In the classic calcium citrate route for citric acid recovery, the precipitated calcium citrate is treated with which mineral acid to regenerate free citric acid and simultaneously remove calcium as an insoluble salt?

Introduction / Context:
Following precipitation of calcium citrate from the fermentation broth, an acid “cracking” step liberates citric acid and removes calcium. Choosing the correct acid determines whether the calcium leaves as an easily filterable salt and minimizes contamination of the citric acid solution.

Given Data / Assumptions:

• Intermediate: calcium citrate solid.
• Objective: release citric acid and precipitate calcium as an insoluble mineral.
• Operational need: simple solid–liquid separation post-reaction.

Concept / Approach:
Calcium citrate reacts with sulfuric acid to produce free citric acid in solution and calcium sulfate (gypsum) as a sparingly soluble, easily filterable precipitate. This step is central to the traditional recovery flowsheet and yields a clear citric acid liquor suitable for decolorization and crystallization.

Step-by-Step Solution:

Verification / Alternative check:
Industrial practice universally cites sulfuric acid for this conversion due to the low solubility of CaSO4 and its ease of removal.

Why Other Options Are Wrong:

• HCl/HNO3 keep Ca2+ soluble as chlorides/nitrates, complicating removal.
• Carbonic acid is too weak for efficient conversion.
• “None of these” contradicts standard methodology.

Common Pitfalls:
Over-acidification leading to losses; inadequate filtration of gypsum causing carryover.

Final Answer:
Sulfuric acid (H2SO4)