Use of dextran as a filtration aid for thickener overflow filtration in Kelly filters in the Bayer process

In the Bayer process for producing alumina from bauxite of the type wherein the bauxite is added to a caustic solution to produce a slurry of sodium aluminate solution and an insoluble red mud fraction which is then subjected to a thickener and separation steps to produce a thickener overflow, wherein said thickener overflow results from solid liquid separation, whereby said thickener overflow is then subjected to a filter press which contains a filter aid to polish the thickener overflow the improvement which comprises adding to the thickener overflow and the filter aid as the only treating agent from 0.05 to 15 mg/l of dextran.