Process operating technologies
Most of the developments in process operating technologies are in AC- electric arc furnaces as these furnaces are popular.
i) Bottom stirring
In convectional arc furnaces there is little natural electrical turbulence within the bath. Due to absence of stirring large piece of scrap can take a long time to melt and may require oxygen lancing. Argon or nitrogen stirring
- Eliminates temperature and concentration gradients
- Shortens tap-to-tap times
- Reduces refractory, electrode and power consumption and
- Improves yield of iron and alloys
Industrial systems for bottom stirring are either with direct contact plug or with indirect contact plug. In direct contact plug, the plug is in contact with molten metal, whereas in indirect one the plug is embedded in a porous bottom refractory. In the indirect contact, the plug is not directly in contact the molten metal. The gas enters the bath via the porous refractory hearth which results in stirring over a large area when compared with direct plug as shown in the figure 16.2. Figure 16.2 shows the direct contact and indirect contact plug for bottom stirring. Note that in indirect contact large area of the bath is stirred as compared with direct contact plug.
| Figure 16.2: |
Industrial bottom stirring systems in electric arc furnace |
ii) Foamy slag practice
In EAF steelmaking, progressive melting of scrap increases the irradiative heat transfer from arc to the side walls of the furnace. By covering the arc in a layer of slag, the arc is shielded and more energy is transferred to the bath. The foaming slag during this period is beneficial. The effectiveness of slag foaming depends on slag basicity,  content of slag, slag temperature and availability of carbon to react with either oxygen or of slag.
Slag foams in steelmaking due to entrapment of gas bubbles. Gas producing reactions in steelmaking are:
a) Reaction between of slag with carbon
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(1) |
b) Between carbon and oxygen dissolved in metal
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(2) |
c) Between chromium oxide and carbon:
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(3) |
Reactions 1 and 2 are important in carbon steelmaking whereas reaction 3 is important in stainless steel making. Slag foaming is discussed in lectures 4 and 5.
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