Operation Characteristic of Super-Large Blast Furnace Slag in China

Abstract

Operational smoothly of ironmaking process is determined to a great extent by its slag performance especially for the large blast furnace. Operation characteristic of super-large blast furnace slag in China was summarized and the appropriate adjustment measures for better performance of BF were analyzed based on the operation characteristic by using the MPE. The results show that MgO content ranges from 6.5–9.5 wt% and Al₂O₃ ranges in 11–16 wt% of the super-large blast furnaces. The hot metal temperature ranges between 1490–1520°C and C/S is controlled within 1.12 to 1.24. A slag operation concept is built for reference by the operation viscosity and the superheat temperature. The operation viscosity is 0.34±0.02 Pa·s and the superheat temperature is 80±10°C. Based on this, Mg/Al ratio can be reduced with the increasing C/S in the production. Mg/Al ratio of 15 wt% Al₂O₃ slag can be reduced to 0.4 in the high basicity of 1.3 and should be increased to 0.67 in the low basicity of 1.1. Mg/Al ratio of 0.53 (8 wt% MgO) is ideal if the basicity of slag changes in a wide range near 1.2. The rational Mg/Al ratio of the slag (C/S=1.2) can be reduced to 0.33, 0.47 and 0.58 when the Al₂O₃ in the slag are 12 wt%, 15 wt% and 18 wt%, respectively.

1. Introduction

Enlarging the effective volume of blast furnace (BF) will be the main tendency in China in consideration of the economic conditions, environmental pressures and market requirements in future.¹⁾ Recently, super-large BFs (larger than 4000 m³, the same as below) are becoming an important symbol of ironmaking development, and the super-large BFs have some technology advantages in energy saving and emission.²⁾ The super-large BFs have achieved low carbon smelting and smooth operation. In addition, molten slag has a significant effect on the stability and productivity of BF operation. Therefore, analyzing and understanding the characteristic of large-scale BF slag has an important guidance for the optimization of BF slag. In recent years, utilization of high-aluminum-content iron ore has increased the content of Al₂O₃ in the slag. It is important to analyze the slag properties of BF slag with different Al₂O₃ content for maintaining stable BF operations. Much of attentions have been paid on the final slag viscosity due to its great effect on understanding the fluid dynamic of molten slag during the pyrometallurgy process. Zhang et al.³⁾ studied the effect of Mg/Al ratio on the viscosity, the stability and break point temperature of BF slag. The rational of Mg/Al was recommended in a range of 0.6–0.7 in the high Al₂O₃ content. Shiau et al.⁴⁾ indicated that the lower liquidus temperature and the better viscosity stability laid in the area of MgO = 5.4 wt%, Al₂O₃ = 10–15 wt%, and C/S (CaO/SiO₂) = 1.2. Lu et al.⁵⁾ investigated that the viscosity and the melting temperature of the slag. The appropriate Mg/Al ratio was among 0.90 to 0.75. Shen et al.⁶⁾ considered that the suitable Mg/Al was 0.2–0.5 based on the analysis of the slag viscosity and assuming the melting temperature of molten slag was 1400±25°C. Thus, different researches had different opinions on the ratio of Mg/Al ratio. However, the previous study were based on the laboratorial experiments. Few researches regarded to the viscosity and liquidus temperature of the actual operation characteristic of BF slags with various compositions and temperature.

Moreover, a number of viscosity models have been developed to predict the viscosity of the SiO₂–Al₂O₃–CaO–MgO (typical BF slag components) system over the last decades. These viscosity models can be generally classified into two groups, empirical models and structural models.^{7,8,9,10,11,12)} The Multi-Phase Equilibrium (MPE) is a thermodynamic package developed by CSIRO for simulating reactions between phases in multi-component and multiphase systems.^13,14,15) Through application of the thermodynamic model MPE, the viscosity and liquidus temperature of BF slags were calculated and compared with published experimental data. These comparisons showed the predicted values by the model were to be in good agreement and within experimental uncertainty.

In order to specify the actual operation characteristic of the super-large BF slag, the characteristic of Chinese super-large BFs slag with different compositions and temperature were conducted based on the actual production data by using MPE. The objective of this study is to evaluate the operation characteristic of super-large BFs slag and find the appropriate adjustment measures for better performance of BF.

2. Current Operation of Chinese Super-large BF Slag

2.1. Overview of Chinese Super-large BFs in Operation

The development of Chinese super-large BF started late and entered in the flourishing period during the last ten years. In recent years, by Chinese super-large BF operators’ continuous efforts, the BF production indexes were improved constantly, and fuel rate was declined ceaselessly. At present, there are 21 super-large BFs running in China, as shown in Table 1.^16,17) The BFs are ordered by the effective volume.

Table 1. Overview and production indexes of Chinese super-large BFs in operation.

	Furnace No.	Effective volume m3	Commissioning date	Hearth diameter m	Productivity t/(m3·d)	[Si] wt%	Fuel rate kg/tHM	Coal rate kg/tHM	Slag Volume kg/tHM
1	Shagang	5800	2009.10.20	15.3	2.27	0.46	522	184	296
2	Shougang Jingtang Steel No. 1	5500	2009.5.21	13.3	2.20	0.40	498	153	304
3	Shougang Jingtang Steel No. 2	5500	2010.6.26	15.5	2.26	0.38	498	154	305
4	Baogang zhangjiang No. 1	5050	2015.9.25	14.5	2.28	0.50	490	180	280
5	Baogang zhangjiang No. 2	5050	2016.7.15	14.5	2.28	0.39	477	180	280
6	Baosteel No. 1	4966	2009.2.15	14.5	1.96	0.50	492	149	296
7	Baosteel No. 3	4850	2005.4.27	14.2	2.11	0.39	487	183	266
8	Anyang Steel No. 3	4836	2013.3.19	13.3	1.89	0.45	514	146	330
9	Baosteel No. 4	4747	2005.4.27	14	2.11	0.45	494	181	284
10	Angang Bayuquan No. 1	4747	2008.9.6	13.3	1.82	0.32	520	140	358
11	Bengang New No. 1	4747	2008.10.9	14.2	2.19	0.57	520	137	304
12	Baosteel No. 2	4706	2006.12.7	14.5	2.10	0.56	492	177	280
13	Tisco No. 5	4350	2006.10.13	14.2	2.11	0.45	529	195	304
14	Tisco No. 6	4350	2013.11.7	14.2	2.20	0.25	524	194	306
15	Baogang No. 7	4150	2014.5.27	12.9	1.85	0.25	542	105	294
16	Wisco No. 8	4096	2009.8.1	13.6	2.58	0.48	504	171	352
17	Baosteel Meigang No. 5	4070	2012.6.2	15.5	2.24	0.47	496	140	329
18	Angang Bayuquan No. 2	4038	2008.9.6	13.3	2.20	0.59	531	158	338
19	Magang No. A	4000	2007.5.8	13.5	2.17	0.43	548	130	319
20	Magang No. B	4000	2007.5.24	13.5	2.20	0.43	527	122	315
21	Shougang Qianan Steel No. 3	4000	2010.1.8	13.5	2.20	0.39	508	155	316

The description appears in Table 2 and Fig. 1 showing the slag compositions and hot metal temperature (T_HM) of the Chinese super-large BFs.^1,18,19,20) C/S (CaO/SiO₂) of slags is controlled within 1.12 to 1.24 except No.10 BF, which is 1.08 and lower than others. Mg/Al ratio ranges from 0.45 to 0.7. MgO content in the slag is among 6.5–9.5 wt% and Al₂O₃ ranges in 11–16 wt%. It also illustrates that the temperature of hot metal (T_HM) are various, which ranges among 1490–1520°C.

Table 2. Slag compositions (wt%) and hot metal temperature (°C).

Furnace No.	CaO, wt%	SiO2, wt%	MgO, wt%	Al2O3, wt%	THM, °C	C/S	Mg/Al
1	40.8	33.6	8.0	15.0	1501	1.21	0.54
2	37.8	32.7	7.9	13.9	1490	1.16	0.57
3	37.8	32.7	7.9	13.9	1493	1.16	0.57
4	42.7	35.9	7.2	14.2	1508	1.19	0.51
5	42.6	35.5	7.4	14.5	1510	1.20	0.51
6	41.4	33.7	6.8	15.0	1507	1.23	0.46
7	41.3	33.7	7.0	15.0	1515	1.23	0.47
8	40.3	33.1	6.7	15.5	1505	1.22	0.43
9	41.3	34.0	7.1	14.8	1515	1.22	0.48
10	38.1	35.1	8.4	12.1	1500	1.09	0.70
11	41.3	33.7	6.9	15.0	1509	1.23	0.46
12	42.4	35.2	6.5	11.4	1513	1.21	0.57
13	42.8	35.8	6.4	11.0	1517	1.20	0.58
14	39.4	35.2	9.3	14.3	1502	1.12	0.65
15	40.0	33.8	7.2	14.6	1505	1.18	0.50
16	40.2	33.3	7.7	15.6	1515	1.21	0.49
17	40.8	33.0	6.9	12.7	1493	1.24	0.54
18	40.8	33.0	7.0	12.8	1493	1.24	0.54
19	38.6	34.0	8.6	14.9	1500	1.13	0.58
20	38.5	34.0	8.6	15.0	1506	1.13	0.57
21	39.8	33.7	7.7	14.0	1518	1.18	0.55

Fig. 1.

Slag compositions and hot metal temperature (T_HM) of Chinese super-large BFs.

Figure 2 illustrates the relationship between C/S and Mg/Al ratio based on the slag analysis of Chinese super-large BFs. Mg/Al ratio decreases with increasing C/S. In addition, all of the BFs including BFs with low Mg/Al ratio are in the condition of smooth operation. Thus, it is possible to be concluded that the BF can also achieve a smooth operation with low Mg/Al ratio in the high basicity (C/S) condition.

Fig. 2.

Relationship between the C/S and Mg/Al ratio of Chinese super-large BFs slag.

2.2. Characteristic of Chinese Super-large BFs Slag

The bad permeability for liquid flow is usually caused by the bad slag fluidity, which can be expressed by using the indexes such as slag liquidus temperature and viscosity. The developed MPE are used in the calculation of all the Chinese super-large BFs slag to present the characteristic of slag fluidity.

Figure 3 shows the operation viscosity (slag viscosity in the temperature of hot metal temperature) and liquidus temperature of the slag estimated based on the actual slag chemical compositions. In terms of the Chinese super-large BFs, the operation viscosity of the slag ranges from 0.29 to 0.38 Pa·s and the average of the operation viscosity is 0.34 Pa·s in the various temperatures and slag compositions. The liquidus temperature ranges from 1380 to 1440°C and the slag composition varies in a relative wide range, which means that a lower and a higher liquidus temperature could obtain in the complex BF operation. In the actual BF, there is a temperature distribution and a composition distribution in the furnace lower part and thus, a slag operation concept is needed for the guidance of BF slag operation. In the present study, the difference between the hot metal temperature and the slag liquidus temperature are calculated and named as superheat temperature. The higher the superheat temperature is, the better the slag flows. The BF can have a smooth operation with fluidly slag. Superheat temperature of the super large BFs range in a small scale between 70–90°C excluding four of the BFs although both the liquidus temperature and hot metal temperature fluctuate in a wide range.

Fig. 3.

Viscosity, liquidus temperature and superheat temperature of Chinese super-large BFs slag.

Moreover, comparing Figs. 4(a), 4(b), and 4(c), the operation viscosity and the liquidus temperature of the actual BF slag are influenced by many factors. Operation viscosity and liquidus temperature are the results of comprehensive influence by the hot metal temperature and slag compositions. However, the operation viscosity appears to be more pronounced by the hot metal temperature, which is the dominant factor for operation viscosity. The slag liquidus temperature fluctuates with the hot metal temperature and shows a great influence on the C/S and Mg/Al ratio.

Fig. 4.

Viscosity and liquidus temperature change with temperature of hot metal (a), C/S (b) and Mg/Al (c). (Online version in color.)

Based on the analysis of the Chinese super-large BFs slag in the production, a slag operation concept is built for reference by the operation viscosity and the superheat temperature. The operation viscosity and the superheat temperature of the Chinese super-large BFs slag are 0.34±0.02 Pa·s and 80±10°C, respectively.

3. Optimization of BF Slag

3.1. Rational Mg/Al

The effects of MgO on the superheat temperature and operation viscosity have been evaluated by using MPE for slags having basicity (C/S) of 1.2 various levels of Al₂O₃. The Al₂O₃ content was fixed at 12, 15 and 18 wt% while MgO was increased from 4 to 12 wt%. Figure 5 shows that the addition of MgO can alter the superheat temperature quite significantly depending on the concentration of Al₂O₃. It indicates that at low Al₂O₃ contents (12 wt%), the slag is in mellilite (an alumino-silicate solid solution phase) saturated region and the liquidus temperature decreases with increasing MgO. The superheat temperature decreases with MgO higher than 9 wt% and enters into a region of Ca₃MgSi₂O₈ saturation. It is clear that at high Al₂O₃ contents (15 and 18 wt%), the increasing MgO leads to increase the superheat temperature of the slag.

Fig. 5.

Effect of Al₂O₃ and MgO on superheat temperature, phase equilibrium and viscosity in slags with CaO/SiO₂ is 1.2 and hot metal temperature is 1500°C. (Online version in color.)

The viscosities of slags with basicity (C/S) of 1.2, Al₂O₃ content of 12–18 wt% and MgO of 4–12 wt% are calculated at 1500 °C. Figure 5 shows that the viscosity of the slag increases with increasing the Al₂O₃ content. The trend was supported by the experimental data.^21,22,23,24) The slag with high Al₂O₃ will lead to less fluidity of the slag, hence affect the BF operation. As marked in Fig. 5, with the Al₂O₃ increasing from 12 to 18 wt%, the slag viscosity can be maintained at the same level by increasing the MgO level.

Figure 5 also gives the range of superheat temperature and operation viscosity of the BF operation area. When Al₂O₃ is 12 wt% and MgO is lower than 10 wt%, the superheat temperature is above the operation range. When MgO is higher, the viscosity of the slag is lower than the operation range. So the BF can have a smooth operation when MgO varies from 4–12 wt%. However, in order to have a lower carbon consumption, the hot metal temperature can be reduced to a lower level than 1500°C.

When Al₂O₃ is 15 wt%, the viscosity is not in the operation range as MgO is lower than 7 wt% and the superheat temperature is not in the operation range as MgO is higher than 10 wt%. So the rational MgO is 7–10 wt%. And the corresponding Mg/Al is 0.47–0.67. When Al₂O₃ is 18 wt%, the superheat temperature is higher than the operation range as MgO is lower than 10.5 wt% and the viscosity is also higher than the operation range as MgO is lower than 10.5 wt%. Therefore, the BF slag is reasonable when MgO is higher than 10.5 wt%. The corresponding Mg/Al is 0.58. And it is a good measure to reduce the Mg/Al ratio further by increasing the hot metal temperature.

3.2. Effect of C/S on the Viscosity and Superheat Temperature

The effects of basicity (C/S) on the viscosity and superheat temperature have been evaluated by using the MPE for slags with various levels of MgO. The Al₂O₃ content is fixed at 12, 15, and 18 wt% while the corresponding hot metal temperature are 1490°C, 1500°C and 1510°C. Figure 6 shows the calculation results of 12 wt% Al₂O₃. A lower C/S corresponds a higher slag viscosity and higher superheat temperature regardless of the MgO content. The slag viscosity as well as superheat temperature decrease with the increasing MgO. It indicates that MgO in the slag can be a relative low lever (4–8 wt%) when the basicity is high at 1.3. When the basicity is low at 1.1, the MgO should be increased to 10–12 wt% to have a smooth operation.

Fig. 6.

Viscosity and superheat of slags of CaO-SiO₂-(4–12) wt% MgO-12 wt% Al₂O₃.

The viscosity and superheat temperature at 15 wt% Al₂O₃ is plotted in Fig. 7. The viscosity of the slag decrease with the increasing C/S and the variation of the superheat temperature is relative complex. The low C/S corresponds high superheat temperature and high C/S corresponds to low superheat temperature regardless of the effect of MgO. It indicates that the effect of viscosity is more pronounced than the effect of superheat temperature as the range area of viscosity is much narrower. Mg/Al ratio in the slag can be reduced to 0.4 in the high basicity of 1.3 and should be increased to 0.67 in the low basicity of 1.1. The Mg/Al ratio of 0.53 (8 wt% MgO) is ideal if the basicity of slag changes in a wide range near 1.2.

Fig. 7.

Viscosity and superheat of slags of CaO-SiO₂-(4–12) wt% MgO-15 wt% Al₂O₃.

As illustrated in Fig. 8, when Al₂O₃ content is 18 wt%, the viscosity decreases with the increasing C/S and the superheat temperature varies in a wider range. Many of the viscosity and superheat temperature values are not in the operation range when MgO is lower than 10 wt%. So it is much more difficult to have a smooth operation at low Mg/Al ratio. When MgO content is above 10 wt%, the viscosity and superheat temperature can satisfy the requirement of BF operation and the Mg/Al ratio is 0.56.

Fig. 8.

Viscosity and superheat of slags of CaO-SiO₂-(4–12) wt% MgO-18 wt% Al₂O₃.

3.3. Optimization Strategies for Actual BF Slag

As a result of the complicated changes of the actual parameters in BF production, it is difficult to find the unified regularity. In order to specific the optimization of Chinese super-Large BF slag, No. 13 BF is taken as an example. Only the influence of single factor of the slag composition or hot metal temperature on the slag characteristic is analyzed in the following respectively and individually.

Calculation values of the viscosity and superheat temperature of No. 13 are shown in Fig. 9. From Fig. 9(a), the temperature of the hot metal can be reduced to the range that the superheat temperature and the viscosity can satisfy the requirement of the No. 13 BF slag. Thus, in the BF slag composition range of the study, the temperature can be reduced by almost 20°C. From Fig. 9(b), MgO can be reduced for the viscosity of the slag is lower and the superheat temperature is higher than the specific area. By reducing MgO content from 7.0 to 4.0 wt%, the superheat temperature of No.13 slag can be decreased. With the decreasing MgO, the slag volume of the BF operation can be obtained at a lower level, which can result in the lower fuel ratio and higher productivity. Figure 9(c) illustrates the effect of C/S ratio on the slag viscosity and the superheat temperature. With C/S of the slag increasing from 1.1 to 1.25, the viscosity is found to first decreases rapidly and then increases, and then decreases. The viscosity decreases largely with C/S increasing. It suggests that C/S can be adjusted in a wide range. Figure 8(d) shows the effect of Al₂O₃ content on the viscosity and the superheat temperature of slag. Slag viscosity increases and superheat temperature decreases with increasing Al₂O₃ content at a fixed temperature. In order to reduce the production cost, the low price ore with higher Al₂O₃ can be partly used, which has small influence on the slag operation and BF can also be smooth operation at a lower cost. Therefore, the optimization strategies for actual BF slag can be obtained through the concept of operation viscosity and superheat temperature.

Fig. 9.

Reasonable superheat temperature and the operation viscosity range area for the No. 13 BF slag: (a) hot metal temperature, (b) MgO, (c) C/S and (d) Al₂O₃.

4. Conclusions

Operation characteristic of super-large BF slag in China was summarized and the appropriate adjustment measures for better performance of BF were evaluated. The conclusions of the present work can be drawn as follows:

(1) The average data of Chinese super-large BF slag are summarized. MgO content ranges from 6.5–9.5 wt% and Al₂O₃ ranges in 11–16 wt%. The hot metal temperature ranges between 1490–1520°C. C/S is controlled within 1.12 to 1.24 and the Mg/Al is ranged from 0.45 to 0.7. Mg/Al ratio reduced with the increasing C/S.

(2) The operation characteristic of super-large BFs slag is identified based on the analysis of slag properties of production data. A slag operation concept is built for reference by the operation viscosity and the superheat temperature. The operation viscosity is 0.34±0.02 Pa·s and the superheat temperature is 80±10°C.

(3) In the condition of C/S is 1.2, the rational Mg/Al is 0.58 and can be reduced further by increasing the hot metal temperature when Al₂O₃ is 18 wt%. The rational Mg/Al is 0.47 when Al₂O₃ is 15 wt%. When Al₂O₃ is 12 wt%, the rational Mg/Al can be 0.33 and the hot metal temperature can be reduced to a lower level than 1500°C.

(4) The rational Mg/Al changes with C/S. Mg/Al ratio in the slag can be reduced to 0.4 in the high basicity of 1.3 and should be increased to 0.67 in the low basicity of 1.1. The Mg/Al ratio of 0.53 (8 wt% MgO) is ideal if the basicity of BF changes in a wide range near 1.2.

(5) The appropriate adjustment measures for better performance of actual BF slag are analyzed based on the operation characteristic of the slag to have a lower carbon and lower cost ironmaking.

Acknowledgements

This work was financially supported by the National Science Fund for Distinguished Young Scholars (51304014), the Key Program of the National Natural Science Foundation of China (No. U1260202) and the 111 Project (No. B13004).

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