The invention relates to a foreign matter isolating device, in particular to a foreign matter isolating device for a reinforced concrete or steel alumina storage bin having a middle or small diameter of 8 to 18 meters. In the foreign matter isolating device for the alumina storage bin, steel I-beams are arranged under the roof of the alumina storage bin, two ends of each steel I-beam are fixed onthe wall of the bin, and a steel barrier is arranged between each two adjacent steel I-beams. The foreign matter isolating device separates alumina powder from various foreign matters mixed in the alumina powder, so that the foreign matters can be cleared in time to obtain more pure alumina powder. The foreign matters separated by the steel barrier stay above the barrier; while the pure alumina powder falls into the bottom part of the storage bin through the steel barrier. Foreign matters such as refractory brick blocks, concrete fragments, package bags, PVC thin films, metal wires and threadballs which are mixed in the alumina powder during alumina production and transportation are stopped above the barrier to be cleared easily, so the normal operation and use of the alumina powder in super dense phase conveyance, chute scales and volume fixing devices are ensured and the overhaul and maintenance workload is reduced.