Nano calcium carbonate production methods are mainly physical and chemical methods. The physical method refers to a method in which no chemical reaction takes place from the raw material to the entire particle, that is, a method in which a calcium carbonate product is obtained by mechanically pulverizing natural limestone with a high content of calcium carbonate. However, it is very difficult to pulverize it to less than 1.0 μm using a general pulverizer. It is only possible to achieve a size of less than 0.1 μm using special methods and machinery. Therefore, chemical methods are often used in actual production. The chemical method includes two methods of double decomposition and carbonization.
2.1 Double decomposition method
The double decomposition method uses a water-soluble calcium salt (such as calcium chloride, etc.) and a water-soluble carbonate (such as ammonium carbonate or sodium carbonate, etc.) and reacts under appropriate process conditions to produce nanometers through a liquid-solid reaction process. Grade calcium carbonate products. This method can control the concentration of the reactants, the temperature and the degree of supersaturation of calcium carbonate, and add appropriate additives to obtain spherical, small particle size, large specific surface area, very good solubility of amorphous carbonic acid calcium. The resulting product has high purity and good whiteness. However, since a large amount of chloride ions adsorbed in calcium carbonate is difficult to be eliminated, the cleaning method used in production often requires a large amount of time and washing water, so it is rarely used at present in China.
2.2 Carbonization
The carbonization method is to calcine the selected limestone to obtain calcium oxide and kiln gas. The suspended calcium hydroxide produced by calcium oxide digestion is crushed under the action of high shear force, and the multipolar hydrocyclone is separated to remove particles and impurities to obtain a certain concentration of purified calcium hydroxide suspension; then, carbon dioxide gas is introduced to add an appropriate amount of crystal form The control agent is carbonized to the end point to obtain a calcium carbonate slurry in a desired crystal form; finally, the nano calcium carbonate product is obtained through surface treatment and dry crushing. In the carbonization process, the carbonization process determines the particle size and crystal form of the light calcium carbonate. This method has the advantages of good product quality, low cost, etc. It is currently the main method for producing nano-scale calcium carbonate at home and abroad. He mainly includes several production methods such as batch carbonization, continuous spray carbonization, and high-gravity carbonization.
2.2.1 Intermittent Carbonization
The production of nanometer calcium carbonate by batch carbonization is carried out by using carbon dioxide and calcium hydroxide as raw materials, and stirring the bubbling carbonization tower or the kettle to pass the carbon dioxide and reacting with calcium hydroxide after the temperature is reduced by the refrigerator to generate calcium carbonate by emulsion reaction. The reaction control is performed intermittently, so it is called intermittent (frozen) type carbonization method. According to the different contact modes of carbon dioxide and calcium hydroxide, it can be divided into two types: intermittent bubble carbonization and intermittent stirring carbonization. The intermittent bubbling carbonization method is to reduce the temperature of the 5~8 Baumite lime mortar cooler below 25°C, pump it into the carbonization tower, maintain a certain liquid level, pass the bottom of the tower into the kiln gas bubbling for carbonization reaction, Control the reaction temperature, concentration, gas-liquid ratio, additives and other process conditions, and prepare nano-calcium carbonate intermittently. This method has the advantages of low equipment investment, simple operation, high energy consumption, difficult control of process conditions, and wide particle size distribution. The intermittently agitated carbonization method, also called the kettle type carbonization method, is to reduce the temperature of the lime milk to below 25° C. by a freezer, place it into a carbonization reactor, pass a carbon dioxide gas mixture, and carry out the carbonization reaction under stirring. Nano calcium carbonate was prepared intermittently by controlling the reaction temperature, concentration, stirring speed, additives and other conditions. This method has large equipment investment and complicated operation, but due to the large gas-liquid contact area, the reaction is more uniform, and the product particle size distribution is narrower.
2.2.2 Continuous Spray Carbonization
Continuous spray carbonization is a process developed by Japan's Shiraishi Industrial Co., Ltd. in the late 1970s. Usually, multiple carbonization towers are used. The kiln gas containing carbon dioxide enters from the bottom of the tower and is sprayed with a certain amount of droplets of lime milk. Countercurrent contact, carbonization reaction. The carbonization process can be two-stage or three-stage to multi-stage, with 2 to 4 segments generally being preferred. The number of segments to be selected depends on the user's performance requirements for the product. Using a spray-type carbonization process, an average particle size can be obtained by adjusting the concentration of the calcium hydroxide suspension during the reaction, the droplet diameter of the spray, the concentration of the carbon dioxide gas mixture, the superficial velocity, the gas-liquid ratio, and the carbonization rate per column of the slurry. Calcium carbonate with a diameter of less than 100 nm. Similarly, by adjusting the reaction parameters of the reactor, a single ultra-fine product such as cubic, chain-shaped, spherical-like can be obtained. For example, a three-stage carbonation method is used to control the concentration of calcium hydroxide suspension in the carbonization process to be 0.1% to 10% (mass), a temperature of 1 to 30°C, a certain droplet diameter, and a certain superficial velocity, and less than 100nm can be obtained. Square calcium carbonate. The method produces nano calcium carbonate with high efficiency, good economic benefits, and can realize automatic large-scale production. The disadvantage is that the equipment investment is relatively large.
2.2.3 Hypergravity carbonization
Hypergravity carbonization is a new method developed by the Hypergravity Engineering Technology Research Center of Beijing University of Technology in recent years. It uses carbon dioxide and calcium hydroxide as raw materials. According to the theory of molecular reactions, it utilizes a gravitational environment in which rotation generates much more than the gravity of the earth. It effectively controls the carbonization reaction and crystallization process at the molecular scale, and produces small particle size and distribution. Uniform nano powder calcium carbonate. The high-speed rotating filler cuts the calcium hydroxide solution into fine droplets, liquid filaments, and liquid film. The powerful centrifugal force field quickly releases the calcium carbonate particles from the calcium hydroxide solution once they are formed, and cannot continue to grow. At the same time, the hydroxide The contact area between the calcium solution and the carbon dioxide gas is greatly increased and quickly updated, resulting in a greatly increased reaction rate. Production of nano-calcium carbonate by over-carbonization has the following advantages: (1) It can strengthen the transfer and reaction of carbon dioxide and calcium hydroxide, and can control the carbonization and crystallization process of carbon dioxide and calcium hydroxide; (2) Uniform particle size and small average particle size , narrow particle size distribution, the average particle size of the product obtained is 15 ~ 30nm; (3) without the need to add crystal inhibitors, so that production costs greatly reduced; (4) carbonization time is greatly shortened than the ordinary carbonization method, only the traditional process 10 %~20%; (5) Small reaction device, easy operation, low investment; (6) High product purity and stable quality.
2.2.4 Multi-stage spray carbonization
In traditional gas-liquid equipment, the mass transfer rate must be increased. When other conditions are constant, only the gas-liquid phase contact area is increased. In the spray carbonization tower, it is through the action of the centrifugal force that the mass transfer area between the gas and the liquid phase is increased, and because the gas-liquid phase is in countercurrent contact, and the nebulizer itself is caused by high-speed rotation. The cutting atomizes the liquid into very fine uniform mist particles, so the microscopic mixing degree between the gas and the gas is extremely high, the mass transfer surface area between the gas and the liquid phase is greatly increased, and the mass transfer coefficient is higher than that between ordinary gas and liquid phases. The increase of 100 times or more ensures that the average volumetric diameter of nano-CaCO3 produced in the spray carbonization tower is about 35 nm. The basic steps for preparing nanometer calcium carbonate by multi-stage spray carbonization method are as follows: the refined lime milk suspension is configured to the concentration required by the process, an appropriate amount of additives is added, and the mixture is thoroughly mixed and pumped into the atomizer at the top of the spray carbonization tower. Under the action of the huge centrifugal force generated by high-speed rotation, the emulsion is atomized into fine-grained droplets; after mixing and drying, the mixed gas containing an appropriate amount of carbon dioxide enters from the bottom of the tower and is evenly distributed in the tower by the gas distributor. Drops in the tower react instantaneously with the gas and react chemically to produce calcium carbonate. The particle size of the calcium carbonate products prepared by the multi-stage spray carbonization method is fine and uniform. The average particle size is in the range of 30 to 40 nm, and the crystal form of the particles can be controlled and controlled.