Pulp and paper
Black liquor
Also known as kraft black liquor, weak black liquor, heavy black liquor.
Black liquor is the concentrated spent cooking liquor from kraft pulping, containing dissolved lignin, hemicellulose, sodium carbonate, sodium sulphate and other inorganic compounds. After pulping, the weak black liquor (~15% solids) is concentrated in a multi-effect evaporator train to heavy black liquor (~70–75% solids) and burned in the recovery boiler. Combustion serves three purposes simultaneously:
- Generate steam and electrical power for the mill
- Recover the sodium and sulphur chemicals as smelt for re-use in pulping
- Destroy the organic-loaded waste stream
Why it matters for cleaning
Burning concentrated black liquor produces uniquely sticky, alkali-rich carry-over that deposits on the recovery-boiler generating bank, superheater and economiser tubes. Black-liquor combustion is what makes recovery boilers the iconic application for sonic horns — no other industrial-boiler fuel produces fouling so aggressive yet so responsive to acoustic cleaning.
Related terms
Related terms
- Recovery boilerA recovery boiler burns kraft black liquor to generate steam, electrical power and recovered pulping chemicals. Iconic application for sonic horns on superheater cleaning.
- Smelt (recovery boiler)Smelt is the molten sodium carbonate and sulphide mixture that accumulates in the bottom of a kraft recovery boiler. It is dissolved into green liquor and recausticised to pulping reagent.
- Multi-effect evaporatorA multi-effect evaporator train concentrates weak kraft black liquor from 15% solids to 70-75% solids before it can be burned in the recovery boiler.
- RecausticisingRecausticising converts green liquor (sodium carbonate) and burnt lime back into white liquor (sodium hydroxide and sulphide) for re-use in kraft pulping.