Method for the production of green liquor
The method is for production of green liquor in association with a causticizing process during recovery of chemicals in manufacture of sulphate pulp. A smelt ( 14 ) of chemicals principally consisting of Na2S and Na 2 CO 3 from a soda boiler ( 1 ) is provided. Weak liquor ( 15 ) that contains dissolved NaOH and CaO is provided. A dissolving tank ( 2 ) is provided in which the smelt ( 14 ) from the soda boiler ( 1 ) is dissolved in the weak liquor ( 15 ) it order to form green liquor ( 16 ). A solution of NaOH is added to the weak liquor ( 15 ), at a position before the weak liquor ( 15 ) is added to the dissolving tank, in order to increase the concentration of NaOH in the weak liquor ( 15 ) such that a fraction of CaO that is presently dissolved in the weak liquor ( 15 ) precipitates.
1. A method for the production of green liquor in association with a causticizing process during recovery of chemicals during the manufacture of sulphate paper pulp, comprising:
providing a smelt of chemicals consisting essentially of Na 2 S and Na 2 CO 3 from a soda boiler, conveying the smelt from a soda boiler to a dissolving tank, providing a weak liquor in a vessel that contains dissolved NaOH and CaO, the weak liquor having a first concentration of NaOH of not higher than 25 g/l, adding a solution of NaOH to the weak liquor in the vessel in such an amount that the first concentration of NaOH in the weak liquor increases to a second concentration of NaOH that is at least 20% higher than the first concentration and until a main portion of the dissolved CaO starts precipitating into particles in the weak liquor,
conveying the weak liquor and the particles to the dissolving tank, and
dissolving the smelt in the weak liquor in the dissolving tank to form a green liquor.
2. The method according to claim 1 wherein the temperature of the weak liquor lies in an interval of 20-80° C.
3. The method according to claim 1 , wherein the weak liquor is obtained from a process of washing a sludge.
4. The method according to claim 1 . wherein the weak liquor is obtained from a process of washing a green liquor sludge.
5. The method according to claim 1 , wherein NaOH is added to the weak liquor in the vessel, such that a main portion of the CaO in the weak liquor in the vessel precipitates as particles in the vessel and that any remaining dissolved CaO precipitates in the dissolver tank on the surface of the particles.
6. The method according to claim 5 wherein at least a fraction of the particles is removed from the weak liquor before the weak liquor is led onwards to dissolving tank.
7. The method according to claim 1 wherein the NaOH that is added to the weak liquor is added at such a temperature that a temperature of the weak liquor is essentially maintained.
8. The method according to claim 1 wherein the NaOH that is added to the weak liquor is added at such a temperature that a temperature of the weak liquor increases.