Because of their very low solubility products, aluminum hydroxide and ferric hydroxide are almost impossible to precipitate slowly. Adding a solution of the chemical for coagulation to the water produces relatively high local concentrations as solutions of these chemicals contact the water. Even with violent mixing, these local concentrations persist long enough to initiate an uneven precipitation. This effect, and the very rapid precipitation, result in an opalescent precipitate that is gelatinous in character. Fortunately, it tends to settle quite well. The sedimentation of ferric or aluminum hydroxide captures other particles and aids greatly in their collection. This gives rise to the name sweep floc for a gelatinous precipitate that carries just about all colloidal material with it.
Not only does a sweep floc entrap other particles, it incorporates foreign ions into the precipitate itself. When ionic salts precipitate very slowly, each ion finds its proper place in the crystal lattice. A well-formed crystal tends to be extremely pure- this is why the electronics industry makes silicon wafers by slicing crystals that have been melted and recrystallized several times. When a crystalline material must form very rapidly, as in the case of aluminum or ferric hydroxide driven by great insolubility, there is insufficient time for putting each ion in the correct place. In fact, a foreign ion may be substituted for the correct ion. This gives the desirable effect of removing some metal ions by jamming them into the precipitate.