A plant produces 8640 tonnes per day (100 kg/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 1 kg of salt solution, containing 0.55 kg 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 (200 kg/s) and the solid discharged from each thickener removes 0.5 kg 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? Concentration of solution (kg solute/kg solution) 0 0.1 0.2 0.3 0.4 0.5 Amount of solution removed (kg solution/kg pigment) 0.30 0.32 0.34 0.36 0.38 0.40 The concentrated wash liquor is mixed with the material fed to the first thickener.