Application and Characteristics of Calcium Carbonate as Filler in Paper Industry
At present, in the papermaking process, the amount of fillers and pigments is second only to fibers, and is the second most important raw material. Common paper fillers include calcium carbonate, talc, kaolin, wollastonite, bentonite, etc. As the acidic papermaking process gradually shifts to neutral and alkaline papermaking processes, calcium carbonate, as an important papermaking raw material, is used more and more in the papermaking industry.
Paper fillers are fine, white mineral powders that are generally insoluble or slightly soluble in water. The main purposes of adding fillers to paper are: fillers are dispersed in the pulp fibers to fill the gaps, improving the optical properties of paper such as opacity and whiteness; improving the ink absorption of paper, so that the paper has better printability; Meet some special performance requirements of paper, such as improving the electrical conductivity and heat resistance of paper, etc.; reduce the use of fiber raw materials and reduce production costs; the filler has a large specific surface area, which helps to overcome resin barriers.
Filler adding methods are divided into two types: intermittent and continuous. Intermittent filling is used in the section of intermittent beating or batch mixing and pulping. When using a trough beater, add to the beater after beating is complete. Continuous beating equipment is used, and it is usually added in the mixing tank or mixing machine. Continuous filling is to continuously add filler liquid in the slurry supply system. Generally, it is added at the outlet of the high-level pulp stabilization box, which can not only ensure uniform mixing but also prevent the calcium carbonate filler from decomposing due to acidic papermaking.
The advantages of calcium carbonate as a paper-making filler are: abundant sources, cheap price; fine particles, high whiteness, which can significantly improve the opacity of paper; fast ink absorption speed, which can increase the ink absorption of paper; can make paper soft and compact , high gloss; less impact on the physical strength of the paper. The Chinese chemical industry standard stipulates the technical requirements for filler calcium carbonate used in papermaking. Light calcium carbonate used as paper filler is available in powder, paste and slurry form, and heavy calcium carbonate is available in powder form and slurry form. The main quality indicators include purity, D65 brightness, pH value, hydrochloric acid insoluble matter, viscosity and particle size distribution, etc. The amount of filler added is mainly determined by factors such as the type of paper, the type of pulp, the nature of the filler itself, and the papermaking conditions of the paper machine. In order to save plant fibers and reduce production costs, the amount of filler should be appropriately increased on the premise of ensuring the papermaking and use requirements.
Ground calcium carbonate is mainly used as a filler in printing paper, writing paper, office paper and advertising paper other than cigarette paper, filter paper and special low-quantity information paper. Light calcium carbonate can be used in all papers in principle, and is currently mainly used as a high-grade paper filler. For example: cigarette paper, office information paper (such as fax paper, instrument recording paper, inkjet printing paper), low-weight newsprint, low-weight coated paper, etc.
Heavy calcium carbonate and light calcium carbonate have their own advantages as paper-making fillers. Therefore, the choice of calcium carbonate should be determined according to the actual paper production process requirements.