Chemical Residue
Laboratory analysis of the non-carbohydrate fraction of wood pulp measures the acid-insoluble organic polymers that remain after acid hydrolysis. Klason lignin is the insoluble residue obtained by treating pulp with seventy-two percent sulfuric acid, which dissolves the cellulose and hemicellulose. This measurement determines the residual lignin content in chemical pulps, providing a direct measurement of the pulping and bleaching efficiency.
The test is limited to chemical and semi-chemical pulps, as it does not distinguish between lignin and synthetic organic additives like plastics or hot-melt adhesives.
Digestion Method
The extraction process starts by washing the pulp with organic solvents to remove any resin or fat soluble extractives before the acid treatment. During the digestion with sulfuric acid, the pulp is kept at a controlled temperature to prevent the carbonization of the sugars. The mixture is then diluted with water and boiled to complete the hydrolysis, leaving the Klason lignin as a dark brown precipitate.
This precipitate is filtered, washed, dried, and weighed to calculate the percentage of acid-insoluble lignin in the pulp.
Bleaching Optimization
Reducing the residual lignin content improves the brightness and brightness stability of the finished paperboard. High lignin levels cause the board to yellow when exposed to light, which is unacceptable for premium packaging. The Klason method provides the baseline data needed to optimize the bleaching chemicals in the mill.
