Patent ID: 7713422

Claim:
A black liquor treatment method, comprising: diluting alkaline black liquor discharged from paper pulping operation with water, adding acid to the liquor so as to adjust pH in a range from 2.5 to 3.5, adding an aggregating agent to solidify lignin contained in the black liquor to be settled, or settled and floated, so as to separate a solid substance of lignin and clean water, filtering the clean water, and contacting ozone gas with the clean water in a reaction tank so as to oxidatively decompose at least one component selected from the group consisting of unsettled lignin contained in the clean water, organic acids and resins contained in wood substances, soluble carbohydrate substances contained in hemi-cellulose, and sulfur compounds, wherein the ozone gas is blown into a part of the clean water and stirred by a stirrer so as to form a micro bubble mixed water of the ozone gas, and the micro bubble mixed water is discharged into the clean water, the stirrer is made of stainless material, and comprises an impeller type stirrer rotating at a speed of 3000 to 20000 rotations per minute into which the part of the clean water is introduced and the ozone gas is blown, thereby mixing the clean water and the ozone gas with the stirrer so that the ozone gas becomes the micro bubble, and a mixture of the micro bubble and the clean water is discharged into the clean water in said reaction tank from below so that the micro bubble remains in the clean water for an effective time to decompose said component.