Tag: metallurgy of aluminium

Questions Related to metallurgy of aluminium

Identity the principle behind the concentration of bauxite ore.

chemistry production of metals extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. Magnetic property of impurities in white bauxite

  2. Low specific gravity of $Al _{2}O _{3}$

  3. Amphoteric nature of $Al _{2}O _{3}$

  4. High melting point of $Al _{2}O _{3}$

Show answer
Correct Option: C
Explanation:

$Al _2O _3 + 2NaOH + 3H _2O \rightarrow 2Na[Al(OH) _4]$
$Al _2O _3 + 6HCl \rightarrow 2AlCl _3 + 3H _2O$

Hence, in both the ways, Al can be concentrated, which owes to its amphoteric nature. 

Use the relationship $\Delta \,G^{\circ}\,=\,- nFE^{\circ} _{cell}$ to estimate the minimum voltage required to electrolyse $Al _2O _3$ in the Hall-Heroult process.

$\Delta G^{\circ} _{f}(Al _2O _3)\,=\,-1520\,kJ\,mol^{-1}$

$\Delta G^{\circ} _{f}(CO _2)\,=\,-394\,kJ\,mol^{-1}$

The oxidation of the graphite anode to $CO _2$ permits the electrolysis to occur at a lower voltage than if the electrolysis reactions were :

$Al _2O _3\,\rightarrow\,2Al\,+\,3O _2$.

What is the approximate value of low voltage?

chemistry production of metals extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. 2 V

  2. 3 V

  3. 4V

  4. 5V

Show answer
Correct Option: A
Explanation:

Net reaction in Hall-Heroult process is

$3C\,+\,2Al _2O _3\,\rightarrow\,4Al\,+\,3CO _2$

Or $4Al^{3+}\,+\,12e^{-}\,\rightarrow \,4Al,$

Number of electrons (n) = 12

$\Delta\,G^{\circ }\,=\,3 \Delta\, G^{\circ } _f (CO _2)\,-\,2 \Delta\,G^{\circ } _f(Al _2O _3)$

$=-3\,\times\,394\,-\,2(-1520)$

$= 1858\ kJ$


$\Delta G^{\circ}\,=\, -nFE^{\circ} _{cell}$

$-E^{\circ} _{cell}\,=\, \displaystyle \frac {\Delta G^{\circ}}{nF}\, =\, \displaystyle\frac {1858\,\times\, 1000}{12\, \times\, 96500}$

$= 1.60\ V$

Thus, Hall-Heroult process takes place at lower voltage.

In electrtolysis of $Al _2O _3$ by Hall-Heroult process:

chemistry production of metals extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. Cryolite $Na _3[AIF _6]$ lowers the melting point of $Al _2O _3$ and increases its electrical conductivity.

  2. $Al$ is obtained at cathode and probably $CO _2$ at anode

  3. both (a) and (b) are correct

  4. none of the above is correct

Show answer
Correct Option: C
Explanation:

The electrolysis of alumina by Hall and Heroult's process is carried by using a fused mixture of alumina and cryolite  along with minor quantities of aluminum fluoride and fluorspar. The addition of cryolite and fluorspar increases the electrical conductivity of alumina and  lowers the fusion temperature. Thus, the option A is correct. Also at anode, alumina reacts with fluorine to give oxygen. The liberated oxygen reacts with carbon to form $CO$ and $CO _2$. These gases are liberated at anode.

Which one of the following statements is incorrect?

chemistry production of metals extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. In the aluminothermite process, aluminium acts as reducing agent.

  2. Lead is extracted from its chief ore by both carbon reduction and self-reduction.

  3. Zinc is extracted from its chief ore by carbon reduction.

  4. Extraction of gold involves the leaching of ore with cyanide solution followed by reduction with zinc.

  5. In Hall-Heroult process, the electrolyte used is a molten mixture of alumina, sodium hydroxide and cryolite.

Show answer
Correct Option: E
Explanation:
(1) $Cr _{2}O _{3}+2Al\rightarrow Al _2O _3+2Cr^{3+}$
(2) $2PbS+2PbO\rightarrow 2PbO+2SO _{2}$
$PbS+2PbO\rightarrow 3Pb+SO _{2}$ (self reduction)
$PbO+C\rightarrow Pb+CO$ (carbon reduction)
(3) $ZnO+C\rightarrow Zn+CO$ (carbon reduction)
(4) $4Au(S)+8CN(aq)+2H _{2}O(aq)+O _{2}(g)\rightarrow 4[Au(CN) _{2}]^{-}(aq)+4OH^{-}(aq)$ - leaching
$2[Au(CN] _{2}]^{-}(aq)+Zn(S)\rightarrow 2Au(S)+[Zn(CN) _{4}]^{2-}(aq)$ - reduction
(5) It is molten mixture of alumina and cryolite

The following question is relevant to the extraction of Aluminium:
Along with cryolite and alumina, another substance is added to the electrolyte mixture. the substance is ___________.

chemistry industrial inorganic chemistry extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. Fluorspar

  2. Mica

  3. NaCl

  4. None of these

Show answer
Correct Option: A
Explanation:

Fluorspar is the substance act as a solvent and it increases conductivity. (Pure alumina melts at about $2000^oC$ and is a bad conductor of electricity. If fused cryolite and fluorspar is added, the mixture melts at $900^oC$ and  $Al _2O _3$ becomes a good conductor of electricity.)

A compound added to lower the fusion temperature of the electrolytic bath in the extraction of Al is ___________.

chemistry industrial inorganic chemistry extraction of aluminium metallurgy of aluminium extraction of metals by electrolysis

  1. cryolite and fluorspar

  2. Bauxite

  3. Iron and fluorspar

  4. none of these

Show answer
Correct Option: A
Explanation:

Pure alumina melts at about $2000^oC$ and is a bad conductor of electricity. If fused cryolite and fluorspar is added, the mixture melts at $900^0C$ and $Al _2O _3$ becomes a good conductor of electricity.