Current situation of drying technology for the hot

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Current status of drying technology for nano calcium carbonate powder

nano calcium carbonate powder refers to inorganic powder products with particle size of 1~100mm and different performance from conventional calcium carbonate. Because of its unique electrical, magnetic, thermal and optical properties, it is widely used as pigments and fillers in plastics, rubber, coatings, inks, papermaking, cosmetics, pharmaceuticals, adhesives and other industries. Nano calcium carbonate not only has filling effect, but also has semi reinforcing or reinforcing effect, which has attracted worldwide attention

when nano calcium carbonate powder is synthesized by wet chemical method, the post-treatment process of sediment drying is generally involved. Due to the small particle size of nano calcium carbonate powder, the safety belt is one of the indispensable parts in the automobile, with high surface energy and in a thermodynamic unstable state. If the drying method is not selected properly, the agglomeration of nano calcium carbonate particles will occur, which will have a negative impact on its service performance. Therefore, one of the key steps in the production of nano calcium carbonate powder is how to ensure that the nano calcium carbonate particles remain dispersed and do not agglomerate during the drying process. Here, in the application of nano calcium carbonate powder, special emphasis is placed on its hydrophobic and lipophilic properties, that is, the final water content of the product is very high (that is, the water content of the product is very low)

1. Current status of industrial drying technology for nano calcium carbonate

1.1 primary drying

in the industrial production process of nano calcium carbonate, in addition to preventing agglomeration between particles, the biggest problem in the drying operation process is how to reduce the drying temperature and make the device have a certain production scale. Nano calcium carbonate powder usually needs to be coated on the particle surface. However, the thermal stability of organic matter is generally poor, and high temperature is easy to cause coking and deterioration, which directly affects the dispersibility and whiteness of particles. For example, under the condition of 135 ℃, stearic acid or stearate can be coking in a very short time, reducing the whiteness of the product by 7~8%. The degree of coking is corresponding to time and temperature. Therefore, to ensure that the product is not coking, the drying temperature and time must be controlled, and the appropriate drying temperature and time must be sought according to the characteristics of surface modifiers

at present, domestic enterprises producing nano calcium carbonate often use primary drying equipment, such as drying room drying, conveyor belt drying, disc drying, rotary cylinder drying, spray drying, vacuum rake drying, rotary rapid drying, etc. However, almost every drying equipment encounters the problem of secondary agglomeration of product particles, which makes some nano-sized calcium carbonate particles unable to exist as primary particles or close to this state, and ultimately affects the function and application of its products. Here, after primary drying, regrinding, classification, screening and other processes are generally required

although drying in the drying room is economical and simple, it is an intermittent operation, resulting in excessive coal production capacity, low production capacity, high energy consumption, poor operating environment, high labor intensity of workers, easy to introduce impurities, and uneven drying. After drying, there is also a need for crushing, classification and other operations. Therefore, this drying equipment has been gradually replaced by new drying technology

conveyor belt drying is actually an improved drying room, which makes the drying process continuous. However, the equipment covers a large area and its thermal efficiency is not high

although the tray dryer solves the problems of large land occupation and low thermal efficiency of the conveyor belt dryer, the equipment structure is complex and difficult to maintain. The additional plate making to reduce the adhesion of materials on the plate has serious wear on the plate, and the resulting impurities have a great impact on the product quality

the rotary cylinder dryer is widely used in the production of ordinary light calcium carbonate. Its production capacity is high. The annual output of a rotary cylinder dryer with a diameter of 1.8m and a length of 21m can reach 15000-20000 tons. However, its drying temperature is generally 600-700 ℃, which is not suitable for the production of wet activated nano calcium carbonate, because at this high temperature, it is almost impossible to find a surface modifier that will not coking. Here, rotary cylinder drying is only applicable to the drying of nano calcium carbonate with water content of 30% - 40%

spray drying can quickly dry wet calcium carbonate with water content of more than 60% to products with water content of less than 0.5%, and the product is in good dispersion state without grinding. However, it has poor adaptability to active nano calcium carbonate, because the temperature required for spray drying of nano calcium carbonate is high (generally above 400 ℃), and it is very easy to burn off the surface modifier

vacuum rake drying is rarely used in the production of nano calcium carbonate. Its advantages are low drying temperature and high thermal efficiency, but there are many auxiliary equipment, most of which are intermittent operation, which restricts the production capacity

at present, rotary rapid drying (also known as rotary flash drying) is widely used in the production of nano calcium carbonate. This technology has made some progress in solving the problem of particle agglomeration, but there are still problems such as sticking to the wall and grinding after drying

1.2 combined drying

in order to solve the industrial drying technology problem of nano calcium carbonate, the two-stage drying technology is more and more favored by many enterprises. It has better solved the contradiction between low-temperature drying and production capacity. At the same time, the particle dispersion and depolymerization function is introduced into the second stage drying equipment to reduce the regrinding process after drying. Common two-stage drying combinations include conveyor belt dryer and rotary rapid drying, paddle drying and rotary rapid drying, paddle drying and micro powder drying, double screw conveyor drying and disc drying

1.2.1 combination of conveyor belt drying and rotary rapid drying

the two-stage combined process flow of conveyor belt drying and rotary rapid drying of nano calcium carbonate is shown in Figure 1. The nano calcium carbonate after pressure filtration is shaped by the extruding machine and then enters the conveyor belt dryer to remove most of the moisture. When the moisture content of semi-finished products is ≤ 20%, they leave the conveyor belt dryer, enter the feeder of the rotary rapid dryer through the screw conveyor, and then are sent to the rotary rapid dryer, where they are further dried and broken under the action of mixing and rotating hot air flow. The dried fine powder and the rising air flow enter the cyclone separator and bag filter through the classifier, and the separated product is the product. Exhaust gas is discharged through induced draft fan and chimney

1.2.2 combination of paddle drying and rotary rapid drying

the two-stage combined process flow of paddle drying and rotary rapid drying of nano calcium carbonate is shown in Figure 2. Comparing Fig. 1 and Fig. 2, it can be seen that the main difference between the two processes is that the conveyor belt dryer is replaced by the paddle dryer. The filter cake after pressure filtration is first added to the paddle dryer and dried while moving forward under the rotating agitation of the hollow paddle. Water vapor (or heat transfer oil) passes through the hollow blade and dryer jacket to dry the nano calcium carbonate wet material by means of heat conduction. After leaving the paddle dryer, the semi dry products enter the rotary rapid dryer and are further dried into qualified products

1.2.3 combination of paddle drying and micro powder drying

if the rotary rapid dryer in the above two-stage combination of paddle drying and rotary rapid drying is replaced by a micro powder dryer, this constitutes a two-stage combination process of paddle drying and micro powder drying. The loose materials obtained after the first stage paddle drying are continuously and evenly added into the micro powder dryer by the screw conveyor, and further dried to become qualified products. The micro powder dryer is mainly composed of a cylinder, a rotary agitator and a classifier. It is a multifunctional dryer integrating drying, crushing and grading. The main function of the rotary agitator is to disperse the soft agglomerated particles, so that the large soft agglomerated particles can become products with the required particle size. The classifier is a centrifugal air flow classification structure, which is produced by the high-speed rotation of the impeller, but the use of plastics on Chinese cars is far from enough. The particles are classified by the action between the centrifugal force and the centripetal force of the air flow. The materials with qualified particle size pass through the classifier and are sent into the cyclone separator and bag filter by hot air to obtain qualified products; Under the action of centrifugal force, the materials with large particle size impact the inner wall of the equipment barrel and return to the bottom. Under the action of rotary agitator and hot air, they are further dried and broken until they are qualified and then enter the subsequent collection device

1.2.4 the basic reason for the double screw conveying drying and disc drying is the combination of the drying of materials or implants in vivo in the presence of foreign bodies.

the two-stage combined process flow of double screw conveying drying and disc drying of nano calcium carbonate is shown in Figure 3. The filter cake is sent to the silo by the belt conveyor, and is broken by the crusher and discharged at the tail of the double screw conveyor dryer; After being measured by bucket elevator, silo and electronic belt weigher, it is continuously added to the top of the disc dryer, and then dropped in the disc dryer one by one, and the dried products are discharged from the bottom

2. Conclusion

the existing domestic nano calcium carbonate drying technology has not yet solved the problem of particle agglomeration in the drying process, and almost every drying equipment has the problem of secondary particle agglomeration. Therefore, the problem of poor particle dispersion caused by agglomeration has become an urgent focus and difficulty in the production and research of nano calcium carbonate. In addition, the drying technology of nano calcium carbonate has lagged behind other technologies, and is becoming the bottleneck of its industrial production. Therefore, it is necessary to strengthen the research on the drying mechanism and drying theory of nano calcium carbonate powder, so as to provide the basis and direction for the development of new drying devices, and then develop large-scale nano calcium carbonate drying equipment suitable for large-scale industrial production; The research and development of new drying technologies (such as azeotropic distillation drying, freeze drying, etc.) should be strengthened

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