The rheo-reactor consists of a cylindrical vessel equipped with a double helical ribbon impeller installed in a RS150 controlled stress rheometer. It was used to investigate the theological behaviour and the heat transfer in aqueous mineral residue suspensions during the phosphatation process. The absolute viscosity was determined using a Couette analogy allowing the quantitative analysis of the torque-rotational speed. The results show that the suspensions exhibit a viscoplastic or Binghamian behavior. The experimental operating conditions for the phosphatation process were then selected in order to ensure a good macro-mixing, even in laminar regime (Re < 30). Heat was generated as the combined result of the exothermic phosphatation process and the viscous dissipation inside the rheo-reactor. The heat produced needed to be efficiently removed to reduce its effect on the behaviour of the suspension. This was evaluated using the Nusselt number and the heat transfer coefficient h.