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in the smelting process for the extraction of tin,excess of lime must be avoided.why?

in the smelting process for the extraction of tin,excess of lime must be avoided.why?

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Suraj Prasad IIT Patna
askIITians Faculty 286 Points
9 years ago
4 The tin concentrate is placed in a furnace along with carbon in the form of either coal or fuel oil. If a tin concentrate with excess impurities is used, limestone and sand may also be added to react with the impurities. As the materials are heated to about 2550° F (1400° C), the carbon reacts with the carbon dioxide in the furnace atmosphere to form carbon monoxide. In turn the carbon monoxide reacts with the cassiterite in the tin concentrate to form crude tin and carbon dioxide. If limestone and sand are used, they react with any silica or iron present in the concentrate to form a slag.
5 Because tin readily forms compounds with many materials, it often reacts with the slag. As a result, the slag from the first furnace contains an appreciable amount of tin and must be processed further before it is discarded. The slag is heated in a second furnace along with additional carbon, scrap iron, and limestone. As before, crude tin is formed and recovered along with a certain amount of residual slag.
[The tin concentrate is placed in a furnace along with carbon in the form of either coal or fuel oil. It is heated and forms a slag along with the crude tin. The slag and crude tin are heated several more times to remove impurities and recover tin hardhead.]
The tin concentrate is placed in a furnace along with carbon in the form of either coal or fuel oil. It is heated and forms a slag along with the crude tin. The slag and crude tin are heated several more times to remove impurities and recover tin hardhead.
6 The residual slag from the second furnace is heated one more time to recover any tin that has formed compounds with iron. This material is known as the hard head. The remaining slag is discarded.

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