A drum mixer is used for continuous mixing of the substrates. It is a rotating horizontal cylinder driven by 2 electric geared motors. The axis of rotation of the mixer is inclined from the inlet of the raw materials towards the outlet. Inside the cylinder, paddles are installed to help to mix the blend thoroughly and to convey it towards the outlet. The material of the mixer is carbon steel with a protective coating. The inside of the cylinder is lined with anti-abrasive material. The substrate is fed into the mixer by a conveyor belt and is discharged in the axis of the drum. At the other end of the mixer, the perfectly mixed substrate falls out onto the conveyor belt.