A re-appraisal of the dusty gas model.

Experimental measurements have been made of gaseous diffusion and flow in porous materials of metallurgical relevance. Hollow spheres of the porous materials, prepared by isostatic compaction, sintering and reaction, have been placed in an apparatus able to maintain gases of known composition and pressure inside and outside the spheres. Two separate types of experiment were carried out. In the first set, the permeation of pure gases across the spheres was determined at a range of different total pressures. In the second set, the counter diffusion of two gases was studied at a single mean pressure but under a range of different pressure gradients. Measurements made with a zero pressure gradient - isobaric experiments - constituted a key component in these binary diffusion experiments. Experiments were conducted at high and low temperatures using helium, argon, carbon dioxide and nitrogen and in porous iron produced by reduction, in sintered porous iron and in lime produced by the decomposition of calcium carbonate. The experiments were analysed in terms of the Dusty Gas Model, a fresh development of this model being presented to emphasise its phenomenological nature and include a general statement of the influence of mechanically driven gas flows. The standard application of these equations is used to analyse the single gas permeability experiments and thus to determine the Knudsen diffusion coefficient and a parameter quantifying the porous material's resistance to mechanically driven gas flow. A new unified method of solution is then introduced for binary diffusion, combining a previously obtained solution for isobaric diffusion with a new analysis for non-isobaric diffusion and flow. Using this method, a parameter determining effective binary molecular diffusion coefficients has been obtained from the isobaric experiments and used to predict non-isobaric diffusion rates, these predictions being compared with the corresponding experimental results. Further development of the method has allowed it to be applied to the non-isobaric equi-molar counter diffusion process that occurs during the reduction of hematite and to non-isobaric diffusion during the decomposition of calcium carbonate.