The key chemical in the process is ammonium fluoride (NH4F). 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 NH4F2 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, (NH4)2SiF6), while the aluminum forms a similar ion that remains behind as a solid, (NH4)3AlF6). Each of these is processed separately.
Using everything
We’ll start with the aluminum chemistry, which is one of the simpler pathways. Initially, heating the (NH4)3AlF6 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.
Again, 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.
We’ll just note here that this is probably the worst aspect of the whole process, given the energy requirements for these temperatures and the highly dangerous chemicals involved.
By contrast, the silicon purification is a walk in the park. Simply adding more ammonia to the solution caused the starting chemical (NH4)2SiF6) to react with water, releasing silicon dioxide and ammonium fluoride. Again, an ammonium fluoride solution is one of the starting materials; the silicon dioxide simply precipitates out of this solution. That has a variety of applications, but the team showed that it’s quite effective at strengthening concrete.
