Phase Composition and Formation Mechanism of Slag Crust in Blast Furnace

Abstract

Copper stave damage is common problem in blast furnace operations, and the formation of slag crust is beneficial to reduce the damage of copper stave. Therefore, an in-depth understanding of phase composition and formation mechanism of slag crust is helpful to clarify the protection mechanism of copper stave, so as to control the growth of the slag crust and to increase the service life of copper staves. In this study, the slag crust from a copper stave blast furnace was sampled, and the phase composition and structure of the slag crust were characterized in detail through XRD analysis and SEM-EDS. The results indicated that the slag crust presented apparent layer structure as the solid slag layer and viscous layer, which primarily consisted of gehlenite (Ca₂Al₂SiO₇), calcium aluminate (CaAl₄O₇), magnesia-alumina spinel (MgAl₂O₄), pleonaste (Mg_0.7Fe_0.23Al_1.97O₄), kaliophilite (KAlSiO₄) and metallic iron. In addition, the ternary phase diagram analysis of CaO–SiO₂–Al₂O₃ showed that the primary crystal phase of the slag is in the gehlenite region, and that the primary crystal region migrates to the calcium aluminate region with the increasing of Al₂O₃ content, which are beneficial to the slag crust formation. Finally, the formation mechanism of slag crust was proposed.

1. Introduction

The blast furnace shutdown caused by damage has been recognized as one of the most serious issues faced by the economic and efficient development of the blast furnace in a long time.^1,2,3) And the damage of the copper stave of a blast furnace body is one of the restrictive links of blast furnace longevity.⁴⁾ In recent years, Chinese commercial blast furnaces have suffered a large amount of economic losses due to the damage of copper stave.^5,6) The formation of slag crust on the cooling stave of a blast furnace plays an important role in protecting the copper stave.^{4,7,8,9,10,11,12)} On the one hand, the slag crust can effectively protect the copper stave by avoiding mechanical wear, thermal shock and chemical corrosion of the copper stave caused by the solid charge, high-temperature melt and gas flow. On the other hand, a reasonable slag crust thickness plays a key role in maintaining a reasonable operating shape of the blast furnace and provides good conditions for the smooth operation of a blast furnace. Therefore, it is necessary to study the phase composition and formation mechanism of slag crust, which will be helpful to further clarify the protection mechanism of the copper stave.

In recent years, researchers paid attention to the study of the protection mechanism and monitoring of slag crust. Researchers^8,9) studied the effects of slag thickness on heat transfer and stress of copper staves with different cooling stave design parameters, cooling conditions, and different edge gas flow temperature conditions by establishing a three-dimensional heat transfer model. The results show that the slag crust can significantly reduce the thermal load and thermal stress of the copper stave, and that the slag crust can effectively reduce the temperature of the hot surface of a copper stave. There are also some researchers dedicated to the research of slag crust condition monitoring methods. Wu^10,11) implemented the monitoring of slag crust thickness by establishing a three-dimensional heat transfer model, which can monitor water temperature difference and thermocouple temperature. An¹²⁾ proposed a real-time monitoring method of slag crust based on a two-dimensional decision fusion which was based on the characteristics of the slag crust and the temperature of the cooling wall. Due to the high temperature and high pressure inside the blast furnace, it is difficult to sample and analyze the slag crust during the normal production process of the blast furnace. Also, the previous dissection investigation of the blast furnace was mainly to study the internal state and the protective layer of the hearth,^{13,14,15,16,17,18,19,20)} but there are few reports on the slag crust of the blast furnace copper stave.

In this work, we attempt to determine the phase composition and formation mechanism of slag crust in the blast furnace. XRF and XRD were used to study the chemical composition and phase composition of the slag crust of the commercial blast furnace. The microstructure of the slag crust was analyzed by a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), and the layered structure of the slag crust was proposed. In addition, phase diagram analysis was performed by FactSage thermodynamic software. Finally, the formation mechanism of slag crust in the blast furnace was proposed based on the phase characterization and theoretical analysis, which might provide new insights into the slag crust growth behavior.

2. Materials and Methods

2.1. Sampling of Slag Crust

In the present paper, the slag crust samples were taken from a Chinese blast furnace with its volume of 2650 m³. The blast furnace started in October 8, 2004, and shut down for overhaul in June 27, 2019. The iron output per unit volume reached 11604 t. The main operating data before the blast furnace overhauled are shown in Table 1.

Table 1. Main Operating Data before the blast furnace overhauled.

Parameter	Productivity (t/m3/d)	Coke rate (kg/tHM)	Coal rate (kg/tHM)	Slag per (kg/tHM)	Blast temperature (K)
Values	2.33	375	123	306	1450

The blast furnace uses three sections copper stave as cooling equipment in the bosh, the belly and the lower part of the blast furnace body. During 14 years and 9 months operation of the blast furnace, the copper stave has been operated well without damage, and the shape of the copper stave ribs have been preserved, as shown in Fig. 1. During the overhaul, the slag crust of the copper stave was sampled, and the sampling locations and morphology of slag crust are shown in Fig. 2. As shown in Fig. 2, the average thickness of slag crust is between 8–35 mm.

Fig. 1.

Macromorphology of copper stave. (Online version in color.)

Fig. 2.

Schematic diagram of sampling locations and morphology of slag crust. (Online version in color.)

2.2. XRF and XRD Measurements

The slag crust samples were ground to under 74 μm with an agate mortar, and its chemical composition and mineral composition were detected by using X-ray fluorescence (XRF, Shimadzu XRF-1800, Japan) and X-ray diffraction (XRD Ultima IV; Rigaku), respectively. During the XRD analysis, Cu (Kα) radiation was used to obtain an X-ray diffraction spectrum, the powder sample was scanned at a scan rate of 10°/min in the range from 10° to 90°(2θ), and then the XRD pattern was analyzed by using Jade6.5 software.

2.3. SEM–EDS Analysis

A piece of about 20–30 mm in length and 15 mm in width were cut from the cross-section direction of the slag crust samples under dry environment and then fixed in a 25 mm or 30 mm plastic mold with epoxy resin. The surface of the slag crust samples was grounded and polished by silicon carbide paper and diamond paste to ensure that the samples surface meet the requirements of scanning electron microscope (SEM) analysis. After drying, the surface of the slag crust was spray-coated with gold, and then the morphology and the element distribution of the sample were analyzed by using a FEI Quanta 250 scanning electron microscope equipped with energy dispersive X-ray spectroscopy (EDS).

2.4. Thermodynamic Calculation

In this study, Factsage 6.4 software was used for thermodynamics. Based on FToxid and FactPS databases, the phase diagram of CaO–SiO₂–Al₂O₃ ternary system was drawn by using the phase diagram module.

3. Results and Discussion

3.1. Chemical Composition and Mineral Phases Composition of the Slag Crust

The slag crust and the final slag composition measured via XRF are shown in Table 2. Compared with the final slag, the slag crust contains a large amount of Al₂O₃ and Fe element, while the MgO content is relatively low and varies between 1.63% and 2.89%. It should be noted that the contents of K, Na and Zn in the slag crust are relatively high, and the aggregation of alkali metal oxides are beneficial to the formation of low-melting compounds, which has a significant effect on the formation of the slag crust. However, the deposition of ZnO and alkali metal oxides are not conducive to the stability of the slag crust. Figure 3 shows the XRD analysis of the slag crust. Results indicate that the mineral phase composition of the slag crust samples are not exactly identical, and its main phase is gehlenite (Ca₂Al₂SiO₇). As shown in Fig. 3(a), a small amount of kaliophilite (KAlSiO₄) exists in sample 1, and Fe element mainly forms compounds with CaO, Al₂O₃, MgO, and SiO₂ in the form of FeO. The phases in sample 2–5 also contain a certain amount of C, metallic iron, MgAl₂O₄ and CaAl₄O₇, as shown in Figs. 3(b) and 3(c). Due to the low content of some phases, the XRD pattern did not show its diffraction peaks. Its detailed analysis will be performed by SEM-EDS.

Table 2. Chemical composition of the slag crust and final slag (mass ratio, %).

Items	Al2O3	CaO	SiO2	FeO	TiO2	MgO	S	K2O	ZnO	Na2O	R2
Sample 1	26.92	22.00	22.53	19.61	1.66	2.89	0.35	3.09	0.27	0.67	0.98
Sample 2	33.20	28.07	18.12	14.54	2.65	1.65	0.39	0.77	0.30	0.30	1.55
Sample 3	32.91	27.91	20.22	12.59	2.04	1.91	0.36	1.21	0.46	0.39	1.38
Sample 4	30.61	28.91	18.79	13.39	2.74	2.01	0.55	1.71	0.80	0.51	1.54
Sample 5	31.21	25.85	20.15	16.83	2.60	1.63	0.41	0.76	0.32	0.25	1.28
Final slag	13.86	40.27	34.64	0.60	1.84	7.73	1.05				1.16

Fig. 3.

XRD pattern for the slag crust. (Online version in color.)

3.2. Microstructure of Slag Crust

3.2.1. Microstructure Analysis of Slag Crust at Lower Part of the Shaft

The slag crust was characterized by using SEM and EDS mapping, as shown in Fig. 4. The layered structure of the slag crust is clearly observed in Fig. 4(a), and it is divided into three layers according to the phase composition and morphology. The formation of slag crust is accompanied by a series of complex physical and chemical reactions, which can be confirmed from the phase composition and structure changes in different regions of the slag crust (Figs. 4(b)–4(d)). From the EDS mapping on the cold side of the slag crust, it can be seen that the slag crust is mainly composed of Ca, Al, Si, Fe, S, K and Na. The cyclic enrichment of alkali metal oxides in the lower part of the shaft forms a low melting point K, Na aluminosilicate with Al₂O₃ and SiO₂, which is beneficial to the formation of slag crust. Also, EDS mapping shows that the Fe element mainly exists in the form of FeS and FeO. The slag-metal interface shown in Fig. 5 further confirms the existing state of Fe element, and the metal iron droplets are surrounded by a layer of FeO and FeS. Among them, the FeO-containing layer indicates that FeO in the slag is enriched at the metal/slag interface and reduced by the carbon in the liquid metal at the metal/slag interface.²¹⁾ The layer containing FeS is between the FeO-containing layer and the slag, which may be formed by the oxidation-reduction reaction between the gas containing sulfur vapor and the layer containing FeO. In addition, the EDS maps in Fig. 5 shows that Si is not clearly observed in the iron phase, which depends on whether SiO₂ is more difficult to reduce than FeO in the upper part of the blast furnace. Interestingly, the presence of ferrosilicon was observed inside the metallic iron droplets, as shown in Figs. 6(d) and 6(P7). It indicates that the reduction reaction of SiO₂ in the upper part of the blast furnace. At the slag-iron interface, SiO₂ and FeO in the slag contact with the carbon in the liquid metal to form FeSi and diffuse into the liquid metal, as shown in Eq. (1).   

$$\begin{array}{l}\text{3[C]}+{\text{(SiO}}_{\text{2}}\text{)}+\text{FeO(l)}=\text{FeSi(s)}+\text{3CO(g)}\\ \mathrm{\Delta }{G}^{\theta }=700\mathrm{\hspace{0.17em} }731.04-458.95T\end{array}$$(1)

Fig. 4.

SEM showing the layered structure of slag crust. Image (a) shows the overall structure of slag crust; Images (b–d) were taken from image (a); The EDS maps were taken from image (e). (Online version in color.)

Fig. 5.

SEM image and EDS maps showing the interface between slag and metal. Image (a) shows the interface between slag and metal; Image (b) was taken from image (a); The EDS maps were taken from image (b). (Online version in color.)

Fig. 6.

The occurrence state of iron in slag crust. Images (a, b) shows the FeS in slag crust; Image (c) shows the FeS and FeSi in metal; Image (d) was taken from image (c). (Online version in color.)

Moreover, the hot side of the slag crust has obvious granular crystals (Fig. 4(d)), and the EDS results (P2) show that the solid particles are Mg–Fe–Al spinel, which is consistent with the XRD results. The slag crust maintains the mineral composition under high-temperature conditions during blast furnace water quenching. It confirms that the hot surface of the slag crust is a viscous layer of solid-liquid mixing.

3.2.2. Microstructure Analysis of Slag Crust in the Belly

The structure and phase composition of the slag crust in the belly are shown in Figs. 7 and 8. It can be seen from Fig. 7 that the slag crust in the belly is mainly composed of slag and metallic iron, and it is consistent with the result of the slag crust in the lower part of the shaft, which also has an obvious layered structure. The middle layer and cold side of the slag crust contain a large amount of metal iron droplets (Figs. 7(c) and 7(d)), and the flake-like graphite was precipitated in the metallic iron. It is interesting to notice that, no precipitated flake-like graphite was found in the metallic iron droplets on the hot surface of the slag crust, and the metallic iron was wrapped by FeO-containing slag (Figs. 7(d) and 7(P1)), which means the FeO in the slag was reduced by CO. The metal particles (mainly iron) in the slag crust are not completely reduced as oxygen can be clearly observed by the EDS results(Figs. 7(f) and 7(P4)), which is mainly due to the reduction reaction occurs inside the metal drops and the produced reducing gas formed the bubble in the metal phase.²²⁾ As shown in Fig. 8(d), titanium carbonitride (Ti(C_X, N_1−X)) was also found in the metal particles. The presence of Ti(C_X, N_1−X) in the iron phase confirms the reduction of Ti in the lower part of the cohesive zone, which has also been reported by Fan et al.²³⁾

Fig. 7.

SEM images and EDS maps of slag crust in the belly. (Sample 2) Images (a–c) represents hot side, middle layer and cold side of the slag crust respectively; Images (d–f) were taken from images (a–c) respectively. (Online version in color.)

Fig. 8.

SEM images and EDS maps of slag crust in the belly (Sample 3). Images (a, c) shows hot side, middle layer and cold side of the slag crust; Image (b) was taken from image (a); Image (d) shows the Ti(C_X, N_1−X) in metal. (Online version in color.)

Depending on the strong cooling effect of the copper stave, a large number of crystals were formed on the cold side and the middle layer of the slag crust (Figs. 7(e)–7(f) and 8(a)–8(c)). The EDS (Fig. 7(P2)) results indicate that the columnar crystals are gehlenite (Ca₂Al₂SiO₇) and the granular crystals are Al₂O₃ (Melting point, 2327 K). The difference is that the EDS mapping in Fig. 8(b) shows that the granular crystals are MgAl₂O₄ (Melting point, 2523 K), which is mainly caused by the precipitation temperature of MgAl₂O₄ being higher than the precipitation temperature of Al₂O₃. The crystal structure and mineral phase of the slag phase in the slag crust changed. This may be due to the influence of temperature and cooling rate on the nucleation and growth of the crystal. It can be proposed that a certain thickness of the slag phase adheres to the hot surface of the copper stave, and that the temperature gradient formed caused the layered structure of the slag crust, which is important for the growth of the slag crust.

3.2.3. Microstructure Analysis of the Slag Crust in the Bosh

The coke particles are clearly observed in the slag crust of the bosh as shown in Figs. 9 and 10, which is likely attributed to the obvious pulverization of the coke in the bosh. Meanwhile, the interface among slag-metal-coke in Fig. 9(b) was observed, and it can be seen that the surface of the liquid metal iron and coke is surrounded by a slag layer contained FeO. The liquid slag phase enters into the inside of the coke through the damaged pores (Fig. 10(c)). The EDS results show that the flaky phase in Fig. 10(d) is FeO. It may be that FeO in the slag migrated to the coke surface, but the oxidation reaction of metallic iron during the sampling process cannot be ruled out.

Fig. 9.

SEM images and EDS maps of slag crust in the bosh (Sample 4). Images (a, c) represents hot side, middle layer and cold side of the slag crust respectively; Images (b, d) were taken from images (a, c) respectively. (Online version in color.)

Fig. 10.

SEM images and EDS maps of slag crust in the bosh (Sample 5). Image (a) shows the overall structure of slag crust; Images (b–d) were taken from image (a). (Online version in color.)

The crystallization behavior of the slag phase in the slag crust of the bosh is consistent with that of the slag crust of the belly. The main crystal phases in sample 4 are Ca₂Al₂SiO₇ and MgAl₂O₄. The amorphous phase is consisted of Ca, Al, and Si (Fig. 9(P2)), but its Ca content is lower than that of Ca₂Al₂SiO₇ (Fig. 9(P1)). The mineral composition of the sample 5 is shown in Fig. 10(b). The EDS results (Fig. 10(P2)) show that CaAl₄O₇ was formed in the slag phase, which was attributed to the decrease in the basicity of the slag phase.

3.3. Thermodynamic Calculation

In order to better understand the crystallization behavior of the slag phase on the copper stave, the ternary phase diagram of CaO–SiO₂–Al₂O₃ was drawn by Factsage software, and the calculated phase diagram was consistent with the experimentally determined phase diagram,²⁴⁾ as shown in Fig. 11. The slag crust samples in Table 2 are marked in Fig. 11. Since Fe element in the slag mainly exists in the metal, the effects of FeO and trace elements on the crystallization of the slag phase are ignored. It can be seen from the phase diagram that the composition of the samples is located in the Ca₂Al₂SiO₇ region, which indicates that the primary crystal phase of the slag phase is Ca₂Al₂SiO₇. According to the liquidus line, it can be seen that the initial precipitation temperature of Ca₂Al₂SiO₇ increases as the basicity of the slag phase increases, and the increase of the basicity moves to the region of the Ca₂Al₂SiO₇, which is conducive to the precipitation of more melilite. With the increase of the Al₂O₃ content in the slag, the primary crystal phase migrates to the calcium aluminate region, and the precipitation temperature also increases, which is beneficial to the formation of slag crust in the high temperature region.

Fig. 11.

Ternary phase diagram of the CaO–SiO₂–Al₂O₃ system. (Online version in color.)

3.4. Formation Mechanism of the Slag Crust

The slag phase in the slag crust is mainly high-alumina slag, and the basicity of the slag crust at the lower part of the shaft is 0.98, which is consistent with the basicity of the primary slag reported in previous studies,²⁵⁾ but the slag phase in the slag crust has high Al₂O₃ content and low MgO content. Considering that it is difficult for coal ash to enter the lower part of the shaft, it is believed to be mainly due to the segregation of gehlenite (Ca₂Al₂SiO₇) formed during the cooling of the primary slag on the surface of the copper stave. Fan et al.²¹⁾ analyzed the composition of the slag phase of the cohesive zone at the bosh and belly, and it was found that the slag phase was high aluminum slag (CaO/SiO₂:1.75–2.35,Al₂O₃:18%–35%,MgO:0.4%–8.1%).The basicity of the slag crust of the bosh and belly is between 1.28 and 1.55, which may result in the decrease of slag phase basicity due to the ingress of ash.

Based on the detailed analysis of the phase composition and layered structure of the slag crust, the description of its formation mechanism is shown in Fig. 12. The formation of the slag crust on the hot surface of blast furnace copper stave is a multi-phase reaction process with complex heat and mass transfer. Under high temperature conditions, the slag crust was divided into a solid slag layer and a viscous layer according to the existing state of the slag phase. Under the effect of cooling, the liquid slag phase quickly condensed on the hot surface of the copper stave to form a solid slag layer. The increase in the size and amount of crystals in the solid slag layer are attributed to the thermal resistance formed by the growth of the slag crust, as shown in Fig. 12(S1). The precipitation of part of the solid phase in the slag phase causes a decrease in the fluidity of the slag phase and promotes the adhesion of the solid-liquid mixed phase on the surface of the solid slag layer, as shown in Fig. 12(S2). In addition to the solidification of the slag phase, the formation of the slag crust is accompanied by a physicochemical reaction with the gas stream and slag-iron mixed melt, and finally forms a stable layered structure. In short, the cooling strength and gas flow distribution have a significant effect on the structure and stability of the slag crust. In the actual production process of the blast furnace, the variation of the operating parameters, such as cooling system, charging system and raw material quality, could change the temperature of the hot surface of the copper stave and the composition of the slag curst, thereby controlling the thickness of the slag crust.

Fig. 12.

Formation mechanism of slag crust during blast furnace production. (Online version in color.)

4. Conclusion

The phase composition and structural characteristics of the slag crust were studied through XRD and SEM-EDS. Based on the analysis results of the slag crust, the formation mechanism of the slag crust was proposed. The main conclusions are summarized as follows:

(1) The slag crust has a distinct layered structure, which is formed by the solidification of the slag phase and the iron on the copper stave. Its main phases are the gehlenite (Ca₂Al₂SiO₇), calcium aluminate (CaAl₄O₇), magnesia-alumina spinel (MgAl₂O₄), pleonaste (Mg_0.7Fe_0.23Al_1.97O₄), and kaliophilite (KAlSiO₄).

(2) During the formation of the slag crust, FeO in the slag migrated to the slag-metal iron interface and slag-coke interface, and was reduced by C in metal iron and coke, respectively.

(3) Phase diagram analysis showed that the composition of the sample is located in the Ca₂Al₂SiO₇ region. The increase in the basicity of the slag phase was beneficial to the precipitation of Ca₂Al₂SiO₇. With the increase of Al₂O₃ content in the slag, the primary crystal phase migrates to the calcium aluminate region, which is beneficial to the formation of slag crust.

(4) According to the existing state, the slag crust was divided into a solid slag layer and a viscous slag layer. The formation of the slag crust was controlled by the cooling strength and gas flow distribution, which has an important influence on the structure and phase composition of the slag crust.

Acknowledgements

This work was financially supported by the Young Elite Scientists Sponsorship Program by CAST(2018QNRC001).

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