How to obtain small pieces of pure and dry iron from a mixture of iron pieces and sand?

A magnet. Iron is magnetic, so any bits of iron should stick to the magnet, while non-magnetic sand is left behind. The iron will only be as pure as the chunks are, but there shouldn't be any sand mixed in, especially if you take the stuff off and do it again. A refrigerator magnet should be fine if you don't have anything else.