The preparation technology of different forms of ultra-fine calcium carbonate has become a hot spot in many advanced countries. There are physical and chemical methods for preparing nanometer calcium carbonate. The physical method is a method of mechanically pulverizing natural limestone or attapulgite having a high calcium carbonate content to obtain a calcium carbonate product. However, it is quite difficult to pulverize to 1 μm or less with a pulverizer, and it is possible to achieve 0.1 μm or less only by a special method and machine. Therefore, the industry is dominated by chemical methods. The chemical method is further divided into a carbonization method and a metathesis method.
The carbonization process is the main production method of nano calcium carbonate. The selected limestone is calcined to obtain calcium oxide and kiln gas. The calcium oxide is digested, and the generated suspended calcium hydroxide is pulverized under the action of high shear force, and the particles and impurities are separated by multi-stage cyclone separation to obtain a certain concentration of the purified calcium hydroxide suspension; then the CO 2 gas is introduced. Adding an appropriate crystal form controlling agent, carbonizing to the end point, obtaining a calcium carbonate slurry of a desired crystal form; further dehydrating, drying, and surface treatment to obtain a nano calcium carbonate product.
According to the different contact modes of CO 2 gas and calcium hydroxide suspension in the carbonization process, the carbonization method can be divided into batch bubbling carbonization method, continuous spray carbonization method and supergravity carbonization method , and improvement based on intermittent bubbling carbonization method. Non-freezing method.
Intermittent bubbling carbonization
The intermittent bubbling carbonization method is to cool the Ca(OH) 2 slurry to below 25 ° C, pump it into the carbonization tower, maintain a certain liquid level, and carry out carbonization reaction at the bottom of the tower to the mixed gas of CO 2 and air in the tower, and batch preparation. Nano calcium carbonate.
The law has low investment and is easy to convert, and is adopted by most domestic and foreign manufacturers. However, this method has low production efficiency, poor gas-liquid contact, long carbonization time, and coarse and uneven particle size.
Process flow chart for preparing nanometer calcium carbonate by intermittent bubbling method
Continuous spray carbonization
In the spray carbonization method, lime milk is sprayed into a mist by a nozzle, and is sprayed from the top of the tower to raise a certain concentration of CO 2 from the bottom of the tower at a certain speed to react with the misty lime milk. For continuous spray carbonization, the above process is repeated, and finally nano-calcium carbonate having a particle diameter of less than 0.1 μm can be obtained.
The method has high efficiency in producing nanometer calcium carbonate, considerable economic benefits, and can realize continuous automatic mass production, and is highly scientific and technical. However, equipment investment is large.
Hypergravity carbonization
The supergravity carbonization method utilizes rotation to create a stable super-gravity environment that is much higher than the earth's gravitational acceleration, greatly increasing the gas-liquid contact area, and enhancing the mass transfer process between gas and liquid, thereby increasing the carbonization rate. At the same time, since the emulsion is highly dispersed in the rotating bed, the growth of the crystal grains is restricted, and nano calcium carbonate having a particle diameter of 15 to 30 nm can be prepared without adding a crystal form controlling agent.
Non-freezing method
The difference between the non-freezing method for preparing nano-calcium carbonate technology and other preparation techniques is that the intermittent bubbling carbonization method is adopted in the carbonization tower by successively adding a plurality of dispersing agents arranged without changing the device equipment. The slurry reacts together, eliminating the refrigeration system, reducing energy consumption and reducing production costs.
The metathesis method refers to a method of reacting a water-soluble calcium salt (such as calcium chloride) with a water-soluble carbonate (such as ammonium carbonate or sodium carbonate) under suitable conditions to obtain calcium carbonate. This method can obtain amorphous spherical calcium carbonate, which has a spherical shape, a small specific particle diameter, a large specific surface area, and a good solubility by controlling the concentration of the reactants and the supersaturation of calcium carbonate and adding appropriate additives. However, a large amount of chloride ions adsorbed in calcium carbonate are hard to be removed, and the decantation method used in production often requires a large amount of time and washing water.
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