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Metallugical Processes - Zinc
September 18, 2003

Introduction

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Imperial Smelting Process

The National Smelting Corporation (England) developed the Imperial Smelting Process based on their experience with zinc retorts and lead blast furnaces.  The Imperial Smelting Furnace (ISF) as a blast furnace that produced zinc.

The Imperial Smelting process treats primarily zinc concentrates.  The concentrates are heated, pelletised and reduced to zinc metal by coke in a blast furnace.  The Imperial Smelting Furnace can also treat a limited amount of secondary zinc material.

The first step in the process is sintering of the zinc concentrate.  Sintering achieves the following:

• Removal sulphur
• Agglomeration of the material into lumps
• Increase porosity which allows the gas-solid reaction to occur in the furnace
• Yield refractory lumps of sinter which do not soften at temperatures below 1140°C

Off-gas from the sintering process is directed to a sulphuric acid plant.

The sinter is mixed with preheated coke and fed to the top of the blast furnace where it is smelted using a preheated air blast introduced into the base of the furnace via tuyeres.  The air reacts with coke to form carbon monoxide which reacts with zinc oxide to form elemental zinc.

ZnO + CO -> Zn + CO₂

The reaction is reversible at temperature less that 907°C.  For this reason, the top of the furnace is maintained at 1000 to 1050°C.

The zinc vapour leaving the furnace is condenser in a lead splash condenser where the zinc dissolves into a pool of molten lead.   Lead from the condenser is cooled at which point the zinc separates from the lead and is tapped into ladles.  The zinc is then sent to the zinc refinery for further purification.

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