Electrometallurgy_week 7_8.pdf

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1. MME-213
Extractive
Metallurgy
Week 7
Electrometallurgy

2. Extractive Metallurgy
•Electrometallurgy –refers to all
metallurgical processes which utilize
electricity and electrical effects.
•uses electrolysis to extract the metal.
•Electrowinning
extraction of metal from the electrolyte.
used extensively for the extraction of reactive
metals like Al and Mg.
Alternative to pyrometallurgical extraction for
metals like Cu and Zn
Inert anode
Metal ions in solutions
Metal from solution is
deposited on the
cathode

4. Extractive Metallurgy
•The approximate percentages of metals
produced by electrolysis are:
•Aluminium (100)
•Sodium (100)
•Lithium (100)
•Magnesium
•Chromium (70)
•Cobalt (50)
•Zinc (50)
•Copper (10)
•Nickel (10)
Inert anode
Metal ions in solutions
Metal from solution is
deposited on the
cathode

5. Electrolytic decomposition
•Used in both extraction and refining of metals.
Electrolyte
Aqueous solution
Cu, Zn, Sn, Au and Mn
Or a mixture of fused salts
Al, Na, Mg, K, Th, Zr, Ti
Desired properties of electrolyte
•Must have sufficiently high ionic conductivity and conduction must be entirely due to ions (electronic
contribution should not be there)
•Chemically inert towards the electrode, the container material, and the electrolysis products.
•Must be stable at the temperature of operation.
•Metal compound should dissociate at a voltage appreciably lower than that required for dissociation of
solvent.

6. Electrolysis of aqueous solutions
•The fact that water can dissolve relatively high concentrations of salt renders it
suitable for electrolysis.
•Aqueous solutions have high conductivity at ambient temperatures, especially when
acidic.
•The stages involved in the overall extraction process leading to electrowinning of
metals from aqueous solutions
1.Leaching of a roasted ore (i.e., an oxide) to dissolve in acid
MO
x
+ 2xH
+
→M
2x+
+ xH
2
O
2.Purification and concentration of leaching liquor
3.Anode and cathode reactions during electrolysis
x
6
5
6
6
+ 2xH
+
+2xe (anode in acidic solutions)
M
2x+
+ 2xe M (cathode)
4.Recirculation of acid liberated at anode for leaching process

7. Electrolysis of aqueous solutions
•During electrolysis, the reactions takes place at two distinct regions,
namely, at the cathode and the anode.
•At cathode, metal ion is discharged and the metal deposited.
•At anode, the reaction depends on nature of the electrolyte
•The reaction in acidic and alkaline solutions are
2H
2
O → O
2
+ 4H
+
+ 4e (E
o
= -1.23V)(acidic solutions)
4OH
-
→ O
2
+ 2H
2
O +4e (E
o
= -0.4V)(basic solutions)
The operating voltage of a cell should be greater than the difference between the potential difference
between cathode and anode reactions.

8. https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Elect
rochemistry/Electrolytic_Cells/Electrolysis_I

9. https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Elect
rochemistry/Electrolytic_Cells/Electrolysis_I
The direction of the reaction can be reversed by applying a voltage in excess of 0.74V and by attaching Cu to
the positive terminal and Cd to the negative terminal of the battery.

10. Electrowinning of Cu
•In the electrowinning stage of copper extraction,
the solution containing the copper ions is pumped
through a series of tanks.
•Suspended in these tanks are sheets of lead alloy
(anodes) alternating with cathodes made of either
thin copper starter sheets or stainless steel
blanks.
•An external power supply is used to pull electrons
out of the anode and push them to the cathode.
•The animation below represents one of these
electrolytic cells using copper starter sheets.
https://www.ausetute.com.au/thief.html

11. Electrowinning of Cu
Anode (positive electrode)
•Lead alloy plates
•Oxidation occurs at the anode.
•Water is oxidized at the anode.
•H
2
O
(l)
→ 2H
+
(aq)
+ ½O
2(g)
+ 2e
-
•Water is consumed at the anode.
•Oxygen gas and protons are produced at the
anode.
•Electrons are produced at the anode.
•Electrons flow from anode to cathode.
https://www.ausetute.com.au/thief.html

12. Electrowinning of Cu
Cathode (negative electrode)
•thin copper sheets (starter sheets) or
stainless steel sheets (blanks)
•Reduction occurs at the cathode.
•Copper ions migrate to the cathode.
•Copper ions are reduced at the cathode.
•Cu
2+
(aq)
+ 2e
-
→ Cu
(s)
•Solid copper is deposited at the cathode.
https://www.ausetute.com.au/thief.html
Note: H
+
is not reduced to H
2(g)
at the cathode. Cu
2+
is a
stronger oxidant than H
+
.
Cu
2+
is more easily reduced than H
+
.

13. Electrowinning of Cu
https://www.ausetute.com.au/thief.html

15. Electrolysis of fused salts
•Used primarily for reactive metals.
•Any metal can be obtained by direct electrolysis of its salts, for
example its halides.
•It is advantageous to dissolve a compound such as a halide into a
mixture of more stable halides such as sodium chloride and
potassium chloride.
•Operations in high temperature media such as molten salts have
some distinct advantages, for example, a reactive metal may be
readily obtained in absence of hydrogen ions.
•The metal produced if molten can be removed easily.

16. Electrolysis of molten salts
•When Na
+
ions collide with the
negative electrode, the battery
carries a large enough potential to
force these ions to pick up
electrons to form sodium metal.
•Cl
-
ions that collide with the
positive electrode are oxidized to
Cl
2
gas, which bubbles off at this
electrode.
•The net effect of passing an
electric current through the
molten salt in this cell is to
decompose sodium chloride into
its elements, sodium metal and
chlorine gas.
https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch20/faraday.php

17. Electrolysis of molten salts
https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch20/faraday.php
The potential required to oxidize Cl
-
ions to Cl
2
is -1.36 volts
and the potential needed to reduce Na
+
ions to sodium
metal is -2.71 volts. The battery used to drive this reaction
must therefore have a potential of at least 4.07 volts.

18. Electrolysis of molten salts
https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch20/faraday.php

19. Electrolysis of aqueous solutions
https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch20/faraday.php
•The figure below shows an idealized drawing of a
cell in which an aqueous solution of sodium chloride
is electrolyzed.
•Once again, the Na+ ions migrate toward the
negative electrode and the Cl-ions migrate toward
the positive electrode.
•But, now there are two substances that can be
reduced at the cathode: Na+ ions and water
molecules.

20. Electrolysis of aqueous solutions
https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch20/faraday.php
•Under ideal conditions, a potential of 1.23 volts is large
enough to oxidize water to O
2
gas.
•Under real conditions, however, it can take a much larger
voltage to initiate this reaction.
•The overvoltage for the oxidation of water can be as large as
1 volt.