Understanding the Direct Reduced Iron (DRI) Process

With world steel demand projected to grow, the Direct Reduced Iron (DRI) process has emerged as a crucial alternative to traditional blast furnace methods, offering significant environmental and efficiency benefits. This innovative technology allows for the creation of high-quality iron without melting the ore, directly impacting the decarbonization of the steel industry.

Quick Summary

The DRI process produces solid metallic iron by removing oxygen from iron ore at temperatures below melting point, typically using natural gas or coal as a reductant. This technique yields sponge iron, a high-purity feedstock for steel production that reduces energy consumption and emissions compared to conventional ironmaking.

Key Points

• Core Principle: The DRI process produces solid iron from ore by removing oxygen at a temperature below the iron's melting point, creating a porous 'sponge iron'.

• Key Difference: Unlike the traditional blast furnace, DRI does not require melting the ore, leading to a more energy-efficient and cleaner process.

• Two Primary Methods: The process can be gas-based (using natural gas or hydrogen) or coal-based (using solid reductants), depending on resource availability.

• Environmental Advantage: When using natural gas or especially green hydrogen, DRI production significantly reduces carbon emissions compared to conventional coke-dependent methods.

• High-Quality Product: DRI is a superior feedstock for steelmaking due to its high iron content and low levels of impurities, resulting in higher-grade finished steel.

• Processing and Use: The 'sponge iron' is often compacted into Hot Briquetted Iron (HBI) for safe transport and is used primarily in Electric Arc Furnaces (EAFs).

What is the DRI Process?

The Direct Reduced Iron (DRI) process, also known as direct reduction, is a method of producing solid iron from iron ore without melting it. The product, called 'sponge iron' due to its porous structure, is a highly metallized material used primarily as a feedstock for electric arc furnace (EAF) steelmaking. The core principle involves heating iron ore to a high temperature, typically between 800°C and 1,200°C, in the presence of a reducing agent that strips oxygen from the ore. This solid-state reduction is fundamentally different from the traditional blast furnace process, which produces liquid iron or 'hot metal'.

The DRI process is considered more environmentally friendly than blast furnace operations, especially when using natural gas or green hydrogen as the reducing agent, as it can significantly reduce carbon emissions. The resulting direct reduced iron has a high iron content and very low levels of residual elements like sulfur and phosphorus, making it a high-quality material for producing specialty steels.

The Chemistry of Reduction

At a chemical level, the DRI process removes oxygen from iron oxide compounds. The main raw material is typically hematite ($Fe_2O_3$) or magnetite ($Fe_3O_4$). The process can be driven by different reducing gases, primarily carbon monoxide ($CO$) and hydrogen ($H_2$).

The primary chemical reactions involved in reduction using a combination of $CO$ and $H_2$ are:

• Initial Reduction: $3Fe_2O_3 + CO \rightarrow 2Fe_3O_4 + CO_2$ (Hematite to Magnetite)
• Intermediate Reduction: $Fe_3O_4 + CO \rightarrow 3FeO + CO_2$ (Magnetite to Wustite)
• Final Reduction: $FeO + CO \rightarrow Fe + CO_2$ (Wustite to Iron)
• Hydrogen Reduction: $FeO + H_2 \rightarrow Fe + H_2O$ (Wustite to Iron)

In gas-based processes, a reformer creates the hydrogen and carbon monoxide from natural gas, which is then fed into the reduction furnace. The end result is solid, metallic iron and gaseous byproducts, such as carbon dioxide and water vapor.

Key Methods of the DRI Process

The DRI process can be categorized into two main types based on the type of reducing agent used: gas-based and coal-based.

Gas-Based Reduction: Midrex and Hylsa

Gas-based methods are the most prevalent globally, used in regions with abundant and affordable natural gas supplies.

Midrex Process The Midrex process is the leading gas-based DRI technology, accounting for a significant portion of worldwide production. It is a continuous, counter-current process that uses a vertical shaft furnace.

Steps include:

1. Feed preparation: Iron ore pellets or lump ore are fed into the top of the shaft furnace.
2. Gas reforming: Natural gas is catalytically reformed into a reducing gas rich in hydrogen and carbon monoxide.
3. Reduction: The hot reducing gas ascends through the furnace, removing oxygen from the iron ore.
4. Cooling: Reduced iron is cooled by an inert gas in the lower portion of the furnace.
5. Discharge: The resulting DRI is discharged and can be further processed into different forms.

Hylsa (HYL III) Process The Hylsa process uses similar gas-based principles but operates in a batch or semi-continuous mode within one or more reactors. This approach allows for staged reduction and regeneration of the reducing gas.

Coal-Based Reduction: Rotary Kiln and Rotary Hearth

In areas where natural gas is scarce or expensive, solid reductants like coal are used. These methods are often carried out in rotary kilns or rotary hearth furnaces.

• Rotary Kiln: Iron ore and non-coking coal are fed into a rotating, slightly inclined furnace. The heat and tumbling action facilitate the reduction process as the materials move through.
• Rotary Hearth Furnace (RHF): The Fastmet process is a notable example using an RHF, where a layer of iron ore pellets and coal is placed on a rotating hearth. High temperatures accelerate the reduction process.

Benefits and Challenges of the DRI Process

Advantages

• Lower Emissions: Gas-based DRI reduces CO2 emissions significantly compared to traditional blast furnaces, with hydrogen-based methods offering near-zero carbon steelmaking.
• Energy Efficiency: The process is more energy-efficient per ton of iron produced than the blast furnace route.
• High Purity Product: DRI has low levels of tramp elements (impurities) like copper, allowing for the production of high-grade steel.
• Flexibility: DRI can be used in different steelmaking furnaces (EAF, BF, BOF) and can be hot-charged directly into an EAF for additional energy savings.
• Product Consistency: The uniform composition of DRI ensures predictable performance and steel quality.

Challenges

• Raw Material Requirements: The process typically requires high-grade iron ore pellets or lump ore, and a reliable supply is crucial.
• Energy Dependency: Gas-based plants are tied to the availability and cost of natural gas, while coal-based methods require specific coal quality to manage sulfur content.
• Product Handling: Standard DRI (Cold DRI) is pyrophoric and susceptible to reoxidation, requiring careful handling and storage. Compacting into hot-briquetted iron (HBI) addresses this issue.
• Technical Complexity: The technology, especially for gas-based systems, demands advanced equipment and expertise.
• Logistical Issues: The transport of DRI and its raw materials can pose logistical and storage challenges.

DRI vs. Traditional Blast Furnace

Feature	Direct Reduced Iron (DRI) Process	Traditional Blast Furnace (BF) Process
Temperature	Below melting point of iron (solid-state reduction)	Above melting point of iron (liquid phase)
Primary Reductant	Natural gas, hydrogen, or non-coking coal	Coking coal to produce coke
Product Form	Solid 'sponge iron' (DRI, HBI)	Liquid 'hot metal' or pig iron
Energy Source	Natural gas, electricity, coal	Primarily metallurgical coke
CO2 Emissions	Significantly lower, especially with gas/hydrogen	Substantially higher due to coke use
Capital Investment	Often lower for DRI plants	Very high due to need for coke ovens and larger scale
Scale	Economic at smaller scales ('mini-mills')	Best suited for large-scale, integrated steel plants
Product Purity	High iron content, low impurities	Lower purity, requires more refining

Conclusion

The Direct Reduced Iron (DRI) process has cemented its place as a viable and increasingly preferred method for modern steelmaking, offering significant advancements in both environmental performance and product quality. By forgoing the need for high-emission coke and using cleaner reductants like natural gas and hydrogen, DRI production is a key component in the steel industry's global push towards decarbonization. The technological flexibility of DRI, encompassing both gas and coal-based methods, allows it to be adopted in various contexts depending on regional resources. While challenges related to raw material quality, handling, and energy supply exist, continued innovation promises to mitigate these issues. As the world seeks more sustainable industrial practices, the DRI process stands out as a critical and evolving technology for a greener steelmaking future. For more details on DRI technology, the MIDREX website provides comprehensive resources.

Frequently Asked Questions

What is the primary product of the DRI process?

The primary product of the DRI process is solid metallic iron, commonly known as sponge iron. This product has a porous, sponge-like structure because oxygen is removed from the iron ore without melting.

How is the DRI process different from a traditional blast furnace?

The main difference is that the DRI process reduces iron ore in its solid state at temperatures below the iron melting point, while a blast furnace melts the iron ore to produce liquid hot metal. DRI is also more energy-efficient and can have a lower carbon footprint.

What are the two main types of DRI processes?

The two main types are gas-based and coal-based. Gas-based processes predominantly use natural gas or hydrogen in a shaft furnace, while coal-based methods use solid carbon reductants in equipment like rotary kilns.

What is HBI and why is it important for DRI?

HBI, or Hot Briquetted Iron, is a denser, compacted form of DRI. This briquetting process makes the product much safer for shipping and handling, as it is less susceptible to reoxidation and pyrophoric tendencies than standard sponge iron.

What role does hydrogen play in the DRI process?

Hydrogen is an increasingly important reducing agent in the DRI process, offering the potential for near-zero carbon emissions, as the byproduct of the reduction is water rather than carbon dioxide. This makes hydrogen-based DRI a key technology for green steelmaking.

Are there any challenges with using the DRI process?

Yes, challenges include the need for high-quality iron ore, logistical issues with handling and storing pyrophoric DRI, and reliance on potentially volatile natural gas prices for many current installations.

What is the main use of Direct Reduced Iron?

DRI is primarily used as a high-quality, low-impurity feedstock for electric arc furnace (EAF) steelmaking. It can be used to dilute the tramp elements present in steel scrap to produce a better grade of steel.