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