A chemical called a depressant is added which is soluble in water (for example, zinc sulfate) and the zinc ore sinks and the lead ore is skimmed off. Later, a chemical such as copper(II) sulfate is added and the zinc ore now floats and is skimmed off. The lead ore concentrate from the flotation tanks now contains about 50% lead and ca % of silver, a relatively small but valuable amount. (b) Smelting