Ellingham Diagram
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Transcript of Ellingham Diagram
Extraction of Metals-the chemistry behind
http://www.chem.iitb.ac.in/~rmv/ch102/extraction.pdf
17510.019Nickel17890.025Zirconium18860.029Fluorine17970.035Chromium17740.045Chlorine18080.05Bariumancient0.052Sulfur17740.09Manganeseancient0.094Carbon16690.13Phosphorus17760.14Hydrogen17910.62Titanium17552.08Magnesium18072.58Potassium18072.75Sodium18083.65Calciumancient5.05Iron18258.07Aluminum182427.69Silicon177446.71Oxygen
92 %
99.5 %
99.97 %
Need for efficient separation techniques
All other elements = 0.03 %
Methods of Separation / Extraction
1. Mechanical separation
2. Thermal decomposition
3. Displacement of one element by other
4. High temperature chemical reduction
5. Electrolytic reduction
Mechanical separation
Free elemental form – unreactive elements
Coinage & Pt metals
Gold; 19.3 g/cm-3, separated by panning
Unstable compounds ���� Constituent elements
Ag2O ���� 2Ag + ½O2
Marsh test: As, Sb salt + Zn/H2SO4����
As/SbH3 ����Silver mirror of the metal
Decomposition of NaN3 to Na and N2
Mond process; production of nickel
van Arkel process
Thermal decomposition
Displacement of one element by other
In principle, any element may be displaced by another element which has more negative Eo in electrochemical series.
Cu2+ + Fe ���� Fe2+ + Cu-0.44 +0.16
Cl2 + 2Br- ���� 2Cl- + Br2+1.36 +1.09
High temperature chemical reduction
1. Many metals are found as their oxides2. Oxide Ores: Directly reduced (smelted) to
the metal. General reducing agents: C , Al, Si, H2. Carbon is the most widely used reducing agent (can form carbide)
3. Sulfide Ores: First roasted to convert them to oxide and then reduced to the metal (for thermodynamic reasons oxides rather than sulfides used) (SELF REDUCTION)
4. Other metals as reducing agents
(all points will be elaborated)
Electrolytic reduction
1. Electron – the strongest known reducing agent.
2. Highly electropositive metals, e.g. alkaline earth
metals are produced this way (Electrolytic reduction
of their fused halides)
3. Ionic materials (salts) are electrolyzed – reduction at
cathode
4. Excellent method, gives pure metal, but expensive
Methods of Separation / Extraction
1. Mechanical separation
2. Thermal decomposition
3. Displacement of one element by other
4. High temperature chemical reduction
5. Electrolytic reduction
High-T chemical reduction Thermodynamic considerations ….
1. Used to identify which reactions are spontaneous under the prevailing conditions.
2. To choose most economical reducing agent and reaction condition
Criterion for spontaneity
∆∆∆∆Go = − RT ln K
•Negative ∆∆∆∆Go corresponds to K > 1; favorable reaction•Kinetics is not important as reductions are done at high. temp & fast
∆∆∆∆Go = ∆∆∆∆Ho - T∆∆∆∆S
For the formation of metal oxide, 2M(s) + O2(g) ���� 2MO(s)
•∆∆∆∆S is negative; because oxygen gas is used up.
•If temperature is raised, T∆∆∆∆S becomes more negative
•Thus the free energy change (∆∆∆∆Go) increases with an increase in temperature
High-T chemical reduction Thermodynamic considerations ….
The free energy changes that occur when one gram molecule of a common reactant (O2) is used, is plotted against temperature.
This graph is calledEllingham Diagram
∆∆∆∆Go = ∆∆∆∆Ho - T∆∆∆∆S
Properties of Ellingham diagram• All metal oxide curves slop upwards• If materials melt / vaporize, the slope changes • When the curve crosses ∆∆∆∆Go = 0, decomposition
of oxide begins (Ag, Au, Hg)• Electropositive metal curves are at the bottom of
the diagram• Any metal will reduce the oxide of other metal
which is above in Ellingham diagram (the ∆∆∆∆Go
will become more negative by an amount equal to the difference between the two graphs at a particular temperature)
Carbon as the reducing agent
∆∆∆∆Go = ∆∆∆∆Go(C,CO) - ∆∆∆∆Go(M,MO)
C + O2(g) ���� CO2(g) (∆∆∆∆S constant)
710 oC
CO(g) + ½O2(g) ���� CO2(g) (∆∆∆∆S –ve)
C + ½O2(g) ���� CO(g) (∆∆∆∆S +ve )
When C����CO line is below M����MO line, C reduces the MO and produces CO.
When C����CO2 line is below M����MO line, C reduces the MO and produces CO2.
When CO����CO2 line is below M����MO line, CO reduces the MO and produces CO2.
The three curves intersect at 710 oCBelow 710 oC, CO is better reducing agent.Above 710 oC, carbon is better reducing agent.
Using ED, find out what is the lowest temp. at which ZnO can be reduced to Zn by carbon. What is the overall reaction?
What is the minimum temp. required for the reduction of MgO by carbon?
Thermit Process – Sacrificial MethodCr2O3
Al2O3
∆∆ ∆∆Go
(kJm
ol-1
)
Temperature (oC)
-600
-800
-1000
-1200
4/34/34/34/3Al + O2 ���� 2/32/32/32/3Al2O3 ∆∆∆∆H = -266 Kcal/mol4/34/34/34/3Cr + O2 ���� 2/32/32/32/3Cr2O3 ∆∆∆∆H = -180 Kcal/mol4/34/34/34/3Al + 2/32/32/32/3Cr2O3 ���� 4/34/34/34/3Cr + 2/32/32/32/3Al2O3 ∆∆∆∆H = -86 Kcal/mol
∆∆∆∆G ≈ ∆∆∆∆H (since ∆∆∆∆S is similar)
Thermit Process – Details4/34/34/34/3Al + 2/32/32/32/3Cr2O3 ���� 4/34/34/34/3Cr + 2/32/32/32/3Al2O3 ∆∆∆∆H = -86 Kcal/mol
∆∆∆∆G is negative at all temperatures.∆∆∆∆S is very small since there are no gaseous productsHence, ∆∆∆∆G is approximately same at different temperatures
However Al reduction requires higher temperature to trigger off.Kinetic factor: Activation energy
Priming the reaction with Mg-ribbon and barium peroxide / a KNO3+S+Al pellet is necessary.
The reduction is usually exothermic. Once initiated, the whole mass gets reduced spontaneously.
Alloy formation with Al can take place in some cases.
H2 -Poor reducing agentH2O
MO∆∆ ∆∆G
o(k
Jmol
-1)
Temperature (oC)
H2
• 2H2(g) + O2(g) ���� 2H2O; entropy decreases• points upwards and runs parallel to many MO curves.• Up above in the diagram• Metal hydride formation• Dissolved (interstitial) hydrogen – poor properties
Reduction of Metal Sulfides
Self reduction:CuS ���� [CuS + CuO] ����
Cu + SO2
First roasted to MO and then reduced to metal2MS + 3O2 ���� 2MO + 2SO2
Many metals, which are chemically soft, occur as sulfide ores. e.g. Cu, Hg, Zn, Fe, etc.Carbon is not a good reducing agent to for sulfide ores. MS + C ���� CS2 has no slope in ED.
C
H2 is also a poor reducing agent for metal sulfides.
Ellingham diagram – Metal Sulfides
Cas
FesZns
Hgs
0 1000 oC 2000 oC
-40
- 80
- 160
- 200
- 240
∆Go (K
Cal
/mol
eS 2
(g))
- 120SO2
MnS
CS2
H2S
Ellingham diagram – Metal Halides
ZrCL4
300oC 500oC 1500oC 2500oC
-40
-100
∆Gfo
CaF2
NaFMgCl2
UF4
NbF5
NaCl
CaCl2
MgCl2
HCl
CF4CCl4
HF
TiCl4
SnCl4
Purification of Elements
1. Fusion, distillation, crystallization.– Fusion removed adsorbed gases (SO2, O2, etc.)– Distillation of volatile metals to remove impurities – Fractional distillation of OsO4 and RuO4 from other Pt-metals in the
presence of oxidising agents.– Fractional Crystallization of Pt/Ir as (NH4)2MCl6
2. Oxidative refining– When impurities have more affinity to oxygen than the metal.– Pig iron contains C, Si, P, and Mn, which can be purified by
blowing air through the molten metal in Bessimer Convertor.– CO, SiO2, P4O10, MnO formed combine with added CaO to give slag
- Ca3(PO4)2, MnSiO3
3. Thermal Decomposition– Carbonyl (Mond process) for purification of Fe, Ni, etc.– Van Arkel de Boer’s filament growth method (ZrI4, BI3, etc.)– Decomposition of Hydrides (AsH3, SbH3 etc.)
Special attention to metals
Purification of Elements
4. Electrolytic refining5. Zone refining6. Chromatographic methods7.Solvent Extractions8. Ion-Exchange Methods
Special attention to metals