Björn Saxén, Marko Lahtinen, Juho Savikangas
Jarosite precipitation is a widely utilized iron removal process in the zinc industry. However, in its
standard realization the process has a significant drawback of causing losses of valuable metals like
zinc, silver and lead to the jarosite. This comes mainly from the content of these metals in the calcine
used for neutralization.
Outotec has developed an improved version of the jarosite precipitation process, which minimizes the
need for neutralization, minimizing also the metal losses to the jarosite. Thus, the pure jarosite also
contains less amount of environmentally hazardous metals. The main feature of this process is to start
the precipitation from ferrous iron after having reduced the iron by zinc concentrate in a separate step
before jarosite precipitation. Iron is then oxidized back to the ferric form simultaneously with jarosite
precipitation. By this method, the formation of sulphuric acid is only half of that compared to the conventional
method and neutralization with calcine is avoided. Outotec has now verified this pure jarosite
process by extensive laboratory and continuous bench-scale tests.
The change to pure jarosite is not only a change of chemistry and flowsheet, as it also emphasizes the
requirements for reactor performance. High performing reactors ensure the precipitation efficiency
and maximize oxygen utilization; i.e., minimize operation costs. The required efficient mixing and
oxygen dispersion capabilities are found in the Outotec OKTOP® reactor series. Optimal solution to
guarantee the best technical and operation result is the OKTOP® plant product including not only the
reactors, but also all the related equipment for the reactor series.
In addition to the jarosite precipitation itself, the change to pure jarosite implies new process steps for
reduction and neutralization. However, our case studies show that the increased metal recoveries
achieved with the pure jarosite process easily make the investment highly attractive, with a payback
time of only a few years. Outotec can design all needed process and equipment changes, and typically
delivers at least the new key equipment. One responsible technology supplier ensures efficient and fast
delivery and can provide both mechanical and process performance guarantees for the entire delivery.