In ironmaking practice, the lower (hearth/bottom) portion of a hot metal mixer is lined with which class of refractory bricks to withstand metal washing and slag attack?

Introduction / Context:
Hot metal mixers receive molten iron from the blast furnace and hold it for homogenisation before further processing. The lining must survive continuous contact with liquid metal, fluctuating temperatures, and chemical attack by slags. This question focuses on which refractory class is typically selected for the lower (hearth/bottom) region of a mixer where the most severe washing and corrosion occur.

Given Data / Assumptions:

• Zone: lower portion (hearth/bottom) of a hot metal mixer.
• Exposure: intense washing/erosion by molten iron and contact with basic slags.
• Objective: choose a brick class having high hot strength, corrosion resistance, and adequate thermal shock tolerance.

Concept / Approach:
For mixer bottoms, refractories must combine good structural integrity with chemical compatibility. High alumina bricks, with elevated Al2O3 content, offer high refractoriness under load, respectable resistance to slag corrosion in this service, and better mechanical strength than standard fireclay. Silica, while excellent at very high crown temperatures, is unsuited to basic slag contact and thermal cycling in the hearth. Silicon carbide (carborundum) bricks have excellent abrasion resistance but are typically used selectively (e.g., wear-prone areas) and require atmosphere considerations; they are not the default full-bottom lining choice. Superduty fireclay is widely used in less severe areas, flues, or intermittent furnaces, but not preferred where the worst washing occurs.

Step-by-Step Solution:
Identify service conditions: washing + basic slag contact + high temperature.Match to materials: high alumina provides hot strength and better chemical durability than fireclay.Exclude silica (acidic) and general-use fireclay for the most severe hearth duty.Select high alumina as the standard lining for lower mixer regions.

Verification / Alternative check:
Industry lining schedules commonly specify high alumina compositions in hearth/bottom zones, with mullite or alumina-silicate products higher up where splash and slag corrosion are less intense.

Why Other Options Are Wrong:
Superduty fireclay: adequate in many services but inferior hot properties versus high alumina for the hearth.Silica: poor choice against basic slags and cycling in lower zones.Carborundum: excellent abrasion resistance yet not the default full hearth lining in mixers.

Common Pitfalls:
Assuming a single brick type suits all mixer zones; in practice, zones are graded to service conditions.

Final Answer:
High alumina