Practical Tips for Efficient Charge Melting in EAF: Lessons from the Melt shop Floor

This article is part of a series of technical insights into Electric Arc Furnace (EAF) steel making that I’ll be sharing here on LinkedIn. As a steel making process engineer with practical experience in the melt shop, I aim to share the lessons I’ve learned directly from the shop floor. My goal is to exchange ideas, best practices, and knowledge with other industry experts and professionals. In this first article, we’ll explore key tips for efficient charge melting, which can significantly enhance productivity and optimize time utilization within the melt shop.

1. Hot Heel Management

The hot heel, which refers to the amount of molten metal remaining in the furnace after tapping, plays a critical role in the melting process and energy efficiency in Electric Arc Furnaces (EAF).

• For 100% scrap-based furnaces, maintaining a hot heel equal to 10% of the total heat weight is generally sufficient.
• In furnaces where DRI makes up more than 25% of the charge, the hot heel should be increased to 15–25% of the total heat weight.
• A good rule of thumb is to ensure that the hot heel just covers the EBT sand level. Maintaining this balance is key to an efficient melting process, as too much or too little hot heel can have operational drawbacks.
• Avoid Excessive Hot Heel: large hot heel can lead to excessive splashing of molten metal through the furnace door and result in arc instability due to slag loss.

2. Efficient Scrap Selection and Charging

• It’s important to understand the average chemical composition of each type of scrap being in scrap yard. For instance, flat steel requires cleaner scrap, while billet production can tolerate more impurities. To avoid unwanted residual elements, it’s important to understand the average chemical composition.
• Dust, non-metallic substances, explosives, and non-conductive materials are all potential sources of production loss. Removing these materials prior to charging helps avoid downtime, damage to the furnace, and energy waste.
• Poor-quality scrap can significantly affect slag chemistry. Low-grade scrap often contains high levels of Aluminum and Silicon will result in a watery slag that requires additional lime and dolomite to adjust the composition. Percentage of 10 to 14% Al2O3 will impact greatly the fluidity of slag causing very watery slag.
• The scrap mix plays a major role in energy consumption, with an ideal range of energy usage between 330 kWh to 350 kWh per ton charge. A well-chosen scrap mix also supports arc stability, which is crucial for efficient melting.
• The bulk density of the scrap charged into the furnace must be carefully controlled. Too high a density can reduce the melting rate, while too low a density means not enough material is charged. Ideally, bulk density should range between 0.7 and 1 ton/m³ to strike a balance between efficient charging and effective melting.
• The ratio of arc voltage to scrap size is key to effective electrode regulation and arc stability. Larger and heavier scrap pieces tend to cause poorer arc regulation compared to smaller or shredded scrap.
• Scrap loading into the bucket should be optimized by considering both furnace and bucket volume. Choose scrap types that meet the following requirements:

1. Maintain good bulk density for efficient melting.
2. Ensure the right weight of scrap can be loaded.
3. Arrange the scrap types in layers for optimal melting performance.
4. Avoid overloading that could obstruct the furnace roof from closing.

3. Scrap Layering Strategy

Proper layering of scrap inside the bucket is essential for furnace performance:

• Bottom Layer: Light scrap to protect the furnace bottom.
• Next Layer: Heavy scrap directly under the arc for efficient melting in the heat spot.
• Carbon Addition: Add carbon if needed at this stage.
• Bulk Layer: Medium scrap should make up the main bulk of the load.
• Top Layer: Shredded scrap should be placed at the top to ensure easy boring. Avoid placing heavy pieces here to protect the furnace roof and shell panels.
• Avoid Over-Length Scrap: Over-length scrap can create voids that cause scrap collapse during arcing, leading to electrode damage.
• Lime and Dolomite: Placed along the sides of the bucket to avoid contact with the electrodes.

4. Furnace Charging

One of the most crucial steps in Electric Arc Furnace (EAF) operations is the proper charging of scrap into the furnace. Both the crane operator and the signalman must work in coordination to ensure that the bucket is precisely centered above the furnace before releasing the scrap.

1. The bucket must be carefully centered, with the opening positioned about 1 meter above the furnace shell before releasing the clamps. Proper positioning ensures that the scrap falls directly into the furnace without disrupting the carefully arranged layers of materials.
2. The process of releasing the scrap should be quick and done with precision. Misalignment or slow charging can disrupt the scrap’s layering inside the furnace.
3. Off-center charging can cause the scrap to accumulate unevenly in one area of the furnace. This imbalance can lead to several operational issues, such as:

• Electrode Breakage: Scrap accumulating on one side can cause uneven load distribution, increasing the risk of electrode breakage.
• Uneven Melting: Off-center scrap piles lead to uneven melting, reducing efficiency and extending the melting cycle.
• Damage to Equipment: Scrap spilling outside the furnace shell can damage vital components like hoses, pipes, and gantry bearing.

5. Boring Phase Tips

The boring phase is a critical step in Electric Arc Furnace (EAF) operations, where electrodes descend into the scrap to initiate the melting process:

1. Boring begins by lowering the electrodes 1 to 3 meters into the scrap. Light scrap on the top layer facilitates easier penetration.
2. The initial arc should be applied with low power and a short arc length. This minimizes the risk of arc deflection, which can lead to roof or panel leakages.
3. Once the electrodes have descended slightly into the scrap, the arc voltage should be gradually increased to lengthen the arc. This increase creates a larger boring pit, which is crucial for effective melting. However, during this phase, there’s a higher risk of short circuits caused by scrap falling under the electrodes. A larger boring pit ensures safe electrode operation, reducing the likelihood of such short circuits.
4. The diameter of the boring pit is vital for optimal performance. It is directly related to arc length and electrode tip diameter.
5. Feeding the furnace with carbon through the 5th hole during the boring phase helps stabilize the arc and supports early foaming slag formation.

6. Main Scrap Melting Phase

Once the electrodes reach their lowest position, the main scrap melting phase begins:

1. At this point, maintaining maximum power and a longer arc length is essential for melting the scrap effectively. A longer arc helps ensure that the scrap surrounding the boring pit melts quickly, while also reducing the risk of scrap pieces falling under the electrodes, which could cause short circuits causing electrode breakages.
2. Building up a good slag layer early in this phase is important to enhance furnace efficiency. By quickly melting the slag builders (like lime and dolomite), you can achieve slag foaming, which improves arc coverage and reduces energy losses.
3. Feeding dolomite especially into the furnace early during this phase is crucial for rapidly reaching slag saturation.
4. As the main melting phase progresses, operators can monitor the sound of the furnace and the behavior of the cables. A steady decrease in furnace noise, along with stable cables and a fully covered arc, indicates that the scrap has been fully melted.
5. As melting nears completion, it is important to reduce the arc length while maintaining moderate power to protect the furnace walls and panels from excessive heat and wear. However, if the water-cooling circuit is functioning properly with no signs of overheating, and the arc is well-covered by slag, higher power can still be used without concern for furnace damage.

7. Regulation Control Strategy for Efficient Melting

Effective regulation control is crucial for maintaining stable furnace operation and optimizing energy consumption during the scrap melting process.

1. During the scrap melting phase, impedance regulation is typically the most efficient control strategy. Impedance regulation adjusts the arc length automatically to maintain consistent power delivery, which is ideal for the varying resistance as scrap melts and the arc position changes.

2. Once the furnace reaches a flat bath condition or when melting Direct Reduced Iron (DRI), current regulation becomes more effective to achieve smooth heating, current regulation ensures steady melting and prevents overloading.

8. Burner, Oxygen Lancing and Carbon Injection Utilization

• Gas burners, which mix natural gas and oxygen, play a critical role during the initial stages of scrap melting in Electric Arc Furnaces (EAF). Their primary purpose is to preheat the scrap, making it easier for the oxygen lancing process to cut through the scrap efficiently. Once 60% of the scrap charge has melted, the effectiveness of gas burners significantly decreases. Continuing to use burners after this point leads to wasted energy and time.
• Burners should be used when the furnace is already in a hot state, maximizing their efficiency. The gas-to-oxygen ratio should be carefully controlled, starting at 1:1.8 and gradually increasing to 1:2.4 as the process progresses.
• After the gas burners have completed their job in preheating the scrap, oxygen lancing takes over to cut through the remaining scrap, speeding up the melting process. Initially, when oxygen lancing begins, it’s important to use a low flow rate. Starting with too high a flow can cause flashbacks which can cause severe leakage of modules.
• As the scrap melting progresses and the furnace reaches flat bath conditions, you can begin to increase the oxygen flow rate to its optimum level. At this stage, higher flow rates are more effective.
• It is important to calculate all carbon entering the bath including carbon in scrap, hot heel, DRI and carbon charged from fifth hole to balance the injected oxygen.
• Based on the calculated carbon entering the bath and calculated oxygen, you can calculate the required carbon injection flow rate entering the bath.
• Carbon injection shall be enough to keep bath foaming to enhance arc coverage. Carbon injection flow rate shall be within limits. Too high flow rate will cause excessive foaming, slag getting out the door and arc instability due to bath boiling. Also too little flow rate will not be enough for keeping slag foaming.
• Furnace operator shall follow up the sound of the furnace. If the sound is getting high indicating the arc is uncovered and carbon injection needs to be increased or supporting with carbon from fifth hole with moderate flow rate.

9. DRI Melting Tips

In Electric Arc Furnaces (EAF), Direct Reduced Iron (DRI) can be charged in two ways: either in buckets, limited to 25% of the bucket weight, or through continuous charging via the fifth hole. For furnaces that use 20% to 100% continuous feeding of DRI, the following tips are important:

1. DRI quality must be monitored regularly, as changes in its composition or size can significantly impact the melting process. Despite the advantage of a known chemical composition, variations in quality must be considered.
2. Size: Optimal DRI size is between 9 mm and 16 mm. Particles below 5 mm are considered fines and can negatively affect the process.
3. Gangue Content: This should be kept below 4%, as higher gangue levels increase slag formation and reduce yield. Higher silica in the slag means more lime and dolomite are required for adjustments.
4. Metalization: Monitoring FeO levels is important, as higher FeO means increased energy consumption during melting.
5. Carbon Content: DRI carbon levels are critical. Carbon content between 2% and 2.5% is beneficial for the melting process as it contributes to energy savings and slag foaming.
6. Process engineers must regularly check the DRI analysis, including size, gangue, metalization, and carbon content. This allows for fine-tuning of the EAF melting program to ensure optimal performance and efficiency.
7. DRI requires a hot, well-mixed slag for effective melting. It's crucial not to start feeding DRI into a cold bath, as this will reduce melting efficiency and delay the process.
8. DRI fines tend to float on top of the slag layer and can be lost through the fume extraction system, leading to material loss. Moreover, these fines can disturb arc stability and affect electrode positioning, causing unwanted voltage spikes.
9. The DRI feeding index, expressed as Kg/Min/MW, is a key factor for efficient DRI melting. This index must take into account the total energy in the bath, including both electrical and chemical energy.
10. The best practice is to start with a low feeding index (13–16 Kg/Min/MW) during the initial stages to heat the bath and complete the scrap melting. Once the bath is ready, increase to a moderate index (30–40 Kg/Min/MW) to maintain bath temperature. It's essential to avoid a high feeding index that surpasses the power input capacity, as this can lead to icebergs—large, unmelted DRI masses that disrupt the melting process.
11. High DRI feeding rates can severely affect arc stability.
12. DRI feeding typically starts after 60% to 70% of scrap melting has been completed. However, in some cases, starting earlier can be beneficial for establishing arc stability sooner.
13. Maintaining an adequate hot heel, typically 25% of the heat size, is essential for efficient DRI melting.
14. Regarding DRI spot inside the furnace, it has to be within the pitch diameter as this the most heating spot.

10. De-slagging Time & Bath Sampling

• Delaying deslagging is important to maintain a stable and effective slag layer inside the furnace. Early deslagging is not recommended because it can disrupt several critical factors like slag foaming, heating rate of the bath and DRI melting.
• Keeping the bath temperature around 1560 to 1580 deg C is very important for good phosphorus removal, good slag viscosity, good slag foaming and avoiding loading the furnace thermally. Working with temperatures higher than 1600 deg C will cause high thermal load on EAF refractory and furnace water cooling circuit.
• Meltdown carbon% has to be limited to 0.06 to 0.08%. Percentages lower than that indicating bath over oxidizing which is not good from yield point of view.

11. General Tips

1. Toward the end of the heat, slag foaming often decreases, leading to high noise levels and arc instability. To avoid this: Increase lime and dolomite flow rate/addition. Increase carbon injection while keeping oxygen flow steady to avoid power loss.
2. It's important to avoid overheating the bath. The ideal tapping temperature should range between 1640°C and 1660°C to prevent bath overheating.
3. If icebergs (unmelted DRI or scrap) form in the furnace, perform small tilting cycles. This will help the skulls melt effectively.
4. In case of a bath reaction, take the following steps: 1. Shut off oxygen injection. 2. Reduce Power. 3. Open slag door.
5. Never work with short electrodes, as they can reduce the ability to maintain optimal arc length and power control.
6. Monitor the performance of electrode regulation to ensure the electrodes can maintain set points and function efficiently.
7. Monitor the spray cooling system, particularly during scrap melting. Excessive cooling can affect electrode regulation.

12. Conclusion

The key parameter for the success of any melt shop is the standardization of processes. Consistency is crucial: using the same charge mix, following the same practice, and adhering to the same melting program ensures predictable and optimized outcomes. Beyond process standardization, the furnace lifetime, including the condition of the hearth and walls, greatly influences process parameters.

Each phase of the EAF’s life requires careful planning—adjusting scrap charge weight, oxygen utilization, power usage, and more. The ability to adapt the process as the furnace ages is vital to maintaining operational efficiency and ensuring longevity of equipment

Thank you for taking the time to read this article. I hope you found the tips useful for improving your EAF operations. I would love to hear your thoughts and experiences—feel free to share your insights or ask any questions in the comments. I'm always open to feedback and discussions. Let's exchange ideas and grow together.