The Role of the Lining in a Smelting Furnace
In a smelting furnace the refractory lining is not a passive shell. It separates molten metal and slag from the steel shell, limits heat loss, and controls the shape of the reaction zone. A lining that wears unevenly changes the thermal profile of the furnace and forces operators to compensate with extra energy. High alumina bricks are widely chosen for this duty because their alumina content gives them a high melting point, good refractoriness under load, and useful resistance to slag attack, while their brick format allows damaged zones to be replaced without rebuilding the entire vessel.
Composition and Firing Route
The mix is built on calcined bauxite or fused alumina, combined with plastic clay for workability and small additions that adjust firing shrinkage. After batching and dry mixing, a controlled amount of water is added and the body is shaped by high pressure pressing or vacuum extrusion. Green bricks are dried slowly in a tunnel dryer to avoid lamination cracks and then fired between 1400 ℃ and 1550 ℃. During firing, mullite needles grow and bond the corundum grains, producing a matrix that keeps strength at temperature and resists slag penetration.
Key Properties and Typical Values
Grade designations follow GB/T 2988, in which the number indicates the minimum alumina content. The table shows values normally specified for smelting duty.
| Property | LZ-55 | LZ-65 | LZ-75 |
|---|---|---|---|
| Al2O3 content, % min | 55 | 65 | 75 |
| Bulk density, g/cm3 | 2.30 to 2.45 | 2.45 to 2.60 | 2.60 to 2.75 |
| Apparent porosity, % max | 22 | 21 | 20 |
| Cold crushing strength, MPa min | 45 | 50 | 55 |
| Refractoriness under load at 0.2 MPa, ℃ min | 1450 | 1480 | 1500 |
| Thermal expansion at 1000 ℃, % | 0.6 | 0.7 | 0.8 |
Higher alumina content raises refractoriness and slag resistance but also increases cost and thermal conductivity, so grade selection is a balance rather than a case of always choosing the highest figure.
Selecting a Grade for Different Smelting Duties
Slag line and tap hole areas: LZ-75 or better is preferred, because a dense low porosity brick slows slag penetration and limits chemical erosion.
Side walls above the slag line: LZ-65 offers a practical balance of refractoriness, strength and cost where gas erosion dominates over slag contact.
Upper walls and roof: LZ-55 with lower density reduces structural load on the shell and improves thermal insulation.
Back up layers: insulating firebrick or castable behind the working lining cuts heat loss and protects the steel shell from overheating.
Installation, Back Up Design and Inspection
Working linings are laid with thin joints, normally 1 mm to 2 mm, using a mortar whose alumina content matches the brick. Expansion joints are placed at fixed intervals and filled with ceramic fibre so that brick growth during heat up does not close the joints and lift the courses. A back up layer of insulating brick or lightweight castable reduces shell temperature and thermal loss. First heat up follows a staged schedule with holding points above 100 ℃ so that free and chemically bound water leaves the lining gradually. Once in service, the lining is inspected at planned shutdowns for joint opening, hot face glazing, slag build up and crack width, and single zones are replaced before wear reaches the shell.
Frequently Asked Questions
Q: How do I choose between LZ-65 and LZ-75 for a slag line?
Compare slag basicity and temperature first. Aggressive basic slag at high temperature favours LZ-75, while a milder duty allows LZ-65 to meet campaign life at lower cost.
Q: Does a higher alumina content always give a longer life?
Not by itself. Alumina content raises refractoriness, but joint design, expansion allowance and back up insulation often decide whether a lining reaches its expected campaign life.
Q: Why is porosity important in a smelting lining?
Open pores let slag and gas penetrate behind the hot face, where they cause spalling and structural cracking. Lower porosity slows that penetration.
Q: What is the purpose of the back up layer?
It lowers shell temperature and heat loss, and it gives the working lining some freedom to expand without loading the steel shell directly.
Q: How should a new lining be dried out?
Slowly and in stages. A hold above 100 ℃ allows free water to escape as vapour, and further holds release chemically bound water before the furnace reaches operating temperature.
