New online measurement method helps monitor and manage abnormal operating conditions
United Arab Emirates: Evidencing the commitment to operations excellence for which it is renowned, Emirates Global Aluminium (“EGA”) is pleased to announce yet another invention that supports a corporate philosophy of continuous improvement, this time in the form of an online measurement method to monitor and manage abnormal operating conditions in the Hall-Héroult electrolytic reduction cells used to produce primary metallic aluminium from aluminium oxide[i]. The invention, developed collaboratively by researchers from EGA operating subsidiary Dubai Aluminium (“DUBAL”) and the University of New South Wales (“UNSW”), is particularly useful for anode effects (“AEs”), a distinct working state of these cell characterised by high voltage, interrupted aluminium production and emissions of tetrafluoromethane (CF4) and hexafluoroethane (C2F6) – perfluorocarbons (“PFCs”) that rank among the most noxious greenhouse gases. The invention is equally adept at detecting problematic anodes, blocked alumina feeders, defective crust breakers, errors in manual setting of anodes, data input errors in the pot control system as well identifying background PFC emissions at a very early stage – thereby improving both process productivity and environmental performance.
Sergey Akhmetov (Senior Vice President, Midstream), who led the team of Ali Jassim Banjab (Manager, Plant Optimisation & Process Development, Technical at EGA) and Jie Bao, Barry Welch, Yuchen Yao, Cheuk-Yi Cheung and Maria Skyllas-Kazacos of Process Control Group at UNSW, says, “As responsible corporate citizens, EGA and its subsidiaries are committed to protecting the environment wherever we operate. One of our key strategies is to contain our carbon footprint, where a fundamental objective is to decrease the frequency and duration of AEs, thereby minimising the generation of CF4 and C2F6 gases. Enhanced monitoring of AE frequency and duration is integral to this.”
AEs are usually triggered by a decrease in the alumina concentration in the electrolyte bath to below 2%, interrupting the normal reactions that produce aluminium and causing other electrochemical reactions to take place resulting in the formation of CO, CF4 and C2H6. Bubbles of these gases accumulate under the anodes, forming an electrical insulating layer that leads to an increase in anode potential of 10V to 50V – so measuring voltage change is a strong indicator. Akhmetov explains that, under normal operating conditions, bubbles of CO2 form on the under surface of the anode and need to be released continuously. “The dynamics of these bubbles causes perceptible perturbation of the anode currents, which is detectable as electric noise on the current signal,’” he says.
Improved monitoring and control of cell operations also has priority ranking in EGA’s quest for operational excellence, with monitoring and management of anodes representing a key component. “The carbon anodes are consumed during electrolysis and therefore need to be replaced,” explain Akhmetov. “As the thickness of the anode reduces as a result of burn-off, so the height of the anode in the electrolyte needs adjustment. The ability to monitor individual anode currents and their distribution would give a clear indication of spatial variations in cell operating conditions, contributing to more effective anode management.”
None of the available cell monitoring methods fitted the bill. For example, methods reliant on cell resistance cannot detect spatial variations, such that localise abnormalities – such as AEs – are not detected until they become severe. Other methods to measure the currents that pass the anode busbar either side of each anode rod require interpolation from electric potential drops across a specific length. And methods measuring voltage points across the anode busbar require complex calculations for meaningful interpretation. While the latter methods offer the most promise in terms of system reliability (less maintenance is required), the structure of the apparatus and the susceptibility of the transmitted signals to noise are detracting factors.
“To overcome these shortcomings and address our needs, we set about finding a reliable way to determine and monitor individual anode currents in the cells,” says Akhmetov. “We identified that the transmission of signals in analogue form in the existing methods was a major drawback as carrying raw voltage signals of a few mV from the sensors to the central unit for amplification and data acquisition requires numerous long signal wires. Analogue transmission is also more susceptible to noise. We therefore determined that our solution should use simple equipment and digital communication for easy wiring, installation and maintenance; and so that noise issues in signal transmission would be eliminated.”
The method developed by the EGA-UNSW team involves the installation of sensing assemblies on the anode beam for each of the anode assemblies. Each multifunctional sensing assembly measures and samples a voltage drop on the anode busbar either side of the anode assembly. The sensing assemblies are connected to the central unit through a local controller network. The signal from each anode assembly is converted to digital form in the respective sensing assembly before transmission to the central unit. Being digital, the signals are virtually immune to electro-magnetic interference and other noise sources; and fewer cables are required. The temperature of the anode beam is measured simultaneously, to enable more precise determination of the anode current. The digital signals received at the central unit are then computed to derive the individual current flow of each individual anode assembly, the data is analysed, and then the cell operating information is transmitted to a cell controller and or potroom computer server.
“Our method of monitoring individual anode currents offers a reliable means to detect and monitor abnormal conditions of cell operations,” says Akhmetov. “It has substantially enhanced our ability to detect AEs at an early stage and avert them before they become severe. This is demonstrated in the low frequency and short duration of AEs recorded at EGA’s operations.”
Based on this success, the inventors filed a patent application for their new method of monitoring individual anode currents in Hall-Héroult electrolytic cells in February 2016, after working closely with Takamul to develop and submit the required patent application documentation. An innovation support programme developed and operated by the Abu Dhabi Technology Development Committee (“TDC”), Takamul’s mission is to help Emirati individuals, universities and enterprises in Abu Dhabi and the wider UAE, to protect and commercialise their innovative ideas.
[i]Hall-Héroult Electrolytic Process
In an aluminium smelter, direct current is fed into a line of electrolytic reduction cells that are connected in series. Each cell is a large carbon-lined metal container, maintained at a temperature of about 960˚C, that forms the cathode. The cell contains an electrolytic bath of molten salt into which aluminium oxide is fed (along with aluminium fluoride, to balance the chemistry). Large carbon blocks are suspended in the solution, and serve as the anode. The anodes and cathodes are connected to external busbars. An electrical current passes through the cell, typically at voltage of 3.7V to 5V, which splits the aluminium oxide into aluminium ions and oxygen. The aluminium ions move to the cathode, forming a metal pad of pure liquid molten aluminium at the base of the cell, creating a liquid metal pad of pure molten aluminium on the cathode surface that is extracted periodically (usually by suction). The oxygen reacts with the carbon in the anode, forming CO2that is emitted from the cells.