The present invention relates to an in-line mixing apparatus and use therein for adding a polymer solution and dewatering an aqueous mineral suspension. Said method comprises statically mixing the aqueous mineral suspension with a poly(ethylene oxide) (co) polymer to form a dough-like material. The viscous mixture material is then dynamically mixed in an in-line reactor 40 to reduce the mixture viscosity and to form microflocs and release water. Said method is particularly useful for the treatment of suspensions of particulate material, especially waste mineral slurries, especially for the treatment of tailings and other waste material resulting from mineral processing, in particular, the processing of oil sands tailings.