394
J. Salminen et al.
in the landfill in zinc production. However due to large space requirements, tightening
legislation, and loss of valuable metals, landfilling is not perhaps the best option in
the medium- and long-term future.
Investigations to minimize the waste generation and possibilities on how to process
them in the best way have been carried out in this work. Smelting the residues like
jarosite, goethite, ferrites, etc. is widely in use in non-ferrous industries instead of
landfilling of stabilized waste like Jarofix. The motivation of processing iron residues
can be legislation, direct ban of landfilling, scarcity of available land area, and targets
to achieve better yields and recoveries of critical by-product metals.
Generally, the pyrometallurgical residue smelting processes work in the same way.
After smelting at temperatures around 1250–1350 °C, the residue is decomposed
and volatile compounds such as Zn, Pb, Ag, In, Ge, As, Sb, Hg, Se, Cd, F, Cl, and S
are evaporated. The outcome is mixed metal oxide-containing dust (fume) and iron
silicate base slag. Slag forming materials include Fe, Si, Ca, Mg, Al, Mn, Na, and
K. Portion of Cu and Co will report into slag, and the distribution depends mainly
on the content of these metals in the feed material.
Boliden Kokkola has made several pilot tests with jarosite, combined residue as
well as with other feeds. The target has been to recover Zn, Pb, Ag, In, Ge, and
producing a slag for further utilisation. Moreover, the final safe disposal of elements
of concern such as As, Hg, and Cd needs to be in place.
The ScanArc plasma process marketed under the name ArcFume has been successfully demonstrated for smelting other types of zinc residues [5]. In this work
ScanArc plasma pilot plant in Hofors, Sweden has been piloted.
The recoveries of the valuable metals have been high, and good stable slag has
been obtained. The final slag has been low in target metals like Zn, Pb, and Ag. In
addition, the standard leaching tests have also given promising results.
Challenges
Jarosite and combined residue material is a sludge-like, fine material that needs to be
pre-treated prior to the smelting stage. The moisture content should be lowered down
to 25 wt% or less to avoid stickiness in the conveyer belt. In addition, the material
cannot be too dry as it becomes dusty. Suitable levels of moisture are within 10–25
wt%.
Smelting combined waste, i.e. jarosite plus sulphur residue, is challenging due to
the high level of elemental sulphur in the residue (30–40 wt%). Therefore, the right
oxidising conditions in the furnace are required during the smelting stage. Sufficient
amount of air must be fed into the reactor to avoid formation of FeS matte in the
smelting stage. In the smelting stage, the sulphur is burned out as SO 2 gas and jarosite
is decomposed into FeO and forms FeSiO 4 with SiO 2 present in the feed. Most of the
volatile metals are vaporized and recovered as oxide dust in the smelting stage. Still,
significant amount of Zn (2.5–1 wt%, typically about 1 wt%) and Pb (<0.3 wt%,
typically under 0.1 wt%) is left after the smelting stage. Subsequent steps in the
J. Salminen et al.
in the landfill in zinc production. However due to large space requirements, tightening
legislation, and loss of valuable metals, landfilling is not perhaps the best option in
the medium- and long-term future.
Investigations to minimize the waste generation and possibilities on how to process
them in the best way have been carried out in this work. Smelting the residues like
jarosite, goethite, ferrites, etc. is widely in use in non-ferrous industries instead of
landfilling of stabilized waste like Jarofix. The motivation of processing iron residues
can be legislation, direct ban of landfilling, scarcity of available land area, and targets
to achieve better yields and recoveries of critical by-product metals.
Generally, the pyrometallurgical residue smelting processes work in the same way.
After smelting at temperatures around 1250–1350 °C, the residue is decomposed
and volatile compounds such as Zn, Pb, Ag, In, Ge, As, Sb, Hg, Se, Cd, F, Cl, and S
are evaporated. The outcome is mixed metal oxide-containing dust (fume) and iron
silicate base slag. Slag forming materials include Fe, Si, Ca, Mg, Al, Mn, Na, and
K. Portion of Cu and Co will report into slag, and the distribution depends mainly
on the content of these metals in the feed material.
Boliden Kokkola has made several pilot tests with jarosite, combined residue as
well as with other feeds. The target has been to recover Zn, Pb, Ag, In, Ge, and
producing a slag for further utilisation. Moreover, the final safe disposal of elements
of concern such as As, Hg, and Cd needs to be in place.
The ScanArc plasma process marketed under the name ArcFume has been successfully demonstrated for smelting other types of zinc residues [5]. In this work
ScanArc plasma pilot plant in Hofors, Sweden has been piloted.
The recoveries of the valuable metals have been high, and good stable slag has
been obtained. The final slag has been low in target metals like Zn, Pb, and Ag. In
addition, the standard leaching tests have also given promising results.
Challenges
Jarosite and combined residue material is a sludge-like, fine material that needs to be
pre-treated prior to the smelting stage. The moisture content should be lowered down
to 25 wt% or less to avoid stickiness in the conveyer belt. In addition, the material
cannot be too dry as it becomes dusty. Suitable levels of moisture are within 10–25
wt%.
Smelting combined waste, i.e. jarosite plus sulphur residue, is challenging due to
the high level of elemental sulphur in the residue (30–40 wt%). Therefore, the right
oxidising conditions in the furnace are required during the smelting stage. Sufficient
amount of air must be fed into the reactor to avoid formation of FeS matte in the
smelting stage. In the smelting stage, the sulphur is burned out as SO 2 gas and jarosite
is decomposed into FeO and forms FeSiO 4 with SiO 2 present in the feed. Most of the
volatile metals are vaporized and recovered as oxide dust in the smelting stage. Still,
significant amount of Zn (2.5–1 wt%, typically about 1 wt%) and Pb (<0.3 wt%,
typically under 0.1 wt%) is left after the smelting stage. Subsequent steps in the