Tables 3 and 4 detail the separation performance obtained
when using a low fluidization flux of 0.7 cm/s and high gas flux
of 2.6 cm/s. Despite the relatively low level of fluidization water
applied, the high permeability of the fluidized bubbly bed clearly
permitted a uniform downward flow of the wash water, with
98.8% of mass fraction reporting to the reject of particle size less
than 38 lm, and with an ash of 87.1%. The overall recovery was
83.6% with the product ash at a respectable 9.8%. Note however,
the combustible recovery was an impressive 98.6% for the particles
greater than 38 lm, implying the loss in combustibles to tailings
was in the 38 lm size fraction. This drop in recovery in the finest
size fraction is thought to be a consequence of approaching the
kinetic limit of the very finest of the particles (perhaps less than
5 lm) in the downcomer. This could be improved by increasing
the shear rate in the downcomer by increasing the feed rate and/
or by narrowing the annulus width. Using a finer media grade frit
to produce finer sized bubbles, and extending the length of the
downcomer to increase the residence time is also recommended.