Study on Direct Preparation of Titanium-Chromium Alloy by Molten Salt Electrolysis

Inter-metal compounds as quite a potential for development of high-temperature structural materials have been widely aroused interest. The Laves phase is the largest group of intermetallic compounds. Laves phase TiCr ₂ is an intermetallic compound easily formed in the hypereutectic titanium chromium alloy. It still exhibits excellent creep resistance at 1100 ° C and has excellent With good oxidation resistance, TiCr-based alloys not only have excellent mechanical properties, but also have potential superior hydrogen storage properties. TiCr-based hydrogen storage alloys were first discovered in the early 1980s by the Brookhaven laboratory in the United States. These alloys have received great attention since their discovery due to their complex hydride composition. TiCr-based hydrogen storage alloy has a high hydrogen storage density, and its maximum hydrogen storage mass ratio exceeds 2.4% (mass fraction). Japan is juxtaposed with TiCr alloy and Mg-based hydrogen storage alloy in the study of classification and development trend of hydrogen storage alloys. It is a third-generation hydrogen storage alloy.

At present, the preparation of TiCr alloy is mainly based on pure metal, and then a dense alloy is prepared by powder metallurgy or high temperature vacuum melting. Due to the complicated production process of raw material sponge titanium, high energy consumption and low efficiency, coupled with the need to increase the new energy consumption in the alloying process, the production cost of titanium-chromium alloy is high, so reducing the smelting and processing cost of titanium alloy is the material industry and The titanium industry has been striving for the goal. The molten salt electrolysis of metal oxides is a new electrolysis process, first proposed by Fray et al. of the University of Cambridge in the United Kingdom at the end of the 20th century. The biggest feature of this method is that the process is simple, non-polluting and adaptable. The metal oxide mixture directly produces the alloy; the method has less equipment investment and the cost is expected to be lower than the conventional production method. Around this method, the study reported electrowinning a metal from a metal oxide of titanium, niobium, chromium, silicon and the like in the world. At home and abroad, the preparation of Nb ₃ Sn alloy, TiW alloy, TiNi, TiFe, etc. by molten salt electrolysis has been reported, but there is little research on titanium chromium alloy. This paper explores the feasibility of directly preparing titanium-chromium alloy by molten salt electrolysis.

    First, the experiment

(1) Equipment and raw materials

The experimental device is shown in Figure 1. In the experiment, a resistance heating furnace is used, and a temperature controller is provided. The electrolytic cell is a graphite crucible, which is built in a stainless steel reactor, and the electrolysis power source is a WYK-3010 DC stabilized power supply.

Figure 1 Schematic diagram of the electrolysis experiment device

The electrolytic raw materials used in the experiment were analytically pure TiO ₂ and Cr ₂ O ₃ ; the molten salt was analytically pure anhydrous calcium oxide, and the content was >96%, wherein the content of other impurities was not more than 0.5% except for water.

The electrolysis process is carried out under the protection of high purity argon, wherein the Ar content is >99.999%, the O ₂ content is <3×10 ^-4 %, and the H ₂ O content is <3×10 ^-4 %.

The main analytical equipment was: the phase and composition of the product were analyzed by the Dutch PHILIPS X'Pert Pro Super X-ray diffractometer (Cu Ka target, tube voltage 40 kV, current 40 mA); using Japanese HITACHI S-4800 field emission scanning electron microscope The sample was analyzed by X-ray energy spectrometer (EDS) for elemental analysis; the oxygen content of the electrolysis product was analyzed by the American LECO TC-436 nitroxeter.

(two) experimental steps

The titanium dioxide and chromium oxide powders were mixed with a molar ratio of 1:1 and then added with a certain amount of adhesive. After mixing, the mixture was pressed into an electrode having a diameter of 10 mm, and the electrode molding pressure was 4 to 10 MPa. It was allowed to stand for 2 days at room temperature, allowed to dry naturally, and then sintered in a muffle furnace at a temperature of 900 to 1200 ° C for several hours before being used for the electrolysis experiment. The electrolysis experiment was carried out in a device as shown in Fig. 1, using a high-density graphite crucible wall as an anode, a mixture of sintered metal oxide as a cathode, and a calcium chloride melt under the protection of argon (100 ml·min ^-1 ). Electrolysis is carried out in the salt. First, the graphite rod is used as the cathode, and the graphite crucible is used as the anode. The pre-electrolysis is carried out at a voltage of 1.5 V. The purpose is to remove the residual moisture and impurities in the molten salt, and then carry out constant pressure electrolysis at a specified voltage, and the electrolysis temperature is controlled at 900. °C. After the end of the electrolysis, the electrolysis product was naturally cooled to room temperature under argon gas protection.

(three) sample testing

After electrolysis, the surface was rinsed with water, and the salt was washed with distilled water under ultrasonic assistance. After drying, the obtained sample was subjected to SEM, EDS, XRD analysis and oxygen content analysis.

    Second, the results and discussion

(1) Preparation of titanium chromium alloy

The microstructure obtained by sintering the electrode with TiO ₂ +Cr ₂ O ₃ (molar ratio 1:1) at 1050 °C for 2 h is shown in Fig. 2(a). The results of XRD analysis indicate that the electrode consists of TiO ₂ and Cr ₂ O ₃ . Figure 3 (a) illustrates that TiO ₂ and Cr ₂ O ₃ do not undergo a chemical reaction during sintering. Fig. 2(b) shows the microstructure of the product obtained by electrolysis at 2.8 V for 6 h. The particles grow up to about twice the initial electrode. The XRD analysis electrolysis products are mainly TiCr ₂ and a small amount of Cr, as shown in Fig. 3(d). The DES analysis of the electrolysis product showed that the molar ratio of Cr to Ti in the electrolysis product was 1.95. Considering the analysis error, the ratio of Cr and Ti in the electrolysis product was close to the ratio of the raw materials in the initial electrolysis, indicating the mixed oxidation of titanium and chromium in molten salt electrolysis. The composition can be directly prepared to form a controllable titanium chromium alloy.

Figure 2 SEM image of the electrode before and after electrolysis

(a) - initial electrode; (b) -2.8V electrolysis 6h electrolysis product

Figure 3 XRD spectra of the initial electrode and electrolysis products at different times

(2) The formation process of titanium-chromium alloy under constant pressure

In order to better understand the reduction process of the mixed oxides of TiO ₂ and Cr ₂ O ₃ , the control cell voltage was 2.8 V, and electrolysis was carried out for 10 min, 1 h and 6 h, respectively, and the XRD pattern of the obtained product is shown in Fig. 3. It can be seen from the figure that the electrolytic reduction of mixed oxides has undergone an alloying process from the preferential formation of Cr to the formation of TiCr ₂ . According to the product composition and thermodynamic calculation at different stages of electrolysis, it is speculated that TiO ₂ and Cr ₂ O ₃ mixed oxides are presumed. The main reactions that occur during the reduction process are as follows:

1. The products of electrolysis for 10 min are mainly Cr, CaTiO ₃ and a small amount of CaO, as shown in Fig. 3(b). Since it is thermodynamically analyzed that Cr ₂ O ₃ is more easily reduced than TiO ₂ , Cr ₂ O _{3 is} first reduced to Cr in the initial stage of the reaction. Electrolysis is carried out at a voltage of 2.8 V. The reduction mechanism of Cr ₂ O ₃ is similar to that of TiO ₂ , and is also carried out by oxygen ionization and calcare reduction, and the reaction may occur as (1) to (3).

Cr ₂ O ₃ +6e=2Cr+3CO (1)

Ca ^{2 +} +2e=Ca (2)

Cr ₂ O ₃ +3Ca=2Cr+3CaO (3)

Electrolytic reduction releases a large amount of O ^{2 -} diffuses to the anode, while Ca ^{2 +} in the molten salt diffuses toward the cathode. If the rate at which the oxide cathode is reduced to form O ^{2 -} is greater than the rate at which O ^{2 -} diffuses toward the molten salt and the anode, Reaction (4) occurs to form CaTiO ₃ , so that there is CaTiO ₃ present in the electrolysis product.

Ca ^{2 +} +O ^{2 -} +TiO ₂ =CaTiO ₃ (4)

2. The new phase of TiCr ₂ is formed in the electrolysis product obtained by electrolysis for 1 h, and contains Cr, as shown in Fig. 3(c), which contains several unknown peaks. Since the electrode used in the electrolysis experiment is relatively thin, only about 1 mm, it is advantageous for calcium and oxygen to be quickly removed from the electrode, and CaTiO ₃ is not found in the electrolysis product. The CaTiO ₃ formed as an intermediate product in the reduction process has a very short lifetime, and in the subsequent electrolysis, CaTiO ₃ reacts on the newly formed Cr particles to form TiCr ₂ , and thus CaTiO ₃ is not detected in the electrolysis product. With the formation of TiCr ₂ alloy and the reduction of CaTiO ₃ phase, the CaO concentration in the porous liquid layer decreases, and the CaO originally precipitated gradually melts with the reduction of CaTiO ₃ and migrates out of the electrode.

When electrolyzing large TiO ₂ tablets, it is often found that CaTiO _{3 is} formed. Due to the occurrence of on-site perovskite, the volume of solid particles is expanded, thereby reducing the ion transport channels between the particles and hindering the ions in the porous layer. Migration, before the TiO ₂ tablet is completely electrolyzed, even if a voltage higher than 3.0 V is applied, a partially reduced sandwich structure can often be seen, but when the TiO ₂ and Cr ₂ O ₃ mixed oxide electrodes are electrolyzed, due to Cr ₂ O ₃ is easily reduced to Cr. The presence of Cr increases the conductivity of the electrode and increases the porosity of the electrode. Therefore, the sandwich structure often occurring when electrolytic TiO ₂ is not found.

3. The electrolysis product obtained by electrolysis for 6h is a titanium chromium alloy and still contains a peak of chromium. It can be seen from Fig. 3 that the peak of TiCr ₂ in the electrolysis product increases when the electrolysis time is extended from 1 h to 6 h, while the peak of Gr decreases and the peak disappears. It can be seen from the phase diagram of TiCr binary system that the uniform composition of C15 phase at room temperature is TiCr _1.75 (65.5%Cr) to TiCr _1.95 (68%Cr). Since the raw material is prepared according to TiCr ₂ , it may contain a small amount of unalloyed. Cr.

In conclusion, under the experimental conditions mixed oxide titanium reduction chrome experienced the following process: first generation reaction Cr, CaO ₂ reaction by-products generated with TiO CaTiO _3, generated in the subsequent electrolysis processes and CaTiO ₃ / or TiO ₂ reacts on the newly formed Cr particles to form a TiCr ₂ alloy.

(III) Effect of electrolysis time on oxygen content of electrolysis products

In order to study the effect of electrolysis time on the oxygen content of the product, a small piece of TiO ₂ and Cr ₂ O ₃ (molar ratio 1:1) was used as an electrode for electrolysis at 1,2.8 V for 1, 2, 4, 6 and 8 h, respectively. The change in oxygen content in the electrolysis product over time is obtained. It can be seen from the figure that the oxygen content in the electrolysis product has been reduced from 38.81% of the initial electrode to 11.50% after electrolysis for 1 h at 2.8 V cell voltage, indicating that the electrochemical reaction rate is fast at the first 1 h, and the first 1 h is removed. Oxygen accounts for 74.56% of the total oxygen. After 2 h of electrolytic reduction, the oxygen content in the product is reduced to 0.64%, and the oxygen removed in the first 2 h accounts for 98.98% of the total oxygen content. When the electrolysis time was extended from 2h to 6h, the electrode reaction rate became slower, the oxygen content decreased from 0.64% in 2h to 0.20%, and the oxygen removed in the first 6h accounted for 99.68% of the total oxygen content. This may be because the main reaction occurring after 2 h is the deoxidation process from the alloy, so the reaction becomes slow. The deoxidation reaction occurs during the subsequent electrolysis, and the oxygen content is further lowered, but the rate of oxygen removal is slow.

Fig. 4 The oxygen content of the electrolysis product changes with time (electrolysis voltage 2.8V, electrolysis temperature 900 °C, Ar100ml·min ^-1 )

In this paper, only the direct preparation of titanium-chromium alloy by molten salt electrolysis is carried out. The electrolysis conditions used are not optimal. The next research focus is to prepare pure titanium-chromium alloy for hydrogen storage performance test and element substitution. Thereby improving its hydrogen storage performance and optimizing the electrolysis conditions to improve product purity and current efficiency.

    Third, the conclusion

(1) In the molten CaCl ₂ system, a mixture of TiO ₂ and Cr ₂ O ₃ is directly electrolyzed, and a titanium-chromium alloy having an oxygen content of 0.20% can be obtained by electrolysis at a cell voltage of 2.8 V for 6 h, indicating that the TiO is directly oxidized by direct electrolytic reduction. _It is feasible to prepare a titanium chromium alloy from a mixture of ₂ and Cr ₂ O ₃ .

Reduction (ii) a mixed oxide subjected to preferential formation of Cr alloying course gradually TiCr _2, the first reaction is the formation of Cr, byproduct CaO TiO ₂ reacts with CaTiO _3, generated in the subsequent electrolysis of CaTiO ₃ And/or TiO ₂ reacts on the newly formed Cr particles to form a TiCr ₂ alloy.