Method and apparatus for obtaining strong white liquor and lime mud with low residual alkali level

A method of extracting strong white liquor and lime mud with low residual alkali content from causticized liquor emanating from causticizing vessels in the recovery process of a pulp mill, comprising the following steps in sequence:

• adding burnt lime to green liquor to start a causticizing reaction to produce a causticized liquor;
• feeding the causticized liquor to a first part of a vat of a pressurized disc filter wherein the first part of the vat has a first set of filter discs and a separate first filtrate reception system, thereafter
• extracting strong white liquor from the causticized liquor in the first filtrate reception system by passing the liquid part of the causticized liquor through filter media of the first set of filter discs, while collecting the residual lime particles of the causticized liquor as a first lime cake on the face of the filter media of the first set of filter discs and using no wash water on the formed first lime cake on the filter media, thereafter
• scraping off at least a part of the first lime cake from the face of the filter media on the first set of filter discs, wherein the dry matter content of the scraped off first lime cake is higher than 50 wt-% and collecting the scraped off first lime cake in a first lime cake collection system while diluting the scraped off first lime cake with a suitable low alkali liquid to a dry matter content of the diluted first lime cake slurry less than 25 wt-%;
• feeding at least a part of the diluted first lime cake slurry to a second part of the vat of the pressurized disc filter wherein the second part has a second set of filter discs and a separate second filtrate reception system, thereafter
• extracting weak white liquor from the diluted first lime cake slurry in the second filtrate reception system by passing the liquid part of the diluted first lime cake slurry through filter media of the second set of filter discs, while collecting the residual lime particles of the diluted first lime cake slurry as a second lime cake on the face of the filter media of the second set of filter discs and using wash water on the formed second lime cake on the filter media in order to displace residual alkali in said second lime cake, thereafter
• scraping off at least a part of the second lime cake from the face of the filter media on the second set of filter discs, wherein the dry matter content of the scraped off second lime cake is higher than 50 wt-% and collecting the scraped off second lime cake in a second lime cake collection system while diluting the scraped off second lime cake with a low alkali liquid to a dry matter content of the diluted second lime cake less than 25 wt-% and passing the diluted second lime cake slurry to at least one lime mud thickening stage before feeding the thickened lime mud to a lime kiln.