Rotary dryer, the solid-gas heat exchanger, can use waste heat of smoke gas to remove moisture from materials. It uses high-temperature smoke gas to dry materials evenly through direct or indirect heat exchange. Direct heat exchange refers to the direct contact heat exchange between high-temperature waste gas and materials to be dried. Indirect heat exchange refers to the heat transfer through the rotary drum wall, so as to avoid the direct contact between high-temperature waste gas and materials to be dried. Due to its high drying efficiency and large capacity, rotary dryer is widely used in fields such as chemical, building materials, mining, chemical fertilizer, etc.