Method for precipitating lignin from black liquor by utilizing waste gases
The method is for separation of lignin from original black liquor. Lignin is precipitated in a first acidification stage followed by dewatering a first lignin suspension while forming a first filter cake. The first lignin filter cake is suspended in a second acidification stage whereupon a second lignin suspension is obtained. The second lignin suspension is dewatered to form a second filter cake. The second filter cake is washed and the washed second lignin cake is dewatered to obtain a lignin product. Waste gases from the second acidification stage is recirculated back to the first acidification stage and mixed with the original black liquor.
1. A method for separation of lignin from original black liquor, comprising:
precipitating lignin in a first acidification stage of an original black liquor by adding a first acid or mixture of acids and obtaining a first lignin suspension,
dewatering the first lignin suspension while forming a first filter cake with a high content of lignin,
suspending the first filter cake in a second acidification stage using a second acid or mixture of acids,
obtaining a second lignin suspension,
dewatering the second lignin suspension while forming a second filter cake with a high content of lignin,
washing the second filter cake,
dewatering of the washed second filter cake and obtaining a lignin product,
re-circulating waste gases emitted from the second acidification stage, and
mixing the waste gases with the original black liquor.
2. The method according to claim 1 wherein the waste gases are treated in a conversion process wherein at least a part of waste gas components is converted to a stronger acidity having a lower pK a value compared to an acidity and pK a value of the waste gases before being re-circulated and mixed with the original black liquor.
3. The method according to claim 2 wherein the conversion process takes place in a combustion chamber wherein a combustion is assisted with an air or oxygen supply.
4. The method according to claim 2 wherein the conversion process takes place in an oxidization reactor with a catalyst with or without a flame.
5. The method according to claim 1 wherein the waste gases are collected by a pump that establishes a sub-atmospheric pressure during the second acidification stage.
6. The method according to claim 1 wherein the first acid is a weak acid having a positive pK a .
7. The method according to claim 6 wherein the second acid is a strong acid having a negative pK a .
8. The method according to claim 7 wherein a separation process from the first acidification stage until obtaining the lignin product is kept at acidic conditions below pH 6.
9. The method according to claim 8 wherein the separation process from the first acidification stage is kept at acidic conditions below pH 4.
10. The method according to claim 1 wherein at least the dewatering of the first lignin suspension comprises blowing through the first filter cake of the second filter cake by gas or a mixture of gases.
11. The method according to claim 1 wherein a pH level adjustment is combined with an adjustment of an ion strength.
12. The method according to claim 1 wherein at least a part of a third filtrate obtained from washing the second filter cake is returned to the second acidification stage to further reduce a consumption of acid and water.
13. The method according to claim 1 wherein the method further comprises re-circulating the waste gases to the first acidification stage.
14. The method according to claim 1 wherein the method further comprises re-circulating a first portion of the waste gases to the first acidification stage and a second portion of the waste gases are mixed with the original black liquor.