Industry and refractories
A refractory is a non-metal material that stays strong and does not melt or crumble at high temperature, usually above 1000 °C and often up to 1800 °C or more. Refractories are used wherever things are made very hot: iron and steel (the biggest user), cement kilns, glass tanks, ceramic kilns, power plant boilers and chemical reactors.
A good refractory must: (1) resist heat, (2) carry weight when hot, (3) survive sudden heating and cooling (thermal shock), (4) resist attack by hot liquids called slag, and (5) sometimes be a poor heat conductor so heat does not escape.
Raw materials and making fire bricks
- Fireclay (kaolin-type clay): cheap and common.
- Bauxite or alumina: for high-alumina bricks.
- Silica (quartz): for silica bricks.
- Magnesite and dolomite: give magnesia (MgO) and lime-magnesia bricks.
- Others: chromite, zircon, silicon carbide and graphite.
Steps: crush and sort grains by size, mix with a binder and a little water, press at high pressure into bricks, dry, then fire at 1400-1800 °C. Some are chemically bonded or left unfired. Unshaped refractories (castables, mortars, ramming mixes) are poured or packed on site and used for odd shapes and repairs.
Various refractories
By chemistry:
- Acidic (silica, fireclay): resist acidic slag; attacked by basic slag.
- Basic (magnesia, dolomite): resist basic slag such as steel-making slag.
- Neutral (alumina, chromite, carbon): stable with both.
By brick:
- Fireclay brick: about 1500 °C; kilns, chimneys, furnace backs.
- High-alumina brick: about 1800 °C; glass tanks, steel ladles.
- Silica brick: strong when hot (about 1650 °C); coke ovens and glass furnace roofs.
- Magnesia brick: over 2000 °C; steel furnaces and cement kiln hot zone.
- Insulating firebrick: light and full of tiny pores; keeps heat in.
Try it
Try it. Ask a tandoor maker or look at a brick kiln wall. What material is the inside lining? Why is there a thick wall? In the 3D, press each job and see which of the four bricks hold. Write the cheapest brick that works for each job.
Key formulas and definitions
- Refractoriness: highest heat at which the brick keeps its shape
- Bricks needed = wall area / area of one brick face
- Magnesite: MgCO3 -> MgO + CO2 (84 g gives 40 g of MgO)
- Acidic slag needs acidic or neutral brick; basic slag needs basic or neutral brick
Worked examples
1. A furnace wall is 6 m2. One brick face is 0.02 m2. How many bricks are needed?
6 / 0.02 = 300 bricks.
2. Each brick weighs 5 kg. What is the mass of 300 bricks?
300 x 5 = 1500 kg.
3. A steel furnace runs at 1650 °C with basic slag. Which brick: fireclay, silica or magnesia? Why?
Magnesia. Fireclay (about 1500 °C) fails at 1650 °C and silica is acidic, so basic slag would attack it. Magnesia is basic and holds above 2000 °C.
Common mistakes
- Thinking refractory bricks are just ordinary bricks. They have special raw materials and a much higher firing heat.
- Using an acidic brick against basic slag. It gets eaten away.
- Forgetting thermal shock: heating or cooling too fast can crack even strong bricks.
- Believing the highest heat limit is always best. Costly bricks should be used only where needed.