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Direct Production of Ti–Fe Alloy in Liquid by Electrowinning in Molten Slag
(Toshihide TAKENAKA, Hidetaka MATSUO and Masahiro KAWAKAMI)
(Toyohashi University of Technology, Japan.
(August 1, 2011)
Aiming at the direct production of Ti–Fe alloy, its direct electrowinning has been attempted in a CaF2– CaO–TiO2–FeO bath by using a direct-current electro-slag remelting unit. The experiments were carried out in the slag where the molar ratio of CaO to TiO2 was fixed as 1.5, and the influence of the molar ratio of FeO to TiO2 was mainly investigated. The experimental results proved that Ti–Fe alloy in liquid could be obtained at the limiting bath compositional range. At the molar ratio of FeO to TiO2 was 0.90, both the Ti content in the deposit and the cathodic current efficiency were the best under the conditions in this study; the Ti content reached about 50 wt%, and the cathodic current efficiency also did about 50% at best.
The addition of Ti in steel is effective in the elimination or inactivation of C, N, S and O because Ti has a strong affinity with these elements. Ferro-titanium (Ti–Fe alloy) is widely used for this purpose as well as Ti metal itself. Steel makers are one of the major users of Ti actually, and their Ti consumption must increase remarkably considering a great demand of high quality steel in future.
Titanium is not a rare element in the crust, and the natural resource of Ti is oxide ore because of its strong affinity with O. Conversely, this strong affinity limits the process of Ti metal production, and Kroll process is only the current industrial process. Kroll process which has been well developed gives Ti metal of very high quality, but includes inefficient steps. Therefore, the production cost of Ti metal is hardly lowered in the present situation, which obstructs the wider use of Ti metal. A breakthrough in Ti production has been strongly required to reduce the production cost. Actually, many well-designed processes have been proposed, but they have not been industrialized yet.
Ferro-titanium is produced by remelting of iron/steel with Ti metal from Kroll process or Ti scrap. Ferro-titanium production from primary Ti metal is restricted by the issues of Kroll process, and that from Ti scrap has to depend on uncertain supply of Ti scrap. In the latter case, the contamination of alloying elements in Ti scrap must be also taken account of. The direct production of ferro-titanium from Ti natural resource is strongly required, though it has not been actualized.
Our group has been investigated the direct electrolysis of Ti by using a DC-ESR (direct current electro-slag remelting) unit. In the process, a very high direct current passed through a slag bath containing TiO2, and Ti metal was electrodeposited on a cathode at the bottom of the container. In the case that the bath temperature was sufficiently high, Ti metal was deposited in liquid, and the cathodic current efficiency became rather high. Not only pure Ti metal, but also
Ti–Al alloy could be directly obtained in a bath with TiO2 and Al2O3 by the same process.5) It is reported that the cathodic current efficiency of the Ti–Al electrolysis was better than that in the Ti electrolysis; it was inferred that the higher cathodic current efficiency was caused by the higher bath temperature due to the bigger consumed electric power with the change in the electric conductivity of the bath.
Electrolytic co-deposition of Ti with Fe is difficult normally in molten slag as well as in an aqueous solution considering the large difference between their redox potentials. However, there is a possibility of the co-deposition of Ti with Fe by the process using a DC-ESR unit because the very high cathodic reaction rate according to large direct current in the process may cause the insufficient supply of Fe ion to the cathode.
Considering that major natural Ti resources are complexes of Ti and Fe oxides, the efficient process using Ti resources can be proposed. It is also regarded as the advantage of the proposed process that the process can be started and stopped easily, namely can be operated on demand.
In this study, direct electrodeposition of Ti–Fe alloy was attempted in a molten CaF2–CaO–TiO2–FeO bath by using a small DC-ESR unit. From the analysis of the electrodeposit, the necessary conditions for effective Ti–Fe electrolysis were discussed. Since it had been shown that the composition of the slag bath had a remarkable influence on the Ti electrolysis and Ti–Al electrolysis, the dependence on the bath composition was investigated in detail.
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