beneficiation (separation and concentration); (3) smelting; and (4) refining. Each stage is
discussed in detail in Section 3.5.1 through 3.5.3.
Comminution is the method by which the size of solid materials is reduced by crushing,
grinding, and other processes. Breaking the rock into smaller fragments helps to either liberate
certain particles of interest or increase the surface area to facilitate processing.
This part of the process occurs at the mine site, sometimes very early in the mining process, such
as in the open pit or underground (with a crusher) to break the mineral-bearing ore to a small
enough size to facilitate its transport to the treatment plant. Once the material is transported into
the plant, several types of crushers, mills, and screens are often used in sequence (with feedback
loops) to reduce the material to a fine enough fraction for recovery.
The exact transition between comminution and beneficiation is not clearly defined, but generally,
beneficiation is the process whereby extracted ore from mining is separated into mineral and
(the former being suitable for further processing or direct use).
In the processing of many base metals, such as copper, lead, zinc, and nickel, this first stage of
beneficiation occurs at the mine site. But full separation/beneficiation cannot be completed at the
mine due to the metallurgical complexities and scale of operation required.
Instead, the mill (or treatment plant on site) separates the compounds from the ore by flotation to
produce concentrates, which are the typical products from a base metal mine. The concentrates
are then transported to a smelter that is usually a long distance from the mine. The concentrates
often contain small quantities of precious or special metals such as gold, indium, or tellurium,
which can improve the value, and may contain undesirable impurities such as mercury, sulfur, or
arsenic, which reduce the value.
In the case of indium-bearing zinc sulfide ores (sphalerite), the indium may only be as
concentrated as 1–100 ppm in the sphalerite; the zinc grade might be as low as 2% (20,000 ppm).
After passing through the treatment plant, 50%–70% of the indium contained in the ore may
report to the concentrate. Typically, indium occurs in the concentrate at 120 ppm
to 170 ppm
(Alfantazi and Moskalyk 2003). This concentration varies greatly by mine (i.e., the technology in
the Peruvian and Bolivian zinc concentrates have 187 ppm and 630 ppm of indium content,
respectively, which makes these two countries major indium players compared to their share of
world zinc production (Moss et al. 2011).
Gangue refers to material of little to no economic value that surrounds, or is closely mixed with, a wanted mineral in an ore
The zinc residues feeding the Akita plant in Japan are reported to have this indium concentration level (Alfantazi 2003).
As anticipated by Ausmelt of Australia (Alfantazi and Moskalyk 2003).
Once zinc concentrate is transported to a smelter, zinc metal can be produced via either
pyrometallurgical or hydrometallurgical processes. About 90% of total zinc refining is done
through hydrometallurgical processes; thus, we describe and focus on this process.
The hydrometallurgical zinc smelting generally consists of four separate stages (see Figure 8):
1. The Waelz process (often referred to as calcining/roasting). A mixture of zinc
concentrate and coal is heated at high temperatures to produce a calcine of impure zinc
leach process to produce a zinc sulfate solution.
solution. During this stage indium and other elements such as copper or cadmium can be
4. Electrowinning. The aqueous zinc solution is contained in an electrolytic cell and an
electric current from a lead-silver alloy anode is used to deposit zinc onto the aluminum
cathode. Zinc can then be stripped from the aluminum cathodes and melted and cast into
ingots (ILO 2012).
Roasting & calcine
Zn( 3-11%), Cd (0.001-0.2%) In -(0.0001-0.01%) - (Fthenakis et al., 2007)
In (0.001% to 0.002%) - (Alfantazi and Moskalyk 2003)
Melting & casting
Precipitates of: Cd,
sludge, Ge, In, Ga,
Pb & Zn.
30%Zn, 30%Pb, 3.5%As, 3%Cd,
0.4%In (Fthenakis et al., 2007)
0.32% Ga, 0.58% In (Roskill, 2010)
Figure 8. Smelting of zinc ores to yield zinc slabs and indium precipitate
The main source of indium for primary refining is from the fumes, dusts, slags, and residues in
zinc smelting. Refineries can sometimes be located near the smelters in a “combined”
metallurgical complex, as was the case with Xstrata’s Kidd Creek smelter, or they can be
separate standalone refineries that source their feed materials on global markets, as is the case