Topic

Zone refining:
This method is based on the principle that impurities are more soluble in the molten state of metal (the melt) than in the solid state. In the process of zone refining, a circular mobile heater is fixed at one end of a rod of impure metal. As the heater moves, the molten zone of the rod also moves with it. As a result, pure metal crystallizes out of the melt and the impurities pass onto the adjacent molten zone. This process is repeated several times, which leads to the segregation of impurities at one end of the rod. Then, the end with the impurities is cut off. Silicon, boron, gallium, indium etc. can be purified by this process.

The Gibbs free energy of formation (73916;_fG) of Cu₂S is less than that of H₂S and CS₂. Therefore, H₂ and C cannot reduce Cu₂S to Cu.
On the other hand, the Gibbs free energy of formation of Cu₂O is greater than that of CO. Hence, C can reduce Cu₂O to Cu.
C + Cu₂ → 2 Cu + CO
Hence, the extraction of copper from its pyrite ore is difficult than from its oxide ore through reduction.

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 froth, but prevents ZnS from coming to froth. This happens because NaCN reacts with ZnS to form Na₂[Zn(CN)₄].
4 NaCN + ZnS → Na₂[Zn(CN)₄] + Na₂S

The reduction potentials of zinc and iron are lower than that of copper. In hydrometallurgy, zinc and iron can be used to displace copper from their solution.
Fe + Cu²⁺ → Fe²⁺ + Cu
But to displace zinc, more reactive metals i.e., metals having lower reduction potentials than zinc such as Mg, Ca, K, etc. are required. But all these metals react with water with the evolution of H₂ gas. 2K + 2 H₂O → 2KOH + H₂
As a result, these metals cannot be used in hydrometallurgy to extract zinc.
Hence, copper can be extracted by hydrometallurgy but not zinc.

Mg + ½ O₂ → MgO_s [ΔG_(Mg,MgO)]
Si + O₂ → SiO₂ [ΔG_(Si,Si2)]
The temperature range in which ΔG_(Mg,MgO) is lesser than ΔG_(Si,Si2) , Mg can reduce SiO₂ to Si.
2Mg +SiO₂ → 2MgO + Si ; ΔG⁰ = -ve
On the other hand, the temperatures range in which [ΔG_(Si,Si2)] is less than [ΔG_(Mg,MgO)], Si can reduce MgO to Mg. 2Mg +SiO₂ → 2Mg + SiO₂; ΔG⁰ = -ve
The temperature at which Δ_fG curves of these two substances intersect is 1966 K. Thus, at temperatures less than 1966 K, Mg can reduce SiO₂ and above 1966 K, Si can reduce MgO.

The change in Gibbs energy is related to the equilibrium constant, K as
ΔG = -RT ln K
At room temperature, all reactants and products of the given reaction are in the solid state. As a result, equilibrium does not exist between the reactants and the products. Hence, the reaction does not take place at room temperature. However, at a higher temperature, chromium melts and the reaction takes place. We also know that according to the equation
ΔG =ΔH – TΔS ,
Increasing the temperature increases the value of TΔS making the value of ΔG more and more negative. Therefore, the reaction becomes more and more feasible as the temperature is increased.

In the extraction of aluminium, the significance of leaching is to concentrate pure alumina (Al₂O₃) 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 (Al₂O₃) dissolves as sodium meta-aluminate and silica (SiO₂) dissolves as sodium silicate leaving the impurities behind.
Al₂O₃ + 2 NaOH + 3 H₂O → 2 Na[Al(OH)₄]
SiO₂ + 2NaOH → 2 NaSiO₃ + H₂O
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[Al(OH)₄] + CO₂ ? Al₂O₃. xH₂O + 2NaHCO₃
Hydrated alumina thus obtained is filtered, dried, and heated to give back pure alumina (Al2O3).
Al₂O₃. xH₂O ? Al₂O₃ + xH₂O

If the ore or the gangue can be attracted by the magnetic field, then the ore can be concentrated by the process of magnetic separation. the ores of iron such as haematite (Fe₂O₃), magnetite (Fe₃O₄), siderite (FeCO₃), and iron pyrites (FeS₂) can be separated by the process of magnetic separation.