Research Status and Development Direction of Wet Grinding of Heavy Calcium Carbonate

Calcium carbonate is generally classified into heavy calcium carbonate and light calcium carbonate. Heavy calcium carbonate, also known as ground calcium carbonate, referred to as heavy calcium, is obtained by physical processing; light calcium carbonate, also known as precipitated calcium carbonate, referred to as light calcium, is obtained by chemical processing.

Heavy calcium is widely used in many fields because of its many advantages. The real value of heavy calcium application is its deep processing products, namely ultra-fine, high-purity, surface-modified products. Therefore, the preparation of high value-added ultra-fine, high-purity weight Calcium products have good industrial application prospects.

First, the production method of heavy calcium carbonate

The production method of heavy calcium carbonate is generally classified into two types, dry method and wet method. Characteristics of dry production: 1) Low investment and high-quality production of stable products; 2) Flexible control of production conditions for different use requirements, production of products with different requirements at lower cost; 3) Surface modification of the product to improve the performance of the product.

The wet process produces the natural calcium carbonate extracted by primary and secondary pulverization and washing. After being selected, it is ground into a wet grinding equipment, and then mixed with a dispersing agent and then ground twice. After screening and sedimentation, the product is obtained. Features: 1) Large-scale production, low cost; 2) Product quality parameters can be adjusted according to product requirements; 3) Extremely fine particle size can be achieved; 4) Product transportation is difficult, such as drying, investment and production costs are increased Therefore, wet products should be used locally for local production. In addition, the dry method is suitable for the production of fine heavy calcium with an average particle size of more than 3um. It is technically feasible for powders below 3um, but the cost is too high and economically unfeasible; and wet grinding is mainly used to produce average particle size. Fillers and coated grades of calcium less than 2 um, with a maximum particle size of less than 10 um, are mostly used in paper and high-grade coatings. Therefore, different production methods can be used according to different needs.

Second, the main process of wet grinding of heavy calcium carbonate

The main processes for wet grinding of heavy calcium carbonate are:

(1) Raw ore → smashing → Raymond grinding → wet mixing grinding or stripping machine (intermittent, multi-stage or cycle) → wet classifier → sieving → drying → activation → bagging (coating grade heavy calcium). The wet process super-segmentation level is added to the process, which can timely separate qualified products and improve efficiency. The wet super- segmentation equipment mainly has a small-diameter cyclone, a horizontal spiral classifier and a dish classifier. The classified pulp concentration is relatively thin, and sometimes a sedimentation tank is needed. The economic indicators of the process are good, but the classification is difficult to operate. At present, there is no very effective wet super-segmentation equipment.

(2) Raw ore → smashing → Raymond grinding → wet mixing grinding → sieving → drying → activation → bagging (filler grade heavy calcium).

(3) Raw ore → smashing → Raymond grinding → wet mixing mill or stripping machine (intermittent, multi-stage or cycle) → sieving (paper coating grade heavy calcium slurry).

Wet production of heavy calcium can produce both filler and coating grades depending on the process chosen. Filler grade heavy calcium can generally meet the requirements of the product by grinding once. The coated grade heavy calcium needs to be ground again after grinding. In addition, in wet production, the process control is relatively complicated, and the appropriate production process should be selected according to raw materials, media, grinding equipment, product use, and the like.

Research Status of Wet Grinding of Heavy Calcium Carbonate

In order to achieve the technical specifications of the wet-processed paper-making grade calcium products, we must select a number of key factors affecting raw materials, grinding equipment, grinding aids, grinding media.

(1) Selection of raw materials. Calcium carbonate crystals with high purity, crystal form and high utilization value in nature are mainly calcite , marble and glacial stone. At present, the raw materials of domestic heavy calcium powder are formed by the regional metamorphism and thermal contact metamorphism of carbonate rock. The marble type calcite deposit and a few hydrothermal vein calcite deposits can use the mineral as calcite. Calcite is mostly in the form of granular and flake, and has excellent optical properties. It is suitable for high-grade occasions, such as papermaking, especially flake calcite, commonly known as calcite. Practice has proved that due to its flaky, three rhombohedral cleavage structure, it has high whiteness and hiding power, which is very suitable for the filling and coating of advanced white cardboard, and its mineral source is relatively small, so the price is relatively Higher. Relatively speaking, granular calcite, commonly known as small square solution, is second to opacity, and more importantly, the settlement rate of the generous solution is smaller and slower, which is conducive to the dispersion of pulp.

(2) Selection of grinding equipment. As mentioned above, the grinding equipment for wet grinding of heavy calcium mainly includes a stripper, a stirring mill and a sand mill, but no matter which equipment is selected, the purity and whiteness of heavy calcium must be ensured, such as a cylinder. The lining and grinding disc or agitator are made of polyurethane, ceramic or other polymer wear-resistant materials, and the mill can run stably under the conditions of high solid content, high viscosity and small grinding medium. Wet superfine grinding a too large vertical rule Changsha Institute developed a new and efficient ultra-fine grinding apparatus, may be used to wet superfine grinding non-metallic ore, but also the current number of ultra-fine grinding apparatus could be A device with high utilization rate and fine product granularity has a simple structure and convenient operation and maintenance. The mill consists of a transmission part (including a motor and a reducer), a cylinder and a combined agitator. The motor is driven by a speed reducer to fill a certain grinding medium ( corundum ball, zirconium ball or glass ball) and a high-speed rotation in the barrel of the material to be ground, and the centrifugal pressure generated by the heavy pressure of the grinding medium and the rotation of the agitator Under the force, the material is effectively pulverized by friction, impact and shear. After testing and production practice verification, the ultra-fine grinding equipment can be used to produce filler grade and primer grade (-2um>=60%-90%) papermaking heavy calcium products after a period of grinding. Therefore, the ultrafine grinding equipment has very Good industrial application prospects. In addition, wet grinding of heavy calcium application of more grinding equipment and sand mill, the current domestic advanced sand mill is a modified MB300 sand mill, which is characterized by the mill's input power directly acting on the mixer, Thereby, the grinding medium is pushed at a high speed to achieve the purpose of ultra-fine grinding. The high-speed rotation of the agitator causes the grinding medium to play the dual role of impact and grinding, effectively grinding the material and dispersing evenly. The ultra-fine grinding equipment can achieve the filling grade and the primer-grade papermaking heavy calcium product after the first or second grinding, and the three-stage grinding can reach the surface coating grade papermaking heavy calcium product.

(3) Selection of grinding aids. Grinding aids are essential for wet superfine grinding of powders. We generally divide grinding aids into grinding aids and dispersants, and their mechanism of action is not the same.

According to the research results of the existing grinding aids, there are two representative theories: the first one is the theory of “adsorption reduction hardness” led by Rebinger, that is, the grinding aid molecules are on the particles. The adsorption reduces the surface free energy of the particles, or causes dislocation migration of the surface lattice, resulting in point defects or line defects, thereby reducing the strength and hardness of the particles, promoting the production and expansion of the cracks, thereby reducing the energy consumption of the grinding and improving The grinding effect; the second is the theory of "mineral rheology regulation" led by Cranpal, which believes that the grinding aid can prevent the flow of particles, prevent particles, particles and grinding media and liners. Agglomeration and adhesion between.

In the production practice, the role of grinding aids is mainly reflected in the following three aspects: 1) changing the rheological properties of the slurry and the surface electrical properties of the particles, thereby reducing the viscosity of the slurry, improving the dispersibility of the slurry, and reducing production. The pipeline and various inlet and outlet resistances in the system improve the stability of the system; 2) reduce the resistance of the medium movement due to the decrease of the viscosity of the slurry, and at the same time increase the impact and grinding effect of the relative motion of the medium on the slurry particles; 3) The adsorption of molecules on the surface of the new particles greatly reduces the unsaturated surface energy of the surface of the microparticles, thus preventing the agglomeration of the particles. For the grinding of ultrafine calcium by wet method, the grinding aid is generally a dispersing agent, and a polycarboxylic acid or polyacrylic acid having a high purity and stability and a sodium salt thereof should be used as much as possible.

4) Selection of grinding media. The hardness of the wet-milled heavy calcium raw material is between 2.6 and 3.0, so the grinding media must have the proper hardness and toughness. If the hardness is too large, the wear of the mill cylinder and the agitator or the grinding disc is too large; and if the toughness is not good, the grinding medium is easily broken. According to experience, the grinding media has an optimum hardness of 3 times that of the material. For heavy calcium, commonly used grinding media are corundum balls, zirconium balls and glass beads. For the size of the grinding medium, generally, the size is small, the contact points are large, and the grinding efficiency is high, but the too small grinding medium increases the small particle portion of the slurry, while for the large particles, the small size grinding medium cannot Provide sufficient kinetic energy to break, favoring the widening of product particle distribution and easy to block the separation system of grinding media and slurry. Therefore, for products with a certain range of fineness, there is an optimal grinding media diameter, which has been practiced in production. It was verified. In the case where a plurality of mills are connected in series, a correspondingly small grinding medium is selected as the feed particles of each stage become smaller.

5) Optimal configuration of some process parameters. Such as feed particle size, mill spindle speed, grinding concentration, grinding medium and raw material mass ratio, grinding time, dispersant and grinding aid dosage.

Fourth, the development of wet grinding of heavy calcium carbonate

China's heavy calcium application is still a primary product, and the real value of heavy calcium application is its deep processing products, namely ultra-fine, high-purity, surface-modified products, while the wet-produced heavy calcium products have high purity, fine and uniform grain size. The grain shape is good and the quality is stable, which achieves the purpose of deep processing of the product. Therefore, wet grinding of heavy calcium has broad prospects.

At present, the production and application of ultra-fine calcium in the world is mainly concentrated in the United States, Western Europe and Japan. In terms of production capacity and consumption, the annual production capacity and consumption of ultra-fine heavy calcium in the United States are the highest in the world, followed by Western Europe and Japan, and although our country has achieved encouraging results every year, but with the United States Compared with countries such as Western Europe and Japan, there is still a big gap. For example, there are many manufacturers in China, small scale and poor product quality; there are many coarse material diameter products and few ultra-fine products; there are many common products and few special products; the old equipment of most factories is backward, the self-control level is poor, the product quality is unstable; There are many powders, ultra-fine dry or wet products, which can not meet the market demand, resulting in a large amount of foreign exchange and high-priced imported ultra-fine calcium. Therefore, the development direction of China's ultra-fine heavy calcium is:

(1) Scale of production, large-scale equipment, automation and intelligent production process, and improve economic efficiency;

(2) Using advanced equipment to improve the grinding effect and reduce energy consumption;

(3) Adopt advanced process flow to achieve optimal configuration of various process parameters and improve production efficiency;

(4) Research and develop more effective grinding aids to better improve the rheology of the slurry and provide the paper mill with excellent dispersion of heavy calcium slurry;

(5) Establish a satellite supply system to reduce losses caused by transportation and subsequent processing.

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