Carbochlorination of Low-Grade Titanium Slag to Titanium Tetrachloride in Molten Salt

Abstract

Thermodynamic analysis and experiments were conducted in order to verify the feasibility of preparing crude titanium tetrachloride (TiCl₄) via the carbochlorination of low-grade titanium slag in molten salt . Titanium slag , assaying 74.6 wt% TiO₂ with high calcium and magnesium oxide impurities, was treated by an optimized carbochlorination process in NaCl molten salt . These impurities in the titanium slag were chloridized simultaneously, and chlorination products FeCl₂, MnCl₂, MgCl₂, CaCl₂ and CrCl₃ were collected in the furnace slag . XRD and SEM/EDS analysis of residue shown that SiO₂ and Al₂O₃ in titanium slag were difficult to chlorinate completely. Theoretical calculations and industrial-scale experimental studies reveal the content of TiCl₄ in the products was more than 98.8 wt% and thus proved the feasibility of utilizing low-grade titanium slag for TiCl₄ preparation by molten salt chlorination technology.

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

The procedure for the chlorination of low-grade titanium slag is shown in Fig. 1.

../images/468727_1_En_59_Chapter/468727_1_En_59_Fig1_HTML.gif — Fig. 1
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

Low-grade titanium slag with particle size between 50 and 100 μm was produced by Pangang Titanium Industry Company. The composition of titanium slag is listed in Table 1.

Table 1

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

As seen in Table 1, the Titanium slag is rich in calcium oxide and magnesium oxide impurities. Additionally, the composition of petroleum coke is described in Table 2. Chloride (Cl₂) in the off-gas discharged from MgCl₂ electrolysis in the Pangang titanium sponge plant was used as the chlorinating agent. The Cl₂ volume fraction was around 85% sodium chloride (NaCl), with a grade of 98%. In addition, a small amount of potassium chloride (KCl) was used as molten salt material.

Table 2

Composition of petroleum coke/mas. %

Fixed carbon	Ash content	Vdaf	Moisture
98.45	0.36	0.4	0.07

The main phase composition of low- grade titanium slag used was shown in Fig. 2. The Fig. 2 shows that the main component of the low-grade titanium slag is M₃O₅ (pseudobrookite), which is a solid solution consisting of M₂(Ti₄)₂O₅ and (M₃)₂Ti₄O₅. It accommodates divalent and trivalent ions together with the tetravalent titanium [10]. The divalent ions in this solution are Fe²⁺, Mg²⁺, and Mn²⁺, and the trivalent ions are Ti³⁺, Cr³⁺, Al³⁺, and V³⁺. Diffraction peaks of TiO₂ also were observed in the XRD patterns.

../images/468727_1_En_59_Chapter/468727_1_En_59_Fig2_HTML.gif — Fig. 2
XRD pattern of low-grade titanium slag

Results and Discussion

Thermodynamic Analysis of Carbochlorination

Based on the chemical compositions achieved from the testing results of titanium slag and XRD analysis, it reveals that various chemical reactions probably occur in the titanium slag carbochlorination process. These corresponding reactions are listed in Table 3.

Table 3

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)

As seen in Table 3, the main products of titanium slag carbochlorination reaction are TiCl₄, metal impurity chloride , CO_2, and CO . Chlorination of TiO₂ may occur via the reactions of (1) and/or (12). The standard Gibbs free energies of reactions (1)–(11) and reactions (12)–(19) are presented in Figs. 3 and 4 respectively.

../images/468727_1_En_59_Chapter/468727_1_En_59_Fig3_HTML.gif — Fig. 3
Gibbs free energy of reactions (1)–(11)

../images/468727_1_En_59_Chapter/468727_1_En_59_Fig4_HTML.gif — Fig. 4
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.

The melting point and boiling point of generating chloride products are shown in Table 4.

Table 4

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 VOCl_3, SiCl_4, 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.

Data representing temperature changes of different test batches in the chlorination process is shown in Fig. 5. The temperature of molten salt fluctuated at 1050 K. In return, this confirms that due to the high temperature it is possible to get rid of the chloride impurities from volatilizing into crude titanium . That means FeCl₂, MnCl₂, MgCl_2, CaCl₂ and CrCl₃ can be dissolved in the molten salt , and then consequently discharged together with furnace slag at the stabilized temperature of 1050 K. The temperatures of the mixed flue-gas and the flue-gas after dust removal range between 650–750 K and 608–670 K respectively. Moreover, it should also be noted that the temperature changes of the mixed flue-gas and the flue-gas after dust removal experience a similar trend, meaning temperature can be steadily controlled in the carbochlorination process of low-grade titanium slag .

../images/468727_1_En_59_Chapter/468727_1_En_59_Fig5_HTML.gif — Fig. 5
Temperature changes in the chlorinetion process

The dust slag chemical composition are shown in Table 5. Table 5 indicates that most of the unreacted fine solid material particles and produced high boiling point impurities can be collected in the dust slag between the temperatures of 608–670 K.

Table 5

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

To study the components of unreacted titanium slag in the molten salt , some of the test samples were washed with deionized water to remove chloride before the XRD test. XRD analysis of molten salt and rinsed furnace slag after chlorination is shown in Fig. 6. The results in Fig. 6 indicate that the main components in molten salt are NaCl and KCl. Solid solution phase of (Fe,Mg)₂TiO₅ and (Ca,Mg,Al)SiO₃ were also detected in the residue. This demonstrates that impurities in titanium slag , such as SiO_2, Al₂O_3, are difficult to chlorinate completely.

../images/468727_1_En_59_Chapter/468727_1_En_59_Fig6_HTML.gif — Fig. 6
XRD pattern of molten salt and washed furnace slag

The SEM and EDS spectra of the residue are shown in Fig. 7.

../images/468727_1_En_59_Chapter/468727_1_En_59_Fig7_HTML.gif — Fig. 7
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

To make a comparison, the high-grade titanium slag with 85 wt% TiO₂ was used as a reference to produce crude TiCl₄. The chemical composition of crude TiCl₄ product after condensation and sedimentation is shown in Table 6.

Table 6

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 CrCl_3, 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.