Journal of Korea TAPPI ISSN:0253-3200(Print) 펄프종이기술

Although the domestic high-grade limestones are depleting, the demand for high brightness calcium carbonate has continually increased in the domestic market. In this study, recovered pigments were evaluated for internal fillers and coating pigment in the papermaking process in order to use lime mud, a by-products in the kraft mill, as a substitute for commercial calcium carbonate. In order to prepare the pigments, the lime muds taken from causticizing plant was reacted with sulfuric acid to form gypsum (CaSO₄·2H₂O). The prepared gypsum showed the higher whiteness than that of the commercial GCC and had an average particle size of 30 μm in spindle shape. As a result of applying the gypsum as an internal filler, it showed very poor wet-end performance compared to the commercial GCC. The resulting retention efficiency at the wet-end was not significantly improved even when applied to the wet-end system by various retention aids and filler pretreatment. In addition, gypsum was applied to the coating color to replace the 100% of GCC, however the physical properties of the coating color and printing qualities were not sufficient for the printing and writing paper. The pulverization with the mixture of the powdered GCC and gypsum at a ratio of 50:50 could get the smaller particle size and more narrow the particle size distribution than the earlier ones. Therefore, these properties could result from improvement of the concentration of the pigment itself in the wet status. Such properties could also make the coating color to higher solid content. Consequently, the water retention of the coating color was improved similar to GCC. Therefore, the physical properties and printability of the coated paper were improved to a similar level as with GCC.