Salt is often purified by dissolving it in water and precipitating NaCl back out. This process doesn't always make sense economically, so it's understandable t

Salt is often purified by dissolving it in water and precipitating NaCl back out. This process doesn't always make sense economically, so it's understandable that Detroit uses their own salt for deicing and imports food-grade salt from places where it's cheaper to make. Or maybe in the past they just didn't care about a bit of contamination.