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Calcium Carbide for Steel Desulfurization: How Can a Particle Size of 2-10mm Improve Desulfurization Efficiency?
2026-01-13
Seeing slag form on top of molten steel during ladle metallurgy is a beautiful sight that signals progress. It means the synthetic refining layer has formed, and steel cleaning has already begun. However, the thick layer that absorbs impurities and prevents contact between steel and oxygen also acts as a physical barrier, preventing the desulfurizer (Calcium Carbide) from reaching the molten steel below. That is where the particle size of calcium carbide becomes essential!
Removing sulfur from steel is a critical secondary refining step. It ensures that the formed steel is machinable and has minimal impurities. Adding CaC2 is a reasonably mature technique going back to the 1970s. However, the use of small particle size remains mainstream for control.
In this article, we will explore how the calcium carbide size (2-10mm) affects the desulfurization process. Also, analyze its effectiveness, safety, and cost-related aspects. By the end, the reader will have a clear idea of the ideal particle size for their steel desulfurization process. Let us begin with the basics: why sulfur is removed from steel.
Background on Sulfur in Steel
The source of sulfur in steel is typically from the raw materials, such as iron ore, scrap steel, or ferroalloys. Molten metal will have sulfur content from the sources mentioned. It needs to be removed to ensure the steel's mechanical properties remain intact. Typically, a blast furnace output contains 100-800 ppm of sulfur, which needs to be reduced to 35-10 ppm, depending on the process requirements.
The Need for Desulfurization
High-quality steel with low sulfur content is also called “clean steel”. The output of Electric Arc Furnace (EAF) and Basic Oxygen Furnace (BOF) must undergo a secondary metallurgy process to improve their purity levels. Here are the top reasons why steel must undergo desulfurization:
● Hot Shortness: The iron present in steel reacts with sulfur to form FeS. Iron sulfide has a low melting point of 988 °C, which affects the machinability of steel, particularly during hot-rolling and forging. The FeS melts, weakening the steel and making it brittle and prone to cracking.
● Poor Weldability: During welding, the intense heat causes the metal to melt. The good portion solidifies, forming internal tensile stresses. While FeS remains liquid for a period, the stresses become unbearable, leading to a solidification crack or hot tear.
● Corrosion Resistance: Small, deep holes can form on the surface as sulfur reduces the steel's corrosion resistance.
● SO2 Emissions: Removing sulfur helps keep sulfur dioxide emissions within the allowable range. It is key to ensuring compliance with strict air regulations that typically require 20mg/m3
● Recycling Byproduct: Adding CaC2 causes the formation of carbide slag, which is recyclable, ensuring sustainability (Qi et al., 2022)
Science Behind Calcium Carbide Desulfurization
Before diving into the impact of Calcium Carbide (CaC2) particle size on desulfurization, let us understand how the process works. We will cover the impact of calcium carbide size and how it enhances these mechanisms in a later section.
Reaction Mechanism
The core reaction that results in the slug formation on top of the molten steel is:
CaC2 + S → CaS + 2C
The Calcium Carbide reacts with sulfur to form calcium sulfide, releasing carbon into the steel. The CaS is insoluble, so it floats to the surface of the molten steel, which can be conveniently removed after a suitable time.
Effect of Process Parameters
Reagent Quality
● Purity of Reagent: It is simple. A pure CaC2 will result in a stronger reaction. A >63-68% pure calcium carbide and low levels of Si (<2%) and P (<0.02%) ensure maximum reaction and low contamination.
● Reagent Flow Rate: Controlling the addition of reagent is necessary to achieve high removal rates. 200-300g/min is considered ideal.
Chemical Factors
● Deoxidation State: To prevent CaC2 from being consumed by oxygen, the molten steel should be kept in a low-oxygen state.
● Incubation Time: Calcium carbide does not react immediately. It needs an initial incubation period of 20–40 seconds before the reaction kinetics kick in.
● Rate-Limiting Step: The diffusion of sulfur to the CaC2 is the rate-limiting step in the process.
● Temperature: The effect is minimal, with 3% above 100 °C. Therefore, focusing on other areas, such as mixing, is key.
Mechanical Aspects
● Penetration Ratio (β): The efficiency of reaction is dependent on the penetration ratio, which is the gas flow (for stirring) relative to the reagent flow (for injection).
● Process Variant: Choosing between hot-metal pretreatment and Ladle Furnace refining determines the output.
● Mixing/Stirring: The configuration of reagent injection systems, such as dual-lance systems, and the level of inert gas stirring determine the sulfur's contact with calcium carbide.
The Impact of Particle Size 2-10mm on Desulfurization
After learning all about the process of desulfurization and the impacts of various parameters on its efficiency, we can move to the most critical part of our discussion, i.e., the impact of calcium carbide particle size on desulfurization efficiency. In the high-productivity processes, the optimum particle size is 2-10mm.
Kinetics and Sustained Reactivity
We will analyze how increasing the reactive surface area affects the overall reaction kinetics. The calcium carbide must remain stable in the molten metal throughout the process. The 2-10mm range is crucial for efficiency.
● Sustained Reaction Rate
Although reducing calcium carbide to dust for addition to molten steel can lead to the fastest reaction, the effect fades quickly. The 2–10 mm size provides a sustained, effective mass-transfer diameter, ensuring a reliable reaction throughout the entire refining time. (Coudure & Irons, 1994).
● Avoiding Chemical Slowdown
The moderate surface area of the 2–10 mm particles limits the rapid formation of a calcium sulfide (CaS) product shell that "chokes" ultra-fine particles within seconds (Chiang et al., 1990). This slower shell growth ensures the carbide core continues to react effectively, following predictable diffusion kinetics.
Utilization and Physical Control
The physical robustness of 2-10mm calcium carbide particles results in superior material utilization, unlike dust.
High Utilization Rate
Particles in the 2–10 mm range achieve 55–75% reagent utilization versus only 40–55% for <1mm dust. This is because fewer particles are immediately blown out by the carrier gas (Coudure & Irons, 1994).
Excellent Penetration Ratio (β)
At typical injection rates, the penetration ratio β reaches 25–35%—significantly higher than for fine powders. The higher mass of these particles helps them resist being entrained by the gas bubbles and ensures they penetrate deep into the molten metal.
Sufficient Residence Time
Although the residence time in the injection plume is short (0.8–3 seconds), billions of particles are injected, providing sufficient cumulative exposure for 75–90% desulfurization in 180–240 seconds (Guo et al., 2023).
Predictable Results
Tight size control (>90% within specification) ensures repeatable K and β values shift after shift, making the reaction easier to control and the results highly predictable.
Advantages of using 2-10mm Particle Size
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Optimized Reaction Balances surface area for effective kinetics while preventing rapid CaS shell formation, sustaining the reaction longer. |
Better Penetration High penetration ratio (β=25-35%) in pneumatic conveying ensures deep contact, leading to substantial desulfurization gains per reagent addition. |
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Enhanced Mixing Granules enable efficient stirring and circulation with gas bubbling, reducing inert "dead zones" and maximizing sulfur diffusion. |
Higher Safety Significantly lowers the risk of acetylene gas explosion from moisture exposure compared to highly reactive, dusty, fine powders. |
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Process Efficiency Tight size control delivers repeatable rate constants K and penetration, ensuring highly predictable results and process efficiency. |
Ease of Handling Granules are less abrasive, enabling stable, higher flow rates and protecting equipment like injection lances from wear. |
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Reduce Reagent Cost Improved utilization and reduced loss lead to an estimated 25% lower reagent consumption compared to micron-sized powder methods. |
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Case Study: Example of TYWH
The 2-10mm granule size is a successful commercial product, with major producers like TYWH specializing in its production and global export. TYWH has an annual production capacity of 120,000 tons and exports specialized 2-4 mm and 4-7mm grades worldwide. These products feature tight size consistency (>90% qualified) and are packaged securely in N2 drums to maintain quality and safety.
If you are interested in finding more about TYWH, then visit their website!
Conclusion
In a nutshell, we can conclude that optimizing calcium carbide particle size results in high utilization, better penetration, and a sustained optimized reaction for a longer time. It leads to lower reagent cost and higher safety by reducing explosion risks and ensuring predictable results.
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