Evaluating the lignin content in the fibreline of a birch kraft pulp mill with a TDS sensor

In previous washing studies on brown stock and bleaching it has been shown that a refractometer can be used as an indicator of wash loss, since it measures the total dissolved solids (TDS) reliably.
This current study was performed on both the mill and laboratory scale. On the mill scale, lignin concentrations and also organic and inorganic materials in the hardwood fibre line were examined. On the laboratory scale, TDS concentration values using two different commercially available lignins with different
concentrations in alkaline conditions were measured.
In the mill-scale tests of the fibre line it was noticed that the lignin content changed with the pulp and pulp suspension liquor streams during the various process steps, with a good relationship between lignin and total dissolved solids in the liquor streams. A very good relationship between dissolved lignin
and organics (measured as COD) and a moderate correlation between dissolved lignin and inorganic material in the filtrates (measured as conductivity) in the different process steps were found.
In the laboratory tests, it was observed that differently processed lignins differ from one another and have different solubility behaviours. This phenomenon was also identified using a refractometer.
These observations can help to better control the processes and improve their efficiency in the fibre line of a Kraft pulp mill. More research will be needed to prove all the findings however.