Steel vacuum degassing — The refractory lining typically used in vacuum steel degassing units is:

Introduction:
Vacuum steel degassing involves exposing molten steel to low pressure to remove dissolved gases like hydrogen and nitrogen. The lining must withstand thermal shock, steel/slag chemistry, and vacuum operation. High alumina refractories are a common choice for their chemical stability and temperature capability.

Given Data / Assumptions:

• Service: contact with steel and slag at high temperature.
• Environment: reduced pressure (vacuum) with potential for thermal cycling.
• Need: chemical compatibility and mechanical integrity.

Concept / Approach:
High alumina bricks (e.g., 60–90% Al2O3 grades) offer higher refractoriness under load and better corrosion resistance to basic slags than ordinary fireclay. Silica bricks are acid refractories better suited to silica environments and can be problematic with basic slags. Graphite is used where carbon linings are intended (e.g., blast furnace hearth), but not typically as the general lining for vacuum degassing vessels.

Step-by-Step Solution:
Evaluate chemistry: steel slags often basic → high alumina performs better than silica or low-duty clay.Consider temperature and mechanical strength: high alumina exhibits superior RUL and hot strength.Assess vacuum service: high alumina bricks are well-established for vacuum degassing linings.

Verification / Alternative check:
Industrial practice and supplier catalogs list high alumina as the default material for ladles and vacuum tanks unless specific chemistries require magnesia-carbon or other specialized refractories.

Why Other Options Are Wrong:

• Silica bricks: acid nature; not preferred with basic slags.
• Low duty firebricks: inadequate hot strength and corrosion resistance.
• Graphite blocks: used in different applications (e.g., BF hearth), not standard for vacuum degassing vessel linings.

Common Pitfalls:
Assuming one refractory type fits all steelmaking vessels; overlooking slag chemistry.

Final Answer:
High alumina bricks