Introduction
Titanium is the seventh most abundant metal and the ninth most abundant element in the earth’s crust [1]. At least 90.5% of China’s titanium resources can be attributed to the vanadium -titanium magnetite ore deposits located in the Panzhihua region. The ore has a very high comprehensive utilization value since it contains value-added titanium and vanadium [2, 3]. However, it is quite difficult to completely remove the calcium and magnesium impurities in the smelting process to achieve a high-grade titanium slag . High ratios of CaO and MgO, varying between 6 and 9%, limit direct application of Panzhihua’s titanium slag in the chloride process for the purpose of titanium dioxide synthesis [4–6]. It is well known that Titanium tetrachloride (TiCl4) has been widely utilized as an intermediate material to produce titanium white and titanium sponge, both of which are major products in the titanium industry [7]. Therefore, the preparation of titanium tetrachloride using Panzhihua titanium resources is very promising for future use in the synthesis of titanium oxide and titanium sponge preparation.
Two main chlorination processes, fluidized bed chlorination and molten salt chlorination, are commonly used to produce titanium tetrachloride [8]. Currently, most TiCl4 is produced by the chlorination of rutile and high-grade titanium slag in a fluidized-bed reactor at 1272 K [9], where TiO2 content is more than 90%. Previous studies found that when the molten salt chlorination technique was conducted in a higher content of magnesium and calcium, the fluidized beds became plugged [10]. However, at the Pangang titanium sponge plant in China, titanium slag with 85 wt% TiO2 was successfully carbo-chlorinated in a molten salt bath at 1023–1323 °C. In this work, lower grade titanium slag , smelted from Panzhihua’s titanium resources, with a TiO2 content of about 74.6 wt%, was carbo-chlorinated in NaCl and KCl molten salt . The goal of this study is to develop a process to treat low-grade titanium slag with a higher impurity content of CaO and MgO for producing purified TiCl4 which can be further used as raw material to produce titanium sponge.
Experimental
Experimental Method

Procedure for chlorination of low-grade titanium slag
The molten salt chlorination process consists of the following four steps: the mixture of materials, the chlorination of titanium slag , semi-circulating of TiCl4 pulp, and off-gas treatment. In this process, a molten salt chlorinating furnace with an inner diameter of 3.3 m was employed. A mixture of titanium slag , petroleum coke, and solid NaCl were added in the middle-upper part of the furnace at a rate of 1.8–2.0 t/h. The high temperature flue gas mixture generated in the chlorination process was dusted and then consequently condensed to crude TiCl4 liquid. In the meantime, furnace slag was discharged regularly. The concentration of element analysis was performed by ICP-OES(ICAP6300 Therom, USA). X-ray diffraction (XRD) patterns were carried out with Cu Kα radiation at 40 kV and 44 mA (Rikagu, Japan). The morphology and components of furnace slag were achieved by a scanning electron microscope (SEM) using FEI Quanta 650(USA) with an energy -dispersive spectrometer (EDS).
Materials and Analysis
Composition of low-grade titanium slag /mas. %
TiO2 | Al2O3 | CaO | MgO | MnO | Cr2O3 | V2O5 | FeO | SiO2 |
---|---|---|---|---|---|---|---|---|
74.6 | 1.71 | 1.85 | 6.42 | 1.25 | 0.06 | 0.18 | 8.30 | 4.5 |
Composition of petroleum coke/mas. %
Fixed carbon | Ash content | Vdaf | Moisture |
---|---|---|---|
98.45 | 0.36 | 0.4 | 0.07 |

XRD pattern of low-grade titanium slag
Results and Discussion
Thermodynamic Analysis of Carbochlorination
Chemical reactions of carbochlorination process
Reactions of carbochlorination | No. |
---|---|
1/2TiO2(s) +1/2C(s) + Cl2(g) = 1/2TiCl4(g) +1/2CO2(g) | (1) |
2/3FeO(s) + 1/3C(s) + Cl2(g) = 2/3FeCl3(g) + 1/3CO2(g) | (2) |
1/3Fe2O3(s) + 1/2C(s) + Cl2(g) = 2/3FeCl3(g) + 1/2CO2(g) | (3) |
FeO(s) +1/2 C(s) + Cl2(g) = FeCl2(s) + 1/2CO2(g) | (4) |
CaO(s) + 1/2C(s) + Cl2(g) = CaCl2(l) + 1/2CO2(g) | (5) |
MgO(s) + 1/2C(s) + Cl2(g) = MgCl2(l) + 1/2CO2(g) | (6) |
MnO (s) + 1/2C(s) + Cl2(g) = MnCl2(l) + 1/2CO2(g) | (7) |
1/2SiO2(s) +1/2C(s) + Cl2(g) = 1/2SiCl4(g) + 1/2CO2(g) | (8) |
1/3Al2O3(s) +1/2C(s) + Cl2(g) = 2/3AlCl3(g) + 1/2CO2(g) | (9) |
1/3V2O5(s) +1/2C(s) + Cl2(g) = 2/3VOCl3(g) + 1/2CO2(g) | (10) |
1/3Cr2O3(s) + Cl2(g) + 1/2C = 2/3CrCl3(s) + 1/2CO2(g) | (11) |
1/2TiO2(s) +C(s) + Cl2(g) = 1/2TiCl4(g) +CO(g) | (12) |
2/3FeO(s) +2/3C(s) + Cl2(g) = 2/3FeCl3(g) +2/3CO(g) | (13) |
CaO(s) +C(s) + Cl2(g) = CaCl2(l) + CO(g) | (14) |
MgO(s) +C(s) + Cl2(g) = MgCl2(l) + CO(g) | (15) |
MnO (s) + C(s) + Cl2(g) = MnCl2(l) + CO(g) | (16) |
1/2SiO2(s) +C(s) + Cl2(g) = 1/2SiCl4(g) + CO(g) | (17) |
1/3Al2O3(s) +C(s) + Cl2(g) = 2/3A lCl3(g) + CO(g) | (18) |
1/3V2O5(s) +C(s) + Cl2(g) = 2/3VOCl3(g) + CO(g) | (19) |

Gibbs free energy of reactions (1)–(11)

Gibbs free energies of reactions (12)–(19)
The results in Figs. 3 and 4 indicate that the Gibbs free energy of metal oxide carbochlorination reactions is negative at temperatures from 875 to 1175 K. This means both of TiO2 and other oxides can be chloridized into metal chlorides. The carbochlorination reactions of oxide impurities in titanium slag , such as MnO, CaO , MgO, FeO and V2O3, are more likely to occur than that of TiO2, which results in an increased level of impurities in titanium tetrachloride . In contrast, impurities, such as Cr2O3, Al2O3 and SiO2 are energetically unfavorable and are less likely to chloridize than TiO2. When comparing the Gibbs free energies of reaction (12) with that of reaction (1), it is shown that the main oxycarbide product is CO2, rather than CO when the temperature is lower than 1075 K. Expectedly, the result matches well with the analysis reported in literature 9.
Melting point and boiling point of products
Items | Melting point/K | Boiling point/K |
---|---|---|
TiCl4 | 244 | 404 |
VOCl3 | 189 | 395 |
SiCl4 | 198 | 329 |
AlCl3 | 461 | 449 |
FeCl3 | 572 | 600 |
FeCl2 | 945 | 1280 |
MnCl2 | 918 | 1499 |
MgCl2 | 982 | 1686 |
CaCl2 | 1040 | 2268 |
CrCl3 | 1425 | 1573 |
According to the summary in Table 4, the boiling points of VOCl3, SiCl4, AlCl3 and FeCl3 are similar to that of TiCl4 under atmospheric conditions. On the contrary, the boiling points of FeCl2, MnCl2, MgCl2, CaCl2 and CrCl3 are much higher than that of TiCl4. This indicates that the impurities of FeCl2, MnCl2, MgCl2, CaCl2 and CrCl3 transform into molten salt furnace slag when the chlorination temperature is lower than 1280 K. Meanwhile, the major impurities in crude TiCl4 are SiCl4, VOCl3, AlCl3 and FeCl3. Thus, it is believed that a lower chlorination temperature is energetically favorable when attempting to lower the impurity content of crude TiCl4. In order to increase impurities in the molten state and maintain a stable mobility of the molten salt system, the chlorination temperatures could perform between 1000 and 1075 K.
Temperature Control of Chlorination Process
The chlorination of titanium slag in the molten salt bath releases large amounts of heat. Thus, it is necessary to reflux the condensed TiCl4 mud or pulp into chlorination furnace to ensure a stable temperature of both molten salt and mixed flue-gas.

Temperature changes in the chlorinetion process
Chemical composition of dust slag /mas. %
TiO2 | FeCl2 | FeCl3 | MnCl2 | AlCl3 | C |
---|---|---|---|---|---|
24.13 | 1.77 | 31.21 | 0.97 | 23.02 | 4.18 |
Furnace Slag Characteristics

XRD pattern of molten salt and washed furnace slag

SEM and EDS analysis of the washed residue
It can be seen that the unreacted titanium slag particles are dense and many noticeable particles are attached onto the surface of the residue. Point analysis was conducted using EDAX of SEM on the particle surfaces. This analysis shows that both spot 1 and spot 2 are mainly composed of titanium oxide, calcium oxide, aluminum oxide and silicate oxide. A good agreement is achieved between the thermodynamic analysis predictions and the experimental data. It further verified aluminum oxide and silicate oxide were more difficult to chloridize than titanium oxide.
Composition of Crude TiCl4
Chemical composition of crude TiCl4/mas. %
TiCl4 | FeCl3 | SiCl4 | VOCl3 | AlCl3 | |
---|---|---|---|---|---|
Titanium slag with 74 wt% TiO2 | 98.83 | 0.0068 | 0.0406 | 0.1292 | 0.0093 |
Titanium slag with 85 wt% TiO2 | 98.92 | 0.0050 | 0.0250 | 0.1045 | 0.0061 |
Requirements of refining processes | 98.50 | 0.0100 | 0.4000 | 0.2000 | 0.0500 |
The average solid impurities dissolubility content in crude TiCl4 is 4.5 g/L. From Table 6, it can be seen that the percent content of TiCl4 in the final products was more than 98.8%, which meet the requirements for refining processes. Additionally, the impurities, including FeCl3, SiCl4, VOCl3 and AlCl3, were slightly lower than that of titanium slag with 74.6 wt% TiO2. This is probably due to the relatively lower content of impurities initially found in the titanium slag . However, both of them meet the requirements of crude TiCl4 refining processes. This in turn confirms that the titanium slag with 74.6 wt% can also be used as raw material for the production of TiCl4. Although titanium slag with 74.6 wt% TiO2 can be directly utilized for the TiCl4 production, studies are still necessary to further remove impurities in the crude TiCl4. Possible solutions may be to decrease the chlorination temperature or optimize the process of crude TiCl4 sedimentation.
Conclusions
Based on the results obtained in this study, the following conclusions can be drawn. The molten salt chlorination technique can be used to dispose low-grade titanium slag with 74.6 wt% TiO2 for TiCl4 preparation. Experimental results were consistent to the thermodynamic analysis. The oxide impurities in titanium slag can also be chloridized into FeCl2, MnCl2, MgCl2, CaCl2 and CrCl3, all of which can be collected in the molten salt furnace slag when the chlorination temperature is below 1280 K. SiO2 and Al2O3 in titanium slag are less likely to chloridize than TiO2 and difficult to chlorinate completely. The content of TiCl4 in the final products was found to be greater than 98.8 wt%. The impurity content of FeCl3, SiCl4, VOCl3 and AlCl3 still meet the requirements for the crude TiCl4 refining processes.
Acknowledgements
Authors are grateful to the financial support of Panzhihua Iron and Steel (Group) Company Limited (China) for this research.