Understanding Reactions in Iron Ore Pellets

The induration furnace for iron ore pellets was originally designed to process concentrates with high contents of magnetite. To support the use of haematite concentrates, internal fuel was added to the pellets in the form of ground coke breeze or anthracite. To ensure that the addition of coke breeze was not excessive, the ‘heat of magnetite equivalent’ or heat of magnetite equivalent (HME) relationship was developed, guaranteeing that the fi ring temperature requirements remained the same. However, this oversimplifi es the different reaction paths of magnetite and carbon in iron ore pellets. At fi ring temperatures, gas diffusion controlled reactions such as the combustion of coke, the calcination of carbonates and the reduction and oxidation of iron oxides can all occur in different parts of the pellet simultaneously. The interaction of these reactions produces a range of atmospheres across the diameter of the pellet that vary from oxidising at the pellet shell to reducing at the pellet core. Mathematical modelling and pellet fi ring experiments to preheat temperatures before quenching have shown that a higher proportion of coke relative to magnetite slows down the oxidation process, irrespective of temperature. A higher temperature, however, results in an increase in the rate of haematite reduction in the pellet core and a more reducing atmosphere, which could lead to different sintering rates in different parts of the pellet and shrinkage cracks forming. To observe these gas-solid reaction rates at a larger scale, the COREM pot grate has been updated with an additional oxygen analyser. This, along with other improvements, will allow better optimisation of straight-grate firing cycles for different pellet compositions.