The reduction of haematite by hydrogen

On integration and evaluation of die integration constant diis yields 

Laboratory studies of some important industrial reactions 271 

In this derivation, the diffusion coefficient which is used is really a parameter, since it is not certain which gas diffusion rate is controlling, that of hydrogen into a pore, or that of water vapour out of die pore. The latter seems to be the most probable, but the path of diffusion will be vety tormous drrough each pore and drerefore the length of the diffusion path is ill-defined. 

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