NOVELTY - An apparatus for compaction of silicon powder in free space reactor to minimize oxidation of silicon comprises reactor having duct (10), where powder is passed along duct towards exit (1) of reactor, filter (2), where compaction is in part effected by using drop in pressure across filter for exit gases from reactor, and piston (3) arranged at gas exit, where compaction in addition is effected mechanically with aid of piston. The filter is arranged on piston, where filter is permeable to gases, but collects powder. A replaceable container (6) is connected to ducts via vertical duct (7).    USE - As an apparatus for compaction of silicon powder in a free space reactor to minimize oxidation of the silicon (claimed) and provide solar cell grade silicon.    ADVANTAGE - The apparatus compacts the powder and does not contaminate the powder or expose it to oxygen. The apparatus avoids contamination of silicon powder and contact with oxygen.    DETAILED DESCRIPTION - An apparatus for compaction of silicon powder in a free space reactor to minimize oxidation of the silicon, comprises a duct (10), where the silicon powder is passed along the duct towards an exit (1) of reactor, filter (2), where compaction is in part effected by using the drop in pressure across the filter for exit gases from the reactor, and at least one piston (3) arranged at the gas exit, where the compaction in addition is effected mechanically with the aid of the piston. The filter is arranged on the piston, where the filter is permeable to the gases, but collects the powder. A piston (4) is arranged axially opposite the piston for compaction of the powder, where the pistons are arranged in a common straight duct (5). A replaceable container (6) is connected to the ducts via a vertical duct (7) which is arranged at an angle relative to the duct. The container is equipped with a valve (8) and a gas lock, where the container is capable of being replaced without altering the atmosphere in the reactor or in the container. An INDEPENDENT CLAIM is included for method for compaction of silicon powder in a free space reactor for minimizing oxidation of the silicon, involving: passing the silicon powder along a duct towards an exit of a reactor; depositing the silicon powder on a filter for exit gases from the reactor, where the method further involves compacting the silicon powder mechanically with the aid of at least one piston arranged at the gas exit. By pushing a piston arranged axially opposite to the piston towards the piston, the pistons are arranged in a common straight duct arranged at an angle relative to the duct.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of apparatus.    Exit of a reactor (1)    Filter (2)    Piston arranged at the gas exit (3)    Oppositely axially arranged piston (4)    Common straight duct (5)    Crucible or replaceable container (6)    Vertical duct (7)    Valve (8)    Duct towards exit of reactor (10)