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What is polling?

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Hint: We come across the term ‘polling’ in metallurgy. It is a purification method. It is generally carried out for metals like copper or tin.

Complete step by step answer:
Polling or Poling is a method of purification for metals having their own oxide as impurity. We use polling generally to get rid of oxide impurities of the metal like copper oxide,$Cu{{O}_{2}}$ or tin oxide$Sn{{O}_{2}}$.
For the removal of oxide impurities, we use two methods which include oxidation and then reduction.
The first step is oxidation where we pass air through the furnace in which the molten metal is present and the iron oxide and sulphur dioxide impurities are removed. Sulphur dioxide is a gas and therefore exits the system itself and iron oxide slag is light therefore floats on the top and can be poured off.
The next step is polling which is a reduction process.
For this process, the liquid metal is stirred with a green log of wood. The heated, molten metal causes the wood to decompose and release the hydrocarbons present in wood. The majority of the hydrocarbon gas is methane.
As we know, methane is a hydride of carbon and we can use carbon as a reducing agent. Thus, it acts as a reducing agent and reduces the metal oxide to give us the pure metal. $Cu{{O}_{2}}+C{{H}_{4}}(g)\to Cu(s)+CO(g)+{{H}_{2}}O(g)$
$Sn{{O}_{2}}+C{{H}_{4}}(g)\to Sn(s)+CO(g)+{{H}_{2}}O(g)$
However, nowadays instead of the wood log, we use natural gases or diesel. These gases are also hydrocarbons and reduce the metal oxide to give us the metal.

Note:
We use a fresh green pole of wood here because if we use a dry log, it will burn easily as the temperature is very high inside the furnace.
During the reaction, it is possible that carbon dioxide is released instead of carbon monoxide. It depends on the oxygen hydrogen ratio in the reaction.