Concentration Questions in English

(A) The cyanide process,also known as the MacArthur-Forrest process,is primarily used for the extraction of precious metals like gold $(Au)$ and silver $(Ag)$ from their ores.
In this process,the crushed ore is treated with a dilute solution of sodium cyanide $(NaCN)$ in the presence of air (as a source of oxygen).
For silver $(Ag)$,the chemical reaction is: $4Ag(s) + 8CN^-(aq) + 2H_2O(aq) + O_2(g) \rightarrow 4[Ag(CN)_2]^-(aq) + 4OH^-(aq)$.
Thus,$Ag$ is extracted using the cyanide process.

(A) Electrostatic separation is based on the difference in the electrical properties of the ore components.
$PbS$ (Galena) is a good conductor of electricity,whereas $ZnS$ (Zinc blende) is a poor conductor (insulator).
When the powdered ore is passed over a charged roller,the conducting particles $(PbS)$ get attracted to the roller and fall separately,while the non-conducting particles $(ZnS)$ fall away from the roller.

(C) The froth flotation process is primarily used for the concentration of sulfide ores.
Among the given options,$HgS$ (Cinnabar) is a sulfide ore.
Therefore,it is concentrated using the froth flotation process.
$Horn \ silver$ is $AgCl$,$Bauxite$ is $Al_2O_3 \cdot 2H_2O$,and $Hematite$ is $Fe_2O_3$.

(B) Gravity separation is based on the difference in the densities of the ore and the gangue particles.
This method is also known as hydraulic washing or levigation.
In this process,an upward stream of running water is used to wash away the lighter gangue particles,while the heavier ore particles settle down.

(A) In the froth flotation process,stabilizers are added to stabilize the froth. $Cresol$ and $Aniline$ are commonly used as stabilizers to enhance the stability of the froth formed during the concentration of sulfide ores.

(C) In the Bayer's process for the extraction of aluminum from bauxite,the ore is treated with a concentrated solution of $NaOH$ at $473-523 \ K$ and $35-36 \ bar$ pressure.
$Al_2O_3 \cdot 2H_2O + 2NaOH + H_2O \rightarrow 2Na[Al(OH)_4]$.
During this process,impurities like $Fe_2O_3$ (ferric oxide),$TiO_2$ (titanium dioxide),and $SiO_2$ (silica) remain insoluble and are removed as 'red mud'.
Among the given options,$Fe$ and $Ti$ compounds ($Fe_2O_3$ and $TiO_2$) are the primary insoluble impurities that precipitate out.

(B) The froth flotation process is used for sulfide ores.
$(1)$ The ratio of water and oil is adjusted to ensure proper wetting and frothing,which is true.
$(2)$ Depressants are used to selectively prevent one sulfide ore from forming froth,which is true.
$(3)$ $HCN$ is used as a depressant to prevent $ZnS$ from floating while allowing $PbS$ to float,which is true.
$(4)$ Since $HCN$ prevents $ZnS$ from floating,only $PbS$ comes up with the froth,which is true.
Therefore,all statements $(1), (2), (3),$ and $(4)$ are true.

(D) Statement $I$ is incorrect because Hall's process involves the conversion of $Al_2O_3$ into $NaAlO_2$ using $Na_2CO_3$. Silicon is removed as $Na_2SiO_3$ in the Bayer's process,not Hall's process.
Statement $II$ is correct because Bayer's process is specifically used for bauxite ores containing $Fe_2O_3$ as the main impurity.
Statement $III$ is correct because Serpek's process involves heating bauxite with coke and nitrogen at high temperatures to form aluminum nitride $(AlN)$.

(IRON ORES) The magnetic separation method is used when either the ore or the gangue particles are magnetic in nature.
Among the ores listed,the iron ores are magnetic.
Specifically,$Haematite (Fe_{2}O_{3})$,$Magnetite (Fe_{3}O_{4})$,$Siderite (FeCO_{3})$,and $Iron pyrites (FeS_{2})$ are iron-containing ores that exhibit magnetic properties and can be concentrated using this method.

In the extraction of aluminium,the significance of leaching is to concentrate pure alumina $(Al_2O_3)$ from bauxite ore.
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 $(Al_2O_3)$ dissolves as sodium meta-aluminate and silica $(SiO_2)$ dissolves as sodium silicate,leaving the impurities like iron oxide and titanium oxide behind.
$Al_2O_{3(s)} + 2NaOH_{(aq)} + 3H_2O_{(l)} \xrightarrow[35-36 \, bar]{473-523 \, K} 2Na[Al(OH)_4]_{(aq)}$
$SiO_{2(s)} + 2NaOH_{(aq)} \xrightarrow[35-36 \, bar]{473-523 \, K} Na_2SiO_{3(aq)} + H_2O_{(l)}$
The impurities are then filtered and the solution is neutralized by passing $CO_2$ gas. In this process,hydrated $Al_2O_3$ gets precipitated and sodium silicate remains in the solution. Precipitation is induced by seeding the solution with freshly prepared samples of hydrated $Al_2O_3$.
$2Na[Al(OH)_4]_{(aq)} + CO_{2(g)} \to Al_2O_3 \cdot xH_2O_{(s)} + 2NaHCO_{3(aq)}$
Hydrated alumina thus obtained is filtered,dried,and heated to give back pure alumina $(Al_2O_3)$.
$Al_2O_3 \cdot xH_2O_{(s)} \xrightarrow{1470 \, K} Al_2O_{3(s)} + xH_2O_{(g)}$

(B) In the froth flotation process,the role of the depressants is to separate two sulphide ores by selectively preventing one ore from forming froth.
For example,to separate two sulphide ores ($ZnS$ and $PbS$),$NaCN$ is used as a depressant which selectively allows $PbS$ to come with the froth,but prevents $ZnS$ from coming to the froth.
This happens because $NaCN$ reacts with $ZnS$ to form the soluble complex $Na_2[Zn(CN)_4]$.
$4NaCN + ZnS \longrightarrow Na_2[Zn(CN)_4] + Na_2S$

(N/A) To separate alumina from silica in bauxite ore,the powdered ore is digested with a concentrated $NaOH$ solution at $473-523 \ K$ and $35-36 \ bar$ pressure.
This process leaches out alumina $(Al_2O_3)$ as sodium aluminate and silica $(SiO_2)$ as sodium silicate,leaving behind other impurities.
$Al_2O_{3(s)} + 2NaOH_{(aq)} + 3H_2O_{(l)} \to 2Na[Al(OH)_4]_{(aq)}$
$SiO_2 + 2NaOH_{(aq)} \to Na_2SiO_{3(aq)} + H_2O_{(l)}$
Next,$CO_2$ gas is passed through the resulting solution to neutralize the aluminate,which causes the precipitation of hydrated alumina. The solution is seeded with freshly prepared samples of hydrated alumina to induce precipitation.
$2Na[Al(OH)_4]_{(aq)} + 2CO_{2(g)} \to Al_2O_3 \cdot xH_2O_{(s)} + 2NaHCO_{3(aq)}$
During this step,sodium silicate remains in the solution. The hydrated alumina is filtered,washed,dried,and heated to obtain pure alumina.
$Al_2O_3 \cdot xH_2O_{(s)} \xrightarrow{1470 \ K} Al_2O_{3(s)} + xH_2O_{(g)}$

(N/A) In the case of low-grade copper ores,leaching is carried out using acid or bacteria in the presence of air. In this process,copper dissolves into the solution as $Cu^{2+}$ ions.
$Cu_{(s)} + 2H^{+}_{(aq)} + \frac{1}{2}O_{2(g)} \to Cu^{2+}_{(aq)} + H_2O_{(l)}$
The resulting solution is then treated with scrap iron or $H_2$ gas to recover metallic copper.
$Cu^{2+}_{(aq)} + H_{2(g)} \to Cu_{(s)} + 2H^{+}_{(aq)}$

(N/A) $(i)$ Hydraulic Washing :
Principle : The method is based on the difference between the specific gravities of the ore and the gangue particles.
In hydraulic washing,an upward stream of running water is used to wash the powdered ore. The lighter gangue particles are washed away by the water current,while the heavier ore particles are left behind. This method is also referred to as Gravity Separation.

(N/A) Principle: This method is based on the difference in magnetic properties of the ore components (ore or gangue).
Procedure: The powdered ore is dropped over a conveyor belt which moves over a magnetic roller.
Mechanism: If either the ore or the gangue particles are magnetic,they are attracted towards the magnetic roller and fall closer to it,while the non-magnetic particles fall further away,forming a separate heap.
Example: This method is used for the concentration of iron ores like magnetite $(Fe_3O_4)$ and hematite $(Fe_2O_3)$.

(N/A) Principle: The process is based on the difference in wetting properties of ore and impurities in the presence of water and a frothing agent. The following components are used in the process:
$(i)$ Frothers: These substances create froth. Examples: Pine oil,turpentine oil,etc.
$(ii)$ Collectors: Collectors help in the adhesion of ore particles to air bubbles in the froth. Example: Sodium ethyl xanthate.
$(iii)$ Froth stabilizers: These substances stabilize the froth. Examples: Aniline,cresol,etc.
$(iv)$ Depressants: These substances prevent the formation of froth with air bubbles for specific minerals. Example: $NaCN$ (sodium cyanide).
Process: In this process,a suspension of the powdered ore is made with water. The mineral particles become wet by oils,while gangue particles are wetted by water. $A$ rotating paddle agitates the mixture and draws air into it. As a result,froth is formed which carries the mineral particles. The froth is light and is skimmed off. It is then dried for the recovery of ore particles.
Role of depressants: Sometimes,it is possible to separate two sulphide ores by adjusting the proportion of oil to water or by adding depressants. For example,in the case of an ore containing $ZnS$ and $PbS$,the depressant used is $NaCN$. $NaCN$ forms a layer of zinc complex $Na_2[Zn(CN)_4]$ on the surface of $ZnS$,thereby preventing it from froth formation and allowing $PbS$ to come with the froth.
Conclusion: The sulphide ores,being lighter and easily wetted by oils,come to the surface,leaving behind the gangue. Hence,this method is used for the concentration of sulphide ores.

(N/A) Leaching is a process of concentration of ore,which is used when the ore is soluble in a suitable solvent.
Principle: The powdered ore is treated with a suitable solvent in which the ore is soluble,but the impurities are not. The ore is then recovered from the solution by suitable chemical methods.
Example-$1$: Leaching of Alumina from Bauxite.
Bauxite,the principal ore of aluminium,usually contains $SiO_2$,iron oxides,and titanium oxide $(TiO_2)$ as impurities.
Concentrated $NaOH$ solution is used to digest the powdered ore at $473-523 \ K$ and $35-36 \ bar$ pressure. This process extracts $Al_2O_3$ as sodium aluminate.
$Al_2O_{3(s)} + 2NaOH_{(aq)} + 3H_2O_{(l)} \rightarrow 2Na[Al(OH)_4]_{(aq)}$
The impurities like $SiO_2$ also dissolve in $NaOH$ to form sodium silicate,while other impurities remain insoluble.
The aluminate in solution is neutralised by passing $CO_2$ gas,which results in the precipitation of hydrated $Al_2O_3$. The solution is seeded with freshly prepared $Al_2O_3$ samples to induce precipitation.
$2Na[Al(OH)_4]_{(aq)} + 2CO_{2(g)} \rightarrow Al_2O_3 \cdot xH_2O_{(s)} + 2NaHCO_{3(aq)}$
The sodium silicate remains in the solution,and the hydrated alumina is filtered,dried,and heated to get pure $Al_2O_3$.
$Al_2O_3 \cdot xH_2O_{(s)} \xrightarrow{1470 \ K} Al_2O_{3(s)} + xH_2O_{(g)}$
Example-$2$: Leaching of Silver and Gold.
In the metallurgy of silver and gold,the respective metal is leached with a dilute solution of $NaCN$ or $KCN$ in the presence of air $(O_2)$,which supplies the necessary oxygen.
$4M_{(s)} + 8CN^-_{(aq)} + 2H_2O_{(aq)} + O_2(g) \rightarrow 4[M(CN)_2]^-_{(aq)} + 4OH^-_{(aq)}$ (where $M = Ag$ or $Au$)
The metal is then recovered by displacement using zinc.
$2[M(CN)_2]^-_{(aq)} + Zn_{(s)} \rightarrow [Zn(CN)_4]^{2-}_{(aq)} + 2M_{(s)}$

(D) The concentration of an ore is the process of removing gangue from the ore. The common methods used are:
$1$. Hydraulic washing (gravity separation): Based on the difference in densities of the ore and gangue.
$2$. Magnetic separation: Used when either the ore or the gangue is magnetic.
$3$. Froth flotation process: Used for sulphide ores based on the difference in wetting properties of the ore and gangue with oil and water.
$4$. Leaching: Chemical method used when the ore is soluble in a suitable solvent.

(B) The froth flotation process is based on the principle of the difference in the wetting properties of the ore and the gangue particles with water and oil.
In this process,the ore particles are preferentially wetted by oil,while the gangue particles are preferentially wetted by water.

(B) The froth flotation method is primarily used for the concentration of $Sulphide$ ores.
In this process,the ore particles are wetted by oil (pine oil) and rise to the surface with the froth,while the gangue particles are wetted by water and settle at the bottom.

(B) Magnetic separation is based on the difference in magnetic properties of the ore and the gangue particles.
If either the ore or the gangue is attracted by a magnetic field,this method can be used for separation.
For example,magnetic ores like magnetite $(Fe_3O_4)$ or chromite $(FeCr_2O_4)$ can be separated from non-magnetic impurities using this technique.

(N/A) In the Froth Flotation Method,two sulphide ores can be separated by using substances called depressants.
Depressants selectively prevent one sulphide ore from forming froth while allowing the other to come to the froth.
For example,in an ore containing both $ZnS$ (zinc blende) and $PbS$ (galena),$NaCN$ is used as a depressant.
$NaCN$ reacts with $ZnS$ to form a soluble complex,$Na_2[Zn(CN)_4]$,which prevents $ZnS$ from forming froth,whereas $PbS$ is allowed to come with the froth.

(N/A) Principle: It works on the difference in the solubilities of ore and impurities in a suitable solvent.
$(i)$ Leaching of Alumina from Bauxite (Bayer's process): Bauxite is the principal ore of aluminium. It contains impurities of ferric oxide $(Fe_{2}O_{3})$,silica $(SiO_{2})$,and titanium oxide $(TiO_{2})$. The powdered ore is digested with concentrated $NaOH$ solution at $473-523 \ K$ temperature and $35-36 \ bar$ pressure. The alumina dissolves as sodium aluminate,while $Fe_{2}O_{3}$ and $TiO_{2}$ remain as solid residue. $SiO_{2}$ dissolves as sodium silicate.
$Al_{2}O_{3(s)} + 2NaOH_{(aq)} + 3H_{2}O_{(l)} \xrightarrow[473-523 \ K]{35 \ bar} 2Na[Al(OH)_{4}]_{(aq)}$
$SiO_{2(s)} + 2NaOH_{(aq)} \rightarrow Na_{2}SiO_{3(aq)} + H_{2}O_{(l)}$
The sodium aluminate solution is neutralized by passing $CO_{2}$ gas,causing hydrated $Al_{2}O_{3}$ to precipitate. $A$ small amount of freshly prepared hydrated $Al_{2}O_{3}$ is added to induce precipitation,a process called seeding.
$2Na[Al(OH)_{4}]_{(aq)} + 2CO_{2(g)} \rightarrow Al_{2}O_{3} \cdot xH_{2}O_{(s)} + 2NaHCO_{3(aq)}$
Sodium silicate remains in the solution,and the hydrated alumina is filtered and heated to obtain pure $Al_{2}O_{3}$.
$Al_{2}O_{3} \cdot xH_{2}O_{(s)} \xrightarrow{1470 \ K} Al_{2}O_{3(s)} + xH_{2}O_{(g)}$
$(ii)$ Cyanide Process (Leaching of Gold and Silver): In the metallurgy of gold and silver,the metal is leached with a dilute solution of $KCN$ or $NaCN$ in the presence of air (which supplies $O_{2}$). The metal is later recovered by a displacement reaction.
$4M_{(s)} + 8CN^{-}_{(aq)} + 2H_{2}O_{(l)} + O_{2(g)} \rightarrow 4[M(CN)_{2}]^{-}_{(aq)} + 4OH^{-}_{(aq)}$
$(M = Ag \text{ or } Au)$

(B) The removal of unwanted materials such as sand,clay,and rocks from the ore is known as ore benefaction or concentration (dressing) of ore.
Before concentration,the ores are crushed and pulverized into fine particles.
The selection of the concentration method depends on the differences in physical properties between the metal compound and the gangue.
Factors like the type of metal,available facilities,and environmental impact are also considered.

(A) The method of concentrating ore based on the difference in densities of the ore and the gangue (impurities) is known as $Levigation$ or $Gravity$ $Separation$ or $Hydraulic$ $Washing$. In this process,the powdered ore is washed with a stream of water,where lighter gangue particles are carried away,leaving behind the heavier ore particles.

(A) The concentration of sulphide ores is often carried out by the froth flotation process.
In this process,depressants are used to selectively prevent one sulphide ore from forming froth.
For an ore containing $ZnS$ (Sphalerite) and $PbS$ (Galena),sodium cyanide $(NaCN)$,which is a group $1$ cyanide salt,is used as a depressant.
$NaCN$ selectively prevents $ZnS$ from coming to the froth by forming a complex,$Na_{2}[Zn(CN)_{4}]$,while $PbS$ forms froth.
Therefore,$ZnS$ (Sphalerite) is the ore associated with this process.

(B) Statement $I$ is true: Sphalerite is $ZnS$ (zinc sulphide) and copper glance is $Cu_2S$ (copper sulphide).
Statement $II$ is true: In the froth flotation process,the separation of two sulphide ores can be achieved by adjusting the ratio of oil to water or by using 'depressants' (e.g.,$NaCN$ is used to separate $ZnS$ from $PbS$).

(B) In bauxite,impurities of $Fe_{2}O_{3}$,$TiO_{2}$,and $SiO_{2}$ are present.
$Fe_{2}O_{3}$ and $TiO_{2}$ are basic oxides,therefore they do not react with or dissolve in $NaOH$.
$SiO_{2}$ is an acidic oxide; it reacts with $NaOH$ to form soluble sodium silicate,hence it leaches out.
The chemical reaction is: $SiO_{2} + 2NaOH \rightarrow Na_{2}SiO_{3} (aq.) + H_{2}O$.

(C) In the froth flotation process,$NaCN$ is used as a depressant to separate $ZnS$ from $PbS$.
$NaCN$ selectively prevents $ZnS$ from forming froth by forming a soluble complex $Na_2[Zn(CN)_4]$,while $PbS$ forms froth and is collected.

(A) In the Froth Flotation process,depressants are used to selectively prevent one component of the ore from forming froth.
For example,in the separation of $PbS$ (Galena) from $ZnS$ (Zinc blende),$NaCN$ is used as a depressant.
$NaCN$ reacts with $ZnS$ to form a soluble complex $Na_2[Zn(CN)_4]$,thereby preventing $ZnS$ from coming to the froth,while $PbS$ forms the froth.

(C) Leaching is a process of concentrating ore by dissolving it in a suitable solvent in which the ore is soluble but the impurities are not.
The reaction $Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]$ represents the Bayer's process,which is a classic example of leaching bauxite ore $(Al_2O_3)$ using a sodium hydroxide solution.

(A) Earthy and undesired materials present in the ore,other than the desired metal,are known as gangue.

(B) Statement $I$ is false because in the froth floatation method,the rotating paddle agitates the mixture to draw air into it,not to drive air out of it.
Statement $II$ is true because iron pyrites $(FeS_2)$ contain a high percentage of sulfur,which leads to the release of $SO_2$ gas upon roasting,causing severe environmental pollution and acid rain. Therefore,it is generally avoided for the extraction of iron.

(C) The concentration of ore (also known as beneficiation or dressing) involves removing gangue from the ore. The common methods are:
$(i)$ Hydraulic Washing (Gravity separation)
$(ii)$ Froth Flotation
$(iii)$ Magnetic Separation
$(iv)$ Leaching
Liquation $(A)$ is a method used for the refining of metals with low melting points.
Electrolysis $(C)$ is a method used for the refining of metals.
Therefore,the methods $NOT$ involved in the concentration of ore are $A$ (Liquation) and $C$ (Electrolysis).

(D) The alkali leaching process is commonly used for the extraction of $Al$ (from bauxite ore) and $Au$ or $Ag$ (from their respective ores).
In the case of gold $(Au)$,the ore is treated with a dilute solution of sodium cyanide $(NaCN)$ in the presence of air $(O_2)$,which acts as a source of oxygen.
The chemical reaction is: $4Au(s) + 8CN^-(aq) + 2H_2O(aq) + O_2(g) \rightarrow 4[Au(CN)_2]^-(aq) + 4OH^-(aq)$.
Thus,$Au$ is extracted using this leaching technique.

(D) In the froth flotation process,stabilizers are added to stabilize the froth. $Cresols$ or $Aniline$ are commonly used as stabilizers.

(B) Cassiterite $(SnO_2)$ is the principal ore of tin.
It often contains magnetic impurities of wolframite,which is a mixture of iron tungstate $(FeWO_4)$ and manganese tungstate $(MnWO_4)$.
These magnetic impurities are separated from the non-magnetic $SnO_2$ ore using electromagnetic separation.

(D) The correct answer is $Galena$.
$Galena$ $(PbS)$ is a sulphide ore.
The froth flotation process is specifically used for the concentration of sulphide ores because sulphide ores are preferentially wetted by oil,while gangue particles are wetted by water.
Since $Galena$ is lead$(II)$ sulphide,it is concentrated using this method.

(A) In leaching of alumina from bauxite by Bayer's process,the ore is treated with $NaOH_{(aq)}$.
Concentration is carried out by heating the powdered ore with a concentrated solution of $NaOH$ at $473-523 \ K$ and $35-36 \ bar$ pressure.
$Al_2O_3$ is extracted out as sodium aluminate.
$Al_2O_{3(s)} + 2NaOH_{(aq)} + 3H_2O_{(l)} \rightarrow 2Na[Al(OH)_4]_{(aq)}$
The sodium aluminate formed is neutralised by passing $CO_2$ gas and hydrated $Al_2O_3$ is precipitated.
$2Na[Al(OH)_4]_{(aq)} + 2CO_{2(g)} \rightarrow Al_2O_3 \cdot xH_2O_{(s)} + 2NaHCO_{3(aq)}$

(D) Cassiterite $(SnO_2)$ is a non-magnetic ore.
Wolframite $(FeWO_4)$ is a magnetic impurity.
These can be separated by the magnetic separation method,where the ore is passed over a magnetic belt that attracts the magnetic wolframite while the non-magnetic $SnO_2$ falls off.

(B) In the process of leaching gold,the ore is treated with a dilute solution of sodium cyanide $(NaCN)$ in the presence of air or oxygen $(O_2)$.
This results in the formation of a soluble complex,sodium dicyanoaurate$(I)$,represented by the equation: $4Au(s) + 8CN^-(aq) + 2H_2O(aq) + O_2(g) \rightarrow 4[Au(CN)_2]^-(aq) + 4OH^-(aq)$.

(A) Bauxite ore is finely divided and heated under pressure with a concentrated solution of sodium hydroxide $(NaOH)$ at $150^{\circ} C$ to obtain sodium meta-aluminate.
The chemical reaction is as follows:
$Al_{2}O_{3} \cdot 2H_{2}O + 2NaOH \xrightarrow{150^{\circ} C} 2NaAlO_{2} + 3H_{2}O$
Thus,the product formed is sodium meta-aluminate.

(C) Bauxite ore $(Al_2O_3 \cdot 2H_2O)$ is treated with a concentrated solution of sodium hydroxide $(NaOH)$ at $473-523 \ K$ and $35-36 \ bar$ pressure.
This process dissolves the aluminium oxide to form soluble sodium meta aluminate $(NaAlO_2)$:
$Al_2O_3(s) + 2NaOH(aq) + 3H_2O(l) \rightarrow 2Na[Al(OH)_4](aq)$ or $2NaAlO_2(aq) + 3H_2O(l)$.

(B) Bauxite $(Al_2O_3 \cdot 2H_2O)$,an ore of aluminium,is purified by the leaching process known as Hall's process (or Bayer's process) to remove impurities like $Fe_2O_3$,$SiO_2$,and $TiO_2$.

(A) In the Bayer's process,$NaOH$ is used to leach bauxite ore $(Al_2O_3 \cdot 2H_2O)$ to separate it from impurities like $Fe_2O_3$ and $SiO_2$.
$NaOH$ is not a primary standard because it is hygroscopic and absorbs $CO_2$ from the air.
Manganous hydroxide,$Mn(OH)_2$,is insoluble in excess $NaOH$.
$NaOH$ reacts with $Cl_2$ to form sodium chloride and sodium hypochlorite or chlorate depending on the temperature.

(B) In the froth floatation process,collectors like potassium ethyl xanthate are added to the suspension.
These molecules adsorb onto the surface of the mineral particles,making them water-repellent or $hydrophobic$.
This allows the mineral particles to attach to air bubbles and rise to the surface with the froth,effectively separating them from the gangue.

(A) In the froth floatation process,potassium ethyl xanthate acts as a collector.
It attaches itself by its polar group to the grains of minerals,making the mineral surface hydrophobic.
This increases the non-wettability of the mineral particles,effectively making the ore water repellant.

(D) The froth floatation process is primarily used for the concentration of sulphide ores.
Among the given options,chalcopyrites $(CuFeS_{2})$ is a sulphide ore,whereas cryolite $(Na_{3}AlF_{6})$,cuprite $(Cu_{2}O)$,and calamine $(ZnCO_{3})$ are not.
Therefore,chalcopyrites is concentrated by the froth floatation process.

(B) Cinnabar $(HgS)$ is a sulphide ore.
Froth floatation process is specifically used for the concentration of sulphide ores.
Therefore,cinnabar is concentrated by this process.

(A) When $Al_2O_3$ is leached from bauxite using concentrated $NaOH$ solution,the complex formed is $Na[Al(OH)_4]$.
The chemical reaction involved in the leaching process of bauxite ore is:
$Al_2O_3 \cdot 2H_2O + 2NaOH + 3H_2O \xrightarrow{473-523 \ K} 2Na[Al(OH)_4]$