Extraction of Metals -the chemistry within Elemental Composition of - - PowerPoint PPT Presentation

Extraction of Metals

• the chemistry within

Elemental Composition of earth’s crust

These are mostly in the form of compounds - ores All other elements = 0.03% Need for efficient separation techniques

92% 99.5% 99.97%

• 1. Mechanical separation
• 2. Magnetic separation
• 3. Thermal decomposition
• 4. Displacement of one element by other
• 5. Electrolytic reduction
• 6. High temperature chemical reduction

and so on ……

Methods of Separation / Extraction

Free elemental form – unreactive elements Coinage & Pt metals

• 1. Mechanical separation

Based on the density variation by sieving method Example: Gold; 19.3 g/cm3, separated by panning

• 2. Magnetic separation

Removal of ferrous impurities/materials

• 3. Thermal decomposition

Note: Azides are explosives ! Zr or B forms ZrI4 or BI3

Thermal decomposition

Monds Process: (Reduction followed by thermal)

NiO (s) + H2 (g) → Ni (s) + H2O (g) 200ºC impure Ni, mixed with iron Ni (s) + 4 CO (g) → Ni(CO)4 (g) 50ºC Ni(CO)4 (g) → Ni (s) + 4 CO (g) 230ºC

Kroll Process: (Reductive separation)

Kroll produced Ti by reducing TiCl4 with Ca/Mg (950-1150oC).

Van Arkel-deBoer process:

Crude zirconium (Zr) + Iodine → ZrI4 200 oC ZrI4 → Zr (s) + Iodine 1300 oC

• 4. Displacement of one element by other

Metal with lower electrode potential Greater ability for acting as reducing agent Can displace other metal of higher electrode potential from ore

Cu + 2AgNO3 → 2Ag + Cu(NO3)2 Fe + Cu(NO3)2 → Fe(NO3)2 + Cu

JD Lee Page 181

In principle, any element may be displaced by another element which has more negative Eo in electrochemical series.

Cu2+ + Fe  Fe2+ + Cu Cd2+ + Zn  Cd + Zn2+ Cl2 + 2Br-  2Cl- + Br2

COST and SAFETY !!!!

• 5. Electrolytic reduction
• 1. Electron – Strongest known reducing agent
• 2. Highly electropositive metals, e.g. alkaline earth

metals are produced this way (Electrolytic reduction

• f their fused halides)

but very expensive

• 3. Less electropositive elements, viz., Cr, Cu & Zn can

be made by electrolysis even from aqueous solution

• 4. Ionic materials (salts) are electrolyzed – reduction at

cathode

• 5. Excellent method, gives pure metal,

• 1. Mechanical separation
• 2. Magnetic separation
• 3. Thermal decomposition

4. Displacement of one element by other 5. Electrolytic reduction 6. High temperature chemical reduction

Methods of Separation / Extraction

• 1. Many metals are found as their oxides. Some are found as

sulfides and halides.

• 6. High temperature chemical reduction
• 2. 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

High-T chemical reduction Thermodynamic considerations ….

• 1. Used to identify which reactions are spontaneous under the conditions
• 2. Kinetic equilibrium is reached easily at such high temperatures

3. To choose most economical reducing agent and reaction condition

ΔGo = − RT ln K

• Negative ΔGo corresponds to K > 1; favorable reaction
• Kinetics is NOT important as reductions are done at high
• temp. & are fast

Criterion for spontaneity

ΔGo = ΔHo – TΔS

For the formation of metal oxide, 2M (s) + O2 (g)  2MO (s)

High-T chemical reduction Thermodynamic considerations ….

• ΔS is negative; because oxygen gas is used up.

But we need negative ΔGo for a spontaneous reaction

• If temperature is raised, TΔS becomes more negative & hence

(– TΔS) is more positive

• Thus the free energy change (ΔGo) increases (+ve) with increase

in the temperature

ΔGo = ΔHo – TΔS

Ellingham Diagram

The free energy changes that occur when

• ne gram mole of a common reactant (O2)

is used, is plotted against temperature.

Ca + MgO  CaO + Mg

2M (s) + O2 (g)  2MO (s)

Properties of Ellingham diagram

• All metal oxide curves slope upwards (ΔS is negative, ΔGo becomes +ve).
• If materials melt / vaporize, slope changes (ΔS is more –ve, ΔGo becomes more +ve)
• When the curve crosses ΔGo = 0, decomposition of oxide (Ag, Au, Hg) begins

ΔGo = ΔHo – TΔS

• 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 ΔGo will become more negative by an amount

equal to the difference between the two graphs at a particular temperature)

Carbon as the reducing agent

ΔGo = ΔGo(C,CO) - ΔGo(M,MO)

ΔGo = ΔHo – TΔS

C + ½O2(g)  CO(g) (ΔS +ve )

CO (g) + ½O2 (g)  CO2 (g) (ΔS –ve)

Carbon as the reducing agent

C + O2 (g)  CO2(g) (ΔS constant) C + ½O2 (g)  CO (g) (ΔS +ve )

The three curves intersect at 710 oC Below 710 oC, CO is better reducing agent. Above 710 oC, carbon is better reducing agent.

When C  CO line is below M  MO line, C + MO  CO + M

C or CO as the reducing agent ??

When C  CO2 line is below M  MO line, C + MO  CO2 + M When CO  CO2 line is below M  MO line, CO + MO  CO2 + M

Example:

Using ED, find out what is the lowest temp. at which ZnO can be reduced to Zn by carbon. What is the overall reaction?

Work out

Check out Webtoolat http://www.engr.sjsu.edu/ellingham/

What is the minimum temp. required for the reduction of MgO by carbon?

Thermit Process – Sacrificial Method

4/3 Al + 2/3 Cr2O3

•  4/3 Cr + 2/3 Al2O3 ΔH = -86 Kcal/mol

ΔG is negative at all temperatures. ΔS is very small since there are no gaseous products Hence, ΔG is approximately same at different Temps

However Al reduction requires higher temperature to trigger

• ff. Kinetic factor: Activation energy

Thermit Process – Details

Priming the reaction with Mg-ribbon and barium peroxide / 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 agent

Reduction of Metal Sulfides

Many metals, which are chemically soft, occur as sulfide

• res. e.g. Cu, Hg, Zn, Fe, etc.

Self reduction: CuS [CuS + CuO] - Cu + SO2 First roasted to MO and then reduced to metal 2MS + 3O2  2MO +2SO2 H2 is also a poor reducing agent for metal sulfides.

C

Carbon is not a good reducing agent for sulfide ores. MS + C  CS2 has no slope in ED.

Ellingham diagram – Metal Sulfides

Ellingham diagram – Metal Halides

• 1. Fusion, distillation, crystallization.

– Fusion removes adsorbed gases (SO2, O2, etc.) – Distillation of volatile metals to remove impurities – Fractional distillation of OsO4 and RuO4 from

• ther Pt-

metals in the presence of oxidising agents. – Fractional Crystallization of Pt/Ir as (NH4)2MCl6

Purification of Elements-Special attention to metals

• 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 will combine with the added CaO to give slag - Ca3(PO4)2, MnSiO3

Purification of Elements

• 3. Thermal Decomposition

– Carbonyl (Mond process) for purification of Fe, Ni – Van Arkel de Boer’s filament growth method (ZrI4, BI3) – Decomposition of Hydrides (AsH3, SbH3 etc.)

• 4. Zone refining (refinement of pure silicon to ultrapure silicon)
• impurities are more soluble in the liquid phase as

compared to the solid phase 5. Electrolytic refining 6. Chromatographic methods 7. Solvent Extractions 8. Ion-Exchange Methods

Zone refining

Zone refining

The widely held philosophy that a theory can never be proved, Scientific Method, and that all attempts to explain anything are therefore futile.

• nly disproved,