The Chemistry of Zinc Smelting
Use this information to create a FLOWCHART of the zinc smelting processes. Under each step
write a BALANCED CHEMICAL EQUATION of the chemical reaction taking place in that step.
Identify each TYPE of reaction (synthesis, decomposition, single replacement, double
replacement, combustion).
Zinc comes out of the ground as zinc ore which consists mostly of zinc sulphide (ZnS) and needs to
be smelted in order to obtain pure zinc. There are two processes used globally to smelt zinc from zinc
ore. These two processes are the pyrometallurgical and the hydrometallurgical (also called
electrolytic) processes. The electrolytic process is used in approximately 90% of smelting plants
worldwide. This process is more energy efficient, has higher recovery rates of zinc and is easier to
automate which results in higher productivity. The pyrometallurgical processes (there are a few) are
different from the hydrometallurgical process because they allow simultaneous production of zinc and
lead metals – roughly 1 ton of lead for every 2 tons of zinc. It is an energy intensive process which
has become very expensive with the rise of energy prices, and it produces lower grade zinc – only
98% pure as compared to the 99.995% pure zinc resulting from the hydrometallurgical process. The
pyrometallurgic process accounts for approximately 10% of global zinc production today and is in use
mostly in China, Japan, and Poland.
Before you start, contrast the two zinc refining processes.
ROASTING
Once raw zinc material is received at the smelting plant, the first step is to roast it in a roasting
furnace to remove the sulphur. It is heated to 950oC. About 90% of the zinc from the zinc sulphide
(ZnS) converts into the more active zinc oxide (ZnO) in the presence of oxygen in the air. Some zinc
also reacts with iron in the concentrate forming zinc ferrite (ZnFe2O4). The zinc oxide contains many
impurities such as minerals and waste materials which are known as gangue. Common gangue
minerals are silica (SiO2) as quartz or various silicates, aluminum oxide (Al2O3), calcite (CaCO3)
magnesium carbonate (MgCO3), barites (BaSO4), fluorspar (CaF2) and various iron compounds. The
impure zinc oxide produced by roasting is called calcine.
At the same time the sulphur (S) that is released from zinc sulphide reacts with oxygen (O2) to
produce sulphur dioxide gas (SO2).
The sulphur dioxide is then further oxidized to sulphur trioxide (SO3) which is then dissolved in water
to produce strong sulphuric acid (H2SO4).
During the roasting process, the gangue components decompose releasing various harmful gaseous
impurities such as Hg, Se, F, Cl, As. One example of a harmful gas released is hydrogen fluoride.
Zinc ores frequently contain fluorspar which is a halide mineral composed of calcium fluoride (CaF2).
When calcium fluoride reacts with the sulphuric acid byproduct, calcium sulfate (CaSO 4) precipitates
and hydrogen fluoride (HF) is released as a gas.
HYDROMETALLURGIC PROCESS
Then, if the process is the hydrometallurgical process, the following steps occur.
LEACHING
The next step is leaching. The point of this process is to dissolve the impure zinc oxide (calcine) and
transform it into zinc sulphate. Sulphuric acid is used in a two-part process using weak acid and then
strong acid. The reason why there needs to be two steps is because weak acid won’t dissolve the
zinc that is combined with iron (zinc ferrite is not soluble in weak acid). Strong acid will dissolve the
zinc AND the iron and then it’s a problem to separate the two.
First, weak sulphuric acid is added to the calcine. The zinc oxide (ZnO) dissolves readily in dilute
sulphuric acid (H2SO4) forming soluble zinc sulfate liquid (ZnSO4). The zinc sulphate liquid is impure
because it contains metallic impurities such as arsenic, antimony, cobalt, germanium, nickel and
thallium. So it needs to be further purified. The zinc ferrite in the calcine does not dissolve in the
weak acid.
Next, strong, hot sulphuric acid (H2SO4) is added to the remaining calcine to dissolve the zinc ferrite
(ZnFe2O4) and any remaining zinc oxide. The iron separates from the zinc forming zinc sulphate,
ferric oxide (Fe2O3), also known as rust, and water. A number of processes, one of which being the
jarosite process, can be used to precipitate the dissolved iron in a readily filterable crystalline form.
Other metals (lead and silver) and gangue components also precipitate out of solution. The zinc
sulfate that was separated out is added to the impure zinc sulphate liquid from the weak acid.
PURIFICATION
The zinc sulphate solution (ZnSO4) then needs to be purified to
remove other non-iron dissolved impurities such as cadmium,
copper, cobalt or nickel which exist as soluble compounds (not
by themselves). Because zinc is a highly reactive metal this
property can be used to remove the impurities. As all the
elements to be removed lie below zinc in the reactivity series
(see diagram) they can be precipitated by a process called
cementation.
By adding powdered zinc (Zn) and steam to the solution the
zinc is oxidized and dissolves, with the opposite side of the
reaction being the reduction of the other dissolved metals back
to their solid metallic form which can be separated from the
zinc sulphate (ZnSO4) solution by filtering.
For example, when powdered zinc (Zn) is added to the zinc
sulphate solution the zinc bumps the copper from copper
sulphate (CuSO4) – one of the impurities in the zinc sulphate
solution. This results in pure copper precipitating out of
solution and leaves a purified liquid zinc sulphate (ZnSO4). It
works in the same way with the other dissolved metal
compounds.
The purified zinc sulphate (ZnSO4) solution then moves on to
the next step which is electrolysis.
Activity Series of the Elements
Activity of Metals
Li
Rb
K
Can react with cold
Ba
water and acids,
replacing hydrogen
Sr
Ca
Na
Mg
Al
Can react with
Mn
acids or steam, but
Zn
not usually liquid
water, to replace
Cr
hydrogen
Fe
Cd
Co
Can react with
Ni
acids but not water,
Sn
to replace hydrogen
Pb
H2
Sb
React with oxygen,
forming oxides
Bi
Cu
Hg
Fairly unreactive.
Ag
Form oxides only
Pt
indirectly
Au
ELECTROLYSIS
During electrolysis, an electrical current is passed through the solution of zinc sulphate (ZnSO4) in
water. This is done to obtain pure zinc. A current is passed between lead alloy anodes and
aluminium cathodes and metallic zinc is deposited onto the aluminium cathodes by a reduction
reaction; in electrolysis there always has to be an opposing oxidizing reaction at the anode and, in this
case, it is the liberation of oxygen gas from water. Periodically the cathodes are removed and the
99.995% pure metallic zinc is harvested. This phase uses lots of electrical energy. Sulfuric acid is
also formed as a byproduct.
The sulphuric acid produced during this process is returned to the leaching plant to leach zinc oxide
(calcine).
PYROMETALLURGIC PROCESS
The other process is the pyrometallurgic process. This process involves thermal reactions in which at
least one product is a molten phase. The main feature of pyrometallurgic zinc smelters (also called
thermal zinc smelters) is that zinc is produced by the use of heat and carbon. Zinc was first produced
by this method. This was the process used in the Donora Zinc Works.
SINTERING
After roasting, the zinc oxide calcine is sintered. Combustion gases are drawn through the sinter
material. The carbon in the combustion gases (CO2) reacts with some impurities such as lead oxide
(PbO), cadmium oxide (CdO), and halides such as fluorine (F) forming lead carbonate (PbCO3),
cadmium carbonate (CdCO3), carbon fluoride (CF4) etc. which are given off as dust and gas.
RETORTING
The zinc oxide is then heated to approximately 1500°C with coke (fuel) or charcoal (forms of carbon)
in an electric retort furnace. The coke (C) combines with oxygen forming carbon monoxide. The
carbon monoxide reacts with zinc oxide to form zinc vapor and carbon dioxide which is released into
the atmosphere or regenerated. Then 95% of the zinc gas is condensed in a vacuum condenser to
molten zinc.