Ammonium fluoride (NH4F) is used to fluorinate lithium, silicon, and aluminum. Heating molten NH4F produces hydrogen fluoride, so NH4F is dissolved in water and heated to about 70°C to form NH4F2 ions while releasing ammonia gas. The fluorine transfers to lithium, producing a water-based lithium fluoride solution. Silicon forms a soluble (NH4)2SiF6 ion, while aluminum forms a solid (NH4)3AlF6. Each compound is processed separately. Heating (NH4)3AlF6 to about 300°C yields aluminum trifluoride and releases ammonia and hydrogen fluoride. Heating to about 700°C reacts aluminum trifluoride with water to form aluminum oxide and more hydrogen fluoride. Hydrogen fluoride is then neutralized with ammonia to regenerate ammonium fluoride, with aluminum oxide used as a feedstock for aluminum metal production at over 98% purity.
"The key chemical in the process is ammonium fluoride (NH 4F). It's possible to use the salt directly in a molten form, but heating it invariably leads to some production of hydrogen fluoride, which is extremely dangerous stuff (although they end up using some later). So instead, they used it dissolved in water, which apparently keeps these reactions from occurring."
"In this process, heating the solution to about 70° C results in the formation of NH 4F 2 ions, releasing ammonia gas that's used later in the process. This ion donates a fluorine to the lithium, leaving a water-based solution of lithium fluoride. The silicon also forms a soluble ion, (NH 4) 2SiF 6), while the aluminum forms a similar ion that remains behind as a solid, (NH 4) 3AlF 6)."
"Initially, heating the (NH 4) 3AlF 6 to about 300° C produces aluminum trifluoride and releases ammonia and hydrogen fluoride. Then, raising the temperature to 700° C causes the aluminum trifluoride to react with water, leaving behind aluminum oxide and releasing yet more hydrogen fluoride. Hydrogen fluoride is dangerous stuff and needs to be handled carefully."
"But it's also easy to react it with the ammonia (which is produced during two different reactions here) and reform the ammonium fluoride that was used to start the whole process. So, aside from minor losses due to inefficiencies, the process regenerates one of the key ingredients. Meanwhile, aluminum oxide is one of the key starting materials for the production of aluminum metal, and so can be fed into that, given that the purity of the end product here was over 98 percent."
#ammonium-fluoride #fluorination-chemistry #hydrogen-fluoride-handling #aluminum-oxide-production #silicon-and-lithium-fluorides
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