Thermal (electric-furnace) route for elemental phosphorus:\nWhen a mixture of phosphate rock, coke (carbon) and silica sand is heated in an electric furnace at high temperature, what principal product is formed?

Introduction / Context:
The electric-furnace (thermal) process is a classic high-temperature route for producing elemental phosphorus (P4) from phosphate rock. Unlike wet processes that yield phosphoric acid or superphosphate via acidulation, the thermal route relies on carbothermic reduction in the presence of silica, followed by condensation of phosphorus vapor. This question checks whether you can distinguish the thermal reduction product from the acidulation products used in fertiliser manufacture.

Given Data / Assumptions:

• Feed mixture: phosphate rock (fluorapatite or related Ca phosphates), coke (carbon), and silica sand (SiO2).
• Equipment: electric furnace operating at very high temperature (well above 1200 °C).
• Objective: identify the principal chemical product of the reduction.

Concept / Approach:
In the furnace, carbon reduces the phosphate to elemental phosphorus, while silica combines with calcium to form a fusible calcium-silicate slag. Phosphorus is volatilized as P4 and later condensed under controlled conditions. This is a reductive route, not an acid leach; therefore the products are entirely different from orthophosphoric acid or superphosphate obtained by sulphuric or phosphoric acid digestion of rock.

Step-by-Step Solution:

Verification / Alternative check:
Standard furnace reactions are summarized as Ca3(PO4)2 + SiO2 + C → P4 (vapor) + Ca-silicates + CO/CO2. Process flow diagrams show electrothermal furnaces, condenser trains, and slag handling units in place of acid reactors and gypsum filtration used in wet processes.

Why Other Options Are Wrong:

• Orthophosphoric acid: product of wet-process digestion with strong H2SO4, not high-temperature reduction.
• Ammonium phosphate: made by neutralizing phosphoric acid with ammonia, not in the furnace.
• Single superphosphate (SSP): formed by direct acidulation of rock with dilute H2SO4.
• Calcium phosphate glass only: slag does form, but the valuable product is P4, not just glassy calcium silicates.

Common Pitfalls:
Confusing thermal reduction (elemental P) with acidulation routes (fertilisers or H3PO4). Also, overlooking silica’s role as a flux for CaO can obscure the overall reaction logic.

Final Answer:
Elemental phosphorus (white phosphorus, P4)