Treatment of Ferrite Residue

For this treatment both pyro- and hydrometallurgical processes are available, the latter being the most frequently used and comprising leaching and iron precipitation steps. The processes are named after the precipitated iron compound. 

Hot (high)-acid leach (HAL) of ferrite. This step is compulsory in almost every ferrite treatment process regardless of the iron precipitation method used. The zinc in the ferrite is brought into solution in an acid leaching step at close to 100 C and an acid concentration above 30 g/L H SO, the so-called hot-acid leach (HAL)  

Because the leaching also dissolves the iron, a removal step is needed. Moreover, silver and lead remain in the residue and are either recovered or disposed of depending on the economy of the process. Recovery of silver and lead is performed by smelting, in lead smelters, either the entire acid leach residue or the silver-rich lead concentrate produced by a flotation on the residue. In the case of flotation, the tailing is mixed with the iron residue. For precipitating iron from the leaching Hquor, three different processes are available  

The ferrite in the neutraUzed calcine remains unleached in the conditions normally used in the precipitation, and the corresponding zinc value is either lost with the jarosite or can be recovered in a subsequent acid leach step of the precipitated jarosite in conditions that will leach the ferrite but do not affect the jarosite. This is possible because of the high stabOity of jarosite in acid solutions. Extractions of zinc can thus be raised to 99%, even with an iron content of up to 10% in the concentrates. 

Conversion process. In this process, introduced by Outokumpu Zinc, the leaching of zinc ferrite and the precipitation of iron as jarosite are combined in a single step with the concentration of the sulfuric acid kept at 30 g/L by adding spent acid according to the following chemical reaction  

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