Northeastern University, Liaoning Dongda Powder Engineering Technology Co., Ltd., 1990 in absorbing foreign equipment and technology, based on the design and production of suitable conditions, with technical characteristics of flash drying equipment, has formed a specialized series of products, chemical, building materials, metallurgy and other fields to form the industrial scale application, The successful experience of drying multi-species alumina and the perfect process supporting system are obtained. In this paper, the drying process of rotary flash dryer is introduced in order to help the selection and application of multi-species alumina drying.

1. Drying process mechanism of granular materials

After the pellet material enters the dryer, the hot gas flow first passes the heat to the grain surface, the water evaporates immediately and spreads to the outside world. Surface water evaporation, causing the particle surface and internal moisture difference, moisture will from the inside of the particles continue to spread to the surface, and then from the surface to the outside evaporation, the process of cyclic, and finally to the whole particle drying.

1.1-liter-speed drying stage

Particles placed at a higher temperature and relative humidity of less than 100% of the heat transfer medium, in a short time, the surface is heated to dry medium wet bulb temperature, water evaporation rate of rapid growth, the particles absorb heat and evaporation of water consumption of equal heat, to achieve balance. This stage time is very short, the discharge volume is small, then enters the equal speed stage.

1.2 Equal-speed drying stage

At this stage, the evaporation of the surface of the particles of water, from the internal to the surface continuously replenished, so that the particle surface is always maintained wetting state. At this point the drying speed remains constant and the surface temperature of the particles remains unchanged. The evaporation of water in this stage can theoretically calculate the drying speed according to the outer diffusion (evaporation) formula and the Heat transfer formula:

Outside i = M/(t? F) = a(η (t wet ball-t table)),kg/m2h

It can be seen from the above that the evaporation speed (drying speed) is related to the water vapor concentration and temperature difference of the particle surface and the surrounding medium. The greater the difference, the greater the drying speed. In addition, the drying speed is related to the air velocity of the particle surface. The particle surface always has a layer of air film which is not easy to flow, and its thickness decrease is advantageous to water evaporation and heat exchange. Thus increasing the velocity of the particle surface airflow, so that the air film thinning, can significantly improve the drying speed.

1.3 Spin down drying stage

At this stage, the evaporation speed and heat consumption are greatly reduced, the surface temperature of the particles is higher than the wet bulb temperature of the medium, and the temperature difference between the heat carrier is reduced to close or the same.

1.4 Balance Stage

At this time the particle surface moisture absorption and evaporation to achieve a balance, drying speed of zero.

The moisture in the particles, which is dry final moisture, should not normally be lower than the equilibrium moisture at the time of storage. Rotary Flash dryer Due to the particle size of the material after drying is very small, the material in the drying cylinder has a very short stay time, usually in 1~3s. Therefore, the drying of particles in the drying stage, the surface temperature is the drying medium wet bulb temperature. The use of rotary flash drying equipment, the particle size uniformity, surface without cracking, deformation and overheating, to ensure product quality.

2. The moving state and drying process of the particles in the rotary flash dryer

Hot gas flow from the lower part of the tube along the tangential direction of the tube into the tube, in the barrel of high-speed rotation rise, and wet materials meet, rotating blades will crush the material, hot gas flow will be heating materials, blowing scattered. The moisture content of fine granular material is evaporated and rises with the spiral of hot air flow, which is discharged from the exhaust outlet, and the dry product is formed after separating device. Coarse particle material spiral rise a certain height, because its suspension speed is smaller than the operating speed of the dryer, so it will stop rising and fall, after crushing into fine particles, blown away by the hot wind and then repeat the process. The unique design and layout of the rotating blades in the dryer is advantageous to the quick breaking and drying of the materials and the reasonable design of the graded ring structure, which can guarantee the final water content and particle size of the products.

The heat exchange in the dryer is mainly manifested by two kinds of heat exchange of air flow and particle, cylinder wall and granule.

As mentioned above, the essence of drying process is the diffusion process of water, which is carried out by external diffusion and internal diffusion.

The movement of water molecules can be divided into wet conduction and heat conduction according to the different motive force.

2.1 Wet Conduction

During the drying process, due to the evaporation of surface water, the surface moisture of particles and the formation of internal water concentration is poor, so there is a water gradient in the particle radius direction, causing moisture to move from the internal to the surface. This diffusion, conduction is caused by the water difference.

2.2 Heat conduction

Due to the evaporation of the particle surface moisture needs to absorb heat, resulting in the particle interior and surface temperature difference, that is, in the radius of a temperature gradient temperature difference. The resulting moisture movement is called heat conduction.

When used in hot air drying, the wet diffusion is carried out from the inside of the particles to the external surface. At the same time, because the surface temperature of the particles is often higher than the internal temperature, heat diffusion causes the water to move from the particle surface to the interior. Therefore, it can be regarded as thermal diffusion to hinder the wet diffusion and reduce the drying speed.

Rotary Flash dryer by high-speed hot air flow along the tangential direction into the cylinder, because of the spiral movement in the cylinder, on the one hand, reduce the medium temperature around the particles, while increasing the surrounding medium velocity and temperature, improve the speed of external diffusion, on the other hand, high-temperature air impact on the lower part of the barrel of the granular aggregates In addition, the effect of stirring blades in the cylinder makes the aggregates break loose, the granularity becomes smaller, and the length of internal capillary is reduced, the effect of inner diffusion is strengthened and the resistance is reduced. The process is repeated, eliminating the caking of the material, strengthening the evaporation of the particle moisture.

The flow of particles and hot air flows in the lower part of the cylinder has both convection and downstream (and flow). For coarse-grained aggregates, convection and downstream heat exchange are repeated. For fine-grained materials, the process is carried out in the same way as the thermal airflow, so that the drying process can be completed instantaneously. For the coarse aggregates of dry, in fact, the use of high temperature and low humidity hot air. These coarse aggregates are mainly adsorbed by the water molecule, filled in between the particle voids, the use of high temperature and low humidity conditions, so that the whole collective heat conduction slow, causing local stress concentration and make it dry, broken scattered, accelerate the drying process, improve thermal efficiency.

3. Flash Drying Machine Process

Rotary Flash Dryer is a continuous working drying equipment, consisting of a host and a helical feeding mechanism, respectively driven by the motor alone.

Rotary Flash drying System, in addition to the host must also have: heating parts, gas supply parts, finished product collection part. In general, the three parts of the heating use coal, fuel, gas, hot air stove or the use of electric power, steam heat exchanger; The gas supply system is composed of centrifugal ventilator and its pipeline, valve, etc. the finished part is composed of cyclone separator and bag filter.

The above system configuration is the basic configuration of Flash dryer system, the special materials should be adjusted to adapt to the requirements of different materials drying.

4. Technological properties and parameter selection of multi-species alumina drying

Many varieties of alumina drying habits used in the dry way for box, belt or rotary drum drying, these dry materials are more static or semi-dynamic drying process, drying time, high energy consumption, and dry materials also need to be crushed, graded process, resulting in low product quality, high energy consumption, serious product waste, the formation of dust pollution, Flash Dryer is a full dynamic drying method. High moisture content into the dryer after a few seconds of instant drying into powder, complete drying, dispersing, grading process, dry hot and dry medium and dried material is in the maximum specific surface area heat transfer condition to show the instantaneous heat-transfer drying characteristics, so that the energy utilization rate increased significantly, product apparent, fluidity, uniformity outstanding, Drying process is carried out in closed state, improve product recovery rate, reduce environmental pollution, this drying method in the drying of many varieties of alumina products show remarkable characteristics.

Many varieties of alumina products drying process, the first will be pressure filter or centrifugal dehydration after the product, by the adjustable speed feeder into the dryer, the feeding device with feeding stability and can overcome the viscosity of different materials, the heat source of the dryer can be used indirect or direct combustion heaters. Fuel for natural gas, diesel, liquefied gas, such as coal should be used in a machine-burning coal-fired hot air stove, to ensure stable coal temperature, burning exhaust gas in line with environmental requirements, the system stable setting according to the Drying product physical property selection, high temperature easily caused by individual material surface sintering, crystallization water precipitation and heat-sensitive color change, The material that enters the flash dryer is dispersed, dry, granular grading process by gas-solid separation after the gas capture system, the separation parameter selection to determine the size of the material, to limit the ultra-fine materials into the bag filter wind speed is not greater than 1 m/s appropriate to prevent the running material, to flame retardant aluminum hydroxide For example, will be filtered after the water 50 ?? The above filter cake into the flash drying machine, the inlet air temperature of 250~300℃, dry material end water 1?? The following, the SKSZ125 type per hour output is 500~700kg/h, System configuration Power 65-kilowatt, covers an area of 120m2.

Rotary Flash dryer operation, dynamic, continuous, fast, uniform product quality, stable system operation, and energy saving, consumption, pollution-free, in many varieties of alumina drying, as well as in the light metal industry products will be more widely used.