The batteries are first drained of acid and the outer casing removed manually or by crushing followed by magnetic separation of the ferrous materials and density separation of the lead. The lead is subsequently smelted with coke and fluxes in a rotary, reverberatory or blast furnace. Copper is removed by addition of sulphur (drossing) producing Cu2S as a dross, tin is removed as SnO2 and SnCl2 by addition of PbO2 and PbCl2 while arsenic and antimony are removed as Na3AsO4 and NaSbO3 by addition of NaOH and NaNO3 (Harris process) ; tin is also removed in this process as Na2SnO3. Each of these refining techniques is another example of preferential oxidation of the impurity element.