CH111_Topic_I_Metal extraction processes_DK (1).pdf

CH111 Inorganic Chemistry Dr. Deepti Kalsi [email protected] Inorganic Chemistry Universe….. Scope of Inorganic Chemistry Real World applications of Inorganic Chemistry  Course Coverage 1. Basic principles of extraction of metals from...

CH111 Inorganic Chemistry Dr. Deepti Kalsi [email protected] Inorganic Chemistry Universe….. Scope of Inorganic Chemistry Real World applications of Inorganic Chemistry  Course Coverage 1. Basic principles of extraction of metals from ores & purification 2. Transition metal chemistry & some applications 3. Magnetism & some applications 4. Bioinorganic Chemistry At the end of the course: we will celebrate by conducting an exam  Course Coverage Interpretations, Explanations and Substantiations Are not necessarily reflected in the slides, but are reflected in the lecture. Please DO NOT miss any class Pre requisite (self study topics) Electronic Configuration (s, p, d, f blocks) Penetration Shielding Effective Nuclear Charge Atoms and Ions Size & Charge Ionization Potential Electron Affinity Electronegativity  Recommended Text Books (1) Concise Inorganic Chemistry - J.D. Lee (2) Shriver & Atkins’ Inorganic Chemistry P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong (3) Chemistry 4th Edition, Catherine E. Housecroft Edwin C. Constable (4) Principles of structure and reactivity James E. Huheey, E. A. Keiter, R. L. Keiter & O. K. Medhi  Topic I Basic principles of extraction of metals from ores & purification Extraction of Metals -the chemistry within  Minerals: Ores All ores are minerals but all minerals are not ores Minerals: Ores Iron Pillar of Delhi ❖The History of metallurgy in the Indian subcontinent dates back to 1700 BC. ❖ Metals and related concepts were mentioned in various early Vedic age texts. ❖ The Rig-Veda already uses the Sanskrit term Ayas (metal). Composition: Iron Pillar of Delhi [Fe(O)(OH)]  Elemental Oxygen Silicon Composition 46.71 1774 27.69 1824 of earth Rust Aluminum 8.07 1825 92 % Iron 5.05 ancient All other Calcium 3.65 1808 99.5 % elements = Sodium 2.75 1807 0.03 % Potassium 2.58 1807 Magnesium 2.08 1755 Titanium 0.62 1791 Hydrogen 0.14 1776 99.97 % Phosphorus 0.13 1669 Carbon 0.094 ancient Manganese 0.09 1774 Need for Sulfur 0.052 ancient Barium 0.05 1808 Chlorine 0.045 1774 efficient Chromium 0.035 1797 separation Fluorine 0.029 1886 techniques Zirconium 0.025 1789 Nickel 0.019 1751  Why Different Methods The method used to extract metals from the ore depends on their reactivity. For example, reactive metals such as Na, K, Al etc. are extracted by electrolysis, while a less-reactive metal such as iron may be extracted by reduction with carbon or carbon monoxide. Metals (decreasing order of Extraction method reactivity) Potassium, Sodium, Calcium, Electrolysis Magnesium, Aluminium Zinc, Iron, Tin, Lead Reaction with carbon or carbon monoxide Copper, Silver, Gold,* Platinum Various chemical reactions *Gold – so unreactive, thus found in its native form  Methods of Separation / Extraction 1.Mechanical separation 2. Thermal decomposition 3. Displacement 4. High temperature chemical reduction 5. Electrolytic reduction And so on ……  Mechanical Separation Concentration of Ore Hydraulic /Gravity Separation  Mechanical Separation Free elemental form – unreactive elements Coinage & Pt metals Gold; 19.3 g/cm-3, separated by panning  Magnetic Separation Concentration of Ore  Froth Floatation Concentration of Ore  Thermal Decomposition Unstable compounds → Constituent elements Ag2O → 2Ag + ½O2 Marsh test: As, Sb salt + Zn/H2SO4→ As/SbH3 →Silver mirror of the metal Decomposition of ammonium compounds Ammonium dichromate on heating yields nitrogen, water and chromium(III) oxide. (NH4)2Cr2O7(s) → Cr2O3(s) + N2(g) + 4H2O(g) Please refer to text book or other resources for more examples of thermal decomposition reactions: Self study Thermal Decomposition of Carbonate and Azide Carbonate decomposition CuCO3 → CuO + CO2 CaCO3 → CaO + CO2 Azide decomposition / Life saving reaction 2NaN3 + 1/2O2 → Na2O + 3N2 NaN3 → Na + 3/2N2 0.03 SECONDis all it takes to inflate an air bag. 130 g. needed (~ Rs. 100) 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, 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  Thermal Decomposition: Chemical Reduction Mond’s Process (For pure nickel): Ni(CO)4(g)+ Ni(CO)4(g) Kroll’s Process (for pure titanium):  First Industrial Process For Zr and Ti Van Arkel-deBoer’s Process (for metallic Zr / Ti): Vacuum Mo electrode And net Chamber raw material W- Wire 1400 0C Extraction of Metals  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 Cd2+ + Zn → Cd + Zn2+ How does this work? Cu2+ +2e-→ Cu +0.34 Fe2+ + 2e-→ Fe -0.44 But Fe → Fe2+ +2e- +0.4 Total = +0.78 Exothermic reaction…  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 Cu2+ + Zn → Cu + Zn2+ Cl2 + 2Br- → 2Cl- + Br2  Demonstration !!! Cu2+ + Fe → Fe2+ + Cu -0.44 +0.16 Demonstration II Which one can oxidize Cu then? AgNO3 → Ag(s) + NO3- couple E0 =+ 0.8 V  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 And so on ……  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, 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 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  High –T Chemical Reduction- Thermodynamic Considerations…. Go = Ho - TS For the formation of metal oxide, 2M(s) + O2(g) → 2MO(s) ΔS 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 (ΔGo) increases with increase in the temperature  Go = Ho - TS The free energy changes that occur when one gram molecule of a common reactant (O2) is used, is plotted against temperature. ~-900 This graph is called Ellingham Diagram  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)  CO(g) + ½O2(g) → CO2(g) (S –ve) 710 oC C + O2(g) → CO2(g) (S constant) 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 oC Below 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 Method Cr2O3 -600 Go (kJmol-1) -800 Al2O3 -1000 -1200 Temperature (oC)  Al + O2 →  Al2O3 H = -266 Kcal/mol  Cr + O2 →  Cr2O3 H = -180 Kcal/mol  Al +  Cr2O3 →  Cr +  Al2O3 H = -86 Kcal/mol G ≈ H (since S is similar)  Thermit Process – Details  Al +  Cr2O3 →  Cr +  Al2O3 H = -86 Kcal/mol G is negative at all temperatures. S is very small since there are no gaseous products Hence, 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- A Poor Reducing Agent H2O Go (kJmol-1) H2 MO Temperature (oC) 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 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 Self reduction: then reduced to metal CuS → [CuS + CuO] → 2MS + 3O2 → 2MO + 2SO2 Cu + SO2 C H2 is also a poor reducing agent for metal sulfides. Ellingham Diagram - Metal Sulfides HgS Hgs H2S CS2 -40 Zns ZnS Fes FeS - 80 Go( KCal/ mole S2(g) ) MnS - 120 SO2 - 160 CaS Cas - 200 - 240 0 1000 oC 2000 oC Ellingham Diagram - Metal Halides CCl4 CF4 SnCl4 ZrCL4 MgCl2 TiCl4 -40 HCl NaCl HF NaF MgCl2 NbF5 CaCl2 UF4 CaF2 -100 Gfo 300oC 500oC 1500oC 2500oC Figure 2. Graph of Go/T for halide formation  Purification of Elements Special attention to metals 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.)  Purification of Elements 4. Electrolytic refining 5. Zone refining 6. Chromatographic methods 7.Solvent Extractions 8. Ion-Exchange Methods