I. Working Principle of Spray Drying
Spray drying is a process that uses an atomizer to disperse the liquid material into fine droplets, and then rapidly evaporates the solvent in a hot drying medium to form a dry powder. Spray drying generally consists of four stages: a) atomization of the liquid material; b) contact and mixing of the droplet cloud with the hot drying medium; c) evaporation and drying of the droplets; d) separation of the dried product from the drying medium.
The raw material liquid can be a solution, emulsion, or suspension, or any liquid form that can be pumped, such as a molten liquid or paste. The product is in powder, granular, hollow sphere, or agglomerated form.
II. Common Spray Dryers
Spray dryers use an atomizer to spray dilute liquid materials (such as solutions with a water content of 75%-85% or more) into fine droplets dispersed in a hot airflow, allowing the water to evaporate rapidly to obtain a solid product. The drying time is generally only a few seconds to tens of seconds. The atomizer is a key part of the spray dryer, mainly including centrifugal, pressure, and airflow atomizers.
1. Pressure Spray Dryer
The liquid material (filtered liquid) is fed into a pressure atomizer at a high pressure of 2-20 MPa to atomize it into small droplets. The atomized droplets (with a greatly increased surface area) are in full contact with the hot air, and the drying process is completed rapidly within 10-30 seconds, resulting in a powder or fine granular product after separation.
2. Centrifugal Spray Dryer
Air passes through a filter and heater and enters the air distributor at the top of the centrifugal spray dryer. The hot air enters the dryer uniformly in a spiral pattern. The liquid material is pumped from the liquid tank through a filter to a centrifugal atomizer at the top of the dryer, where it is sprayed into extremely fine droplets. The liquid material and hot air flow concurrently, causing the water to evaporate rapidly, drying into a finished product in a very short time. The finished product is discharged from the bottom of the drying tower and the cyclone separator, and the exhaust gas is discharged by a fan.
3. Airflow Spray Dryer
This type of dryer mainly uses high-speed air or steam ejected from a nozzle to separate the liquid material into fine droplets through friction, allowing it to come into contact with hot air for heat exchange. The entire process takes less than half a minute. It works very well for highly viscous materials and is easy to operate.
III. Application Fields of Spray Dryers
1. Spray Drying for Special Ceramic Granulation
Before the forming and sintering of ceramic materials, the forming of the green body directly affects the performance of the product. The characteristics of the powder greatly influence the uniformity of the green body and the density after dry pressing. Spray granulation technology is widely used in the preparation of special ceramic powders. Pressure spraying is usually used for spray granulation of special ceramic powders. Under suitable process conditions, the prepared powder has good chemical uniformity, high fineness, good fluidity, and high compaction density, making it suitable for dry pressing or isostatic pressing of special ceramic powders.
2. Application of Spray Drying in Lithium Iron Phosphate Cathode Materials for Lithium Batteries
Lithium batteries mainly consist of cathode materials, anode materials, electrolytes, and separators. Cathode materials constrain the development of lithium-ion batteries in terms of cost and performance; therefore, the research and development of cathode materials are key to promoting the technological advancement of lithium-ion batteries. Currently, domestic cathode materials mainly include lithium cobalt oxide, lithium nickel oxide, lithium iron phosphate, and ternary materials. Among them, lithium iron phosphate has the advantages of good safety, long cycle life, good thermal stability, wide availability of raw materials, and low price, but it suffers from low tap density and poor conductivity at low temperatures.
Simple preparation process of lithium iron phosphate/C cathode material: Take LiOH and FePO4, add them to a ball mill jar with pure water, ball mill, then add a certain amount of carbon source, ball mill for 3 hours, then take out the slurry and feed it into a spray dryer for spray drying. Spray drying was performed at specific inlet and outlet temperatures (inlet 200-250℃, outlet temperature <100℃). After drying, the material was placed in an atmosphere furnace for low-temperature calcination for 2 hours, followed by high-temperature calcination for 6 hours to obtain LiFePO4/C cathode material.
Experiments showed that the spray drying step imparted excellent sphericity to the precursor material before sintering, with a particle size of 20-20 micrometers. Under spray drying conditions, the chemical composition of the material was relatively uniform. During calcination, no solid-phase transformation occurred between the spherical particles, and the growth of LiFePO4/C crystallites was confined within each spherical particle. Therefore, small LiFePO4/C crystallites were obtained, and these fine crystallites are beneficial to the electrochemical performance of LiFePO4/C.
3. Spray Drying for the Drying of Precipitated Silica
Precipitated silica, also known as white carbon black, is an important reinforcing material in the rubber industry. Because its microstructure and aggregation morphology are similar to carbon black, and it has similar reinforcing properties in rubber, it is called white carbon black. White carbon black can be produced by gas-phase and precipitation methods. Gas-phase products have high purity and good performance, but the production process consumes a lot of energy, is technically difficult, and has high costs.
In contrast, the precipitation method for preparing white carbon black is mature, simple to operate, and has significant cost advantages. However, during the drying and dehydration process, ultrafine particles are prone to agglomeration, affecting performance. Studies have shown that spray drying can produce white carbon black with uniformly dispersed particles, high specific surface area, and high oil absorption value, conforming to the recommended oil absorption value and specific surface area correlation curve, making it suitable as a reinforcing agent for high-performance tires and colored tires.
4. Application of Spray Drying in Pharmaceutical Preparations
Drying plays an important role in pharmaceutical production, especially in traditional Chinese medicine production. Pharmaceutical preparations prepared by spray drying have the following characteristics:
a. Good uniformity of the finished product: During spray drying, the drug solution is sprayed into an atomized dispersion under continuous stirring, and drying is completed instantaneously, resulting in good uniformity. b. The finished product has good fluidity, looseness, and solubility: During spray drying, the water evaporates rapidly, resulting in small particle size, good fluidity, and smooth tablet formation; this allows for more accurate dosing during packaging. When in contact with water, it is easier for the water to penetrate the inside of the particles, which facilitates drug dissolution.
c. Simplified production process: Spray drying allows for concentration and drying to be completed in one step, and the drying time is relatively short, which helps maintain the efficacy of heat-sensitive drugs.
d. Clean production: The use of a closed spray environment prevents drug contamination by bacteria and reduces dust pollution in the workshop.
Traditional Chinese medicine products require high levels of cleanliness, so multiple air purifiers are used. Therefore, spray drying is very suitable for the drying process in the manufacturing of traditional Chinese medicine.