The charging phase of an Electric Arc Furnace represents the primary stage of the electric steelmaking process route as clearly shown in the picture here below:
As already explained in our previous article (ELECTRIC ARC FURNACE AC (PART 2) – The Raw Materials) the main raw materials used are listed in the scheme here below:
Charge mix optimization strategy is extremely important considering that it has an incidence of about 75-85% on the total cost of liquid steel in electric steelmaking route.
The main technical and practical limits in the using of different raw materials are described in the table here below:
Now we’re going to explain in details the main key factors that are involved in the process of charging of raw materials.
2. SCRAP CHARGING
Scrap charging can be done using two different methods:
- SCRAP CHARGING BY BUCKET
- CONTINUOUS SCRAP CHARGING
2.1. SCRAP CHARGING BY BUCKET
The most common way to charge scrap into the EAF is through the using of proper metal bucket (or basket) that is a simple vessel with an opening mechanism at the bottom in order to release the scrap while the bucket is put in correspondence of the furnace vessel.
The number of buckets required to produce a heat is dependent primarily on the volume of the furnace and on the scrap density that also impacts the electrical and chemical energy profiles.
In general, dense scrap is slower to melt and if aggressive practices of burners and oxygen lancing are used, dense scrap may reflect the jet back onto the furnace side walls resulting in damage.
The scrap must be charged into the bucket considering its size and density in order to:
- PROMOTE THE FORMATION OF LIQUID LAYER OF STEEL
- PROTECTION OF EAF SIDEWALL AND ROOF FROM ARC RADIATION
- AVOID GRAPHITE ELECTRODES MECHANICAL BREAKAGES
- REDUCE BURNERS AND OXYGEN JET BACK TO AVOID DAMAGE
If we consider a case of using 100% scrap to charge the furnace the optimal disposition of the material into the bucket is shown in the following picture:
2.2. CONTINUOUS CHARGING OF SCRAP
Systems of continuous charging of scrap have been developed in the last decades in order to avoid the practice of charging scrap by buckets. Those systems have been introduced into steelmaking plants in order to decrease the production costs, while increasing the productivity and personnel safety guaranteeing a high flexibility at the same time.
2.3. CONSTEEL SYSTEM
The Consteel system (developed by TENOVA) is a continuous charging of scrap in the EAF connecting the scrap yard to the EAF.
The scrap is loaded onto conveyors by the yard cranes through a lateral belts. Then the conveyors move the scrap, and the conveying surface oscillates forward slowly and backward faster. This movement allows the scrap to move together with the conveyor during the forward stroke, while the scrap slides over the surface when the conveyor oscillates back. The end result is the movement of the scrap towards the furnace.
Before reaching the furnace the scrap arrive to the pre-heating area where the scrap is heated-up by the hot gases exiting the EAF that are moving in the direction opposite to the scrap. In the pre-heating section the carbon monoxide in the exhaust gas is oxidized by an automatically controlled injection of air, allowing more energy to be recovered by the system.
During the continuous feeding operations the steel bath in the EAF is kept constantly liquid, therefore the scrap entering the furnace is melted by immersion. Hence, the electric arc is working in a liquid bath environment. Consequently the arc is very stable as it is not affected by the presence of solids elements like in the case of bucket scrap charges.
Another system for semi / complete continuous charging of scrap (and relevant pre-heating) has been developed by FUCHS company in late 1980s under the name of shaft furnace (SF).
Through the utilization of the furnace off-gas during the heat cycle, scrap can be preheated to approximately 600 °C prior to final melting in the furnace vessel. This means considerable energy and cost savings with a substantial reduction in TTT (tap to tap) times. This technology is available with a single-shaft or double-shaft types.
One of the most efficient type of shaft furnace is called finger shaft furnace (FSF), which has been implemented by a special scrap retaining system with fingers to preheat 100% of the charge.
3. SCRAP PRE-HEATING
The scrap pre-heating is an energy recuperation measure which influences the energy balance and the productivity of the electric arc furnaces directly positively.
Scrap pre-heating systems preheat the scrap to about 600 °C using waste gases and are typically either of a vertical shaft design or a horizontal conveyor design as used by Consteel. With the shaft design, scrap is pre-heated then charged as a batch. With the Consteel system, scrap is heated and fed continuously into the furnace.
With these integrated systems a saving of about 60 to 100 kWh/ton can be easily achieved.
Pre-heating the charge has different advantages such as:
- DECREASING SPECIFIC ENERGY CONSUMPTION
- SHORTENING MELTING STAGE TIME
- LOWERING GRAPHITE ELECTRODES CONSUMPTION
4. DRI / HBI CHARGING
Similar to the scrap, also DRI (direct reduced iron) and HBI (hot briquetted iron) can be charged into the furnace in two ways:
- DRI / HBI CHARGING BY BUCKET
- CONTINUOUS DRI / HBI CHARGING
In this case, the EAF electricity consumption will be affected by the following key factors:
- DRI / HBI QUALITY that includes metallization (and relevant metallic Fe content), carbon and gangue contents, other elements content (such as sulphur and phosphorous). The physical properties of DRI / HBI and the allocation inside the EAF bucket affect the quality and metallurgical yield. Especially in Europe and North America DRI and HBI are used in order to lower the concentration of the metallic residuals such as copper and tin;
- DRI / HBI TEMPERATURE
4.1. DRI / HBI CHARGING BY BUCKET
Bucket charging is adopted by steel mills when the percentage of DRI / HBI is less than 25-30% compared to the total charge mix.
We’ve to consider that DRI / HBI melt fast when it’s charged in molten metal due to the excellent conductive heat transfer properties. For this reason, generally, DRI / HBI has to be charged in the second bucket.
Considering that HBI has a high bulk density (about 2.4 to 2.8 t/m3), it’s advised to charge HBI above the heavy density scrap to maximize the densification of the bucket. HBI is usually loaded in different baskets.
DRI is usually charged just higher up to prevent DRI falling towards the bucket bottom.
Here below we’re going to represent a possible way to charge the bucket when we’re having a mix between scrap, DRI and HBI:
4.2. CONTINUOUS CHARGING OF DRI / HBI
Continuous charging of DRI / HBI results efficient when there’s more than 25-30% of DRI / HBI compared to the total charge mix.
Continuous charging can be usually done using two different methods:
- FIFTH HOLE (EAF ROOF) CHARGING Usually it facilitates the coordination of the feed rate with the power input and flux feeding to ensure slag control (such as foaming height, viscosity and so on) and prevent ferrobergs, which will occur when cold DRI is charged too quickly. Normally, continuous charging of hot DRI (about 600 °C) can easily reduce the EAF energy consumption by 15 to 20%;
- SIDE CHARGING (like CONSTEEL or equivalent) Usually is more appropriate for HBI charging and can be combined with the continuous feeding of DRI through the fifth hole of the EAF roof. The HBI is pre-heated on the belt as it travels to the EAF.
As discussed in our previous article (ELECTRIC ARC FURNACE AC (PART 2) – The Raw Materials) charging hot metal into the EAF can be of benefit to operations. By bringing thermal and chemical (carbon) energy into the EAF, it reduces electrical energy consumption and increases productivity.
However a risk of strong reaction while charging hot metal into EAF has to be considered. In fact there’s an interaction between the oxygen (included in the steel and supplied by the lances) and the carbon (included in the steel, in the hot metal and provided by the lances).
For this reason the control of carbon content into the bath is an essential thing.
With the continuous charging of the scrap the hearth of the EAF will always contains liquid and this is an ideal situation in order to achieve a fast distribution of the carbon charged with the hot metal. By keeping the carbon level between 0.15 and 0.25%, foamy slag practice is optimized and the violent oxygen/carbon reactions in the bath are avoided, thus achieving more energy-efficient operations, fewer problems for the equipment and safer operating conditions for personnel.
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Author: Eng. Matteo Sporchia
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