Question

Black liquor is converted into the white liquor by

• A. Evaporation and burning the concentrate followed by causticisation of products
• B. Multi-effect evaporation only
• C. Selective liquid extraction
• D. Extractive distillation

Hint:

Kraft Recovery Loop Steps: \[ \text{Spent Black Liquor} \xrightarrow{\text{Evaporate + Burn}} \text{Green Liquor} (\text{Na}_2\text{CO}_3) \xrightarrow[\text{Lime}]{\text{Causticize}} \text{Regenerated White Liquor} (\text{NaOH}) \]

Answer 1

The Correct Option is A

Concept: In the Kraft (sulphate) pulp process, economic viability and environmental compliance rely on recycling cooking chemicals through an industrial recovery loop.
• White Liquor: The initial fresh cooking liquor containing active $\text{NaOH}$ and $\text{Na}_2\text{S}$.
• Black Liquor: The spent byproduct solution containing dissolved wood lignin, hemicellulose residues, and spent inorganic salts.

Step 1: Breaking down the multi-stage chemical recovery loop.
The conversion of black liquor back into white liquor follows a standard industrial sequence:
• Evaporation: Weak black liquor ($\approx 15\%$ solids) is processed through multi-effect evaporators to remove water, producing concentrated heavy black liquor ($\approx 65\text{--}80\%$ solids).
• Recovery Boiler Combustion: This concentrated organic slurry is sprayed into a high-temperature recovery furnace. The organic wood wastes burn to generate high-pressure process steam. The inorganic chemical components melt into a molten salt mixture called "smelt," where sodium sulfate ($\text{Na}_2\text{SO}_4$) is chemically reduced to sodium sulfide ($\text{Na}_2\text{S}$): \[ \text{Na}_2\text{SO}_4 + 2\text{C} \rightarrow \text{Na}_2\text{S} + 2\text{CO}_2 \] The resulting smelt (predominantly $\text{Na}_2\text{CO}_3$ and $\text{Na}_2\text{S}$) is dissolved in water to produce green liquor.
• Causticization: The green liquor is reacted with slaked lime ($\text{Ca(OH)}_2$) to convert inactive sodium carbonate into active sodium hydroxide: \[ \text{Na}_2\text{CO}_3\text{(aq)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow 2\text{NaOH(aq)} + \text{CaCO}_3\text{(s)}\downarrow \] Filtering out the insoluble calcium carbonate precipitate yields regenerated white liquor ready to return to the digesters. This complete process sequence corresponds directly to option (1).