Understanding Coring in Iron Ore Pellets: Causes, Effects, and Solutions

FeO (ferrous oxide) and magnetite (Fe3​O4​) are closely related because magnetite contains FeO as a component in its molecular structure. Magnetite is a mixed oxide of iron, chemically represented as FeO·Fe2​O3​ .

Relationship Between FeO and Magnetite

1. FeO Content in Magnetite:

   • The FeO component constitutes approximately 31% by weight of magnetite. This value comes from the molar masses of FeO and Fe3​O4​.

   • Magnetite’s overall Fe content is about 72.4%, of which FeO contributes to a portion through the ferrous state.

2. Impact of Magnetite Oxidation:

   • When magnetite (Fe3O4​) oxidizes to hematite (Fe2O3​), the FeO component is removed: 4Fe3​O4​+O2​→6Fe2​O3​

   • The FeO percentage decreases as magnetite transforms into hematite.

3. Unoxidized Magnetite and FeO Percentage:

   • Incomplete oxidation of magnetite during induration leads to residual FeO in the final pellet. Higher FeO percentages indicate that some magnetite has not fully oxidized.

   • Residual FeO is often used as a metric to evaluate the oxidation efficiency of iron ore pellets during their production.

What is Coring in Iron Ore Pellets?

Coring in iron ore pellets is a common issue that significantly impacts pellet quality during the induration process. It occurs when incomplete oxidation takes place in the pellet’s core, leaving magnetite (Fe₃O₄) at the center, while the outer layers convert to hematite (Fe₂O₃). This results in a less efficient iron production process and can affect the stability of the pellets. Let’s break down the causes of coring and explore methods to control it, with a focus on the chemical reactions involved.

What Causes Coring?

1. Excess Carbon Addition (Coke)

   • Adding excess carbon in the green ball promotes the formation of reducing gases like CO during induration. This leads to localized reducing conditions in the pellet core, forming wustite (FeO) instead of allowing complete oxidation to hematite (Fe₂O₃). The reaction is:
     Fe2​O3​+CO→2FeO+CO2​

2. Limited Oxygen Availability

   • Chemical Reaction: The key chemical reaction during pellet induration is the oxidation of magnetite (Fe₃O₄) to hematite (Fe₂O₃):

     4Fe3O4+O2→6Fe2O3

     Oxygen must penetrate the pellet's structure to complete this reaction. However, in the pellet’s core, oxygen flow can be restricted, preventing the full oxidation of magnetite. Poor airflow distribution in the furnace, insufficient oxygen in the atmosphere, and low residence time for the pellets exacerbate this issue, leaving unoxidized magnetite in the center.

3. High Pellet Density

   • Chemical Impact: Pellets with high density and low porosity reduce the ability of oxygen to diffuse into the core. High-Feo% pellets are often formed when the green ball size is too large or when the pellet mix contains a higher percentage of ultrafines (very fine particles). This denser structure makes it more difficult for oxygen to penetrate, causing the core to remain in its magnetite form.

   • Green Ball Size & Chemical Influence: Larger green balls with low porosity pellets that are harder for oxygen to penetrate up to center, allowing the core to remain unoxidized. The Blaine number, a measure of particle fineness, also plays a role. Higher Blaine numbers (finer particles) can increase pellet density and reduce porosity, which restricts the oxygen's ability to oxidize magnetite in the core.

   • High Negative Pressure in Windboxes and Machine Bed Height:
     Higher negative pressure in windboxes compacts the green ball structure in the bottom layer of pellets. Less compact pellets, with more internal voids, allow better oxygen penetration during the oxidation process. Similarly, an increase in bed height exerts more pressure on the bottom-layer pellets, influencing their structural integrity.

4. Temperature Variations

   • Chemical Reaction Considerations: Non-uniform furnace temperatures can lead to incomplete oxidation of the core. In areas of the pellet that are not exposed to sufficient heat like [ bottom layer pellets, Pellets near to the side hearthlayer or pallet car having non uniform bed height], the oxidation reaction slows down or even reverses. For instance, in reducing zones of the furnace, gases like carbon monoxide (CO) may cause a reduction reaction, converting hematite back into magnetite (Fe₂O₃ → Fe₃O₄). This can prevent the complete oxidation process and contribute to coring.

   • Reducing Gas Reactions: The presence of CO gases in the furnace can reverse the oxidation process. In these reducing zones, the following reaction may occur:

     Fe2O3+CO→2FeO+CO2

     This reduction reaction converts hematite back to magnetite or iron oxide, contributing to the core's persistence in its unoxidized state.

How to Control Coring in Iron Ore Pellets

Uniform Coke addition

• Cause: Probable reasons for excess carbon addition include:

  1. Flushing of Ground Coke and Lime: Spillage or overflow during transfer through loss-in-weight (LIW) feeders.

  2. Wrong Calibration of LIW and Filter Cake Weighfeeder: Inaccurate calibration causes deviations in the intended coke-to-ore ratio.

  3. Nonuniform Mixing in the Mixer: Improper blending leads to uneven distribution of coke across the batch, with some pellets containing higher concentrations of carbon.

• Solution:

  • Frequently calibrate loss-in-weight and filter cake weighfeeders to ensure the correct dosage of coke and lime.

  • Implement logics to reduce excessive flushing or overflow. Deploy AI/ML models to predict flusing in advance.

  • Conduct regular maintenance of the mixer to prevent material segregation.

  • Implement mixer cleang logic.

Optimize Pellet Porosity

• Cause: High-density pellets restrict oxygen flow.

• Solution:

  • Increase porosity by controlling the Blaine number. Monitor grinding and ultrafines production in the beneficiation plant to avoid excessive fines, which can lead to denser pellets. Avoid regrinding unless necessary to maintain optimal particle size distribution.

  • Optimize the addition of binders and additives in the pellet plant.

  • Use coke with low fixed carbon content for increased porosity. Higher coke addition creates more pore volume during combustion, enhancing oxygen penetration.

  • Incorporate finer binders and additives that increase pore density while maintaining the same pore volume, ensuring better oxygen diffusion.

  • Adjust green ball size to balance porosity. Larger green balls can have reduced oxygen penetration due to the longer diffusion path, leading to a more significant core. Optimize the size to ensure better air permeability without compromising pellet strength.

Ensure Proper Airflow

• Cause: Limited oxygen availability.

• Solution:

  • Ensure sufficient cooling air and adequate airflow in the oxidation zone.

  • Adjust the firing zone burners by turning off the last burners to extend the after-firing zone. This increases the length of the cooling and oxidation zone, providing more time for oxygen to penetrate the pellets.

  • Lower the temperature in the after-firing zone to increase air density, allowing for higher oxygen availability. Lower temperatures ensure that the air in this zone carries more oxygen compared to the hotter firing zone, facilitating oxidation in the pellet core.

  • Maintain airflow distribution across the furnace bed to minimize dead zones where oxygen diffusion may be insufficient.

Optimize Residence Time and Temperature

• Cause: Short residence time and non-uniform furnace temperatures.

• Solution:

  • Extend the residence time of pellets in the oxidation zone to ensure that oxidation reactions proceed uniformly throughout the pellet layers.

  • Maintain a consistent temperature profile within the furnace to prevent uneven oxidation. Avoid excessive heating that can lead to sintering of the pellet surface, reducing porosity.

  • Avoid temperature drops in the firing zone that may cause incomplete oxidation or even reversal of oxidation due to the presence of reducing gases like CO .

  • Use real-time temperature monitoring and adjust airflow and burner controls to ensure that all parts of the furnace maintain the desired temperature range.

Minimize Reducing Gas Formation

• Cause: Reducing gases like CO reverse oxidation.

• Solution:

  • Maintain an oxidizing atmosphere throughout the furnace by carefully adjusting the fuel-air ratio. Excess fuel without sufficient air can produce CO gas, leading to reducing zones that hinder oxidation.

  • Ensure complete combustion in the firing zone to reduce the formation of CO gas.

  • Avoid introducing excessive fuel into areas where oxygen levels are already low, such as the Firing zone,after-firing zone, to prevent the buildup of reducing gases.

  • Regularly monitor gas composition in the furnace using gas analyzers to detect reducing gases and adjust the air and fuel inputs in real time.

By optimizing green ball size, airflow, temperature, residence time, and fuel-air ratios, manufacturers can effectively increase porosity, ensure complete oxidation, and reduce the occurrence of coring in iron ore pellets. These measures, combined with advanced monitoring and process control, lead to higher-quality pellets and improved furnace efficiency.

Leveraging Technology to Address Coring

Modern technology and data science provide powerful tools for detecting and mitigating coring in iron ore pellets.

• Real-Time Monitoring Systems: Using advanced sensors such as oxygen sensors and gas analyzers, pellet producers can monitor furnace conditions in real time. These sensors help track the oxygen content, CO and CO₂ levels, and other critical parameters, allowing for immediate adjustments to furnace conditions. This ensures the pellet's core undergoes complete oxidation.

• Data Science and Predictive Modeling: By analyzing historical furnace data, manufacturers can use machine learning models and heatmaps to predict which areas of the furnace are prone to coring. Predictive models can also optimize furnace settings such as temperature, airflow, and residence time based on past trends, making the process more efficient.

• Automated Process Controls: Integrating AI-based controllers with real-time sensor data allows dynamic adjustments to the furnace parameters, ensuring that the oxidation process remains consistent and that coring is minimized.

• Predict coke flushing: Data science techniques combined with IoT-based sensors can proactively detect and predict coke flushing events by analyzing critical parameters such as:

  • Bag Filter Differential Pressure (DP): Unusual increases or fluctuations in bag filter DP can indicate excessive coke flushing or clogging.

  • Pumping Pressure: Variations in pumping pressure during material transfer provide early warnings of irregular coke flow.

  • Loss-In-Weight (LIW) Feeder Data: Real-time tracking of weight loss in feeders helps detect inconsistencies in coke addition.

Conclusion

Coring in iron ore pellets is a complex issue that stems from factors like limited oxygen availability, high pellet density, temperature variations, and the presence of reducing gases. By understanding the chemical reactions that drive the oxidation process and implementing measures to optimize pellet porosity, airflow, residence time, and temperature, pellet manufacturers can significantly reduce or eliminate coring. Furthermore, embracing advanced technologies like IoT and data science can help monitor and control the conditions that lead to coring, ensuring optimal pellet quality and a more efficient iron production process.