---
title: "Black liquor"
description: "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:"
canonical_url: "https://sylio.co/glossary/black-liquor"
last_updated: "2026-06-28T02:29:35.808Z"
---

**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](/glossary/multi-effect-evaporator) train to heavy black liquor (~70–75% solids) and burned in the [recovery boiler](/glossary/recovery-boiler). Combustion serves three purposes simultaneously:

1. Generate steam and electrical power for the mill
2. Recover the sodium and sulphur chemicals as [smelt](/glossary/smelt) for re-use in pulping
3. Destroy the organic-loaded waste stream

## Why it matters for cleaning

Burning concentrated black liquor produces uniquely sticky, alkali-rich [carry-over](/glossary/carry-over) that deposits on the recovery-boiler [generating bank](/glossary/generating-bank), [superheater](/glossary/superheater) and [economiser](/glossary/economiser) tubes. Black-liquor combustion is what makes recovery boilers the iconic application for [sonic horns](/glossary/sonic-horn) — no other industrial-boiler fuel produces fouling so aggressive yet so responsive to acoustic cleaning.

## Related terms

- [Recovery boiler](/glossary/recovery-boiler)
- [Smelt](/glossary/smelt)
- [Multi-effect evaporator](/glossary/multi-effect-evaporator)
- [Recausticising](/glossary/recausticising)