Slag fuming zinc recovery

Indium may be recovered from zinc ores by several patented processes. Usually it is recovered from residues obtained from zinc extraction. The residues, slags, fume, or dusts from zinc smelting or lead-zinc smelting are treated with a mineral acid. Other steps involved in recovery often vary, but mostly use solvent extraction and precipitation steps. In some processes, treatment with caustic soda yields indium hydroxide. The hydroxide is calcined to obtain oxide, which then is reduced with hydrogen at elevated temperatures to obtain the metal. Distillation or electrolysis are the final steps to... 

The complex system of recycle streams betwe i and within each of ftie operating units ensures a higih overall recovery of the major products. For example, lead, bismuth and precious metals are recovered firom both the lead concentrates and the zinc concentrates treated. Recovery of lead is also significantly enhanced by operation of ftie slag fuming and oxide leaching circuits. Lead not recovered directly to bullion in the KIVCET furnace is recycled... 

There is only one day of capacity between the zinc fume recovery process in the smelter and the fume leach process in the zinc plant. Therefore, if there is an upset in the leach circuit, it is necessary to stop the slag fuming process for zinc recovery and stockpile smelter slag for recovery in the future. The same is true for the flow of residue from the zinc plant circuits to the smelter. Residue slurry is transported from the zinc plant to the smelter, where it is... 

From Table 7.4 bullion with less than 0.5 per cent sulfur can be achieved as well as slags containing around one per cent lead. However, where slag fuming facilities are available for zinc recovery, low lead levels can be sacrificed for furnace throughput as in the case of the Teck-Cominco owned Trail plant. The lead content of slag will be almost fully recovered with the zinc oxide fume and will be recycled in the form of zinc plant leach residues containing lead as lead sulfate. 

Off-gas from the electric furnace can contain significant amounts of lead and zinc, which are oxidised to a fume by the addition of air in an after burner chamber. If high zinc recovery is required from the slag the electric furnace temperature must be raised, but this will cause a substantial increase in the amount of lead to fume. Bullion and slag are separately and continuously tapped from the electric furnace. 

If the zinc level in the slag after lead reduction is sufficiently high to make its recovery economically attractive, or if the slag is cleaned to a low metal content to facilitate its use in a specific market requiring such low metal contents, a zinc fuming stage is carried out. 

Recovery of zinc was not required by Gold Fields of Namibia and the slag contains approximately 11% zinc. Levels below 4% lead are not required in this design in order to minimise fuming of zinc, though in practice, levels of 2% lead were routinely obtained. 

As well as removing zinc, fuming will also remove most of the lead content of the slag and if this is recycled by return of zinc smelter residues to the lead smelting stage, then almost complete lead recovery can be achieved in an integrated operation. This ability of lead to fume is illustrated by the vapour pressure of lead compounds at 1250°C ... 

As with zinc, lead will fume efficiently, possibly assisted if some sulfur is present in the slag. A slag containing 2.5 per cent Pb will be reduced to less than 0.05 per cent with a lead recovery in excess... 

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