Metal and Metallurgy

Difference Between metal and non-metal:

Metalloids:

Those elements which can show some properties of metals and some properties of nonmetals.

These are also called border line elements because they lies in between metal and non-metal in the periodic table. E.g. Arsenic (As), Sb, Ge, etc.

They can form acidic oxides like non-metals and can conduct heat and electricity like metals.

Metallurgy:

The science which deals with the study of extraction and purification of metal from ore is known as metallurgy.

Difference between Minerals and Ores

*Note: from Bauxite (Al₂O₃. 2H₂O) metal Aluminum can be extracted economically because it only contains Aluminum. So, it is an ore.

But clay (Al₂O₃. 2SiO₂. 2H₂O) also contains the Aluminium but there is less presence of Aluminium in it. So, we cannot extract Aluminium from it economically. So, it is a mineral.

Alloy

They are the homogeneous mixture of metals with metals or metals with non-metals. The homogeneous mixture of non-metal and non-metal is not called alloy. Basically, alloys are of two types and they are:

a) Ferrous Alloys.

b)Non- Ferrous Alloys.

a)Ferrous alloys:

Those alloys which contain Iron in it are called ferrous alloys.

E.g. Steel (Fe+ C), stainless steel (Fe+ Ni+ Cr+ C), etc.

b) Non- ferrous alloys:

Those alloys which do not contain Iron in it are known as non-ferrous alloys.

E.g. Brass (Cu+ Zn), Bronze (Cu+ Sn)

Importance of Alloys:

1. They are less rusting in nature in comparison to the pure metals. So, using alloys we can save the loss from rusting.
2. Alloys are stronger than metal i.e. by making alloys we can increase the mechanical strength of metals.
3. Alloys are useful for making colorful utensils and materials i.e. we can import color if we make the alloy.

Amalgam:

The homogeneous mixture of Mercury with metal is known as amalgam. Mercury is in liquid form so by dissolving metal on Mercury we can obtain an Amalgam.

E.g. Hg+ Na ,Na.Hg [Sodium amalgam]

Hg+ Cu, Cu.Hg [Copper amalgam]

Note: metals like Iron (Fe), Cobalt (Co) and Nickel doesn’t form amalgam because they do not dissolve in Mercury.

Uses of Amalgam:

• Copper amalgam is used for filling the cavity in teeth.
• Sodium amalgam (NaHg) is used as reducing agent instead of Sodium (Na).

EXISTENCE OF METAL

Depending upon the reactivity of metal they can either exist in combined state or in Free State. Those metals which are highly reactive in nature are located in top position of electrochemical series and non-reactive metals are in bottom of the electrochemical series. Metals like Na, K, Ca are most reactive and they combine with most reactive non-metals i.e. halides. Similarly metals like Fe, Mg, Al, etc. exits in oxides form. Similarly Cu, Zn, Pb, etc. exist in sulphide form and Au, Ag, Pt exist in Free State.

Metallurgy:

The branch of science that deals with the study of extraction and purification of metal is known as Metallurgy.

Gangue/ Matrix:

The unwanted earthy particles present in the ore are called Gangue.

Process of Metallurgy:

Metallurgical process involves the following processes:

a) Mining

The collection of Ore from the mines is mining. It is 1^st step of Metallurgy.

b) Crushing and Pulverization

The process of converting big lumps of ore into smaller pieces by using crush mill is known as crushing.

The process of changing crushed ore into a fine powder by using ball mill is known as pulverization.

CONCENTRATION OF ORE

Concentration of ore

This is the method of removing impurities present in powder ore in order to increase the extent of ore. Depending upon the nature of ore and impurities present on given ore concentration can be carried out by following ways:

a) Hand picking

If impurities present in given ore are distinct in color and with different particle size they can be removed by hand picking. This is the lengthy process.

b) Electromagnetic separation:

If one of impurities or ore be magnetic and another is non-magnetic then such types of ores can be concentrated by using electromagnetic separation. During this process powdered ore is transferred into the rotating belt fitted with rotator or wheel where one of the rotator is magnetic in nature. On passing the ore in rotating belt magnetic particles will be attracted by magnetic rotator collected near the rotator. Whereas non-magnetic particles will be collected away from rotator. During concentration of tin stone (SnO₂) iron impurities present can be separated by this method.

c) Gravity separation:

If the density of the impurities and ore is different then such types of impurities can be separated out by using gravity separation method. During the separation given ore is transferred into the vessel where stream water is passed from the bottom. Inside the vessel lighter impurities will be washed away with water whereas heavier ore particles get settled down at bottom. Generally oxide and hydroxide ore can be concentrated by this method. E.g. Bauxite ore of aluminum, Hematite or Magnetite of Iron etc.

d) Forth floatation Method:

This is the specific method for concentration of Sulphides ore like PbS (Galena), ZnS (Zinc blend), CuFeS₂ (copper pyrite), HgS (Cinnabar).

This method is based on the principle that solids have the different attraction towards liquids.

In this method, powdered ore is mixed with water along with little amount of oil like a pine oil or Eucalyptus oil in a vessel known as flotation. On passing a compressed air inside a vessel agitation takes place inside and vigorous bubbling occurs. Sulphide ore which get preferentially wetted with oil forms bubbles and collected at a surface as a scum or forth whereas impurities which get preferentially wetted with water get settled down at a bottom of vessel from a forth one can obtain a concentrated ore. In order to increase the stability of bubbles chemicals like Aniline or Cresol can be added.

e) Leaching

This is a chemical method of concentration of ore. In this method powdered ore is treated with suitable chemical (Acid, Base) so that metallic portion present in given ore get dissolved with chemical but impurities don’t get dissolve. By filtration impurities can be separated as a residue while a filtrate will contain the metallic portion. From filtrate concentrated ore can be recovered by suitable chemical technique like precipitation. Generally, ores of Aluminum, gold and silver can be concentrated by leaching.

• Leaching of Bauxite ore of Aluminum

Al₂SiO₃. 2H₂O + NaOH→ NaAlO₂+ H₂O

\( Impure Form \: \: \: \: \: \:\:\:\:\:\:Sodium Metal Aluminate\)

NaAlO₂+ H₂O Al (OH)₃ + NaOH

• CALCINATION & ROASTING

Calcination is a process of heating a concentrated ore in the absence of air below the melting point of ore. The purpose of calcination is to convert a concentrated ore into oxide form. Calcination is mainly carried out for oxides, hydroxides, carbonates and Nitrate ores. During calcination following changes takes place.

a) Moisture gets removed.

b) Volatile impurities gets removed.

c) Water of crystallization gets removed

Al₂O₃.XH₂O Al₂O₃+ 2H₂O

d)Decomposition of ores may also take place

ZnCO₃ZnO+ CO₂

[Calamine]

Roasting is the process of heating a concentrated ore in the presence of ore below the melting point. The purpose of roasting is to convert a concentrated ore into oxide form. It is mainly carried out for the oxidation of sulphide ores.

During roasting similar changes take place

a)Oxidation of ores takes place

ZnS+ O₂→ ZnO+ SO₂

PbS+ O₂ → PbO+ SO₂

HgS+ O₂ → HgO+ SO₂

_b) Mixture volatile impurities and organic matters get removed.

c) Elements like S, P, As etc. gets oxidized.

S+ O₂ → SO₂

4P+ 5O₅ →2P₂O₅

As+ O₂ → As₂O₃

Refractory Impurities

Those impurities which have the high melting point so that they cannot melt easily is known as refractory impurities. E.g. CaO, MgO, SeO, SiO₂, etc.

Flux

Flux are the substances added for the removal of refractory impurities.

Types of flux:

a) Acidic Flux

The flux used for the removal of basic impurities is known as acidic flux. E.g. SiO₂, Borax (Na₂BuO₇.10H₂O)

CaO+ SiO₂ → CaSiO₃ (Slag)

b) Basic flux

The flux used for the removal of acidic impurities is known as basic flux. E.g. CaO, MgO, FeO, etc.

SiO₂+ CaO → CaSiO₃

Differences between Calcination and Roasting:

Slag

The resulting compound obtained after treating the refractory impurity with suitable flux is called slag. The melting point of slag is low. So, it can be fused easily.

REDUCTION/ EXTRACTION OF METAL FROM METALLIC OXIDE

Metals can be extracted from metallic oxide removing oxygen present in the metallic oxide. Such process of removal of oxygen with the help of suitable reducing agent is known as reducing. Depending upon the nature of metal reduction can be carried out by chemical method or electrolytic method.

There are two types of reduction they are:

1. CHEMICAL METHOD

In this method, reduction is carried out by using suitable chemicals like Carbon, Aluminum, Hydrogen, water gas (CO+ H₂) etc. depending upon the reactivity of metal, chemical method of reduction can be carried out in the following ways:

a) Reduction with Carbon:

Metals which lies in central or lower position in electrochemical series can be extracted by using carbon as a reducing agent. In this method calcined or roasted ore is mixed with Carbon (as a source of Carbon we can use coke, coal or charcoal) along with suitable flux. On providing a sufficient heat, reduction proceeds and metal is obtained in a molten state. So, this method is also known as smelting. Flux combines with impurities resulting a slag which also exist in molten state. Taking an advantage of immiscibility of slag with metal. Slag can be removed easily and almost pure metal is obtained.

Fe₂O₃+ C → Fe+ CO

ZnO+ C→ Zn+ CO

SnO₂+ C→ Sn+ CO

In certain case, carbon monoxide gets formed in situ which helps for the reduction of remaining oxide.

Fe₂O₃+ CO→ Fe+ CO₂

Refractory impurities get removed with flux resulting a slag.

E.g. During extraction of Fe, an ore may contain CaO as a refractory impurity so acidic flux like SiO₂ is added for the removal of impurity.

CaO+ SiO₂→ CaSiO₃

Limitations:

i) Highly reactive metals like Na, K, Co cannot be extracted from this method.

ii) Metal can’t be extracted from metallic oxide in which oxygen has the strong affinity with metal than with Carbon.

b) Reduction with Aluminium

Metallic oxides in which Oxygen has strong affinity with metal than Carbon, such type of oxides can’t be reduced by Carbon. So, aluminum which is powerful reducing agent than carbon can be used as a reducing agent instead of Carbon. Generally metals like Manganese, Chromium and Nickel used to be extracted by this method. In this method, calcine or roasted ore is thoroughly mixed with Aluminum powder along with little amount of Barium Peroxide (BaO₂) and kept in a suitable crucible fitted with magnesium ribbon. Heat energy required to initiate reduction is supplied by burning of BaO₂ and magnesium ribbon. Once the reduction is started, heat is evolved inside a crucible because the reaction is exothermic in nature. In this process also metal is obtained in molten state.

E.g. 3MnO₂+4Al → 3Mn+ 2Al₂O₃+ Heat

Cr₂O₃+ 2Al → 2Cr+ Al₂O₃+ Heat

2) Electrolytic Reduction

Highly reactive metals like Na, K, and Ca etc. can’t be extracted from their oxide by chemical method of reduction because in order to reduce their oxide there is necessary of huge amount of heat and at very high temperature these metals forms an undesired products. Extraction of such metal can be done by carrying out an electrolysis of molten salt. During electrolytic reduction aqueous solution of salt can’t be used because moisture is easily captured by metals and ferrous hydroxide, after electrolysis of molten salt metal is collected at cathode.

E.g. Na can be extracted by electrolysis of molten NaCl. During this process, Na is collected from the cathode.

NaCl → Na⁺+ Cl ^-

At cathode: Na⁺+ e^- → Na

At anode: 2Cl ^- → Cl₂ (gas) + 2e^-

PURIFICATION OF METAL

Metals obtained by the chemical method of extraction is not in the pure state. Following impurities may be associated with metals.

i) Residual slag or flux

ii) Metals other than the desired one.

iii) Unreduced metallic ores.

iv) Elements like P, As, S , etc.

In order to remove such types of impurities, metals should be purified.

Depending upon a nature of impurities following methods can be applied for the purification.

a) DISTILLATION:

Metals with comparatively lower boiling point can be purified by carrying out distillation. Generally Zn, Cd (cadmium), Hg can be purified by this process. In this process metals with low boiling point distilled out first and impurities remains in retort.

b) LIQUATION:

Metals with low melting point can be purified by heating in a sloping furnace. After heating metal with low melting point, it melts earlier and drains down while impurities remains unchanged in furnace. E.g. Sn, Pb.

c) OXIDATION:

Impurities present in metal can be removed by heating an impure metal in the presence of air. During heating, stirring of molten metal is done so that impurities get oxidized.

d) POLLING:

In this process molten metal is stirred with pole of green wood. Hydro carbon present in green wood acts as reducing agent so that oxide impurities get reduced into respective metal. This method is specially used for purification of Copper.

CuO+ C→ Cu+ CO

e) ELECTROLYTIC REFINING:

This method is cheap and convenient metals like Cu, Ag, and Zn can be refined electrolytically. The impure metal is made anode and a thin strip of pure metal is made cathode in electrolytic cell. The cell is filled with solution of suitable salt of concerned metal or passing electricity, pure metal gets deposited at cathode as the anode undergoes dissolution. Insoluble impurities fall behind the anode in the form of anode mud and soluble impurities go to the solution.