u2014 al u2014 ca u2014 mg u2014 fe u2014 or organic sulfate caso 4 k 2 so 4 al 2 so 4 iron sulfates or mgso 4 said compound may also be comprised in recovery boiler ashes which is a mixture or it may be essentially pure na 2 so 4 it is intended throughout the present description that the expression u201c acidifying u201d embraces any means for acidifying the lignin containing liquid slurry such as black liquor preferably the acidifying is performed by adding so 2 g organic acids hcl hno 3 carbon dioxide or sulphuric acid in the form of fresh sulfuric acid or a so called u201c spent acid u201d from a chlorine dioxide generator or mixtures thereof to said liquid slurry preferably black liquor most preferred by adding carbon dioxide or sulphuric acid it is intended throughout the present description that the expression u201c dewatering u201d embraces any means for dewatering preferably the dewatering is performed by using centrifugation a filter press apparatus a band filter a rotary filter such as a drum filter or a sedimentation tank or similar equipment most preferred a filter press apparatus is used according to a preferred embodiment of the first aspect of the invention the dewatering of step c is performed in a filter press apparatus according to a preferred embodiment of the first aspect of the invention the addition of step a is done by adding recovery boiler ashes i e ashes emanating from a soda recovery unit which is a steam generator combined with a smelting furnace for the utilization of the heat of combustion of the black liquor and the recovery of the greater part of.