In the first series of tests, the effect of initial acid concentration on impurity concentration was investigated by weighing amounts of bauxite slime and adding acid solutions to give a 20% pulp density. The concentrations of acid used were 1.0 s, 1.1 s and 1.2 s acid, representing 1, 1.1 and 1.2 times the stoichiometric amounts of acid required whilst the temperature was set at 110 °C as before. Samples were taken as before and analysed.