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Wednesday, April 13, 2022

Parting process : removal of Zn and Pb from Gold which is obtained by MacArthur-Forrest cyanide process:

Crude gold obtained by MacArthur-Forrest cyanide process and chlorination process contains Ag, Cu, Zn, and sometimes Pb as impurity. 
(A) Zn and Pb are removed by cupellation process. 
(B) Cu and Ag are removed by parting process.

(1) Parting with sulfuric acid or nitric acid:Gold is not attacked by these acids while Cu and Ag dissolve. If, however, the Au content in an impure sample is more than 30%, the Cu and Ag are also not attacked by the acid of any strength. Hence, before the acid treatment, the impure sample is melted with necessary amount of Ag to reduce its gold content to about 25% (quartation). The resulting alloy, after being granulated in water, is boiled with H2SO4 or nitric acid when Cu and Ag pass into solution, leaving Au undissolved . Au is separated and
fused again with borax and nitre when 100% Au is obtained.

(2) Parting with CI2 : Sometimes chlorine is used for the purification of Au. The impure sample of Au is fused with borax and CI2 gas is forced through it. The base metals are converted into chlorides that pass out as fumes at this high temperature, and AgCI forms a separate layer between the fused layer of Au and borax, which is skimmed off and the Au left behind cast into ingots.




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Bessemerisation : Purification of iron from cast Iron or pig iron :

The iron obtained from a blast furnace is a brittle material called cast iron or pig iron. It contains about 4% elemental C and smaller amounts of other impurities such as elemental Si, P, S, and Mn that are formed from their compounds in the reducing atmosphere of the
furnace. 
The most important of several methods for purifying the iron and converting it to steel is the basic oxygen process or oxidative refining. 
Molten iron from blast furnace is exposed to a jet of pure O2 gas for about 20 minutes in a furnace that is lined with basic oxide such as CaO. The impurities in the iron are oxidized and the acidic oxides that form react with basic CaO to yield a molten slag that can be poured off.
For example Phosphorous,  is oxidized to P4O10, which then reacts with CaO to give molten Ca3(PO4)2.

P4(l) + 5 O2(g)--> P4O10(l)

6 CaO (g) + P4O10(l) -->2 Ca3
(PO4)2(l)

Basic oxide acidic oxide slag
Mn also passes into the slag because its oxide is basic and reacts with SiO2 yielding molten manganese silicate.
This process produces steel that contains about 1% carbon but only very small amount of P and S. Usually the composition of liquid steel is monitored by chemical analysis and the amount of oxygen and impure iron used are adjusted to achieve the desired concentration of carbon and other impurities






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Cupellation : lead (Pb) present in Silver removed by Cupellation process:

In this process the molten impure metal is heated in a cupel, which is boat-shaped dish made of bone ash or cement, and a blast of air is passed over the molten metal. The impurities are oxidized and the volatile oxides thus produced escape with the blast of air.

The pure metal remains behind in the cupel. Pb present in silver is removed by cupellation process.

2 Pb(g) + O2 --> 2 PbO(g)



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Polling process: This process is used for the purification of copper and tin.


(1) Purification of impure copper : Impure copper is remelted in a reverberatory furnace lined with SiO2 and a blast of O2 is blows into the furnace. O2 oxidises S, Sb and As to their respective oxides which, being, volatile, get volatilised and are thus removed. Fe is oxidised to FeO which forms a slag of FeSiO3
with SiO2 lining of the furnace. Molten copper left behind contains CuO as impurity. This molten copper is treated with powdered anthracite and then stirred with a pole of green wood. Green wood, at high temperature, liberates hydrocarbon gases, which are converted into methane (CH4). Methane thus obtained reduces CuO to free Cu–metal, which is about 99.5% pure and is called tough pitch copper.

Green wood--> Hydrocarbons--> CH4
4CuO + CH4 --> 4Cu (pure metal) + CO2 + 2H2O

(2) Purification of impure tin : Impure tin metal contains the impurities of Cu, Fe, W and
SnO2. The impurity of SnO2 is due to the incomplete reduction of tin stone ore (SnO2) during smelting. In order to remove these impurities, the impure molten tin metal is taken in a big pot and stirred with a pole of green wood. 
Green wood, at high temperature liberates hydrocarbon gases, which are converted into methane CH4. Methane thus obtained reduces SnO2 to pure metal while the impurities of Cu, Fe, W etc. come up to the surface, where they come in contact with air and are oxidised to their respective oxides.
The oxides form a scum on the surface of pure tin metal. This scum is removed from the surface. Tin metal obtained by this method is 99% pure. 
Green wood --> Hydrocarbon-->CH4
2SnO2 + CH4 --> 2Sn + CO2 + 2H2O


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Kroll's Process and Imperial metal industries (IMI) Process of reduction;

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