysis. Under what conditions did the drying take place
Ans. 6). The manuscript has been revised according to the comments.
Page 3, line 86- 89: The filtered residues were washed several times with distilled water until the filtrate became completely clean and then dried at 60oC for 48 h. The morphology of the residue after adsorption was investigated by Field Emission scanning electron microscopy (FE-SEM) (S- 4800, Hitachi, Tokyo, Japan).
Page 7, line 162: Besides that, the morphology of adsorbent after the adsorption of silicate(IV) was studied by scanning electron microscopy (see Figure 7). It can be observed that the shape of hydrocalumite changed from plate-like morphology and hexagonal crystallites into polyhedral crystal block. Therefore, it might be said that most hydrocalumite after absorption exhibits the phases Ca3Al2(OH)12-SiO2.
Ques.7. This process is possibly taking place but you have only 0.0016 mol Si in solution, while using 30 gram hydrocalumate, this amounts to 0.1 mol. You can then only convert about 1.5 mol % of the hydrocalumate. This does not make sense relative to the XRD in figure 6. Hydrocalumate should still be your main phase. SiO2 would also be a minor phase since it is also formed from the added silica of 150 mg/L
Ans. 7). Thank you for the comments. However, the phase change of hydrocalumite after absorption is clearly shown through XRD and SEM. Therefore, we think the XRD in figure 6 and the SEM image are necessary to demonstrate this phase change.
Modification
Page 7, line 162: Besides that, the morphology of adsorbent after the adsorption of silicate(IV) was studied by scanning electron microscopy (see Figure 7). It can be observed that the shape of hydrocalumite changed from plate-like morphology and hexagonal crystallites into polyhedral crystal block. Therefore, it might be said that most hydrocalumite after absorption exhibits the phases Ca3Al2(OH)12-SiO2.
Ques.8. Your equation 2 suggests a phase change, while the figure is only showing the ion exchange. Write the ion exchange equation as well as indicated by the figure 5.
Ans. 8). As discussed in page 6, the removal of silicate(IV) might occur at both the surface and interlayer sites through anion exchange.
Equation (2) describes silicates(IV) are adsorbed onto the surface by replacing the chloride ion on the surface of hydrocalumite. Figure 5 shows that these adsorbed silicates(IV) H2SiO42− are then intercalated into the interlayer via exchanging with the chloride ion in the interlayer. This is the reason why equation 2 suggests a phase change on the surface and Figure 5 is only showing the ion exchange in the interlayer.
Ques.9. Line 158: Need SEM photo to show the cubic crystals! What is the reason that you have quartz precipitation? Is there a drop in pH?
Ans. 9). The manuscript has been revised according to the comments.
Modification
Page 7, line 162: Besides that, the morphology of adsorbent after the adsorption of silicate(IV) was studied by scanning electron microscopy (see Figure 7). It can be observed that the shape of hydrocalumite changed from plate-like morphology and hexagonal crystallites into polyhedral crystal block. Therefore, it might be said that most hydrocalumite after absorption exhibits the phases Ca3Al2(OH)12-SiO2.
Ques.10. Line 182 and line 211: There could have been more Al removed than silica, but the accuracy of the analysis is not good, sentence needs to be changed to reflect this more properly. Common uncertainty is abut 5-10wt.% 5 percent would amount to 750 mg/L that could have been sorbed without you noticing it.
Ans. 10). Thank you for the comments. Most of the experiments were repeated several times to ensure their accuracy. In most of the case, the errors associated with the percentage of silica removal was within ± 4%. The concentration of Al (III) before and after absorption is shown in Table 1. The manuscript has been revised according to the comments.
Modification
Page 194: When reaction time was less than 2 hs, more than 20% of aluminate(III) was also removed with silicate(IV). However, the removal of aluminate(III) rapidly decreased when the reaction time was 2hs.
Page 234: In these experiments, aluminate(III) was not adsorbed onto hydrocalumite and thus pure aluminate(III) solution can be obtained.
Page 10, line 222: The mass balance and the purity of aluminate in the solution after removal of silicate(IV) are shown in Table 1.
Table 1. Change in the concentration of Al(III) and Si(IV) in the solution and the corresponding purity of Al(III) before and after adsorption by hydrocalumite.
|
Composition |
Al(III) |
Si(IV) |
Purity of Al(III) (%) |
|
Before adsorption (ppm) |
14954.8 |
150 |
99 |
|
After adsorption (ppm) |
15028.0 |
0.6 |
99.99 |
Ques.11. Line 217: this is not applicable when you have ion exchange or especially phase change. You need to have your processes understood as sorption before using this.
Ans. 11). Thank you for the comments. In this work, adsorption of silicate(IV) follows anion-exchange mechanism. Frist, silicates(IV) are adsorbed onto the surface by replacing the chloride ion on the surface of hydrocalumite. These adsorbed silicates(IV) H2SiO42− are then intercalated into the interlayer via exchanging with the chloride ion in the interlayer as illustrated in Figure 5. The removal of silicate(IV) might occur at both the surface and interlayer sites through anion exchange. The intercalation of silicates(IV) leads to the phase transformation of hydrocalumite. Adsorption isotherms are an important parameter which elucidated the interaction of adsorbent with an adsorbate.
Ques.12. Line 256-258. Remove last sentence unless better justification in the discussion.
Ans. 12). This sentence has been deleted.
Round 2
Reviewer 2 Report
There paper has been upgraded well, however, there are still some issues. 1. The table should have all the solution concentrations data that was used for the calculations resulting in the figures 4, 8, and 9.
2. The mass balance of the silica addition to the solution full conversion of hydrocalumite to the silica form with excess SiO2 has not been addresses.
3. Does the the new silica containing phase has a name? I was quick looking but could not find it?
Author Response
Thank you for the comments. The manuscript has been revised according to the comments.
Ques.1. The table should have all the solution concentrations data that was used for the calculations resulting in the figures 4, 8, and 9.
Ans. 1) Table 1 has been added to the manuscript according to the comments.
Modification
Page 10, Line 213:
Table 1 shows the concentration of Al(III) and Si(IV) after the specified adsorption treatments. The removal percentage of Al(III) and Si(IV) in Figures 4, 8, and 9 was calculated on the basis of this data.
Table 1. The concentration of Al(III) and Si(IV) in the solution after adsorption treatment with hydrocalumite. (The initial concentration of Al(III) and Si(IV) was 14954.8 ppm and 150 ppm)
Ques.2. The mass balance of the silica addition to the solution full conversion of hydrocalumite to the silica form with excess SiO2 has not been addresses.
Ans. 2). In this work, hydrocalumite was added directly to the solution for the purpose of removing silicate(IV). In this manuscript, we present only the concentration of Al (III) and Si (IV) before and after absorption. .
Ques.3: Does the the new silica containing phase has a name? I was quick looking but could not find it?
Ans. 3). In the removal of silicate(IV) by adsorption onto hydrocalumite,silicate might influence the transition of hydrocalumite to Ca3Al2(OH)12. Here, the new phase can be understood as the silicate form of Ca3Al2(OH)12 (Tricalcium Dialuminium Oxide).
