Calcium carbonate is also gradually dominant in pigments

　　In the past 10 years, with the development of medium alkaline papermaking and coating processing technology, calcium carbonate has been widely used as a coating pigment and filler in North America and Europe. In China, calcium carbonate has gradually occupied a dominant position in coating pigments.

　　Calcium carbonate has many forms of existence in nature, and the most useful for paper coatings is calcite. However, natural calcite must be mechanically pulverized, ground, and classified to produce fine particles (we call it heavy calcium carbonate or ground calcium carbonate, GCC) for the preparation of paper coatings.

　　The dispersant is adsorbed on the surface of the pigment particles to generate electrostatic repulsion and steric hindrance, so that the particles can stably exist in a suspended state. With the help of dispersant in calcium carbonate grinding, high-solids, low-viscosity calcium carbonate slurry can be obtained, the grinding efficiency can be improved, and the energy required for grinding can be saved. There are three general types of dispersing agents, namely, Inorganic, organic and polymeric, and currently widely used in polymeric dispersants. In contrast, polymeric dispersants have the following advantages:

　　(1) Inorganic dispersants may adversely affect electrical conductivity and dielectric constant, and are limited in some fields, while polymeric dispersants are not subject to this limitation.

　　(2) Unlike organic dispersants, polymer dispersants have a controlled molecular weight during production, while higher molecular weights have better dispersion stability.

　　(3) The polymer dispersant has strong dispersing ability, good stability effect, and low sensitivity to ions, pH value and temperature in the dispersion system.

　　(4) The polymer dispersant can significantly reduce the viscosity of the dispersion system, improve the rheology of the dispersion system, and save the energy required for mechanical operation.

　　Based on the above advantages, polymer dispersants are widely used in the grinding of calcium carbonate or in the preparation of paper coatings.

　　2, the grinding process

　　Adding water to the calcium carbonate raw material to form a certain solid content (generally from 70 to 75%), and then adding the dispersing agent in proportion (from a wide range, the dry ratio is not more than 1%), and then passing through several sand mills in sequence Each machine is equipped with a certain amount of quartz sand or zirconium beads for continuous grinding to finally obtain the finished product. The most important characteristics of ground calcium carbonate are particle size distribution and viscosity, which are discussed below.

　　2.1 Particle size distribution

　　The particle size of the calcium carbonate raw material before grinding is usually about 350 mesh (about 45 μm), and the calcium carbonate after the grinding by the dispersing agent can reach the order of 90 or 60 grades according to the required fineness. The so-called 90 or 60 grade means that particles having a diameter of less than 2 μm in the ground calcium carbonate account for 90% or 60% of the whole. The particle size distribution of the calcium carbonate raw material is shown in Figure 1:

　　Figure 190 and 60 calcium carbonate particle size distribution

　　The determinants of particle size distribution are: grinding time, solid content and equipment.

　　(1) Grinding time

　　Obviously, the length of the grinding time fundamentally determines the fineness of the particle size. The longer the grinding time, the longer the action of calcium carbonate by the sand, and the calcium carbonate will be finer.

　　(2) Solid content

　　The solid content determines the efficiency of the grinding. The solid content is high, the collision chance between the calcium carbonate particles and the sand is more, and the calcium carbonate particles are more easily crushed.

　　(3) Equipment

　　Grinding equipment also plays a pivotal role in grinding efficiency. The continuous grinding efficiency of the combined sand mill is the highest, so it is currently widely used by calcium carbonate grinding plants.

　　2.2 Viscosity

　　The viscosity of the ground calcium carbonate is generally desirably controlled between 500 and 1000 cps. If the viscosity is more than 1000 cps, it is difficult to achieve a high solid content in the formulation of the coating, and the apparatus is prone to heat during the grinding process resulting in greater loss. If the viscosity is less than 500 cps, the ground calcium carbonate is easily precipitated during storage.

　　Factors affecting viscosity are: dispersant dosage, solid content and temperature.

　　(1) Dispersant dosage

　　The amount of dispersant determines the degree of coating of the calcium carbonate particles, and the degree of coating of the calcium carbonate particles determines the stability and viscosity of the entire system. However, if the coating is sufficient, the amount of the dispersant is increased, and the viscosity of the system is not significantly reduced, and may even have the opposite effect.

　　(2) Solid content

　　Viscosity is very sensitive to changes in solids content, as shown in Figure 2:

　　Figure 2 shows the relationship between viscosity and solid content at different initial viscosities

　　Curve 1 shows that at higher viscosity, the solid content decreases by 1%, the viscosity decreases by more than 20% on average, and the decrease is very obvious. Curve 2 indicates that at a lower viscosity, the solid content decreases by 1%, and the viscosity decreases. 20%, the decline is also significant. As the solid content decreases, the calcium carbonate particles appear looser and the viscosity decreases.

　　(3) Temperature

　　Temperature has an effect on viscosity, but it is very small.

　　2.3 Relationship between particle size distribution and viscosity

　　There is a close relationship between the particle size distribution and viscosity of ground calcium carbonate. Under the same dispersant dosage and the same solid content, if the particle size is relatively small, the amount of calcium carbonate particles in a certain volume will be denser and the viscosity will increase. On the other hand, if the particle size is relatively large, the amount of calcium carbonate particles in a certain volume is relatively loose and the viscosity is lowered.

　　3 Conclusion

　　In the process of grinding calcium carbonate, if there is no dispersant, the grinding cannot be carried out at all. So the work that must be done for grinding is to choose a highly efficient dispersant that optimizes several key indicators of ground calcium carbonate and saves costs and energy. At present, the research on the promising dispersant is the research on the hyperdispersant. Through the study of the anchoring section and the solvation section, the most suitable dispersing agent for grinding is obtained.