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Temperature Sensing in the Copper Industry

22 October, 2009

Realizing the significant energy and cost savings available through temperature control best practices. Improper or lax temperature control creates inefficient energy usage, wasting money and damaging equipment. Correct temperature sensing strategies improve reliability, can help avoid costly repairs and save energy to reduce harmful CO2 emissions.

“Optimized temperature sensing is more important than ever to increase reliability, lower emissions and reduce plant operation costs,” said John Shanahan, Pyrosales CEO. “The move to best practices can help manufacturers meet sustainability goals and fulfill their commitment to innovative, energy-efficient solutions by understanding where they may be wasting energy.”

Pyrosales is positioned to work with plant managers, industrial maintenance engineers and environmental professionals who stand to benefit from increased awareness of the importance of temperature control as part of a proactive and planned innovation program. 

Pyrosales has gained a wealth of experience and exceptional results partnering with copper industry professionals to solve temperature sensing issues during the last decade.   In working closely with Mt Isa Mines in Australia and subsequent work with copper smelters all over the world, Pyrosales has developed an in-depth understanding of temperature sensing challenges and solutions in the many different processes used to produce copper. 
Traditionally, temperature measurement in most of the processes has relied on the results from manual dip sensors using the disposable type temperature sensors.

In more recent times, infrared temperature measurement at the tapping points or launders exiting the furnaces or converters have been employed in addition to the manual dip process.   Both the manual dip and the infrared sensing processes have been shown to be inaccurate and unreliable, although with present technology do provide some form of temperature monitoring. 

The manual dip is a not a closed loop solution. Typically, readings are taken every 40 minutes to 2 hours and heating or cooling of the process is initiated by these readings.  This method allows for overshooting optimal temperature at both the high and low end of the desired temperature range.  Results of this overshoot will include inefficient and excessive use of energy and a potential product degradation or rework.
Infrared thermometers pose some challenges. It relies on stable emissivity, clear line of site, and a clean environment to allow accurate measurement of the process. All of these points are problem areas when measuring in and around copper furnaces and converters.

Direct immersion into the bath is the option that would yield the best opportunity to control the temperature – reducing energy costs, saving operator time and improving product quality.  There has been a reluctance to use direct immersion sensors due to the typically poor performance of these sensors. These are easily broken, usually noble metal thermocouples which have high cost and as they have been subject to continual breakage, do not give a consistent or cost effective solution.  The atmosphere of high sulphur and molten metal as well as the operating temperature of the furnace or converter, presents a substantial challenge to the design of any temperature sensor solution.

It is apparent that if there is a desire to measure the bath or slag temperature that it is not going to be practical to mount a sensor from the roof - although this may seem to be the obvious place to install it. Practically speaking, ceramic based solutions or high chromium cast protection sheaths are the only materials that are able to withstand the atmosphere. If one was to consider a roof mount installation, the length and weight of these forms of assemblies make these options impractical. Both need substantial hardware support to allow them to reach the bath, and there is no metal that will last in the atmosphere. The length also causes logistical problems with head height above the furnace, and the general handling of a sensor of this size and weight creates more logistical problems. In addition, there is also the potential to introduce the problem of sensor droop due to the high temperature environment.

Challenged with the above considerations and the goal of monitoring and controlling bath temperature, Pyrosales has engineered sensors using the extensive experience gained in the design of sensors in oxygen lance furnaces with companies such as Ausmelt®. Typically these have specially designed protection sheath assemblies made from metal ceramic (LT-1) which is able to be immersed in liquid copper, and has a known performance in sulphur atmospheres making it an ideal solution for sensor protection in these furnaces. The special bore diameter allows copper to freeze in the assembly ensuring that the contents of the furnace will not unload in the event of a failure of the sheath. Additional safety measures with the direct immersion below the “water line” designs can include using positive seals or angled immersion to allay those concerns.

It is important that the immersion is kept to a minimum, to ensure that the sheath is not subject to abnormal shear forces that would cause the sheath to break, and due consideration is also needed where large solid material is loaded in the furnace, or likely to be formed. Unlike immersion from the hood which is physically cumbersome and extremely expensive, immersion from the side of the furnace that allows easy access for removal or replacement and optimum position to maintain immersion in the bath means that the sensor can be kept to a minimal length. This is critical in keeping the cost down, especially in applications where the control temperature is likely to exceed 1250o C, which necessitates a noble metal thermocouple.
Where it can be established that the temperature can be controlled below 1250o C, the use of a type N thermocouple with a Pyrosil ® sheath is recommended.  This allows the use of a base metal thermocouple which is substantially more cost effective than a noble metal thermocouple of the same length.

With this new design there is the ability to get a continuous measurement of the process temperature allowing either semi closed loop control or full closed loop control of the process temperature.

Temperature control has the advantage of better energy or feed control and a more consistent product output from the furnace. Making sure that the temperature swings or maximum temperate is minimized has implications also on refractory performance and rework if the product is out of specification for the downstream processing.