Concentrated sulphuric acid reaction: Phosphate rock treated with about 98% H2SO4 primarily yields which product under wet-process conditions?

Introduction / Context:Phosphate processing follows different routes depending on the acid used. The wet-process route to phosphoric acid treats phosphate rock with strong sulphuric acid, forming phosphoric acid while precipitating calcium sulphate (gypsum or anhydrite) as a by-product. The concentration of H2SO4 influences by-product hydration state but the primary product is H3PO4.

Given Data / Assumptions:

• Reagent: Concentrated H2SO4 (~98%).
• Feed: Phosphate rock (fluorapatite).
• Industrial wet-process conditions assumed.

Concept / Approach:Under these conditions the rock is digested to release phosphoric acid. Calcium from the rock is converted to calcium sulphate, which precipitates (dihydrate or anhydrite depending on temperature/acid strength), and the filtrate contains orthophosphoric acid (H3PO4). Superphosphate refers to direct acidulation of rock with dilute sulphuric acid to form monocalcium phosphate and gypsum within a solid product matrix; white phosphorus is obtained via the thermal electric furnace process, not via acidulation.

Step-by-Step Solution:

Verification / Alternative check:Wet-process flowsheets show reactor–filter systems producing H3PO4 liquor and gypsum/anhydrite solids at higher acid strengths.

Why Other Options Are Wrong:

• Superphosphate: produced by dilute H2SO4 directly on rock, not strong-acid digestion for H3PO4.
• White phosphorus: requires thermal reduction with silica and coke.
• Triple superphosphate: produced by reacting rock with phosphoric acid (H3PO4), not H2SO4.

Common Pitfalls:Confusing SSP/TSP manufacture with wet-process acid; overlooking the role of acid concentration in determining the by-product phase (gypsum vs anhydrite).

Final Answer:Orthophosphoric acid