A novel method for washing lignin precipitated from kraft black liquor - Laboratory trials

A previous study on the washing of lignin precipitated from kraft black liquor encountered problems with complete or partial plugging of the filter cake and/or filter media that resulted in uneven washing and high sodium contents in the final lignin product. This effect was ascribed to large gradients in pH and ionic strength during the washing process. This paper presents an improved method that aims at solving these problems by reducing pH and ionic strength gradients through the introduction of a re-suspension stage before the final washing. Laboratory results indicate successful washing with sodium contents of the lignin well below the requirements for the intended use as a biofuel. No plugging problems were encountered when the pH was below 3.75. An additional important positive effect was a net yield gain in precipitated lignin over the process.