Abstract: A fluid mixing device comprises a housing provided with fluid entry ports for fluids being treated, and for treatment fluids, as well as a fluid discharge port for the discharge of treated fluids. A compound valve arrangement is positioned in the entry port, comprising one valve for adjusting the amount of treatment fluids entering the mixing device, and in series therewith a toggle valve actuated by a rod attached to the toggle valve which extends into the housing and which is actuated by the impact of fluids passing through the device. The mixing device is particularly suited to the neutralization of mine waste waters with caustic soda and for the introduction of polymer flocculents to induce the settling of solids in liquid suspension.