Precious metals and secondary metals

1. Module - 8 Precious metals and secondary metals Precious metals ( Ag, Au, Pt- group), all with low affinity for oxygen are found either in free state or, in rare cases, as very easily reducible compounds. Secondary metals are metals obtained not from ores but from scraps. We will talk mainly about metallurgical wastes.

2. Learning Objectives • Production of Au, Ag and Pt group metals • Production of secondary metals ( Cu, Pb, Zn, Al and Sn) • Utilization of metallurgical wastes

3. Precious Metals • Gold is most attractive, worth being judged not in terms of rarity or usefulness • Gold has ornamental value, prestige value and an exchange value. A symbol of wealth and power this metal is the basis of international monetary transactions.

4. Au – outstanding ability to keep its shine • Amazing ductility ( one rupee coin size gold can be drawn into 1.5 km long wire. Can be beaten to thinnest foils ( 0.000002 cm). Used also for dental alloys, special containers, ayurvedic medicines

5. Ag – This also has ornamental value. Can be alloyed with gold. • Outstanding electrical and thermal conductivity • Used in electroplating, photography ( silver halide), dental alloys, reflectors , solders etc. Silver and copper additions harden gold and impart a wide variety of colours that range from red through yellow to green – colours highly valued in jewellery Silver is associated with gold in nature- normally gold to silver is one is to thirteen. They can be separated.

6. Platinum Group • High temperature melting point, resistance to corrosion and oxidation make the metals suitable for special applications ( e.g. melting of salts and slags) Other attractive properties – High ductility, low vapour pressure, low co-efficient of thermal expansion, high temperature coefficient of electrical resistivity.

9. Processes for gold extraction • Amalgamation process • Cyanidation process • Chlorination process • Cyanidation 2 Au + 4CN- + 2H2O + O2 = Au(CN)-2 + 2OH- +H2O2 2Au + 4CN- + H2O2 = 2Au(CN)-2 +2OH- ----------------------------------------------------------- 4Au + 8CN- +2H2O +O2 = 4Au(CN)-2 +4OH-

10. NaCN is poisonous. If H2SO4 is present then more poisonous HCN is produced as gas. 2NaCN + H2SO4 = Na2SO4 + 2HCN Gold is recovered from solution by • Cementation by adding Zinc dust or wires or aluminium powder. Residual Zn is removed by heating • Eletrolytic process • Selective absorption on specially prepared carbon powder surface.Aucyanicle is absorbed , then released into aqueous phase by heating carbon powder

12. In ores of Pb, Cu, Zn , Silver can be present as agentite ( Ag2S) 75% of total world output of silver is from refining of base metals and only about 25% from silver ore proper.

13. Pt group metals are found with sulphide deposits (SA) and Cu-Ni sulphide ores (Canada, Russia). They are obtained from the slimes during electrorefining of copper

16. Secondary Metals Importance arises from the following If scrap is recycled then environment is cleaned Producing metals from scrap requires less energy Does not cause depletion of primary resources

17. Unit energies for production of primary and secondary metals -------------------------------------------------------------- Metal Primary from Ore Secondary 106k.Cal/t 106k.Cal/t -------------------------------------------------------------- Magnesium 90.2 3.0 Aluminium 61.5 3.0 Nickel 36.3 3.8 Zinc 16.4 4.5 Steel 8.1 3.3 Lead 6.8 3.0