I’m not counting this as a Nuffield day, although it was interesting. It included a brief look around a salt…farm? factory? I’m not sure what it’s called. But they make salt, from seawater. 220,000 tonnes a year, which sounds like a lot.
Sea water contains three main cations, calcium, sodium and magnesium. As the water evaporates off, the first one to drop out of suspension is the calcium. It forms crystals of Gypsum (which is calcium sulphate I think, so there must also be sulphate ions present). This picture is one of those crystals, which are just left in the evaporations pans. Perhaps one day they will need cleaning out, but not for a long time.
At this point the water goes into a crystallisation pan, and it’s here that the sodium ions combine with chlorine to form salt, which settles on the bottom of the pond. The water, by now very concentrated with magnesium ions, turns this disconcerting shade of red. There is actually a use for this brew; it is packed into cardboard IBCs and shipped off to Japan to help make tofu.
When all of the sodium has precipitated, the water is drained off. A special machine then comes in and strips the layer of salt off the floor, and it ends up in these heaps. At this point it can either be used as is for certain industrial purposes, or it is washed, graded, and shipped out as food grade salt. So when you buy sea salt, it probably once sat in a pile like this, somewhere in the world.