A Monte Carlo simulation procedure was used to generate a probability distribution of the solids feed rate to a coal preparation plant thickener. Variations about an average value in the raw coal feed rate, proportion of minus 0.5 mm material, amount of out of seam dilution, and fine coal yield resulted in a large range of thickener feed loadings. The effect of the distribution of thickener feed loadings on thickener operation was then assessed using experimental data. Such an analysis is helpful in deciding the level of automation or control required for a particular operation. It was shown that thickeners with a small diameter, similar to the so called high rate units, must have automatic control equipment for bed level detection, underflow rate variation and flocculant addition in order to maintain steady operation. For conventional thickeners automatic control was shown to be necessary in some cases.