Solubility,,,,,add water to the mixture,salt will dissolve to form a solution while sand is insoluble,,,,,decant the salt solution as filtrate and remain with sand as residue,,,,,if salt crystals are needed,,,,evaporate the filtrate to dryness.
Dissolve the mixture of sand and salt in water, make sure all salt is dissolved. Filtrate the solution a couple of times and use vacuum filtration at the end to obtain a clear solution. At this point you will have wet sand and a salty solution. Take the salty solution and evaporate water to obtain the salt.