Reoxidation and ignition behaviour of DRI to improve safety

Sponge iron (DRI) as a virgin raw material is of great interest due to the developments in regard of producing high quality flat products with low tramp elements content in scrap-based steelmaking . Sponge iron may react with water and air to produce hydrogen and heat. The heat produced may cause ignition. Precautions are necessary to avoid hazards in transport and storing. In the research work on reduction, carburization and reoxidation at the Institute of Technical Chemistry and Macromolecular Chemistry (ITMC) at RWTH Aachen Carajas fine ore was used as a model substance investigating the influence of the gas composition and reduction temperature on the ignition of DRI (Direct Reduced Iron). The internal surface area of the DRI, depending on reduction temperature and reduction agent, has a strong influence on the ignition temperature of DRI. The influence of carburization and carbon-load, respectively, on the reoxidation behaviour was determined by different methods. Different equations were tested with respect to estimate the ignition hazards of large stock piles of DRI. At the MPIE the kinetics of reoxidation of direct reduced iron was investigated by thermogravimetry in dependence on temperature and partial pressures of oxygen, water vapor and carbon dioxide, also effects of vapor phase inhibitors were studied. The material investigated, mainly DRI pellets and Comet DRI, turned out to be rather different concerning composition and content and distribution of carbon and oxygen. Studies were conducted on the determination and distinction of carbon in its different states: graphitic, amorphous and carbidic (i.e. cementite). Reoxidation of the pellets was studied at first at temperatures < 200 °C. The kinetic tests were continued at 350 °C, here the oxidation reached higher extents. The independence of oxidation kinetics on the oxygen pressure was confirmed, but at the higher temperature there was no effect of water vapor, nor of carbon dioxide or sulfur dioxide addition on the rate. Obviously the reaction is of zero order, due to saturation of the oxidized surface with oxygen. Inhibition of reoxidation by vapor phase inhibitiors might be of interest to suppress reoxidation and ignition of DRI. Benzylamine, ammonium benzoate, ammonia and a commercial inhibitor were tested in thermogravimetric studies at 150 and 200 °C. Generally the effects were obtained with low inhibitor concentrations concentration due to saturation of the reactive surface with the inhibitor. - Also the effect of inhibitors on ignition was tested and a notable decrease of ignition temperature was observed. CRM prepared samples of Comet DRI made from two different iron ores. These samples were distributed to the partners of the research. A test-rig for carrying out bulk reoxidation trials simulating the conditions occurring when DRI is wetted in a pile during storage or in a ship's hold during transportation was constructed. The trials carried out in that test-rig showed that a low porosity of the DRI and a high reduction temperature are main factors for limiting the risk during the transportation of the DRI. Corrosion trials carried out on small DRI samples in a programmable climatic chamber did not assess correctly the reoxidation behaviour of DRI. The ignition temperature of DRI for the given ore basis was 312 °C. The ignition temperature of Comet-DRI and of HBI was higher than 750 °C.