If you are really concerned, you might drag a magnet through it to remove the metal. When we were kids we used to drag a magnet through sand in the high jump pit at the local school to collect the iron filings. I have wondered whether anyone has tried that with play sand. A small amount steel in the sand isn't going to hurt anything although you might not like the potential staining. More than likely it is from the crushing process, and not iron slag. From the pictures it looks like there is quite a bit, which is surprising.