Enhancing Fines Recovery by Hybrid Flotation Column and Mixed Collectors

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COMMENT: Flotation is a bit energy consuming process, and air/bubblesgeneration takes significant part of the energy consumption, adding electrolysis to the column to produce smaller bubbles willincrease the energy consumption.

ANSWER: The comment of the reviewer is considered an the following text is included in the manuscript: It is believed that employing electrolysis for micro-bubble generation in support of fine particle flotation enhancement will be used economically, even on an industrial scale, by employing alternative energy resources from renewable processes e.g., solar power. Therefore, research must continue a  larger scale.

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COMMENT:134 – the Electrolysis cell should be described, not only referenced.

ANSWER: The electrolysis unit is described in the text (red fond): The electrodes were placed horizontally and parallel and the unit is supported on the inner wall of the flotation column 6 cm above the columns bottom (Photo 2) so as the produced microbubbles can be dispersed homogeneously in the column’s volume. Electrolysis unit is connected to an external power supply.

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COMMENT: Stable tests, with no downflow of the water with suspended material. How this can be scaled up to continuous flow system?

ANSWER: The goal of the present study was the investigation of the microbubbles’ role on the enhancement of fine particles recovery. As a second step, continuous flow experiments are planned as a future work.

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COMMENT:What is the energy consumption of the system, and calculated per

mass unit of floated material?

ANSWER: Several published papers mention the energy consumption. However, in this study the concept is based on renewable energy sources e.g., solar power.

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COMMENT:How the size of the microbubbles (190) was obtained?

ANSWER: The following paragraph has been included in the manuscript with corresponding reference: The size of bubbles is determined by capturing bubble images using a high-resolution digital camera (a 20MP Canon EOS 70) equipped with macro lenses and extension tubes for efficient image magnification A custom made image analysis software (BubbleSEdit software) is used to automatically detect the bubbles contour and measure their size and consequently obtain the corresponding bubble size distributions. It is worth mentioning that in order to calculate the size of the bubbles from the photos taken it is necessary to convert the pixels to μm.

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COMMENT: The pH range 2-12 covers the acidic solution, so MgCO3 will be eatched to MgCl soluble salt and CO2 will be released. Was this considered? 215 – perhaps this is the reason!

ANSWER: The following sentence has been included at the corresponding paragraph: The solubility of magnesite in acidic pH values cannot be excluded.

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COMMENT:Use of both big and micro bubbles should increase the recovery

as turbulence occurs from big bubbles. And with only smallbubbles turbulence is very limited.

ANSWER:  We agree with the reviewer that the large bubbles are necessary to create turbulence and move the slurry. A clarification has been added to the revised manuscript. Comment 7 can be combined with comment 17 and the following paragraph has been included on manuscript: The recovery rate of the finest particles is faster and this is attributed to the fact that flotation columns favor the flotation of finer particles due to the fact that they produce bubbles with smaller diameters (spargers) and in addition, a milder turbulent flow field prevails. Fine and ultrafine particles are more likely to attach to smaller bubbles and rise to the froth phase, whereas in presence of larger bubbles they tend to follow the flow lines around them. However, their existence is important for creating turbulence increasing the frequency of bubble particle collisions and for moving the slurry. First and second order flotation kinetic models were fitted to the experimental data of Figure 6 and the results are given on Table 1. The second-order kinetic model failed to fit the results and therefore the corresponding model parameters are not presented."

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COMMENT:230-232 the conclusion is not clear for me, not supported by

evidence. I would say it is other way around!

ANSWER: the sentence has been corrected as: Adsorption of the collector on magnesite surface is not favored in acidic conditions, so a very small percentage of particles recover.

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COMMENT:239 – optimum is not maximum! Optimal is in function of efficiency.

ANSWER: We replaced optimum by maximum.

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COMMENT:252 the figures are not clear in small spectrum of the graph,

additional right scale should be added and present the data, oradding the values on the graph, allowing precise presentation ofvalues.

ANSWER:  The figures have been extended to be more clear

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COMMENT:257 – what are the parameters of the electrolysis? Amperage,

volts, power, surface, amount of electrolysed water?

ANSWER: The following paragraph has been included in the manuscript: The current density (J) of the electrolysis unit (I=0.1 Ampere, a= 50 mm = 5 cm , b= 25 mm = 2.5 cm, c= 1 mm = 0.1 cm) is: J= 2 x (current intensity/ surface area) = 2 x (0.1 A / 26.25 cm² )= 0.008 A/cm² = 80 A/m², where a,b,c are the dimensions of the electrodes. The voltage used was 15 V and the volume of electrolysed water was 1,750L.

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COMMENT: What was the temperature of the solution during the experiments?

ANSWER: The following sentence has been included in the manuscript: Experiments were performed at room temperature (RT; ~20-25 °C).

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COMMENT:282 – it is absurd to increase the salt amount in the process water

to allow micro use of electrolysis, in industrial practice it will not bepossible. This is only reasonable for lab tests.

ANSWER: The following sentence has been included in the manuscript: In industrial scale the salt usage expense could be equilibrated by the higher floated material and/or possibility of using seawater, recycled water or renewable energy sources.

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COMMENT:300 – particle size analysis should be presented also before the

flotation, ie on the laser particle analyser.

ANSWER: The following sentence has been included in the manuscript: Τhe technique of laser diffraction to measure the particle size and particle size distribution of materials. This is achieved by measuring the intensity of light scattered as a laser beam passes through a dispersed particulate sample. This data is then analyzed to calculate the size of the particles that created the scattering pattern.

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COMMENT:Future 4 why it is volume % ? Not mass? Or share of total?

ANSWER: Laser diffraction uses Mie theory of light scattering to calculate the particle size distribution, assuming a volume equivalent sphere model.

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COMMENT:318 why 2 arrows? And one picture enlarged? And 2 boxes? What

is the magnification, scale, size of the pictures?

ANSWER:Two arrows were used to indicate that the photo on the right is a magnification of the first photo. The two boxes on Fig. 5b were used to show the hetero-aggragates formed and only one was enlarged just to ostensive show the formation of the microbubbles-particles aggragates.

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COMMENT:331 no space between words.

ANSWER:Corrected on the manuscript.

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COMMENT:335 – You are missing the collision effect, in the stady conditions.

Only big bubbles moves the slurry. To effectively flote the slurrythere has to be energetic interaction between the particles and thebubbles.

ANSWER: The current comment has been answered on comment 7.

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COMMENT:350 whree is the calculation of the model? And the model

presentation?

ANSWER: First order model R=Rmax∗(1-e kt) k corresponds to the first-order rate constant (deter-ministic) and Rmax to the maximum achievable recovery at t → ∞ [56].

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COMMENT:What is the bubbles size distribution when generated? And how

does it affect the recovery of which particles, this would be verry

interesting for next article.

ANSWER: In the current research no coalescence of the bubbles was considered, to that end it is considered that the the bubbles size distribution when generated was that presented. Of course, further investigation regarding the hydrodynamics aspects is a future work planned in the laboratory.

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COMMENT:392 – figures should be presented more clearly, one or max 2 side

by side. This is mess.

ANSWER: Figure 8 has been converted in the manuscript

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COMMENT:What is the BDDHC

ANSWER:BDDHC stands for Boron Doped Diamond Hybrid Column

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COMMENT:430-456 this is summary not Discussion.

ANSWER: Section 4 has been change to Conclusions.

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COMMENT: If 397 – 424 would relate to presented data on figured around 392

this could be discussion.

ANSWER: The paragraph “Mainly, carbonate minerals are recovered by using anionic collectors..” which may be more introductory has been moved before the Figure 8.

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COMMENT:438 this is not optimal as no optimization was carried out. And no

optimization parameters and goals was selected and min/maxcalculated.

ANSWER:  The word optimal has been changed to maximum

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COMMENT:445 – weakly presented nothing about kinetic study.

ANSWER: The kinetic study has been presented on paragraph3.6

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COMMENT:450 – again, this is not optimal. It is just “best” result.

ANSWER: it is corrected on the manuscript

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COMMENT: I am afraid these lines are too general. Suggest omitting general

information. Maybe to introduce the problem in the beginning of

the abstract to make readers to understand the importance of the

further research.

ANSWER: the following sentence has been added on the Abstract section: Moreover, fine particles require higher reagent consumption due to the fact that have higher specific area and finally their flotation is limited by low kinetic energy.

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COMMENT: Readers will appreciate if authors describe the samplespreparation procedures to flotation tests.

Also, please mention the masses of the samples that were usedfor flotation tests, the solid content in the suspension and flotationconditions (pH, reagent dosages (g/L) or concentration of theirsolutions, purity of reagents). Please specify if you used distilled,tap or process water in the tests.

Readers will appreciate if authors describe the procedure forflotation kinetics tests as the flotation kinetics findings arepresented in the “Results” section.

The paper might benefit if authors state the number of replicas inthe tests, mean and SD.

ANSWER: The preparation of the experiments and the flotation conditions are given in detail in the  manuscript in the Materials and methods section. The following paragraph has been added in the manuscript: Moreover, flotation experiments were performed in triplicate, and the values represent the mean value of independent experiments. The obtained data were presented as average and standard error mean (SEM) values of multiple sets of independent measurements. Recovery percentages and SEMs were calculated for each individual group.

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COMMENT: Fig.1 Readers will appreciate if authors explain the reasons for

stabilization of flotation recovery at conditioning times of 20 min

and 25 min

ANSWER:The corresponding experiments have depicted that less than 20 min fines recovery did not reach the maximum recovery value and moreover, when treatment exceeded 20-25 min recovery value reached a plateau. The following sentence is added in the manuscript: It is considered that 20 min is the time required for the electrolytic bubbles to act as selective flocculants, promoting the formation of large aggregates, which are easier to float by dispersed-air bubbles.

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COMMENT:The paper will benefit if author discuss the reasons for differences

in the effects of pH and SO concentration on flotation recovery.

ANSWER: the following has been added in the manuscript:  When pH values are lower than adsorption of sodium oleate (anionic collector) does not occur effectively, so flotation is not promoted at pH values lower than 6due to the fact that the surface is positive charged. This is probably because adsorption of the collector on magnesite surface occurs possibly through chemisorption.

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COMMENT:Does Fig.4 show volume or mass particle size distribution? Please

specify the apparatus and procedures the were used in the study

to obtain particles size distribution.

ANSWER: the following comment is answered on paragraph 3.4

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COMMENT:would recommend to discuss in more detail the findings in the

Fig. 5 as it represents the strong points of the research. Also,please specify in the “Materials and Methods” section theapparatus and procedures that were used for flotation mechanisminvestigations.

ANSWER:The apparatus used for capturing the photos is the same used for capturing bubbles size and is described on paragraph 3.1.

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COMMENT:Iwould recommend to simplify the description of the Fig.8. Also,

there are too many graphs in the figure.In the current state readers are spending too much time tounderstand the meaning of the description and to compare theresults. This might distract readers from the further reading of the

paper. Maybe to split Fig. 8 into two separate figures or to

rearrange data in more simple way?

ANSWER: Figure 8 has been rearranged in the manuscript.

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COMMENT:I would recommend to add this text in the "Results and

Discussion" section.

Also, please add the "Conclusions" section just after the "Results

and Discussion" section and state the main findings.

The electroflotation is a well-known technology, however, it lacks

of economic efficiency. Maybe to add a paragraph describing

economic evaluation of the hybrid flotation column, or at least

mentioning this issue in the end of the paper.

ANSWER: Section 4 has been changed to Conclusions. Moreover, the economic efficiency of the procedure is added on the las paragraph of Introduction.