The impure metal, usually in the form of molten blister copper, is placed in an anode furnace for two stages of refining.
[2] 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.
This involves using a reducing agent, normally natural gas or diesel (but ammonia,[2] liquid petroleum gas,[2] and naphtha[3] can also be used), to react with the oxygen in the copper oxide to form copper .
Care must be taken to avoid removing too much of the oxygen from the anode copper, as this will cause other impurities to change from their oxide to metallic states and they will remain in solid solution in the copper, reduce its conductivity and change its physical properties.
Also upper surface can be covered with coke to prevent reoxidation of metal.