The fine powder flows into the cyclone collector together with air flow and is expelled from the powder output pipe as product. In the grinding chamber, since the stuff contains moisture to some extent, the heat generated during grinding makes the moisture evaporate; since the pipeline joints are not airtight, the external air is sucked in and the circulation air mass increases. In order to enable the grinder to work in negative pressure, the increased air flow is guided to the duster and sequentially to the atmosphere after purification.