Zinc Plant Expansion and Modification for Increased Metals …
371
First Operational Results
Direct leaching was started at 50% capacity at the very end of 2018, at that time
utilizing five of totally eight leaching reactors. The capacity was then gradually
increased as all reactors were taken into use. Shorter periods with nominal capacity
were run from March 2019 onwards; however, there were limitations from other
plant areas as the stabilization of the integrated plant still needed time. At the time
of writing, nominal DL capacity is mostly reached. The leaching recovery of zinc
has been at a very good level, approx. 98%. Some issues with extensive foaming
in reactors and launder were experienced in the start-up, but the situation is now
controlled by increased dosing of anti-foaming agent.
The new solution purification plant was started up simultaneous with DL at the end
of 2018. The solution feed was initially around half of nominal. There were some
challenges in reaching the required Co removal efficiency in the start-up, which
in turn lead to high zinc powder use. The situation was improved by tuning some
operational parameters, and zinc powder use is now at designed level when operating
roughly with nominal feed capacity.
Modifications for Improved Metals Recovery
This section presents the ongoing modifications aiming at higher metal recoveries,
especially for silver. The calcine leaching sections are modified to a process for
precipitating pure jarosite. A feed pre-treatment system for the roaster furnaces is
installed, partly because of a new intermediate from the pure jarosite process that
is to be fed to the roaster. Differently from the DL project, these changes will not
significantly affect zinc production capacity.
Pure Jarosite Process
The traditional process for precipitating jarosite starts with iron in the ferric (Fe
3+ )
form according to
3Fe
3+ (aq) + X
+ (aq) + 2SO
2−
4 (aq) + 6H 2 O → XFe 3 (SO 4 ) 2 (OH) 6 (s) + 6H
+ (aq) (X = Na, NH 3 , K)
(3)
For each mole of iron precipitated, there are two moles of hydrogen ions (H
+ )
formed. As jarosite formation occurs favorably only in a certain acidity range, typically 10–20 g/l H 2 SO 4 , there is a need to neutralize the produced acid during the
precipitation. Typically, and as well at Torreón plant, the neutralizing agent is ZnO
of zinc calcine. However, the calcine also contains components that will contaminate
371
First Operational Results
Direct leaching was started at 50% capacity at the very end of 2018, at that time
utilizing five of totally eight leaching reactors. The capacity was then gradually
increased as all reactors were taken into use. Shorter periods with nominal capacity
were run from March 2019 onwards; however, there were limitations from other
plant areas as the stabilization of the integrated plant still needed time. At the time
of writing, nominal DL capacity is mostly reached. The leaching recovery of zinc
has been at a very good level, approx. 98%. Some issues with extensive foaming
in reactors and launder were experienced in the start-up, but the situation is now
controlled by increased dosing of anti-foaming agent.
The new solution purification plant was started up simultaneous with DL at the end
of 2018. The solution feed was initially around half of nominal. There were some
challenges in reaching the required Co removal efficiency in the start-up, which
in turn lead to high zinc powder use. The situation was improved by tuning some
operational parameters, and zinc powder use is now at designed level when operating
roughly with nominal feed capacity.
Modifications for Improved Metals Recovery
This section presents the ongoing modifications aiming at higher metal recoveries,
especially for silver. The calcine leaching sections are modified to a process for
precipitating pure jarosite. A feed pre-treatment system for the roaster furnaces is
installed, partly because of a new intermediate from the pure jarosite process that
is to be fed to the roaster. Differently from the DL project, these changes will not
significantly affect zinc production capacity.
Pure Jarosite Process
The traditional process for precipitating jarosite starts with iron in the ferric (Fe
3+ )
form according to
3Fe
3+ (aq) + X
+ (aq) + 2SO
2−
4 (aq) + 6H 2 O → XFe 3 (SO 4 ) 2 (OH) 6 (s) + 6H
+ (aq) (X = Na, NH 3 , K)
(3)
For each mole of iron precipitated, there are two moles of hydrogen ions (H
+ )
formed. As jarosite formation occurs favorably only in a certain acidity range, typically 10–20 g/l H 2 SO 4 , there is a need to neutralize the produced acid during the
precipitation. Typically, and as well at Torreón plant, the neutralizing agent is ZnO
of zinc calcine. However, the calcine also contains components that will contaminate