VERFAHREN ZUR RÜCKGEWINNUNG VON ALKALI AUS DER SCHWARZLAUGE DER PAPIERHERSTELLUNG

摘要

An improved process for recovering alkali from the black liquor of papermaking includes evaporating the black liquor before burning and dissolving the residues to give a green liquor for causticization with lime to form alkali solution as white liquor, wherein the causticization process is modified by incorporating a lime cleaning step and green liquor treatment as well as a 2-step causticization. An improved process for recovering alkali from the black liquor of papermaking includes evaporating the black liquor before burning and dissolving the residues to give a green liquor for causticization with lime to form an alkali solution as white liquor, wherein the causticization process is modified by incorporating a lime cleaning step and green liquor treatment prior to a 2-step causticization. In the lime cleaning step, lime is digested with water or light washing solution to give lime cream and optionally sieved (180-800 mesh) for causticization while using the residues for mixing with coal powder as mineralizing agent for coal combustion in the burner. The green liquor in its container is treated with a green liquor treatment agent [main active ingredient being calcium(II) ion, Ca 2>+>] at 0-80[deg]C, followed by clarifying, filtering the upper layer to remove some solid, and causticization of the filtrate. During such alkali recovery, the precipitated calcium carbonate is directly used for producing coated paper and in neutral sizing whereas calcium carbonate particles of not > 1 Microm in size are applicable as calcium carbonate for various industrial applications, in which the first causticization is performed at 0-70[deg]C by mixing the fine lime cream (1-1.6 times based on calcium oxide for chemical reaction) with the cleaned green liquor before separating the solid from the liquor known as white liquor for recycling in the boiling stage of the papermaking process, reacting the separated solid with such purified green liquor at 0-70[deg]C for second causticization that is based on the weight ratio of sodium carbonate in the green liquor to the excess calcium oxide in the first causticization being 106n: 56, where n is 1-50, to complete conversion of all calcium oxide into calcium carbonate, and returning the solution for first causticization to form a solid which is washed to provide pure calcium carbonate precipitate that contains water. An independent claim is also included for a green liquor treatment agent for the recovery including substances that can provide calcium (II) ion, e.g. soluble calcium salt, calcium oxide, calcium hydroxide, or their mixture, as a suspension or solid, and when applying calcium hydroxide (calculated by basing on calcium oxide) paste as the green liquor treatment agent alone, weight ratio of the calcium oxide in the treatment agent to sodium carbonate in the green liquor is 0.01-1:106.