After the completion of pulping, water-insoluble cellulose and soluble calcium lignosulfonate are separated by filtration.
The brownish filtrate, containing the lignosulfonates, will also contain residual amounts of sulfite salts and reducing-sugar monomers formed from wood cellulose during pulping. The pH of the filtrate is adjusted by
addition of concentrated sulfuric acid.
The water and sulfite (as sulfur dioxide) content are reduced by subsequent evaporation.
After the first evaporation step, the filtrate may be diluted with water before it is subjected to further purification by ultrafiltration at moderately elevated temperatures. Ultrafiltration is a liquid/liquid separation
method whereby the filtrate is separated by molecular size through a semi-permeable membrane.
The ultrafiltration step separates the high-molecular weight lignosulfonate fraction from depolymerisation products, such as low-molecular weight lignosulfonates, and reducing-sugar monomers.
According to the sponsor, the use of other than softwood as a source material would not yield a product with the desirable high-molecular weight distribution.
The purified calcium lignosulfonate (40-65) from ultrafiltration may be pH-adjusted by addition of dilute sulfuric acid. This solution is then evaporated at a temperature of 95-105° to a dry-matter content that is appropriate for spray-drying.
The final product is spray-dried to a moisture content in accordance with the specification for Loss on drying and filled into containers suitable for holding food.
A process flow diagram is shown below: