Method for lignin separation from black liquor comprising removal of sulfur compounds from formed water effluent
The method is for separation of lignin from original black liquor (BL IN ) having a first precipitation phase (PR1/PR2) for precipitation of lignin by a first acidification using acidifier, CO 2 , at alkaline conditions, then separating a lignin cake with subsequent suspension of the lignin cake in a strong acid to leach our metals from the lignin followed by dewatering and obtaining a clean lignin product LP. The acidic liquid phase (FL 1 ) subjected to a sulphur removal process wherein a calcium containing compound is added to the acidic liquid phase, whereby sulphur in the acidic liquid phase is reacting with the calcium compound forming solid gypsum which could be separated and bled off from the process. The remaining acidic liquid could then be returned to the recovery process without negatively affecting the sulphur balance of the mill.
1. A method for separation of lignin from original black liquor (BL IN ) handled in chemical recovery operations with evaporation stages receiving black liquor from a digester, said original black liquor having a first pH value, comprising the following phases in sequence:
a first precipitation stage (PR) wherein an acidifier charge, comprising carbon dioxide, is added to the original black liquor in order to decrease the first pH value of the original black liquor to a second pH level and to initiate precipitation of lignin wherein the second pH level is above pH 7 and below 11.5,
followed by a separation phase (FP 1 ) wherein the precipitated lignin is separated as a lignin cake from a remaining alkaline liquid phase (BL out ) of the acidified original black liquor, and sending the remaining liquid phase (BL out ) of the acidified original black liquor back to the evaporation stages in the chemical recovery operations,
suspending the lignin cake in a strong sulfuric acid solution establishing a pH level below 5 and maintaining the acidified lignin slurry in this slurry state for at least 2 minutes, dewatering the acidified lignin slurry (FP 2 ) obtaining a second lignin cake (LP) and at least one acidic liquid phase (FL 1 );
subjecting the acidic liquid phase (FL 1 ) to a sulfur removal process wherein a calcium containing compound is added to the acidic liquid phase to reach a pH level of 5-8, whereby sulfur in the acidic liquid phase is reacting with the calcium containing compound forming solid gypsum,
separating the solid gypsum (WG) from the acidic liquid phase and bleeding out the solid gypsum from the chemical recovery operations while sending the remaining liquid phase (FL 2 ) with reduced sulfur content to alkaline recovery operations; and
using at least a part of the carbon dioxide released in the sulfur removal process as part of the acidifier charge added to the original black liquor.
2. A method according to claim 1 wherein the calcium containing compound is calcium carbonate and that carbon dioxide is formed during the reaction.
3. A method according to claim 1 wherein the calcium containing compound is calcium acetate and that carbon dioxide is formed during the reaction and wherein at least a part of the carbon dioxide released in the sulfur removal process is used as part of the acidifier charge added to the original black liquor.
4. A method according to claim 2 wherein in the sulfur removal process acid is added in order to maintain the sulfur removal process acidic and decrease the solubility of gypsum.
5. A method according to claim 4 wherein the acid added to the sulfur removal process is an organic acid.
6. A method according to claim 5 wherein the organic acid is acetic acid or formic acid or citric acid.
7. A method according to claim 1 wherein the separated solid gypsum is washed in order to wash out any residual sodium from the solid gypsum.
8. A method for separation of lignin from original black liquor (BL IN ) handled in chemical recovery operations with evaporation stages receiving black liquor from a digester, the original black liquor having a first pH value, comprising the following phases in sequence:
a first precipitation stage (PR) wherein an acidifier charge, comprising carbon dioxide, is added to the original black liquor in order to decrease the first pH value of the original black liquor to a second pH level and to initiate precipitation of lignin wherein the second pH level is above pH 7 and below 11.5,
followed by a separation phase (FP 1 ) wherein the precipitated lignin is separated as a lignin cake from a remaining alkaline liquid phase (BL out ) of the acidified original black liquor, and sending the remaining liquid phase (BL out ) of the acidified original black liquor back to the evaporation stages in the chemical recovery operations,
suspending the lignin cake in a strong sulfuric acid solution establishing a pH level below 5 and maintaining the acidified lignin slurry in this slurry state for at least 2 minutes, dewatering the acidified lignin slurry (FP 2 ) obtaining a second lignin cake (LP) and at least one acidic liquid phase (FL 1 );
subjecting the acidic liquid phase (FL 1 ) to a sulfur removal process wherein a calcium containing compound and an acid are added to the acidic liquid phase to reach a pH level of 5-8, whereby sulfur in the acidic liquid phase is reacting with the calcium containing compound forming solid gypsum, and
separating the solid gypsum (WG) from the acidic liquid phase and bleeding out the solid gypsum from the chemical recovery operations while sending the remaining liquid phase (FL 2 ) with reduced sulfur content to alkaline recovery operations.