A plant produces 8640 tonnes per day (100kg/s) of titanium dioxide pigment which must be 99.9 per cent pure when dried. The pigment is produced by precipitation and the material, as prepared, is contaminated with 1kg of salt solution, containing 0.55kg of salt/ kg of pigment. The material is washed countercurrently with water in a number of thickeners arranged in series. How many thickeners will be required if water is added at the rate of 17,400 tonnes per day (200kg/s) and the solid discharged from each thickener removes 0.5kg of solvent /kg of pigment? What will be the required number of thickeners if the amount of solution removed in association with the pigment varies with the concentration of the solution in the thickener, as follows? The concentrated wash liquor is mixed with the material fed to the first thickener