Solved Examples on General Principles & Processes of Isolation of Metals:

Question: 1, Write short notes on

(a) Amalgamation    (b) Hydrometallurgy

(c) Self reduction     (d) Electrolytic reduction

Solution: a) This process is used for the extractioin of noble metals like gold and silver from the native ores. The finely crushed ore is brought into contact with mercury which combines with the metal forming its amalgam. The metal is then recovered by distilling the amalgam.

b) Hydrometallurgy is the process of bringing the metal into solution by the action of suitable chemical reagents (e..g., sodium cyanide solution or chlorine in presence of water etc.) followed by recovery of the metal by the use of a proper precipitating agent which is a more electropositive metal. For example, poor ores of silver are suspended in a dilute solution of sodium cyanide and air is blown through it when the silver present goes into solution as the argentocyanide complex. From this solution the metal is precipitated by adding zinc turnigs.

4Ag + 8NaCN + O₂ (air) + 2H­₂O --> 4Na [Ag(CN)₂] + 4NaOH

or AgCl + 2NaCN --> Na[Ag(CN)₂] + NaCl

2Na [Ag(CN)₂] + Zn --> Na₂[Zn(CN)₄] + 2Ag¯

c) In some cases no extra reducing agent is required. A part of the sulphide ore is changed to oxide or sulphate which then reacts with the remaining part to give the metal and sulphur dioxide. Copper and lead are obtained by this method.

2Cu₂S + 3O₂ --> 2Cu₂O + 2SO­₂

Cu₂S + 2Cu₂O --> 6Cu + SO₂

2PbS + 3O₂ --> 2PbO + 2SO₂

PbS + 2O₂ --> PbSO₄

PbSO₄ + PbS --> 2Pb + 2SO₂

d) The carbon reduction methods are not applicable for the highly electropositive and chemically active metals such as alkali metals, alkaline earth metals, aluminium etc. The oxides of these metals are very stable and have to be heated very strongly with carbon in order to reduce them to metals. But at high temperatures these metals combine with carbon to form carbides, thus creating problem. Hence these metals are extracted by the electrolytic reduction of their fused halides. The metal is liberated at the cathode. Aluminium is however obtained by the electrolytic reduction of alumina (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆).

Question: 2. What is solvent extraction?

Solution: Solvent extraction is the latest separation technique and has become popular because of its elegance, simplicity and speed. The method is based on preferential solubility principles.

Solvent or liquid-liquid extraction is based on the principle that a solute can distribute itself in a certain ratio between two immiscible solvents, one of which is usually water and the other an organic solvent such as benzene, carbon tetrachloride or chloroform. In certain cases, the solute can be more or less completely transferred into the organic phase. The technique can be used for purpose of preparation, purification, enrichment, separation and analysis.

Question: 3. A sulfide ore (A) on roasting leaves a residue (B). (B) on heating with chlorine gives (C), soluble in water, addition of excess potassium iodide to a solution of (C) gives a solution (D). A brown precipitate (E)is formed when a solution of ammonium sulfate is added to an alkaline solution of (D). Identify (A) to (E).

Solution: