Introduction

With an estimated average concentration in the earth’s crust of 0.42%, titanium is widely distributed in the earth’s crust as a silver white transition metal [1, 2]. Owing to its high strength, low density, high hardness, high melting point, and strong corrosion resistance, titanium and its compounds would be ideal metallic materials employed in many fields. China has the most abundant titanium resources in the world, and about 90% of which are in the form of vanadium-titanium magnetite in Panzhihua area, the southwestern part of China [3]. As a solid waste for smelting vanadium-titanium magnetite in blast furnace, titanium-bearing blast furnace slag contains a lot of titanium oxide (TiO2 about 23%) [4]. However, the utilization of titanium-bearing blast slag is still an unsolved problem now.

It is well known that the composition of titanium-containing blast furnace slag is quite complex, and the chemical composition of Panzhihua titanium-containing blast furnace slag was shown in Table 1 [5]. It can be seen that the titanium-bearing blast furnace slag contains a large amount of impurity oxides, which would combine with the titanium oxides to form a variety of titanium-containing phases during the cooling process of slag, such as perovskite (CaTiO-3), anosovite (M3O5), and so on. Related studies have shown that the crystalline behavior of perovskite and anosovite in the slag is affected by the cooling rate and slag basicity [6, 7]. So, titanium element in titanium-bearing blast furnace slag can be enriched and precipitated in the form of perovskite or anosovite by adjusting these factors. Then, the remaining problem is how to extract titanium from perovskite and anosovite.

Table 1 Chemical compositions of Panzhihua titanium-containing blast furnace slag (wt.%)
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Methane (CH4), as an efficient and clean reducing-carbonizing agent, has been widely concerned by researchers in recent years. CH4 would decompose rapidly to solid-free carbon and hydrogen at elevated temperatures over 1000 °C. Hydrogen is widely used for the reduction of metal oxides, while the solid-free carbon can be used as reducing agent and carbonizing agent. In recent research, researchers have successfully used methane gas as reductants to reduce metal oxides, such as MnO2, WO3, Fe2O3, Cr2O3 [8,9,10,11], and so on. These studies showed that methane gas could reduce metal oxides to its metal and metal carbides at lower temperatures (compared with conventional carbothermal reduction).

A few reports have been published on the reduction of titanium-containing phases with methane gas. Zhang et al. reported that the optimum conditions for TiCxOy formed from titanium dioxide in CH4–H2–Ar atmosphere were in the temperature range of 1300–1450 °C [12]. In our previous work [13, 14], we also found that titanium compounds in the titanium slag and ilmenite can be reduced to titanium oxycarbide by methane-containing gas at 1200 °C and above, while perovskite could be reduced completely to TiCxOy by CH4-H2 gas mixture at 1400 °C and above [15]. However, to the best of our knowledge, no report has been published on the reduction of anosovite with CH4. Therefore, the present study aimed to explore the possibility of reduction and carbonization of anosovite (MgTi2O5) at low temperature with CH4 by thermodynamically and experimentally.

Experimental

Materials

Due to the complex and diverse mineral phase composition of anosovite, the raw material used in this study was the most common magnesium-containing anosovite phase (MgTi2O5), which was homemade by mixing and sintering MgO powders (99.8%, Chengdu Kelong Chemical Co, Ltd) and TiO2 powders (99.8%, Chengdu Kelong Chemical Co, Ltd) in a molar ratio of 1:2. The mixed powders were first sintered at 1400 °C for 5 h, and then broken and sifted to obtain different sizes of particles. The particle size used in this study was in the range of 100–200 mesh (75–150 μm). XRD patterns of the particles are shown in Fig. 1. The assisted additive was reagent-grade Fe2O3 powders (99%, Adamas Reagent Co., Ltd). The employed gases were CH4 (99.99 vol.% Chongqing Ruixin Gas Co., Ltd), H2 (99.999 vol.% Chongqing Ruixin Gas Co., Ltd), and Ar (99.999 vol.% Chongqing Ruixin Gas Co., Ltd).

Fig. 1
figure 1

XRD patterns of homemade anosovite

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Experimental Procedure

The experimental apparatus employed in this study basically includes vertical sealed furnace (heated by silicon-molybdenum heater) with a corundum furnace tube (60 mm in diameter, 1000 mm in length) and gas flow controllers (Alicant, Model MC-500SCCM-D and MC-1SLPM-D), as shown in Fig. 2.

Fig. 2
figure 2

Schematic diagram of the experimental apparatus: 1—furnace, 2—Al2O3 crucible, 3—filter, 4—infrared spectrometer, 5—data collector, 6—temperature controller, and 7—mass flow controller

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A crucible (inner height 30 mm x inner diameter 16 mm) with around 0.5 g sample was placed in the center of the furnace tube (the typical depth of samples in the crucible was about 3 mm). Samples were first heated in flowing Ar (99.999 vol.%, without any other treatment before it entered the furnace) to the required temperature, and then Ar flow was stopped with methane-hydrogen gas mixture (the volume ratio of methane to hydrogen is 8:92) flowing into the system at a flow rate of 500 sccm/min. After reduction for a certain time, the reaction products were allowed to cool quickly to room temperature at a flowing Ar atmosphere. And the reaction products would be characterized by X-ray diffraction (XRD, Cu Kα radiation, PANalytical X’Pert Powder, and Panalytical B.V.).

Thermodynamic Analysis

In order to predict the reduction process of MgTi2O5 by methane–hydrogen gas mixture, thermodynamic analysis based on the method of equilibrium calculations has been carried out in this study. The phase equilibrium processes in the MgTi2O5–CH4–H2 systems were estimated by FactSage software using the database of Cao et al [16]. And the partial pressure of methane and hydrogen in the systems was set to 0.08 and 0.92 atm, which was consistent with the experimental conditions.

Figure 3 displays the equilibrium phase composition of MgTi2O5 at 1200 °C, 1250 °C, 1300 °C, and 1350 °C, respectively, with various CH4/MgTi2O5 molar ratios (the molar number of MgTi2O5 is 1 mol). It can be found that the predicted changes in phases upon reaction at four temperatures are similar, while the molar ratios of CH4/MgTi2O5 required to each reduction steps decreases a little as the temperature increases. And MgTi2O5 can theoretically be completely reduced by methane-hydrogen gas mixture at 1200 °C and above. As the CH4 content increases, MgTi2O5 is first converted to anosovite (mostly MgTi2O5, with trace Ti2O3) which is immediately reduced to geikielite (mostly MgTiO3, with trace Ti2O3). Geikielite would be reduced to TiCxOy and spinel (contains Mg3O4 and MgTi2O4) when the reduction process of anosovite to geikielite is completed, and the final reduction products are TiCxOy and MgO. However, MgO does not appear immediately with the formation of TiCxOy. The initially formed MgO would enter the spinel phase, and then it appears with the reduction of spinel. Deposited carbon is found to form after the completions of reduction and carbonization processes, meaning that the reduction rate of MgTi2O5 is faster than the cracking rate of CH4. Note that when MgTi2O5 is completely reduced, MgO would be reduced to magnesium vapor at 1300 °C and above thermodynamically, which may not happen in actual experiments.

Fig. 3
figure 3

Calculated equilibrium solid phases for reaction of MgTi2O5 with CH4-H2 mixture. a 1200 °C, b 1250 °C, c 1300 °C, and d 1350 °C

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The red dotted line in Fig. 3 represents the mole fraction of TiC in the TiCxOy phase. The composition of initially formed TiCxOy at 1200 °C, 1250 °C, 1300 °C, and 1350 °C would be approximately TiO0.47C0.53, TiO0.48C0.52, TiO0.5C0.5, and TiO0.52C0.48- unchanged, respectively. With the reduction of spinel, the mole fraction of TiC starts to increase and then pause with the formation of MgO. When spinel runs out, the mole fraction of TiC increases quickly as the CH4/MgTi2O5 mol ratio rises. However, the increase rate almost flattens out when deposited carbon starts to form, suggesting that the further carbonization of TiCxOy presents a thermodynamic limitation. With mole ratio of 10 for CH4/MgTi2O5, TiC in TiCxOy obtained at 1200 °C, 1250 °C, 1300 °C, and 1350 °C would increase to 0.698, 0.732, 0.764, and 0.794, respectively. It means that the increase of temperature can also increase the final mole fraction of TiC significantly.

Results and Discussion

Effect of Reaction Temperature

Higher temperature is helpful to the reduction reaction in thermodynamics. Therefore, to study the effect of temperature on the reduction, samples were reduced for 8 h at a set of temperatures of 1200 °C, 1250 °C, 1300 °C, and 1350 °C, respectively. XRD patterns of the reduced samples and the summarized phases are shown in Fig. 4 and Table 2. As presented, almost all the MgTi2O5 in the samples was reduced to MgTiO3 and MgTi2O4 at 1200 °C, while a small amount of TiCxOy and MgO also formed in the reduced samples. This means that MgTi2O5 could be reduced to TiCxOy by methane-hydrogen gas mixture at 1200 °C. As the reduction temperature increased to 1250 °C, major phases in the reduced samples were MgTi2O4 and TiCxOy, with a small amount of MgTiO3. It suggests that TiCxOy formation from MgTi2O5 in CH4-H2 atmosphere follows the path of MgTi2O5 → MgTiO3 → MgTi2O4 → TiCxOy, which was similar to the thermodynamic calculation results. When the reduction temperature increased to 1300 °C and above, samples were substantially completely reduced and carbonized to TiCxOy. The residual trace amounts of MgTi2O4 in the reduced samples may be caused by the unreduced sample at the bottom of crucible because of the poor gas permeability. Obviously, the increase of temperature was conducive to the reduction reaction of MgTi2O5 by methane–hydrogen gas mixture, which would also promote the formation of deposited carbon.

Fig. 4
figure 4

XRD patterns of MgTi2O5 samples reduced for 8 h at different temperatures

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Table 2 Phases in samples reduced for 8 h at different temperatures (corresponding to Fig. 4)
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Effect of Iron Oxides Addition

Based on our previous work [17], iron addition as catalyst was found to effectively promote the reduction rate and carbonization extent of TiO2 at a lower temperature (1200 °C and above). And related catalytic reaction mechanisms of iron can be summarized as follows:

methane pyrolysis reaction:

$$ {\text{CH}}_{4} = {\text{C}} + 4 \left[ {\text{H}} \right]/2 {\text{H}}_{2} \left( {\text{iron as the catalyst}} \right) $$
(1)

water-gas-shift reaction:

$$ {\text{C}} + {\text{H}}_{2} {\text{O}} = {\text{CO}} + {\text{H}}_{2} \left( {\text{iron as the catalyst}} \right) $$
(2)

Boudouard reaction:

$$ {\text{C}} + {\text{CO}}_{2} = 2{\text{CO }}\left( {\text{iron as the catalyst}} \right) $$
(3)

where [H] represents the dissociated hydrogen atom.

Therefore, in order to study the effect of addition on the reduction reaction, parallel tests were set up during studying the effect of reaction temperature. Pure samples and samples with adding 9 wt% Fe2O3 were tested simultaneously (the weight of both samples was 0.5 g).

Figure 5 shows the XRD patterns of pure samples and samples with adding 9 wt% Fe2O3 reacted at different temperatures. It can be seen that the sample of MgTi2O5 with adding 9 wt% Fe2O3 was completely reduced to TiCxOy, MgO, and Fe at the temperature of 1200 °C and above. Compared with the reduction of pure MgTi2O5 under the same experimental conditions, it could be affirmed that the reduction of MgTi2O5 was greatly promoted by the addition of Fe2O3. Meanwhile, the addition of iron oxides would obviously intensify the production of carbon deposition at 1200 °C and above.

Fig. 5
figure 5

XRD patterns of pure samples and samples with 9 wt% Fe2O3 reduced for 8 h at different temperatures

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Conclusions

The reduction behaviors of MgTi2O5 by CH4–H2 gas mixture were preliminarily studied thermodynamically and experimentally. Both thermodynamic results and experimental results indicated that MgTi2O5 could be reduced to TiCxOy at 1200 °C and above. The reduction rate and extent of MgTi2O5 could be greatly improved by increasing the reaction temperature. Addition of iron oxides could effectively promote the reduction rate of MgTi2O5, as well as the generation of deposited carbon.

In general, this study provided a possibility for the low-temperature carbonization of MgTi2O5, and thus proposed a possible way to recycle titanium in the titanium-bearing blast furnace slag.