Correlation of Oxygen and Aluminum Contents of Molten Titanium-Aluminum Alloys in Alumina and Calcia Crucibles
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UM with its high ratio of strength/density, high corrosion resistance and biocompatibility is a very attractive metal for various applications, e.g., transport industry. TiO2 is very abundant in earth’s crust, mostly in combination with other oxides like with FeO in Ilmenite (TiO2ÆFeO), with CaO in Perovskite (TiO2Æ CaO) and with both SiO2 and CaO in Titanite (TiO2ÆSiO2ÆCaO). The main reason that Ti alloys are used less often than expected regarding their properties is the high price as a result of extraction costs of Ti. Although alternative methods like ITP/Armstrong,[1] FFC,[2–4] and OS[5] have been proposed and investigated considerably, the main commercially applied route is still the Kroll process[6,7] which begins with TiO2 (Rutile, synthetic Rutile, some upgraded Ti slags), its chlorination, and purification into TiCl4 and produces titanium sponge by reduction through molten magnesium. The sponge has to be vacuum-distilled or leached, crushed,
MAHDI FARHANI, GEREON HILS and KARL-HEINZ SPITZER are with the Institute of Metallurgy, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany. Contact e-mail: [email protected] Manuscript submitted July 16, 2018. Article published online March 27, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B
pressed, and often manually modified to form an electrode. This electrode is melted several times by vacuum arc remelting process (VAR). Not only the remelting itself is costly, a machining step before each remelting is necessary to remove impurities from the electrode surface. Thus, the final ingot is only affordable for high-tech applications like aerospace and medical. Other above-mentioned proposed methods also begin with either pure TiO2 or TiCl4 but finally produce Ti powder. Therefore, they are preferred for the still-limited powder metallurgy applications. A. Pyrometallurgical Titanium Alloy Production The fundamentals of an alternative cost-effective production route for titanium alloys have been investigated at the Institute of Metallurgy at Clausthal University of Technology, a schematic view of which is given in Figure 1. Instead of pure TiO2, this process uses the cheap and abundant Ilmenite to produce a modified titania slag. Ilmenite pellets are fed into a carbothermic direct reduction process, through which the iron is reduced selectively and nearly completely. Afterwards the pellets are melted for iron separation. Lime (CaO) is added to get a sufficiently low melting titania slag. This slag is then reduced by aluminum and produces molten titanium pre-alloy with Al and O impurities. This pre-alloy is subsequently refined to a VOLUME 50B, JUNE 2019—1193
hot metal for steelmaking and calcium aluminate slag for cement production. The slag can also be recycled partly in the form of the cement for pelletizing of Ilmenite in this route. The focus of this work is on the investigation of the refining step for removal of the excess Al and O after the aluminothermic production of titanium pre-alloy. The details about the other steps are presented elsewhere
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