1 Introduction

Many important metallic resources, such as Fe, V and Ti, exist in vanadium–titanium magnetite ore [1, 2]. Such ore now constitute a hot spot in the study of smelting technology [3,4,5,6,7]. There are two typical vanadium–titanium magnetite ores in China: high chromium vanadium–titanium magnetite (HCVTM) and ordinary vanadium–titanium magnetite (VTM). HCVTM contains TiO2 of 5.55% and Cr2O3 of 0.62%, while, TiO2 content can reach 13% in VTM. HCVTM is mainly reserved in the Kuranakh deposit and used in Jianlong steel. However, VTM is mainly reserved in the Panzhihua mining area and used in Panzhihua steel. Compared to the ordinary iron ore, compositions of HCVTM and VTM are so complex that process of slagging and smelting in blast furnace is difficult. Currently the blast furnace process is the major method to extract valuable elements [1, 2]. In this process, most of iron and vanadium are reduced into hot metal, while almost all of Ti passes to the slag [2].

Over recent years, researches on the properties of sinter, pellet and slag with vanadium titanium magnetite have been conducted. Bristow et al. [8] showed that increasing the titanium magnetite levels to over 3% had a significant deterioration in sinter RDI. Liu et al. [9] studied reduction process of pellet containing high chromium vanadium titanium magnetite in cohesive zone. Cheng et al. [5] studied the effect of TiO2 on the crushing strength for high chromium vanadium–titanium magnetite pellets. Qiu et al. [10] studied the effect of Cr2O3 addition on viscosity and structure of Ti-bearing blast furnace slag. In previous studies [8,9,10,11,12,13,14,15], researchers have altered some variables to find out how the modified sinter, pellet and slag would change under laboratory conditions. It was reported that the phase composition and microstructure of the laboratory and on-site samples had definite differences [16, 17]. However, the on-site samples received little attention. So, the high temperature metallurgical properties of on-site sinter and pellet have been investigated in the laboratory. This article gives results about the phase composition and microstructure of on-site samples and non-dripped slag. The obtained results have been analyzed and compared with the phase diagram in the literature.

2 Experimental

2.1 Sample Preparation

Table 1 shows that the chemical composition of vanadium–titanium magnetite. Sinter, pellet and slag, which were made of HCVTM (provided by Jianlong steel) or VTM (provided by Panzhihua steel), were taken out for future use, respectively, when the operation of the blast furnace was stabilized. The chemical compositions of the above samples were analyzed by chemical analysis. The chemical compositions of samples are shown in Table 2.

Table 1 Chemical composition of vanadium–titanium magnetite
Full size table
Table 2 Chemical composition of on-site samples
Full size table

2.2 Procedure

In the present work, phase constitution and microstructure of the processed on-site samples were studied by X-Ray diffraction (XRD, model X’Pert Pro, Max power 3 kW, PANalytical B.V., Netherlands) and scanning electron microscope (SEM, model Ultra Plus, resolution ratio 0.8 nm, Carl Zeiss AG, Germany) analysis techniques. The softening–melting–dripping experiments of on-site samples were carried out to measure high temperature properties. The schematic experimental apparatus is shown in Fig. 1a. The device is mainly composed of heating system, gas control system, temperature control system, and data recording system. MoSi2 heating elements were used as the core components in the heating system to heat and melt the burden. A R-type thermocouple was placed in the bottom of the pressure lever to control the experimental temperature. The crucible and pressure lever were both made of graphite. The dimension of the crucible is shown in Fig. 1b.

Fig. 1
figure 1

Schematic of the experimental apparatus

Full size image

In order to simulate the blast furnace burden conditions, 500 g on-site sinter and pellet (as shown in Table 3) were put in the crucible and coke layers (size: 10–12.5 mm) of 20 and 40 mm height were placed below and over the samples, respectively. The displacement changes and contraction of the charging were measured by the displacement sensor when the experiment started. Experimental conditions are shown in Table 4. The heating rates were 10 °C/min below 900 °C, 3 °C/min at 900 °C to 1020 °C, and 5 °C from 1020 °C to the end. Ar gas was introduced immediately to prevent the dripped and non-dripped products being oxidized again after the reduction was finished.

Table 3 The burden conditions of on-site sinter and pellet
Full size table
Table 4 Temperature profile and reduction atmosphere of experiment
Full size table

3 Results and Discussion

3.1 Phase Constitution and Microstructure of Sinter and Pellet

Figure 2 shows the XRD pattern of sinter1 and sinter2 which are made from HCVTM and VTM, respectively. The main phase constitutions of sinter1 and sinter2 include Fe3O4, Fe2O3, CaTiO3 and CaFe4O7. The basic phases are Fe3O4 and Fe2O3, and the diffraction peak intensity of the basic phases in sinter2 is larger than sinter1. Whereas, the diffraction peak intensity of CaTiO3 and CaFe4O7 in sinter2 is a bit less than sinter1.

Fig. 2
figure 2

XRD pattern of sinter1 and sinter2

Full size image

Figures 3 and 4 show the SEM–EDS mapping analysis of sinter1 and sinter2, respectively. According to Fig. 3a, sinter1 is mainly composed of A, B, C and D-phase. From EDS results shown in Fig. 3b, it may be inferred that phase A is schorlomite. In Fig. 3c, point B contains an abundance of O and Ca and a small number of Si, Ti, Fe, which can be speculated as CaO·nFe2O3 and CaTiO3. Figures 3d, e show that the main elements are O and Fe, thus C-phase and D-phase may be Fe2O3 and Fe3O4 phase.

Fig. 3
figure 3

SEM and EDS mapping analysis of sinter1

Full size image
Fig. 4
figure 4

SEM and EDS mapping analysis of sinter2

Full size image

As shown in Fig. 4a, the main microstructure of sinter2 contains A, B, C and D-phase. Figure 4b shows that point A is composed of almost Fe and O, which can be inferred iron oxide phase (Fe2O3 and Fe3O4). Point B consists mainly of O, Ca, Ti and Fe in Fig. 4c, which can be inferred as CaTiO3 and CaO·nFe2O3. The main elements are O, Si, Ca and a little Al, Ti, V, Fe in Fig. 4d and e; thus C-phase and D-phase may be schorlomite. Because of the low melting point of schorlomite, the performance of sinter can be improved as binder phase. Comparing with Fig. 3, the microstructure of sinter2 contains much more binder phase. So, sinter2 is a more dense structure.

Figure 5 shows the XRD pattern of pellet1 and pellet2 made of HCVTM and VTM, respectively. The main phase compositions of pellet1 and pellet2 include Fe2O3, MgTiO3 and Fe2O3·TiO2. In pellet2, the diffraction peak intensity of phase is a bit less than in pellet1.

Fig. 5
figure 5

XRD pattern of pellet1 and pellet2

Full size image

Figures 6 and 7 show the SEM–EDS mapping analysis of pellet1 and pellet2, respectively. In Fig. 6a, the microstructure of pellet1 includes a lot of A-phase and a small amount of B, C and D-phase. In Fig. 6b, the main elements are O and Fe, thus A-phase could be Fe2O3. In Fig. 6c, point B contains mainly O, Ti and Fe, which can be inferred as Fe2O3·TiO2. In Fig. 6d, there is an abundance of O, Si and a small number of Al, Ca, Ti, V, Fe, which can be speculated as silicate mineral. The main elements are O and Ti in Fig. 6e, thus D-phase may be TiO2.

Fig. 6
figure 6

SEM and EDS mapping analysis of pellet1

Full size image
Fig. 7
figure 7

SEM and EDS mapping analysis of pellet2

Full size image

As is depicted in Fig. 7a, pellet2 is mainly composed of A-phase. B-phase remains embedded in the A-phase, and C-phase is around A-phase. Figure 7b shows that point A includes mainly O and Fe, which could be Fe2O3. In Fig. 7c, the main elements are O, Ti and Fe, which can be inferred as Fe2O3·TiO2. There are lot of O, Si, Ca and a small amount of Mg, Al, Ti, V and Fe in Fig. 7d, thus point C may be silicate mineral.

The low melting point silicate mineral has grown due to interaction among particles at high temperatures. At the same time, the ilmenite of vanadium–titanium magnetite is oxidized to pseudobrookite (Fe2O3·TiO2) in pelletizing process, as seen in Eq. (1).

$$2{\text{FeO}} \cdot {\text{TiO}}_{2} + \frac{1}{2}{\text{O}}_{2} \to {\text{Fe}}_{2} {\text{O}}_{3} \cdot {\text{TiO}}_{2}$$
(1)

Taken together, mineralogical compounds of on-site sinter and pellet are listed in Table 5.

Table 5 Mineralogical compound of sinter and pellet
Full size table

3.2 Softening–Melting–Dripping Behaviour

The inner shape of the cohesive zone of the blast furnace is determined by the melt-down of ores, and the permeability and distribution of gas there is also affected. Softening start temperature (T4), softening temperature (T40), melting start temperature (Ts), and dripping temperature (Td) are measured to evaluate the softening and melting characteristics [18]. T4 and T40 are the temperatures at which the contraction ratio of charging reaches 4 and 40%, respectively. The temperature accompanying differential pressure with a massive jumping is considered as Ts. The hot metal drips from the graphite crucible at a certain temperature as Td. The results of softening and melting characteristics of burden are shown in Fig. 8.

Fig. 8
figure 8

The softening and melting characteristics of on-site sinter and pellet

Full size image

Figure 8 shows that T4, T40, softening zone (T40 − T4) of burden1 and burden2 are 1157, 1267, 110 °C and 1134, 1228, 94 °C, respectively. It can be seen that the softening property of burden1 is better than that of burden2. That is because a higher softening start temperature and a wider softening zone are favourable for the gas–solid reduction reaction in the blast furnace. Cheng et al.’s study [5] reports that T4, T40 and (T40 − T4) of pellet with 2.47 mass percent of TiO2 are 1088, 1201 and 113 °C. Comparing with the previous results, it can be shown that T4 of the burden1 with 2.45 mass percent of TiO2 in this study is higher by 69 °C, T40 is higher by 66 °C, and (T40 − T4) is higher by 3 °C. T4 and T40 of burden1 are significantly higher than pellet with 2.47 mass pct TiO2, but the difference of (T40 − T4) is very small. Beside the TiO2 difference, burden structure can also contribute immeasurably to the result change. It is also found that Ts, Td and melting–dripping zone (Td − Ts) of burden2 and bruden1 are 1246, 1480, 134 °C and 1205, 1490, 185 °C respectively. By the contrast analysis, melting start temperature of burden1 is higher and melting–dripping zone is narrower than burden2, which is beneficial to the blast furnace smelting.

The results of pressure drop and contraction of burden1 and burden2 are shown in Fig. 8. Permeability index (S) can be calculated by the equation \({\text{S}} = \int_{{T_{s} }}^{{T_{d} }} {\left( {P_{m} - \Delta P_{s} } \right) \cdot dt}\), where Pm is the pressure drop at any temperature and ΔPs is the pressure drop at the Ts. The calculated S of burden1 and burden2 are 2426 and 2952 kPa °C, respectively. It can be seen that the permeability of burden2 is better than burden1.

In summary, the softening–melting–dripping property of burden1 with HCVTM is better than that of burden2 at a suitable ratio.

3.3 Non-Dripped Slag and On-Site Slag

The chemical compositions of non-dripped slag (ndslag1 and ndslag2) in the bottom of crucible at the end of dripping stage were analyzed by chemical analysis and are shown in Table 6.

Table 6 The chemical compositions of non-dripped slag
Full size table

Compared with Table 2, it is found that the mass percent of Fe in the non-dripped slag is significantly higher than on-site slag. It is necessary to analyze the phase constitution and microstructure of on-site slag and non-dripped slag.

Figure 9 shows the XRD pattern of on-site slag (slag1 and slag2) and non-dripped slag (ndslag1 and ndslag2), respectively. The main phase constitutions of ndslag1 and slag1 include CaTiO3, melilite, MgAl2O4 and proxene. Ndslag2 and slag2 include CaTiO3, MgAl2O4 and proxene. The diffraction peak intensity of CaTiO3, proxene of ndslag2 and slag2 is larger than ndslag1 and slag1. In ndslag2 and slag2 the diffraction peak intensity of MgAl2O4 is less than in ndslag1 and slag1. However, melilite does not appear in ndslag2 and slag2. It is also found that in the diffraction peak intensity of every phase, there exists difference between non-dripped slag and on-site slag.

Fig. 9
figure 9

XRD pattern of slag1, slag2, ndslag1 and ndslag2

Full size image

Figure 10 shows SEM and EDS analysis of non-dripped slag1 (ndslag1) for HCVTM. It can be seen obviously from Fig. 10a that ndslag1 is mainly composed of A, B and C-phase. From Fig. 10b and c, A-phase is a complex silicate matrix phase, and B-phase remains embedded in A-phase as CaTiO3.

Fig. 10
figure 10

SEM and EDS mapping analysis of ndslag1

Full size image

Similarly, Fig. 11 presents on-site slag1 (slag1) for HCVTM at 2000×, together with the EDS analysis of A, B, C and D-phase. When compared with SEM and EDS analysis of non-dripped slag1, they all have silicate matrix and CaTiO3 phase, but slag1 exists as MgO·Al2O3 phase.

Fig. 11
figure 11

SEM and EDS mapping analysis of slag1

Full size image

Figure 12 shows SEM and EDS analysis of non-dripped slag2 (ndslag2) for VTM. As is shown in Fig. 12a, ndslag2 is mainly composed of A, B, C, D and E-phase. It can be inferred from Fig. 12b–e that point A and C are silicate matrix phase, and point B, D and E are CaTiO3, metallic iron and TiC, respectively. It is proposed that TiC is generated through the reaction of Ti and C.

Fig. 12
figure 12

SEM and EDS mapping analysis of ndslag2

Full size image

In a similar way, Fig. 13 presents on-site slag2 (slag2) for VTM at 1000×, together with the EDS analysis of A, B, C, D and E-phase. On the basis of Fig. 13a–f, point A is silicate matrix phase, and B, C and E are CaTiO3, FexO and metallic iron, respectively. When compared with SEM mapping of ndslag2, TiC does not appear in slag2. This is because non-dripped slag has been in a reducing atmosphere.

Fig. 13
figure 13

SEM and EDS mapping analysis of slag2

Full size image

To explore the phase of slag, the Factsage 7.0 package was used to calculate the phase diagram. The V2O5 and Cr2O3 content were not considered in the calculations of the phase diagram as slag1 and slag2 contain small amount of V2O5 and Cr2O3. So the compositions of on-site slag were converted into CaO–SiO2–TiO2–Al2O3–MgO system, as shown in Table 7. The CaO–SiO2–TiO2–Al2O3–MgO system were calculated by Factsage 7.0 by setting an oxygen partial pressure by 10−5. The FactPs and FToxid database were used, and the possible species of solid phases were considered, such as perovskite, melilite, spinel and so on. The phase diagram of CaO–SiO2–TiO2–13.05wt%Al2O3–9.29wt%MgO slag1 is shown in Fig. 14. The phase diagram of CaO–SiO2–TiO2–14.31wt%Al2O3–8.69wt%MgO slag2 is shown in Fig. 15.

Table 7 Oxide composition of target slag
Full size table
Fig. 14
figure 14

Phase diagram of CaO-SiO2-TiO2-13.05wt%Al2O3-9.29wt%MgO slag

Full size image
Fig. 15
figure 15

Phase diagram of CaO-SiO2-TiO2-14.31wt%Al2O3-8.69wt%MgO slag

Full size image

As is shown in Fig. 14, the primary crystal field of slag1 with 8.61 wt% TiO2 is melilite, and the liquidus temperature is 1409.81 °C. In Fig. 15, the primary crystal field of slag2 is CaTiO3 when the content of TiO2 is 23.26 wt%. At the same time, the liquidus temperature of slag2 is 1418.51 °C.

4 Conclusions

In this study, the on-site sinter, pellet, slag and non-dripped slag which were made from HCVTM and VTM respectively were analyzed by XRD and SEM–EDS. The high temperature metallurgical properties of on-site sinter and pellet were investigated. Besides, the target slag was discussed by phase diagram. The conclusions are as follows:

  1. (1)

    The phases of HCVTM and VTM sinter contain Fe2O3, Fe3O4, CaTiO3, CaO·nFe2O3, schorlonite; the phases of HCVTM pellet contain Fe2O3, Fe2O3·TiO2, TiO2, silicate mineral, and the phases of VTM pellet contain Fe2O3, Fe2O3·TiO2, silicate mineral.

  2. (2)

    The softening start temperature and softening zone of HCVTM burden are higher than VTM burden; the melting start temperature of HCVTM burden is higher and melting–dripping zone is smaller than VTM burden, which is beneficial to the blast furnace smelting.

  3. (3)

    The phase constitutions of on-site slag are not different from laboratorial slag, but there are some differences in the microstructure. The laboratorial VTM slag shows obviously TiC around metallic iron.

  4. (4)

    The main phase and primary crystal field of slag1 is the melilite, and the liquidus temperature is 1409.81 °C; the main phase and primary crystal field of slag2 is CaTiO3, and the liquidus temperature is 1418.51 °C.