Wet-process phosphoric acid at high temperature: In strong-acid leaching of phosphate rock, maintaining the reactor above ~100 °C tends to form which undesirable calcium sulphate phases or related by-products?

Introduction / Context:
In the wet process for orthophosphoric acid, phosphate rock is digested with sulphuric acid, precipitating calcium sulphate and leaving H3PO4 in solution. The operating temperature influences which CaSO4 hydrate forms and thus the filtration/handling characteristics of the cake. This question probes the effect of running the reactor above about 100 °C under strong-acid conditions.

Given Data / Assumptions:

• Dihydrate process: ~70–80 °C yields CaSO4·2H2O (gypsum), easier to filter.
• Hemihydrate process: ~90–110 °C tends to form CaSO4·0.5H2O (hemihydrate), more difficult to handle unless the plant is designed for it.
• Above ~100 °C and strong acid, anhydrite (CaSO4) may form, which can be dense and less filterable.

Concept / Approach:
Operating at elevated temperature shifts calcium sulphate formation away from dihydrate and toward hemihydrate or anhydrite, which often have inferior filtration properties in a flowsheet designed for dihydrate. Although polyphosphoric species (pyro/metaphosphoric acids) can appear at very high concentrations and dehydration severities, the primary processing concern in standard wet-process operation is the solid CaSO4 phase type, which directly affects filtration and overall plant performance.

Step-by-Step Solution:

Verification / Alternative check:
Process descriptions distinguish dihydrate, hemihydrate, and anhydrite routes, each with dedicated filtration strategies. Cross-operating a dihydrate design at hemihydrate/anhydrite temperatures reduces filter performance.

Why Other Options Are Wrong:

• (b) and (c): Polyphosphoric acids become important in dehydration/concentration steps, not as the primary “undesirable” solid phase in wet digestion.
• (d): Overstates the outcome; the major practical issue is CaSO4 phase.
• (e): Dihydrate formation is typical below ~80 °C; it is not expected above ~100 °C in strong acid.

Common Pitfalls:
Assuming higher temperature is always beneficial for kinetics; overlooking the knock-on effect on solid phase and filterability.

Final Answer:
Calcium sulphate hemihydrate (CaSO4·0.5H2O) and/or anhydrite (CaSO4) crystals