Question

The major chemical recovered from the black liquor in Kraft Pulp Process is

• A. Sodium sulphate
• B. Sodium carbonate
• C. Sodium hydroxide
• D. Sodium bicarbonate

Hint:

In the Kraft Pulp Process, black liquor is burned to recover sodium carbonate, which is then converted back to sodium hydroxide for reuse in pulping.

Answer 1

The Correct Option is B

Step 1: Understand the Kraft Pulp Process and black liquor.

The Kraft Pulp Process is a method for producing paper pulp from wood by digesting wood chips in a solution of sodium hydroxide (NaOH) and sodium sulfide (Na₂S), known as white liquor. After digestion, the spent liquor, called black liquor, contains organic materials (lignin) and inorganic chemicals. The black liquor is processed in a recovery system to reclaim chemicals for reuse.

Step 2: Analyze the chemical recovery process.

In the Kraft process, black liquor is concentrated and burned in a recovery boiler. During combustion:

• Organic materials are burned to generate energy.
• Inorganic chemicals form a molten smelt at the bottom of the boiler, primarily consisting of sodium carbonate (Na₂CO₃) and sodium sulfide (Na₂S).

The smelt is dissolved in water to form green liquor, which mainly contains sodium carbonate (Na₂CO₃) and sodium sulfide (Na₂S). The green liquor is then causticized with lime (Ca(OH)₂) to convert sodium carbonate back into sodium hydroxide (NaOH):

\[ \text{Na}_2\text{CO}_3 + \text{Ca(OH)}_2 \rightarrow 2\text{NaOH} + \text{CaCO}_3. \]

The sodium hydroxide is reused in the pulping process, but the major chemical directly recovered from the black liquor (after combustion) is sodium carbonate (Na₂CO₃).

Step 3: Evaluate the options.

• (1) Sodium sulphate: Incorrect, as sodium sulphate (Na₂SO₄) is not the major chemical recovered; it may be used as a makeup chemical in some processes but is not the primary recovery product.
• (2) Sodium carbonate: Correct, as sodium carbonate (Na₂CO₃) is the major inorganic chemical recovered from black liquor after combustion in the recovery boiler.
• (3) Sodium hydroxide: Incorrect, as sodium hydroxide is produced later in the causticizing step, not directly recovered from black liquor.
• (4) Sodium bicarbonate: Incorrect, as sodium bicarbonate (NaHCO₃) is not a significant product in the Kraft recovery process.

Step 4: Select the correct answer.

The major chemical recovered from the black liquor in the Kraft Pulp Process is sodium carbonate, matching option (2).