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I am assuming, you are talking about something like this https://www.youtube.com/watch?v=kokO6XqJvAE. If so, it's not sand, it's salt.

I am pretty sure it used to be sand. Comments on same youtube video:

> Sand is also used in a similar process, but only for roasting groundnuts/peanuts, which can be cleaned easily afterward.

Interesting, TIL.