In-house developed preheating frame for anode assemblies enables safe and rapid preparation of cells for preheating and allows achieving low energy consumption after start-up.
United Arab Emirates: There is only one continuous industrial process to produce primary metallic aluminium from aluminium oxide. Named after its discoverers, the Hall-Héroult process uses electrolytic reduction cells, where direct current is used to reduce into metallic aluminium the aluminium oxide dissolved in the liquid electrolyte. For leading primary aluminium producers, such as Emirates Global Aluminium (“EGA”), a key operational focus has always been on improving safety, productivity and energy-efficiency.
This quest has been a fundamental motivator for an ongoing commitment to continuous innovation at EGA and which, through substantial investments in research and development, has yielded a host of inventions and technology advances. To protect the associated intellectual property, EGA submits applications to patent its proprietary developments. The business has been working closely with Takamul since 2014 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.
A case in point is the patent application submitted in March 2016 for a unique EGA-developed device to hold mechanically and connect electrically anode assemblies during the electrical preheating of the Hall-Héroult cells, as well as the process for preheating the cells using the device. Electrolytic reduction cells must have their cathode replaced after a period of 5 to 10 years of operation. Before pouring liquid electrolyte at 960 ºC into the cell to begin electrolysis, it is necessary to preheat the cathode and the anodes to avoid any detrimental thermal shock and allow low energy start-up.
The innovative invention offers substantial advantages over prior systems, most notably improving the thermal homogeneity of the cathode during preheating (i.e. prior to start-up). This prevents abnormal thermal stresses on and potential damage to the cathode. A cathode surface that is preheated homogeneously allows reaching a target temperature close to the liquid electrolyte to be poured for start-up and will avoid high energy consumption just after start-up.
Mahmood Abdulmalik (Senior Engineer, Technology Engineering), who developed the device alongside Syed Fiaz Ahmed(Engineer I, R&D) and Mark Jordan (formerly Lead Engineer, R&D), explains that each Hall-Héroult cell is a large carbon-lined metal container that forms the cathode. The cell contains an electrolytic bath of molten electrolyte (cryolite) into which aluminium oxide is fed (along with aluminium fluoride to maintain the chemistry). The resulting electrolyte is maintained at 950˚C to 970˚C. Large carbon blocks are suspended in the solution, and serve as the anode.
“The start-up of a cell after its construction, refurbishment or repair is a critical operation that typically takes several days. If not supervised intensely, degradation of the cell components may occur that, in turn, reduces the cell life,” says Abdulmalik. “One of the most critical aspects is preheating the cell to temperatures of above 900˚C before adding the electrolyte, especially as preheating a cold cell generates strong thermal gradients that may cause abnormal thermal stress on the cathode and could result in cracks to the cathode surface.”
At EGA, as at most modern smelters, cell preheating has traditionally been achieved using an individual flexible connection for each anode rod to allow expansion of the cathode and anode assembly during preheating as well as to accommodate a drop in the level of the preheat carbon-based material placed below the anodes to improve the contact with the cathode. To maintain good contact during the complete period of preheating, the carbon anodes rest on the layer of carbon based conductive material and receive the electrical current through aluminium flexible material. Due to the flexible connection, the anodes ensure good contact by their own weight and receive the electrical current heating the anodes and the cathode by Joule effect.
“The EGA preheating frame is unique due to its design, which combines multiple aluminium flexibles on a single aluminium bar which is mechanically and electrically connected to the anode beam of the electrolytic pot. The frame supplies current to several anodes at a time and only a few frames are necessary to preheat one pot. Handling of the frames is carried out with an overhead crane and equipping one pot is therefore simple and rapid compared to the standard individual flexible arrangement. More importantly, removal of these frames at the end of preheating cycle, and connecting directly the anodes to the anode beam as during normal operation, just before pot start-up, is quick – which allows quick start-up of the pot while maintaining the cathode surface temperature at the optimum level before pouring liquid bath for start-up,” says Abdulmalik.
“The new frame enables quicker and safer preparation of cells for preheating and improves the thermal homogeneity of the process,” concludes Abdulmalik. “This is key, given that the cells are not producing metal during preheating. The reduced potential for damage to the cathode is an excellent advantage, especially in terms of maximising the cell life.”