A device for mixing a powder reagent with a flow of a liquid medium allows reducing the liquid drag by an apparatus for injection of a powder reagent into the flow of a petroleum product, which comprises: a mixing chamber, an inlet for feeding a powder reagent, an outlet to remove the powder reagent mixed with the petroleum product; a powder reagent introducer having a screw with an outlet connected to the inlet of the mixing chamber through a flow restriction device; a pump for feeding the mixture from the mixing chamber to the oil pipeline, in which the specified at least one inlet for feeding petroleum products are tangential, an inlet for feeding the reagent and the outlet to remove the mixture are axial. Based upon a load of the screw, a gear motor controls the opening and closing of the flow restriction device via a coupling system to control the transporting of the powder reagent to the mixing chamber.