Free Access
Issue
Metall. Res. Technol.
Volume 117, Number 2, 2020
Article Number 205
Number of page(s) 4
DOI https://doi.org/10.1051/metal/2020018
Published online 13 April 2020

© EDP Sciences, 2020

1 Introduction

In recent years, the use of limestone instead of lime in the process of converter steelmaking has attracted attention of many researchers [18]. A static model for converter steelmaking using limestone has been established and the optimal limestone substitution ratio was obtained [5]. The hydraulic model of converter powder injection was developed in order to study the behavior of powder injection from the bottom and top as well as the penetration depth of the powders [68].

In particular, Deng et al. studied the decomposition behavior of limestone in slag at 1600 °C [3] and found that the limestone decomposition in the converter slag was slow due to the formation of dense dicalcium silicate layers. These layers affect the heat transfer in the slag and thus limit the rate of its decomposition. Wang et al. also studied the decomposition behavior of limestone in slag and found that the decomposition of limestone in the converter was controlled both by heat transfer and reaction. Therefore, they determined the corresponding reaction rate constant and the thermal conductivity of the product layer [4].

Even though the above-described results were obtained using limestone particles, they still may not provide accurate guidance for industrial converter steelmaking process, because in industrial conditions, the heat transfer of limestone in the converter slag may be affected by the heat transfer from all sides, rather than by the heat transfer from the hot metal only. To address this issue, the rotating cylinder method which is commonly used to study the behavior of slagging and can overcome the limitations of uneven heat transfer in experimental set up [9,10] was used in the present study to examine limestone decomposition in the converter steelmaking process.

2 Experimental materials

The limestone used as the raw material in this study was purchased from the Shenyang Company (China), and cylindrical limestone samples with the dimensions of Φ 14 mm × 50 mm were obtained using a drilling machine.

Lime dissolves rapidly in slags with high FeO content [4]. Therefore, the slag composition is selected so as to obtain ferruginous slag phase. In addition, considering that the limestone should be added to the converter from the beginning of the early stage of the conversion until 1/3 of the converter blowing time, the slag basicity in the early stage of converter, namely binary basicity R = 1.0, was selected. Based on the above two factors and the CaO-FetO-SiO2 ternary phase diagram [11], the composition of the selected synthetic slag was obtained as shown in Table 1.

In this work, the effects of the temperature, rotation speed and reaction time on the thickness of the calcined limestone layer were investigated. Since the converter steelmaking temperature is generally higher than 1250 °C, the temperatures of 1250 °C, 1300 °C and 1350 °C were selected for the experiment. The limestone samples were rotated at the speeds of 0 rpm and 150 rpm, respectively. To ensure the accuracy and continuity of the experimental sampling, the reaction time in the rotation experiment was varied between 30 and 90 s at the intervals of 15 s. Similarly, the reaction time in the static experiment varied between 30 and 120 s. At the end of the experiment, the microstructures of the decomposed limestone samples obtained with different reaction times were examined by scanning electron microscopy (SEM-EDS, Ultra Plus; Carl Zeiss GmbH, Jena, Germany).

Table 1

Composition of slag in the experiment (wt%).

3 Experimental results and analysis

3.1 Results of limestone decomposition in converter slag

Figure 1 shows the cross-section of the limestone sample after the limestone decomposition in the converter slag for 180 s. An examination of Figure 1 shows that the limestone sample can be divided into two regions. The outer region is approximately ring-shaped and the undecomposed region is approximately coin-shaped. Additionally, the decomposed lime was wrapped by the converter slag on its outer layer, most likely due to relatively high viscosity of the slag.

The products of calcination in converter slag were analyzed by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). Figure 2 shows the SEM image of the microstructure of the limestone decomposed in the converter slag for 180 s. Going from the outer to the inner region of the sample, three layers are observed, namely the white reacted layer that can be divided into the purely white complete slag-forming layer and the partial slag-forming layer (for the total reaction layer thickness of approximately 600 μm), the lime layer and the undecomposed limestone layer.

EDS line scan was performed on the limestone sample, and the results are shown in Figure 3. It is observed that when limestone was calcined in the converter early slag for 180 s, iron was found on the outside of the calcined limestone layer with a graded concentration that decreases from the outer to the inner region of the sample. A peak at approximately 10 μm and a dip at approximately 520 μm are observed for the iron content. Therefore, it is inferred that iron diffused roughly to the depth of 520 μm from the outer region to the inner region. This also implies that the decomposed limestone has good porosity. As is well known, Fe diffusion into the lime layer promotes lime dissolution and slagging. Therefore, it is plausible that limestone can be used instead of lime in converter steelmaking.

thumbnail Fig. 1

Cross-section of decomposed limestone at 1300 °C in converter slag for 180 s.

thumbnail Fig. 2

SEM image after limestone decomposition in 1300 °C slag for 180 s.

thumbnail Fig. 3

Counts for Fe by using line scanning after limestone decomposition in the 1300 °C converter slag for 180 s.

3.2 Limestone decomposition mechanism in converter slag

The lime layer is the intermediate layer between the reacted layer and the remaining limestone. Figure 4 shows the thickness variation of the calcined lime layer obtained at different temperatures and rotational speeds.

As shown in Figure 4, for the rotational speeds of both 0 and 150 rpm, the thickness of the calcined layer increases gradually with the reaction time. An examination of the data presented in Figure 4 also shows that for a given reaction time, the thickness of the calcined layer increases with the temperature. Therefore, considering the influence of both the sample rotation speed and the slag temperature on the thickness of the calcining layer, it can be concluded that the rotation speed has a small effect on the calcining of limestone, which is mainly affected, by slag temperature.

Similarly, previous studies have found that for the decomposition of limestone in hot metal, the rotation of the sample does not affect the calcination process [4,12]. Therefore, in the further analysis of the kinetic parameters of the limestone calcination presented below, only the sample decomposition data in the converter slag under static conditions are analyzed.

The relationship between the spherical limestone conversion ratio and time can be expressed as [12]: (1) where r0 is the initial radius of the sample, m; f is the conversion ratio (the ratio of the mass of decomposed limestone to the mass of limestone at the beginning of the decomposition); θ is the time, s; and KL is the kinetic parameter, m2/s.

The method used in previous studies was adopted [12] to obtain the kinetic parameters of limestone decomposition in the converter slag at 1250 °C, 1300 °C and 1350 °C, and were 2.41 × 10−7, 2.74 × 10−7, 3.26 × 10−7, respectively. All of the kinetic parameter values are slightly smaller than the corresponding values for the limestone decomposition in the hot metal at 1250 °C, 1300 °C and 1350 °C (2.66 × 10−7, 3.85 × 10−7, 4.73 × 10−7) [12]. This may be due to the faster heat transfer in hot metal compared to that in the slag. Then, the curves for the evolution of the spherical limestone (d = 14 mm) conversion under different conditions were obtained as shown in Figure 5.

The oxygen blowing time of the converter is 12∼18 min, and the first batch of slag is generally required to be melted in 4∼6 min [13]. Therefore, limestone decomposition should be completed within 4∼6 min. Assuming the converter early slag temperature is 1300 °C, the largest allowed particle size of limestone is approximately 2 cm, in good agreement with the literature data [4].

thumbnail Fig. 4

Effect of slag temperature on the thickness of lime layer at the rotation speeds of 150 rpm and 0 rpm.

thumbnail Fig. 5

Evolution of spherical limestone conversion under different conditions (d = 14 mm).

3.3 Dissolution mechanism of lime generated by limestone calcination in slag

Limestone can decompose rapidly and completely in the converter slag. The rapid decomposition of limestone in the converter slag is attributed to its high porosity that gives rise to high activity, and the ability of limestone to react with slag (ferrous oxide), breaking the dense layers of dicalcium silicate. This enables better heat transfer and energy supply into the interior of the sample, increasing the decomposition rate. This can be observed in Figure 2 that shows that a white region (the main component here is lime) that is beneficial for rapid slagging is present in the sample.

4 Conclusions

The limestone decomposition behavior in converter slag at 1250 °C, 1300 °C and 1350 °C is studied in this work, and its microstructure was analyzed by SEM. The dynamic model is used to determine the reaction bottleneck and a model for predicting the limestone decomposition process is developed. The following conclusions were obtained:

  • under the experimental conditions, the total decomposition time of limestone in converter early slag is approximately 5 min. Going from the outer to the inner region, the cross-section of the limestone sample after the reaction can be divided into the reaction, lime and unreacted limestone zones, which is similar to the observations for the limestone decomposition in hot metal;

  • the FeO in the converter slag can diffuse through the lime pores along the radial direction from the outer region to the inner region, and reaches the depth of approximately 520 μm, while the thickness of the outermost layer that is considered to be complete slag-forming reaction is approximately 10 μm for the decomposition at 1300 °C in the experiment;

  • heat transfer is the bottleneck for the limestone decomposition in converter early slag, and the particle size of the limestone added to the converter slag must be less than or equal to 2 cm.

Acknowledgments

The authors acknowledge financial support from University Nursing Program for Young Scholars with Creative Talents Heilongjiang province (UNPYSCT-2017146), Heilongjiang province youth science foundation project (QC2018071), and Excellent Discipline Team Project of Jiamusi University (JDXKTD-2019001) for financial support.

References

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Cite this article as: Tang Biao, Li Bing, Ma Zhen, Decomposition behavior and kinetics of limestone in early converter slag, Metall. Res. Technol. 117, 205 (2020)

All Tables

Table 1

Composition of slag in the experiment (wt%).

All Figures

thumbnail Fig. 1

Cross-section of decomposed limestone at 1300 °C in converter slag for 180 s.

In the text
thumbnail Fig. 2

SEM image after limestone decomposition in 1300 °C slag for 180 s.

In the text
thumbnail Fig. 3

Counts for Fe by using line scanning after limestone decomposition in the 1300 °C converter slag for 180 s.

In the text
thumbnail Fig. 4

Effect of slag temperature on the thickness of lime layer at the rotation speeds of 150 rpm and 0 rpm.

In the text
thumbnail Fig. 5

Evolution of spherical limestone conversion under different conditions (d = 14 mm).

In the text

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