Aluminum, number 13 on the periodic table, is a popular metal used in the construction, aerospace, automotive, and packaging industries. Aluminum is the second most produced metal in the world, after iron.
Aluminum doesn’t occur in nature as a metal; to make aluminum, first bauxite is mixed with sodium hydroxide to form aluminum hydroxide. Aluminum hydroxide is then heated and transforms into alumina (Al2O3). Aluminum is produced by the electrolyte reduction of alumina, known as the Hall-Héroult electrolytic process. During this process, carbon anodes are dipped in an alumina bath and a high current is applied. This burns the anodes and forms CO2, thus removing oxygen. Conductivity of the melt improves with the addition of fluorides, but the proportion of aluminum fluoride to sodium fluoride is critical and must be monitored in order to avoid “anode effects.”
Anode effects are brief process upsets that occur during the aluminum smelting process when an insufficient amount of alumina is available in the electrolyte bath and a rapid voltage increase results. Anode effects produce perfluorinated compound (PFC) emissions, powerful greenhouse gases associated with global warming and climate change in addition to being poisonous for the environment.
For several years, the aluminum industry has been making serious efforts to reduce PFC emissions resulting from anode effects. The Voluntary Aluminum Industrial Partnership (VAIP) was developed jointly by EPA and the primary aluminum industry to improve aluminum production efficiency while reducing PFC emissions. The Asia-Pacific Partnership, in association with the Aluminum Association, the International Aluminum Institute, and other national associations also is working to decrease the amount of PFC emitted during aluminum smelting. Partners in the project include the United States, China, Korea, Japan, India, Canada and Australia.
Through these organizations, many technologies and methods are underway to eliminate anode effects and their associated PFCs. Perhaps the most critical effort involves finding ways to carefully control the electrolyte bath composition during the smelting process.
A typical aluminum bath is composed of chiolite (Na5Al3F14), cryolite (Na3AlF6) and a number of minor components. The bath ratio determination (NaF/AlF3) requires a combination of elemental and phase analysis. Historically, simple XRD (x-ray diffraction) instruments fitted with an additional calcium detector were used to measure chiolite and total calcium. α-alumina (corundum) was measured by XRD as an approximation of free-alumina.
Recent developments show that a better control of the bath can be achieved by measuring more compounds. On the elemental side, there is a need for total calcium (reported as CaF2), total magnesium (reported as MgF2) as well as total oxygen (reported as Al2O3). On the phase side, chiolite is required together with fluorite (CaF2) and α-alumina. Such a complete analysis, including oxygen, is only possible in a combined XRF-XRD instrument working under vacuum. Tests verify that with integration of XRF and XRD capabilities in a single instrument, aluminum bath samples can be completely quantified with high sensitivity, reliability and excellent stability, allowing better control of the melt.
Comment below and let us know if you have tried this technique and if it resulted in better control of your melt.