Introduction of calcium carbonate as an extender pigment and related applications

Excessive pigments, often referred to as fillers, are derived from nature, directly manufactured, or obtained as by-products. These substances are mainly salts of barium, calcium, magnesium, or aluminum, oxides of silicon or aluminum, or miscible double salts derived from the first two species, in coatings, inks, linoleum, plastic materials, paper, and paper coatings. , tarpaulins, putty, wiping and scrubbing materials, rubber products, cement, plastics, pottery and Other fields have a wide range of uses.

They are close to white opaque pigments, and the types and amounts of extender pigments and their use in traditional industries will be further developed. These pigments have two general properties: the refractive index is lower than 1.75 (usually 1.45 to 1.70), and the color is white or almost white. Other properties of extender pigments vary widely, such as relative density, capacity value, particle shape, particle size, particle size distribution, oil absorption, chemical activity, and the like.

Calcium carbonate

Calcium carbonate is mainly obtained from limestone ore, and it can also be used as a by-product of some manufacturing processes, or it can be directly produced by reacting carbon dioxide with lime slush.

The natural calcium carbonate can be manufactured by dry grinding or wet grinding. The dry-milled product in the dry-grinding method is separated by an air separator to remove excess particles and regrind. The use of wet milling produces finer, more vibrant, better-sieved grades. Limestone is ground in the presence of water, and the milled product is continuously discharged using a conical or tubular grinder and sorted by a centrifuge. Excessive particles are fed back together with the feed and regrind. Another important advantage of wet milling is that the color and chemical purity of the product can be improved by further flotation. In the continuous grinding process, there are several types of grinders that can be used, but in principle, compressed air, high-temperature compressed air, or superheated steam should be used as the liquid. The raw material enters the grinding machine together with the liquid and collides with each other at a high speed to generate the grinding. The grinding process of the liquid and the pigment is controlled so that the fine particles and the liquid flow together. General grinding machines combine the functions of sorting and grinding.

There are several mature processes for the manufacture of precipitated calcium carbonate fillers. Manufacturers can use different methods to produce different grades of pigments, but the common point is that the precipitation method is to use natural calcium carbonate as raw materials, calcined into calcium oxide (lime ), and then hydrolyzed to give calcium hydroxide. There are many methods in this process that can be used to remove almost all large particle impurities and limestone that does not break down into lime. 1. The bicarbonate process controls the reaction process of calcium hydroxide and carbon dioxide to obtain calcium carbonate with different particle sizes, but for all precipitated calcium carbonate, the powder particle size is generally moderate. 2. By-Product Method This method utilizes the reaction of calcium hydroxide slurry and sodium carbonate solution to produce relatively fine calcium carbonate particles. In this process, special attention needs to be paid to the complete cleaning of all the sodium hydroxide components, otherwise the calcium carbonate pigment is alkaline, which may cause problems in the application of the coating. 3. Calcium Chloride Method The reactants in this method are usually derived from other chemical processes. For example, the ammonium chloride obtained when producing sodium bicarbonate is mixed with calcium hydroxide to produce calcium chloride and ammonium hydroxide, and calcium chloride. Reacting with sodium carbonate results in a series of very fine calcium carbonates, with high color purity and similar particle size distribution. Of course, all the sodium chloride components need to be carefully washed out. The calcium chloride method gives calcium carbonate having a calcite crystal structure with a particle size of usually 8 to 10 μm and a minimum range of 0.03 to 0.05 μm. The product obtained by the lime milk method is an aragonite type, and the particle size is generally 0.2 to 2.0 μm. There are many grades of calcium carbonate fillers, relative density between 2.70 and 2.77, oil absorption of 5~45g/100g, average particle size from less than 1μm to very large, 200# sieve (about 300μm) usually 25~30 %, product with particle size distribution (less than 0.1 μm) in the range of colloids can be obtained by controlling the production process.

Calcium carbonate is insoluble in water and is inert to most bases of oily resin coatings, but it is sensitive to dilute acids. This property is beneficial to the application of calcium carbonate in many indoor coatings, such as primers, semi-gloss paints, etc., and also has applications in high gloss paints, spray paints, and many other industrial coatings.