Chemical Recovery Plants: How Black Liquor Recovery Reduces Costs & Pollution

Why Chemical Recovery Is Essential for Kraft and Soda Pulp Mills

In the kraft or soda pulping process, sodium hydroxide (NaOH) and sodium sulfide (Na₂S) cooking chemicals dissolve the lignin binding wood or agro fibres together, releasing the cellulose fibre for papermaking. These chemicals end up in the spent cooking liquor — black liquor — along with the dissolved lignin and hemicellulose. Without recovery, this chemically rich, high-COD liquor must either be purchased fresh (at enormous cost) or discharged (creating a serious pollution problem).

A chemical recovery plant solves both problems simultaneously. It concentrates the black liquor by evaporation, combusts it in a recovery boiler (generating steam from the lignin's heat value), and recovers the inorganic cooking chemicals as a smelt that is reconstituted and processed back to active white liquor for cooking. The recovered chemicals replace 85–92% of fresh chemical purchases, and the combustion of black liquor generates 60–80% of the mill's steam requirement — turning the 'waste' liquor into a valuable energy source.

For any kraft or soda pulp mill above 30 ODT/day, chemical recovery is not optional — it is economically essential and increasingly required by pollution control regulations. The capital cost of a chemical recovery system (INR 15–35 crore for 30–80 ODT/day scale) is recovered in 2–4 years through chemical cost savings alone.

• Chemical recovery: 85–92% of Na₂SO₄ and Na₂S recovered as smelt
• Steam generation: 2.5–3.5 kg steam per kg black liquor solids combusted
• Chemical cost saving: INR 12–25 crore/year at 50 ODT/day scale
• ETP benefit: 70–80% reduction in COD load by removing black liquor from effluent
• Regulatory: required above 30 ODT/day under CPCB norms in India

Multiple Effect Evaporation: Concentrating Black Liquor for Combustion

Black liquor from the pulp washers is dilute — 14–18% dissolved solids. It cannot be combusted at this concentration — the water content is too high to sustain combustion. Concentration to 60–72% dissolved solids in a multiple effect evaporator (MEE) is the essential precursor to recovery boiler combustion.

A multiple effect evaporator uses a cascade of evaporation stages, each operating at progressively lower pressure (and therefore lower boiling point), with steam from the first effect driving evaporation in the second, and so on. A 6-effect system achieves steam economy of 5–6 kg water evaporated per kg steam consumed — essential for energy efficiency in the overall chemical recovery cycle.

For agro-based mills (bagasse, wheat straw) with silica in the black liquor, forced-circulation evaporators are required in the early effects where silica scaling risk is highest. Rajshree designs MEE systems for both wood and agro-based black liquors, with silica management as a primary design parameter for agro-based mills. The evaporator design is the most technically demanding component of the recovery plant — and the one most frequently underspecified by clients buying on price rather than process performance.

• Inlet concentration: 14–18% dissolved solids from pulp washers
• Target outlet concentration: 60–72% for recovery boiler combustion
• 6-effect MEE steam economy: 5–6 kg water/kg steam consumed
• Falling-film evaporator: for wood and low-silica agro black liquors
• Forced-circulation evaporator: required for high-silica (rice straw) black liquors

Recovery Boiler: Combustion and Chemical Recovery

The recovery boiler combusts concentrated black liquor at 850–1,050°C, burning the organic content (lignin and hemicellulose) to generate steam and reducing the inorganic sulfur compounds (Na₂SO₄) to sodium sulfide (Na₂S) in the smelt. The smelt — a molten mixture of Na₂CO₃ and Na₂S — flows from the bottom of the boiler into a dissolving tank where it is quenched and dissolved to form green liquor.

Recovery boilers for pulp mills generate 2.5–4.0 kg of steam at 4–10 MPa per kg of black liquor solids combusted, with steam generation efficiency depending on black liquor heat value and boiler design. For a 50 ODT/day kraft pulp mill generating 80–100 T/day of black liquor solids, a recovery boiler generating 200–300 T/day of steam supplies 60–80% of the mill's total steam requirement — essentially providing free fuel from the pulping process.

Safety is the most critical design parameter for recovery boilers. Black liquor-water explosions (smelt-water contact explosions) are the most severe accident risk in pulp mills. All Rajshree recovery boiler designs include emergency dilution systems, automatic liquor cutoffs, and smelt-water separation protocols consistent with BLRBAC (Black Liquor Recovery Boiler Advisory Committee) safe operating standards.

• Steam generation: 2.5–4.0 kg steam/kg BL solids; 4–10 MPa high-pressure steam
• Combustion temperature: 850–1,050°C in furnace
• Smelt composition: Na₂CO₃ + Na₂S (kraft) or Na₂CO₃ (soda process)
• Safety: BLRBAC safe operating procedures; emergency dilution and cutoff systems
• Steam contribution: 60–80% of mill's total steam requirement

Causticising Plant and Lime Kiln: Completing the Chemical Cycle

Green liquor (dissolved smelt) contains Na₂CO₃ and Na₂S but no active NaOH — it cannot cook pulp until the Na₂CO₃ is converted to NaOH in the causticising plant. The causticising reaction: Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃ — converts sodium carbonate to sodium hydroxide using slaked lime (calcium hydroxide), producing white liquor (active cooking liquor: NaOH + Na₂S) and calcium carbonate sludge (lime mud).

The lime kiln burns the lime mud (CaCO₃) at 950–1,100°C to regenerate active lime (CaO): CaCO₃ → CaO + CO₂. The reburned lime is slaked with water (CaO + H₂O → Ca(OH)₂) and returned to the causticiser — completing the chemical cycle. Make-up lime (fresh CaO) is added to compensate for losses in lime mud washing and handling.

The lime kiln is typically a rotary kiln 25–45 m long and 2.0–3.0 m diameter for 30–80 ODT/day pulp mills, fuelled by natural gas, heavy fuel oil, or biomass. Kiln availability is a critical factor in recovery plant uptime — a lime kiln shutdown forces the causticising plant to operate on purchased lime at significantly higher cost until the kiln is restored.

• Causticising: Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃
• Causticising efficiency: 80–88% (fraction of Na₂CO₃ converted to NaOH)
• Lime kiln: rotary, 950–1,100°C; CaCO₃ → CaO + CO₂
• Lime reburning efficiency: 85–92% CaCO₃ conversion
• White liquor active alkali: 90–120 g/L NaOH equivalent for cooking

Chemical Recovery Plant: Capex and Payback

The capital cost of a chemical recovery plant for a 50 ODT/day kraft or soda pulp mill — covering MEE, recovery boiler, causticising plant, lime kiln, and all auxiliaries — is approximately INR 25–45 crore installed. This is a significant investment, but the payback economics are compelling: chemical savings of INR 12–20 crore/year and steam value of INR 4–7 crore/year combine for INR 16–27 crore/year in value generation, giving payback in 1–3 years.

The decision not to install chemical recovery — treating black liquor biologically and buying fresh chemicals — is financially justified only for mills below 20–25 ODT/day, where the capital cost per tonne of chemical saving is too high to be competitive with purchased NaOH. Above 25 ODT/day, recovery is almost always the correct long-term economic decision, even if the upfront capital is challenging.

Rajshree offers chemical recovery plants as a standalone scope (for mills adding recovery to existing pulp mill installations) or as part of a complete integrated pulp mill project. Phased installation — MEE first, recovery boiler in phase 2 — is possible for projects where capital constraints prevent all-at-once investment.

• Recovery plant capex: INR 25–45 crore for 50 ODT/day scale (installed)
• Chemical saving value: INR 12–20 crore/year at 50 ODT/day
• Steam value from recovery boiler: INR 4–7 crore/year
• Combined value generation: INR 16–27 crore/year
• Payback period: 1–3 years — best return investment in pulp mill project

Frequently Asked Questions