CHAPTER 4 General methods of extraction.pdf

DEFENCE ENGINEERING COLLEGE
Metallurgical And Materials Engineering
Present By:
Weldebrhan Hadush (Maj.) 31 Oct., 2019

Content
i. Introduction
ii. Pyro-metallurgy
 Calcinations
 Roasting
 Smelting
 Slags and fluxes
iii. Hydro- Metallurgy
• Advantages of hydro metallurgy
General methods of extraction
• Advantages of hydro metallurgy
• Leaching operations
iv. Electro-metallurgy
 Electrolytic decomposition
 Structure of solvent media
 Molten(fused) salt electrolytes
 Electrolysis of aqueous solutions
 Role –energy in metal extraction

INTRODUCTION
 In metal extraction, bulk of metal separated from impurities present in the ore-
using a system comprising more than one phase.
 Driving force for the reactions involved in metal extraction depends on differences
in chemical potentials of the chemical elements.
 Specific amount of energy required to reduce or release a metal ion present in
compound (solid or liquid) in association with other elements.
 There are three main methods of extracting metals from their ores
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 There are three main methods of extracting metals from their ores
• Pyrometallurgy: Metallic extraction at elevated temperatures, ore compounds
relatively unstable facilitating the release of metals.
• Electrometallurgy: dissociation or decomposition of a compound in solution
(i.e. reduction of an ion) – by electrical forces.
• Hydrometallurgy: reduction process depends on the judicious manipulation of
chemical reactions taking place in an aqueous solution.
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PYRO-METALLURGY
Pyrometallurgy is a branch of extractive metallurgy. It consists of the thermal (Word
“Pyro” indicates “Fire”) treatment of minerals and metallurgical ores and
concentrates to bring about physical and chemical transformations in the materials to
enable recovery of valuable metals .
Heating and smelting operations are used in this process to recover the valuable
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Heating and smelting operations are used in this process to recover the valuable
metals such as Fe ,Cu ,Zn ,Cr, Mn which are extracted by pyrometallurgy.
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PROCESSES
Pyrometallurgical processes required energy to takes place . The energy provided by
fuel combustion ,electrical heat ,exothermic reaction of materials.
This processes is auto-genous and exothermic.
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Cont’d
Pyro-metallurgy deals with the extraction of minerals from ore by treating them with
heat.
1.Calcination
Heating below MP* in absence or limited supply of air
2. Roasting
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2. Roasting
Heating below MP* in presence of excess oxygen for oxidation
3. Smelting
Reduced to metals by heating with carbon at high temperature
4. Refining
Removal of impurities from materials by a thermal process
MP* = Melting Point

CALCINATION
 Calcination is a process in which the ore is heated in a limited supply of air at a
temperature insufficient to melt it.
 During calcination , organic matter , volatile impurities and moisture present in the
ore are expelled and remaining mass becomes porous.
 Calcination is also done to remove water from hydrated oxide ores or CO from
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 Calcination is also done to remove water from hydrated oxide ores or CO2 from
carbonate ore.
 Calcination is generally done in reverberatory furnace to render the porous and easily
workable in subsequent stages.
CaCO3 → CaO + CO2 (g) Lime stone is heated to give of CO2
PbCO3 → PbO + CO2 (g)
Al2O3.2H2O→ Al2O3+ 2H2O (g) Bauxite is calcinated H2O is removed
Fe2O3.2Fe(OH)3 → 2Fe2O3+ 3H2O (g) Hydrrous Iron is calcinated to remove H2O
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ROASTING
 In roasting, the ore is heated in a regular supply of air in a furnace at a temperature
below the melting point of the metal.
 Roasting is a metallurgical process involving gas–solid reactions at elevated
temperatures with the goal of purifying the metal component(s).
 Often before roasting, the ore has already been partially purified, e.g. by froth
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 Often before roasting, the ore has already been partially purified, e.g. by froth
floatation. The concentrate is mixed with other materials to facilitate the process.
 This process is generally applied to sulphide minerals. During roasting, the sulfide is
converted to an oxide, and sulfur is released as sulfur dioxide, a gas..
For the ores Cu2S (chalcocite) , ZnS (sphalerite) and PbO
2Cu2 S + 3O2 → 2Cu2 O + 2SO2
2ZnS + 3O2 → 2ZnO + 2SO2
2PbS + 3O2 → 2PbO + 2SO2
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SMELTING
A large number of metal oxides obtained during roasting can be reduced to metals by
heating with carbon at elevated temp .This process is called smelting.
The roasted ore is mixed with a suitable quantity of carbon and heated to a high
temperature above the smelting point of the metal. Carbon monoxide reduce the oxide
to the free metal.
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to the free metal.
During reduction ,an additional substance called flux is added to ore . It combines
with impurities to form fusible product slag.
Impurities + flux=slag
Fe2 O3 +3C → 2Fe+3CO
Fe2 O3 +3CO → 2Fe+3CO2
PbO + C → Pb+CO
PbO + CO → Pb+CO2
NB: Smelting is carried out in reverbretory or blast furnace in a controlled supply of air
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PURIFICATION
Purification Occurs after reduction
Types
Mond process for Nickel
Produces extremely pure metals
450- 470°K
330- 350°K
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FLUXES:
 When metal is smelted, the metal is separated from its impurities by melting, with
the impurities forming a molten slag on top of the metal.
 Many of the impurities associated with the ore are difficult to melt, and so they will
not form a proper slag easily, which retards the smelting process.
 To make these impurities easier to melt, fluxes are added.
 To make these impurities easier to melt, fluxes are added.
 Limestone (CaCO3) or dolomite ((Ca,Mg)CO3) are two typical fluxes used in blast
furnaces. When a large amount of sulfur needs to be removed from the furnace
charge, limestone is the preferred flux.
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Cont’d
 Limestone is also a better flux to use if slag from the blast furnace is to be used as a
raw material for cement manufacture.
 An important criterion for flux selection is availability and cost, and dolomite is often
more readily available and less expensive than limestone.
SLAGS
SLAGS
Two main functions of a slag are:
1. To collect the unreduced gangue minerals so as to form a separate layer in a
metal extraction process.
2. To provide a medium in which the impurities in a metal can collect during a
refining process. (often, the impurities get partitioned between the metal and
the slag).
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Cont’d
 To effectively fulfill these functions, a slag must possess the following properties:
1. The difference between the specific gravities of the slag and the metal should be
sufficiently high so that one can be easily separated from the other.
2. The slag must be fluid enough to permit its easy separation from the metal and
also to bring about a mass transfer faster (than when the slag is not fluid enough)
also to bring about a mass transfer faster (than when the slag is not fluid enough)
of the reacting and product species. (If the slag is viscous, the loss of metal by
entrapment in the slag is high).
3. The slag must have a chemical composition which ensures that the activities of the
impurities and of the dissolved gangue minerals are low.
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HYDRO- METALLURGY
Hydrometallurgy takes place with help of aqueous solution or water.
Removal of impurities by different physical methods, grinding are the basic processes
in preparation of ores in hydrometallurgy.
Steps of Hydrometallurgy
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Convert the valuable metals into soluble salts
1. Leaching
Convert the valuable metals into soluble salts
2. Concentration & Purification
Remove impurities to increase metal concentration
3.Metal Recovery
Metals are recovered from solution to solid form

Cont’d
Advantages:
1. Suitable for low grade ore.
2. Valuable byproducts can be recovered.
3. Less environmental pollution
4. Less cost as compared to pyro-metallurgy.
5. It can produce metals like nodules or powder form
6. Even in leaching , low temperatures are involved
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Disadvantages:
1. Process requires large volume.
2. Handling of chemicals require special care.
3. Large space is required.
4. Chances of corrosion and erosion in handling equipments

ELECTRO-METALLURGY:
It is a branch of metallurgy that deals with the application of electric current either
for electrolytic deposition or as a source of heat.
Electrometallurgy is related with electrodeposition.
There are four types of this process:
i. Electro-winning : it represents extraction of metal from ores
i. Electro-winning : it represents extraction of metal from ores
ii. Electro-refining : it represents purification of metals.
iii. Electro-plating : in this deposition of a layer of one metal on another metal is
involved.
iv. Electro-forming : it includes forming of thin metal parts through
electroplating
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Cont’d
The Electrolysis of Molten NaCl
An idealized cell for the electrolysis of sodium chloride is shown in the figure below.
A source of direct current is connected to a pair of inert electrodes immersed in
molten sodium chloride. Because the salt has been heated until it melts, the Na+ ions
flow toward the negative electrode and the Cl- ions flow toward the positive
flow toward the negative electrode and the Cl ions flow toward the positive
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. 15
Negative electrode (cathode (-)):
Na+ + e- → Na
Positive electrode (anode (+)):
2 Cl- → Cl2 + 2 e-

Cont’d
The Electrolysis of Aqueous NaCl
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
Cathode (-): Anode (+):
Na+ + e- → Na Eo
red = -2.71 V 2 Cl- → Cl2 + 2 e- Eo
ox = -1.36 V
2 H2O + 2 e- → H2 + 2 OH- Eo
red = -0.83 V 2 H2O → O2 + 4 H+ + 4 e- Eo
ox = -1.23 V
Then: 2 H2O + 2 e- → H2 + 2 OH- Then: 2 Cl- → Cl2 + 2 e-
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Example 4.1
Using the information below calculate °E for the process
Cu2+ (aq) +Zn(s) → Zn2+ (aq) + Cu (s)
Substance Zn Cu Zn2+ Cu2+
ΔG 0KJ 0KJ -147KJ 64.77KJ
Example 4.2
Based on the electro chemical process
Au+3 (aq) + 3e- → Au (s)
How many grams of Au are plated out by passing current 13.5 amp through a solution of
gold(III) chloride for 4 hrs? (molar mass Au=197 g/mol)
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“The future can not be predicted,
but it can be made !”

CHAPTER 4 General methods of extraction.pdf

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CHAPTER 4 General methods of extraction.pdf