Recausticising

Recausticising is the kraft pulp mill process that converts green liquor from the recovery boiler system back into white liquor for cooking wood chips. It regenerates sodium hydroxide while preserving the sodium and sulphur chemicals that make the kraft cycle economical.

Green liquor contains mainly sodium carbonate and sodium sulphide after recovery-boiler smelt is dissolved in weak wash. In the recausticising plant, clarified green liquor reacts with slaked lime, calcium hydroxide. The main reaction converts sodium carbonate to sodium hydroxide and calcium carbonate. The sodium hydroxide and sodium sulphide solution becomes white liquor; the calcium carbonate solids become lime mud.

Main process steps

The typical sequence is green liquor clarification, slaking, causticising, white liquor clarification, lime mud washing, and lime kiln reburning. The lime kiln converts calcium carbonate back to calcium oxide so it can be reused. Each step depends on good solids separation and stable chemistry.

Key performance measures include causticising efficiency, sulphidity, total titratable alkali, lime mud solids, green liquor dregs removal, and white liquor clarity. Poor control can send solids to the digester, overload the lime kiln, waste sodium, or upset cooking conditions.

Fouling and maintenance concerns

Recausticising equipment handles abrasive lime, scaling liquors, dregs, grits, and mud. Common problems include clarifier carryover, slaker grit, lime mud filter blinding, scaling in pipes, kiln ring formation, and poor green liquor quality after recovery-boiler upsets.

Although acoustic cleaning is not normally the main tool inside wet recausticising vessels, the surrounding kraft recovery area uses sonic horns on recovery boilers, smelt dissolving tank vents, ducts, and dry particulate locations. Recausticising performance is linked to those areas because smelt quality, dissolving tank operation, and liquor clarification all affect the chemical cycle.

Operating and maintenance context

Recausticising links the recovery boiler to the lime kiln and the white-liquor system. Operators track green-liquor clarity, causticising efficiency, lime mud solids, residual carbonate, temperature, filtration rate, and lime availability. Poor control increases dead load in the liquor cycle, raises kiln fuel use, and can send more non-process elements around the mill.

Common maintenance concerns include scaling in pipelines and tanks, clarifier carryover, slaker grit, weak lime mud washing, plugged filters, kiln ring formation, and dust or mud build-up in vents and hoppers. Safety issues include hot caustic liquor, confined spaces, hydrogen sulphide or total reduced sulphur gases in some areas, and chemical burns. Acoustic cleaning has only a narrow role, mainly around dry lime, dust collection, or vent handling equipment. It is not applied to wet liquor reactions or mud filtration itself.

Measurement and reliability context

The main measurements are causticising efficiency, total titratable alkali, active alkali, green-liquor density, lime mud solids, filter differential pressure, and lime kiln feed consistency. These figures show whether the cycle is chemically balanced and whether solids handling is becoming a reliability constraint. A weak clarifier or filter can send solids forward, while poor lime quality can increase dead load and fuel use.

For maintenance teams, the practical split is wet chemical process versus dry solids and vent handling. Pumps, agitators, filters, tanks, and piping need chemical and scaling control. Lime dust collectors, vents, and kiln feed bins may have dry build-up where acoustic or vibration devices can help. Keeping that boundary clear prevents cleaning hardware from being applied to a problem that is actually liquor chemistry, mud washing, or kiln operation.

Plant interfaces

Recausticising performance affects the recovery boiler and lime kiln even though the equipment is outside the boiler house. Poor clarification, weak causticising efficiency or mud-washing problems can change liquor quality, kiln loading and non-process-element circulation. Operators follow green-liquor clarity, effective alkali, causticising efficiency, mud solids, lime availability and kiln fuel demand. Deposits and plugging can appear in slakers, causticisers, clarifiers, mud filters and lime-handling equipment. Acoustic cleaning is not a chemical-control tool, but it may be relevant to dry lime, dust or hopper handling around the kiln and solids systems. The glossary distinction matters because recausticising is a chemical conversion loop, not just a set of tanks.

Related terms

• Black liquor
• Smelt
• Smelt dissolving tank
• Lime kiln