In Pursuit of Purity, Technology Goes with the Flow

Molten metal demands a lot of attention, to ensure purity while it is in the furnace and stability as its filling the mold. In between time, when the furnace is tapped or poured out, the metal must be monitored to ensure its quality is not degraded – and there is technology to assist with that.

Liquid metal tapping may involve various metals and alloys (steel, iron, aluminum, copper, brass) for operations of varying scales. Accurate measurements of liquid metal temperature are required to ensure the quality of the final product.

Non-contact temperature measurements

Thermocouples often take these measurements, typically by dipping thermocouples that contact the liquid metal. It is impossible to measure the metal stream, so the reading must be taken in the ladle or converter, where the metal runs out into the mold or another vessel.

Dipping the thermocouple into the metal delivers an average temperature for the liquid metal within seconds, and at its dipping depth. It dips into the metal, avoiding any surface deposits that could affect the measurement.

However, a manual measurement requires the operator to be close to the metal stream, which is a safety hazard in that hot and smoky atmosphere. Also, thermocouple readings are likely to be inconsistent – two or three different thermocouples are likely to produce two or three different measurements, so accuracy is an issue. Cost is another, as the dipping tip for each thermocouple must be replaced after one or two measurements.

Non-contact measurement of the liquid metal can be done using a hand-held portable pyrometer, a stationary pyrometer, or a thermal imager, taken on the surface of the metal or of the metal pouring/tapping stream. However, oxygen in the atmosphere may lead to an oxide layer forming on the liquid metal surface, creating slag/dross. This affects the emissivity of the metal, making it more difficult to achieve an accurate reading.

Additionally, as the slag layer becomes thicker, the surface temperature decreases compared to the liquid metal temperature.

So, for non-contact temperature measurements it is important to maintain a view of the metal surface where it is nearly (or completely) free of metal oxide.

Automated or manual tapping processes where liquid metal is poured into molds is such an application. A thermal imager or pyrometer is able to measure the temperature of the liquid tapping stream, which typically has only a thin oxide layer (a “pouring skin”) on its surface. This ensures the temperature being measured is that of the liquid metal, as it runs into the mold to create the final product. It is the latest and most exact point to measure the temperature of the liquid metal in the tapping/pouring process, before it solidifies, so it provides the best, 24/7 assessment of the quality and metallurgical properties of the metal product.

Several non-contact liquid metal temperature measurement packages are offered by AMETEK Land, including portable and stationary pyrometer systems, and thermal imagers:

•  The Cyclops C055L is a portable, hand-held pyrometer specially designed for liquid temperature measurements. It is easy to use and can be operated one-handed, so the user can keep hold of a safety rail while taking readings.

•  Auto Pour is a pyrometer-based stationary temperature measurement system for liquid metal and pouring streams that combines shortwave infrared technology with expert signal processing to reduce operating costs.

•  Thermal imagers – like the NIR-656 or NIR-2K – offer high-resolution images and temperature readings unaffected by heat or smoke near the tapping point.

For instance, the hand-held Cyclops 055L Meltmaster is in use by Snam Alloys Pvt. Ltd., an Indian foundry and manufacturer of ferrosilicon, ferrosilicon magnesium, and ferrosilicon-based inoculants. Snam Alloys needed a device for accurately measuring the temperature of liquid metal in the furnace and ladle, a device that is also consistent, with information clearly displayed on the shop floor.

The Cyclops 055L Meltmaster paid for itself within 12 months, as Snam Alloys eliminated the cost of dip-type thermocouples, and the user-friendly, highly accurate portable pyrometer also positively affects workers’ health and safety.

Slag detection measurements

High-quality pouring means minimizing slag carry-over during tapping. Slag reduction also decreases refractory wear in the vessels and significantly reduces the requirement for costly additives to remove the slag.

Traditionally, operators monitored the tap visually, wearing a dark mask to prevent eye damage. However, this method depends on the operator’s skill and/or experience in spotting the slag and reacting quickly; using different operators leads to different results. Even with just one operator, results may vary according to factors like fatigue and/or fumes obscuring the operator’s view.

Another solution involves an induction coil fitted around the tap hole. The induction field changes according to the composition of the flow, so as slag begins to carry over the coil signal is used to determine when reversal should start. While effective, the coil life is much shorter than the vessel campaign life, so coil failure means extra downtime or a switch back to manual slag detection.

Modern coil designs are more modular, enabling easier, quicker replacement, but still create a period of time between coil failure and replacement, with associated device and labor costs.

Non-contact thermal imaging cameras provide more reliable slag detection, with no consumables cost.

Optical and infrared detectors produce more reliable, repeatable results than an operator’s eye, but optical detectors may be obscured by steam or fumes.

Mid-wavelength thermal imagers, however, offer significant advantages, with an imager using a 3.9 μm waveband able to view the vessel through smoke and dust. The shorter waveband also allows for the use of extremely durable optical components, like sapphire protection windows.

The AMETEK Land SDS-640 slag-detection system allows plants to improve product quality, reduce slag carry-over and improve operator safety.

Designed to perform in a hostile environment, it has a high-resolution thermal imaging camera to detect the transition between liquid metal and slag, using the 3.9 μm wavelength see through smoke, dust and fumes. Quick termination of the tap after an alarm is triggered is necessary to prevent excessive levels of slag in the ladle.

The system’s IMAGEPro-SDS software presents data to operators in real-time, so they can make informed decisions about the tapping process and automatically control tapping or stop it if a certain amount of slag starts to run out from the ladle.

SDS-640 installations have improved operator response time and consistency at the end of each tap, resulting in a typical reduction in slag depths of up to 25% versus traditional methods of stream monitoring.

As the tap commences, IMAGEPro-SDS records it using a stream identification algorithm and produces a data log and graph for quality control.  When slag appears and exceeds an operator-defined amount, an alarm automatically triggers.

Using SDS has improved operator response time and consistency at the end of each tap, typically reducing slag in depths of up to 25%, compared with traditional methods of stream monitoring.

The cost of additional downstream processing time and materials can be a significant burden on an operating plant. By controlling slag carry-over, downstream processing can be reduced or eliminated, improving plant throughput and operating margins.