# Use of cryolite and fluorspar in electrolytic reduction of pure alumina by Hall-Héroult process

Cryolite $$\ce{(Na3AlF6)}$$ and fluorspar $$\ce{(CaF2)}$$ are added to alumina to lower the temperature of the mixture from $$\pu{2000 °C}$$ to about $$\pu{900 °C}$$ and also to increase the conductance.

But my question is about the specific effect these compounds are producing. I mean, out of the two mentioned effects, which compound is doing what?

• It is my understanding that Hall originally used cryolyte to dissolve alumina so he could perform electrolysis. The molten cryolite was both solvent and electrolyte and electrolyzing cryolite itself was not a problem. My guess is that the fluorite just helped lower the melting temperature. Fluorite (from 'to flow') has been used for centuries to help 'liquify' oxide slag, in metallurical processing, so the slag could be more readily scraped aside. Maybe someone will provide a definitive answer. – Ed V Jul 8 at 18:13
• I don't know enough to post a full answer but as Ed V says $\ce{(CaF2)}$ is used "generally" in slag-metallurgical processes to control viscosity. It also provides a surplus of flourine ions, which I guess would be useful for dissolving and solvating the alumina - since the cryolite natively cannot handle much more aluminium ions, you need a surplus of fluoride. – Stian Yttervik Jul 9 at 7:36
• @ChakravarthyKalyan What was the point of your edit? Units should be depicted upright and with proper spacing; representation you've suggested contradicts both rules. If you are having trouble with a mobile app not being able to render \pu{…} macros, please see the corresponding Meta post. The short version is not to use a mobile app at all as it's pretty outdated. – andselisk Jul 9 at 11:04
• @andselisk ,in my mobile, these values are shown as "undefined control sequence\pu" twice. I can post screen shot of the same.Hence the edit. – Chakravarthy Kalyan Jul 9 at 11:48
• @ChakravarthyKalyan I think I already addressed this in my comment, please read the Meta post and suggestions within carefully. If your device doesn't support formatting features available in every web browser (and mobile web browsers, too), it's not a reason to downgrade the quality of the posts. – andselisk Jul 9 at 11:55