When aluminium metal is used as a reducing agent for reducing oxides of metals such as Mn, Fe, Cr etc., the process of reduction is known as thermite process.
Example:
Cr₂O₃ + \(\longrightarrow\) Al₂O₃ + 2Cr

ZnS forms soluble complex Na₂ ​[Zn(CN)₄​] while PbS forms froth.
When the minerals are expected to contain PbS and ZnS, suitable depressants such as NaCN are added.
The added NaCN, selectively prevents ZnS from coming to the froth by forming soluble complex Na₂​[Zn(CN)_4​] and PbS is removed along with the froth.

(i) Cast iron is made by melting pig iron with scrap iron and coke using hot air blast. It has slightly lower carbon content (in pig iron, C is 4%) and in cast iron, C is about 3% and is extremely hard and brittle.
(ii) Leaching of alumina from bauxite ore Bauxite usually contains SiO₂, iron oxide and titanium oxide (TiO₂) as impurities.
Powdered ore is digested with cone. NaOH solution at 473-523 K and 35-36 bar pressure.Al₂O₃ and SiO₂ are dissolved in the solution while impurities do not.
\(Al_2O_3(S) +2NaOH(aq) + 3H_2O(l) \xrightarrow[35-36\ bar]{473-523K}\underset{Sodium\ aluminate}{2Na[Al(OH)_4](aq)}\)
Filtrate is neutralised by passing CO2 gas and hydrated Alp j is precipitated. Some freshly prepared hydrated Al₂O₃ is also added to solution to induce precipitation.
\(2Na [Al(OH)_4] (aq)+ 2CO_2(9)⟶\underset{(ppt.)}{Al_2O_3.xH_2O(s)}+2NaHCO_3(aq)\)
Precipitate is filtered, washed, dried and heated to give back pure Al₂O₃.
\(Al_2O_3. xH_2O( s)\xrightarrow{1470K}\underset{Alumina}{Al_2O_3(s)}+xH_2O(g)\)

(i) van-Arkel method Titanium is purified by this process. Crude titanium is heated in an evacuated vessel with iodine, titanium iodide is formed which is more covalent, volatilises.
\(\underset{Ti}+2I_2\xrightarrow{870K}TiI_4\xrightarrow{1800K}\underset{Pure}{Ti}2I_2\)
Titanium iodide is decomposed on a tungsten filament electrically heated to about 1800 K.Pure titanium is deposited on the filament.
(ii) Froth floatation process It is based on the fact that sulphide ores are preferentially wetted by pine oil whereas the gangue particles are wetted by water.

Argentite (Ag₂S). Because of low Ag content, it is not concentrated by froth floatation process

The standard energy of formation (\(\Delta\)_fG⁰) of CO₂ from CO is higher than that of the formation of ZnO from Zn.Therefore,CO cannot be used to reduce ZnO to Zn

Yes, the amorphous silicon is obtained by reduction of SiO₂ with burning Mg wire
\(Si{ O }_{ 2 }+2Mg\rightarrow Si+MgO\)  
Si, when heated with MgO at very high temperature, i.e. 1000^o C, forms SiO₂ and Mg metal is obtained.   
\(2MgO+Si\overset { 1000^{ \circ }C }{ \rightarrow } { 2Mg+Si{ O }_{ 2 }}\) 

(i) The metal should form a volatile compound with a suitable reagents and impurities should not.
(ii) The volatile compound should be easily decomposable so that pure metal can be easily obtained.

They are refined by zone refining. It is based on the principle that the impurities are soluble in the melt (molten state) than in the solid-state of metal.

It is because at high temperature, metals react with 'C' to form metal carbide and with hydrogen, they form metal hydride.

Basic requirements for both processes are:
(i) The metal should form volatile compound with a reagent, whereas impurities should not react.
(ii) The volatile compound should be easily decomposable so that metal can be recovered easily.

Fe₃O₄ (Magnetite), MnO₂ (Pyrolusite) and FeCr₂O₄(Chromite) can be concentrated by magnetic separation as these are attracted by magnet but impurities do not.