Abstract: A cryogenic grinding mill for grinding organic base material pieces into sub-micron-sized powder particles. An upper grinding block is rotated relative to a stationary lower grinding block by a motor, and is maintained at a temperature below ?150° C. by a cryogenic system including an annular liquid nitrogen chamber disposed around the grinding blocks. The upper grinding block defines a trench for receiving base material pieces fed by a feed system, and includes through-holes that extend from the trench to a grinding region formed between the grinding surfaces of the upper and lower blocks. When the upper grinding block is rotated, the base material pieces are gravity-fed from the trench to the grinding region, and ground powder material is forced to a peripheral edge of the grinding region. The powder material is then filtered, and particles having an undesirably large size are fed back into the trench for re-grinding.