Method for separating lignin from black liquor, a lignin product, and use of a lignin product for the production of fuels or materials

A method for separation of lignin from black liquor, comprising:

• precipitating lignin by acidifying black liquor with carbon dioxide resulting in a first lignin suspension;
• separation to form a lignin cake of a solid material of a first filter cake and a first filtrate,
• suspending the first filter cake to obtain a second lignin suspension,
• separating the second lignin suspension to form a second lignin cake of a solid material of the second filter cake and a second filtrate,
• adding a washing liquid for displacement washing of the second filter cake,
• enhancing a dryness of the second filter cake,
• obtaining filtrate from the displacement washing, the filtrate forming a part of the second filtrate,
• increasing an ionic strength by adding recovery boiler precipitator dust to at least a first portion of the second filtrate and adjusting the pH of the second filtrate to a value below 6,
• washing the first filter cake with the first portion of the second filtrate,
• the recovery boiler precipitator dust of the second filtrate keeping lignin in the first filter cake in solid form, and
• sending the first portion of the second filtrate to recovery after use as washing liquid, and
• adding a second portion of the second filtrate to the washed first filter cake to form the second lignin suspension, adjusting the pH level of the second lignin suspension to a value below 6 and approximately the pH level of the washing liquid added to the second filter cake.