Question

Separate wax powder from sand

Answer 1

Melting wax changes it's phase from a solid to a liquid and you could filter out the sand.  

For sand and gravel, use a filter just fine enough for the sand to filter through.  

You could also do this for rice and salt as rice is larger than NaCl. NaCl ionizes in water as it forms hydrogen bonds, which would be an additional method of separation as it dissociates in H20