Filtration properties of lignin precipitated from black liquor

The filtration properties of lignin suspensions precipitated from black liquor is a very important area, which, to the authors knowledge has not been studied thoroughly before, Precipitation experiments were conducted using black liquor from a Swedish mill, with mainly sulphuric acid but also carbon dioxide as acidifying agent. In these experiments the precipitation pH and temperature were varied to study the effect of these parameters on the filtration properties of the resulting lignin suspensions. The results show that the precipitation conditions, especially the temperature but also the precipitation pH, are very important factors influencing the filtration properties. At high pH and low temperatures (below 70 degreesC) the lignin suspensions are difficult to filter. At lower precipitation pH and a precipitation temperature of 80 degreesC the lignin suspensions are readily filterable.