For nanometer calcium carbonate, crystal form is a very important technical indicator. Because products of different crystal types are suitable for different application fields, only by determining the appropriate product crystal type according to the specific application can marketable products be produced. Due to different processing methods and crystallization conditions, the shape of the original particles (also called primary particles) of the product is different, and different effects are reflected in the application. There are three crystal forms of calcium carbonate: calcite, aragonite, and vaterite, which belong to the trigonal, orthorhombic and hexagonal crystal systems, respectively, but there are dozens of morphologies, and the following eight crystal morphologies are common.
Irregular calcium carbonate
That is, natural calcite, limestone, marble, chalk, etc. are used as raw materials, which are crushed by mechanical or airflow to a certain fineness standard. Because it is relatively heavier than precipitated calcium carbonate, it is called heavy calcium carbonate, heavy fine calcium carbonate, etc. All are irregular. Under the electron microscope, the difference in particle size can be observed, and there are certain edges and corners outside the particles. The specific surface area of GCC is small, about 1m2/g, and the oil absorption value is about 20-27ml/100g. The average particle size of FGCC can reach below 3μm, the specific surface area is 1.45～2.1m2/g, and the oil absorption value is about 48ml/100g.
In short, GCC has the characteristics of irregular shape, wide particle size distribution, high density, small specific surface area, and low oil absorption value.
Fusiform calcium carbonate
Ordinary light calcium products do not need to add any crystal form directing agent, and the crystal form is pointed at both ends, such as a spindle. The long diameter is 5-12 µm, and the short diameter is 1 to 3 µm. If the added crystallization control agent is H2O2, chelating agent, etc., a small spindle with a short diameter of 0.1 to 1 μm can be obtained, with a particle size of 100 to 1000 nm and an aspect ratio of 3 to 4, which will not produce secondary aggregation during the drying process. , The dispersion is very good.
Cubic calcium carbonate
In the early stage of the carbonization reaction, adding sulfuric acid, or sulfates such as aluminum sulfate and zinc sulfate, or crystal type directing agents such as sodium polyphosphate to the calcium hydroxide slurry can produce cubic ultra-fine calcium carbonate products with a particle size of 5-100nm, with uniform particle size, good dispersion, and low oil absorption.
Needle-shaped calcium carbonate
Also known as whisker-like calcium carbonate: using sodium pyrophosphate solution, or strontium chloride, or sodium sulfate solution as a crystal orientation agent, ultrafine needle-like calcium carbonate crystals can be obtained. In a steam pressure cooker, under constant high temperature and high pressure, high-purity whisker calcium carbonate can be prepared without adding crystal form directing agent.
Needle-shaped nano calcium carbonate has the advantages of high whiteness, low production cost, high strength, and good filling performance. It is expected to replace fiber materials such as glass and asbestos and expensive whisker materials such as potassium titanate and titanium carbide (TiC). It is widely used in industrial fields such as papermaking, plastics, rubber and coatings.
Chain Calcium Carbonate
By adding additives such as sodium hexametaphosphate, ethylenediaminetetraacetic acid and aluminum chloride, or maleic acid in the early stage of the carbonization reaction, or adding aluminum trichloride during the carbonization process, the nano-scale chain shape can be obtained. Superfine calcium carbonate. After the formation of single-particle crystallites at the early stage of the carbonization reaction, AlCl3 is added, and AlCl3 is hydrolyzed to form Al(OH)3. Due to the bonding effect of Al(OH)3, each small particle crystallite is connected into several to dozens Superfine cubic crystals are loosely combined in chain-shaped calcium carbonate in a certain direction. The average particle size is 10-100nm and the aspect ratio is 1:5-50.
Spherical calcium carbonate
In the metathesis reaction of calcium carbonate production with CaCl2 and Na2CO3 as raw materials, EDTA and Na2HPO4 are added to the Na2CO3 solution, and then the CaCl2 solution is added dropwise to the Na2CO3 solution with stirring until the reaction is complete. The average particle size can be obtained by suction filtration and drying. Spherical nano calcium carbonate product with diameter of 50-70nm.
Flake calcium carbonate
Pass CO2 into the calcium hydroxide slurry containing a small amount of tributyl phosphate and borax for carbonization, and then centrifuge to filter and dry to obtain flake calcium carbonate; most crystallographic scholars believe that adding organic phosphine as a crystal form directing agent, using carbonization The method can be used to obtain flaky fine calcium carbonate. That is, the aqueous solutions of sodium carbonate and sodium chloride containing a small amount of xylene, sodium dodecyl sulfonate and n-pentanol are respectively prepared, and the W/O type sodium carbonate emulsion and the W/O type calcium chloride emulsion are prepared by ultrasonic emulsification. After the two are mixed and stirred, the emulsion is layered by heating in a water bath, and the upper solution is filtered and dried to obtain fine flake calcium carbonate with a thickness of 100 nm and a plane size of 10-20 μm.
Amorphous body
This is an amorphous calcium carbonate that does not exist in nature. It is mainly prepared by using soluble carbonate and soluble calcium salts, such as potassium carbonate and calcium chloride, by the method of interface low-temperature metathesis reaction. It is formed by rapid precipitation; others have synthesized amorphous calcium carbonate in the CaO-CH3OH-CO2 system. Compared with other crystalline products, its specific surface area is as high as 600m2/cm3, which is about 20 times that of crystalline products. It has very high adsorption of color and odor, and can release adsorbed gas under certain conditions. Good adsorbent; in addition, the product has excellent solubility in water, which is 30 times that of crystalline products, breaking the concept of calcium carbonate insoluble in water, and can be used as food.
