Neutral leaching of zinc calcine
The Neutral Leaching step is the most important section of the leaching, because the zinc oxide is leached in this step. The main equipment of this step are the leaching reactors and thickeners. The reactors 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.
Conversion process
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 a low environmental impact.
In this process simultaneous leaching of zinc ferrite, in the neutral leach residue, and the precipitation of jarosite and other impurities has led to a very high recovery.
The process is applied at New Boliden's Kokkola zinc plant (former Outokumpu's), at Valleyfield of Canadian Electrolytic Zinc and at Debari of Hindustan Zinc Ltd.
Hot acid leaching
If the neutral leach residue contains high lead and silver contents, the residue is leached in two counter-current Hot Acid I & II Leaching steps, where the main purpose is to break up the incoming zinc ferrite to obtain an overall high zinc recovery. Further, a lead-silver residue with high lead and silver content will be produced. The overflow of the Hot Acid Leach is sent to the iron precipitation step, while the lead/silver-residue is pumped to filter presses and for further treatment in lead processes, such as shaft furnace or QSL reactor processes.
Iron precipitation by jarosite or hematite process
For precipitating iron from the process we can engineer and build the Iron Precipitation plant by using the Jarosite Process and alternatively by using the Hematite Process. During the last decades we have gained wide experience in this area.
During the jarosite precipitation step the iron is precipitated selectively by addition of ammonia (or alternatively sodium, potassium or lead) out of the solution as a basic iron sulfate jarosite. Jarosite formation allows the co-precipitation of other impurities, which are brought into solution during the Hot Acid Leaching.
For further information, please contact  zinc(at)outotec.com
Brochure
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