Why High Alumina Bricks Suit Silicon Metal Smelting
Silicon metal is produced in submerged arc furnaces, where quartz and a carbon reductant react at high temperature to give liquid silicon, silicon monoxide vapour and carbon monoxide. The refractory lining has to survive that chemistry while the furnace runs continuously for months between shutdowns. High alumina bricks are the established hot-face material for the lower sidewall, taphole, hearth and roof because they keep a stable volume at temperature and resist the reducing, vapour-laden atmosphere far better than silica-based products.
Duty varies from zone to zone. The hearth carries the molten metal and slag pool and therefore needs volume stability, high cold strength and slag resistance. The sidewalls see rapid temperature swings during tapping and thermal shock from charging, so resistance to thermal shock cycling matters more than ultimate refractoriness. The roof and upper walls work under load at temperature and are governed by refractoriness under load and creep behaviour.
Grades and Typical Properties
High alumina bricks are supplied in the grades defined by GB/T 2988, from LZ-48 to LZ-80, and in the alumina-silica classes of ISO 10081-1. Alumina content is the first selection parameter, but porosity, cold crushing strength and refractoriness under load decide how a brick behaves in service.
| Grade | Al2O3 (%) | Apparent porosity (%) | Cold crushing strength (MPa) | Refractoriness under load at 0.2 MPa (°C) |
|---|---|---|---|---|
| LZ-48 | ≥ 48 | 18-24 | 40-55 | 1420-1480 |
| LZ-55 | ≥ 55 | 18-23 | 45-60 | 1450-1520 |
| LZ-65 | ≥ 65 | 19-23 | 50-65 | 1480-1550 |
| LZ-75 | ≥ 75 | 18-22 | 55-70 | 1520-1600 |
| LZ-80 | ≥ 80 | 17-21 | 60-80 | 1550-1650 |
The ranges above are indicative and are given for orientation only; the agreed supply specification governs acceptance. Apparent porosity and bulk density are determined to GB/T 2997, cold crushing strength to GB/T 5072 and refractoriness under load to GB/T 3002. Permanent linear change on reheating, measured to GB/T 5988, should be small and slightly expansive for hearth service, because shrinkage opens joints and allows metal and slag to penetrate the lining.
Lining Zones and Performance Requirements
Hearth and taphole: dense, high alumina bricks from LZ-65 to LZ-80 with low apparent porosity and near-zero reheat shrinkage; joints are kept thin and use a mortar matched to the brick grade.
Lower sidewall: bricks selected for thermal shock resistance and slag resistance, usually one alumina grade above the upper wall to slow chemical attack in the reaction zone.
Upper sidewall and roof: bricks chosen for refractoriness under load at 0.2 MPa and creep resistance, often LZ-55 to LZ-65 so that lining weight and thermal expansion stay manageable.
Backup insulation: insulating firebrick with mineral wool or ceramic fibre board behind the working lining lowers shell temperature and heat loss without loading the hot face.
Expansion control: expansion joints, paper joints and sliding interfaces absorb thermal expansion so that the steel shell is not distorted.
Selection, Installation and Quality Control
Selection starts from the furnace operating data: shell diameter, power input, tapping temperature, slag basicity and the intended campaign length. A brick that performs well in a small furnace on a short campaign is not automatically right for a high-power furnace on a long campaign. A purchase specification should therefore state the alumina grade, dimensional tolerances, the mortar to be supplied and the test certificates required.
The points that most often decide lining life during installation are:
storage under cover, with bricks kept dry and stacked on pallets;
ring layout with staggered vertical and radial joints, avoiding joints that run continuously through the lining thickness;
consistent mortar thickness, worked with a full bed and closed joints;
expansion allowances checked against the design drawing as each ring closes;
a documented dry-out and heat-up curve for the first campaign, with intermediate plateaus to release moisture gradually.
Works quality control covers raw material batch, batching accuracy, pressing density, firing curve and final inspection of dimensions, appearance and physical properties. Inspection records and sample test results should accompany each shipment so that lining behaviour in service can be traced back to the delivered batch.
Frequently Asked Questions
Q: Which alumina grade is best for the hearth of a silicon metal furnace?
Dense grades from LZ-65 to LZ-80 are normally used, because they combine low apparent porosity with high cold crushing strength and very small reheat change. The final choice follows the slag chemistry and the required campaign length.
Q: Why are high alumina bricks preferred over silica bricks in this furnace?
Silica bricks offer very good refractoriness under load but are attacked by the reducing atmosphere and by silicon monoxide vapour in the reaction zone. High alumina bricks tolerate those conditions and also withstand the thermal cycling that occurs during tapping.
Q: How should two suppliers offering the same grade be compared?
Compare the actual test values rather than the grade name: alumina content, apparent porosity, bulk density, cold crushing strength, refractoriness under load, permanent linear change and dimensional tolerance. Batch consistency across successive deliveries is the strongest indicator of a reliable supplier.
Q: What causes brick spalling during tapping?
Spalling usually follows rapid heating or cooling of the hot face, excessive moisture in the mortar or the brick, and joints that cannot absorb expansion. Correct dry-out, thin and well-filled joints, and a stable tapping temperature greatly reduce the risk.
Q: Can high alumina bricks be used in the roof and upper walls as well?
Yes. Grades with high refractoriness under load are used in roofs and upper walls, where creep resistance and load-bearing capacity at temperature matter more than slag resistance. Roof bricks are usually one grade lower in alumina content than the hearth bricks.
Q: What lining thickness is appropriate for the sidewall?
Thickness is set by furnace diameter, power input and campaign target, and is normally stated in the furnace design. Increasing thickness beyond the design value adds weight and expansion load without extending life, because the wear rate is driven by chemistry and temperature rather than by thickness alone.
