Roasted iron sulfide particles dispersed in asbestos fibers and found that the

time for complete conversion was related to particle size as R$1\mathrm{.}5$

$.$ Particles

remained as hard solids of unchanging size during reaction. A fluidized$-$bed

reactor is planned to convert ore to the corresponding oxide. The feed is to

be uniform in size, $=20$ min, with mean residence time $=60$ min in the

reactor. What fraction of original sulfide ore remains unconverted?