A mixing apparatus comprises first and second opposing mixing members rotatable relative to one another about an axis and which have facing surfaces which extend away from the axis and which define a mixing chamber therebetween. Means are provided for rotating at least one of the mixing members to provide relative rotation between the first and second mixing members in a first rotational direction. An array of mixing formations on at least one of said surfaces interact to mix material within the mixing chamber and are configured to propel material within the mixing chamber towards the axis. The number of mixing formations on at least one of the surfaces increases with radial distance from the axis.