General Principles And Processes of Isolation Of Elements

General Principles And Processes of Isolation Of Elements

Question

Describe the principle controlling each of the following processes:

(i) Preparation of cast iron form pig iron.

(ii) Preparation of pure alumina (Al2O3) from bauxite ore.

Answer

(i) The iron obtained from blast furnaces is known as pig iron. It contains around 4%carbon and many impurities such as S, P, Si, and Mn in smaller amounts.

 Cast iron is obtained by melting pig iron and coke using a hot air blast. It contains a lower amount of carbon (3%) than pig iron; cast iron is extremely hard and brittle.

(ii) Bauxite usually contains silica, iron oxide, and titanium oxide as impurities. In the process of leaching, alumina is concentrated by digesting the powdered ore with a concentrated solution of NaOH at 473-523 K and 35-36 bar. Under these conditions, alumina (Al2O3) dissolves as sodium meta-aluminate and silica (SiO2) dissolves as sodium silicate leaving the impurities behind.
stack Al subscript 2 straight O subscript 3 left parenthesis straight s right parenthesis with alumina below space plus 2 NaOH left parenthesis aq right parenthesis space plus 3 straight H subscript 2 straight O left parenthesis straight l right parenthesis space rightwards arrow from 35 minus 36 space bar to 473 minus 523 straight K of space stack 2 Na left square bracket Al left parenthesis OH right parenthesis subscript 4 right square bracket left parenthesis aq right parenthesis with sodium space aluminate below

stack SiO subscript 2 left parenthesis straight l with silica below right parenthesis space plus 2 NaOH space left parenthesis aq right parenthesis space rightwards arrow from 35 minus 36 space bar to 473 minus 523 straight K of space stack NaSiO subscript 3 with sodium space silicate below left parenthesis aq right parenthesis space plus straight H subscript 2 straight O left parenthesis straight l right parenthesis

The impurities are then filtered and the solution is neutralized by passing CO2 gas. In this process, hydrated Al2O3 gets precipitated and sodium silicate remains in the solution. Precipitation is induced by seeding the solution with freshly prepared samples of hydrated Al2O3.
2 Na left square bracket Al left parenthesis OH right parenthesis subscript 4 right square bracket left parenthesis aq right parenthesis space plus CO subscript 2 left parenthesis aq right parenthesis space rightwards arrow space Al subscript 2 straight O subscript 3 xH subscript 2 straight O left parenthesis straight s right parenthesis space plus 2 NaHCO subscript 3 left parenthesis aq right parenthesis

Hydrated alumina thus obtained is filtered, dried, and heated to give back pure alumina (Al2O3).
Al subscript 2 straight O subscript 3 xH subscript 2 straight O left parenthesis straight s right parenthesis space rightwards arrow with 1470 straight K on top space Al subscript 2 straight O subscript 3 left parenthesis straight s right parenthesis space plus xH subscript 2 straight O left parenthesis straight g right parenthesis

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