ONE STEP AHEAD The STARprobe™
A new technology allowing simultaneous measurements of four cryolitic bath properties in only four minutes
5 1- Cart 2- Reusable probe tip 3- Probe 4- Electronic probe head 5- Interface computer 6- Power unit 7- Battery pack 1 8- Telescopic 7 towing arm
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How it is used in the potroom
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Figure 1: STARprobe™ device kit
During the last 10 years, Alcoa has developed a new device called the STARprobe™ [1, 2, 3]. This technology, now available through STAS, is a portable device that takes real time measurements of four bath properties in electrolysis cells:
Superheat Temperature Alumina concentration Ratio (excess AlF3) (STAR) This synchronicity of measurements is the most important step forward in improving the control and efficiency of electrolysis cells. Figure 1 shows one STARprobe™. The device kit consists of a mobile cart and two probes, one communication panel, one interface computer, one power unit, one battery pack and a telescopic towing arm in the front. How it works The probe concept consists in making a Differential Thermal Analysis (DTA), which is a proven method [4], on a bath sample and a reference. The reusable
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The correlation algorithm is very fast, and the calculated results are equivalent to the XRD analysis, as presented [1] and as independently verified on many occasions so far by the author in demonstrations performed in smelters all around the world (for example, see [5]).
The reusable, consumable probe tip can take around 100 measurements. It is connected to the probe head through a probe lance, as seen in Figure 3. An operator can use two of those assemblies to measure cells simultaneously. That way, a trained operator can obtain an average time of just under 4 minutes per measurement.
probe tip (Figure 2) includes two high-precision type K thermocouples, dynamically paired for compatibility. The thermocouple on the left records the cooling rate of the bath sample, while the thermocouple on the right records the cooling rate of the reference. During the cooling process – the bath sample liquidus temperature – cryolite starts to solidify, which slows the bath sample cooling rate even further. At the eutectic temperature of the bath cryolite-alumina phase diagram, the alumina starts to solidify as well. Finally, at a much lower temperature -that has reduced down to the eutectic temperature of the bath cryolite AlF3 phase diagram (Figure 1 of [1]) -the excess AlF3 finally solidifies. The reference temperature is selected as the X coordinate instead of the time, so the analysis results of the bath sample are not affected by fluctuating ambient conditions [1]. In fact, the shape of the curve depends on two things only: the design of the probe tip and the composition of the bath sample. This means that for a given probe tip design, the shape of the curve is solely dependant on the composition of the bath sample.
PRIMARY SMELTING AND PROCESSES
Figure 3: Probe assembly The probe head includes a very high resolution electronic thermocouple reader that reads the thermocouples and transmits data via Wifi to the tablet PC running the STARprobe™ application. One tablet PC can simultaneously process the data from the two probe heads. After a few seconds, the results are displayed on the tablet screen (left side of Figure 4), stored in a file on the tablet and can be transmitted automatically to the plant database and/ or to the pot control system.
Figure 4: STARprobe™ application displaying the results
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STAS is the exclusive supplier for the STARprobe™ technology and offers demonstrations upon request: www.stas.com/en/starprobe
Figure 2: Reusable probe tip 1
Sample cup
Contains bath sample
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Reference side
Provides a reference cooling curve
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Thermocouple connectors
Measures temperatures
Potential of process control improvements using the STARprobe™ The conventional way to control the bath ratio and the temperature is to regularly take bath samples and measure the bath temperature. Most of the time, bath sampling is not synchronized with bath temperature measurement. In any case, due to the delay in getting the bath sample analyzed, the cell controller typically never receives new temperature and ratio data at the same time. This lack of synchronicity between the bath ratio and bath temperature data and the lag in getting the ratio data can be totally eliminated by using the STARprobe™ to measure both parameters at the same time and by immediately transmitting the results to the database and to the pot control system via Wifi . Furthermore, a typical bath ratio control logic uses the data for both the bath temperature and bath ratio in order to control the bath ratio by adjusting the amount of AlF3 added to the cell, assuming a given and constant cell superheat. As presented in [7], any inconsistencies between the target bath ratio and the target bath temperature can create instabilities in the feedback control loop. Since the STARprobe™ also measures the bath superheat, the bath ratio control and the bath temperature control – or rather the bath superheat control – can
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be decoupled. The bath ratio can be controlled by adjusting the amount of AlF3 and the bath superheat by adjusting the target cell pseudoresistance independently [8]. Improvements have already been achieved in more than 10 of Alcoa’s plants in terms of process control. In parallel with the development of the STARprobe™, Alcoa has developed a new cell controller called QLC which takes full advantage of its STARprobe™ bath properties measurement technology. The QLC automatically acquires the results of STARprobe™ measurements in real time via Wifi [1,6]. The gains guaranteed by Alcoa [6] are the following: • 0.5% current efficiency (proven) • 35 mV voltage savings (proven) • 5% AlF3 savings (proven) • 100-150 day potlife improvement (still to be established) • One time capital cost saving (X-ray equipment) (proven) • Labor savings for sampling/ analysis (proven) • Improved understanding by operators (proven) The STARprobe™ technology is a new way to control electrolysis cells. Other companies around the world have already taken advantage of this new opportunity.
References [1] Xiangwen Wang, Bob Hosler and Gary Tarcy, Alcoa STARprobe™, Light Metals, (2011), 483-489. [2] Bob Hosler, Xiangwen Wang, Jay Bruggeman and Patrick O’Connor, Molten Cryolytic Bath Probe, US patent no. 2005/0069018 A1. [3] Xiangwen Wang, Bob Hosler and Gary Tarcy, Systems and Methods Useful in Controlling Operation of Metal Electrolysis Cells, US patent no. 2007/0295615 A1. [4] R.C. Mackenzie, Differential Thermal Analysis, Academic press London, 1970. [5] M. Dupuis, P. Bouchard and J.-P. Gagné, Measuring bath properties using the STARprobe™ , 19th International ICSOBA Symposium (2012). [6] Wang, X., Tarcy, G., Batista, E. and Wood, G. Active pot control using Alcoa STARprobe™, Light Metals, (2011), 491-496. [7] M. Dupuis, Excess AlF3 concentration in bath control logic, National Conference on Advancements in Aluminium Electrolysis, Indian Institute of Metals, Angul Chapter, (2006). [8] M. Dupuis and J-P. Gagné, Testing a new STARprobe™, ALUMINIUM 89 (2013) 1/2, 76-79. Contact Information: Pierre Bouchard, Eng., M.Sc. - President Telephone Office: + 1-418-696-0074 ext. 2222 bouchard.pierre@stas.com www.stas.com Jean-Pierre Gagné, Eng., M. Eng. – Technical Manager, Electrolysis and Carbon Technologies : Telephone Office: + 1-418-696-0074 ext. 2417 jpgagne@stas.com www.stas.com
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ONE STEP AHEAD WORLD CLASS ELECTROLYSIS EQUIPMENT FROM STAS Manufactured by STAS, the Covered Anode Tray, or CAT, is a specifically designed container that confines the cooling anode butts in an enclosed container shortly after their removal from the cells in order to reduce HF emissions from the anode butts by up to 50% [1].
CAT / Covered Anode Tray STAS has developed new technologies dedicated to the potrooms and the rodding shop, among which the AAPS / Automatic Anode Positioning System and the CAT / Covered Anode Tray, which are used in the potroom to improve operational and safety aspects of the anode changing process, and the EIC / Electrically Isolated Platform which provides a fully safe environment for the operators working above the pots. On the existing conveyor system of the rodding shop plant, STAS provides an Anode Butt Inspection System (ABIS) and also an Anode Stub Inspection System (ASIS) to perform the automatic inspection of anode butts and stubs. The Automatic Anode Positioning System In modern aluminium smelters, huge amounts of prebaked anodes need to be replaced continually and properly positioned in electrolysis cells. However, manual positioning methods – even with the best trained operating crews – are quite conducive to variability and a lack of consistency, exposing operators to safety hazards. The Automated Anode Positioning System, or AAPS, is a fully automated system developed by Alcoa Canada & STAS which can be implemented on existing or new Pot Tending
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Machines (PTMs) to ensure optimal anode positioning. Based on laser measurements using the anodic beam as a reference, the AAPS also relies on load sensing devices to avoid positioning errors related to mechanical play. With the AAPS, the duration of the anode replacement cycle can be reduced by up to 25%. The system is also designed to minimize the period of time between the removal of the anode butts from the electrolysis cell and their storage in the transport tray. The system therefore contributes in improving the health and safety conditions for floor operators by reducing their exposure to HF emissions, gases and particulates. Using the AAPS in combination with the Covered Anode Tray (see below), the anode butts can be quickly loaded into closed containers, which further reduces HF emissions in the potroom. The Covered Anode Tray Once the used anodes (anode butts) are removed from the cells, they are usually placed on anode trays for transportation and cooling, a process during which Hydrogen Fluoride (HF) – a particularly toxic gas for the environment and human beings – is generated in large quantities.
ANODE PLANT TECHNOLOGY
The panels of the closed container are automatically activated by the movement of the anodes when they are inserted or removed. The CAT features a patented mechanism used to seal the gap around the anode rod and to ensure optimum confinement while minimising downtime and maintenance costs. The geometry of the containers can be easily adapted for use with any plant anode trays and transport vehicles. The Electrically Isolated Platform Live parts up to 2 KV are within reach in certain layouts of a Hall-Heroult potroom, special tools are required during operation and maintenance activities to protect workers against electrical hazards. Thanks to its triple level of isolation, the Electrically Isolated Platform or EIP virtually eliminates electrical hazards by preventing the circulation of an electric current between the live parts and the ground, thus ensuring the safety of the operator. STAS uses industrial boom lifts and modifies them with very specific electrical insulation material at the first joint hinge, which eliminates the possibility of short circuits if a boom lift were to come into contact with two different electrical potentials in the potroom. An important feature of the STAS EIP is its Realtime Isolation Supervisory System (RISS) that validates the insulation of the boom lift in real time, using STAS’ proprietary instrumentation. If for any reason the electrical insulation is broken, the platform operator, while still protected against electrical shocks, is immediately informed by an alarm that an insulation barrier is non-functional
cells, and such defects can reduce the efficiency of, or jeopardize the anode assembly process, or cause major downtimes when faulty stubs have to be removed. To take all these problems into consideration, STAS supplies two automated systems based on highresolution artificial vision: the ABIS / Anode Butt Inspection System [2] and the ASIS / Anode Stub Inspection System [3]. EIP / Electrically Isolated Platform
and that maintenance is required. STAS Anode Butt Inspection System and Anode Stub Inspection System At the end of their life cycle, anode butts need to be cleaned from the bath residue before recycling the residual carbon. Inadequate cleaning leads to high sodium levels in the residual carbon (which is reused in the anode fabrication process), causing problems such as cracks in the anodes and shortened life expectancy of the anode baking furnaces. In addition, abnormal shapes (defects) could indicate the need to adjust process parameters such as the thickness of the carbon that remains underneath the cast iron thimbles, which gives information on the anode cycle, etc. Such process data provides relevant information for optimizing the fabrication of new anodes and the electrolysis process. Typically, only a few anode butts are manually inspected, and limited useful information is available from such an inspection scheme. After the removal of residual carbon and cast iron thimbles, the stubs of each anode rod assembly also have to be carefully inspected to determine if the assembly can be used again or if it needs to be routed to the repair area. Stubs tend to deform and corrode in the
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The ABIS measures a certain number of geometrical characteristics of the anode butts and is easily integrated into the conveyor system of the rodding shop. The ABIS measures and analyses all the anode butts processed at the rodding shop, identifies a certain number of defects, and performs the monitoring of anode butt cleanliness with regard to the residual bath remaining on the surface. The ASIS measures more than 30 geometrical parameters associated to the anode rods and stubs. These measurements are used to route rod assemblies through the repair area or to the equipment used to align up the external stubs if repairs are needed. The measured parameters are stored in the plant database. The characteristics of each anode rod can be followed up, and statistical analyses are made available for process control. The ABIS and the ASIS are available separately but can be combined to ensure the best possible performance regarding anode cycle optimization. About STAS STAS Inc. (www.stas.com) is a well established company and a recognised leader in its innovative abilities to develop, fabricate and commercialise new technologically advanced equipment for the aluminium industry. STAS is serving a global market place on a custom-made basis, with quality specifications that comply with world standards.
ASIS / Anode Stub Inspection System
ABIS / Anode Butt Inspection System Founded in 1989, STAS employs more than 150 persons, including 80 engineers and technicians with broad expertise in process and design. STAS’ headquarters are in the heart of Quebec, Canada, where about 2.5 million tonnes of aluminium are produced by some of the world’s most technologically advanced plants. STAS holds licenses with Rio Tinto Alcan and Alcoa.
References [1] Jean-Pierre Gagné et al, HF Emission Reduction from Anode Butts Using Covered Trays, Light Metals, (2012), 557-560. [2] Jean-Pierre Gagné et al, ANODE B U T T AU TO M AT E D V I S UA L INSPECTION SYSTEM, Light Metals, (2008), 895-898. [3] Jean-Pierre Gagné et al, ANODE STUB INSPECTION SYSTEM, Light Metals, (2007), 1021-1024. Contact Information: Jean-Pierre Gagné, Eng., M. Eng. – Technical Manager, Electrolysis and Carbon Technologies : Telephone Office: + 1-418-696-0074 ext. 2417 jpgagne@stas.com www.stas.com
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