Extraction of Metals

Transcription

1 Extraction of Metals -the chemistry behind

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3 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

4 Thermal decomposition Nickel tetracarbonyl formation (50 o C) and its decomposition (230 o C) Kroll Process {Reduction of titanium tetrachloride by magnesium (~ o C) van Arkel-deBoer process Crude zirconium is treated with iodine at 200 o C and decomposition of the zirconium tetraidide at 1300 o C

5 Displacement of one element by other In principle, any element may be displaced d by another element which has more negative E o in electrochemical series. Cu 2+ +Fe- Fe 2+ +Cu Cl 2 + 2Br - - 2Cl - + Br

6 High temperature chemical reduction 1. Many metals are found as their oxides 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 (all points will be elaborated)

7 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. Less electropositive elements, viz., Cr, Cu & Zn can be made by electrolysis is aqueous sol. 4. Ionic materials (salts) are electrolyzed reduction at cathode 5 E ll t th d i t l b t 5. Excellent method, gives pure metal, but expensive

8 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

9 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 ΔG o = RT ln K Criterion for spontaneity Negative ΔG o corresponds to K > 1; favorable reaction Kinetics is not important as reductions are done at high. temp & fast

10 High-T chemical reduction Thermodynamic considerations. ΔG o = ΔH o TΔS For the formation of metal oxide, 2M(s) + O 2 (g) - 2MO(s) ΔS is negative; because oxygen gas is used up. If temperature is raised, TΔS becomes more negative & hence TΔS ismore positive Thus the free energy change (ΔG o ) increases with an increase in temperature

11 ΔG o = ΔH o TΔS The free energy changes that occur when one gram molecule of a common reactant (O 2 ) is used, is plotted against temperature. This graph is called Ellingham Diagram

12 Properties of Ellingham diagram All metal oxide curves slope upwards If materials melt / vaporize, the slope changes When the curve crosses ΔG o = 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 ΔG o will become more negative by an amount equal to the difference between the two graphs at a particular temperature)

13 Carbon as the reducing agent ΔG o = ΔG o (C,CO) - ΔG o (M,MO)

14 CO(g) + ½O 2 (g)- CO 2 (g) (ΔS ve) C + O 2 (g)- CO 2 (g) (ΔS constant) C + ½O2(g) CO(g) (ΔS +ve )

15 When C- CO line is below M- MO line, C reduces the MO and produces CO. When C- CO 2 line is below M- MO line, C reduces the MO and produces CO 2. When CO- CO 2 line is below M- MO line, CO reduces the MO and produces CO 2. 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.

16 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?

17 Thermit Process Sacrificial Method

18 Thermit Process Details 4/3 Al + 2/3 Cr - 2 O 3 4/3 Cr + 2/3 Al 2 O 3 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 off. Kinetic factor: Activation energy Priming the reaction with Mg-ribbon and barium peroxide / a 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.

19 H 2 -Poor reducing agent

20 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 to for sulfide ores. MS + C CS2 has no slope in ED. First roasted to MO and then reduced to metal 2MS + 3O 2-2MO +2SO 2 C Self reduction: CuS [CuS + CuO] - Cu + SO 2 H 2 is also a poor reducing agent for metal sulfides.

21 Ellingham diagram Metal Sulfides

22 Ellingham diagram Metal Halides

23 Purification of Elements Special attention to metals 1. Fusion, distillation, crystallization. Fusion removed adsorbed gases (SO 2, O 2, etc.) Distillation of volatile metals to remove impurities iti 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 h can be purified by blowing air through the molten metal in Bessimer Convertor. CO, SiO 2, P 4 O 10, MnO formed combine with added CaO to give slag - Ca 3 (PO 4 ) 2, MnSiO 3

24 Purification of Elements Special attention to metals 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. Electrolytic refining 5. Zone refining 6. Chromatographic methods 7.Solvent Extractions 8. Ion-Exchange Methods