Why Sinter Chemistry is Important and How to Get it Right

When sinter chemistry fluctuates, it can significantly impact both the sintering process and the properties of the final product. These fluctuations can lead to variability in sinter quality, necessitate changes in sintering temperature, and affect the mechanical properties of the sintered material. Additionally, they can cause microstructural changes, influence sintering kinetics, introduce environmental and safety concerns, and increase production costs. To mitigate these issues, it is crucial to closely monitor and control the chemical composition of the materials used in the sintering process through regular analysis and strict quality control measures.

Let’s delve more into what sinter actually is, why it is needed, why the chemistry matters, and how that chemistry is analyzed.

What is Sinter?

Sinter is typically made from a mixture of fine particles of various materials that are agglomerated and then heated to form a solid mass. The specific composition of sinter can vary depending on its intended use, but common components include iron ore fines (which are small particles of iron ore that are generated during the mining, crushing, and processing of iron ore.) These fines are typically less than 6.3 mm in size and are too fine to be used directly in a blast furnace for ironmaking. Instead, they need to be agglomerated into larger, more manageable pieces through processes like sintering or pelletizing.

Other primary raw materials include fluxes like limestone and dolomite to adjust the chemical composition and remove impurities, and coke or coal to provide necessary heat and act as a reducing agent. Additionally, recycled materials such as mill scale and blast furnace dust are often included to reduce waste and improve efficiency.

Binders like bentonite may also be added to help the fine particles adhere during the initial stages of sintering. This carefully proportioned mixture is then heated to a temperature below its melting point, causing the particles to bond into a porous, solid mass known as sinter, which is widely used in various applications.

Common Uses of Sinter

In the iron and steel industry, sinter is primarily used as a feedstock for blast furnaces. Iron ore fines, along with fluxes and coke, are agglomerated into sinter to produce pig iron*. Pig iron, produced in a blast furnace, contains about 3.5-4.5% carbon and other elements like silicon, manganese, sulfur, and phosphorus, making it brittle and not directly useful for most applications. It is primarily used as a raw material for steel and cast iron production, where it is refined to reduce carbon content and remove impurities.

Using sinter in blast furnaces improves the efficiency of iron-making by providing a more uniform and permeable burden, enhancing gas flow and heat distribution. Sintering is also widely used in powder metallurgy to produce high-precision metal parts for automotive, aerospace, and machinery industries, essential for manufacturing tooling and machinery parts with specific mechanical properties.

The Importance of Getting the Chemistry of Sinter Right

When sinter chemistry fluctuates, it can have several impacts on the sintering process and the properties of the final product. Here are some key effects:

Variability in Sinter Quality: Fluctuations in the chemical composition of the sinter can lead to inconsistencies in the quality of the sintered product. This includes variations in strength, density, and porosity.
Changes in Sintering Temperature: Different chemical compositions can alter the melting points of the materials being sintered. This can require adjustments to the sintering temperature to achieve optimal results.
Impact on Mechanical Properties: The mechanical properties of the sintered product, such as hardness, tensile strength, and wear resistance, can be affected by changes in the chemistry. This is particularly important in applications where specific mechanical properties are critical.
Microstructural Changes: Fluctuations in chemistry can lead to changes in the microstructure of the sintered material. This includes grain size, phase distribution, and the presence of defects or impurities.
Impact on Sintering Kinetics: The rate of diffusion and the overall kinetics of the sintering process can be influenced by the chemical composition. This can affect the time required to achieve full densification and the overall efficiency of the process.
Environmental and Safety Concerns: Certain chemical fluctuations might introduce hazardous elements or compounds, which could pose environmental or safety risks during the sintering process.
Cost Implications: Variations in the chemistry might necessitate the use of additional or different raw materials, potentially increasing the cost of production.

To mitigate these issues, it is important to closely monitor and control the chemical composition of the materials used in the sintering process. This can involve regular testing and adjustments to the raw material feed, as well as implementing strict quality control measures.

Analyzing Sinter

Throughout the process, technology is utilized to analyze, measure, and control sintering process fluctuations, inhomogeneous mixtures, and other parameters that affect productivity, physical and metallurgical quality, and raw material consumption and costs.

Online Elemental Analyzers provide high frequency online elemental analysis of an entire raw material process stream using Prompt Gamma Neutron Activation Analysis (PGNAA) and Pulsed Fast Thermal Neutron Activation (PFTNA). By measuring the entire sinter feed on a belt conveyor in real time, sampling errors and control information measurement lag are nearly eliminated.

PGNAA and PFTNA are non-contact, non-destructive analytical techniques used in online analysis systems to determine the elemental composition of bulk raw materials. Both of these techniques are known collectively as neutron activation analysis and function by bombarding materials with neutrons.

The neutrons interact with elements in the materials, which then emit secondary, prompt gamma rays that can be measured. Similar to X-ray fluorescence (XRF), each element emits a characteristic energy signature as it returns to a stable state. (Visit this page to learn more about PGNAA and PFTNA technology.)

PGNAA analyzers are situated directly on the conveyor belt and penetrate the entire raw material cross-section, providing minute-by-minute, uniform measurement of the entire material stream, not just the surface or a sub-sample sample. This is the reason PGNAA analyzers are valuable tools for optimizing sinter feed basicity. Surface analysis technologies such as XRF, X-ray diffraction (XRD), and other spectral analysis technologies measure limited depths and surface areas that may never be representative of the entire volume of material on the belt. With PGNAA, sample errors are reduced, and the high-frequency of analysis helps reduce variability in material quality.

Summary

Maintaining consistent sinter chemistry is crucial for optimizing the sintering process and helping to ensure the quality of the final product. Fluctuations in chemical composition can lead to variability in sinter quality, necessitate changes in sintering temperature, and affect mechanical properties. These fluctuations can also cause microstructural changes, influence sintering kinetics, introduce environmental and safety concerns, and increase production costs. To mitigate these issues, it is essential to closely monitor and control the chemical composition of the materials used in sintering through regular testing and strict quality control measures. Advanced analysis technologies, such as PGNAA and PFTNA, play a vital role in providing real-time, accurate measurements of raw material composition, helping to reduce variability and improve overall efficiency in the sintering process.

Additional Resources

Watch the 6-minute Sinter Feed Blending and Analysis Video
Download eBook: Guide to PGNAA and PFTNA technology for non-scientists
Learn more about PGNAA and PFTNA Technology
Visit our center for Improving Steel Manufacturing Processes and Production
*Fun Note: According to the US National Park Service, that metal is called pig iron because the molten iron flowed directly into the “pig bed” in the cast house floor (it looked like a litter of nursing pigs), where it hardened into pig iron ready for market.