Poling (metallurgy)
Poling is a method employed in the purification of copper which contains cuprous oxide as an impurity. The impure metal, usually in the form of molten blister copper, is placed in an anode furnace for two stages of refining. In the first stage, sulfur and iron are removed by gently blowing air through the molten metal to form iron oxides and sulfur dioxide. The iron oxides are skimmed or poured off the top of the copper and the gaseous sulfur dioxide exits the furnace via the off-gas system. Once the first oxidation stage is complete, the second stage (reduction or poling) begins. This involves using a reducing agent, normally natural gas or diesel (but ammonia, liquid petroleum gas, and naphtha can also be used), to react with the oxygen in the copper to form copper oxide. In the past, freshly cut ("green") trees were used as wooden poles. The sap in these poles acted as the reducing agent. The heat of the copper makes the pole emit a gas that reduces the cuprous oxide to copper.