Using high temperature thermocouple sensors in pyrometallurgy

Broadly speaking, metallurgy refers to the processes by which metals are extracted from their ores (or recycled materials) and prepared for use in metal production and refining. Pyrometallurgy is the term for processes where extremely high heat is used to extract the metals through smelting, followed by fire-refining and possibly casting. In some cases, such as with aluminum extraction, electrolysis (electrometallurgy) is required as a final step.

These processes all involve temperature control for molten (liquid) metals, which are broadly divided into three categories:

  • iron (ferrous)
  • steel
  • non-ferrous (aluminium, copper, lead, nickel, tin, titanium and zinc, as well as copper alloys like brass and bronze).

To ensure product quality and plant productivity, metal manufacturing and recycling plants need to keep the molten metal within exact temperature limits before the melt can be refined and cast. This is essential in order to avoid various process problems, for example: A temperature that is too low can result in casting issues, while temperatures that are too high involve extra energy costs. The accuracy, reproducibility and reliability of molten metal temperature measurement is therefore an essential part of pyrometallurgy and will ultimately determine the quality of the end-product.

On this page we will look at how Kamet’s (ultra) high temperature thermocouples, also known as exotic thermocouples, can be used as metallurgical thermometers. Their function is to monitor and control the smelting, refining and casting processes in huge refineries or production plants or, for example, as part of research or recycling applications.

Which high temperature thermocouples are best suited for pyrometallurgy?

These are the thermocouples most commonly used in molten metal applications, such as foundries and steel plants:

TypePositive LegNegative LegTemperature rangeMetallurgic application
Type KNiCrNi-270 to 1260°CSuited to copper and steel’s slightly lower melting point. Also able to withstand the harsh environment of aluminum melting furnaces and molten electrolyte baths.
Type NNiCrSilNiSil-270°C to 1260°CWithstand extreme heat and provide reliable measurements in the harsh environments of furnaces. Excellent accuracy, and thermoelectric stability particularly at temperatures above 900°C.
Type SPt10%RhPt0°C to 1450°CMonitor and control temperatures in holding furnaces during metal melting and casting. Commonly used for steel, aluminum, and zinc melts. Type S offers relatively long term stability and accuracy in harsh environments. Note: the output voltages for continuous operation are only stable to about 1300°C      
Type RPt13%RhPt0°C to 1450°CVery similar to type S, however due to the higher percentage of rhodium it is slightly more stable. It should be noted that one significant difference between S and R thermocouples is that type R have a temperature gradient about 12% higher (seebeck-coefficient) than the type S.

The exact choice of thermocouple depends on the specific application environment, its position, the temperature range to be measured and the choice of compensating wires. It is important to also take thermal and mechanical stressors into account.

If you’re measuring a specific spot within the molten metal, a smaller thermocouple may suffice. For broader coverage, opt for a larger size. Our ultra high temperature (exotic) thermocouples are available in a variety of lengths, diameters and connectors. Different classes of thermocouples suit local instrumentation and configuration.

Thermocouple location in metal melting furnaces

Generally speaking, there are three options for thermocouple location when smelting metal. Each has specific advantages and disadvantages.

  1. The most basic design is one where the thermocouple is placed in the furnace chamber, behind the melting crucible. This allows for fast heating times, however due to the indirect temperature measurement it tends not to be energy efficient, with higher than necessary temperatures often being used.
  2. A much more accurate temperature measurement takes place by placing the thermocouple directly in the molten metal (immersion thermocouple). However, due to the extreme conditions of such a placement, the thermocouple has more chance of failure and the sheaths are considered disposable, as they can only be used once.
  3. The thermocouple can also be placed inside a pocket in the wall of the crucible (this requires an especially designed crucible). This is not as accurate as an immersion thermocouple, however it is a more direct measurement than placement in the furnace chamber (1) while extending the thermocouple lifespan compared to immersion (2).

Where possible, a combination of at least two of these three positions offers the most accurate readings. If one thermocouple fails, then there are still readings for the other.

Whichever (combination of) positions is chosen, it is very important to ensure appropriate electrical insulation and a good connection between the thermocouple and the measuring instrument.

Protecting the thermocouple in pyrometallurgy

It is essential to consider the appropriate assembly to protect the thermocouple in metallurgic applications: Molten metals and salts corrode unprotected sensors. The thermocouples can also be affected by the layers of slag or dross as well as the smoky dirty environment. There is also the challenge of striking a balance between faster response time (thinner sheath) and more thermocouple protection (thicker wall) while ensuring an accurate and reliable measurement is delivered. Making a choice in this regard depends on the positioning and assembly of the thermocouple, the type of molten metal being measured as well as other environmental factors.

Please note that for immersion thermocouples being used in pyrometallurgy, special housings are required to protect the sensor from the smelt. This could also be the case for thermocouples that might be exposed to splashing. For this purpose Kamet has chosen to stock the following options:

Nitrite bonded silicon carbide (without steel inner tube)

This thermocouple housing is suitable for non-ferrous applications up to 1500°C. In applications where there is little mechanical wear, the life span potential is excellent, making them a very economical choice. Some of the beneficial characteristics include:

  • a range of sizes
  • durable
  • preheat is not necessary
  • excellent erosion resistance
  • good response times

Silicon carbide (with inner steel tube)

Suited for brass and aluminum smelting furnaces at up to 1000°C, not only do our silicon carbide tubes protect the thermocouple from the high heat and the aggressive chemicals in the smoke. They furthermore:

  • offer exceptional thermal shock resistance thanks to the high graphite and silicon carbide content (approximately 90%)
  • can withstand repeated immersion in molten aluminum without cracking
  • are available in a range of diameters and lengths
  • require no additional coating

Cast iron

The cast iron thermocouple protection tubes, available with or without ceramic coating, are designed for use in environments with high temperatures and high mechanical loads. These tubes are specially formulated for applications in furnaces for melting metals such as aluminum, magnesium, and zinc. Their robust design also makes them suitable for other industrial applications where durability and temperature resistance are essential.

Sialon/Sylon

This is a ceramic alloy and is a popular and well-established choice in metal industries, especially for aluminum, copper and brass. Our thermocouple protection tubes made with this material have some key characteristics:

  • excellent thermal conductivity
  • high resistance to temperature shock
  • no erosion nor chemical reaction in aluminum smelts
  • suited for immersion as well as indirect molten metal temperature measurement
  • non-wetting

We furthermore offer a range of thermocouple sheaths designed for high temperature applications, of which the following can also be used in certain pyrometallurgic applications (please note the temperature are indications and depend on application parameters):

  • 304 SS (900°C)
  • 316 SS ( 900°C) offers better corrosion resistance than 304 SS
  • 310 SS (1500°C) is the most suited for metal melts. It includes properties such as thermal shock resistance, does not contaminate the melt, resistance to degradation even in aluminum lithium alloys.
  • Haynes alloy 556 (1000°C) is one of the few sheaths that can survive in molten zinc
  • Alumina Al2O3 (1650°C)
  • INC600 (1175°C)
  • INC601 (1200°C)
  • Pyrosil D (1250°C)
  • Syalon/Sialon (up to 1400°C, depending on the type)

Some specific metallurgic applications of our thermocouples

Aluminum smelting

Aluminum production is a complex process. Although pure aluminum (Al) is a naturally abundant element, it is extremely difficult to extract as it is always attached to other chemical elements in the form of salts or oxides. Ultra high temperature thermocouples have an important role to play in temperature measurement and control during direct extraction using aluminum melting furnaces. However the very high temperatures required and the lengthy process necessitates very specialized and expensive equipment with huge energy costs.

For this reason, a more common method is extraction using a chemical separation reaction, which is achieved through electrolysis pots (also called reduction pots). This high temperature process (900°C–1000°C) is controlled by computers connected to remote high temperature thermocouples to ensure optimal operation of the electrolysis pot. It’s a challenging environment for sensors as oxidized alumina reduction is very aggressive and the chemical bath corrodes thermocouples quickly. Type K is the most suited for this process, especially with heavy walls of at least 5.5mm outside diameter.

Aluminium scrap metal recycling is another application of our exotic thermocouples. The metal is re-melted which is much cheaper and more energy efficient than producing virgin aluminum by electrolysis or melting furnaces. In fact recycling scrap aluminium involves only 5% of the energy used to extract it from the raw ore.

Thermocouples are exposed to extreme environmental conditions in all these aluminium production processes. As such, a protective housing is required such as our Nitrite bonded Silicon Carbide tubes or syalon ceramics.

Copper smelting and extraction

Copper production usually takes place either in batch smelting or continuous casting processes. It is the hottest pyrometallurgical process at 1150°C-1200°C. In both cases either disposable high temperature thermocouples or continuous sensor thermocouples have an important role to play in making sure the required temperatures for molten copper are maintained throughout this process.

Especially in the second stage of turning copper into its final form, known as copper making, the main goal is to use oxygen-rich air to turn the initial copper product into blister copper, which is nearly pure copper (99–99.5% copper). This step involves certain chemical reactions that release a lot of heat, making the process self-sustaining in terms of temperature. To keep the temperature steady at about 1250°C, scrap metal or recycled copper that’s not heated up (called cold charge) is added.

Related to copper smelting is the process of resource recovery from copper slag. Copper slag is a potential pollutant due to elements gradually leaching into the soil, (ground)water and air. Recent research has focused on the fact that the copper and iron content of copper slag is higher than that of the ore. Processing this slag to recover resources can be done in different ways, including through pyrometallurgy.

We offer two protective housing options that are well-suited for immersion thermocouples used in copper extraction: nitrite bonded silicon carbide or syalon/sialon tubing.

Steel works

Pig iron is converted into steel by removing the impurities from it, such as carbon and slag, depending on what type of steel is being made. The very high temperatures involved (up to 1600° C) make our silicon carbide (SIC) and alumina (A1203) tubes a good match for protecting the thermocouples. These can be used in any of the following steel extraction processes:

  • oxygen steel-making: molten pig iron is converted into steel by blowing oxygen over it inside an oxygen converter. Knowledge of the temperature is important for process optimization and to extend the life of the inner furnace lining.
  • blast furnaces: precise temperature monitoring and control is crucial for efficient iron production and to optimize the process.
  • stove domes: part of the blast furnace process. Only once it has been heated to a certain temperature before being put on blast.
  • electric arc furnaces: these utilize electrical energy to produce steel from scrap metal. Thermocouples help ensure efficiency and uniform melting.
  • continuous casting: molten steel is solidified to a semi-finished slab and then in the finishing mill it is used to cast metals of uninterrupted lengths. As with other metal production means, the right temperature needs to be maintained for producing a high-quality product (in this case steel billets).
  • soaking pits – these are furnaces for holding hot steel ingots to equalize their temperature before further processing. Ultra high temperature thermocouples are needed to monitor the temperature equalization process

Zinc pyrometallurgy

The first step in zinc making is ‘roasting’ the zinc sulfide at high temperatures (860°C and 960°C) in order to oxidize it and produce impure zinc oxide, also known as zinc calcine. Blast furnaces and electric furnaces, such as used for steel extraction (see above) can be used for zinc extraction with high temperature thermocouples playing an important role in measurement and process temperature control. The temperature of the furnaces has to be kept above the boiling point of zinc (906°C). The second step is usually low temperature electrolysis to produce zinc cathodes.

Sometimes a third step is called for: smelting and casting. Zinc smelting is more challenging than with other metals due to its lower boiling point. This makes the role of thermocouples very significant for control to make sure temperatures do not go high enough for zinc to become a gas and escape from the furnace.

Lastly exotic thermocouples are important in zinc galvanising – a process at around 400°C-500°C whereby steel or iron is coated with zinc to provide protection from rusting.

Metallurgical science and research

Ultra high temperature thermocouples also have function in any pyrometallurgical research applications involving furnaces and metal smelts. One such area is research into efficient recovery of valuable metals from lithium-ion battery cathodes. With the ever increasing prevalence of electric vehicles this is an essential area of research and high temperature thermocouples provide the consistent and accurate means of temperature monitoring and control in these processes.

Kamet’s thermocouple range and support services

In conclusion, exotic thermocouples, such as those here at Kamet, are an outstanding choice for the extreme temperatures environments in the varied field of pyrometallurgy. Not only are they able to monitor the extreme temperatures, but they also have a high degree of accuracy. This robustness and accuracy is the ideal combination for temperature sensing in metallurgy

Detailed specifications on the full range of high temperature thermocouples and sheaths is beyond the scope of this article. The information is available on other parts of our website: click for more about our ultra high temperature thermocouples and you can also read about our different sheaths. Would you prefer personalized advice specific to your application? Our in-house thermocouple specialists are happy to help – please contact us with your questions.