Summary:Calcium carbonate has three common crystal forms: aragonite, vaterite, and calcite. From the perspective of thermodynamic stability, the calcite type is the mos
Calcium carbonate has three common crystal forms: aragonite, vaterite, and calcite. From the perspective of thermodynamic stability, the calcite type is the most thermodynamically stable crystal type, which exists widely in nature; while the vaterite type is the most unstable, in a metastable state, and only exists in some fishes in nature. Otolith organs, spicules of sea squirts, and crustacean tissues.
Vaterite-type calcium carbonate mainly has two formation pathways, that is, the dissolution recrystallization pathway and the solid-solid phase direct conversion pathway. At present, it is believed that the dissolution and recrystallization pathway is the main pathway for the formation of vaterite-type calcium carbonate, that is, amorphous calcium carbonate is formed as the initial phase in the solution, and due to the loose structure inside, it will rapidly undergo phase transformation into vaterite-type calcium carbonate. However, the solubility of vaterite-type calcium carbonate is relatively high, and it will dissolve and then nucleate and grow calcite-type calcium carbonate. This process continues to occur, so that vaterite-type calcium carbonate gradually changes into calcite-type calcium carbonate.
Starting from the formation route and mechanism, at present, high-purity vaterite-type calcium carbonate is mainly prepared by inhibiting the dissolution and recrystallization process.
At present, the common preparation methods can be divided into three types: carbonization method, double decomposition method and thermal decomposition method according to the principles involved in the synthesis process.
1. Carbonization Method
The carbonization method uses an alkaline solution containing soluble calcium salts as a calcium source, and prepares vaterite-type calcium carbonate by introducing CO2 gas into the solution and controlling the process conditions.
The essence of the carbonization method is to use the CO32- obtained from the dissolution reaction of CO2 gas in an alkaline solution to precipitate and react with Ca2+ in the solution to prepare calcium carbonate. Therefore, the generation rate of calcium carbonate is closely related to the dissolution rate of CO2 gas. The product quality of calcium carbonate It is also closely related to the dispersion of CO2 gas in water. And under normal temperature and pressure, CO Dissolution rate and dispersibility of gas in water are low, so the efficiency and productive rate of preparing vaterite type calcium carbonate by carbonization method are low.
However, the carbonization method has the advantages of low cost and simple process equipment, and has been the main industrial production method for preparing various types of calcium carbonate products both at home and abroad. At the same time, researchers at home and abroad have increased the mass transfer rate and dispersion of CO2 gas in the solution by using devices such as gas dispersers, and improved the efficiency and yield of vaterite-type calcium carbonate. Therefore, the carbonization method is used to prepare vaterite-type calcium carbonate. Calcium has great application prospects.
2. Double Decomposition Method
The metathesis method refers to mixing a calcium salt solution and a carbonate solution under certain conditions to undergo a metathesis reaction, and at the same time adding a crystal form regulator and controlling factors such as reaction temperature and concentration to regulate and prepare vaterite calcium carbonate. In general, during preparation, one solution can be quickly mixed into another solution for reaction, or one solution can be introduced into another solution by controlling the addition rate for reaction, and stirring is required at the same time Promote the metathesis reaction.
At present, the calcium salt solution is often introduced into the carbonate solution by dropping to carry out metathesis reaction to prepare vaterite calcium carbonate.
So far, there have been many studies on the preparation of vaterite-type calcium carbonate by metathesis. The obtained product has the advantages of high purity, uniform particle size, and good dispersibility, but has disadvantages such as low production efficiency, high production cost, and difficult precise control of the reaction process. . At the same time, although the use of crystal form regulators is beneficial to the generation of products and the optimization of particle size, it will increase the cost and affect the purity. Therefore, the defects of the metathesis method itself limit the practical application, which leads to most of the current research only staying in the laboratory.
3. Thermal Decomposition Method
The thermal decomposition method is a new method for preparing vaterite-type calcium carbonate, which mainly refers to the preparation of vaterite-type calcium carbonate by thermally decomposing calcium bicarbonate and controlling the conditions. Usually, the saturated aqueous solution of calcium bicarbonate is utilized to achieve the purpose of preparing vaterite-type calcium carbonate by controlling the decomposition temperature, decomposition time, stirring mode and additions.
The preparation principle of the thermal decomposition method is simple, the process is short, and the equipment requirements are low, but the purity of the product vaterite-type calcium carbonate is low, the decomposition takes a long time, and the decomposition reaction is difficult to control; at the same time, the temperature required in the production process is high, and the energy consumption is high. too large to be practically applied. There are few domestic and foreign studies on this method, and a lot of work still needs to be done in theory and practice.
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