An investigation of process parameters during lignin precipitation from kraft black liquors: A step towards an optimised precipitation operation

The main focus of a number of recent articles has been on the investigation and optimisation of the filtration and subsequent washing processes of a lignin extraction process, known as the LignoBoost process, with the aim of minimising the filtration area. However, no evaluation of the parameters during the precipitation step, other than the pH, temperature and origin of the black liquor has been performed. This investigation deals with other important parameters during precipitation that may influence not only the absorption of gas into the black liquor but also the subsequent filtration step, e.g. mixing conditions and concentration of CO2 in the precipitation gas. The CO2 absorption rate and average specific filtration resistance of the lignin after conditioning have been evaluated. It was found that the decrease in pH above 10.5 is more dependent of the chemical reaction rate than parameters controlling physical absorption. It was also found that the CO2 concentration in the precipitation gas affected the pH change significantly: a gas with a lower CO2 content decreased the pH lowering rate. Moreover it was shown that the specific filtration resistance of precipitated lignin slurries subjected to a conditioning procedure prior to filtration was lower than slurries filtrated directly after precipitation.