+2 UNIT 9 PAGE- 2
Tendency to form complexes
In contrast to representative elements, the transition elements form a large number of co-ordination complexes. The transition metal ions bind to a number of anions or neutral molecules in these complexes. The common examples are : [Ni(NH3)6]2+, [Co(NH3)6]2+,[Fe(CN)6]3, [Fe(CN)6]4, [Cu(NH3)4]2+ etc.
The high tendency of transition metal ions to form complexes is due to :
i) Small size of the atoms and ions of transition metals.
ii) High nuclear charge .
iii) Availability of vacant d-orbitals of suitable energy to accept lone pairs of electrons donated by other groups (called ligands)
Formation of interstitial compounds
Interstitial compounds are those which are formed when small atoms like H, N, or C are trapped inside the crystal lattices of metals. They are usually non-stiochiometric and are neither typically ionic nor covalent. Many of the transition metals form interstetial compounds particularly with small non-metal atoms such as hydrogen, , boron , carbon and nitrogen. These small atoms enter into void sites between the packed atoms of the crystalline metal , e.g., TiC, Mn4N, Fe3H, TiH2, etc. The formulae quoted do not , correspond to any normal oxidation state of the metal and often non-stiochiometric material is obtained with such a composition as VH0.56 and TiH1.7 . Because of the nature of their composition, these compounds are refered to as interstitial compounds. The principal physical and chemical characteristics of these compounds are as follows :
(i) They have high melting points, higher than that of pure metals.
(ii) They are very hard, some borides approach diamond in hardness.
(iii) They retain metallic conductivity.
(iv) They are chemically inert.
Catalytic Property
The transition metals and their compounds are known for their catalytic activity. The activity ascribed to their ability to adopt multiple oxidation states and to form complexes. Vanadium (V) oxide (Contact process) , finely divided iron (Haber process) are some examples. Catalysts at a solid surface invove the formation of bonds between reactant molecules and the atoms of the surface of catalyst (first row transition metals utilise 3d and 4s electrons for bonding) ; this has the effect of increasing the concentration of the reactants at the catalyst surface and also weakening of the bonds in the reactant molecules (activation energy is lowered). Also because the transition metal ions can interchange their oxidation states , they become more effective as catalysts. For example, iron(IIII) catalysts the reaction between iodide and persulphate ions,
2 I + S2O82 I2 + 2 SO42
An explanation for this catalytic action can be given as :
2 Fe3+ + 2 I 2 Fe2+ + I2
2 Fe2+ + S2O82 2 Fe3+ + 2 SO42
Alloy formation
An alloy is a blend of metals prepared by mixing the components. Transition metals form a large number of alloys. The transition metals are quite similar in size and therefore , the atoms of one metal can substitute the atoms of other metal in its crystal lattice giving solid solutions. Thus transition metals are miscible with each other in the molten state and on cooling a solution of different transition metals, alloys are obtained. Because of similar radii and other characteristics of transition metals, alloys are readily formed . They are hard and have often high melting points. The best known are ferrous alloys ; chromium, vanadium , tungsten , molybdneum and manganese are used for production of variety of steels and stainless steel. Alloys of transition metals with non-transition metals as brass(copper-zinc), and bronze(copper-zinc) are also of considerable industrial importance.
Problem
04. For first row transition metals the E values are :
E V Cr Mn Fe Ni Co Cu
M2+/M 1.18 0.91 1.18 0.44 0.28 0.28 +0.34
Explain the irregularity in the above values.
05. Explain why the E value for the Mn3+/Mn2+ couple is much more positive than that for Cr3+/Cr2+ or Fe3+/Fe2+ .
COMPARISON OF THE FIRST ROW TRANSITION METALS THROUGH THE d-ELECTRON CONFIGURATION
The d0 configuration
Of the simple ions, only Sc3+is known to have this configuration. This configuration then occurs for those metals in which the formal oxidation states equal the total number of 3d and 4s electron. This is true for Ti(IV) , V(V), Cr(VI) and Mn(VII), but for Fe(VIII) is unknown.
The d1 configuration
Except for vanadium(IV) , all others with this configuration are either reducing or undergo disproportionation. For example, disproportionation occurs for Cr(V) and Mn(VI) as follows:
3 CrO43 + 8 H+ 2 CrO42 + Cr3+ + 4 H2O
3 MnO42 + 4 H+ 2 MnO4 + MnO2 + 2 H2O
The d2configuration
This configuration ranges from TiII which is very strongly reducing, to FeVI, which is very strongly oxidising. Vanadium(III) is also reducing.
The d3configuration
Chromium (III) is an important species with this configuration which is stable and well known for complex formation. In other cases, this configuration is relatively unimportant.
The d4configuration
There are really no stable species with this configuration. The chromium (II) is strongly reducing and manganese(III) disproportionates.
The d5configuration
The two Important species with this configuration are Mn2+ and Fe3+ , the latter may however, be reduced to Fe2+.
The d6configuration
Iron(II) and cobalt(III) are important species with this configuration. Iron(II) is quite stable although a mild reducing agent and cobalt(III) is stable in presence of strong complexing reagents.
The d7configuration
The species with this configuration is cobalt(II) which is stable in aqueous solutions but gets oxidised to form cobalt(III) complexes in the presence of strong ligands.
The d8configuration
Nickel(II) is the most important species with this configuration.
The d9configuration
This configuration is found in Cu2+ compounds and is the most important in the chemistry of copper. Otherwise this configuration is is of little importance.
The d10configuration
The two species Cu+ and Zn2+ are important with this configuration. Whereas, copper(I) is easily oxidised to copper(II) , zinc(II) is the only state known for zinc.
GENERAL GROUP TRENDS IN THE CHEMISTRY OF THE d-BLOCK METALS
GROUP 4
The Group 4 consists of elements , Titanium (22Ti), zirconium(40Zr) and hafnium(72Hf). The most stable oxidation state is +4. The +3 oxidation state is known only for Titanium. Zirconium and hafnium are silvery white metals. The typical compounds are chlorides : TiCl4, ZrCl4 and HfCl4 and oxides : TiO2, ZrO2 and HfO2.
The oxide of zirconium is refractory material. Zirconium and hafnium occur together in minerals and they exhibit similar properties.
GROUP 5
Group 5 consists of vanadium(V) , niobium(Nb) and Tantalum(Ta). The most stable oxidation state is +5. Vanadium has a wide range of oxidation states such as +5, +4, +3, and +2 in its compounds. The chemical properties of Nb and Ta are similar because of intervening lanthanide elements. Niobium alloys are used in jet engines. Tantalum is used in surgical vessels and analytical weights because of its resistance to corrosion.
GROUP 6
This group of transition elements consists of chromium(Cr), molybdenum(Mo) and Tungsten(W). The maximum oxidation state of the elements of this group is +6. The other oxidation state is +3. Chromium is used in making alloys such as chrome steel ( 2 - 4% Cr), stainless steel(18% Cr), nichrome(Ni = 60%, Fe = 25% and Cr = 15%). It is also used in plating of metals called chrome plating.
The compounds of chromium are very useful. For example, potassium dichromate is used as volumetric reagent in the laboratory ; sodium dichromate is used in tanning leather; lead chromate as yellow pigment ; ammonium dichromate is used in fire works.
Molybdenum and tungsten are also useful although they occur rarely. These are used as catalysts.
GROUP 7
The manganese group consists of three elements namely, manganese(Mn), technetium(Tc) and rhenium(Re).
Of these elements, only manganese is important. The maximum oxidation state of manganese is +7. It is used in the manufacture of manganese steel (10-18% Mn) which is used in making rail lines, rock crushers, steel helmets etc. It is also used in the preparation of manganese bronze(alloy of Mn with Cu and Zn) which is used for making parts of ships.
GROUP 8 , 9 and 10
The elements of Groups 8, 9 and 10 constitute three triads. They are known as Iron Group metals. These elements are :
Group 8 9 10
Iron(Fe) Cobalt (Co) Nickel (Ni)
Ruthenium (Ru) Rhodium(Rh) Palladium (Pd)
Osmium (Os) Iridium (Ir) Platinum (Pt)
The first triad comprising of iron, cobalt and nickel is known as ferrous metals. Iron and cobalt exhibit oxidation states of +3 and +2 in their compounds, while nickel compounds are generally in the +2 oxidation states. After aluminium iron is the most abundant of all the metals. The important ores of iron are haematite(Fe2O3), magnetite(Fe3O4) and iron pyrites (FeS2).
The elements of second triad and third triad , ruthenium, rhodium Rh, palladium Pd, osmium Os, iridium Ir and platinum,Pt are collectively known as platinum metals. These elements are relatively less abundant and exhibit wider range of oxidation states. They are inert and serve as good catalysts.
GROUP 11
The copper group includes elements copper, silver and gold. These metals are known as coinage metals. They form alloys with many metals. The most stable oxidation state for copper is +2. For silver and gold the oxidation state of + 1 is relatively more stable. The metals of this group have highest electrical and thermal conductivities. Among the metals of this group copper is the most abundant. Metals of this group occur in combined as well as free state.
GROUP 12
The group 12 consists of metals zinc (Zn), cadmium(Cd) and mercury (Hg). This group has characteristic oxidation state of +2, except for mercury which also forms +1 compounds. Mercurous ion ,Hg(I) is unique in the sense that it consists of two atoms of mercury linked by a covalent bond. Mercurous ion is therefore written as Hg22+. For example, mercurous chloride is written as Hg2Cl2 [ ClHgHgCl ]. Hg+ does not exist.
Problem
06. What is meant by disproportionation of an oxidation state ? Give one example.
OCCURRENCE AND PRINCIPLE OF EXTRACTION OF SOME d-BLOCK ELEMENTS
Iron
Iron is the second most abundant metal occuring in earth’s crust. It is a reactive metal and does not occur in free state. In combined state , it occurs as oxides, carbonates and sulphides. The common ores of iron are :
(i) Haematite : Fe2O3 (red oxide of iron)
(ii) Magnetite : Fe3O4 (magnetic oxide of iron)
(iii) Limonite : Fe2O3 . 3 H2O (hydrated oxide of iron)
(iv) Iron pyrites : FeS2
(v) Siderite : FeCO3
Commercial variety of iron
There are three varieties of iron :
1. Cast iron or pig iron
It contains 2 to 4.5% of carbon , along with impurities such as sulphur, silicon, phosphorus, manganese etc. It is the least pure form of iron. It is brittle and cannot be welded.
2. Wrought iron
It is the purest form of iron and contains carbon and other impurities not more than 0.5% . It is malleable and and can be welded.
3. Steel
It contains 0.5 to 1.5% carbon along with small amounts of other elements such as manganese, chromium, nickel, etc. and other impurities. It comes in between cast iron and wrought iron and exhibits intermediate properties.
Extraction of iron
The cast iron is usually extracted from its oxide ore (haematite). This process involves the following steps:
1. Concentration
The ore is crushed in jaw crushers and is broken to small pieces of about one inch in size. The crushed ore is concentrated by gravity separation process in which it is washed with water to remove clay, sand, etc.
2. Calcination
The concentrated ore is then calcined (heated strongly in presence of limited supply of air ) in a reverberatory furnance . During this process, the following changes take place:
(i) Moisture is removed.
(ii) The impurities such as sulphur, phosphorus and arsenic are converted to their gaseous oxides which is volatile and escape.
S + O2 SO2
4 As + 3 O2 2 As2O3
P4 + 5 O2 P4O10
If some ferrous carbonate is present , it changes to oxide.
FeCO3 FeO + CO2
4FeO + O2 2 Fe2O3
Ferric oxide
This prevents the loss of iron due to the formation of ferrous silicate (slag) during smelting.
(iii) The entire mass becomes porous which helps in the reduction process at a later stage.
3. Smelting
The calcined ore is reduced with carbon, i.e is smelted in the blast furnace (Fig)
Blast furnace for manufacture of cast iron
The calcined ore (8 parts) is mixed with coke(4 parts) and lime (1 part) and is introduced from the top through the cup and cone arrangement. At the same time , a blast of hot air is blown upwards with the help of tuyers arrangement. The added coke serves as a fuel as well as a reducing agent while added lime seves as a flux. The following reactions take place in the furnace :
(i) Combustion zone
At the base, coke burns to produce carbon dioxide which starts rising upward during the reaction. The reaction is exothermic and heat produced raises the temperature to about 1775 K. This region is called combustion zone.
C + O2 CO2 : H = 393.4 kJ
(ii) Fusion zone
As carbon dioxide rises upward, it comes in contact with layers of coke and gets reduced to carbon monoxide.
C + CO2 2 CO : H = + 163.2 kJ
This is an endothermic reaction and therefore the temperature is lowered to 1475 – 1575 K. The iron produced in the upper region melts here. Any Fe2O3 if present undergoes reduction by hot coke to iron. This region is called fusion zone.
Fe2O3 + 3 C 2 Fe + 3 CO + heat
(iii) Slag formation zone
In the middle portion of the furnace, the temperature is about 1075 to 1275 K. In this region lime stone decomposes to produce lime(CaO) and carbondioxide(CO2). The lime thus produced acts as a flux and combines with silica (present as impurity) to produce slag.
CaCO3 CaO + CO2
(lime stone)
CaO + SiO2 CaSiO3
Flux (lime)
The molten slag forms a separate layer (being lighter) above the molten iron. This region is called slag formation zone.
(iv) Reduction zone
The temperature near the top of the furnace is of the order of 875 K. The oxides of iron are reduced by carbon monoxide to iron.
Fe2O3 + 2 CO 2 FeO + CO2
FeO + CO Fe + CO2
The region of the furnace is called reduction zone.
The spongy iron produced in the reduction zone moves slowly and melts in the fusion zone. It dissolves some carbon , silicon and phosphorus and forms the lower layer at the base of the furnace. It is removed from the tapping hole from time to time. The iron thus obtained is called cast iron or pig iron.
Preparation of wrought iron
Wrought iron is the pure form of iron and contains less than 0.5% impurities. The cast iron obtained above contains about 2.5 – 5% carbon and other impurities such as S, P, Si and Mn. In order to convert cast iron into wrought iron, the percentage of carbon and that of other impurities has to be decreased. This is done by heating the cast iron on the hearth of a reverberatory furnace (known as puddling furnace) with haematite (Fe2O3). The haematite supplies the oxygen and oxidises carbon, silicon, manganese and phosphorus present in cast iron to carbon monoxide (CO) , silica (SiO2), manganese oxide(MnO) and phosphorus pentoxide (P2O5) respectively.
Thus,
Whereas CO and SO2 escape, MnO and silica (SiO2) combine to form manganous silicate(MnSiO3) as slag.
Similarly, phosphorus pentoxide combines with Fe2O3 to form ferric phosphate slag.
2 Fe2O3 + P4O10 4 FePO4
ferric phosphate (slag)
Wrought iron thus prepared contains about 0.2% of carbon and some traces of P and Si in the form of slag.
Steel
Steel may be broadly classified as mild steel (0.1 – 0.5% C ) and hard steel (0.6 – 1.5% C). Nowadays , bulk of pig iron is converted into steel. The mild steel is cheaper than the wrought iron and stronger and more workable than cast iron ; it has also the advantage over both in that it can be hardened by heating to redness and then cooled rapidly (quenching ) in water and ‘tempered ‘ by reheating to 473 K to 573 K and cooling more slowly. The hardness , resilience and ductility can be controlled by varying the temperature and rate of cooling as well as the precise composition of the steel.
Alloy steel
Alloy steel with their enormous variety of physical properties are prepared by the addition of the appropriate alloying metal or metals( e.g., Mn, Cr, Ni , W) . Thus, stainless steel contains 18% of chromium ; tungsten steel (which is very hard) about 5% of tungsten ; manganese steel (which is very tough) about 13% of manganese .
Steel Making Processes
(i) Bessemer Process
The molten pig iron is fed into a converter at a temperature of about 1473 K. A blast of oxygen diluted with either steam or carbondioxide is blown through the converter. Oxygen reacts with impurities and raises the temperature to 2173 K. Carbon is oxidised to carbon monoxide which burns off at the mouth of the converter; oxides of silicon and manganese form slag. After about ten minutes , the flame dies down indicating that all carbon has been removed. The flame is stopped, slag is tapped off and then other metals (Mn, Cr, Ni and W) may be added towards the end of the operation to produce the required type of steel.
(ii) Open-Hearth Process
In this process , a mixture of molten pig iron , scrap steel , iron ore and lime stone is heated on a shallow hearth furnace by producer gas. The furnace is adapted for different types of pig iron feed by using acidic or basic lining. The impurities are oxidised by the iron oxide present which form a slag by combining with the lining. Thus,
3 C + F2O3 2 Fe + 3 CO
Oxides of P and Si + lining (CaO + MgO)
Phosphate and silicate slag
Towards the end (after about 10 hours) an alloy of Mn, Fe and C (speiegelesin) is added together with alloying metals.
The Open-Hearth process has advantages over Bessemer’s process largely because of greater ease with which the composition of the steel can be controlled and greater fuel economy.
(iii) The Oxygen Top-Blowing Process
In this process , liquid iron from the blast furnace is charged into a converter, scrap steel is added and jet of oxygen is blown through retractable steel ‘lance’ into or over the surface of the liquid metal. The impurities are oxidised and with the addition of lime form slag, which is usually removed by tilting the converter. When steel of desired composition is obtained, the oxygen is turned off and the molten steel is poured into laddler for casting into ingots.
The oxygen top-blowing process
(iv) The Electric Arc Process
A charge of scrap steel and turnings is fed into the furnace and is melted by electric arc struck between adjustable carbon electrodes. Again acidic or basic linings are employed for scrap differeing in phosphorus content. This method is widely used in the manufacture of alloys and other high quality steels such as stainless steel and high-speed cutting steel.
Electric arc process
(v) The High-Frequency Induction Process
A charge of alloy scrap of known composition, together with iron is fed into the furnace. Alternating current at 500-2000 Hz passes through the insulated water-cooled copper coils. The resulting magnetic field sets up steady current, which generates heat. The circulation of the metal caused by these currents produces strong stirring effect. The induction furnace is capable of producing high quality alloy steels containing tugsten , vanadium , chromium , manganese, molybdenum, cobalt and nickel for making ball-bearing, magnets, dies and tool steel etc.
High –frequency induction process
Properties of steel
The properties of steel and its hardness depends upon its its carbon contents and heat treatment.
(a) Properties based on carbon conents
Based on carbon content, there are three types of steel :
(i) Mild steel : It has the least percentage of carbon (0.1 – 0.5%). It is used for the manufacture of wires and sheets.
(ii) Medium steel : It contains 0.2 to 0.5% carbon and is harder than mild steel. It is used for constructing rails, wheels and also used in buildings.
(iii) Hard steel : The carbon contents vary from 0.6 to 1.5% . It is very hard and it is used in making parts of machines.
(b) Properties based on Heat Treatment
The hardness of steel can be increased or decreased by heat treatment as described below :
(i) Quenching : If a steel article is heated to redness and then suddenly cooled by plunching into water or some oil, the steel becomes hard and brittle. This treatment is called quenching or hardening of steel.
(ii) Tempering : When the quenched or hardened steel is heated to 500 – 575 K temperature and kept at that temperature for some time and then cooled slowly, the steel obtained becomes slightly less hard and tough. This process is called tempering of steel and steel thus obtained is called tempered steel. The properties of tempered steel depend on the temperature and time of tempering.
(iii) Annealing : If quenched steel is heated to temperature below red hot and then allowed to cool slowly, it becomes soft. This process is called annealing.
Passive Iron
When a piece of iron is dipped in concentrated nitric acid, a reaction takes place which stops after some time completely. The iron does not appear to undergo any change in appearance. But it becomes unreactive. It is due to the formation of a thin insoluble and invisible Fe3O4 film on the surface which prevents its further reactions.
COPPER
Occurrence
Copper does not occur abundantly in nature (about 1 x 104 % of the earth’s crust). The chief ores of copper are :
(i) Copper glance Cu2S
(ii) Copper pyrites CuFeS2
(iii) Malachite Cu(OH)2 CuCO3
(iv) Cuprite Cu2O
(v) Azurite 2 CuCO3 Cu(OH)2
Extraction of copper
Copper is mainly extracted from copper pyrites (CuFeS2). The various steps involved in the extraction are :
1. Crushing and concentration : The ore is crushed in jaw crushers and finely powdered. It is concentrated by froth floatation process. In this process , finely divided ore is mixed with water and some pine oil in the tank. The mixture is agitated by blowing compressed air into it. The particles are preferentially wetted by oil and rise to the surface of tank in the form of froth (a foam) from where these are skimmed off. The silicious and earthy impurities are preferentially wetted by water and sink to the bottom of the tank.
2. Roasting : The concentrated ore is roasted i.e., heated strongly in the presence of excess air in a reverberatory furnace. During roasting the following changes occur.
(i) Moisture is removed from the ore and it becomes dry.
(ii) The impurities of sulphur , arsenic, antimony and phosphorus are removed as their volatile oxides.
S + O2 SO2
P4 + 5 O2 P4O10
4 As + 3 O2 2 As2O3
4 Sb + 3 O2 2 Sb2O3
(iii) Copper pyrites is converted to ferrous sulphide (FeS) , cuprous sulphide (Cu2S) which are partially oxidised.
2 CuFeS2 + O2 Cu2S + 2 FeS + SO2
2 FeS + 3 O2 2 FeO + 2 SO2
2 Cu2S + 3 O2 2 Cu2O + 2 SO2
3. Smelting : The roasted ore is mixed with some powdered coke and sand and is heated strongly in a blast furnace. The blast furnace is made up of steel and is lined inside with fire bricks . A blast of hot air is introduced at the lower part of the furnace. The following changes occur during smelting.
(i) Most of the ferrous sulphide gets oxidised to ferrous oxide which combines with silica (flux) to form fusible slag.
2 FeS + 3 O2 2 FeO + 2 SO2
FeO + SiO2 FeSiO3
Flux ferrous silicate (slag)
The slag being lighter floats and forms the upper layer. It is removed through the slag hole from time to time.
(ii) During roasting or in the blast furnace if the oxide of copper is formed, it combines with FeS and is changed back into its sulphide.
2 Cu2S + 3 O2 2 Cu2O + SO2
Cu2O + FeS Cu2S + FeO
Ferrous oxide thus formed again combines with silica to form more slag.
FeO + SiO2 FeSiO3
flux Slag
As a result of smelting, two separate layers are formed at the bottom of the furnace. The upper layer consists of slag and is removed. The lower layer of the molten mass contains mostly cuprous sulphide and some traces of ferrous sulphide. It is called matte and is taken out from the tapping hole at the bottom.
(iv) Bessemerisation : The molten matte from the blast furnace is transferred into a Bessemer converter (Fig).
Bessemer converter
The vessel is made of steel and is lined inside with lime or magnesium oxide. A blast of air mixed with sand , is blown into the moten matte. During this process :
(i) Traces of ferrous sulphide present in the matte is oxidised to FeO which combines with silica to form slag.
2 FeS + 3 O2 2 FeO + 2 SO2
FeO + SiO2 FeSiO3 (slag)
(ii) Copper sulphide is partially oxidised to cuprous oxide which further reacts with remaining copper sulphide to form copper and sulphur dioxide.
2 Cu2S + 3 O2 Cu2O + 2 SO2
Cu2S + 2 Cu2O 6 Cu + SO2
After the reaction has completed, the converter is tilted and the molten copper is poured into sand moulds. The copper thus, obtained is about 99% pure and is known as blister copper. The name blister comes from the fact that as the metal solidifies, the dissolved sulphur dioxide escapes producing blisters on metal surface.
(iii) Refining : The blister copper is purified as follows :
(a) Poling : The blister copper is purified by heating it strongly in a reverberatory furnace in presence of excess of air. The impurities are either removed as volatile oxides or converted into slag.
Some of the copper also changes to cuprous oxide. This is reduced back to copper by stirring the molten metal with green poles of wood. The hydrocarbons present in these freshly cut poles reduce cuprous oxide to copper which is about 99.5% pure. Further purification is done by electrolytic refining.
(b) Electrolytic refining : The crude copper is further purified by electrolytic method. In this method, a thin sheet of metal is made as cathode and block of crude metal is made as anode. Both the electrodes are placed in an acidified copper sulphate solution. When electric current is passed through the solution, impure copper from anode goes into the solution and pure copper from the solution gets deposited on the cathode.
At anode :
Cu 2 e Cu2+
At cathode
Cu2+ + 2 e Cu
The impurities of zinc, nickel, iron etc. get collected below the anode as anode mud.
Alternate method
Prelonged exposure of copper pyrites to air and rain leads to formation of a dilute solution of copper sulphate, from which the metal is precipitated by addition of scrap iron. It is always refined electrolytically.
SILVER
Occurrence
Silver occurs in the combined as well as in free state. The important ores of silver are :
(i) Argentite or silver glance , Ag2S
(ii) Horn silver, AgCl
(iii) Pyrargyrite or Ruby silver Ag3 SbS3
(iv) Proustite Ag3 AsS3
The silver content in these ores are very small(about 1%). Silver is also worked up from the residues remaining after the isolation of copper, lead and gold.
Extraction
Silver metal is extracted from argentite ore (Ag2S) by cyanide process. The various steps involved in the process are :
1. Crushing and concentration of the ore : The ore is crushed and powdered. The powdered ore is concentrated by froth floatation process. In this process the finely powdered ore is mixed with water and pine oil in a tank. The contents are agitated vigorously by blowing air. As a result, the particles are preferentially wetted by oil and rise to the surface of the tank in the form of froth (or foam) from where these are skimmed off. The impurities are preferentially wetted by water and thus sink to the bottom of the tank.
2. Treatment of the ore with sodium cyanide : The concentrated ore is treated with a dilute solution ( about 0.5%) of sodium cyanide solution for several hours. The solution is continuously agitated by passing a current of air. Silver sulphide goes into solution in the form of soluble complex, sodiumdicyanoargentate(I).
Ag2S + 4 NaCN 2 Na[Ag(CN)2] + Na2S
Sodium sulphide formed is oxidised to sodium sulphate by the air blown into the solution. This helps the reaction to occur in the forward direction.
Na2S + 5 O2 + 2 H2O 2 Na2SO4 + 4 NaOH + 2 S
3. Precipitation of silver : The above solution is filtered to remove insoluble impurities. It is then treated with zinc dust. Silver being less electropositive , gets displaced by more electropositive zinc and is precipitated.
2 Na[Ag(CN)2] + Zn Na2[Zn(CN)4] + 2 Ag (ppt)
4. Fusion : The precipitated silver is separated and purified by fusion with borax or potassium nitrate to get pure silver.
5. Electrolytic refining : In this process crude silver is made anode in the electrolytic tank containing silver nitrate solution acidified with about 1% nitric acid. A thin sheet of pure silver is made cathode. On passing current, pure silver gets deposited at the cathode. Zinc and copper, if present , go into solution while gold if present , falls down as anode mud. The reaction taking place in the cell may be represented as :
At cathode : Ag+ + e Ag
At anode : Ag Ag + + e