Pressure drop for flowing mixtures of sized sand particles in water through a vertical tubular leacher has been measured. When the particle size is between 60 and 80 mesh

the volume fraction of solid smaller than 0.175

and the velocity between 0.09 and 1.5m/s

the operation is stable. For different solid/liquid ratios

all experimental curves for pressure drop versus flowing velocity show minima

and the greater the solid/liquid ratio

the higher the flowing velocity at which the minimum occurs. The gravitational pressure drop for vertical tubes is given by △pg = △ε(ρs-ρf)gH. The difference in voidage between the up-and down-flow tubes may be calculated by equations for generalized fluidization

and is approximately equal to △ε= 2q/(1 + q)"+2R. The frictional pressure drop for vertical tubes may be calculated by treating the solid-liquid mixture as a continuum by other existing correlations.