The document discusses principles of non-ferrous metal extraction metallurgy. It specifically focuses on aluminum extraction and the Hall-Héroult process. It describes how bauxite is leached using caustic soda to produce aluminum oxide, which is then dissolved in cryolite salt and electrolyzed to extract aluminum metal. Key aspects of the electrolysis process like temperature, solubility, conductivity are discussed. Additives to the cryolite like calcium fluoride and aluminum fluoride are also summarized in terms of how they modify properties to optimize the electrolysis.

1. Principles of Non Ferrous
Extraction MetallurgyExtraction Metallurgy
Date 12th November, 2014

2. Syllabus
• General methods of extraction in Pyrometallurgy - Drying, Calcination,
Roasting, Smelting, Carbothermic and Metllothermic reduction, Refining
techniques like Liquation, Distillation, Vacuum Distillation etc. Principles of
hydro and electrometallurgy with suitable examples.
• Leaching techniques, Leaching solvents, Theory of leaching, Bacterial
leaching, Electrochemical nature of leaching, Gold and silver extraction.
• Pressure leaching, Sherritt - Gorden process for Copper, Nickel, Cobalt
ores; Solvent extraction, Ion exchange.
• Pressure leaching, Sherritt - Gorden process for Copper, Nickel, Cobalt
ores; Solvent extraction, Ion exchange.
• Electrometallurgy - Electrolysis of aqueous solutions and fuses salts, Cell
design, Recovery of metal values by Cementation, Electro-winning,
Electro-refining etc. Principles and important applications.
• Extraction of metals from oxides - Magnesium and Titanium extraction,
Bayer’s process, Hall Heroult process.
• Extraction of meals from sulphides, Extraction of Copper, Lead, Zinc,
Nickel.

3. Aluminium
• India is fortunate to have lot of deposit of
bauxite but Al extraction is energy intensive
• 40% of the price of Al we pay is the price of
electricity (thermal or hydel)
• Problem with thermal power plant due to• Problem with thermal power plant due to
generation of CO and CO2
• Hydel power requires dams!! Earthquakes,
displacement of people, submerges huge
areas
• Other option for electricity is Nuclear Energy,
Wind power (seasonal)

4. You must know: We have Al ore
but we don’t have sufficient power
for expansion of Al Plant
Interest: Strength to weight ratio
for Al (also resist corrosion) is
Interest: Strength to weight ratio
for Al (also resist corrosion) is
much stronger than steel (easier
and cheaper to produce)
Al has an advantage of low MP, formability, corrosion
resistance…only problem is costlier

5. Application of Al
• Vessels and containers
• Kitchen wares
• Equipment for chemical and brewing industries
• Milk processing
• Packaging
• Protective surfaces• Protective surfaces
• Structural applications (advantage is strength to
weight ratio)
• Deoxidizer (in steel)
• Electrical conductors
Excellent corrosion resistance of Al is due to formation of AlON on surface of pure
Al which does oxidize further..although the formation of Al2O3 from Al is highly
feasible.. Thermodynamics of Al to Al2O3 does not consider Nitrogen present in
surrounding..

6. Aluminium Ores
• Most common:
• Gibbsite (Al2O3.3H2O) (Al2O3- 65.4%)
• Diaspore (Al2O3. H2O) (Al2O3- 85.4%)
• You can say from above that Diaspore is good• You can say from above that Diaspore is good
..but is not so..it is difficult to leach Diaspore
with caustic soda..the one with more water is
easier to leach..
• Bauxite is the mixture of Gibbsite and Diaspore
with different amounts of Fe2O3, TiO2 and SiO2

7. Problems??
• Bauxite will be leached with caustic soda solution
to get enriched Al2O3
• Fe2O3 present is not dissolved in leach solution
and is separated along with TiO2 and SiO2..as red
mudmud
• Red mud is of no commercial importance as it
contains alkali ion..unsuitable for making brick or
use in blast f/c for producing ion..
• This problem can be solved with CO2 produced by
captive plants in Al industry..but again most of
this plants are far from place where red mud is
dumped

8. Red Mud

9. Excellent example
• Enriched alumina obtained from leach liquor is
electrolyzed by using cryolite (Na3AlF6, natural
mineral)
• Generally oxide does not dissolve in halides…oxides
dissolve in oxides and halides dissolve in halides..
• We can think of dissolving Al2O3 in slag (mixture of• We can think of dissolving Al2O3 in slag (mixture of
oxides)..but remember slags cannot be electrolyzed
• At around 1000 deg C, Al2O3 can dissolve in cryolite
and later can be electrolyzed…
• Graphite is used as electrode..so CO and CO2 will be
produced…now this CO2 can be used to with red mud
to neutrilize alkali..but industrial adoption is in
nascent stage…

10. Bayer
Process
This means this is above 100 deg
(catch is steam)..why..because
higher the temperature..faster
will be leaching..this is under
pressure using autoclaves
This is one of the
important challenge
Open to market
for other products
too
Here we prefer incomplete
precipitation because some
silica gets invovled during
leaching process..goal is.. in
electrolysis only pure Al2O3
must be introduced
Al(OH)3 if formed is broken
into Al2O3 at 1400 deg C, this
is calcination

11. Timings
Interestingly, Hall-Heroult process is
still continuing with little modification
from original one…

12. Just an year before Bayer came with
process than can produce pure Al2O3
Aluminium was once so expensive that an aluminium bar was displayed next to the
French Crown Jewels in Paris in 1855. Karl Bayer was an Austrian chemist who
developed a process for extracting alumina from bauxite, and along with the Hall-
Heroult process was able to reduce the cost of producing aluminium to drop by 80%
from 1854 to 1890.

13. Electrolytic Cell for Aluminium
Electrolysis
Al2O3 dissolved
Anodes are
graphite
blocks
Solid crust
Al2O3 dissolved
in cryolite

14. Hall-Heroult Process at around 1000
deg C
Al2O3
2. Al2O3 can dissolve up to 15% at
1000 deg C in cryolite
This is at the top because
you are continuously adding
from top
ρ=3.96 g/cc Al2O3
cryolite
aluminium
ρ=2.1 g/cc
ρ=2.3 g/cc
1. Density will increase
if more Al2O3 goes in
cryolite and that will
come to the bottom
3. This is the reason addition of Al2O3
is maintained so that it does not sunk
into liquid crylolite..
ρ=3.96 g/cc

15. Requirements of electrolysis process
• Lower liquid temperature (for higher current
efficiency..we don’t want current to generate
heat)
• Decrease solubility of metal
• Increase solubility of Al2O3• Increase solubility of Al2O3
• Increase electrical conductivity
• Decrease density for better separation of
metal/salt i.e between cryolite and Al
• Decrease vapor pressure and metal loss

16. Solvent cryolite needs some additives
to suitably modify solvent properties
• For example CaF2, AlF3, LiF, MgF2, NaCl decreases
the solubility of Al2O3 in cryolite (do we require
this?? If Yes than why??)
• Another if we increase temperature solubility of• Another if we increase temperature solubility of
Al2O3 in cryolite will increase. (here is trade off.)
• CaF2, AlF3, LiF, MgF2, NaCl, NaF, Al2O3, Temp will
decrease electrical conductivity. Why?
• Density will increase with CaF2, MgF2, otherwise,
AlF3, LiF,, NaCl, NaF, Al2O3, Temp will decrease
density

17. Solvent cryolite needs some additives to
suitably modify solvent properties..contd
• Viscosity will help in separation of metal and
salt again some salt increases and some salt
decreases
• Melting point will get affected..MP decrease is• Melting point will get affected..MP decrease is
an advantage..except NaF all will decrease MP
(Decrease in MP is to our advantage??)
• Metal solubility, Surface tension, vapor
pressure are all effected by additives
• Most common additives are CaF2 and AlF3.

18. Additives added must meet the
requirements of electrolysis process..
Current efficiency is determined by
CE= 254.92-
1.70CAl2O3+0.39C2
Al2O3+0.45XAlF3+0.055X2
AlF3+0.3CCaF2-0.23T+
129/(D+1)+25i
T is cell temp in deg CT is cell temp in deg C
i anode current density
D is inter electrode separation
C is concentration, wt %
X is wt% AlF3
(Note nature of + and – sign on CE)
More the inter electrode (cathode and anode) distance, D
will decrease

19. Key changes done in modern
operation in Hall Heroult process
• More continuous feeding of Al2O3 to cell..this
also mean continuous removal of Al from cell
• High excess AlF with some CaF• High excess AlF3 with some CaF2
• CE is increased from 7% to 96% today