A New Process to Produce High-Quality PCC by the Causticizing Process in a Kraft Pulp Mill

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A new process has been developed to produce high-quality precipitated calcium carbonate as a paper filler by using the causticizing process in a kraft pulp mill. The causticizing process converts green liquor into white liquor in the kraft pulp mill, producing calcium carbonate as a by-product. If the calcium carbonate (CCC) produced by causticizing could be extracted from the process and used as a papermaking material and if makeup quick-lime for white-liquor generation were supplied, rotary lime-kiln operation could be reduced or eliminated, and the consumption of fuel oil and the resulting discharge of carbon dioxide gas could be reduced. However, conventional CCC includes nonuniform particles with large diameters, which cause severe abrasion of plastic wire on paper machines and poor opacity of paper when it is used as a paper filler. To overcome these disadvantages, the challenge was to find a way to control the morphology of CCC produced from causticizing in the same manner as that of commercial precipitated calcium carbonate (PCC). In conventional causticizing, green liquor is mixed with quicklime to produce CCC and white liquor in two steps: slaking of quicklime with water, and causticizing of the slaked lime with sodium carbonate. When these two reactions were separated and precisely controlled, it was found that CCC was obtained in various forms such as rice-, spindle-, and needle-like particles, some of which demonstrated high opacifying ability and little plastic-wire abrasion when used as paper fillers. In particular, the needle-like aragonite CCC crystal produced in this way demonstrated higher opacity in papermaking than commercial aragonite PCC. Based on these laboratory findings, the reactor was scaled up successfully to a mill-scale plant producing needle-like CCC at the Iwakuni mill of Nippon Paper Industries. This plant has been producing good-quality CCC for paper filler and good-quality white liquor for the last five years.