Oxidation of a Pyrophoric Iron: Part 2. Direct Measurement of Self-heating Accompanying Chemisorption of Oxygen on the Finely Divided Iron-Carbon Substrate

Abstract

The self-heating accompanying the exposure to oxygen of a pyrophoric iron (here a finely divided iron-carbon preparation from an iron mellitate) has been studied quantitatively. In particular, temperature histories have been measured directly by means of a fine thermocouple located at the reactant centre. The dependence of the course of events on (i) initial temperature, from 193 to 720 K, (ii) oxygen pressure, from ca. 0.5 to 200 Torr and (iii) sample mass, from 16 to 40 mg has been investigated. Particular attention has been directed toward attaining really satisfactory reproducibility. All the phenomena observed correspond to sub-critical behaviour; ignitions have not been studied. The results are interpreted against the background knowledge (Part 1) of the kinetics of oxygen adsorption on this pyrophoric iron preparation. From the observations it is concluded that below 373 K and 4 Torr oxygen pressure, the extent of self-heating is determined by the rate of heat loss through the walls of the glass containing vessel. Above 4 Torr, the behaviour is more complex and is influenced by the occurrence of two distinct chemisorption reactions. Reaction above 500 K seems to be governed by gas diffusion processes and the overall temperature rise is moderated both by the containing vessel and by the diffusion of oxygen by inter- and intra-crystalline paths to the active surfaces within the particles of the reactant assemblage.