A mixture of coal and sand particles having sizes smaller than 1×10-4 m in diameter is to be separated by screening and

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A mixture of coal and sand particles having sizes smaller than
1×10-4 m in diameter is to be
separated by screening and subsequent elutriation with water.
Recommend a screen aperture
such that the oversize from the screen can be separated completely
into sand and coal particles
by elutriation. Find the:
a) Settling velocity of larger diameter of coal particle.
b) Diameter of sand particle corresponding to the settling
velocity.
Assume that Stokes law is applicable. Use the following data:
Density of sand = 2650 kg/m3
;
density of coal = 1350 kg/m3
; density of water = 1000 kg/m3
; viscosity of water = 1×10-3 kg/m.s.