Chemistry of metals

Metal, any of a class of substances characterized by high electrical and thermal conductivity as well as by
malleability, ductility, and high reflectivity of light (lustrous).

Approximately three-quarters of all known chemical elements are metals. The most abundant varieties in
the Earth’s crust are aluminum, iron, calcium, sodium, potassium, and magnesium. The vast majority of
metals are found in ores (mineral-bearing substances), but a few such as copper, gold, platinum,
and silver frequently occur in the free state because they do not readily react with other elements.

Metallurgy, art and science of extracting metals from their ores and modifying the metals for use.
Metallurgy customarily refers to commercial as opposed to laboratory methods. It also concerns the
chemical, physical, and atomic properties and structures of metals and the principles whereby metals are
combined to form alloys.

Metallurgy is defined as a process that is used for the extraction of metals in their pure form from their
ores. The compounds of metals mixed with soil, limestone, sand, and rocks are known as minerals. Metals
are commercially extracted from minerals at low cost and minimum effort. These minerals are known as
ores.

The Earth is the main source of minerals and ores. Most of the elements do not found in Free State
because of their reactive tendencies. Potassium, sodium, calcium, magnesium, aluminium, zinc, iron,
lead etc. are the metals found in combined state. Minerals are the natural materials in which the metals
and their compounds are found in earth. Ores are those minerals from which metal are extracted
conveniently and profitably. These ores contain good percentage of metal.
Pyrometallurgy is suitable for the less reactive metals like iron, copper, zinc, chromium, tin, manganese in
presence of high temperature.

Electrometallurgy is carried over by passing electric current to the molten or aqueous solution of ore. For
example- Al, Na.

Hydrometallurgy is carried over by dissolving the ore in a suitable reagent and precipitating the metal.
For example- Au, Ag.

Some terms

Mineral- Minerals are the natural materials in which the metals and their compounds are found in
earth.

Ores- Ores are those minerals from which metal are extracted conveniently and profitably. These ores
contain good percentage of metal.

Remember that every ore is the mineral but every mineral is not the ore as extraction of metal from the
mineral will not be convenient and profitable.

A gangue or matrix in chemistry can be defined as an unwanted material or impurities in the form of
sand, rock or any other material that surrounds the mineral in an ore deposit.

Flux- The substance when mixed with the ore and heated, combines with earthy impurities to form easily
fusible mass (slag) is called flux. Flux may be acidic (SiO2, Na2B4O7) or basic (MgO, CaO, CaCO3).
Acidic flux is added if the impurity is basic and basic flux is added if the impurity is acidic. The slag is
easily fusible glassy material formed by the reaction between flux and impurities.
∆
Flux + gangue → slag
∆
Acidic flux + basic impurity/gangue → slag
∆
SiO2 + CaO → CaSiO3
∆
Basic flux + acidic impurity → slag
∆
FeO + SiO2→ FeSiO3

Alloy- Homogeneous mixture of metal with other metals or sometimes with non-metals is called alloy.
An alloy exhibits metallic property but may be hard and brittle as well in some cases. Metals are liquified
by heating and mixed together to make it homogeneous. Some examples: brass (Cu + Zn), bronze (Cu +
Sn)/ bell metal (bronze- Cu 80% + Sn 20%), stainless steel (Fe, C, Cr, Ni), steel (Fe, C), German silver
(Cu, Zn, Ni), aluminium bronze (Cu, Al) etc.

Alloys are made to increase the hardness and strength, to improve colour, to lower the melting and boiling
points, to make the metals corrosion resistant etc.
Metallic mineral (in different parts of Nepal)
Mineral Location

Lalitpur (Phulchoki), Thoshe (Ramechhap), Labdhikhola (Tanahu), Jirbang (Chitwan),

Iron
and Dhuwakot (Parbat)

Copper Gyazi (Gorkha), Okharbot (Myagdi) and Wapsa (Solukhumbu)

Ganesh Himal area (Rasuwa), Phakuwa (Sankhuwashbha), Libangkhairang, Damar, and

Baraghare (Makawanpur), Pangum (Solukhumbu), Salimar valley
Zinc
(Mugu/Humla),Phulchoki (Lalitpur), Sishakhani and Khandebas (Baglung), Duwakot
(Parbat) Bhalu Danda (Dhading), Kholakhani (Taplejung)

Cobalt Netadarling andTamghas (Gulmi) and Samarbhamar (Arghakhanchi)

Bamangaun (Dadeldhura), Beringkhola (Ilam), Bauligad (Bajhang) Khoprekhani

Nickel
(Sindhuli)

Mahakali, Chamaliya, Jamari gad, Seti, Karnali, Bheri, Rapti, Lungrikhola and
Gold Phagumkhola (Rolpa), Kaligandaki, Myagdikhola, Modi, Madi, Marsyandi, Trishuli,
Bhudhi Gandaki, and Sunkoshi

Silver Ganesh Himal (Rasuwa), Barghare (Makawanpur),Bering Khola (Ilam)

Tin Meddi and Genera (Dadeldhura) and Mandhukhola area (Makawanpur)

Thumki, Jagat, Panchmane, Gagalphedi, and Chunikhel in Shivapur area, Tinbhaangale,

Uranium and
Chandi. Khola and Chiruwakhola (Makawanpur), Bukakhola (Sindhuli) Mardarkhola and
other radio active
Panpakhola (Chitwan), Jamari gad, Bangabagar, Baggoth, Gorang (Baitadi), and Traces
minerals
in different section of Chamaliya River (Darchula)

Bismuth Dadeldhura and Baraghare and Mandukhola area in Makawanpur district

Cinabar Khimti River

 Mineral Location

Khaptad and different parts of Manang, Kathmandu, Nuwakot, Rasuwa, Phakuwa

Berllium
Hyakule, Ilam and Taplejung district

Tungsten Dadeldhura and Makwanpur

Amalgams are the alloys of metals with mercury. Except Fe, Co, Ni, most of the metals form amalgams
with mercury. Sodium amalgam (NaHg) is used as source of nascent hydrogen in organic reactions. CuHg
is used in filling the teeth.

(Consult Modern chemistry for the ores and alloys pages 413 and 416 respectively.)

Metallurgy consists of three general steps: (1) mining the ore, (2) separating and concentrating the metal
or the metal-containing compound, and (3) reducing the ore to the metal. Additional processes are
sometimes required to improve the mechanical properties of the metal or increase its purity.

Steps involved in metallurgy-

After collection of ore from the mine, the steps performed are discussed below.

Crushing and Pulverization

Big lumps of ore are broken into pieces by jaw-crushers and powdered (pulverized) in a ball mill or a
stamp mill. First figure is jaw crusher and second one is stamp mill.

Concentration of ore

For the concentration of ore, hand picking, lavigation or froth floatation, magnetic separation,leaching
process are applied which depends on the type of ore.

Hand picking is applied for the separation of distinct type of impurities from the main deposit of the ore.

Gravity separation method is applied for the separation of heavier ore particles from light gangue
particles. The ore is continuously washed with a fine spray of water on vibrating slanting table fixed with
riffles (Wilfley tables). The lighter particles are wased away and heavier are retained in the riffles on the
table.

Hydrolic classifier is sometimes employed for this purpose. (for bauxite Al2O3.2H2O,haematite Fe2O3)
 Hydrolic classifier

Froth floatation process is applied for the concentration of the sulphide ore (copper pyrite- CuFeS2, zinc
blende- ZnS, cinnabar- HgS, galena- PbS etc). The powdered ore is taken in a big tank and mixed with
water alongwith pine oil, preferrentially wetted by oil than water and a film is formed on vigorous stirring
with blowing air and the oil film of ore comes to the surface forming froth which is skimmed off and the
earthy materials settle down.

Magnetic separation is applied if one of the ore or gangue is magnetic. The powdered ore is dropped
over a conveyor belt which moves over two rollers. The magnetic one (ore or impurity) sticks to the
magnetic roller and falls near whereas the non-magnetic one falls a bit far making two separate heaps and
get separated. SnO2 from wolframite (FeWO4) is separated by this method.

In Leaching process, the powered ore is treated with suitable solvents like acid or alkali or some other
suitable reagent. The dissolved ore is filtered and the filtrate contains the ore. For example, bauxite is
dissolved in hot NaOH, gangue remains unaffected.
 Al2O3 + NaOH → 2NaAlO2 (water soluble) + H2O

NaAlO2 + 2H2O → Al(OH)3 + NaOH

∆
2Al(OH)3→ Al2O3 + 3H2O

Similarly argentite (Ag2S) or horn silver (AgCl) is concentrated by leaching on employing cyanide
process. Gold also can be concentrated by this process.
𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑜𝑓 𝑎𝑖𝑟
Ag2S + 4NaCN (aq)→ 2Na[Ag(CN)2] + Na2S

2Na[Ag(CN)2] + Zn → Na2[Zn(CN)4] + 2Ag

AgCl + 2NaCN → Na[Ag(CN)2] + NaCl

2Na[Ag(CN)2] + Zn → Na2[Zn(CN)4] + 2Ag

Calcination and Roasting

The concentrated ore is converted to the metallic oxide by either calcination or rasting.

In Calcination, the concentrated ore is strongly heated in absence of air or limited supply of air to a
temperarure below melting point with the purpose of driving away the volatile impurities. Absorbed
moisture, water of crystallization, CO2 and volatile impurities are removed.
∆
Al2O3.2H2O → Al2O3 + 2H2O (g)
∆
Fe2O3.2H2O → Fe2O3 + 2H2O (g)
∆
ZnCO3 → ZnO + CO2 (g)
∆
CaCO3.MgCO3 → CaO + MgO + 2CO2 (g)

In roasting (usually for sulphide), the ore is strongly heated to a temperature below melting point in the
presence of sufficient supply of air with a view to oxidise the ore into oxide.

By roasting

- Moisture, other volatile impurities are washed away.

- Impurities like S, P, As are oxidized and removed as their oxide.
∆ ∆ ∆
S + O2 → SO2, 4P + 5O2 → 2P2O5, 4As + 3O2 → 2As2O3
- Sulphide ore are oxidised to their oxide.
∆
2PbS + 3O2 → 2PbO + 2SO2 (g)
∆
2ZnS + 3O2 → 2ZnO + 2SO2 (g)
Calcination is concerrned with decomposition of ore to remove CO2, SO2, water and volatile impurities
while roasting is mainly concerned with oxidation of ore besides the removal of volatile impurities.
Roasting or calcination is done in a reverberatory furnace or in a blast furnace.

Extraction of Metals (Reduction)

Extraction of metal involves

- Removal of earthy impurities

- Reduction of the ore to the metal

By reduction, metals are obtained from its ore. During reduction, metal ions get electrons and convert to
metal.

M n+ + ne -→ M

C, H, Na, Al, Mg are good reducing agents which reduce the metallic ore to metal. If electricity is passed
through molten ore, this also reduces the ore to metal.

The method of reduction and nature of reducing agent to be used depends upon the activity of metals. Fe,
Zn, Sn, Pb, Cd which are at centre or lower parts of Electrochemicl series are reduced by carbon
reduction process and the process is called smelting.

The refratory impurities left after concentration are forced to react with another substance called flux to
form another fusible substance called slag. If refractory impurities are acidic, basic flux is used and if
basic, acidic flux is used to form the slag.
∆
Acidic flux + basic impurity → slag
∆
SiO2 + CaO → CaSiO3
∆
Basic flux + acidic impurity → slag
∆
FeO + SiO2→ FeSiO3
During smelting (heat and melt), the ore is mixed with flux and carbon in the form of coke. Flux removes
refractory impurities as slag. Coke burns and produces heat and also reduces the metal oxide to metal. CO
formed in situ reduces metal oxides more efficiently. Thus, during smelting of iron,
∆
Fe2O3 + 3C → 2Fe + 3CO
∆
Fe2O3 + 3CO → 2Fe + 3CO2

Carbon reduction process is one of the most important process for mining chief metals.

Aluminothermite process or Goldsmith process is carried for elements which are not reduced by
carbon reduction process. Metals like Na, Mg or Al is used as reducing agent. In these processes, oxide
ore of metal is intimately mixed with aluminium powder and the reduction reaction is initiated by burning
Mg to the mixture which starts reaction vigorously and the tremendous heat produced melts the metal. Cr
is extracted by this process.

Cr2O3 + 2Al → 2Cr + Al2O3 + heat

Metals like Na, K, Ca, Mg, Al (are at the top of ECS) are extracted using electrolytic reduction from
their molten salt and produced at cathode. This is called electrometallurgy.

NaCl → Na+ + Cl – (in molten state)

At cathode, Na+ + e - → Na and at anode, 2Cl - → Cl2 + 2e –

Hydrometallurgy is employed for metals like Ag or Au. The ore containing Ag or Au is extracted with
sodium cyanide and the resulting solution is treated with zinc which displaces silver.

2Na [Ag (CN)2](aq.) + Zn → Na2[Zn(CN)4] + 2Ag ↓

This is an example of metal displacement.

The reduction process is categorized into various types. Such as-

a) Smelting (carbon reduction method)

This method is used for the extraction of Pb, Zn, Fe, Cu and Sn.
∆ 𝑡𝑜 𝑣𝑒𝑟𝑦 ℎ𝑖𝑔ℎ 𝑡𝑒𝑚𝑝
The roasted oxide ore + carbon (charcoal, coal or coke) + flux→ carbon reduces
the metal oxide to metal.
∆
PbO + C → Pb + CO
∆
Fe2O3 + 3C → 2Fe + 3CO
∆
ZnO + C → Zn (vapour) + CO
∆
SnO2 +2C → Sn (molten) + 2CO
b) Hydrogen reduction (reducing by using hydrogen)
∆
WO3 + 3H2 → W + 3H2O
∆
MoO3 + 3H2 → Mo + 3H2O
c) Carbon monoxide reduction method- CO gas produced act as reducing agent
∆
Fe3O4 + 4CO → 3Fe + 4CO2
∆
ZnO + CO → Zn + CO2
d) Mg-reduction method-
∆
Rb2O3 + 3Mg → 2Rb + 3MgO
∆
TiCl4 + 2Mg → Ti + 2MgCl2
e) Al- reduction method (Aluminothermic process)- aluminium powder is used when carbon
reduction cannot be used. Applied for- TiO2, Cr2O3, Mn3O4.
∆
3TiO2 + 4Al → 2Al2O3 + 3Ti
∆
Cr2O3 + 2Al → 2Al2O3 + 2Cr
∆
3 Mn3O4 + 8Al → 4Al2O3 + 9Mn
f) Self-reduction - the compound (oxide) formed initiates the reduction with the impurity left on
heating in presence of air. For example-
∆
2HgS + 3O2 → 2HgO + 2SO2
∆
2HgO + HgS → 3Hg + SO2
∆
2Cu2S + 3O2 → 2Cu2O + 2SO2
∆
2Cu2O + Cu2S → 6Cu + SO2
∆
2PbS +3O2 → 2PbO + 2SO2
∆
2PbO + PbS → 3Pb + SO2
g) Reduction by more electropositive metals – For example –
∆
2Cu2S + 5O2 + H2O → 2CuSO4 (aq.) + 2CuO
∆
CuSO4 (aq.) + Fe → Cu + FeSO4 (aq.)
(look at the examples of leaching)
h) Electrolytic reduction method – (electrometallurgy)- reduction done by passing the electricity
NaCl ⇌ 𝑁𝑎+ + 𝐶𝑙 − (molten)
At cathode, 𝑁𝑎+ + 𝑒 − → 𝑁𝑎0
At anode, 𝐶𝑙 − → 12𝐶𝑙2 + 𝑒 −
i) Amalgamation method-
Metals form amalgam with mercury. Noble metals are allowed to react with mercury in slanting
position and is distilled to recover mercury whereas it leaves noble metal.
Refining of metals

The metals extracted by above reduction methods are not pure as expected and purification or refining is
done by various methods such as –

i) Liquation –
The low melting point having metals like Bi, Sn, Pb, Hg which have lower melting point than
gangue. The process is done on sloping hearth of reverberatory furnace.

ii) Distillation –

Hg, Zn, Cd can be separated by this method. Vapour on condensation collects and give pure metal.

iii) Poling –

Green log of wood is used to stir the molten impure metal which reduces the remaining unreacted oxide
and removes the impurity. Refining of copper is done by poling.

iv) Oxidation –

Lead from silver can be separated by this method. Lead oxide formed there on oxidation is driven away
by air current while unoxidized Ag is left behind.

v) Electrolytic method –

Pure sample is made cathode and impure reduced extracted metal is made anode using electrolyte of
having the purifying metal. Cu, Ag, Au, Ni, etc. are purified by this method. On passing current reduced
pure metal is collected at cathode.