Beneficiation of iron ore . United States Patent . Abstract: ... In practicing the invention, low grade ores such as taconite ore is fine ground in a rod and ball mill (not shown) so that at least 70% and preferably up to 95% of the ground ore is 500 mesh or less. Once the ore is ground to the fine grained state, it is introduced into a flotation cell such as that shown in FIG. 2 ...