Phase Equilibrium Diagram for Electric Arc Furnace Slag Optimization in High Alloyed Chromium Stainless Steelmaking

Round 1

Reviewer 1 Report

The structure of this paper is very good, and it is well written and presented. I dont feel fully able to completely judge some aspects of the chemical metallurgy, but the methodology looks ok, (although there is no information as to how the compositions of the 3 EAF slag samples were measured), the findings look sound, and appear to be supported by evidence, and the section on validation using three different EAF slags is interesting because it illustrates how this study could potentially be used in an industrial setting. My only real question for the authors would be about the novelty of this work. Have similar attempts been made before, and if so, what therefore is the particular novelty of this study ? Could it be applied for real time feedback ? I would like the authors to address this at a relevant point in the paper. It would also have been useful to have information about the different EAF process strategies that supplied the three slag samples, although I appreciate this is not possible because of confidentiality reasons.

Author Response

Dear reviewer,

thank you for your helpful comments.

Concerning your comment on novelty: Diagrams for carbon steelmaking slags are not applicable für Cr stainless steelmaking due to very different slag compositions. Proposed diagrams are based on rather rough simplifications or projections to teh slag compositions. With the phase equilibrium approach, we provide now a diagram that is very close to the real slag compositions in CaO-MgO-SiO2-Al2O3-Cr2O3. We added a paragraph in the discussion section:

>>>

Published slag diagrams for carbon steelmaking slags (e.g. isothermal stability diagrams [1] or phase equilibrium diagrams [12]) are not appropriate for Cr stainless steelmaking due to the different FeO content. The proposed diagram in Figures 4-6 can be applied at the stainless steel plants in order to monitor the slag operation, to check strategy of slag operation and to adjust slag former addition.

>>>

Concerning process strategy and details: We have the permission from the 3 steel plants to use their slag data, but we were asked not to provide other process details in the paper.

Best regards

 

Reviewer 2 Report

Phase equilibrium diagram for EAF slag optimization in high alloyed Chromium stainless steelmaking by Marcus Kirschen, In-Ho Jung and Gernot Hackl manuscript is a demonstration of the validity of the multicomponent phase diagrams calculated with a specific thermodynamic software (FactSage 7.3) for the monitoring stainless slag chemistry. Different EAF were compared with the calculated diagrams.

The manuscript has valuable data and it’s very interesting, but the level of novelty is not so high. Anyway I suggest to accept the manuscript in the present way.

Author Response

Dear reviewer,

thank you for your valuable comments.

We addressed the level of novelty with an additional sentence in the discussion. Applying Factsage calculations is not that sophisticated, I agree, we want to provide a precise diagram to the steelmakers to improve their operation.

>>>

Published slag diagrams for carbon steelmaking slags (e.g. isothermal stability diagrams [1] or phase equilibrium diagrams [12]) are not appropriate for Cr stainless steelmaking due to the different FeO content. The proposed diagram in Figures 4-6 can be applied at the stainless steel plants in order to monitor the slag operation, to check strategy of slag operation and to adjust slag former addition.

>>>

Best regards

 

Reviewer 3 Report

In this work, it is well demonstrated that the multicomponent phase diagrams calculated from FactSage thermodynamic software are very valuable for the purpose of monitoring stainless slag chemistry. The plant data from three different EAF operations were compared with the calculated diagrams and the impact of the slag chemistry to operation and valorisation of slag recycling were discussed. Since the design of experiment, data collection, data analysis and modelling have been carried out carefully this paper can be accepted in the current form.

Author Response

Dear reviewer,

thank you for your supporting comments.

Best regards

Reviewer 4 Report

Dear authors,

here is a few recommendations:

In introduction (Line 46) or in chap.2: Please, be so kind and describe more precisely the principle (basics) of reduction of Cr₂O₃ (resp. Cr₃O₄) from slag and the principle of origination of C₂S using chemical formulas.

Table 1. – Please, add the explanation of “n.a.”

Line 176: Could you explain more clearly how did you reach the red line of MgO saturation?

Figure 3: Please, use more clearly lines in diagrams with different content of Cr₂O_3.

Thank you for cooperation

 

Reviewer

Author Response

Dear reviewer,

thank you for your helpful comments. We corrected the manuscript accordingly:

1. the introduction section was indeed not precise and a bit confusing. We rewrote the section and added 2 references on CrOx reduction:

>>>

The ratio of the relevant Cr oxides in the slag, CrO and Cr₂O₃, depends on temperature and oxygen partial pressure. In this paper, we use Cr₂O₃ as synonym for both Cr oxides in the slag, due to the dominant Cr₂O₃ species at low oxygen partial pressure in the molten metal. An option to control Cr oxidation and to decrease Cr losses in the EAF is keeping elevated C and Si levels in the steel melt by decreased oxygen injection [1] and mixing of the steel melt with the Cr₂O₃-bearing slag during tapping. High slag basicity, and pronounced mixing dynamics during tapping via spout is known to improve Cr₂O₃ reduction [2]. In case of elevated final Cr₂O₃ concentrations, reducing agents as FeSi, C, or Al may be added before tapping in order further reduce Cr₂O₃ and to liquefy the oversaturated stiff slag by lowering the basicity of slag. Procedures for the optimum Cr recovery are still under discussion. For example, the combination of FeSi and carbon as reducing agents have been shown the best results on final Cr₂O₃ content of the slag [3]. Mees, however, found best efficiency of Cr₂O₃ reduction with Al addition [4]. In the 2-step...

>>>

C2S now explained in line 76

n.a. explained at Table 1 footnote (not available)

A new sentence added how to reach MgO saturation (line 186-188):

>>>

The MgO level of the slag depends on the addition of MgO-bearing slag former besides lime, e.g. doloma. At significantly undersaturated MgO slag composition, the dissolution of the MgO hearth lining may increase the MgO content of the slag until saturation is achieved. 

>>>

Figure 3: The 3 saturation are very close and difficult to distuingiush which is the relevant point of Figure 3. I used colors now instead of dashes for better quality.

Best regards