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1 Working Principle The working principle of the micro-powder dryer is basically the same as that of the rotary flash dryer; both utilize the large relative motion generated between the rotating airflow and the material particles to enhance heat and mass transfer, thereby achieving the drying of the material. The difference is that the micro-powder dryer has a stronger crushing and dispersing capability, making it highly suitable for drying ultra-fine materials ； The micro-powder dryer is equipped with an adjustable classifier, which allows for easy control of the product’s particle size ； The structure of the micro-powder dryer is more advanced, making maintenance and repair easier. The micro-powder dryer is a multi-functional dryer that combines drying, crushing, and grading in one unit. 2 Equipment Structure The micro-powder dryer is mainly composed of a feeder, mixer, classifier, drive mechanism, and drying chamber, as shown in Figure 1. The details are as follows. 2.1 Feeder: A screw conveyor is generally used to feed the micro-powder dryer. For paste-like viscous materials, the conveyor needs to be specially designed to prevent material blockages and bridging. For slurry materials, a positive displacement pump can be used for feeding. 2.2 Agitator The function of the agitator is to break up lumpy materials and fluidize them with the help of hot air. For different materials such as paste-like, viscous, and granular ones, stirrers of various designs can be created. Unlike rotary flash dryers, the agitator of a micro-powder dryer can be designed in a vertical or horizontal configuration, suitable for different applications respectively. Adopting a horizontal structure can **reduce the equipment height and facilitate maintenance and repair. 2.3 Classifier: Rotary flash dryers are generally used for materials for which there are no strict requirements regarding product particle size; their classification mechanism consists of a fixed sieving ring located at the top of the dryer. The top of the micro-powder dryer is equipped with a classification wheel whose speed can be adjusted, making it suitable for materials that require strict control over particle size, such as ultra-fine and nanoscale particles. For materials with very strict requirements regarding particle size range (such as d90≥2μm), multiple classification wheels can be used if necessary. 2.4 Transmission mechanism: Rotary flash dryers generally use belt-driven vertical transmission, with the motor usually fixed on a frame at the bottom of the drum to enhance the stability of the equipment. The drive mechanism of the micro-powder dryer is fixed to an integrated frame, resulting in almost no vibration. Corresponding to vertical and horizontal mixers, the drive systems also come in vertical and horizontal versions. The design of the transmission system has a significant impact on the stability of the equipment. Lanhua Company uses a type 10 rotary flash dryer manufactured by a domestic manufacturer; due to an unreasonable design of the transmission mechanism, the equipment experiences significant vibration during operation. 2.5 Drying Chamber: The drying chambers of micro-powder dryers and rotary flash dryers are basically the same. The difference lies in the fact that, due to the different air intake methods, the air intake section of the cylinder varies. Micro-powder dryers do not require complex air inlet volutes and distributors. 3 Design calculations: The process flow of the micro-powder dryer is similar to that of the rotary flash dryer. The cylinder diameter is generally determined by referring to the calculation method for rotary flash dryers. Here, the design methods for mixers and classifiers are mainly introduced. 3.1 Design of the mixer The type and structure of the mixer, as well as its rotational speed, have a significant impact on the drying process. The agitators in rotary flash dryers all adopt vertical structures such as anchor type, paddle type, and rotating blade type; they are designed with multiple layers to ensure good dispersion of the material. The agitator of the micro-powder dryer has evolved from an initial vertical design to a horizontal one. Their rotors are often designed with reference to the structure of crushers, offering strong crushing and dispersing capabilities. The speed of the mixer can range from 1000 to 4500 rpm, with the specific value determined by the diameter of the drying chamber and the properties of the material. A smaller value is taken when the diameter of the drying chamber is large; a smaller value is used again when the diameter is large. The circumferential speed at the outer edge of the stirrer is approximately between 50 and 80 m/s. Due to the high rotation speed of the stirrer in the micro-powder dryer, a dynamic balance test must be conducted after processing. The stress conditions on the rotor are quite complex, and at present, the power required for it is determined based on experience. 3.2 Design of the classifier: The classifier in the micro-powder dryer is a centrifugal air-flow classifier, which separates fine powder from coarse powder by utilizing the force generated by the high-speed rotation of the impeller, as well as the centripetal force of the air flow. The speed of the classifier can range from 1000 to 2000 rpm, with the circumferential speed at the outer edge of the impeller being approximately 20 to 30 m/s. When designing the classifier, the impeller diameter corresponding to the cylinder diameter and the maximum allowable impeller speed are determined based on the air volume required for drying, in order to ensure maximum classification capacity. During the production process, the impeller speed and air volume can be adjusted to meet the particle size requirements of the finished dust. 4 Application Status: After years of research and development, Tianhua Chemical Machinery and Automation Research and Design Institute successfully developed the φ350 model and the vertical φ1100 model of vertical micro-powder dryers in 2000. In 2003, the φ800 model horizontal and φ1100 model horizontal micro-powder dryers were successfully developed. This dryer has been patented and successfully applied in industrial installations. Micro-powder dryers are highly suitable for drying paste-like viscous materials. When used to dry viscous materials with a high moisture content, and if the required moisture level of the final product is very low, using a micro-powder dryer alone results in low production volumes and high energy consumption; in such cases, two-stage drying is highly advantageous. The application status of micro-powder dryers is shown in Table 1. For materials that are granular or have low viscosity, any material that can be dried using a rotary flash dryer can also be dried with a micro-powder dryer. Moreover, micro-powder dryers perform better when used with these materials. By modifying the rotor structure, the micro-powder dryer can be used for drying organic materials such as nitrile rubber. In a wet state, nanomaterials exhibit high viscosity due to strong surface forces, which causes them to clump together. This results in poor flow properties and difficulties in transportation, thus posing challenges for drying. During the drying process, this bonding force causes the material to agglomerate, affecting the product’s performance. Therefore, during the drying process, bridging of the material should be prevented, and agglomerated material should be effectively broken apart. Furthermore, nanomaterials generally contain modifiers, whether organic or inorganic, which tend to decompose when heated. Therefore, the thermosensitivity of the material must be taken into account when drying nanomaterials. The mixer of the micro-powder dryer possesses strong crushing and dispersing capabilities; it can break up agglomerated materials, which are then sorted using a classifier, thereby fully meeting the required particle size specifications. At the same time, the residence time of the material in the micro-powder dryer is very short, so the material does not deteriorate. The micro-powder dryer developed by Tianhua Institute has been successfully applied to the drying of materials such as nano-calcium carbonate, nano-aluminum hydroxide, micro-powder grade aluminum hydroxide, and nano-magnesium hydroxide, as shown in Table 1. The nano-calcium carbonate production line at a factory in Inner Mongolia contained oversized particles in the product due to the use of rotary flash dryers, failing to meet the required standards. In January 2002, the manufacturer contacted our institute to carry out the renovation. Later, the manufacturer adopted the micro-powder dryer developed by our institute, and the performance of the products **improved**, fully meeting the manufacturer’s requirements. This fully demonstrates the superiority of the micro-powder dryer. Additionally, micro-powder dryers can also be used to directly dry paste-like materials such as molecular sieves. 5 Conclusion The micro-powder dryer is a new type of dryer developed on the basis of the rotary flash dryer. However, the structural design of micro-powder dryers is more advanced, and their performance is superior, making them particularly suitable for drying ultra-fine materials. The manufacturing cost of the micro-powder dryer increases only slightly compared to that of the rotary flash dryer. With the rapid development of nanotechnology, the application scope of micro-powder dryers will become increasingly wide.