Blast furnace thermal zones: In the lower part of a blast furnace (the zone of fusion approaching the hearth), the typical operating temperature range is:

Introduction / Context:
The blast furnace features stratified thermal and chemical zones. Understanding the temperature profile is essential for controlling reduction reactions, slag formation, iron melting, and refractory wear. The lower region includes the zone of fusion and the hearth where hot metal collects.

Given Data / Assumptions:

• Focus: lower part (fusion zone and hearth vicinity).
• Steady-state, well-operated furnace with typical burden and hot-blast conditions.
• Temperatures expressed as approximate ranges.

Concept / Approach:
As burden descends, indirect and direct reduction are followed by softening and melting. The cohesive zone sits above the fusion zone; below, droplets of iron and slag form and trickle to the hearth. This region experiences the highest thermal load from raceways/flames and molten drainage, with temperatures generally in the 1500–1700°C range (or slightly higher locally), sufficient to keep hot metal and slag fully molten.

Step-by-Step Solution:
Identify lower-furnace function: melting and collection of hot metal and slag.Infer temperature requirement: must exceed melting points and sustain fluidity.Match to typical operating data: ~1500–1700°C in the fusion/hearth area.Select the range 1500–1700°C.

Verification / Alternative check:
Operational reports show tuyere raceway peak temperatures even higher locally, but bulk hearth/fusion temperatures align with 1500–1700°C.

Why Other Options Are Wrong:
400–700°C and 800–1000°C correspond to upper furnace zones (drying, preheating, indirect reduction).

1200–1300°C is too low for sustained molten iron flow in the hearth.

1800–2000°C exceeds typical bulk hearth temperatures and would severely stress refractories.

Common Pitfalls:
Confusing cohesive zone softening temperatures with the hotter fusion/hearth conditions; assuming flame (raceway) peak equals bulk hearth temperature.

Final Answer:
1500°C to 1700°C