Extraction of Metals -the chemistry within
Elemental Composition of earth ’ s crust 92% All other elements = 0.03% 99.5% These are mostly in the form of compounds - ores Need for efficient 99.97% separation techniques
Methods of Separation / Extraction 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 ……
1. Mechanical separation Based on the density variation by sieving method Free elemental form – unreactive elements Coinage & Pt metals Example: Gold; 19.3 g/cm 3 , separated by panning
2. Magnetic separation Removal of ferrous impurities/materials
3. Thermal decomposition Note: Azides are explosives ! Zr or B forms ZrI 4 or BI 3
Thermal decomposition Monds Process: (Reduction followed by thermal) Ni O (s) + H 2 (g) → Ni (s) + H 2 O (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 Ti Cl 4 with Ca/Mg (950-1150 o C). Van Arkel-deBoer process: Crude zirconium ( Zr ) + Iodine → Zr I 4 200 o C Zr I 4 → Zr (s) + Iodine 1300 o C
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 + 2 Ag NO 3 → 2 Ag + Cu(NO 3 ) 2 Fe + Cu (NO 3 ) 2 → Fe(NO 3 ) 2 + Cu In principle, any element may be displaced by another element which has more negative E o in electrochemical series. Cu 2+ + Fe Fe 2+ + Cu Cd 2+ + Zn Cd + Zn 2+ Cl 2 + 2Br - 2Cl - + Br 2 COST and SAFETY !!!! JD Lee Page 181
5. Electrolytic reduction 1. Electron – Strongest known reducing agent 2. Highly electropositive metals, e.g. alkaline earth metals are produced this way (Electrolytic reduction of their fused halides) 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, but very expensive
Methods of Separation / Extraction 1. Mechanical separation 2. Magnetic separation 3. Thermal decomposition 4. Displacement of one element by other 5. Electrolytic reduction 6. High temperature chemical reduction
6. High temperature chemical reduction 1. Many metals are found as their oxides . Some are found as sulfides and halides. 2. Oxide Ores: Directly reduced (smelted) to the metal. General reducing agents: C , Al, Si, H 2 . 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 To choose most economical reducing agent and reaction condition 3. Criterion for spontaneity Δ G o = − RT ln K • Negative ΔG o corresponds to K > 1; favorable reaction • Kinetics is NOT important as reductions are done at high temp. & are fast
High-T chemical reduction Thermodynamic considerations …. Δ G o = Δ H o – T Δ S For the formation of metal oxide, 2M (s) + O 2 (g) 2MO (s) • Δ S is negative ; because oxygen gas is used up. • If temperature is raised, TΔS becomes more negative & hence ( – TΔS ) is more positive • Thus the free energy change ( ΔG o ) increases (+ve) with increase in the temperature But we need negative ΔG o for a spontaneous reaction
Ellingham Diagram 2M (s) + O 2 (g) 2MO (s) Δ G o = Δ H o – T Δ S Ca + MgO CaO + Mg The free energy changes that occur when one gram mole of a common reactant (O 2 ) is used, is plotted against temperature.
Properties of Ellingham diagram Δ G o = Δ H o – T Δ S • All metal oxide curves slope upwards (ΔS is negative, ΔG o becomes +ve ) . • If materials melt / vaporize , slope changes (ΔS is more – ve, ΔG o becomes more +ve ) • When the curve crosses ΔG o = 0 , decomposition of oxide (Ag, Au, Hg) begins • 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 ΔG o will become more negative by an amount equal to the difference between the two graphs at a particular temperature)
Carbon as the reducing agent C + ½O2(g) CO(g) ( Δ S +ve ) Δ G o = Δ H o – T Δ S Δ G o = Δ G o (C,CO) - Δ G o (M,MO)
Carbon as the reducing agent CO (g) + ½O 2 (g) CO 2 (g) ( ΔS – ve) C + O 2 (g) CO 2 (g) (ΔS constant) C + ½O 2 (g) CO (g) ( Δ S +ve )
C or CO as the reducing agent ?? When C CO line is below M MO line, C + MO CO + M When C CO 2 line is below M MO line , C + MO CO 2 + M When CO CO 2 line is below M MO line , CO + MO CO 2 + M The three curves intersect at 710 o C Below 710 o C, CO is better reducing agent. Above 710 o C, carbon is better reducing agent.
Example:
Work out 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? Check out Webtoolat http://www.engr.sjsu.edu/ellingham/
Thermit Process – Sacrificial Method
Thermit Process – Details - 4/3 Cr + 2/3 Al 2 O 3 Δ H = -86 Kcal/mol 4/3 Al + 2/3 Cr 2 O 3 Δ G is negative at all temperatures. Δ S 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 off. Kinetic factor: Activation energy Priming the reaction with Mg-ribbon and barium peroxide / KNO 3 +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.
H 2 -Poor reducing agent
Reduction of Metal Sulfides Many metals, which are chemically soft , occur as sulfide ores. e.g. Cu, Hg, Zn, Fe, etc. Carbon is not a good reducing agent for sulfide ores. MS + C CS 2 has no slope in ED. Self reduction: First roasted to MO and CuS [CuS + CuO] - then reduced to metal Cu + SO 2 2MS + 3O 2 2MO +2SO 2 C H 2 is also a poor reducing agent for metal sulfides.
Ellingham diagram – Metal Sulfides
Ellingham diagram – Metal Halides
Purification of Elements-Special attention to metals 1. Fusion, distillation, crystallization. – Fusion removes adsorbed gases (SO 2 , O 2 , etc.) – Distillation of volatile metals to remove impurities – Fractional distillation of OsO 4 and RuO 4 from other Pt- metals in the presence of oxidising agents. – Fractional Crystallization of Pt/Ir as (NH 4 ) 2 MCl 6 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, SiO 2 , P 4 O 10 , MnO formed will combine with the added CaO to give slag - Ca 3 (PO 4 ) 2 , MnSiO 3
Purification of Elements 3. Thermal Decomposition – Carbonyl (Mond process) for purification of Fe, Ni – Van Arkel de Boer’s filament growth method (ZrI 4 , BI 3 ) – Decomposition of Hydrides (AsH 3 , SbH 3 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
Scientific Method, The widely held philosophy that a theory can never be proved, only disproved, and that all attempts to explain anything are therefore futile.
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