The Effect of Reaction Conditions and Presence of Magnesium on the Crystallization of Nickel Sulfate
Round 1
Reviewer 1 Report
The manuscript reports results from the study of temperature, seeding, and the presence of magnesium on the crystallization of nickel sulfate. Most significantly, experimental data indicate that magnesium promotes NSH7 formation in lieu of NSH6 polymorphs and significantly affects the stability of different phases in the dry state, making it difficult to optimize the crystallization process towards a stable product.
The manuscript is acceptable for publication after minor revision.
1. Page 6, Table 1. Duplicated entries for ‘T = 25°C’. Change the second one to ‘T = 70°C’.
2. Page 7. Delete the sentence in lines 241-242.
3. Page 7, line 251. Change ‘product’ to ‘mixture’.
4. Fix the link for the references to Tables and Figures on page 8, line 271; page 9, lines 287-288; page 9, lines 293-294; page 10 line 232; page 13, lines 433-434; page 13, 437; page 13, 465-466.
5. Pages 15-16. Reformat the references to the format used for Crystals Crystals | Instructions for Authors (mdpi.com).
6. Pages 15-16. Incomplete references with missing page numbers for 6,13,14, 17, and 26.
Author Response
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Reviewer 2 Report
The effect of reaction conditions and presence of magnesium on the crystallization of nickel sulfate
On page 13/16, in chapter 4.2. Effect of magnesium, I think something is missing
“Highest the highest initial concentration of Mg (18 mg g-1), the product contained phase pure NSH7 ( Figure???). It also must… ”
Further suggestions
- Study for crystallization of nickel sulphate and at other temperatures and determining the optimal crystallization temperature
- Extending the conclusions
Scientific characteristics
- Recycling of valuable metals (nickel is an important component in a wide range of materials such as stainless steel)
- Crystallization of nickel sulphate
- Transformation of metastable phases both in suspension and in dry state
Novelty and interest to the readers
- Sketch of the experimental set-up used for crystallization of nickel sulphate
- XRD spectra of phase pure α-NiSO46H2O seeds
- XRD spectra of nickel sulfate crystals produced with 0.2 g -NiSO4.6H2O seeding and without impurities at 25 °C and 70 °C
- Crystallization of nickel sulfate in the presence of magnesium
- Effect of temperature and seed amount in nickel sulfate crystallization
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 3 Report
The research topic is of significance and the reported work is relevant. Nevertheless some clarifications are necessary to ease the reading and to reinforce the conclusions.
Lines 112-117: The authors argue the properties expected from a salt of nickel sulfate for a better stability and easier storage. The authors should explicitly explain here why they selected the alpha-hexahydrate form as seeds for crystallisation.
In the subsection 2.2.1., the authors describe the preparation of alpha-hexahydrate seeds. The purity of the product is checked over time (5 days, 12 days and 4 months) by powder X-ray diffraction. The diffractograms are reported in figure 3. The authors seem to explain the crystal form obtained is pure and corresponds to the alpha-hexahydrate form. However, the diffractograms are not qualitatively identical as some peaks seem to disappear with time (For instance, around 14 deg., a peak disappears with time). The theoretical diffractogram of the alpha-hexahydrate form may help or, if necessary, a superimposing of the theoretical diffractograms.
Note: In supplementary materials, the theoretical diffractograms of the three hydrates are given but the temperature is not specified.
In the subsection 2.2.7. “Phase transformation in solution and in solid-state”, the authors explain how the phase transformation had been monitored by placing a drop of solution on a microscope glass slide.
It is not clear to me if the observation of the phase transition under the microscope is done at the same temperature as the seeding of the solution (25 deg C or 70 deg C).
Is the observation done at RT to discuss the condition of storage after extraction from the solution, or is it done at the temperature of seeding to follow the phase transition in solution?
If it is done at RT, what is the evolution of the crystals in the solution at seeding temperature? Why extracting crystals after 10 min?
In the subsection 3.1.2. “Effect of seed amount”, the authors discuss the effect of an increase of seed amount on the resulting crystals. To me, one interesting information is missing here, it is the mass of crystals produced after 10 min. Is the product mass is dramatically larger than the seed mass? The seed amount alters the mix of produced crystals at 25 deg C., but is there more crystals produced? It is to me a very important point, the ration of the mass of produced crystals over seed mass is a good indicator of the crystallization technique yield.
In subsection 3.2.1. “Effect of initial Mg content”, the authors should remind the readers the temperature and the seed amount considered here.
Did the authors try to perform model refinement to obtain an estimation of the different crystal form fractions in the product?
The composition of the crystalline product changes with the Mg content, but is it also time-dependant? High Mg addition promotes the heptahydrate form at 25 deg. C, but if the sampling is done after 20 or 30 min, can the beta-hexahydrate be found in the sample? Did the authors try extraction after a longer time delay?
In the subsection 3.3.1. “Phase transformation in suspension”, sample samples are taken from solutions and put under a microscope to observe potential phase transition. This subsection should be clarified as it is not obvious if the sample under microscope is kept at a controlled temperature.
Minor corrections:
Line 92: “the rate of dissolution of a metastable form may be retarded” What do you mean here? Do you mean “may decrease”?
The table 1 summaries the test conditions. There is probably a mistake as both temperatures are 25 deg C. I guess the second one should be 70 deg C.
At line 253, the number of the figure is missing.
At line 261, “…in our work correspond well with…”
At line 271, problem with reference to figure
In subsection 3.2.1. “Effect of initial Mg content”, several problems of format and figure reference can be noticed.
At line 322, error in reference
At line 383, missing figure number
At line 391, missing figure number
At line 392, “0.2 g of alpha-NSH6”
At line 405, error in reference
At line 433, error in reference
At line 437, error in reference
At line 465, error in reference
Author Response
Please see the attachment
Author Response File: Author Response.pdf
