Smelt (recovery boiler)

Smelt in a kraft recovery boiler is the molten inorganic chemical mixture that collects on the furnace floor after black liquor combustion. It consists mainly of sodium carbonate and sodium sulphide, with smaller amounts of other sodium salts and impurities.

The recovery boiler burns the organic part of black liquor for steam and reduces part of the sulphur chemistry to sulphide. The inorganic material melts and flows to smelt spouts. From there it drains into the smelt dissolving tank, where it dissolves in weak wash to form green liquor for recausticising.

Why smelt matters

Smelt is the chemical recovery product of the boiler. Its composition affects green liquor quality, recausticising efficiency, sulphidity, and the overall sodium-sulphur balance of the mill. Stable smelt flow also indicates stable lower furnace operation.

Smelt is also a major hazard. Contact between water and molten smelt can generate a violent smelt-water explosion. Recovery boiler leak response, spout handling, wash procedures, and dissolving tank operation are therefore governed by strict safety practice.

Operating issues

Common smelt-related problems include spout plugging, uneven spout flow, char bed instability, poor reduction efficiency, carryover, dissolving tank surging, and green liquor density swings. Changes in black liquor solids, firing pattern, air distribution, and bed temperature can all affect smelt behaviour.

The associated dry-side deposits in a recovery boiler are chemically linked to smelt and black liquor ash. Sodium salts can deposit on superheaters, generating banks, economisers, and vent systems. Their stickiness depends strongly on temperature and chemistry.

Acoustic cleaning context

Sonic horns do not act on molten smelt itself. Their role is in the gas path and vent systems around the recovery process, where dry sodium-rich ash and fume can build up. Keeping those deposits from consolidating supports boiler availability and reduces manual cleaning exposure, but smelt handling remains a separate high-hazard operating function.

Plant and safety context

In kraft recovery, smelt is the molten inorganic material that collects on the furnace floor and flows through spouts to the dissolving tank. Its composition, temperature, and flow behaviour depend on black-liquor chemistry, reduction conditions, bed control, and recovery boiler load. Operators watch spout flow, bed shape, green-liquor quality, and reduction efficiency because smelt handling is both a chemical recovery function and a major safety risk.

The critical hazard is contact between water and molten smelt, which can produce a violent explosion. This shapes leak response, wash procedures, pressure-part inspection, and emergency shutdown practice. Maintenance teams also manage spout plugging, spout cooling, dissolving tank operation, and deposits in vents or nearby gas paths. Acoustic cleaning has no role in moving molten smelt itself. It can be relevant only for dry ash deposits in associated boiler banks, hoppers, or ductwork before that material joins the smelt cycle.

Measurement and maintenance context

Smelt quality is inferred through green-liquor analysis, reduction efficiency, bed condition, and boiler operating stability. Operators do not treat it as a simple liquid stream because it is formed continuously on a reacting char bed and leaves the furnace through spouts that can plug, surge, or cool unevenly. Changes in black-liquor solids, sulphidity, air staging, and bed temperature affect both smelt flow and downstream liquor quality.

Maintenance around smelt focuses on keeping water, weak-wash systems, cooling circuits, and personnel exposure under strict control. Spout work, dissolving tank work, and boiler leak response need defined procedures. Any cleaning system in nearby dry gas paths should be reviewed so dislodged ash does not obstruct spouts, vents, or safe access.

Related terms

• Recovery boiler
• Black liquor
• Smelt dissolving tank
• Recausticising