Introduction

Vanadium titanomagnetite (VTM) is a multi-component mineral that contains Fe, Ti, V, and various rare metals. In addition to Fe, Ti, and V, Hongge VTM (HVTM) found in the deposits in the Panzhihua-Xichang area also has a high chromium content and is the largest VTM resource in China [1, 2]. Currently, two processes are mainly used for the utilization of HVTM, namely, the blast furnace (BF) process and coal-based direct reduction process. However, for both the BF and coal-based direct reduction processes, the recovery rates of titanium, vanadium, and chromium are still low. In addition, it should be noted that it is difficult to deal with BF slag while the coal-based direct reduction process has a relatively low efficiency due to its high energy consumption and high operating temperature [3,4,5]. None of the above techniques can be easily implemented in industrial production and commercial use, hindering the development of sustainable metallurgy.

Therefore, for efficient HVTM utilization, a novel and sustainable smelting process has been proposed by our laboratory that obtains a significant increase in the recovery rates of valuable elements. In this process, HVTMs were first pelletized and oxidized by roasting (HVTMP) and then were reduced in a shaft furnace. Subsequently, the reduced HVTMPs were separated by melting for the comprehensive recovery of iron, titanium, vanadium, and chromium [6]. It is clear that the melting separation is an essential procedure in this novel and sustainable smelting process. It is known that in the melting separation process, the fluidity of the titanium slag at high temperature can affect the separation of iron and slag, thus, affecting the efficiency of the subsequent titanium extraction. Moreover, the extraction efficiency is closely related to the mineral composition and the microstructure of the slag which depend on the chemical composition, initial melting state, and cooling conditions. Therefore, to achieve high extraction efficiency of titanium, it is necessary to improve the fluidity of the slag.

In the melting separation process, CaF2 and B2O3 are commonly used as additives due to their remarkable effect on the fluidity of the slag. CaF2 breaks up into small particles at high temperature and then melts quickly, decreasing the melting temperature and viscosity of slag. Although CaF2 plays an important role in the metallurgical industry, its use leads to significant environmental pollution that is inconsistent with the development of sustainable metallurgy [7,8,9,10]. In the past decades, many studies have proposed to replace CaF2 with B2O3 that not only avoids the disadvantages of CaF2 but also achieves a better metallurgical effect than CaF2. The acidity of B2O3 is clearly stronger than that of SiO2. Therefore, the CaO in the slag reacts preferentially with B2O3 to form nCaO·B2O3 with a low melting point and then promotes the melting of lime. At the same time, the slag viscosity decreases with increasing B2O3 content. This improves the diffusion and mass transfer in the melting separation process, which is beneficial for the slag system and the melting separation process [9, 11,12,13,14,15,16,17,18].

The melting temperature and viscosity are crucial physical properties for the fluidity of slag. The effect of B2O3 on the slag properties has been examined in previous studies [19,20,21,22,23,24,25]. Wang et al. investigated the influence of B2O3 on the melting temperature and viscosity of refining flux. It was demonstrated that B2O3 was beneficial for decreasing the melting temperature and viscosity [20]. Ren et al. investigated the effect of B2O3 on the viscosity of Ti-bearing blast furnace slag and showed that the addition of B2O3 decreased the viscosity and improved the fluidity of the slag [21]. Li et al. found that for the CaO–SiO2–Al2O3–Cr2O3 slag, B2O3 behaved as a network former to decrease the number of non-bridging oxygen atoms per Si atom (NBO/Si). Unfortunately, the addition of B2O3 also resulted in the formation of low-melting-point eutectics and decreased the structural strength of the obtained metal [23]. To summarize, the studies performed to date have mainly focused on the effect of B2O3 on the blast furnace slag, mold flux, or refining slag. However, there have been few reports on the effect of B2O3 on the properties of high-titanium melting slag, particularly in the presence of chromium. In the Cr-containing high-titanium melting slag (CaO–SiO2–MgO–Al2O3–TiO2–Cr2O3), the TiO2 content is higher than that in the Ti-bearing blast furnace slag, and considerable utilization can be obtained if the titanium can be extracted effectively. Therefore, it is urgently necessary to improve the fluidity of the Cr-containing high-titanium melting slag.

In this study, B2O3 was introduced as a fluxing agent into the Cr-containing high-titanium melting slag, and the effect of B2O3 on the melting temperature and viscosity of this slag was investigated. In particular, the evolution of the phase composition and structure was examined. This study provides reference data for an environmental-friendly metallurgical process and also provides the basis for the subsequent extraction of titanium and other valuable elements. Thus, this work contributes to the development of sustainable metallurgy.

Experimental

Sample Preparation

To ensure the accuracy of the experiments, the samples were pre-melted to form a homogeneous slag. Certain amounts of oxides (Table 1) were proportionally weighed, mixed, and placed into a graphite crucible lined with molybdenum flakes and then were placed in a MoSi2 resistance furnace. The mixture was melted under argon atmosphere at 1550 °C. After full stirring, the slag was removed, cooled, and crushed for further experiments.

Table 1 Chemical composition of Cr-containing high-titanium melting slag (wt%)
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Experimental Apparatus

Figure 1 shows a schematic diagram of the instrument used for melting-point and melting-rate measurements that consists of three components, namely a light source, a heat source (an electric furnace with an alumina tube), and a camera system. A U-shaped MoSi2 is used as the heating element, and the highest operating temperature is 1550 °C. The temperature control precision is ± 2 °C and the heating thermocouple is located at the bottom of the sample and can display the temperature change. To reduce the experimental error, two sets of temperature readings were obtained for the same sample. If the difference between the two readings exceeded 3 °C, the sample was tested again.

Fig. 1
figure 1

Schematic diagram of the experimental apparatus for melting temperature

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The viscosity was measured using an RTW-10 melting physical property comprehensive measurement instrument designed by Northeastern University, as shown in Fig. 2. This system enables the continuous measurement of the viscosity during the cooling process and the fixed-point measurement of the viscosity at a constant temperature. The heating element consists of a U-shaped MoSi2 and a Pt–Rh thermocouple to ensure that the temperature deviation in the constant zone is less than 1 °C. The spindle is made of molybdenum and the connecting rod is made of corundum.

Fig. 2
figure 2

Schematic diagram of viscosity furnace and detailed crucible size

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Experimental Procedure

The slag was ground to 74 μm in an agate mortar and the standard sample was prepared by pressing the slag into the cylinder. Then, the standard sample was placed on a corundum sheet in the middle of the tube. The shape change of the standard sample was observed and the melting behavior was described by three characteristic temperatures. When the height of the standard sample decreased one fourth of its original height, the corresponding temperature was recorded as the softening temperature (ST). The hemispherical temperature (HT) was the temperature at which the height of the standard sample decreased to one half of its original height. In addition, this temperature was defined as the melting temperature and this method of temperature measurements is called the hemispherical method [25]. The flow temperature (FT) was the temperature at which the sample was liquefied and its height decreased three fourths of its original height. Figure 3 shows the morphology and characteristic temperature of the standard samples with different B2O3 contents in the heating process.

Fig. 3
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Morphology and characteristic temperature of standard sample with different B2O3 contents

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Prior to carrying out the viscosity experiment, the viscometer was calibrated using castor oil at room temperature [26,27,28]. The distance between the spindle and the bottom of the crucible was maintained at 10 mm. When the furnace temperature reached 1550 °C, the slag was kept at this temperature for 1 h, and then the viscosity measurement was carried out as the temperature decreased. To avoid the measurement interruption caused by the overload of the torque sensor, the viscosity measurement was terminated immediately when the viscosity reached 5 Pa s. During the viscosity measurement, the slag was fully liquid and acted as a Newtonian fluid. Each experiment was conducted twice to ensure the reliability of the results. Argon was used as the protective gas throughout the experiment.

When the viscosity measurement was completed, the furnace temperature decreased to 1100 °C, and the slag solidified. Then, the slag was removed from the furnace and was air cooled to room temperature. The slag sample was broken, and a uniform size was selected. To obtain a smooth surface, the sample was first polished using sand paper with different roughness grades from coarse to fine and then polished using a polishing machine. To improve its electrical conductivity, gold was sprayed onto the detection surface prior to scanning electron microscopy (SEM) observation. The other part was ground in an agate mortar and then filtered by a 200-mesh filter sieve. Then, the obtained powder was used for X-ray diffraction (XRD) analysis.

Characterization

The composition of the precipitated phase was identified by XRD (PANalytical) using Cu-Kα radiation with the voltage and current of 40 kV and 40 mA, respectively. The morphology of the precipitated phase was observed by SEM (Ultra Plus; Carl Zeiss GmbH, Jena, Germany) with energy dispersive spectroscopy (EDS) using a Schottky-type field emission electron source and resolution ratios of 0.8 nm/15 kV and 1.6 nm/1 kV at 20 V to 30 kV.

Results and Discussion

Effect of B2O3 on the Melting Property

The results of the effect of B2O3 on the characteristic temperature of Cr-containing high-titanium melting slag are shown in Fig. 4. With the increase of B2O3 content from 0 to 4%, the ST and the HT clearly decreased from 1204 and 1214 °C to 1144 and 1158 °C, respectively. It should be noted that the FT of the slag with B2O3 was higher than that without B2O3, which may be due to the expansion of the slag after the addition of B2O3. This gives rise to a severe flow deformation. On one hand, it is well known that B2O3 is an acid oxide. When B2O3 was added to the slag, according to ionic theory, the anionic structure became complicated, and the electrostatic attraction acting on the cations decreased, decreasing the surface tension. Therefore, the volume of the slag increased. On the other hand, at higher temperature, when B2O3 was not added, the fluidity of the slag is poor due to its complex structure. However, when B2O3 was added, it gave rise to a “flooding phenomenon” and increased the volume of the slag. The volume and height of the slag decreased only after the “bubble” was broken. Therefore, the flow temperature of the slag with B2O3 was higher than that without B2O3. When the temperature reached FT, the slag melted. However, when B2O3 content increased from 1 to 4%, the FT decreased.

Fig. 4
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Characteristic temperature of Cr-containing high-titanium melting slag with different B2O3 contents

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Since B2O3 is a typical acid oxide with a low melting point (~ 450 °C) [29], the presence of B2O3 is conducive to the fusing of CaO, Al2O3, and other high-melting-point components into slag and decreases the melting point of the slag. Additionally, B2O3 can easily combine with various oxides, forming low-melting-point eutectic crystals such as MgO·B2O3 (988 °C) and CaO·B2O3 (1100 °C). This is also conducive to the decrease of the melting temperature of the slag.

Effect of B2O3 on the Viscosity

Figure 5 shows the viscosity of the Cr-containing high-titanium melting slag with different B2O3 contents. It is clear that the viscosity increased with decreasing temperature. Another important finding was that the viscosity decreased significantly with the increase of B2O3 content, and this trend became more pronounced with higher B2O3 content. Generally, in a certain range of B2O3 content, a slag with high fluidity can be obtained at high temperature and high B2O3 content.

Fig. 5
figure 5

Change of the viscosity of Cr-containing high-titanium melting slag with different B2O3 contents

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According to a previous study, B2O3 is an acid oxide and acts as a network former [30]. In the slag, boron is found in the form of [BO4]5− that gives rise to a more complex structure of the silicate network. This increases the degree of polymerization (DOP) and decreases the NBO/Si [31,32,33,34,35,36,37,38]. While the presence of B2O3 generally leads to the increase of slag viscosity, the opposite tend was observed for the high-titanium melting slag. This unusual behavior can be explained as follows. First, B–O bond is weaker than Si–O bond and breaks more easily, reducing the polymerization strength [17]. Second, although boron forms [BO4]5− with a complex network structure, the increase in the content of this structure is equivalent to the dilution of the complex silicate network structure [18, 39, 40]. Finally, B2O3 has a low melting point and can easily combine with many different oxides to form a low-eutectic mixture. This not only decreases the slag viscosity but also significantly decreases the melting temperature and finally improves the slag fluidity [15, 16, 29].

Crystallization Morphology

The crystallization morphology of the Cr-containing high-titanium melting slag with different B2O3 contents are shown in Fig. 6. It was observed that with increasing B2O3 content, the morphology and size of the precipitated phases presented dense and sparse distribution. When B2O3 was not added, two types of precipitated phases could be found, namely dark-gray matrix phase and light-gray phase. The EDS results show that the matrix phase and the light-gray long-stripe phase were silicate and titanium bearing, respectively. For the B2O3 content of 1%, the light gray phase was almost entirely found in the form of small particles and the long-stripe phase disappeared. The EDS results presented in Fig. 6g indicated that some boron-containing phases precipitated and the dark gray phase was still the silicate matrix.

Fig. 6
figure 6figure 6

SEM–EDS analysis of Cr-containing high-titanium melting slag with different B2O3 contents. a 0%, b 1%, c 2%, d 3%, e 4%, and f–i EDS analysis

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When the B2O3 content increased to 2%, the small light-gray phase gradually grew to long stripe and almost disappeared. However, when the B2O3 content reached 3%, some new boron-containing phase precipitated. Moreover, based on the EDS results, it was determined that the new precipitated phase was borate. To determine the elemental distribution, EDS surface scanning was carried out, and the results are shown in Fig. 7. It was observed that Ca, Si, and Al were mainly concentrated in the matrix phase, while Ti was mainly concentrated in the long-stripe phase, and the other elements were evenly distributed. When the B2O3 content was 4%, the morphology and elemental composition did not change significantly, but the diffraction peak intensity of the precipitated borate phase increased.

Fig. 7
figure 7

Surface scanning of Cr-containing high-titanium melting slag with 3% B2O3

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Crystallization Phase Composition

The crystallization phase composition of the Cr-containing high-titanium melting slag was identified by XRD, and the results are shown in Fig. 8. In the absence of B2O3, six precipitated phases were found, namely anosovite (MgTi2O5), sphene (CaTiSiO5), pyroxene (CaMgSi2O6, CaTiSi2O6), perovskite (CaTiO3), anorthite (CaAl2Si2O8), and spinel (MgAlCrO4, MgCr2O4). After 1% B2O3 was added, the final precipitated phases contained several boron-containing phases such as kotoite (Mg3B2O6), suanite (Mg2B2O5), clinokurchatovite (CaMgB2O5), sinhalite (MgAlBO4), warwickite (MgTiBO4), and danburite (CaB2Si2O8). When B2O3 content increased to 4%, the types of the precipitated phase did not change significantly, but the diffraction peak intensities did change. The spinel phase disappeared completely, and the diffraction peak intensity of the perovskite with a high melting point also decreased. Moreover, based on the diffraction peak intensities, danburite became the main phase, verifying the reliability of the SEM results.

Fig. 8
figure 8

XRD patterns of precipitated phases with different B2O3 contents. a 0%, b 1%, c 2%, d 3%, and e 4%

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Discussion

Enrichment and extraction of titanium are important for the utilization of the Cr-containing high-titanium melting slag. However, the viscosity of the slag will increase due to the presence of chromium and this may greatly increase the difficulty of titanium extraction. Therefore, to improve the fluidity of the slag, B2O3 was introduced into the Cr-containing high-titanium melting slag. As different components have different effects on the slag system due to their different internal structures. Therefore, the elucidation of the relationship between the slag composition and its properties is a key goal of our investigations. Both the phase composition and microstructure of the slag directly reflect the internal structure and affected the melting and viscous properties of the slag. According to our experimental results, when the B2O3 content increased from 1 to 4%, the melting temperature decreased significantly. In addition, the viscosity of the slag gradually decreased with the increase in the temperature and the B2O3 content. This can greatly improve the fluidity of the slag, facilitate the separation of slag and iron, and decrease the iron loss. Moreover, the method of titanium enrichment plays a key role in the extraction of titanium.

As an acid oxide, B2O3 acts as a network former but still improves the fluidity of the slag at high temperature. B2O3 easily combined with other oxides to form low-melting-point eutectics, which was more effective in reducing slag viscosity. Addition of B2O3 depolymerized the complex 3-D [BO4]5− into a simple 2-D [BO3]3− structure, meanwhile, the bridging oxygen O0 transformed into non-bridging oxygen O− during depolymerization, depolymerizing the chain/molecule of the slag. These changes led to a decrease in the viscosity [41]. In addition, according to previous studies, the addition of B2O3 reduced the activation energy EA [42, 43]. The decrease of EA indicated that the (BO3)3– of the planar triangle was the main boron-containing unit in the melt that was looser than that of (BO45–) of the tetrahedral structure, implying that the addition of B2O3 was beneficial for improving the fluidity of slag [44]. In the absence of B2O3, the main enrichment phase of titanium was anosovite. However, after the addition of B2O3, the amount of the precipitated perovskite decreased and the titanium-bearing phase was mainly concentrated in anosovite, which was conducive for the subsequent titanium extraction process.

Conclusion

In this study, the effect of B2O3 on the melting temperature and viscosity of CaO–SiO2–MgO–Al2O3–TiO2–Cr2O3 slag was studied. The following conclusions were drawn:

  1. (1)

    The addition of B2O3 is beneficial for decreasing the melting temperature. When the B2O3 content increased from 0 to 4%, the melting temperature decreased from 1214 to 1158 °C. In addition, the softening temperature and the flow temperature also decreased by 60 °C and 30 °C, respectively.

  2. (2)

    The viscosity decreased with increasing B2O3 content, particularly when the B2O3 content increased from 3 to 4%. Thus, an increase of B2O3 content in a certain range greatly improves the fluidity of the slag.

  3. (3)

    B2O3 had a strong effect on the morphology of the precipitated phase as manifested in the size and shape of the precipitate particles. The morphology of the precipitated phase changed from small sheets to long stripes. In addition, the increase in the B2O3 content led to anosovite and danburite becoming the main precipitated phases and inhibited the precipitation of perovskite, which was beneficial for the subsequent titanium extraction process.

  4. 4)

    The addition of B2O3 to CaO–SiO2–MgO–Al2O3–TiO2–Cr2O3 slag is highly great significant for the improvement of slag fluidity and environmentally friendly, as well as for the development of sustainable metallurgy.