Electric furnace phosphorus route: A mixture of phosphate rock with which additional solid(s) is heated in an electric furnace to produce elemental phosphorus?

Introduction / Context:
Elemental phosphorus (P4) can be manufactured by the thermal (electric furnace) process. This classic route reduces calcium phosphates in phosphate rock using a carbon reductant and a silica flux at very high temperatures to volatilize P4, which is then condensed under water. Knowing the required solid reactants helps understand both the chemistry and the slag formation that removes calcium as a silicate.

Given Data / Assumptions:

• Feed: phosphate rock (chiefly fluorapatite-like calcium phosphates).
• Furnace: electric, high-temperature (typically > 1400°C).
• Additives: a carbon source (coke) and a silica source (sand).

Concept / Approach:
The net reaction reduces phosphate to elemental phosphorus while binding calcium with silica to form a molten calcium silicate slag. Carbon (as coke) provides the reducing environment, generating carbon monoxide. Silica (sand) reacts with calcium oxide to form stable slag, thereby driving phosphorus liberation. Omitting either the reductant or the silica flux prevents efficient phosphorus production or creates handling problems with basic slags.

Step-by-Step Solution:

Verification / Alternative check:
Industrial flowsheets show phosphate rock + silica + coke charged to submerged-arc furnaces; off-gas condenses to white phosphorus; slag tapped as calcium silicate.

Why Other Options Are Wrong:

• Salt & coke: sodium chloride is not used in the standard thermal route.
• “Rock and coke only”: lacks silica to form calcium silicate slag.
• “Rock and sand only”: lacks reductant to produce P4.
• Gypsum & coke: not the conventional flux for P4 production.

Common Pitfalls:
Confusing the thermal process with “wet-process” phosphoric acid manufacture, which uses sulfuric acid and does not produce elemental phosphorus directly.

Final Answer:
Sand & coke.