The reduction step involves reacting the ilmenite in a large, continuously fed, rotary kiln. The residence time in the kiln is approximately ten to twelve hours. The heat source is coal, and the source of the chemical reductant (carbon monoxide) is also coal, which is added as lump coal with the ilmenite at the feed-end of the kiln. Controlled air addition allows enough heat to be generated for reduction reactions to occur (~1,100 degrees Celsius), and carbon monoxide to be generated. The carbon monoxide is oxygen deficient, and scavenges some of the oxygen from the ilmenite structure.
By this process, most of the iron portion of the ilmenite is converted to metallic iron. The formation of iron particles within the reduced ilmenite grain shown Figure 1.
A rotary kiln used to reduce ilmenite. The kiln product is a highly magnetic mineral due to the presence of metallic iron. It is the same size as the original ilmenite and is slightly more porous. Post reduction and cooling, the reduced ilmenite undergoes screening and magnetic separation to remove partially burnt coal (in the form of char), ash from the coal combustion and some of the other mineral impurities which are non-magnetic.
The product is then aerated, a form of accelerated rusting, which involves the removal of the metallic iron from the reduced ilmenite grain. The reduced ilmenite is mixed in tanks with water and air in a batch process. Ammonium chloride is added as a catalyst to accelerate the process. Preheating of liquor using heat recovered from elsewhere in the process, is also used to accelerate the process. In these conditions the metallic iron dissolves, migrates through the pores within the particle and precipitates in the bulk solution as very fine iron oxides, 0.1 μm to 10 μm in diameter. After aeration, the sand grains have lost up to 40 per cent of their original weight and consist of a porous matrix of titanium dioxide of about 88 per cent purity but retain their original particle size. Wet separation of the fine iron oxide from the coarse titanium mineral particles is done using hydro-cyclones. The iron oxides are deposited in settling dams, and the ammonium chloride liquor is recovered and recycled.
Iluka recovers iron oxide from dams once dried and sells this product into the steel making industry.