• Reduction of metal is done in solution or molten state of salt
• Electrochemical equation reveals the principle

$\Delta G\degree=nE\degree F$

• n is no electrons $E\degree$ is the electrode potential of the redux couple formed in the system
• More reactive metals$\rarr large - E\degree \rarr$reduction is difficult
• Difference to two $E\degree$ values corresponds$\rarr +E\degree \rarr -\Delta G\degree$

• Less reactive metal will come out of the solution

• More reactive metal will go to the solution

  $Cu^{2+}(aq)+Fe(s) \rarr Cu(s)+Fe^{2+}(aq)$

• In electrolysis , the $M^{n+}$ ions are discharged at negative electrode (cathode) and deposited there

Aluminium

• Purified $Al_2O_3$ is mixed with $Na_3AlF_6$ and CaF
• Lowers the melting point of alumina and brings conductivity
• The fused matrix is electrolysed

• Steel vessel with lining of carbon acts as cathode graphite anode is used
• The overall reaction is $2Al_2O_3+3C \rarr 4Al+3CO_2$
• It is called hall - heroult process

• The oxygen liberated at anode reacts
• Carbon of anode produces CO and $CO_2$
• For each kg of Al about 0.5 kg of C anode is burnt
• The electrolytic reactions are

Cathode$\quad$ $Al^{3+}(melt)+3e \rarr Al(l)$

Anode$\quad$ $C(s)+O^{2-}(melt) \rarr CO(g)+2e^-$

$C(s)+2O^{2-}(melt) \rarr CO(g)+4e^-$

• Used for low grade ores

• Ore is leached out using acid

• The solution containing $Cu^{2+}$ is treated with scrap iron or $H_2$

  $Cu^{2+}(aq)+H_2(g) \rarr Cu(s)+2H^+(aq)$

  $Cu^{2+}(aq)+Fe(s) \rarr Cu(s)+Fe^{2+}(aq)$

• Iron in the electrochemical series is above Cu

Zinc or iron scraps

• Zinc is above iron in the electrochemical series $\rarr$ more reactive
• Reduction will be faster with zinc scraps
• Zinc is costier than iron $\rarr$ using iron scraps is economical

• It Suitable for non - metals

Example

• Extraction of chlorine from brine (chlorine is abundant in sea water as common salt)

  $2Cl^-(aq)+2H_2O(l) \rarr 2OH^-(aq)+H_2(g)+Cl_2(g)$

  $\Delta G\degree=+422 KJ \rarr using , \Delta G\degree = -nE\degree F$

  $E\degree = -2.2V$

• Required external e.m.f greater than 2.2 V

• Oxidation is used for metals

For example

• In extraction of gold and silver leaching with $CN^-$ is an oxidation reaction $(Ag \rarr Ag^+ or Au \rarr Au^+)$

• The metal is recovered by displacement method

  $4Au(s)+8CN^-(aq)+2H_2O(aq)+O_2(g) \rarr 4 Au(CN)_2 (aq)+4OH^-(aq)$

  $2Au(CN)_2 (aq)+Zn(s) \rarr 2Au(s)+[Zn(CN)_2]^{2-}(aq)$

• Zinc acts as a reducing agent

Isolation of metals

Ore $\rightarrow$ dress up to form convertable to meal (oxide) $\rightarrow$convert oxide/other form to metal$\rightarrow$impure metal$\rightarrow$refine

• Metal extrated by any method is usually contaminated
• Preparation of metals of high purity is refining
• Some of the techniques used are

(a) Distillation

(b) Liquation

(c) Electrolysis

(d) Zone refining

(e) Vapour phase refining

(f) Chromatographic methods

• Method depends upon properties of the metal and the impurity

Distillation

• Very useful for low boiling metals like zinc and mercury
• The impure metal is evaporated
• Pure metal obtained as distillate

Liquation

• Useful for low melting metal like tin
• Molten metal is made to flow on a slopped floor
• In separates from higher melling impurities

Electrolytic refining

• Impure metal is made anode
• A strip of the same metal in pure form is used as cathode
• They are put in a suitable electrolytic bath containing soluble salt of the same metal
• The more basic metal remains in the solution and the less basic ones go to the anode mud
• The reactions are
• Anode $M \rarr M^{n+}+ne^-$
• Cathode $M^{n+}+ne^- \rarr M$

Refining of copper by an electrolytic method

• Anodes are of impure copper and pure copper strips are taken as cathode electrolyte is acidified solution of copper sulphate
• Electrolysis results in the transfer of copper in pure form from the anode to the cathode
• Anode $Cu \rarr Cu^{2+}+2e^-$
• Cathode $Cu^{2+}+2e^- \rarr Cu$
• Impurities Sb, Se, Te, Ag, Au and Pt from the blister Cu deposit as anode mud
• Recovery of these elements may meet the cost of refining
• Zinc may also be refined this way

Zone refining

• Its principle is that the impurities are more soluble in the melt than in the solid state of the metal
• A circular mobile heater is fixed at one end of a rod impure metal

Zone refining

• Molten zone moves along with the heater moved forward
• As the heater moves forward , the pure metal crystallises
• From melt and the impurities pass on into adjacent molten zone
• Process is repeated several times moving heater in the same direction
• At one end , impurities get concentrated
• This end is cut off Method is very useful for producing semiconductor and other metals of very high purity , example Ge , Si , B , Ga , In

Vapour phase refining

• Metal is converted into its volatile compound and collected
• It is then decomposed to give pure metal
• Two requirements are

(i) The metal should form a volatile compound with an available reagent

(ii) The volatile compound should be easily decomposable , so that the recovery is easy

Mond process for refining nickel

• Nickel is heated in a stream of cartion monoxide
• Result a votatile complex , nickel tetracarbonyl
• $Ni+4CO \rarr Ni(CO)_4(330-350 K)$
• Ni carbonyl on heating to 450-470 K
• Decompose giving the pure metal
• $Ni(CO)_4 \rarr Ni+4CO$

Van arkel method for refining zirconium or titanium

• It is very useful for removing all the oxygen and nitrogen present in the form of impurity in metals like Zr and Ti
• The crude metal is heated in an evacuated vessel with $l_2$
• The metal iodide being more covalent , volatilizes

$Zr+2I_2 \rarr ZrI_4$

• The metal iodide is decomposed on a tungsten filament , electrically heated to about 1800K
• The pure metal is deposited on the filament

$ZrI_4 \rarr Zr+2I_2$

• Principle - different components of a mixture are differently adsorbed on an adsorbent (filled in a column)
• The mixture is taken in liquid or gaseous medium and moved through the adsorbent
• Different components are adsorbed at different levels on the column
• Adsorbed components are removed (eluted) by using suitable solvents (eluant)

• Method is called column chromatography
• Metal has to brought in solution
• Useful for purification of the elements present in minute quantities with impurities not very different in chemical
• Properties from the element to be purified
• Eluents complexing agents
• Chloride , EDTA
• They work as different metal ions form complexes of varying solubility

Anion - exchange separation of iron, cobalt and nickel

• From 90% acetone - 10% 6 M hydrochloric acid medium , Co and Ni are strongly adsorbed on the anion - exchange rasin Dowok 1 - ХВ
• Iron is not adsorbed and can be separated from cobalt and nickel
• Cobalt and nickel are then separated by elution with 70% acetone - 30% 2 M hydrochloric acid
• Nickel is eluted before cobalt

• The lanthanoids , are separated on columns of cation exchange resin
• Solutions of citrates , lactates , or other salts whose anions form negatively charged complexes with lanthanoid are used to wash the ions from the column
• The metal ions themselves are held by the resin, the complexes are not
• Ions that form more stable complexes do not adhere to the resin and move off the column quickly
• Ions that complex only weakly come out later in order of stability

Separation of lanthanoids

• Cation exchange in general is not a selective process
• Above process , termed differential complex formation , renders it more so
• An lanthanoid separations the exchanger is like an undiscriminating sponge that simply holds the metal ions
• Real separation of the lanthanoid is accomplished by weakness or strength of the complexes formed