CaCO ₃ is an important inorganic chemical product. Because of its low price, wide raw materials and non-toxicity, it is widely used in rubber, plastic, paper, paint, toothpaste and other fillers. The annual consumption of CaCO _{3 in the} world is about 20 million tons. Common CaCO ₃ as filler since the decrease in production capacity by only the price action. Ultra-fine CaCO ₃ ( particle size less than 0.1um) (hereinafter referred to as nano-calcium carbonate) not only can increase the capacity and reduce the cost, but also has a reinforcing effect in plastics, rubber and paper. The CaCO ₃ product with a particle size of less than 0.02 um has a reinforcing effect and can be compared with white carbon black. CaCO ₃ with a particle size of less than 0.08 um and a narrow particle size distribution is used as a coating for anti- sanding of automobile chassis . Therefore, the research and development of nano- CaCO3 has attracted much attention from both home and abroad. Japan is in an international leading position in the research, production and application of nano- CaCO ₃ , and now has more than 50 varieties of spindle-shaped, cubic, needle-shaped, spherical, chain-locked and amorphous shapes and surface modification . The United States focuses on the application of ultra-fine CaCO ₃ in papermaking and coatings. The UK is mainly engaged in the development of ultra-fine CaCO ₃ for coatings . In the past 20 years, the United Kingdom has occupied a monopoly position in CaCO ₃ for automotive-specific plastics . China has been conducting research on ultra-fine CaCO ₃ since the 1980s . Several domestic units have developed and produced several types of nano- CaCO ₃ products. However, the variety is small and the yield is low. Production technology and equipment are backward, and high-end products mainly rely on imports. Strengthening the research and development and development of new high-grade nano- CaCO ₃ products production process and equipment, reducing production costs, is the hope of CaCO ₃ industry in rubber, plastic products, papermaking and other industries , and also an important goal of China's CaCO ₃ industry development, is calcium carbonate The best way for enterprises to improve their economic efficiency.

   Due to its small particle size, nano-sized calcium carbonate is prone to agglomeration, and the effect of post-treatment is directly related to the application effect of nano-calcium carbonate.

   2   Introduction to current production methods

   According to different carbonization processes, the production methods of nano-scale calcium carbonate in China can be roughly divided into the following four types: batch bubbling carbonization, continuous bubbling carbonization, continuous spray carbonization, and supergravity reaction crystallization.

   2.1   Intermittent bubbling carbonization         

   According to whether there is a stirring device in the carbonization tower, the method can be further divided into a common intermittent bubbling carbonization method and a stirring intermittent bubbling carbonization method. The method comprises adding a refined calcium hydroxide suspension and a suitable additive to a cone-bottom cylindrical carbonization tower, and then passing the carbon dioxide carbonization end point from the bottom of the tower to obtain the desired calcium carbonate product. The reaction conditions such as carbonization temperature, carbon dioxide flow rate, milk concentration and stirring speed are strictly controlled during the reaction, and appropriate additives are added. The method has low investment, simple operation, low production cost, but poor production stability.

   2.2   Continuous bubble carbonization

   The continuous bubbling carbonization process generally adopts a two-stage or three-stage series carbonization process, that is, the refined lime milk is partially carbonized by the first-stage carbonization tower or a reaction mixture is obtained, and the appropriate additive is added to the slurry tank to enter the second-stage carbonization tower. Carbonization produces the final product. The method is carried out step by step according to the carbonization process, and the surface treatment is carried out between the stages, and the carbonization temperature can be controlled by cooling. Therefore, the nucleation, growth process and surface treatment of the crystal form are controlled in stages, so that a better crystal form can be obtained. Small particle size and particle size distribution. Now, some domestic calcium carbonate manufacturers can produce the desired crystal form and particle size products by strictly controlling the concentration of lime milk, carbonization temperature, type and proportion of additives according to the needs of users.

   2.3   Continuous spray carbonization         

   The continuous spray carbonization process generally employs a three-stage tandem carbonization process. Refined lime milk is sprayed from the top of the first-stage carbonization tower to 0.01 -0.1mm The droplets are added, carbon dioxide is passed from the bottom of the column, and the carbonization reaction occurs in countercurrent contact. The reaction mixture flows out from the bottom of the column, enters the slurry tank, is treated with a suitable dispersant, is sprayed into the second-stage carbonization tower to continue carbonization, and then subjected to surface treatment, spraying into a tertiary carbonization tower to obtain a final product. Its product size can reach 40-80nm . Due to the high investment in the process, the complexity of the technology, the difficulty of operation, and the more important problem, the nozzle atomization problem is difficult to solve. Therefore, the application of this law in China is not universal.

   2.4   Hypergravity reaction crystallization

   The technical feature of the supergravity reaction crystallization method is to strengthen the gas-liquid mass transfer process as the basic starting point. The core of the crystallization process is that the carbonization reaction is carried out in a super-gravity centrifugal reactor, using tens to hundreds of times of gravity acceleration generated by high-speed rotation of the packed bed. The super-gravity field environment can be obtained, and the CO2 and Ca(OH)2 suspensions are counter-currently contacted in the special equipment for super-gravity, which greatly enhances the interphase mass transfer and micro-mixing, and creates an ideal for uniform and rapid nucleation of CaCO3. surroundings. In the super-gravity field, various transfer processes are greatly enhanced, the phase interface is rapidly updated, and the volumetric mass transfer coefficient can be increased to 10 to 1000 times that of the normal gravity packed bed , thereby greatly improving Ca(OH)2 dissolution and CO2 absorption. rate, the concentration of Ca2 + and CO32- system increases, increase the degree of supersaturation, while the addition of a suitable dispersing agent, crystal growth control, the final average particle diameter of 15 ~ 30nm nano-calcium carbonate. The particle size distribution of the method is uniform, the reproducibility of different batches of products is good, and the carbonization reaction time is only 1/4 to 1/10 of the traditional method , reaching the international advanced level. However, due to high investment and high production costs, the law is limited in application.

   3    Low-cost production technology features

   3.1 Low-cost nano calcium carbonate production technology

   The intermittent carbonization method is similar to the preparation method of the traditional light calcium carbonate, except that the light calcium carbonate is reacted in a bubble column, and the preparation of the nano calcium carbonate is generally carried out in a stirred reactor, using a strong turbulent flow. In a manner, the mass transfer and heat transfer effect of the reaction system are improved by stirring. The main process of production is: calcining the limestone raw material into quicklime and carbon dioxide, and then adding water to digest the lime to form lime milk. After the lime milk is refined, the carbon dioxide carbonized lime milk is introduced to form calcium carbonate precipitate, and finally the calcium carbonate precipitate is surface modified. Nano-calcium carbonate is obtained by dehydration, drying and post-treatment. The process flow is shown in Figure 1 .

Figure 1 Flow chart of nano calcium carbonate production process

   The reaction conditions are strictly controlled during the production process. The main controlling factors are Ca(OH)2 concentration and CO2.

   Flow rate, reaction temperature, additive amount, additive addition time, etc. By controlling different conditions, monomer particle size has been prepared

   A variety of nano calcium carbonate products larger than 10 nm, the crystal shape is chain, needle, spherical, cuboid, sheet, etc., including all the crystal shapes of calcium carbonate which have been prepared. Most of these products are calcite -type crystals, and some of the spherical crystals are of the nepheline type, which are obtained by adding a polymer additive during the reaction.

   We have comprehensively summarized the technical status of nano-calcium carbonate in China and abroad, and made maximum use of the latest research results at home and abroad. We have done a lot of research on the preparation of nano-calcium carbonate by batch carbonization, and successfully designed a new and efficient synthesis reactor. The efficient and inexpensive particle shape and particle size control agent were found to optimize the process and process conditions. Among them, the synthesis reactor is a key technical link. The requirements of the reactor are: the stirring strength is suitable, uniform and stable; the gas filling is uniform, the bubbles are fine, and the dispersibility is good. There are special special equipment in the peripheral equipment of the reactor. The special equipment is used for strengthening, so that the produced nanometer calcium carbonate has uniform particle size and narrow particle size, and the average particle diameter of the produced nano calcium carbonate is 40 nm.

   The production of the process is all domestically produced, the operation is simple, the operation is reliable, and the control is convenient. With this method, the investment is small, the effect is quick, the conversion is easy, the production cost is low, the production cost is about 700 yuan / ton, and the final total production cost fluctuates in the range of 1000 to 1350 yuan / ton depending on the production scale and product requirements .

   Different crystal form controlling agents are added at different times in the production process according to the needs of production in order to control the particle size and shape of the calcium carbonate particles.

   According to the requirements, special nano-calcium carbonate can be produced, such as nano-calcium for ink, high-value-added nano-calcium for papermaking, special nano-calcium for silicone rubber filler, nano-calcium for medicine, special nano-calcium for plastic filler, Special nano-calcium for coating fillers, special nano-calcium for food, special nano-calcium for cable packing, special nano-calcium for daily chemical fillers, etc.

   The shape of the nano calcium carbonate particles produced by the method has a cubic shape, a spherical shape, an ellipsoidal shape, a chain shape and the like according to the addition of the crystal form controlling agent, and the particle size is uniform, the shape is uniform, and the whiteness is high.

   3.2   Modified dispersion system

   Due to its small particle size, nano-calcium carbonate is extremely easy to agglomerate, so it must be dispersed before application. At present, the surface treatment of nanometer calcium carbonate is only the surface coating treatment of the agent, although the agglomeration has a certain overcoming effect, but the effect is not very obvious. We have developed two types of modification equipment. One is a dry modification device, and the other is a wet modification device.

   The dry modification equipment is completed by surface modification and dispersion treatment in the same equipment.

   The modified dispersion system has the following characteristics:

   1      Continuous operation, large processing capacity;

   2      Uniform modification, less dosage;

   3      The discharge is completely completed by the equipment, and the labor intensity of the workers is low;

   4      The system operates under negative pressure, with good airtightness and no dust pollution;

   5      High production efficiency, high degree of automation, low unit energy consumption;

   6       The modified powder has a smooth surface, good dispersibility and low water absorption.

   The wet modification equipment is a newly introduced special wet modification equipment, which is convenient to use and has the following characteristics:

   1      Continuous operation, processing capacity can be changed as required;

   2      The modification is complete, the coating effect is good, and the dosage of the medicament can be reduced by half compared with the conventional method;

   3      The process is fully mechanized and the degree of automation is high;

   4      The system has low noise, no pollution and good working environment;

   5      Low energy consumption;

   6      Small footprint;

   7      The material after modification and dispersion has good dispersibility, low viscosity and convenient application.

   4    Conclusion

   The nano-calcium carbonate process technology features: 1    A unique production technology, all domestic equipment, the process is simple, safe, reliable, low production cost, less investment and quick, the final total production costs vary with the scale of production and product requirements in 1000 to 1,350 yuan / ton range fluctuations ;2   The product is post-treated by special post-processing equipment, the post-treatment effect of the product is good, and the agglomeration phenomenon is evenly dispersed. As a functional filler, the adding effect is good;   According to different requirements, the product has strong functionalization; 4 can be dry and wet modified; 5 product diversification.

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