chemistry

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Illustration 1:

Explain the following processes as applied to metallurgy i) Amalgamation ii) Hydrometallurgy iii) Solvent Extraction

Solution:

i)

Amalgamation: 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.

ii) Hydrometallurgy or precipitation by a more electropositive metal: 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 present 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 iii) Solvent Extraction: 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 itslf 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.
Illustration 2: Explain the following process as applied to metallurgy i) Zone refining (Fractional crystallization) ii) Electro-refining iii) Van-Arkel Method

Solution:

i)

Zone refining (Fractional crystallization): This method is employed for preparing extremely pure metals. This method is based upon the principle that when a molten solution of the impure metal is allowed to cool, the pure metal crystilises cut while the impurities remain in the melt.

ii) Electro-refining: In this method, the impure metal is converted into a block which forms the anode while cathode is a rod or plate of pure metal. These electrodes are suspended in an electrolyte which is the solution of a soluble salt of the metal usually a double salt of the metal. When electric current is passed, metal ions from the electrolyte are deposited at the cathode in the form of pure metal while an equivalent amount of metal dissolves from the anode and goes into the electrolyte solution as metal ion. The soluble impurities present in the crude metal anode go into the solution while the insoluble impurities settle down below the anode as anode mud. iii) Van-Arkel Method: In this method, the metal is converted into it volatile unstable compound such as iodide leaving behind the impurities. The unstable compound thus formed is decomposed to get the pure metal. Ti(s) + 2F2(s)  TiF4(g)  + 2F2(g) Ti( s ) 540 K
1700 K

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