Evaluating a pulp mill fibre line's lignin content using a refractometer to measure the TDS content of different lignin fractions

In chemical pulping, the purpose of brown stock washing derived from the cooking stage is to remove both organic and inorganic soluble matter from the pulp suspension using the lowest possible amount of wash liquor, and the purpose of bleaching is to achieve the target brightness using the lowest possible amount of bleaching chemicals without losing pulp quality. Improving the cleanliness of the pulp and removing lignin play a major role in brown stock washing, oxygen delignification and bleaching. Previously, we have studied the use of refractometer technology in the area of a pulp mill's brown stock washing as an indicator of wash loss. The process refractometer measures the total dissolved solids (TDS) as a sum parameter. Lignin has a fairly high refractive index, for example, compared to most acids and alcohols. In addition, cooking-based lignin and lignin treated with bleaching chemicals have different refractive indexes. The study was done at both the mill and laboratory scale. At the mill scale, we examined lignin concentrations and also how organic and inorganic material behaves in the hardwood fibre line. At the laboratory scale, we measured TDS concentration values from two different commercially available lignins in different concentrations using a portable process refractometer to better understand lignin behaviour in alkaline conditions. In the mill-scale tests, it could be seen how lignin content changed with the pulp and pulp suspension liquor streams during the various process steps. In addition, we showed a good relationship between lignin and total dissolved solids in the liquor streams in the fibre line. We also noticed a very good relationship between dissolved lignin and organics (COD) and a moderate correlation between dissolved lignin and inorganic (conductivity) material in the filtrates in the different process steps. In the laboratory experiments, it was found that differently processed lignins differ from one another and have different solubility behaviour, and this can be identified using refractometer technology. This may allow us to recognise changes in lignin quality in real time before and after various process steps in the fibre line of a Kraft pulp mill. This observation can help us to better control the processes and improve their efficiency. However, more research will be needed to prove all of the findings.