Process for causticizing green liquor in chemical recovery from black liquor in alkaline pulping

A process for causticizing green liquor in chemical recovery from black liquor in alkaline pulping, comprising the procedures of:

• (1) slaking lime with water or weak wash water to form a lime slurry containing lime grits;
• separating rough lime grits from the lime slurry to obtain the rough lime grits and a lime slurry from which the rough lime grits has been removed;
• sifting the thus obtained lime slurry with a 180-800 mesh sieve screen (excluding 180 mesh sieve screen) to obtain siftings and a fine lime slurry for causticization;
• and the siftings and the rough lime grits are mixed to obtain lime grits that can be used as a mineralizer when coal is burned in a coal burning boiler;
• (2) adding green liquor obtained by dissolving a smelt formed by evaporating and burning black liquor in alkaline pulping into a depurator;
• adding to the depurator a green liquor depurating agent whose active component is Ca2+at a temperature between 0° C. and 80° C. (excluding 80° C.) accompanied with sufficient agitation;
• transferring the green liquor to which the depurating agent is added into a green liquor clarifier to carry out clarification, obtaining a supernatant and dregs at the bottom of the clarifier;
• filtering the dregs at the bottom of the clarifier to obtain solids and a filtrate;
• and subjecting the filtrate together with the supematant obtained in the clarifier to a fine filtration to obtain a fine green liquor;
• and
• (3) conducting a first causticization step by adding the fine green liquor obtained in procedure (2) and the fine lime slurry obtained in procedure (1) 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 causticization reaction;
• subjecting the mixture obtained after the first causticization step to a solid-liquid separation to obtain an alkali liquor that is commonly called as white liquor and solids;
• conducting a second causticization step by adding the fine green liquor obtained in procedure (2) into the solids obtained after the first causticization step at a reaction temperature between 0° C. and 70° C. (excluding 70° C.), wherein the ratio of the weight of Na2CO3 contained in the fine green liquor to the weight of CaO contained in the solids separated after the first causticization step is 106n:
• 56, wherein n =1-50;
• after all CaO is converted into CaCO3, subjecting the mixture obtained after the second causticization step to a solid-liquid separation to obtain a solution and solids;
• returning the obtained solution to the first causticization step;
• and washing the obtained solids to obtain a pure hydrated precipitated calcium carbonate product.