Electric-furnace phosphorus route and feed quality:\nWhy can low-grade phosphate rock still be used effectively in the electric furnace process?

Introduction / Context:
The electric-furnace (thermal) process reduces phosphate rock with carbon in the presence of silica to make elemental phosphorus. Unlike wet processes, furnace operation must also manage slags. This question asks why even low-grade phosphate rock can be suitable, focusing on slag chemistry and fluidity, which are critical for furnace tapping and overall process stability.

Given Data / Assumptions:

• Feed: phosphate rock with variable gangue composition, plus coke (carbon) and silica (SiO2).
• High-temperature electric furnace forms phosphorus vapour and a molten calcium–silicate slag.
• Goal: maintain a slag composition and viscosity that allow smooth tapping.

Concept / Approach:
In the furnace, CaO from the rock reacts with SiO2 to form calcium–silicate slags. A favourable CaO/SiO2 ratio yields a low-viscosity, fluid slag that can be tapped and separated from phosphorus vapour. Low-grade ore often carries additional silica-bearing gangue. With proper burden design, this helps achieve a balanced CaO/SiO2 ratio and improves slag fluidity without resorting to excessive flux additions. Thus, “low-grade” does not automatically mean unsuitable; what matters is whether the final CaO–SiO2 balance supports good slag properties.

Step-by-Step Solution:

Verification / Alternative check:
Phase diagrams for CaO–SiO2 systems show viscosity minima at certain ratios. Operations target these windows to minimise tapping issues; feeds that naturally hit the ratio require fewer corrections.

Why Other Options Are Wrong:

• Lower CaO content (b): not universally true for all low-grade ores.
• Cheaper ore (c): economics alone does not guarantee technical suitability.
• Lower-cost product (d): finished product cost depends on yield/energy, not just ore grade.
• Impurities raising phosphorus vapour pressure (e): not the controlling mechanism.

Common Pitfalls:
Equating “low-grade” with “poor performance” without considering flux balance; ignoring that slag chemistry, not just P2O5 grade, controls furnace operability.

Final Answer:
Because the CaO/SiO2 balance is better for forming a fluid calcium–silicate slag