Abstract: A system for reducing the particle size of particulate substances and classifying the resulting particles includes a compressor, a jet mill and a series of separators. The compressor is connected to the jet mill by a feed pipe, and a hopper for particulate substances discharges into the feed pipe. The compressor, jet mill and separators are arranged in series and a recirculating pipe runs from the last separator back to the compressor. The compressor is driven by a combustion engine having an outlet for exhaust gases. The outlet is connected to the recirculating pipe by way of a catalytic converter and a cooling unit. The catalytically converted exhaust gases provide an inert atmosphere for the jet mill and function as a carrier for particulate substances admitted into the feed pipe.