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Multiple choice chemistry principles of metallurgy concentration of ore concentration of ores general principles of metallurgy

Which of the following reactions does not take place during leaching for concentration of bauxite?

  1. $Al _2O _3+2NaOH+3H _2O\rightarrow 2Na[Al(OH) _4]$
  2. $2Na[Al(OH) _4]+CO _2\rightarrow Al _2O _3.xH _2O+2NaHCO _3$
  3. $Al _2O _3.2H _2O\xrightarrow{{\Delta}}Al _2O _3+2H _2O$
  4. $Al _2O _3.xH _2O\xrightarrow{{\Delta}}Al _2O _3+xH _2O$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
  • Bauxite is the principal ore of aluminium.  The impurities present in bauxite are $SiO _2$, iron oxides and titanium oxide ($TiO _2$). 
  • Concentration is carried out by heating the powdered ore with a concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure. This process is called digestion. 
  •  $Al _2O _3$ is extracted out as sodium aluminate. The impurity, $SiO _2$ too dissolves forming sodium silicate. Other impurities are left behind.
$Al _2O _3+2NaOH+3H _2O \rightarrow 2Na[Al(OH) _4]$
  • The sodium aluminate present in solution is neutralised by passing $CO _2$ gas and hydrated $Al _2O _3$ is precipitated. At this stage, small amount of freshly prepared sample of hydrated$Al _2O _3$ is added to the solution. This is called seeding. It induces the precipitation.
$2Na[Al(OH) _4]+CO _2 \rightarrow Al _2O _3.xH _2O+2NaHCO _3$
  • Sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure$Al _2O _3$.
$Al _2O _3.xH _2O \rightarrow Al _2O _3 + xH _2O$

  • Hence, option C is correct answer.

Multiple choice chemistry principles of metallurgy concentration of ore concentration of ores general principles of metallurgy

Forth floatation process of concentration is based on the:

  1. preferential wetting properties with the frothing agent and water

  2. difference in the specific gravities of gangue and ore particles

  3. difference in solubility of ganaue and ore particles in frothing agent and water

  4. difference in reactivity of gangue and ore particles with water and frothing agent.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
  • Froth floatation process method is used for removing gangue from sulphide ores.
  •  In this process, a suspension of the powdered ore is made with water. Collectors and froth stabilizers are added to it. 
  • Collectors (e.g., pine oils, fatty acids, xanthates, etc.) enhance non-wettability of the mineral particles and froth stabilizers (e.g., cresols, aniline) stabilize the froth.
  • The mineral particles become wet by oils while the gangue particles by water. 
Multiple choice chemistry principles of metallurgy concentration of ore concentration of ores general principles of metallurgy

For which of the following ores forth floatation method is used for concentration?

  1. Haematite

  2. Zinc blende

  3. Magnetite

  4. Carnallite
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
  • Froth floatation method is used for removing gangue from sulphide ores. In this process, a suspension of the powdered ore is made with water
  • Among the given options Zinc blend ($ZnS$) is the only sulphide ore and hence Froth floatation process is used for its concentration.
  • Hence, option B is the right answer.
Multiple choice chemistry principles of metallurgy concentration of ore concentration of ores general principles of metallurgy

Which of the following metals is not extracted by leaching?

  1. Aluminium

  2. Mercury

  3. Silver

  4. Gold
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Leaching is a process where ore is soluble and impurities are insoluble, widely used extractive metallurgy technique which converts metals into soluble salts in aqueous media.
Bauxite, SIlver and gold ores are usually soluble in some solvents and hence leaching is used to concentrate them.
Hence, option $B$ is the correct answer.

Multiple choice chemistry principles of metallurgy concentration of ore concentration of ores general principles of metallurgy

Which of the following statements is correct about the role of collectors added during forth floatation process?

  1. Collectors enhance the non-wettability of ore particles

  2. Collectors enhance the wettability of gangue particles

  3. Collectors help in separating two sulphide ores present in the mixture

  4. Collectors help ore particles to settle down below the froth
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
  • Froth floatation method is used for removing gangue from sulphide ores. In this process, a suspension of the powdered ore is made with water.
  • Collectors and froth stabilizers are added to it. Collectors (e.g., pine oils, fatty acids, xanthates, etc.) enhance non-wettability of the mineral particles and froth stabilizers (e.g., cresols, aniline) stabilize the froth.
  • The mineral particles become wet by oils while the gangue particles by water.
Multiple choice chemistry principles of metallurgy concentration of ore concentration of ores general principles of metallurgy

Which of the following ores is concentrated by chemical leaching method?

  1. Cinnabar

  2. Argentite

  3. Copper pyrites

  4. Galena
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
  • In the metallurgy of silver , the respective metal is eached with a dilute solution of NaCN or KCN in the presence of air, which supplies $O _2$. The metal is obtained later by replacement reaction.
  • The reactions involved are-
$4Ag+8CN^{-}+2H _2O+O _2 \rightarrow 4[M(CN) _2]^{-}+4OH^{-}$

$2[Ag(CN) _2]^{-}+Zn \rightarrow [Zn(CN) _4]^{2-}+2Ag$
  • Hence, Argentite being an ore of SIlver is concenrated by leaching.
Multiple choice chemistry general principles of metallurgy concentration of ore concentration of ores principles of metallurgy

Which type of ores are concentrated by froth flotation process?

  1. Carbonates

  2. Sulphides

  3. Oxides

  4. Phosphates
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The froth flotation method is used to separate and concentrate ores by altering their surfaces to a hydrophobic or hydrophilic condition.


This method is based on the principle of the difference in the wetting properties of the ore and the gangue particles with the water and oil.


Therefore, sulphide ores preferably wetted by the oil and the gangue with the water.
Hence, option $(B)$ is correct.

Multiple choice chemistry general principles of metallurgy concentration of ore concentration of ores principles of metallurgy

When $ZnS$ and $PbS$ minerals are present together, then $NaCN$ is added to separate them in the froth flotation process as a depressant, because:

  1. $Pb(CN) _2$ precipitated while no effect on $ZnS$
  2. $ZnS$ forms soluble complex $Na _2 Zn(CN) _4$
  3. $PbS$ forms soluble complex $Na _2Pb(CN) _4$
  4. both $(a)$ and $(b)$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$NaCN$ prevents $ZnS$ from forming the froth by reacting with it to from complex $Na _2[Zn(CN) _4]$ and acts as a depressant. While it does not prevent $PbS$ from forming froth and both the ores can easily be separated.

Multiple choice chemistry principles of metallurgy concentration of ore concentration of ores general principles of metallurgy

Froth floatation process is used for the :

  1. Oxide ores

  2. Sulphide ores

  3. Chloride ores

  4. All of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Froth floatation process is used for the sulphide ores. In this method, wetting of the ore by an oil takes place. A strong current of air is blown through the suspension, producing a heavy froth or foam on the surface. The metal sulphide is wetted by the oil but the gangues is not and the sulphide-oil mixture is carried to the surface by films of oil The froth is skimmed off, the gangue settles down on the bottom or remains underneath the froth.