.Paper presents theoretical considerations on the process of unloading powder materials (lime) from the tower silos equipped with pneumatic facilities to aid the discharge process. Discharge of lime from a silo is significantly impeded by the "bridges" being shaped over the outlet hole and suspending unloaded material. The bridging may be eliminated by pneumatic rarefaction of lime material to the extent ensuring its free outflow. Using total energy equation to describe the motion process the relationship was determined which describes material output as a function of the other factors. On the basis of theoretical considerations a device aiding the outflow of powdered lime from the bottom part of the silo was designed and built. The device consists of the following assemblies: casing, gate valve and outlet from the silo, filter gauze, loading chamber, reception chamber, porous baffle, gate valve and chute to transport means, separating sieve and container for impurities. The sieve is actuated by harmonic, rectilinear vibration. The experimental tests showed that there are the optimum parameters of sieve vibration ensuring sufficient admixture separation; mass flow of the material and separation efficiency are the function of total air use; the discharge rate is a function of air volume used in material rarefication chamber; the pressure drop on porous baffle is a function of used air.