The Formation and Disintegration of Rafts from Different Aluminas and Fines

Round 1

Reviewer 1 Report

Dissolution of alumina feeding in a cryolitic melt is an important problem in aluminium industry because of complexity of the process and urgent need to shorten the process. The presented research are well designed, thoroughly performed and clearly described. The unexpected behaviour of A alumina dissolution showed that not all physicochemical properties of aluminas were considered. I would suggest closer insight into surface properties of alumina particles, aglommerates in further studies.

In the present studies, caption of Fig.2 should include the meaning of "S" in Fig.2a like for the other symbols. 

In order to compare images of rafts formed from bulk  and fine aluminas (Fig. 8 and 9) I would like to see pictures from the same type of alumina.

Finaly, I would like to know your general conclusion if the type of alumina (fine or bulk) shows more decisive effect on rafts dissolution than process paratemetrs.

Author Response

Point 1:
Dissolution of alumina feeding in a cryolitic melt is an important problem in aluminium industry because of complexity of the process and urgent need to shorten the process. The presented research are well designed, thoroughly performed and clearly described. The unexpected behaviour of A alumina dissolution showed that not all physicochemical properties of aluminas were considered. I would suggest closer insight into surface properties of alumina particles, aglommerates in further studies.
Response 1: Thank you. It is true that the surface properties are not investigated, which might be investigated further. A sentence regarding this is added to further work.

Point 2:
In the present studies, caption of Fig.2 should include the meaning of "S" in Fig.2a like for the other symbols.
Response 2: It was an error in the caption, where “S” was replaced with “L”. The caption is now edited.

Point 3:
In order to compare images of rafts formed from bulk  and fine aluminas (Fig. 8 and 9) I would like to see pictures from the same type of alumina.
Response: The structure of the rafts between the different types A, B and C were not found to vary in any significant degree, and we therefore chose to only choose the images of highest interest, rather than choosing the same type. A sentence commenting the small differences between the types is added.  

Point 4:
Finally, I would like to know your general conclusion if the type of alumina (fine or bulk) shows more decisive effect on rafts dissolution than process parameters.
Response: As mentioned in the introduction, studies suggest that the operational conditions have a larger impact on dissolution of alumina than its properties. The same will probably also be true for raft dissolution, although there are limited studies on this field. This is commented in the last paragraph of the conclusions.

Reviewer 2 Report

The novelty of the contents of the paper is in good quality images of the experimental observations for different types of alumina feed. However dynamic nature of the disintegration and dissolution could be enhanced giving more information of the raft motion and possible interaction with the previously added raft structures. If the raft would stay completely still, there would be a region of saturated liquid solution around it greatly reducing the raft dissolution. The convective motion is known to affect the dissolution significantly in addition to the previous observation. Nevertheless, we appreciate the detailed and instructive presentation given in the paper. Additionally, we could desire more information on the experimental conditions, for instance the rate of the feeding intervals appears to be missing. As noted previously, the raft motion in the experimental cell needs to be more explained, as there is no possibility of adding new feed directly on the existing raft. In summary, the paper is well presented, contains useful results, illustrated by good quality pictures and graphs, therefore recommended for publication with minor enhancements.

Author Response

Point 1:
The novelty of the contents of the paper is in good quality images of the experimental observations for different types of alumina feed. However dynamic nature of the disintegration and dissolution could be enhanced giving more information of the raft motion and possible interaction with the previously added raft structures.
Response: Thank you for a good review. The recording and sampling experiments were conducted separately from each other, as the sampler hindered the view of the surface. That the sampling and recording experiments were conducted separately is now mentioned in the experimental part. It is not possible to observe raft motion during the sampling campaign, as the top of insulated in order to preserve heat. There is no raft structure left in the bath when a new dose of alumina is added, as the structure is removed from the bath, and extra measures were taken before addition of a new dose. This is now more clearly stated in the experimental section.

Point 2:
If the raft would stay completely still, there would be a region of saturated liquid solution around it greatly reducing the raft dissolution. The convective motion is known to affect the dissolution significantly in addition to the previous observation. Nevertheless, we appreciate the detailed and instructive presentation given in the paper.
Response: We agree, and this might be a possible explanation for the slow dissolution observed in the recordings, in addition to low superheat. This is now explicitly stated in the discussion path. For the sampling experiments, removal and addition of samplers will stir the bath, and it is therefore believed that the concentration is close to constant before each feeding.

Point 3:
Additionally, we could desire more information on the experimental conditions, for instance the rate of the feeding intervals appears to be missing.
Response: As mentioned in point 1, temperature and clean bath were the main requirements for adding a new dose. It is therefore no constant rate of feeding intervals, but it was around 30-45 minutes. This is also stated in the experimental section.

Point 4:
As noted previously, the raft motion in the experimental cell needs to be more explained, as there is no possibility of adding new feed directly on the existing raft.
Response: As mentioned in point 1, extra measures were taken prior to addition to avoid the described case.