Triple superphosphate (≈46% P2O5) production: Phosphate rock is reacted with orthophosphoric acid of approximately what concentration (by mass percent H3PO4) to make TSP?

Difficulty: Easy

Correct Answer: 75–80

Explanation:


Introduction / Context:
Triple superphosphate (TSP) is a high-analysis phosphate fertilizer produced by digesting phosphate rock with concentrated orthophosphoric acid rather than sulfuric acid. The acid strength used strongly influences reaction kinetics, product grade, and granulation behavior. Recognizing the approximate acid concentration is a standard fertilizer technology question.


Given Data / Assumptions:

  • TSP product analysis: about 46% P2O5.
  • Reagent: orthophosphoric acid (H3PO4) of high concentration.
  • We are asked for H3PO4 wt% range, not P2O5 equivalent.


Concept / Approach:
Manufacture of TSP typically employs orthophosphoric acid around 70–80 wt% H3PO4 to ensure rapid conversion and produce high-grade mono-calcium phosphate within each granule. Using too dilute acid lengthens reaction time and yields lower-grade products, while ultra-concentrated acid near 100% is impractical and hazardous for routine plant operation. Hence, a practical range of roughly 75–80% H3PO4 is the accepted textbook answer.


Step-by-Step Solution:
Identify target product grade (≈46% P2O5) requiring concentrated acid attack.Recall industry practice: use ≈70–80% H3PO4 for efficient rock digestion.Match to the closest range among choices: 75–80.Select “75–80.”


Verification / Alternative check:
Design manuals and plant experience indicate orthophosphoric acid in the mid-70% range for robust TSP granulation and quality control.


Why Other Options Are Wrong:
25–28 or 52–54: too dilute for making high-grade TSP efficiently.>98: near pure H3PO4, impractical and unnecessary for standard TSP production.


Common Pitfalls:
Confusing H3PO4 wt% with P2O5 wt%; both are used in literature but are not interchangeable numerically.


Final Answer:
75–80

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