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:: A New Diffraction Technique for Analysis of Molten Salt Electrowinning
A multidisciplinary team of engineers and scientists has advanced the analysis of material structures during high temperature extractive metallurgy. The new method relies on x-ray diffraction (XRD) to 'look' through the furnace, inside the operational electrochemical cell. The XRD data can be used to monitor changes inside the electrodes, including identification of the phases that determine electrode performance and therefore, extraction efficiency.
The technique was recently validated by quantitative observations of an EbonexTM anode (which commprises the so-called Magneli phases TinO2n-1 where n = 4 to 6) during the electro-extraction of titanium within a purpose-built electrowinning cell. The apparatus can also be used to study a range of other processes of interest to the minerals and manufacturing industries. For the validation study the cell operated at 950oC and contained highly corrosive molten calcium chloride. Such high temperatures are standard in the electrowinning of light metals, since a salt with a wide electrochemical stability 'window' is needed to carry the current and the most useful salts are liquid only at temperatures above 800oC.
CSIRO's Ian Madsen, who leads the XRD specialists on the Future Manufacturing Flagship team, says that the electrodes of the electrowinning cell were successfully examined using the high energy x-rays available at the Joint Engineering, Environmental and Processing (JEEP) beamline at the Diamond Light Source synchrotron in the United Kingdom.
Data was collected during six electrowinning experiments, each involving eight hours of electrolysis probed by x-rays at one minute intervals. "What we wanted to do was gain insight into what was happening under real industrial conditions," Mr Madsen says. "That means the cell was hot and contained molten calcium chloride which was conducting electricity. Adding to an already tough experiment, given these ferocious conditions, we had to get x-rays in and out of the cell without compromising the electrochemical conditions."
The cell that successfully met all these requirements was designed and built by PhD student Mark Styles, a mechanical engineer from the University of Melbourne and Flagship Collaboration Fund scholarship holder. He travelled to the Diamond Light Source synchrotron, where he was in charge of setting up the cell and associated instruments.
He was joined by electrochemists, XRD scientists and analytical scientists drawn from the collaborating organisations - CSIRO, the Department of Mechanical Engineering at the University of Melbourne, and the Institute of Materials Engineering at the Australian Nuclear Science and Technology Organisation (ANSTO). Over five days at the synchrotron facility, three teams of two people ran the trial - an arrangement that teamed electrochemists Dr Graeme Snook and Andrew Urban with x-ray diffraction specialists Dr Matthew Rowles, Dr Nicola Scarlett and Mr Madsen. They were assisted by Mark Styles, Dr Daniel Ritey from ANSTO and Diamond Light Source synchrotron staff Dr Thomas Connolley and Dr Christina Reinhard.
Dr Kathie McGregor, who leads the project's electrochemists, says titanium was selected for the validation study because its extraction is a process that is well understood by her team from past studies. There is also interest in developing more efficient titanium extraction processes that avoid the use of carbon anodes.
Dr McGregor explains that there are two basic anode strategies that can be used in electrowinning systems. In the first, a carbon anode that is consumed at a rapid rate and produces carbon dioxide emissions is used. The anode requires frequent replacement, which reduces the efficiency of the cell operation. This approach is used for metals such as aluminium.
The alternative is to use a so-called 'inert' anode that is not consumed and evolves only pure oxygen from the melt. As well as reducing costs and greenhouse gas emissions, the 'inert' anode provides greater cell stability and avoids carbon contamination of the product. However, these kinds of electrodes can be prone to failure. "We understand how anode failure happens with an EbonexTM anode in a titanium electrowinning cell, so we used it for this project to test the use of XRD as an analytical tool," Dr McGregor says. "We can now use this new method to look at other materials and processes where we don't know the answer."
With a titanium-based anode, failure occurs when oxygen evolved at the anode causes the formation of a layer of rutile (TiO2) that initially protects the anode but eventually renders it electrically nonconductive. "We examined the anode's transformation into a nonconductive material that eventually halted the cell's operation," Mr Madsen says. "X-rays are a good probe to understand structural changes in the anode material, so part of what we looked at was the growth of these rutile layers on the anode."
With techniques validated for both data collection and analysis, the team now has a new method to study electrochemical and other metallurgical processes in extreme environments. "We validated the method with titanium, but we work with a variety of metals and processing environments," Dr McGregor says. "It boils down to advancing the study of changes in materials at high temperature and possibly high pressure."
Both team leaders believe the experiment has created valuable new skills in their respective research groups. "All our equipment and experimental kit had to be shipped to the UK, creating major logistical complexities," Mr Madsen says. "We then had five days at the facilities. To make the most of the beam time, we worked around the clock in three teams of two people. If we had walked away from this experiment with one successful run I think we would have been pretty happy, but we got six successful runs, which is a testament to the professionalism of the team. It was beautifully done."
Ian Madsen attributes much of the success of getting x-rays in and out of an operating electrowinning cell to Mark Styles. In collaboration with the team, Mr Styles designed and built the multipurpose cell used at the Diamond Light Source synchrotron in the UK. "The most difficult aspect was selecting construction materials, given the chemistry of the molten salt inside the operating cell," Mr Styles says. "Some of the materials I wanted to use would have dissolved in the molten salt or would have corroded too quickly, requiring frequent replacement."
Fully dense EbonexTM was selected as the anode material because it is electrically conductive and shows some stability in the bath for periods of up to eight hours. Moreover, its behaviour is well understood from earlier ex situ studies.
The cell's operation however, is not restricted to EbonexTM anodes; it was designed to accommodate different materials and shapes. Anodes can also be replaced without switching off the furnace and cooling the salt bath - a major time-saving design feature. Instead, the anode is attached to a stalk that is lowered into the molten salt - a process that takes just 30 minutes. "If we had to cool the cell, change the components and reheat, the changeover time would have been about 12 hours, resulting in a large loss of our valuable synchrotron beamtime," Mr Styles says. Mr Madsen is the supervisor of Mr Styles's postgraduate studies at CSIRO and says Mr Styles not only designed a working cell for this experiment but also assisted in proofing it for future experimental work.
Overall, Mr Styles is pleased with the way the cell performed during validation studies. The cell was first conceptualised in a three-dimensional computer-aided design program and then built to specification. "An important goal was to ensure the cell was representative of industrial systems," Mr Styles says. "We could have designed an even better cell for x-ray diffraction studies but we wanted to be sure that electrochemical processes were not compromised by the measurement techniques."
Achieving the right balance meant that the young mechanical engineer had to acquire expertise in electrochemistry as well as learn about how to collect and analyse x-ray diffraction data. "I found the multidisciplinary nature of this work really interesting and I highly recommend the Flagship Collaboration Fund scholarship for future PhD students," he says. "It provides opportunities that you would not normally get as a PhD student at a university, especially access to different skill sets."
Having acquired a taste for multidisciplinary work, Mr Styles intends to stay on at CSIRO as a postdoctoral fellow in metal systems, where he hopes once again to have an opportunity to use his mechanical engineering background.
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