CHAPTER 6 Extraction of metals from oxide members.pdf

DEFENCE ENGINEERING COLLEGE
Metallurgical And Materials Engineering
Present By:
Weldebrhan Hadush (Maj.)
Dec., 2019

Content
Introduction
Magnesium extraction
a. Pidgeon process
b. Magnotherm process
Extraction of metals from oxide members
c. Electrolytic process (Dow process )
Aluminum extinction
a. Baye’s process
b. Hall- heraoult process
c. Methods of titrating low grades ores
d. Newer process for Aluminum production
Tantalum extraction

INTRODUCTION
 During metal extraction, the bulk of the metal is separated from the impurities
present in the ore by using a system which invariably comprises more than one
phase.
 The metal enters the metallic phase while the impurities enter the non metallic
phase or phases.
phase or phases.
 The driving force for the reactions involved in metal extraction depends on the
differences in the chemical potentials of the chemical elements.
 During a chemical reaction, an element tends to move from a high to a low
chemical potential state.
 In each extraction process, the basic aim is to create conditions conductive to such
a transfer, which facilitates the extraction of a metal or its compound
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Cont’d
 The main active substances present in nature, especially in the atmosphere are
oxygen and carbon dioxide.
 In the earth's crust, sulphur (S) and silicon (Si) are found in large quantities.
 Most active metals are highly electropositive and therefore exist as ions. It is for
this reason that most of the important ores of these metals occur as: oxides,
sulphides , carbonates, halides and silicates.
 A metal ore is a rock containing a metal, or a metal compound in appreciable
quantities and from which the metal can be extracted economically.
 For example, Copper is easily extracted, but ores rich in copper are becoming
more difficult to find.
 Aluminum and titanium are metals with useful properties, but they are expensive
to extract.
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Metallurgy of Magnesium( Mg)
Atomic number of magnesium is 12 and its atomic weight is 24.3 and denoted by
symbol Mg.
Magnesium, is light metal has a density of 1783 Kg/m3, (high strength to weight ratio)
which is two thirds of Al and one sixth of steel. Due to magnesium ion’s high solubility
in water, it is the third most abundant element disolved in in sea water.
Occurrence
Magnesium is an active metal and does not occur in free state in nature. Someof its
importance minerals are megnesite (MgCO3), dolomite (MgCO3.CaCO3), Kieserite
(MgSO4.H2O) and Epson salt (MgSO4.7H2O)
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EXTRACTION OF MAGNESIUM (Mg)
Thermal magnesium processes are of two types:
i. Carbothermic: Carbon is used as reducing agent
Carbonthermic reduction of MgO takes place according to the reaction:
2MgO + C → 2Mg (g) + CO(g) @ 1850 °C
EXTRACTION OF MAGNESIUM
The Gibbes energy of reaction become zero arround 1850 °C corrosponding to
PMg = PCO = 1atm/760mmHg
Genarlly to reduce this temperature vacuum can be used, so this is not cost effective.
Unfortuantely the reaction reverse on cooling and, unless the cooling is made very
rappidly , complete revision to MgO and carbon will occur.
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Cont’d
ii. Metallothermic: reducing agent Al, Ferrosilicon
Ferrosilicon as reducing agent
2CaO +2MgO + Si → 2Mg (g) + Ca2SiO2 (s)
P =16mmHg @ 1200 °C (from thermodynamics data calculation)
PMg =16mmHg @ 1200 °C (from thermodynamics data calculation)
PMg =760 mmHg @ 1700 °C
NB: Most of the industries process so far low temperature and reduced
pressure.
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Cont’d
There are three principal magnesium extraction processes:
1. Silicothermic process (Pidgeon process),
2. Magnotherm process and
3. Electrolytic process (Dow process).
3. Electrolytic process (Dow process).
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Cont’d
1. Silicothermic (Pidgeon) Reduction of MgO by Ferrosilicon- Calcined dolomite ,
Pure Si reduces MgO when both reactants are in standard state. To overcome these
difficulties, we will be taken Ferro-Si powder to make in briquette(compressed together)
form (75% Si) with calcite dolomite to give Mg in vapour form at (1100-1200) °C under
vacuum of 0.1 mm Hg in external of retort.
2MgO + 2CaO + (Fe)Si → 2Mg(g) + Ca SiO (s) + Fe
2MgO + 2CaO + (Fe)Si → 2Mg(g) + Ca2SiO4(s) + Fe
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Fig: Retort for Pidgeon Process

Cont’d
The reaction takes place between three solid phases in addition to the gaseous product
fourth solid phase fromed.
And one of them expect slow and added Fluorspar (MgF2 or CaF2)as catalyst to
speed of the evolution of Mg vapour.
In the silicothermic magnesium process, magnesium vapor is the only gaseous product.
Thus there is no danger of back reaction during cooling and condensation of the gas.
This is one of the advantages of the process.
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2. Magnotherm process
Magnotherm process is essentially a ferrosilicon reduction process similar to the
Pidgeon process, except that it is carried out at a temperature of 1500 °C and the bath
is maintained in a molten state by the addition of alumina to form a molten slag.
Magnotherm process has been in operation since 1964 in France.
The production of magnesium from sea-water , which contains MgCl2, is the
largest source of magnesium.
The process for magnesium recovery from sea-water, which is being described, was
developed by the Dow Chemical Company (U.S.A).
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3. Electrolytic process (Dow process)
The major part of the world’s magnesium production is obtained on a large scale by
the electrolysis of fused anhydrous magnesium chloride, which is obtained from
sea water or from hydrous carnallite (MgCl2 .KCl.6H2O).
From sea water: sea water is collected in large vats. A slurry of slaked lime,
Ca(OH)2 is added. Magnesium hydroxide is less solubile than calcium hydroxide and
Ca(OH)2 is added. Magnesium hydroxide is less solubile than calcium hydroxide and
is therefore precipitated and Ca(OH)2 remains dissolved.
The hydroxides of sodium(Na) and potasium (k) , being soluble remains in solution.
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Cont’d
After filtration, the magnesium hydroxide precipitate is converted to a solution of
magnesium chloride (MgCl2) by disolving in hydrochloric acid (10 %HCl).
Mg(OH)2 + 2HCl = MgCl2 + 2H2O
The solution of MgCl2 in water is then concentrated and dried to get a salt.
MgCl2.2H2O the fused mass containing 35% MgCl2 15% CaCl2 and 50% NaCl at
MgCl2.2H2O the fused mass containing 35% MgCl2 15% CaCl2 and 50% NaCl at
about 680-750 °C is electrolyzed to get magnesium.
NB: CaCl2 and NaCl is added to lower MP of anhydrous MgCl2 .
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Cont’d
Electrolysis: The electrolysis is carried out in an iron crucible which server as the
cathode (metallic Mg is formed) . The anode is a graphite rod surrounded by a
porclain tube through which the chlorine gas scapes (chlorine, which is by-product of
the process).
The electrolyte is fused of MgCl2 to which KCl is added to lower the melting point.
The electrolyte is fused of MgCl2 to which KCl is added to lower the melting point.
Fig. Electrolytic cell for
magnesium production
MgCl2→ Mg++ + 2 Cl-
Cathode: Mg++ +2e- →Mg
Anode: Cl- - 2e- →Cl2
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Mixing
Thickening
Filtration
Overflow to sea
Mg(OH)2
Lime
Sea-Water (0.13% Mg)
Mixing
Evaporation
Electrolysis
MgCl2
Mg
MgCl2
Fig: Production of Mg
from Sea-Water
Cl2
HCl
H2 or Steam
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Use of magnesium
 In flashlight photography and mobile frames,
 For igniting Al powder in thermite welding
 As reducing agen in metallurgy
 For removing last traces of air from radio tubes.
 In manufacturing of light alloys . Magnesium contain aluminum (5-15%Mg)
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Metallurgy of aluminium (Al)
Aluminum is one of the most abundant material in the Erth’s crust. Atomic number of
Aluminium is 13 and its atomic weight is 26.96 and denoted by symbol Al. It is a
reactive metal. We might expect it to corrode easily but a strong coating of oxide on
it’s surface prevents this in most everyday situations.
Aluminium, is light metal has a density of 2.7 g/cm3, (high strength to weight ratio)
Occurrence
In nature, aluminum does not exist as a metal because of the high chemical affinity for
oxygen. It occurs in igneous rocks chiefly as alumino silicate in feldspar, feldspathoids,
and mica; in the soil derived from them as clay and upon further weathering as bauxite.
Bauxite is hydrated aluminum oxides and contains varying amount of impurities such as
Fe2O3, TiO2 and SiO2.
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EXTRACTION OF Aluminum (Al)
The extraction of aluminum is called elctro-metallurgy. It deals with the use of
electricity for smelting or refining of metals. In electrometallurgy, the electrochemical
effect of an electric current brings about the reduction of metallic compounds and the
extraction of metals from their ores or the electro-refining of the metals.
The extraction of aluminum, in principle , should be easy because the ore occurs in
EXTRACTION OF ALUMINIUM
oxide form which can be reduced by a suitable reducing agent to give the metal.
However, in practice, aluminum oxide can not be reduced easily. Aluminum has great
affinity for oxygen hence it can not be reduced by usual reducing agents
Aluminum is too high in the electrochemical series ( it is a highly reactive element ) so
it can not be reduced by hydrogen or carbon. If at all reduced by carbon, the
temperature required for the reduction is very high and not economic. Hence aluminum
is obtained by the electrolysis of pure alumina.
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I. BAYER PROCESS
This process is used when bauxite ore contains appreciable amount of Fe2O3 (7-10 %)
and low amount of silica ( < 1 %) that high –purity alumina is extracted from bauxite.
First calcined the ore and then finely ground. Then it is digested with a hot and strong
solution of caustic soda ( 45 % ) in an autoclave under 5-25bar pressure at 150 °C for 2
to 8 hours.
At this stage, aluminum oxide dissolves in NaOH to form sodium meta aluminate (
NaAlO2) while ferric oxide and titanium dioxide remain undissolved.. They are
removed by filtration.
Al2O3 + 2 NaOH → 2NaAlO2 + H2O Sodium metal aluminate ( soluble )
Silica dissolves in the form of silicate. After filtration, sodium meta aluminate solution
is diluted with water, slowly cooled and then mixed with a little freshly precipitated
aluminum hydroxide which acts as a nucleus for precipitation of aluminum
hydroxide. 18

Cont’d
Alternatively CO2 can be passed till the solution becomes acidic it is then digested.
Sodium meta aluminate, NaAlO2 hydrolyses to give precipitate of aluminum
hydroxide.
NaAlO2 + 2H2O → NaOH + Al(OH)3 ↓
Aluminum hydroxide precipitate is then washed, dried and ignited to get pure
Aluminum hydroxide precipitate is then washed, dried and ignited to get pure
alumina (Al2O3 ).The filtrate containing caustic soda is concentrated and used again.
2Al(OH)3 → Al2O3 + 3H2O ↑
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Crushing Grinding Classification
Leaching
settling
Washing
Caustic soda
solution
Pregnant liquor
Starch
Red mud
Bauxite
100 mesh
Steam
Bayer Process Flowsheet
Washing
Heat exchange
Hot liquor
Filtration
Effluent
Hot liquor
Precipitation
Red mud
Rotary kiln Alumina (Al2O3 )
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Bayer Process Flow-sheet
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II. The production of aluminum from bauxites with higher silicon content
The production of alumina(aluminum oxide) from bauxites with the higher silicon
Content (8-15%) . In this process, the bauxite is mixed with the lime stone and the
sodium carbonate. The mixture agglomerates in rotary furnace 1200-1300°C.
The bauxite ore is fused with sodium carbonate(Na2CO3) to give water soluble sodium
meta aluminate( NaAlO2 ) leaving behind Fe2O3 and SiO2.
meta aluminate( NaAlO2 ) leaving behind Fe2O3 and SiO2.
Al2O3 + Na2CO3→ 2NaAlO2 + CO2 ↑
The fused mass of sodium meta silicate is extracted with water and filtered. The
impurities Fe2O3 and SiO2 remain on the filter paper. The filtrate containing NaAlO2 is
warmed and CO2 is passed through it, when Al(OH)3 is precipitated.
2NaAlO2 + CO2 + 3H2O → 2 Al(OH)3 ↓ + Na2CO3
The precipitate is filtered, washed and ignited to obtain pure alumina.
2Al(OH)3 → Al2O3 + 3H2O ↑
Pure alumina 22

Electrolysis of pure alumina-(Hall-Heroults process 1886)
Aluminum can be obtained by electrolysis of pure alumina but it offers two problems.
i. Pure alumina is a poor conductor of electricity and melts at about 2000 °C.
ii. When fused alumina is electrolyzed at 2000 °C , economically not effective.
Aluminium is usually prepared by Hall- Heroult process. Alumina is fused with
Cryolite (Na3AlF6). Alumina dissolves in Cryolite. Cryolite lowers the temperature of
the mixture. Small amount of CaF2 and AlF3 are also added to lower the temperature of
the mixture. Pure alumina melts at 2000 °C while the mixture melts at about 950 °C The
charge consists of Cryolite ( 85 % ), CaF2( 5 % ), AlF3( 5 % ) and Al2O3(5 %) .
The electrolysis is carried out at temperature of 950 °C and with a voltage of 5.5 volts
in a graphite lined steel tank which acts as a cathode. The anodes are made of graphite
Na3AlF6 → 3NaF + AlF3
4AlF3 → 4 A𝑙+3 + 12F-
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Cont’d
The Al2O3 is added from feeder at the top. Some coke is thrown on the surface of
charge to control the oxidation of the metal. The electrode reactions are complicated
and their exact nature is not known(corroded the electrodes) .
The simplified mechanism of electrode reactions is given below.
Fig: Electrolysis of alumina- Hall-Heroults process
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CELL OPERATION
During operation, the cell bath is filled with Cryolite and the anode is lowered into it.
A current is passed through the cell circuit until the Cryolite melts (melting point 950
°C). When both attains a molten state is the addition of alumina commenced.
The alumina decomposes to yield aluminum and oxygen, the necessary heat being
supplied by the resistance offered by both the electrodes and the electrolyte.
supplied by the resistance offered by both the electrodes and the electrolyte.
If the alumina content of the bath (normally 5-10%) falls below 2%, then normal
contact between the anode and the bath is interrupted by a gas film which abruptly
increases the resistance. As a consequence, the normal operation ceases. This effect is
called the anode effect.
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Factors influencing electrolysis
Let us now discuss certain operational factors that have a marked influence on the
electrolysis of alumina:
1. The bath temperature is an important factor. As a rule, an increase in
temperature decreases the current efficiency. For every 4 °C rise in the bath
temperature (above 970 °C), the current efficiency falls by 1%.
NB: It should, however, be noted that a decrease in the temperature does not
improve the current efficiency indefinitely.
2. The higher current density, the greater the current efficiency. Therefore, the
current density is maintained at a very high value.
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Cont’d
3. The lower the density of the bath, the higher the rate of separation of the metal
liberated from the bath. The density can be lowered by using a high AlF3/NaF
ratio. However, when the quantity of AlF3 is large compared with NaF, Certain
drawbacks arise.
4. The current efficiency is also affected by the inter-polar distance, which is the
4. The current efficiency is also affected by the inter-polar distance, which is the
distance from the bottom of the anode to the top of the molten metal bath
5. The cell efficiency can be improved by strictly controlling the addition of
alumina. The current efficiency is a minimum when the Al2O3 content in the bath
is 4%. At values lower and higher than 4%, the current efficiency increases.
However, to maintain a high current efficiency, the cell should be operated
consistently with a high Al2O3 content.
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Uses of aluminum
 Since it is lighter and has high tensile strength, aluminum is used in making
body of air-ships and motor cars.
 On account of its good electrical conductivity, it is used for making electrical
transmission cables.
 On account of its good thermal conductivity, it is used in making cooking
utensils.
 Since it resists corrosion, it is used in aluminium paints.
 Aluminium foils are used in wrapping cigaretts, confectionary items etc.
 Aluminium is used as a deoxidizer and for removing blow holes in metallurgy.
 It is used in thermite welding and in the aluminothermic process.
 Salts of aluminium such as alum are used as mordants in dyeing industries.
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Extraction of Tantalum
Metallurgy of Tantalum
Atomic number of Tantalum is 73 and its atomic weight is 180.95 and denoted by
Symbol Ta. A density of Tantalum is 16.65 g/cm3,
Occurrence
Tantalum occurs as various minerals, it is conveniently represented as the pentoxide
(Ta2O5 )
EXTRACTION of Tantalum
 Refining of tantalum from its ores: crushed the mineral then concentrated by gravity
separation. The chief problem is that tantalum ores contain significant amounts of
niobium, which has chemical properties almost identical to Ta.
 To address this challenge in modern times, the separation is achieved by hydrometallurgy.
Extraction begins with a leaching the ore with hydrofluoric acid together with sulfuric acid
or HCl. This step allows the tantalum and niobium to be separated from the various non-
metallic impurities in the rock.
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Cont’d
A simplified equation for its extraction is thus:
Ta2O5 + 14 HF → 2 H2[TaF7] + 5 H2O
Completely analogous reactions occur for the niobium component, but the hexafluoride is
typically predominant under the conditions of the extraction.
Nb2O5 + 12 HF → 2 H[NbF6] + 5 H2O
Nb2O5 + 12 HF → 2 H[NbF6] + 5 H2O
These equations are simplified: it is suspected that bisulfate (HSO4
−) and chloride
compete as ligands for the Nb and Ta ions, when sulfuric and hydrochloric acids
are used, respectively.
The tantalum and niobium fluoride complexes are then removed from the aqueous
solution by liquid-liquid extraction into original solvent, such as cyclohexanone, octanol.
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Cont’d
This simple procedure allows the removal of most metal-containing impurities (e.g.
iron, manganese, titanium, zirconium), which remain in the aqueous phase in the form
of their fluorides and other complexes.
*Separation of the tantalum from niobium is then achieved by lowering the ionic
strength of the acid mixture, which causes the niobium to dissolve in the aqueous
strength of the acid mixture, which causes the niobium to dissolve in the aqueous
phase.
It is proposed that oxyfluoride H2(NbOF5) is formed under these conditions. Subsequent
to removal of the niobium, the solution of purified H2(TaF7) is neutralized with aqueous
ammonia to precipitate hydrated tantalum oxide as a solid, which can be calcined to
tantalum pentoxide (Ta2O5).
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Cont’d
Instead of hydrolysis, the H2[TaF7] can be treated with potassium fluoride to produce
potassium heptafluorotantalate.
H2(TaF7)+ 2 KF → K2(TaF7) + 2 HF
Unlike H2(TaF7), the potassium salt is readily crystallized and handled as a solid. Then
K2(TaF7) can be converted to metallic tantalum by reduction with sodium, at
K2(TaF7) can be converted to metallic tantalum by reduction with sodium, at
approximately 800°C in molten salt.
K2(TaF7) + 5 Na → Ta + 5 NaF + 2 KF
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Uses of Tantalum
 Tantalum is used in the electronics industries for capacitor and high power
resistors.
 Tantalum is used to make alloy to increase strength, ductility and corrosion
resistance .
 Tantalum is used in dental and surgical instruments and implant, as it
EXTRACTION OF Tantalum
 Tantalum is used in dental and surgical instruments and implant, as it
causes no immune response.
 Tantalum is used diping and overhead condensers as it is inert to sulferic
acid and hydrochloric acid.
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