Composition
Approximately 7 tonnes of black liquor are produced in the manufacture of one tonne of pulp.
The black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process. The black liquor comprises 15% solids by weight of which 10% are inorganic and 5% are organic. Normally the organics in black liquor are 40-45% soaps, 35-45% lignin and 10-15% other organics.
The organic matter in the black liquor is made up of water/alkali soluble degradation components from the wood. Lignin is degraded to shorter fragments with sulphur content at 1-2% and sodium content at about 6% of the dry solids. Cellulose and hemicellulose is degraded to aliphatic carboxylic acid soaps and hemicellulose fragments. The extractives gives tall oil soap and crude turpentine. The soaps contain about 20% sodium.
The residual lignin components currently serve for hydrolytic or pyrolytic conversion or just burning only. Hemicellulosis may undergo fermentation processes, alternatively.
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