TRANSACTION PAPER

Nitriding of ilmenite and high-grade slag fines

D.S. van Vuuren; G.T. Tshilombo

Materials Science and Manufacturing, CSIR

SYNOPSIS

Titanium bearing resources can selectively be chlorinated at temperatures below 200ºC if the TiO₂in the feed is nitrided before chlorination. However, the nitriding reaction is highly endothermic, requires highly reducing conditions, is only thermodynamically favourable at elevated temperatures, and is affected by mass and heat transfer restrictions. At the required processing conditions, problems might also be experienced with sintering and with melting of slag forming components in the raw materials.
A test programme was undertaken to study the effects of raw material type, temperature, gas composition, bed depth, carbon to raw material ratio and residence time in order to simulate conditions required for scaling up a nitriding process.
In contrast to previous experience with low-grade titaniumbearing slag containing about 30% TiO₂, ilmenite and chlorinateable slag do not form melting phases under the studied nitriding conditions and hardly sintered at all. Both feed materials can readily be nitrided with conversions in excess of 90% at temperatures around 1300ºC and using bituminous coal as reducing agent with fixed carbon in excess of 1.1 times the stoichiometric requirement. The conversion decreases as the bed depth of the feed material increases and the N₂/CO ratio in the gas decreases.
Analysis of the relative rates of mass and heat transfer into the reacting beds of ilmenite and slag showed that the heat transfer rates are more limiting than the mass transfer rates. In order to increase the throughput that can be achieved in a kiln, it would be necessary to take special measures to enhance the heat transfer rates into the mass of reacting material. One such a measure is to pre-form the mass of reacting material into thin-walled, hollow brick shapes prior to introducing it into a kiln.
Depending on bed depth, temperature and N₂/CO ratio, required residence times in the high temperature zone of the kiln can vary between about 6 and 10 hours.

Keywords: Titanium dioxide, titanium nitride, carbo-thermic, reduction, ilmenite, slag, tunnel kiln, residence time

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