The mechanism of auto-ignition of direct reduced iron (DRI).

摘要

The storage and transportation of Direct Reduced Iron has been connected with the onset of metal fires that apparently originate with aqueous corrision-processes. Perfectly dry DRI that have been wet, particularly when the water has penetrated to the depths of the pile, have been found to be quite unstable and likely to undergo spontaneous combustion.; A preliminary working model was derived which assumes the existence of some corrosion (up to 100{dollar}spcirc{dollar}C) reaction zones and a high temperature oxidation reaction zone in which ignition eventually becomes self-sustaining. Experimental programmes were developed to study the kinetics of corrosion reaction of iron, iron-carbon foils and DRI. The foils were used to establish the kinetics of formation of corrosion products and their various contributions to the total corrosion process. The products were identified using the X-ray diffraction techniques and their morphological characteristics determined by Scanning Electron Microscope. Similar measurements of the hydrogen evolution kinetics were made using DRI. The results of these two experiments immediately led to the proposal that reduction of the floccullent precipitate, the major product of corrosion could be possible at a dry site by the hydrogen produced from the aqueous corrosion zone. Using thermogravimetry and measuring both the internal and external temperatures of a cluster of DRI pellets, it was shown that reduction is feasible and that the highly activated iron produced can reoxidize to produce temperature excursions that could lead to auto-ignition. The morphological characteristics of the rusted, reduced and reoxidized reaction products were also determined.; These results were then used to refine the model of the auto-ignition mechanism which was shown to be viable on thermodynamic grounds and also on kinetic grounds assuming that a temperature of 200{dollar}spcirc{dollar}C or above can be reached for the reaction to proceed. It is proposed that the catalytic oxidation of hydrogen on the rust product as a catalyst can lead to such temperatures. Various factors that are important in the self-ignition sequence were discussed and in particular, a link between low temperature corrosion reactions and high temperature oxidation reactions at which point, ignition can be self-sustaining is explained.; Even though a self-ignition situation was not reached in this work, it is believed that, with the high thermal insulation offered to an ignition site in an industrial scale pile, the mechanism proposed, is feasible. (Abstract shortened with permission of author.)