GENERAL PRINCIPLES AND PROCESSES OF ISOLATION OF ELEMENTS

MINERAL : a naturally occurring chemical substance in the earth’s crust obtainable by mining

ORE : a mineral from which metal can be extracted profitably

GANGUE : earthly and undesirable impurities which contaminate ore

METALLURGY: entire scientific and technological process used for isolation of metal from its ore

STEPS OF METALLURGY:

(i)Concentration of ore (ii) isolation of metal from its concentrated ore (iii) purification of metal

1.CONCENTRATION OF ORE: removal of undesirable impurities(gangue) from ore . It is also called dressing or benefaction. Difference in physical properties of the compound of metal and those of gangue decides the method to be used for concentration of ore.

METHODS OF CONCENTRATION OF ORES AND THE PRINCIPLE INVOLVED

HYDRAULIC WASHING(or gravity separation): This is based on the differences in gravities of the ore and the gangue particles. The lighter gangue particles are washed away by a stream of water running over powdered ore leaving behind heavier ore particles.

MAGNETIC SEPARATION: This is based on the differences in the magnetic properties of the ore and gangue. The powdered ore moves over a magnetic roller when magnetic and non magnetic particles make separate heaps.

FROTH FLOATATION: This is based on the difference in the wettability of ore and gangue particles. Ore particles are wetted by pine oil/fatty acids/xanthates (a collector) and become light whereas gangue particles are wetted by water and become heavier.

On rotating the mixture of powdered ore and water containing pine oil and a froth stabiliser (aniline/cresols) , heavier gangue particles settle down whereas lighter ore particles form froth which is skimmed off.This method is used for concentration of sulphide ores

Depressant: used to separate two sulphide ores eg. NaCN is used as a depressant to separate ore containing PbS and ZnS. NaCN prevents ZnS to come in the froth but allows PbS .

LEACHING: This is based on the difference in the solubility of ore and gangue particles in a suitable solvent eg. Alumina is separated from silica and other impurities present in bauxite ore by heating powdered ore with concentrated NaOH at high temperature and high pressure. Alumina forms sodium aluminate, silica forms sodium silicate and both remain in solution leaving other impurities behind. From the solution alumina is precipitated in hydrated form by passing CO2 gas whereas sodium silicate remains in the solution.

Al2O3(s) + 2NaOH(aq) + 3H2O(l) ------- 2Na[Al(OH)4](aq)

2Na[Al(OH)4](aq) + CO2(g) ------ Al2O3.xH2O(s) + 2NaHCO3(aq)

Al2O3.xH2O(s) ----- Al2O3(s) + x H2O(g) (on heating at a temp. 1470K)

2.ISOLATION OF METAL FROM CONCENTRATED ORE: The concentrated ore is converted to a form of metal suitable for reduction.Thus isolation of metal from its concentrated ore involves-

(i)conversion to oxide (oxides are easier to reduce and particularly if the metal formed is in liquid state from solid metal oxide at the temperature of reduction because ∆S becomes positive and ∆Go more negative which makes reduction easier)

Calcination – heating of concentrated ore in absence of air eg carbonate ores

ZnCO3(s) ------ ZnO(s) + CO2(g)

Roasting—heating of concentrated ore in a regular supply of air at a temperature below the melting point of the metal eg.sulphide ores

ZnS(s) + O2(g) ------ ZnO(s) + SO2(g)

(ii) reduction of oxide to metal by pyrometallurgy i.e. thermal reduction of metal oxide by heating with a reducing agent like C, CO, a metal or electrolytically ) eg.

FeO + CO ---- Fe + CO2

ZnO + C ---- Zn + CO Cr2O3 + 2Al ---- 2Cr + Al2O3

EXTRACTION OF IRON FROM ITS OXIDE

(i)at lower temperature range in the upper part of blast furnace

3Fe2O3 (ore) + CO ----- 2Fe3O4 + CO2 [ CaCO3 (limestone) ---- CaO +CO2 ]

Fe3O4 + CO ----- 3FeO + CO2 [ CaO + SiO2(flux) -- CaSiO3 (slag) ]

(ii)at higher temperature range in the lower part of blast furnace

FeO + CO ----- Fe + CO2 [ CO2 + C -- 2CO ]

FeO + C (coke) ----- Fe + CO [ C + O2 ---- CO2 ]

(iii)at very high temperature at the bottom of the blast furnace

Iron in the molten form collects with molten slag above it which protects its oxidation by hot blast of air entering the furnace.

PIG IRON – iron collected from blast furnace which contains about 4% C and smaller amounts of S,P,Si,Mn as impurities

CAST IRON – pig iron is heated with scrap iron and coke to get cast iron which contains about 3% C and is extremely hard and brittle.

WROUGHT IRON—it is the purest form of iron and is prepared by heating cast iron with limestone in a furnace lined with haematite. Impurities like S,P,Si are removed as slag and haematite oxidises C to CO and thus contains about 0.5% C and is malleable.

EXTRACTION OF COPPER FROM ITS OXIDE

Cu2O from roasted/smelted sulphide ore is heated with coke (reducing agent) to get Cu

Cu2O + C ---- 2Cu + CO [ 2Cu2S + 3O2 -- 2Cu2O +2SO2] (copper matte) (ore)

Copper matte contains Cu, Cu2S and FeS. It is heated in a converter lined with silica by a hot blast of air to convert FeS(impurity) to FeS which reacts with SiO2 to form slag(FeSiO3). Cu2S oxidises to Cu2O which reacts with remaining Cu2S to form Cu. 2FeS + 3O 2 --- 2FeO + 2SO2

FeO + SiO2 --- FeSiO3(slag)

2Cu2S + 3O2 --- 2Cu2O + 2SO2

Cu2O + 2Cu2S -- 6Cu + 2SO2

Copper obtained solidifies and shows blisters on its surface due to evolution of SO2 and is thus called blister copper.

EXTRACTION OF ZINC FROM ITS OXIDE

Zinc oxide from roasted zinc blende(ZnS) is heated with coke(reducing agent) at temp. 673K to get Zn. (CO is not used to reduce ZnO as its reaction with O2 has less negative ∆rGo value. On the other hand for carbon ∆rGo has high negative value particulary at 673K)

ZnO + C ----- Zn + CO [ ZnS + O2 --- ZnO + SO2 ] (ore) Zinc is distilled off and collected.

EXTRACTION OF ALUMINIUM FROM ITS OXIDE

Alumina(Al2O3) mixed with fluorite(CaF2)/cryolite(Na3AlF6) is electrolysed in a steel cell lined with carbon(cathode) and multi graphite rods(anode). Aluminium collects at the cathode.

Fluorite/cryolite is used to lower the melting point of the mixture.

Multi graphite rods (anode) is used as oxygen liberated at anode reacts with carbon to form CO and CO2 , thus it is burnt away.

Cathode - Al+3 + 3e- ---- Al(l)

Anode - O-2 + C ----- CO(g) + 2e-

2O-2 + C ----- CO2(g) + 4e-

Overall reaction 2Al2O3 + 3C ----- 4Al + 3CO2

HYDROMETALLURGY : extraction of metal from its solution using hydrogen as a reducing agent eg.

Copper is extracted from low grade ores.Copper being less electropositive is reduced by hydrogen.

Cu+2 (aq) + H2 (g) ----- Cu (s) + 2H+1(aq)

Zinc can not be extracted by this method as zinc is more electropositive than hydrogen.

Similarly scrap iron/scrap zinc can also be used to extract copper from low grade ore as iron and zinc

both are more electropositive than copper. However zinc being costlier is not used even though

reduction will be faster. Therefore using iron scraps is advantageous and advisable.

OXIDATION-REDUCTION

EXTRACTION OF GOLD OR SILVER

Gold or silver is extracted from its ore by oxidation – reduction reaction. The ore is leached with

cyanide (CN-1) when the metal forms a complex and goes in the solution. This is an oxidation

reaction(M--- M+1). From the solution metal is obtained by reacting with zinc which reduces the

metal ion to metal( M+1 --- M).

4Au (s) + 8CN-1 (aq) + 2H2O (l) + O2 (g) ----- 4[Au(CN)2 ]-1(aq) + 4OH-1(aq) (oxidation)

2[Au(CN)2 ]-1(aq) + Zn(s) ----- 2Au(s) + [Zn(CN)4]-2 (aq) (reduction)

EXTRACTION OF CHLORINE FROM BRINE(aq NaCl solution)

Electrolysis of aq.NaCl solution releases chlorine gas at the anode , hydrogen at cathode and

NaOH(aq) is formed as by-product. Chlorine is obtained by oxidation of Cl-1 and water is reduced to

hydrogen. NaCl(aq) --- Na+1(aq) + Cl-1(aq)

2Cl-1(aq) + 2H2O(l) -- 2OH(aq) + H2(g) + Cl2(g)

However electrolysis of molten NaCl results in deposition of sodium at cathode instead of hydrogen

and NaOH does not form. Chlorine liberates at the anode.

3.REFINING/PURIFICATION OF METAL: removal of impurities from the metal extracted from its ore

METHODS OF REFINING AND THE PRINCIPLE INVOLVED

DISTILLATION : It is used to separate low boiling metals from high boiling impurities eg. Zn , Hg On heating, the metal distills off leaving impurities behind.

LIQUATION: It is used to separate low melting metals from high melting impurities eg. Sn , Pb On heating the metal flows down the sloping hearth(furnace) leaving impurities behind.

ZONE REFINING: Impurities are more soluble in the molten form of the metal than in its solid form. The metal is heated by a circular heater moving from one end to another repeatedly.The impurities collect on one end and are cut off from pure metal. This method is useful for getting metals of high purity eg. Ge , Si , B , Ga , In

VAPOUR PHASE REFINING: The metal is converted to a volatile compound by reacting with a suitable reagent whereas impurities are left behind. The volatile compound easily decomposes to give metal in pure state on heating.

MOND’S PROCESS: it is used to purify Ni

Ni + 4CO --- ---- Ni(CO)4 (volatile) (on heating at a temp. of 330 – 350 K)

Ni(CO)4 ---------- Ni + 4CO (on heating at a temp. of 450 – 470 K)

VAN ARKEL METHOD: it is used to purify Zr and Ti Zr + 2I2 -------- ZrI4 ZrI4 -------- Zr + 2I2 (on heating at a temp. of 1800 K)

ELECTROLYTIC REFINING: Impure metal is made the anode and pure metal strip is used to make cathode. The two electrodes are put in an electrolytic solution of metal salt. On passing current metal in pure form deposits at the cathode eg. Cu anode: Cu ----- Cu+2 + 2e- cathode: Cu+2 + 2e- ----- Cu Less electropositive metal impurities like Sb, Se, Te, Ag, Au, Pt present in impure copper collect as anode mud below the anode.

CHROMATOGRAPHY (column): Different components of a coloured mixture are adsorbed differently on an adsorbent surface(stationary phase). These coloured components adsorbed at different levels of adsorbent surface are removed using a suitable solvent(mobile phase).

ASSIGNMENT

1.Why is extraction of copper from copper pyrites more difficult than from its oxide ore by

reduction?

2.Is reduction of Cr2O3 possible by Al ?(∆fGo for Cr2O3 is -- 540 kJ/mol and for Al2O3 is --827 kJ/mol)

3.(i)The choice of a reducing agent in reduction of a metal oxide depends on thermodynamic factor.

Support your answer with an example.

(ii)How does electrochemical principle affect reduction of concentrated ore to the metal?

Explain with an example.

4.Predict conditions under which (i) Al can reduce MgO (ii) Mg can reduce Al2O3

5.What criterion is followed for the selection of the stationary phase in chromatography?

6.Give the principle ore (name and formula) from which following metals are extracted (i) Al (ii) Fe

(iii) Cu (iv) Zn

Give use of each of the above metals.

7.Reduction of alumina by Mg below 1350K is thermodynamically feasible but in practice

alumina is not reduced by magnesium. Why?

8.Explain how extraction of silver/gold by cyanide method is an oxidation –reduction process?

9.What is the significance of leaching in the extraction of aluminium from bauxite ore?

10.Which ores can be concentrated by magnetic separation method : bauxite,zinc blende,calamine,

copper pyrites, haematite,cuprite,magnetite,zincite or malachite?

Do all questions asked in exercise(page 163,164 of text book) and previous board papers

and find answers from the notes