Research on the Preparation of Sinter for COREX Reduction Process by Varying Basicity and MgO Content
Round 1
Reviewer 1 Report
In this study, the researchers aimed to reduce the cost and increase the efficiency in the COREX iron production process. For this, they made the necessary tests by changing the MgO content and basicity of the sinter. I think the article is very well organized. The researchers clearly explained the experimental method and interpreted their results with scientific data. The article can be published as it is.
Author Response
We earnestly appreciate the reviewer’ warm work.
Reviewer 2 Report
- Line 20 - double in
- Analysis of the type of SFCA is not given.
- Conclusions based only on SEM observations. Phase diagrams should be analysed to show eutectics or joint crystallization areas etc.
- Conclusion about glass phase and SFCA responsible for higher strength of sinter seems not reasonable.
- Measurement of porosity is not described clearly.
- Does the testing procedure corresponds with any of ISO Standards?
- Parameters of the previously used sinter are not given.
- Cold strength of the sinter is not analysed at all.
- General economical analysis of the benefits (drawbacks) of the design sinter use should be given briefly.
Author Response
Q1:
Line 20 - double in.
Author’s response:
Revision has been made in resubmitted manuscript.
Q2:
Analysis of the type of SFCA is not given.
Author’s response:
The type of SFCA was indeed not given in the paper. According to classical sintering theories and some relative studies, the SFCA-silicoferrites of calcium and aluminum as a kind of alumosilicoferrites is formed during the cooling of high-calcium iron-silicate melts. Generally, the types of SFCA include plate alumosilicoferrite based on CF2, acicular alumosilicoferrite based on CF and C2F. All of them consist of CaO, SiO2, Al2O3 and Fe2O3, as shown in Figure 9 of original manuscript. Different types of SFCA have obvious different amounts of above components, but all of them are regard as main binding phase in sinter with high basicity. In this paper, one of the research priorities was the effect of basicity and MgO content on mineralogy changes of sinter. If we put a lot of attentions to clarify the detailed characters of each kind of SFCA, it would be another research focus on formation of SFCA. So the type of SFCA was nor discussed in the original manuscript. However, in order to clearly present description for mineralogy change, some annotation were added in section 3.3 of the revised manuscript.
Q3:
Conclusions based only on SEM observations. Phase diagrams should be analysed to show eutectics or joint crystallization areas etc.
Author’s response:
In terms of analysis for mineral formation during sintering process and various effects of different chemical composition factors on the mineral formation, a lot of studies that focus on liquidus or crystallization in the CaO–SiO2–FexO phase diagram with high temperature have reached a clear-out conclusion. Although we did not calculated the phase diagram and then further analyzed phenomena occurred in this study, the reference has been made in some sections in the paper, such as section 3.1.We thought this could also be somehow helpful for the reader to understand the effect of MgO content and basicity on the neineralogy of sinter. So the results was not analyzed by a specially calculated phase diagram.
Q4:
Conclusion about glass phase and SFCA responsible for higher strength of sinter seems not reasonable.
Author’s response:
For the strength of sinter and its main contributor, the argument has been demonstrated not only in so many existing researches or literature, but in many our relative and similar research cases. It's well known that sinter strength is related to mineralogy and microstructure, the first studies about how basicity influences the properties and mineral composition of sinter were even really established in the 1980s or earlier, such as “Fedorenko NV, Sokolevskikh GF, Rusakova AG, et al. Properties of sinter with a basicity of 0.6–3.4. Metallurgist+, 1970; 14(9):552-554”, “Malysheva TY, Yusfin YS, Mansurova NR, et al. Mechanism of mineral formation and metallurgical properties of sinter of basicity 1.1–3.1 at OAO MMK. Steel. Transl., 2007; 37(2):126-130” and so on.
As far as we know, the silicate glass phase and SFCA are main binding phase of sinter with low and high basicity respectively. They are the most relevant factors for the sinter strength. In order to avoid forming the misunderstanding in all-or-nothing way, the statements in abstract, main text and conclusions of the paper were all revised according to the comments from reviewer.
Q5:
Measurement of porosity is not described clearly.
Author’s response:
The quantitative analysis of the mineralogy of sinter was performed using the image analysis software QWin of Leica optical microscope. The mineralogical analysis of sinter involved the quantification of phases’ areas of different morphologies by point counting, so the measurement of porosity was also determined by point counting. Actually during point counting, we obtained 20 mineralogical photos which magnified the object 100 times, then each photo were counted by 100+ points, thus we counted above 2000 points per samples. The imperfect thing is that we did not consider the how much it varies between analyses and got average values. Even though, we think the results could still reflect the mineralogical changes of sinter. Corresponding part was modified in revised manuscript according to the comment.
Q6:
Does the testing procedure corresponds with any of ISO Standards?
Author’s response:
The tumble strength of sinter in this work detected according to ISO 3271-1995. For the tests of RI and RDI in COREX shaft furnace, there are no standards to refer to. Actually, reducing conditions used in the experiments for RI and RDI was entirely simulated the practical reducing condition in COREX shaft furnace. These practices are somehow referred to the standard GB/T for the test of RI in traditional ballast furnace. The test condition was regard as a simulation of reducing condition at the belly of blast furnace. Similarly, the reducing conditions in this paper were simulations of reducing condition at the upper and lower middle part of the COREX shaft furnace respectively.
Q7:
Parameters of the previously used sinter are not given
Author’s response:
Actually, the sinter previously used in industrial COREX production was somewhat like the sinter with natural MgO content (1.36%) and basicity 1.8 in the lab experiments. The MgO content of traditional sinter was approximately 1.4% and its basicity was approximately 1.8% as mentioned in the manuscript. The sintering blend structure was also basically same with the lab tests.
Q8:
Cold strength of the sinter is not analysed at all.
Author’s response:
The strength of sinter used in the industrial was not discussed in the paper, because we thought the discussion should focuses on the changes of production technology parameter of COREX shaft furnace, especially for the differential pressure of gas in the shaft furnace and metallization ratio of discharging materials. Because of fluctuations in production, the strength of sinter was also changing all the time. Generally speaking, the average strength of sinter increased by about 3 percentage points after the MgO content of sinter was raised to 2.4% from approximately 1.4%, and the basicity of sinter was raised to 2.4 from approximately 1.8%.
The changes in sinter strength during industrial was added in revised manuscript according to the comment from reviewer.
Q9:
General economical analysis of the benefits (drawbacks) of the design sinter use should be given briefly.
Author’s response:
Like the explanation to question 8, we thought the discussion should focuses on the changes of production technology parameter of COREX shaft furnace. So the general economic analysis of the benefits and drawbacks of the prepared sinter was not conducted. On the other hand, the economic indicators would be not perfectly representative in such relative short industrial testing period. The economic variation would be systematically summarized in another research.
Special thanks to you for your good comments.
We tried our best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper.
We earnestly appreciate for Editors/Reviewers’ warm work, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Reviewer 3 Report
The manuscript of Shi, Zhu and Pan entitled "Research on the preparation of sinter for COREX Reduction Process by varying basicity and MgO content" deals with the search for optimized sinter rawmix compositions to meet the desired quality parameters of the final sinter product to be applied as part of the burden mixture for iron production in COREX reduction shafts. Special attention is given to the reduction degradation indices RDI since sinter degradation is known to be a more sever issue under COREX reduction conditions compared to blast furnace operation. Furthermore, a detailed analysis is given for the variation of the sinter's mineral composition and microstructure under different basicity values. Results of this analysis are used to develop an explanation on the influence of sinter mineralogy on product strength and reduction related parameters.For these reasons the investigation definitely provides a highly relevant contribution for the journal Minerals.
In the referee's view the manuscript is written quite well stilistically, however several key concepts might be not easily to be grasped by readers not so familiar with the subject of iron-ore sintering. Therefore, some procedures and key concepts demand further specification and some additional annotations might facilitate understanding and readibility.
Page 3, line 97; page 4, line 146:
In the text it is mentioned that the return fines balance (RFB) was closed during all experiments. This is an inportant aspect, since the RFB will affect the offset between the predicted and the measured productivity during the experiments. Authors should indicate the organisation of (reciprocating?) experimental procedures needed, which had to be undertaken to achieve a closed RFB during the sinter tests while widely varying raw mix compositions.
Fig. 2:
Key sinter parameters as productivity and tumble index (TI) are indicated and discussed throughout the text; however no definition is given. At least the tumble procedure should be described briefly (ideally within section 2) or alternatively a source for the used norm should be referenced. Additionally: Please term "tumble index" explicitely in Fig. 2 - eventually in the figure's caption or alternatively in the figure's legend - before introducing the abbreviation TI.
Page 4, line 148:
This sentence offers a valuable insight into the planning of the experiments, stating that the burnt lime content (not ratio, which remained undefined somehow) added to the rawmix must be adjusted to keep basicity constant while varaying the MgO-content. However, due to fulfillment of the RFB criterion even further adjustments might have been undertaken during the MgO-variation series of experiments. To make this important point more transparent, in support for Figure 2 and related argumentation a Table should be introduced, specifying the total sinter rawmix composition exemplarily for low, medium and high MgO-content.
Page 7, beginning of section 3.3
To provide better understanding for the significance of the mineralogical analysis a short description of microscopy-sample preparation and mineral phase discrimination procedures (software tools etc.) should be inserted at the beginning of section 3.3.
Minor corrections:
Fig. 7 and 8: Please enhance readibility of the red text in the figures; (hardly readible due to font type or low graphical resolution)
line 275: bsiciry ?
Fig. 10: Statement of the gas pressure drop in the reduction shaft in terms of differential pressure should indicate the height of the grannular bed in the figure caption; alternatively the bed height could be indirectly included by ploting the pressure drop in [kPa/m].
Fig. 10, legend text should read 'Benchmark period'
Ref. 13 corrected citation:
M. Asada, M. Shima, and Y. Omori, Measurement of macro strain in the course of reduction of the skeletal hematite in sinter, Tetsu-to-Hagane, 73(1987), No. 15, p. 1901.
Author Response
Q1:
Page 3, line 97; page 4, line 146:
In the text it is mentioned that the return fines balance (RFB) was closed during all experiments. This is an inportant aspect, since the RFB will affect the offset between the predicted and the measured productivity during the experiments. Authors should indicate the organisation of (reciprocating?) experimental procedures needed, which had to be undertaken to achieve a closed RFB during the sinter tests while widely varying raw mix compositions.
Author’s response:
The return fines balance (RFB) is a very important parameter, it could even be regarded as a precondition for estimating the validity of other experimental results. The RFB was generally understood as a balance between return fines charged into sintering pot and generated from sintering pot, which means the mount of return fines in the sinter mix that charged into sintering pot should be nearly equal to the mount of return fines that generated from the sinter discharged in sintering pot.
For a better understanding of reader regards of the RFB, a brief annotation were added in the revised manuscript according to the comment from reviewer.
Q2:
Fig. 2:
Key sinter parameters as productivity and tumble index (TI) are indicated and discussed throughout the text; however no definition is given. At least the tumble procedure should be described briefly (ideally within section 2) or alternatively a source for the used norm should be referenced. Additionally: Please term "tumble index" explicitely in Fig. 2 - eventually in the figure's caption or alternatively in the figure's legend - before introducing the abbreviation TI.
Author’s response:
The calculation for sintering indexes, such as productivity, tumble index and solid fuel consumption was referred to the literature [19]:
Productivity, r=7.64*104*M/D2•t-1,where M means the mass of product sinter, D means the diameter of sinter pot and t means sintering time, tumble index, TI=M2/M1*100%, where M1 means the mass of sinter charged into drum, M2 means the mass of sinter above 6.3mm after tumbling and screening, solid fuel consumption, EC=m*(1-w)*c/M, where m means the mass of sinter mix charged into sinter pot, w means moisture of sinter mix, c means breeze addition ratio and M means the mass of product sinter).
The meaning of abbreviation TI was added in the caption of Figure 2 in revised manuscript.
Q3:
Page 4, line 148:
This sentence offers a valuable insight into the planning of the experiments, stating that the burnt lime content (not ratio, which remained undefined somehow) added to the rawmix must be adjusted to keep basicity constant while varaying the MgO-content. However, due to fulfillment of the RFB criterion even further adjustments might have been undertaken during the MgO-variation series of experiments. To make this important point more transparent, in support for Figure 2 and related argumentation a Table should be introduced, specifying the total sinter rawmix composition exemplarily for low, medium and high MgO-content.
Author’s response:
A Table was added in section 3.1. This table showed the variation in contents of burnt lime and dolomite in the sinter mix when the MgO content of sinter changed.
Q4:
Page 7, beginning of section 3.3
To provide better understanding for the significance of the mineralogical analysis a short description of microscopy-sample preparation and mineral phase discrimination procedures (software tools etc.) should be inserted at the beginning of section 3.3.
Author’s response:
The microstructure and mineral compositions of different sinter were investigated by means of microscope analysis(Leica DMI5000M, German) and scanning electron microscopy analysis(FEI Quanta200,Netherlandish). The mineralogical analysis of sinter involved the quantification of phases’ areas of different morphologies by point counting. The description was added in the Method section in revised manuscript.
Q5:
Fig. 7 and 8: Please enhance readability of the red text in the figures; (hardly readible due to font type or low graphical resolution)
Author’s response:
The figure7 and figure were replaced in revised manuscript.
Q6:
line 275: bsiciry ?
Author’s response:
The spelling mistake has been modified.
Q7:
Fig. 10: Statement of the gas pressure drop in the reduction shaft in terms of differential pressure should indicate the height of the grannular bed in the figure caption; alternatively the bed height could be indirectly included by ploting the pressure drop in [kPa/m].
Author’s response:
The inner space of shaft furnace was 17m in height and the stock column during reducing stage was 11m in height. The indication of the height of charge column in the COREX shaft furnace.
Q8:
Fig. 10, legend text should read 'Benchmark period'
Author’s response:
Figure 10 has been revised in latest manuscript version.
Q9:
Ref. 13 corrected citation:
- Asada, M. Shima, and Y. Omori, Measurement of macro strain in the course of reduction of the skeletal hematite in sinter, Tetsu-to-Hagane, 73(1987), No. 15, p. 1901.
Author’s response:
The correction of reference 13 has been made in revised manuscript.
Special thanks to you for your good comments.
We tried our best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper.
We earnestly appreciate for Editors/Reviewers’ warm work, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Round 2
Reviewer 2 Report
I am satisfied with the job done by authors. It is very good they understand and accept my comments.
