Alum mine at Ravenscar

I have used alum when teaching about water purification. It is actually aluminium sulphate. Large soil particles in soil sink to the bottom of water under gravity but fine clay minerals don't. The stuff I've read says that the clay minerals are negatively charged. This means that they repel each other which stops them settling. They say that the alum when dissolved in water releases positive charges which attract the negative clay minerals forming particles heavy enough to sink and clear the water. This is called flocculation. This article explains it best https://en.wikipedia.org/wiki/Coagulation_(water_treatment) though I'm still trying to be clear about the difference between coagulation and flocculation. My description above sounds quite like a mix of the two...