Principles of Non Ferrous

Extraction MetallurgyExtraction Metallurgy

Date 12th November, 2014

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.

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)

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

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 conductorsExcellent 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..

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

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

Red Mud

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…

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

Timings

Interestingly, Hall-Heroult process is

still continuing with little modification

from original one…

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.

Electrolytic Cell for Aluminium

Electrolysis

Al2O3 dissolved

Anodes are

graphite

blocks

Solid crust

Al2O3 dissolved

in cryolite

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

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

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

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.

Additives added must meet the

requirements of electrolysis process..

Current efficiency is determined by CE= 254.92-

1.70CAl2O3+0.39C2Al2O3+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

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