Injection of carbon and oxygen at several places in the bath assures slag foaming practice, when carbon content of the bath is insufficient. Typically carbon injection rates for slag foaming are 2.5 to 5 kg/ ton of steel. In high powered furnaces carbon injection is 5-10 kg/ton of steel.
In stainless steel making  forms in preference to due to higher affinity of  to oxygen. The solubility of chromium oxide in the slag is considerably weaker in comparison to for the same basicity and thermal conditions. The oxygen/carbon injection technique in the high chromium alloy steel production and to foam the slag is difficult. Moreover, additional chromium will also be lost due to oxygen injection.
The novel technology utilizes the reduction of iron and chromium oxide by carbon as well as thermal dissociation of limestone contained in small briquette. (See the reference given at the end of the lecture)
iii) Scrap preheating
Preheating of scrap brings thermal energy into the furnace. Preheating of scrap to  brings  of additional energy. Scrap preheating gives the following advantages:
i) Reduction in energy consumption by 40-60 kwh/ton depending on the scrap preheat temperature
ii) Electrode consumption reduces by 0.3 to 0.36 kg/ton
iii) Refractory consumption decreases by 0.9 to 1.4 kg/ton
iv) Tap to tap time reduces by 5 to 8 minutes.
It is important to note that scrap preheating technology needs to be developed. Thermal energy is required to preheat the scrap and is economical only when the waste heat from the furnace is utilized. For this purpose it is important to know the energy balance of the electric furnace. The energy balance of an EAF, as shown in the figure 16.2 indicates that 20% of the total energy leaves the furnace in the
| Figure 16.3 |
Energy balance of an electric furnace steelmaking |
waste gases and represents about 130 kWh/ton of steel produced. Efficient utilization of thermal energy of exit gas is the key to realize the advantages of preheating of scrap. Batch preheating and continuous preheating are the available technologies. In CONSTEEL, scrap and exit gases move counter current to each other. It is possible to preheat the scrap to  .
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