Process of recovering alkali from black liquor of papermaking

An improved process for recovering chemicals from a black liquor in alkaline pulping, comprising obtaining a green liquor by dissolving the smelt formed by evaporating and burning the black liquor, and obtaining an alkali liquor that is commonly named white liquor by causticizing the green liquor with lime, characterized in that some critical technical transformations are introduced in the causticization procedure of the conventional chemical recovery process, and the said improved causticization procedure comprises the following steps:

• (1) Addition of a lime depuration step before the causticization reaction, comprising the following procedures,
• pre-slaking lime with water or weak wash water to form a lime slurry, separating the rough lime grits from the lime slurry, then sifting the lime slurry with a 180-800 mesh sieve screen (excluding 180 mesh sieve screen) to obtain a fine lime slurry that is ready to use for causticization, wherein the siftings and rough lime grits separated from the lime slurry are mixed and incorporated into coal powder as a mineralizer when the coal is burned in a coal burning boiler;
• (2) Addition of a green liquor depuration step before the causticization reaction, comprising the following procedures,
• adding the green liquor obtained by dissolving the smelt formed by evaporating and burning the black liquor into a depurator, adding a green liquor depurating agent (containing Ca2+ ions as major active component) at a temperature between 0° C. and 80° C. (excluding 80° C.), clarifying the green liquor having depurating agent, fine filtering the supernatant of the clarified green liquor to completely remove any remaining solid so as to obtain a fine green liquor, storing the fine green liquor in a tank and being ready to use for causticization;
• (3) Direct production of a series of precipitated calcium carbonate products via a two-step-causticization procedure while the alkali is recovered, wherein said CaCO3 products with various particle size are useful for a plurality of industrial applications, including precipitated calcium carbonate suitable for making coated paper and neutral sized paper, and a superfine precipitated calcium carbonate having an average particle size of less than 1 μm, and so on, said two-step-causticization procedure comprising the following procedures,
• conducting the first causticization step by adding the fine green liquor and the fine lime slurry obtained by depuration into a causticiser at a reaction temperature between 0° C. and 70° C. (excluding 70° C.), wherein the quantity of CaO contained in the fine lime slurry is 1-1.6 times of the theoretical quantity needed for the causticization reaction, separating the solid and liquid after the first causticization step, sending the separated alkali liquor that is commonly called as white liquor to digester for pulp making;
• conducting the second causticization step by adding an excessive amount of the fine green liquor into said separated solids at a reaction temperature between 0° C. and 70° C. (excluding 70° C.), wherein the ratio of the weight of the Na2CO3 contained in the fine green liquor to the weight of the un-reacted excess CaO contained in the separated solids from the first causticization step is 106n :
• 56, wherein n=1-50;
• separating the liquid and solid after all CaO is converted into CaCO3 in the second causticization step;
• returning the separated liquid to the first causticization step;
• and washing the separated solid to obtain a pure hydrated precipitated calcium carbonate.