Answer: Well, I'd personally go with the tedious route and pick them all out by hand, but I know what doesn't really help. Just a suggestion though, sorry.
Explanation: Don't listen to my advice if you want any free time.
Answer:
Start by using a magnet to separate out the metal fillings.
Then put the mixture through a sieve to separate out the stones.
Then mix it into water and drain out the sand.
Finally, boil the water off to get the salt back.
Explanation:
The metal fillings should be magnetic, and so a magnet will attract only the metal and allow you to remove it.
Putting the mixture through a sieve will basically sift it, since the sand and salt will fall through the holes but the rocks will be too big.
Mixing the mixture into water will dissolve the salt but not the sand, allowing you to pour off the water and be left with sand.
Boiling the water will cause the H2O to become a vapor, while leaving the salt behind.
