---
title: "Smelt"
description: "Smelt is the molten inorganic phase recovered from the bottom of a kraft recovery boiler. It consists primarily of sodium carbonate (Na₂CO₃) and sodium sulphide (Na₂S) at ~800 °C and is the chemical-recovery product of black-liquor combustion. Smelt is discharged from the boiler bottom through spouts into a smelt dissolving tank (SDT) where it is quenched into water to form green liquor."
canonical_url: "https://sylio.co/glossary/smelt"
last_updated: "2026-06-28T02:29:36.169Z"
---

**Smelt** is the molten inorganic phase recovered from the bottom of a [kraft recovery boiler](/glossary/recovery-boiler). It consists primarily of sodium carbonate (Na₂CO₃) and sodium sulphide (Na₂S) at ~800 °C and is the chemical-recovery product of black-liquor combustion. Smelt is discharged from the boiler bottom through spouts into a [smelt dissolving tank (SDT)](/glossary/smelt-dissolving-tank) where it is quenched into water to form green liquor.

## Smelt carry-over

A portion of the inorganic burden — sodium sulphate, sodium chloride, fume — does not settle as smelt but is entrained upward in the flue gas as [carry-over](/glossary/carry-over). This carry-over is what fouls the [generating bank](/glossary/generating-bank), [superheater](/glossary/superheater) and [economiser](/glossary/economiser), and is the target of [sonic-horn](/glossary/sonic-horn) cleaning.

## Safety

Molten smelt contact with water is the leading documented cause of catastrophic recovery-boiler explosions. [BLRBAC](/glossary/blrbac) Recommended Good Practices govern smelt-handling procedures and any change to cleaning systems — including acoustic-horn additions — requires review against the smelt-water-explosion protocols.

## Related terms

- [Recovery boiler](/glossary/recovery-boiler)
- [Black liquor](/glossary/black-liquor)
- [Smelt dissolving tank (SDT)](/glossary/smelt-dissolving-tank)
- [Recausticising](/glossary/recausticising)