Summary of drying technology of ultrafine powder

Drying of the oven means that the solvent in the pores of the ultrafine powder material is evaporated in a box drier using atmospheric hot air as a drying medium. Due to the difference in the thermal expansion coefficient of the micropores of the gel and the gas-liquid phase solvent, this drying method is suitable for slow drying, and the safe drying time may even exceed one year. The drying method is simple in operation and is currently used in the laboratory. However, due to the capillary pressure, the powder particles are prone to severe agglomeration, and it is easy to introduce impurities, and the drying is not uniform, and it is difficult to obtain a super good quality. Fine powder.

Microwave drying is a process in which microwaves are directly converted into heat energy by direct interaction with the product to achieve rapid dehydration and drying of the product. The microwave is an electromagnetic wave with a frequency between 300MHz and 300GHz. The water molecules in the material to be heated are polar molecules. Under the action of a rapidly changing high-frequency electromagnetic field, the polarity orientation will change with the change of the external electric field, resulting in molecular Movement and mutual friction effects. At this time, the field energy of the microwave field is converted into thermal energy in the medium, so that the temperature of the material is increased, and a series of physical and chemical processes such as heating and expansion are generated to achieve the purpose of microwave heating and drying. Microwave drying has the characteristics of rapid heating, uniform heating, energy saving and environmental protection, advanced technology and easy control.

The freeze-drying method includes two processes of spray freezing and vacuum drying. Before drying, a certain concentration of saline solution is prepared to be atomized into minute droplets, and the minute droplets enter the refrigerant and are rapidly frozen into solid particles, and the frozen product is filtered. It is transferred to a vacuum freeze dryer, and then heated to sublime the ice, and calcined to obtain ultrafine oxide particles.

The product prepared by freeze-drying has good gas permeability. Theoretically, freeze-drying can greatly improve the agglomeration problem caused by interfacial tension in the drying process. However, this drying method takes a long time, the internal temperature difference of the material is large, and it is easy to cause "disintegration". Phenomenon, that is, the skeleton formed by the dry layer after ice sublimation is very fragile. When a certain temperature is reached, the rigidity of the skeleton is lowered, the water vapor passage is narrowed or blocked, and the powder of the dried layer is affected by the pressure difference. Flying, not only affects the yield of the product, but also makes the drying process impossible.

Spray Drying (Spray Drying) is a mechanical action that treats the material to be dried in a fluidized form, disperses it to form a mist-like particle, instantly removes most of the moisture, and drys the material solid into a shape. Regular spherical powder.

The spray drying process in the production of special ceramics can avoid the sedimentation separation and re-agglomeration of each component, and maintain the uniformity of the slurry; the drying speed of the slurry is fast, the atomization is uniform, and the surface area after atomization is increased. The particles are generally spherical, and the particle size distribution is uniform; the operation control is convenient, simple and stable, easy to realize automation, and the efficiency is remarkably improved. However, there are high requirements for process control and operating conditions, large equipment investment, high energy consumption, and the need to collect particles and treat exhaust gases.

Supercritical drying technology is a new technology developed in recent years. It is a separation process in which a liquid phase component in a porous solid material is removed in a supercritical state in a high-pressure vessel to obtain a powder. Commonly used drying media are methanol, ethanol and carbon dioxide.

The supercritical drying method utilizes the material at a critical temperature and a critical pressure, and there is no interface between the gas and the liquid, so that there is no interfacial tension to eliminate capillary collapse and gel network damage caused by surface tension during drying. The resulting particles agglomerate, so the supercritical drying method can well prevent agglomeration of the particles.

However, in the drying process, it is necessary to pay attention to the replacement of the solute with the drying medium, which must be slow enough to ensure complete replacement. The liquid in the gel reaches a critical state, which requires a stable process and must be kept in a critical state for a certain period of time. In the process, flammable and toxic solvents are often released, posing a safety hazard.

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