Pulp and paper
Smelt (recovery boiler)
Also known as kraft smelt, recovery boiler smelt.
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.
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. This carry-over is what fouls the generating bank, superheater and economiser, and is the target of sonic-horn cleaning.
Safety
Molten smelt contact with water is the leading documented cause of catastrophic recovery-boiler explosions. 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
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.
- Black liquorBlack liquor is the concentrated spent cooking liquor from kraft pulping. It is burned in the recovery boiler to generate steam, power and to recover the pulping chemicals.
- Smelt dissolving tankAn SDT receives molten smelt from the recovery boiler and dissolves it into weak wash to form green liquor. The vent stack accumulates sodium fume; sonic horns prevent stack-line plugging.
- RecausticisingRecausticising converts green liquor (sodium carbonate) and burnt lime back into white liquor (sodium hydroxide and sulphide) for re-use in kraft pulping.