OES Slag Analysis: Off-Line, At-Line, or In-Line - A practical guide

Slag Analysis – Off-Line, At-Line, In-Line: A Guideline and Decision Support

Slag composition plays a key role in understanding and optimizing the steelmaking process. Especially in today's context of fluctuating raw material streams, increased use of DRI and recycled materials, and ongoing pressure to improve efficiency and product quality, the importance of reliable, fast, and easy slag analysis is growing rapidly.

Historically, slag analysis was a post-mortem quality control step—useful, but slow. Nowadays, it is becoming a real-time decision-making tool. In particular, it complements the Spark-OES analysis of steel samples and adds valuable information for process control. This evolution has brought new options for how and where to conduct slag analysis: off-line, at-line, or in-line.

Off-Line Slag Analysis: Proven, Precise – But Not Always the Fastest

Off-line slag analysis has long been the standard method in the steel industry, especially for operations that prioritize accuracy and consistency. In this setup, slag samples are sent to a central laboratory where they are analyzed under ideal conditions—stable temperatures, low dust, and minimal vibration. This environment ensures reliable results using centralized equipment operated by trained lab technicians. With only one or two high-quality instruments needed, redundancy is easy to achieve, and space constraints are rarely a problem in dedicated lab facilities.

However, this approach also introduces delays. Sample transport takes time and adds logistical complexity and cost—especially when vacuum pipe systems are involved. Additionally, the furnace team may feel less responsible for sampling quality, since the analysis is handled remotely. Despite these challenges, off-line slag analysis remains a solid choice for plants that value detailed results and already have the lab infrastructure in place.

Advantages:

• High-quality analysis due to stable lab conditions (temperature, dust, vibration).
• Centralized equipment: only one analyzer and one sample preparation unit needed.
• Skilled laboratory staff ensure consistent results and proper maintenance.
• Easier to implement redundancy (e.g., a 2^nd device for backup).
• Space for installation is rarely an issue.

Disadvantages:

• Time delay due to transport of samples, which adds cost and complexity.
• Less operator ownership over sampling quality and timing.
• Vacuum transport systems (if used) require significant investment and maintenance.

For reference: Modern OES slag analysis can not only improve at-line analysis but also significantly accelerate off-line analysis, potentially combining the best of both worlds.

At-Line rapid slag analysis with the QLX9 in the Ladle Furnace Control room

At-Line Slag analysis (next to furnace) with the QLX9 in combination with the Heavy Duty Option (Temperature and Dust protection)

Off-Line slag analysis in a central laboratory (with a Laser OES slag analyzer)

EAF-slag ready for analysis in a Laser OES slag analyzer

Starting a Laser OES slag analysis

At-Line Slag Analysis: Speed Meets Practical Process Control

At-line slag analysis (analyzer close to the furnace, e.g. in the control room) is gaining popularity as steel producers seek faster, more efficient feedback from their processes. By placing the analyzer close to the furnace, analysis times are dramatically reduced—often to less than one minute when using modern laser OES systems. This minimal delay between sampling and result allows for near real-time adjustments, enhancing process control and helping operators respond to changes more effectively.

Unlike off-line systems, there’s no need for expensive and complex vacuum transport systems. Plus, since the analysis happens close to the source, the furnace team naturally takes more ownership of the results and the sampling quality. However, this setup isn’t without its challenges. Harsh furnace-side environments with dust, heat, and vibration can affect sensitive equipment. Control room space may be limited, making placement and integration more difficult. In addition, although operation is performed by production staff, the system still relies on lab-trained personnel for maintenance and troubleshooting. For operations with several distant sampling points, multiple analyzers might be required, increasing the initial investment.

Despite these limitations, at-line slag analysis offers a compelling balance of speed, accuracy, and cost—especially for plants seeking to modernize without the high barriers of in-line systems.

Advantages:

• Faster results: minimal transport time, almost real-time feedback.
• No need for costly vacuum transport systems.
• Improved process control by furnace team, as responsibility stays near the source.
• With modern laser OES slag analyzers, total analysis time can be under one minute.

Disadvantages:

• Harsh conditions (dust, vibration, heat) can affect sensitive instruments.
• Control room space may be limited, making installation more complex.
• Maintenance still requires laboratory know-how, while daily use is done by process personnel, potentially impacting consistency.
• If multiple sampling points are too far apart, more analyzers may be needed.

In-Line Slag Analysis: A Vision That Hasn’t Delivered

In-line slag analysis is often described as the ultimate goal in steel process control: no manual sampling, real-time data, and fully automated operation. On paper, it promises a future of manless furnaces and seamless process monitoring. But in reality, this approach has not yet proven itself viable for industrial use.

Despite multiple approached througout the year, in-line systems remain expensive to purchase, complex to operate, and costly to maintain. What’s more, they still require regular calibration and validation through traditional laboratory methods. Even so-called “real-time” systems need 30 seconds or more per measurement due to signal integration, purging, and positioning—hardly instant.

The biggest limitation, however, is in the results: current in-line technologies only deliver limited trend information, not full chemical analysis. As a result, they are more suited for academic studies or niche R&D setups than for daily operations in a real steel mill.

While the idea of in-line slag analysis is attractive from a theoretical point of view, the actual benefit compared to modern at-line systems is minimal—especially when weighed against the high investment and complexity. For now, in-line analysis remains more of a vision than a practical solution.

Advantages:

• Theoretical ideal: no sampling, no delay, fully automated—supporting the vision of a manless furnace.
• Real-time trend detection possible.

Disadvantages:

• Not yet ready for wide industrial use.
• Very high purchase and maintenance costs.
• Still requires calibration and reference analysis.
• Analysis times are not truly instant—sensor prep, signal integration, purging, etc., still take 30 seconds or more.
• Only limited elemental information available; trend monitoring rather than full composition analysis.

Summary and Recommendation

In the past, off-line slag analysis was the only viable option, especially due to complex sample preparation requirements for XRF. In-line techniques are being explored at a research level but are not yet mature for broad industrial use.

Modern developments—particularly laser-based OES systems—have dramatically improved the speed and ease of slag analysis. As a result, at-line analysis is becoming increasingly attractive, providing a strong balance between speed, cost, and practicality.

However, off-line solutions remain necessary in many cases:

• Where existing lab infrastructure and transport systems are already in place.
• When operator time or space on-site is limited.
• When multiple sampling points require centralized analysis.

There is no one-size-fits-all solution. But with today’s tools and technologies, producers can select the slag analysis approach that best fits their specific environment and operational goals.