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2. Hot metal

Hot to cold
A hot object will always transfer energy to a cooler object by heating it. If they are in contact with each other, then the energy is transferred by conduction. In conduction, the moving particles pass energy between each other. Let’s see how this happens.
Interactive graphic of conduction at surface
Picture 4. Conduction from a hot surface to cooler surroundings.

Conducting particles
Picture 4 shows the surface of an Electric Arc Furnace. The metal surface of the furnace is extremely hot (~200 ºC). The air around the furnace is not so hot. We can zoom in on the surface to see the particles (roll over on diagram). The metal particles are vibrating vigorously because they are so hot. On the other hand, the air particles are not moving so fast. So, if an air particle comes into contact with an energetic metal particle, it will rebound at a higher speed. It has picked up energy from the surface of the metal. Energy has been transferred by conduction to the surrounding air.
You can find out more about the electric arc furnace in the Chemistry of steelmaking e-source.
Cut away of furnace
Picture 5. Refractories in a blast furnace.
What are refractories?
The inside of a blast furnace is extremely hot (up to 1500 ºC in places). The inside surfaces have to withstand these extremely high temperatures without cracking. The inside is lined with firebricks made from special refractory materials. These can withstand extremely high temperatures without cracking or taking part in any chemical reaction.

Examples of refractory materials include: alumina (Al2O3), silica (SiO2), fireclay (hydrated alumina silicates) and carbon. The inside of a blast furnace is lined with bricks made from different mixtures of these substances. The choice of material for a particular part of a furnace depends on:

  • the chemical conditions (acidic or basic)
  • the temperature ranges
  • the likelihood of sudden shocks or large forces
  • whether it needs to be an insulator or a conductor
You can find out more about the electric arc furnace in the Chemistry of steelmaking e-source.
It takes a lot of energy to heat up the metal inside a furnace. It is important that as little of this as possible is wasted. Therefore the furnaces are well insulated. The refactory bricks that line the furnace are designed to be poor conductors of heat.
Animation of conduction along a rod
Picture 6. A cool lattice. If we heat the left hand end, then the energy will be carried along by conduction.
Conducting across a solid
The particles in the brick are bound together. Energy is transferred across the brick by the vibrations of the particles. The particles on the inside surface are vibrating vigorously because this surface is so hot. They make their neighbours vibrate and, eventually, the particles on the outside of the brick will be vibrating vigorously as well. However, the rate at which energy is passed through the brick is slow. Certainly, it is slower than it would be if the brick were made of metal.
Metals and conduction
Metals are good conductors and so is carbon. Carbon, being a refractory, can be used at the extremely high temperatures in furnaces. Good conductors are useful in situations where we need to keep something cool by conducting the heat away. For example:
  • to stop the hearth of a blast furnace overheating
  • cooling the steel in the mould and rollers of a continuous casting mill

    Metals and carbon are such good conductors because their electrons are free to move. As one side of a slab gets hot, the electrons speed up and move through the lattice, bashing into ions and making them vibrate. The free electrons carry energy across the block much faster than the vibrations in an insulator.

    Question 2-2.

    Fill in the gaps in the paragraph below. Choose the best answer in each case.

    a) Which of the options below is the best description of conduction?

    b) What is a refractory material?

    c) The hearth of a furnace needs to be cooled. The best material to use in its construction would be because it is a .

    Summary                   Close
    • Conduction transfers energy from hot bodies to cold ones
    • And from the hot to cold parts of a material
    • In conduction, energy is transferred by vibrating particles
    • Refractory materials can withstand extremely high temperatures
    • Carbon and metals are good thermal conductors