PROCESSES OF REFINING

Refining: the process of purifying impure metals is called refining.

(A)        Liquation process: the process is based upon the difference in fusability of the metal and the impurities.when the impurities are less fusible the the metal itself, this process is used.the impure metal such as Bi, Sn, Hg, Pb,etc. is placed on the sloping hearth of the furnace and gentely heated. The metal melts and flows down.

(B)         Distillation: the process is used for those metals which are easily volatile. The impure metals are heated and the vapours are seperately condensed in a receiver. This is used for purification of Zn, Cd, Hg, etc.

(C)         Coupellation: the impure metal is heated in the couple or voal shaped crucible made of base ash or cement and removed with the blast of air. Impurityof lead in silveris removed by this process.

(D)        Poling : it is used for the purification of those metals which contains their own oxide as impurity. The molten impure metal is stirred with green poles of wood.

(E)          Electrorefining : metals like Cu, Hg, Au, Ni, Cr, Al, etc.are purified by this method.the impure metal is made as anode and a strip of pure metals is made is cathode while electrolyte solution consists of a solution of a suitable salt of metal.

(F)          Zone refining: when highly pure metals are required, this method is applied for purification. This method is based on the difference in the solubility of impuritiesin the molten or solid state of metal. This method is used in the purification of germanium gallium silicon etc. which are used as semiconductor.

(G)        Van arkel process : this method is used for the purfiaction of ultra pure metals. The impure metal is converted into a volatile compound which is then decomposed electrically to getpure metals. Ti, Zr, Hf, Si, etc are refined by this method.

(H)        Mond process: nickel is purified by this method. The impur nickel is treated with CO at 330-350K. when volatile nickel carbonyl is formed.the carbonyl is subjected to 450-470K when it is decomposes giving pure nickel.

(I)             Chromatographic method: it is based on the selective distribution of various contituents of a mixturebetween two phases, a stationary phase and a moving phase. The stationary phase either be solid or lightly bound liquid on the solid support. The moving phse may be a liquid or a gas. Different types of chromatographic mathods have been developed.

(J)            Leaching: leaching involves the treatment of the ore with is suitable reagent to make it soluble while impurities remain insoluble. The ore or the metal is recovered from solution by a suitable chemical method.

e.g natives ore of Ag and Au are treated with aqueous solution of nacn in the presence of oxygen. The Au and Ag particles get dissolved forming complex cyanides. Ag or Au is recovered from solution by addtion of electropositive metal Zn.

SOLUTIONS (CHEMISTRY)

                                           SOLUTIONS

Dissolution   When a solid solute is added to the solvent, some solute dissloves and its concentration increases in solution. This prosess is known as dissolution.

Saturated solution:  such a solution in which no more solute is dissloved at the same temperature and pressure is called a saturated solution.

Unsaturated solution: such solution in which more solute can be dissloved at the same temperature and pressure.

Factors affecting solubility

1.    Nature of substance

2.    Effect of temperature

3.    Effect of pressure

                      

                 SOLUBILITY OF A GAS IN A SOLUTION

Oxygen dissloves only to the small extent in water. It is this dissloved oxygen which sustains all aquatic life. On the other hand hydrogen chloride is highly soluble in water.the solubility of gases increases with increase in pressure.

Henry was the first to give a quantative relationship between pressure and solubility of gases in the solvent which is known as henry law.

The law states that at a constant temperature the solubility of the gas in the liquid is directly proportional to the pressure of the gas.

Dalton, a contemporary of henry, also concluded independently that the solubility of gas in the liquid is the function of partial prussure of the gas.

The most commonly used  form of henry law states that “the partial preussure of the gas in the vapour phase (p) is directly proportional to the mole fraction of the gas in the solution.

                   P     =K_hX

The solubility of gases increases with decrease in temperature. It is due to this reson the aquatic species sre more comfortable in cold waters rather  than in warm water.

Henry’s law find several application in industry and explain some biological phenolmena:

To increase the solubility of co2 in soft drinks and soda water , the bottle is sealed under high pressure.

Scuba divers must cope with high concentraction of dissolved gases while breathing air at high pressure under water.increased pressure increases the solubility of atmospheric gases in blood. When the diver cames towards surface, the pressure gradually decreases. This releases the dissolved gases and leads to the formation of bubbles of nitrogen in the blood. This blocks capillaries and creates a medical condition known as bend which are painful and dangerous to life. To avoid bend , as well as the toxic effect of high concentrations of nitrogen in the blood , the tanks used by scuba divers are filled with air diluted with helium.

At high altitude the partial pressure of oxygen is less than that of the ground level.this leads to low concentration of oxygen in the blood and tissues of people living living at high altitude or climbers.

Low blood oxygen causes climbers to became weak and unable to think clearly, symptoms of of a condition known as anoxia.