Oxidative process, Parting process and solvent extraction

Parting process.

Gold contains Ag and Cu as impurities. Removal of Cu and Ag from gold is called partition process. Parting process can be achieved by either of following two ways

1. The parting of dry Cl_2.
2. Parting with Conc.H₂SO_4.

Parting with dry Cl₂.

The impure gold containing Ag impurities is melted in a fine clay crucible. Then dry chlorine gas is passed through molten metal.As a result, only Ag reacts with Cl₂ to form AgCl while Au remains as such in a pure state.AgCl form scum on the surface of the molten metal which is skimmed off.

$$Ag+Au+Cl_2→AgCl+Au$$

$$AgCl=Scum$$

$$Au=Alum$$

Parting with Conc H₂SO₄.

Cu and Ag as impurities then parting is carried out with hot and Conc H₂SO₄. The impure gold is treated with hot and Conc H₂SO₄.As a result, only Ag and Cu reacts with H₂SO₄ while gold remains as in pure state.

$$Ag+Cu+Au+H_2SO_4→Ag_2SO_4+CuSO_4+Au$$

$$Au=Pure$$

In this process, the sample of impure gold should contain less than 25% of gold. If the sample contains more than 25% gold then the amount of gold is readjusted to 25% by adding more impurities.Hence, this process is also called quartering process.

Oxidative process.

The oxidative process is used to remove the metallic impurities (Fe, Mn, Cu, Au, Pb, Sn, etc) and non-metallic impurities as S, Si, P etc) from the metal.This method is based on the principle that when oxygen or air is passed through impure molten metal. The impurities get oxidised but the metal should remain inert.This method is employed only when the impurities have high affinity towards oxygen.

The oxides of impurities are removed by following ways.

1. Volatile oxides such as CO, CO_2, SO₂ etc escaped out from the mouth of a furnace.
2. Some oxides forms scum on the surface of the molten metal which is removed by skimming.
3. The refractory impurities are removed in the form of slant by treating with the acidic or basic lining of the furnace.

$$SiO_2+Fe_2O_3→Fe_2(SiO_3)_3$$

$$CaO+SiO_2→CaSiO_3$$

$$CaO=flux$$

$$CaSiO_3=slag$$

The slant floats on the surface of the molten metal which is removed.

Oxidative refining.

The oxidative refining process is a most common separation technique particularly useful in the separation several metallic as well as the non-metallic impurities present along with metal.It is used in those cases when the impurities consisting of other metals or non-metals gets oxidised more readily than the metal itself. This process is used for removing the metallic impurities like Mn, Cu, Pb, Sn, Fe, Ag, etc.as well as the non-metallic impurities like C, P, S, Si etc present in metal.

The oxidative refining process is based upon the principle that, when oxygen or air is passed through the impure molten metal the impurities (metallic or non-metallic) are easily oxidised into their respective oxides.whereas the metal remains inert (unreactive).

The oxides formed during the oxidative refining process are removed by any of the following methods.

1. Some of the impurities may form their volatile oxides such as CO₂, SO₂, etc which escape out from the furnace.
2. The oxides may form a scum on the surface of the metal which is removed by a skimming process.
3. Some of the oxides may combine with each other to form a light fusible mass called slag which floats on the surface and is removed.

The oxidative refining process can be carried out by following ways.

Tossing, Puddling, Isomerisation, and cupellation.

Example : During the manufacture of steel by Bassem erization the impurities are removed as.

$$C+O_2→CO_2↑$$

$$S+O_2→SO_2↑$$

$$4P+5O_2→2P_2O_5$$

$$Mn+O_2→2MnO$$

$$2Ca+O_2→2CaO$$

Some oxides combines to form slag.

$$MnO+SiO_2→MnSiO_3$$

$$P_2O_5+CaO→Ca_3(PO_4)_2$$

Solvent extraction.

Solvent extraction is based on the principle that a solute can distribute itself in the certain ratio between two immiscible solvents such as organic solvent (example benzene, ether, CCl₄ etc). and inorganic solvent like water. This method is based on the preferential solubility of a substance between two immiscible solvents.

In this method, the mixture of metal ions usually aqueous is treated with a suitable solvent usually organic which is immiscible with a solvent of metal ion but should act as a better solvent for the metal ion to be separated. An example-impure solution of Ge(IV) is first treated with HCl to get GeCl₄.When this impure solution is treated with CCl₄ only GeCl₄ dissolved in CCl₄. In this way, CCl₄ selectively extract GeCl₄ while other impurities remain dissolved in water.

$$Ge^{+4}+4HCl\rightleftharpoons GeCl_4+H^++impurities$$

$$Ge^{+4}=impure$$

$$ GeCl_4=extracted\,by\,CCL_4$$

$$impurities=remains\,dissolved\,in\,water.$$

Reference

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.