Preoxidation and hydrogen reduction of ilmenite in a fiuidized bed reactor
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I.
INTRODUCTION
A M O N G the naturally occurring titania-containing minerals, ilmenite, (FeO.TiOz), and rutile (TiO2) are the chief raw materials used for the production of titanium metal and its compounds. Ilmenite due to natural weathering rarely corresponds to the theoretical composition, FeO'TiO2, and rutile is one of the three crystallographic forms of TiO 2, with the other two being anatase and brookite. Rutile and brookite are the common minerals and their reserves are very limited. However, the reserves of ilmenite are abundant world wide, and it is upgraded to produce synthetic rutile, which is in great demand by commercial firms producing titanium metal and pigment-grade titanium oxides. In natural weathering, ferrous iron in ilmenite is oxidized to the ferric state, which is progressively leached out, resulting in the TiO2-rich natural rutile. The natural alteration of ilmenite to rutile is believed to occur through an intermediate metastable pseudobrookite (Fe203.TiO2), which is also termed arizonite,m The alteration of ilmenite in nature is believed to be analogous to high-temperature oxidation,t2] and the intermediate compound is believed to have the stoichiometry[31 Fe2Ti309, known as pseudorutile, which is formed after the ferrous iron in ilmenite is oxidized to the trivalent state and one third of it is leached out. Gupta et aL{ 4~ gave an account on the details of phase transformations of ilmenite concentrates during heating and pointed out that it is difficult to synthesize pseudorutile in the laboratory. Hence, it is apparent that benefieiation of ilmenite to produce rutile involves a complex mechanism of phase transformation. To meet the ever increasing demand of titania-rich mineral for the production of titanium metal and its components, a number of processes have been proposed[5,61 to upgrade ilmenite and produce synthetic futile. These processes have been grouped into four major categories: (1) titania slag; (2) acid leaching;; (3) chlorination, either direct or selective; and (4) reduction followed by selective leach-
P.L. VIJAY, Scientific Officer, RAMANI VENUGOPALAN, Scientific Assistant, and D. SATH1YAMOORTHY, Head, Process Engineering Section, are with the Metallurgy Division, Bhabha Atomic Research Centre, Bombay 400 085, India. Manuscript submitted November 21, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
ing. The titania slag method involves ilmenite smelting, by which the total reduction of iron oxides to metallic iron (pig iron) is achieved, leaving behind titania-rich slag. The slag chemistry controls the operation of the furnace as the amphoteric TiO2 attacks both acidic and basic refractories. The slag composition has to be controlled to meet the stringent chemical compositions imposed by the firms manufacturing titanium dioxide. Thus, this process requires lowash-content reductants and also restricts the use of fluxes or slag modifier. This process, however, requires some minimum level of FeO, which could act as flux and also avoid frothing. A careful thermal balance
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