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Nano calcium and light calcium in the production process, performance and function

With limestone as raw material by the calcine, digestion, carbonic acid, the dry separation and classification of the product is called light calcium carbonate, is one of the most widely use inorganic filler, widely used in rubber, plastic, papermaking, coating, paint, printing ink, printing, cable, food, medicine, cosmetics, daily necessities, feed, oil and other industries.
Ultrafine calcium carbonate, especially nanometer calcium carbonate particles, not only retains the properties of original calcium carbonate, but also has the properties of nanometer particles. By controlling the preparation conditions, calcium carbonate nanoparticles with different particle sizes and crystal shapes can be obtained. Therefore, there are many differences between nano calcium and light calcium in production process, quality control, appearance, particle size and application. Understanding the differences between them helps us to identify and develop non-functional calcium carbonate products according to different needs.

1. Differences in production process and quality control
Although there are many production methods of light calcium carbonate, carbonization is almost the only method to achieve industrial production in China. Calcination of limestone and other raw materials to produce quick lime is mainly composed of calcium oxide and carbon dioxide, and then water digestion of lime to generate lime milk is mainly composed of calcium hydroxide, and then carbon dioxide carbonization of lime milk to generate calcium carbonate precipitation, and finally calcium carbonate precipitation through dehydration, drying and grinding will produce light calcium carbonate. This method is characterized by low energy consumption, simple process and abundant raw materials, which can meet the requirements of different industries.
China began to develop and produce nano calcium carbonate in the early 1980s, and realized industrial production at the end of 1980s. The research and development units in China mainly include Beijing University of Chemical Technology, East China University of Science and Technology, School of Chemical Engineering, Harbin Engineering University, Hefei Institute of Solid State Physics, Chinese Academy of Sciences, Tianjin Research Institute of Chemical Industry, etc.

The production process of nano calcium carbonate is mainly divided into four steps: calcination purification, digestion, carbonization, dry packaging. Production to a selection of limestone ore calcine, calcium oxide and kiln gas and calcium oxide digestion, and will generate suspended calcium hydroxide crushed under high shear force, the multi-stage spiral liquid separation to remove particles and impurities, have a certain concentration of refined bubbled into calcium hydroxide slurry, add the appropriate surface modification agent, the carbide to the finish line, The calcium carbonate slurry is then dehydrated, dried and surface treated to obtain the desired nano calcium carbonate product.

1) digestion
The preparation process of light calcium has low requirements on digestion temperature and quality of digestion water. The recycled filtrate of centrifugal dehydration can be used as hot water for digestion, which can make full use of heat energy to improve digestion reaction temperature and reduce the consumption of water in production. It is generally required that the lime is medium-burned or over-burned, and the viscosity of the raw slurry after digestion is low, which is conducive to improving the carbonization efficiency.

The preparation of nano calcium carbonate should use light burned lime with high activity, high digestion temperature and stable viscosity of raw slurry. Available raw slurry in the process of refining slag washing liquid as absorb water, but cannot use the ripe slurry filtrate digestion, because of the filtrate contained large amounts of calcium carbonate crystal, added to the raw slurry will be involved in the seed carbonation reaction, affect the uniformity of the carbide crystal, easy to produce larger particles in the process of crystal crystal, affect the product particle size distribution.

2) carbide
The preparation of light calcium does not control the initial temperature of carbonization reaction, and almost does not control the concentration and viscosity of raw pulp and concentration. Sometimes in order to improve the carbonization speed, under the condition that the sedimentation volume reaches the requirement, adding a part of cooked pulp in the initial carbonization raw pulp can obviously improve the carbonization reaction speed, and can be used as a means to adjust the sedimentation volume of products.

Nano calcium carbonate is prepared by carbonization reactor different from ordinary light calcium, equipped with cold water jacket to control the reaction temperature, agitator to improve the mixture of gas and liquid and refrigeration equipment. Control the initial temperature of raw pulp carbonization, lime emulsion concentration, concentration, temperature, flow rate and stirring speed and other process parameters, but also can use multi-stage carbonization to control the crystal size and crystal shape. Since many seeds cannot be brought in at the initial stage of the carbonization reaction, the carbonization reactor should be easy to clean so that the necessary cleaning of the system can be carried out after each batch of carbonization. A certain amount of crystal shape control agent can also be added in the process of raw pulp or carbonization reaction to adjust the reaction speed, improve the crystal dispersion performance and control the role of crystallization, the carbonization process control is much more complex than ordinary light calcium or fine calcium.

3) dry
Light calcium is generally controlled as long as the whiteness, moisture, screen residue and value of the product to meet the requirements, so in the product after drying, after a simple screening packaging products. The drying temperature is not high, as long as it does not exceed the decomposition temperature of calcium carbonate, so ordinary light calcium is generally dried at 600-750℃, with high efficiency.

Nano calcium carbonate is mostly after surface treatment, so when the product is dry, the temperature should not be controlled too high, otherwise it will cause the surface active agent coking, the product seriously yellow, affecting the quality of the product. At present, the drying temperature of nano calcium carbonate is generally 200-300℃, and the process control temperature is slightly different depending on the residence time of materials in different drying systems. After drying, crushing, depolymerization and grading are the key methods for quality control of nano calcium carbonate products. For products with different particle distribution and different surfactant treatment, there are different emphases in selecting drying and grinding grading equipment.

2. Different appearance quality

1) the appearance
When rubbing nano calcium carbonate powder with fingers, the particles feel fine and smooth, and the adhesion is strong. It is difficult to wash the powder on fingers. The dust formed by the powder in the air is difficult to settle, and the paste formed by the miscible nano calcium carbonate with small particle size and water is not high in whiteness and has a slightly transparent feeling.

2) white degree
Because ordinary nano calcium carbonate products pay great attention to the control of impurities such as "black and yellow spots" in the product in the production process, the whiteness of the product is relatively high, generally 94%96%, slightly higher than that of ordinary light calcium, but some ordinary light calcium inactive products have high whiteness, which can reach 96% to 97%.

3) Packing density
In general, the bulk density of nano calcium carbonate products is small, mostly 0.4-0.6g/cm3, while the bulk density of light calcium carbonate products is 0.6-0.7g/cm3. However, with the continuous improvement of the application performance of nano calcium carbonate products, the bulk density of some products is gradually increasing, and the bulk density of products produced by different drying equipment varies greatly. At present, it is difficult to distinguish nano calcium carbonate products from ordinary light calcium products in terms of bulk density.

3. Different particle size and crystal shape

1) Different particle sizes
The essential difference between nano calcium carbonate and ordinary light calcium is that the original particle size is also called the primary particle size is different, through transmission electron microscopy analysis and detection can accurately identify the two products.

2) Different crystal shapes
Because of nanometer calcium carbonate with a greater difference between ordinary light calcium carbide process, the nano calcium carbonate crystal microtek shape change crystallization process, the shape of the crystal is given priority to with the cube, the shape of the crystal with the technological conditions and the influence of the shape control agent and diversified, and ordinary light calcium crystal shape is relatively single, give priority to with spindle, chrysanthemum shape crystals formed or reunion together.

4.different application process performance and role
Due to its small particle size, nano calcium carbonate has some special properties in the application materials, such as rubber and plastic has strong reinforcement and modification, can greatly improve part of the mechanical properties and optical properties of the material. Therefore, nano calcium carbonate can be used as both functional filler and volume filler. Ordinary light calcium is obviously inferior to nano calcium carbonate in terms of strengthening performance, so it generally only has volume filling effect.

Nano-sized calcium carbonate has achieved industrialization in China, different carbonization method arises at the historic moment, the increasing scale and increasing production, application domain expands unceasingly, from rubber, printing ink and other industries to plastics, paper, paint, adhesives and other industries rapidly expand, demand is increasing at an annual rate of 20%, high-end products on the market, To meet the growing demand of the two major markets at home and abroad.

Today, functional calcium carbonate has become a big demand point in the calcium carbonate application market. In the face of market demand, different users have different requirements for products. In addition to the particle size of calcium carbonate products, they also look at product performance and quality. Only in this way can various functional calcium products have stronger market competitiveness. Therefore, more efforts can be made on the application performance of nano-sized calcium carbonate, and more functional and special nano-sized calcium carbonate can be developed. The same is true of other inorganic powder materials.
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