Question

Explain its significance in the extraction of aluminium:

Sodium hydroxide

Accepted Answer

Bayer's process:

Bauxite is grinded finely and heated to remove volatile impurities. It is then heated under pressure with conentrated caustic soda solution for $2 to 8$ hours at $140 ° C to 150 ° C$ (leaching).

Here is the chemical reaction:

${Al}_{2} O_{3} . 2 H_{2} O + 2 NaOH \to 2 {NaAIO}_{2} + 3 H_{2} O .$

Bauxite dissolves and forms sodium meta aluminate because of the amphoteric nature of aluminium, leaving behind insoluble impurities called red mud. Red mud consists of ferric oxide, sand, etc., which are removed by filtration.

On diluting sodium meta aluminate with water and cooling to $50 ° C$ , it gets hydrolysed to give aluminium hydroxide as a precipitate. The other soluble impurities remain dissolved in sodium hydroxide.

Here is the chemical reaction:

${NaAlO}_{2} + 2 H_{2} O \to NaOH + Al {(OH)}_{3} ↓$

The precipitate is filtered, washed, dried and ignited at $1000 ° C$ to get alumina $({Al}_{2} O_{3}) .$

Here is the chemical reaction:

$2 Al {(OH)}_{3} \to_{1000^{°} C}^{heat} {Al}_{2} O_{3} + 3 H_{2} O$

The filtrate containing sodium hydroxide is again used to extract aluminium from bauxite.

S P Singh Solutions for Chapter: Metallurgy, Exercise 3: EXERCISE-7C

S P Singh Chemistry Solutions for Exercise - S P Singh Solutions for Chapter: Metallurgy, Exercise 3: EXERCISE-7C

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