A method and apparatus for converting an iron-bearing residue generated in a hydrometallurgical process that contains small amounts of soluble heavy metals into a stable form by means of a neutralizing agent. The residue is elutriated and the elutriated residue is fed into at least one stabilization or homogenization reactor, into which a neutralizing agent is also routed, and the homogenous mixing together of the residue and neutralizing agent takes place by means of a helix mixer, where the ratio of the diameter of the mixer to the diameter of the reactor is 0.75-0.99.