Outotec offers two flexible process options for calcine leaching. Neutral leaching is a common stage for both process options. In the neutral leaching stage, zinc oxide is leached in reactors that are arranged in cascade. The clear overflow of the thickeners is transferred to the Solution Purification while the underflow is pumped to the Hot Acid Leaching and Ferrite Treatment. 
The purpose of the second stage is to leach zinc ferrites.
Long-term intensive research (since early 1970s) on the leaching of ferrites has led to the development of the Outotec® Conversion Process, which combines low investment costs with low environmental impact. In this process, simultaneous leaching of zinc ferrite, in the neutral leach residue, and the precipitation of jarosite and other impurities have led to very high recovery and energy efficiency.
The alternative for the conversion process is counter-current Hot Acid Leaching, which is designed for neutral leach residue with high lead and silver contents. The overflow of the Hot Acid Leach is sent to the iron precipitation step, while the lead/silver intermediate product is pumped to filter presses and then for further treatment at the lead plant.