Metals and Alloys

Metallurgy

Metallurgy is defined as a process that is used for the extraction of metals in their pure form.  It is a process of extraction of metals from their ores economically and profitably. Metals are commercially extracted from minerals at low cost and minimum effort. Metallurgy deals with the process of purification of metals and the formation of alloys.

Minerals

Minerals are substances naturally formed in the earth. A mineral is a naturally occurring inorganic substance with a definite chemical composition and ordered atomic structure. Minerals are native forms in which metals exist. Metals are commercially extracted from minerals at low cost and minimum effort. Minerals are broadly classified as

Primary minerals: Minerals which were formed by igneous process that is from the cooling down of the molten materials called magma are referred to as primary minerals. For example, quartz, mica, etc.

Secondary minerals: Secondary minerals are formed with the help of primary minerals. Primary minerals are altered to form secondary mineral. For example: mica is altered to form illite.

Ores

The metal ores are found in the earth’s crust in varying abundance. The extraction of metals from ores is what allows us to use the minerals in the ground. Ores are very different from the finished metals that we see in buildings and bridges. An ore is a naturally occurring deposit of geologic material (rock) that includes a sufficient quantity of one or more valuable elements or compounds that it can be extracted for economic gain. These valuable elements include metals like copper, platinum, iron, lead, gold, silver, aluminum, nickel, and many more. Ores may also contain nonmetals like silicon or diamond. These valuable substances can be extracted and used in a variety of ways, from use in jewelry making to the manufacturing of electronic devices. Ores vary widely depending on the chemical composition of the parent rock, as well as the natural process by which they were formed.

Difference between Ores and Minerals

Here we have provided the major differences between Minerals and Ores.

Minerals	Ores
All the naturally occurring substances that are present in the earth’s crust are known as Minerals.	Ores are usually used to extract metals economically. A large number of ores are present.
All Minerals are not ores.	All ores are minerals.
Minerals are native forms in which metals exist.	Ores are mineral deposits.

 

Some ores and minerals

Iron	Haematite Magnetite Siderite Iron pyrites	Fe2O3 Fe3O4 FeCO3 FeS2
Copper	Copper pyrites Malachite Cuprite Copper glance	CuFeS2 CuCO3.Cu(OH)2 Cu2O Cu2S
Zinc	Zinc blend/Sphalerite Calamine Zincite	ZnS ZnCO3 ZnO

 

Gangue: Gangue are impurities associated with minerals and ores. These impurities are removed to extract metals

Flux: A substance which is added to the charge in the furnace to remove the gangue (impurities) is known as flux. Flux is a substance introduced in the smelting of ores to promote fluidity and to remove objectionable impurities in the form of slag. Limestone is commonly used for this purpose in smelting iron ores. Other materials used as fluxes are silica, dolomite, lime, borax, and fluorite.

Steps involved in metallurgical process

Generally following operations are performed on ores to extract metals

Step 1: Crushing

Step 2: Concentration

Step 3: Reduction

Step 4: Refining

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Crushing

The ores occur in nature as huge lumps. These lumps are unsuitable for further steps. They are broken to small pieces with the help of crushers or grinders. These pieces are then reduced to fine powder with the help of a ball mill or stamp mill. This process is called pulverisation. The amount of crushing depends on the size of lumps. Different method of crushing is used for different size of lumps.

Concentration

The ores are usually found mixed up with large amounts of non-metallic impurities such as, sand, mica, limestone, felspar, earthy and rocky impurities. These unwanted impurities are called gangue or matrix and have to be removed before extracting the metals. The process of removal of unwanted impurities (gangue) from the ore is called ore concentration or ore dressing or ore benefaction. The powdered ore is concentrated by one of the physical or chemical process. The physical process involves gravity separation, magnetic separation and froth floatation process. Chemical method involves calcination and roasting.

Physical Process

Gravity separation

 

 

 

 

 

 

 

 

 

 

 

 

This method is based on the differences in the specific gravities of metallic ores and the gangue particles. Therefore, this method is known as gravity separation. This method is frequently used when the ore particles are heavier than the earthy or rocky gangue particles. In this method, the fine ore collected after crushing is placed on a slopy surface (wifely table especially designed for gravity separation method) and washed with a running stream of water. The lighter impurities (gangue) are washed away, while the heavier ore particles sink at the bottom of sloping platform. Generally, oxide ores like haematite (Fe₂O₃) or tinstone (SnO₂) are concentrated by gravity separation method.

Electromagnetic separation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This method is used for concentration of magnetic particles from non-magnetic particles. This method is widely used for the separation of two minerals, when one of them happens to be magnetic. The magnetic mineral can be separated from the non-magnetic one by this method. For example, mixture of FeWO₄ (magnetic) and cassiterite SnO₂ (non-magnetic) are separated by this method. The powdered ore is made to fall through a leather conveyor belt. The conveyor belt is placed on a magnetic roll. The leather belt of conveyor moves through electromagnetic roller. The magnetic impurities fall from the belt near the roller due to its electromagnetic property while the non-magnetic impurities fall far away from the roller.

 

Froth Floatation Process

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This method is extensively employed for the preliminary treatment of the minerals especially sulphides. The process is based on the difference in wetting characteristics of the gangue and the ore with water and oil. The ore is preferentially wetted by the oil and the gangue particles by water. Particle sized ore particles are mixed with water. The mixture obtained is called slurry. A collector which acts as a surfactant (a substance which tends to reduce the surface tension of a liquid in which it is dissolved) chemical (usually mixture of water and pine oil) is added to the slurry, this is done to enhance the hydrophobic nature of the mineral. The slurry has now been converted into pulp. This pulp is added to the container filled with water and then air jets are forced into it to create bubbles. The required mineral is repelled by water and thus gets attached to the air bubbles. As these air bubbles rise up to the surface with mineral particles sticking to them, these are called froth. This Froth is separated and further taken for the next process of refining and extraction.

Advantages

ü  Almost all types of minerals can be separated by this process.

ü  Surface properties can be controlled and altered by the flotation reagent.

Disadvantage

ü  Result can vary due to slime

ü  Highly expensive and complex

Chemical Process

Calcination

Metals are usually obtained from oxide ores after going through the electrolysis or reduction process. While oxide ores are easy to reduce, it is not the same with carbonates and sulphides. These ores are turned to metals only after converting sulphides and carbonates to an oxide ore. Calcination is the process of heating ore strongly in the absence of air to a temperature insufficient to melt it. The ore is heated below the melting point either in limited supply or absence of air. Thus, calcination is mostly used in the decomposition of limestone (calcium carbonate) to carbon dioxide and lime (calcium oxide). Calcination is done in hearth of reverberatory furnace (a furnace in which ores are heated by flames of a fuel) when the doors are kept closed. The main purpose of calcination are:

ü  To remove carbonate and hydroxide ore into oxide

Limestone:  

Malachite:

Haematite:

ü  To remove the moisture

ü  To remove the volatile impurities

ü  To make the mass porous, so that it can be easily reduced to the metallic state.

Roasting

Roasting is a process of metallurgy where the ore is converted into its oxide by heating it in the presence of excess air above its melting point. While calcination is the process mostly used in the oxidation of carbonates, roasting is a method that can be used for converting the sulphide ores. Roasting is done in the hearth of a reverberatory furnace when the doors are kept open for the free supply of air. This process is generally used in case of sulphide ores.

The main purpose of roasting are:

ü  To convert sulphide into oxide and sulphate

 

 

ü  To remove the moisture

ü  To remove volatile impurities like sulphur, arsenic, antimony and phosphorus in the form of their oxides.

ü  To oxidise easily oxidisable substances

Difference between calcination and roasting

Calcination	Roasting
It is the process of heating the ore to a high temperature in the absence of air, or where air does not take part in the reaction.	The process of heating the concentrated ore in the presence of air to a high temperature so as not to melt is called roasting.
Usually carbonate ores or ores containing water are calcined	Usually, sulphide ores are roasted
Organic matter, if present in the ore, gets expelled and the ore becomes porous.	The impurities of P, As and S are removed as their oxides which being volatile, escapes as gases.
It is done in reverberatory furnace. The holes of furnace are kept closed.	It is also done in reverberatory furnace but the holes of the furnace are kept open to allow the entry of air into the furnace.

 

Reduction

The oxide of the metal obtained as a result of Calcination or roasting is converted into a free state by reduction.  Reduction of metal oxide means removal of oxygen to refine the metal to its free state. This is done with the help of reducing agents such as Carbon (C), Carbon Monoxide (CO),  Hydrogen (H₂), etc. Reduction can also be carried out by using electricity. Depending on the type used, it can be classified in three processes i.e.

(a) Smelting

(b) Aluminothermic process

(c) Electrolysis

(a) Smelting

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

It is the process of heating the ore or mineral above its melting point. In this process, calcined ore is mixed with coke and flux like CaO and this mixture is strongly heated above its melting point. The coke converts the ore into molten metal while the gangue is removed in the form of slag. Smelting can be carried out in blast furnace. It is also carried out in reverberatory furnace sometimes. The flux added to remove the gangue from ore can be acidic or basic in nature.

The hot blast of dry air is blown into the furnace just above the hearth through a number of pipes called twyers. In the well of the furnace there are two outlets known as tap holes. Furnace charge is comprised of ore, coke, flux, etc. The upper tap hole is used to remove slag and the lower one is used to remove the molten metal. To remove the gangue in ore, certain oxides are added to the ore while melting. These oxides are referred to as flux. If impurity is basic, acidic flux is used and is impurities are acidic then flux used is basic. The flux and gangue are converted to some fusible mass known as slag. This slag is a fusible chemical compound formed by the combination of the added flux and gangue present in ore.

For example, in extraction of copper

So FeO is impurity and it is basic in nature. Now, to remove this impurity, we will use acidic flux like SiO₂. This flux and impurity will react to form slag.

Similarly, in extraction of iron, we have haematite ore

The impurity is SiO₂ which is acidic, so flux will be basic. Therefore, limestone (CaCO₃) will be used as flux. It decomposes at high temperature to give lime. This lime reacts with silica to form slag.

This can be written as

(b) Aluminothermic process

If the oxides of a metal are very stable, aluminium is used as a reducing agent in place of carbon at high temperature. Aluminothermic is a process of extracting metals by reduction of a metal oxide to form metal using aluminium powder, the aluminium acts as a reducing agent. It is an exothermic reaction which liberates a large amount of heat. Practically, this method can be used to reduce all the metallic oxides, with oxide of magnesium being the only exception.

Here are few examples of Alumino-thermite process:

Reduction of ferrous oxide to ferrous metal

Iron (III) oxide is mixed with aluminium powder and is ignited with a burning magnesium ribbon. Aluminium reduces iron oxide to produce molten iron metal with the evolution of heat. The chemical reaction is gives as:

Cr₂O₃ is reduced to chromium metal

A mixture of Chromic oxide and powdered aluminium in ratio of 3:1 and is ignited by a piece of magnesium ribbon. A large amount of heat is liberated during this process and chromic oxide is reduced to chromium. The chemical reaction is gives as:

A mixture of aluminium powder and metallic oxide in ratio 1:3 parts by weight are known as thermite. Aluminium at high temperature has a great affinity for oxygen. The reaction is exothermic; hence it liberates large amount of heat. Metals obtained by this method are highly pure.

 

 

 

 

 

 

 

 

 

 

 

This method is also used to weld the broken rails. The ends of the broken rails are surrounded with a clay mould. A mixture of iron (III) and aluminium powder (thermite) is ignited by magnesium ribbon in funnel above. The molten iron thus obtained runs into the mould. This produces a perfect union upon cooling.

(c) Electrolysis

Electrolysis is nothing but reduction of metal with the help of electricity. This method is used to extract metal from oxides of very active metals. These metals form strong bonds with oxygen, chlorine, etc. so that they are not reducible by carbon.

These metals cannot be obtained from their aqueous solution. If aqueous solution is used, then H⁺ ions is discharged at cathode. This ion may will gain electron and form H₂ gas. The metals can only be obtained if we brought them in their molten form. Suppose we want to extract Sodium from sodium chloride (NaCl).

 

 

 

 

 

 

 

 

 

 

 

Two electrodes are dipped in molten NaCl. These electrodes are then connected to battery. The electrodes dipped are inert in nature. Now, the electrode connected to negative terminal of battery will attract the Na⁺ ions. Now Na⁺ will gain electrons and form solid sodium. Thus, we can say that reduction takes place at cathode. Now, the electrode connected to positive terminal of battery will attract the Cl^– ions. These ions will lose electrons at anode. Thus, we can say that, oxidation takes place at anode.

At cathode, we have

At anode, we have

 

 

Refining

The metal obtained from electrolysis is in pure form. So, it does not require refining. But metals obtained from all the other processes apart from electrolysis contains small amount of other elements. The process of purification of metal to get extra pure metal is referred to as refining. There are several methods to refine a metal. Some of them are as follows:

ü  Poling

ü  Liquation

ü  Distillation

ü  Electrolytic refining

Poling

 

 

 

 

 

 

 

 

 

 

 

Poling is a method that includes a green log of wood to purify a metal. It is typically used to purify metals like copper or tin that are in the impure form of a copper oxide or tin oxide. A log of wood which is still green is used to stir the liquid metal. The hydrocarbons in the green wood can reduce the metal. The molten metal is placed in a concrete container. This hot crude molten metal is stirred with green logs of wood. Also, during stirring, large quantities of air is absorbed by the molten metal and such absorbed air oxidises the oxidisable impurities. The oxidised impurities escape either as vapour or form scum over molten metals. The scum thus formed is then removed by perforated ladle.

Liquation

 

 

 

 

 

 

 

 

 

 

Liquation is another method for refining of metal. This method is particularly suitable for metal whose melting point is comparatively lower. The melting point of the impurities is higher than the metal.  The metals are converted into liquid state by supplying heat at a temperature slightly above their melting point. The crude metal is placed on a slopy surface. The metal with lower melting point melts first and falls down the slope in a container. The impurities are not melted and stays on slope. The impurities left behind are known as slope.

Distillation

 

 

 

 

 

 

 

 

 

 

 

This method is used for the purification of metals which possess a low boiling point such as mercury and zinc. In this process, the impure metal is heated above its boiling point so that it can form vapours. The impurities do not vaporise and hence they are separated. The vapours of the pure metal are then condensed leaving the impurities behind.

 

Electrolytic refining

 

 

 

 

 

 

 

 

 

 

 

 

Electrolytic refining is a process of refining a metal (mainly copper) by the process of electrolysis. As far as the mechanism of the process is concerned, during electrolysis, a large chunk or slab of impure metal is used as the anode with a thin strip of pure metal at the cathode. In this setup, an electrolyte (metal salt aqueous solution) depending on the metal is often used.

The clean or pure metal is formed at the cathode when the electrical current of a sufficient voltage is applied by dissolving impure metal at the anode. When an electric current with a definite voltage is passed through the bath, anode goes on dissolving and only the pure metal is deposited on the cathode which grows in size. Electrolytic refining is also sometimes referred to as Electrorefining. Metals which are less electropositive than the one being refined settle below the anode and is known as anode mud.

Metallurgy of Iron

Iron is most widely used metal. It has occupied its space in our day to day lives like buildings, ships, aeroplanes, etc.  It is the 4^th most abundant element on earth after oxygen, silicon and aluminium. It is found in combined state in nature such as oxide, sulphate, silicates, etc. Iron is also present in our blood and it is very important for our body as well. It is found in combined state in soil, rocks, etc.

The important minerals of iron ore are as follows:

Oxides

Magnetite: Magnetite is a very common iron oxide (Fe₃O₄ or FeO.FeO₃) mineral that is found in igneous, metamorphic, and sedimentary rocks. It is the most commonly mined ore of iron. It is also the mineral with the highest iron content (72.4%). It is magnetic and black in colour. It is also known as Ferrosoferric oxide. This gives high quality of iron and mostly found in USA, Sweden, Canada and India.

Haematite: Haematite Fe₂O₃ is heavy and relatively hard oxide mineral. It is red in colour.  It is an important iron ore because of its high iron content (70 percent) and its abundance. It is mostly found in USA, Brazil, China and India.

Limonite: Limonite, one of the major iron minerals, hydrated ferric oxide Fe₂O₃.nH₂O. It is brown in colour. The limonite ore was the lowest in grade of all the oxides, with an iron content of 40%-60% percent. It is found in Belgium, Germany, France and India. 

Carbonate

Siderite is a mineral composed of iron(II) carbonate (FeCO₃). It is also known as ferrous carbonate. It constitutes 40-45% of iron. It occurs in England and West Bengal.

Sulphides

It is found in form of iron pyrites FeS₂, Copper pyrites CuFeS₂, Arsenical Pyrites FeAsS. The presence of sulphur in ore makes iron hard and brittle and thus useless for many processes. But the sulphide ore is also rich in copper, nickel, etc. So, this ore is not recommended for extraction of iron but can be used to extract other metals.

Iron is produced in three different commercial forms depending upon the number of impurities present, especially carbon. They are:

Pig iron: Iron obtained from reduction using carbon and lime as reducing agent is basically pig iron or cast iron. In more general term, an alloy containing more than 2% of carbon (other impurities are also present) is called as pig iron. Pig iron is direct metallic product obtained from blast furnace. It cannot be shaped into articles by forging or hammering. It is therefore, melted and casted using moulds of desire shape. It is therefore also referred to as cast iron.

Steel: Steel is made from iron ore, a compound of iron, oxygen and other minerals that occurs in nature. The raw materials for steelmaking are mined and then transformed into steel using two different processes: the blast furnace/basic oxygen furnace route, and the electric arc furnace route. Steel consists of 0.2% to 1.5% Carbon.

Wrought Iron: Wrought iron is also known as malleable iron and is very pure form of iron. It does not contain more than 0.5% carbon. It is useful in application like chain, bolts, nails, etc. Wrought iron is composed primarily of iron with 1 to 2% of added slag, the by-product of iron ore smelting.

Extraction of iron

Extraction of iron is mainly done in two major steps:

Step I: Preliminary treatment

(a) crushing and grinding

(b) concentration

(c) calcination or roasting

Step II: Smelting (Reduction)

Preliminary treatment

The first step in preliminary treatment is crushing and grinding.

(a) Crushing and grinding:

The ore obtained in natural form is hard large bulky lumps. These lumps are first broken into smaller pieces using a grinder. They are broken to small pieces with the help of crushers or grinders. These pieces are then reduced to fine powder with the help of a ball mill or stamp mill. This process is called pulverisation.

(b) Concentration:

The ores are usually found mixed up with large amounts of non-metallic impurities such as, sand, mica, limestone, felspar, earthy and rocky impurities. These unwanted impurities are called gangue or matrix and have to be removed before extracting the metals. The process of removal of unwanted impurities (gangue) from the ore is called ore concentration or ore dressing or ore benefaction. The concentration of iron ore is done in two steps. Firstly, the ore is concentrated using gravity separation method. In this method, the fine ore collected after crushing is placed on a slopy surface (wifely table especially designed for gravity separation method) and washed with a running stream of water. The lighter impurities (gangue) are washed away, while the heavier ore particles sink at the bottom of sloping platform. The mass then received free from clay, sand and other earthy impurities are then dried and subjected to magnetic separation. The conveyor belt is placed on a magnetic roll. The leather belt of conveyor moves through electromagnetic roller. The magnetic impurities fall from the belt near the roller due to its electromagnetic property while the non-magnetic impurities fall far away from the roller. This concentrates the ore upto 90% to 95%.

(c) Roasting:

The oxidised metal is brought to its free method by roasting. The concentrated ore is calcined or roasted in a reverberatory furnace at a low temperature, with a little coke. Roastings brings following change to the ore:

Carbonate ore is converted to oxide:

Ferrous oxide is oxidized to ferric oxide and thus its conversion to ferrous silicate (slag) is avoided

The mass is dried up by removing moisture

The lighter non-metallic impurities such as sulphur, arsenic, phosphorus etc. are all removed as volatile substances.

Ore becomes porous and hence easy to reduce.

Reduction

The ore is then reduced to highly concentrated iron.

Smelting

The process of reduction of iron ore to iron is done in blast process. The process is known as smelting. The purpose of a Blast Furnace is to reduce the concentrated ore chemically to its liquid metal state. A blast furnace is a gigantic, steel stack lined with refractory brick where the concentrated iron ore, coke, and limestone are dumped from the top, and a blast of hot air is blown into the bottom. At the top, it is provided with double cup and cone structure to permit charging without escape of waste gases. The upper part of the furnace is called

the throat, the middle part is known as body and the bottom part is referred to as hearth. The region between body and hearth is bosh. The diameter of furnace increases upto bosh. From the bosh downwards, the diameter decreases upto twyers. A little above the base, the furnace is provided with series of water jacketed pipes called twyers.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Working

The charge is provided through double cup and cone mechanism. The charge constitutes of calcined ore, de-sulphurised coke and limestone in ratio of 8:4:1. Various reactions takes place in blast furnace at different temperatures. The reactions occur at different temperatures from 250^o C near throat to 1500^o C near hearth. From throat to bosh, the reaction takes place in three different zones as shown below:

(i) Zone of reduction (300^o C - 800^o C)

(ii) Zone of heat absorption (800^o C - 1200^o C)

(iii) Zone of fusion (1200^o C - 1500^o C)

Zone of reduction (300^o C - 800^o C)

The main reaction that occurs near the top of the furnace is reduction of the iron oxide to metallic iron by carbon monoxide.

The process of reduction continues as charge flows down the furnace. Series of reaction takes place at different stages.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

At the same time, the limestone present in the charge is also decomposed to produce lime

The metal produced at first is spongy. Therefore, simultaneously with the process of reduction, a part of metallic iron reacts with carbon monoxide to form ferric oxide.

 

 

Zone of absorption (800^o C - 1200^o C)

In this region, unreacted iron oxides get reduced to iron by red hot coke

Carbon monoxide gets disproportionate to CO₂ and carbon powder

Lime obtained at the end of first zone combine with impurities to produce slag

Other impurities get reduced to the elementary substances and are mixed with finished iron

 

 

 

 

 

Zone of fusion (1200^o C - 1500^o C)

The temperature of the bottom region is 1500^o C. Iron melts in this region. Molten iron gets collected through the bottom outlet. Slag also melts at lower temperature and since it is lighter than iron, it floats over molten iron and gets removed through another outlet. The molten iron is then sent into sand moulds and cast into pigs or led to steel furnace and converted into different type of steels. The output from blast furnace is cast iron or pig iron whose composition is given below:

Iron	92% to 95%	Phosphorus	0.5% to 1%
Carbon	2.5% to 4.5%	Manganese	0.2% to 1%
Silicon	0.7% to 3%	Sulphur	0.1% to 0.3%

 

This pig iron can then be converted to steel or wrought iron as per requirement. When the molten iron is cooled suddenly, it is known as white cast iron but when it is cooled slowly, it is known as grey cast iron.

Metallurgy of Copper

Copper occurs in native as well as in combined state. Most common ores of copper are in form of sulphides, carbonate and oxides.

Sulphide Ores: Copper Pyrite (CuFeS₂) or copper glance Cu₂S

Oxide ores: Cuprite or ruby copper (Cu₂O)

Carbonate ores: Malachite (CuCO₃, Cu(OH)₂) or azurite 2CuCO₃

The commonest ore used in the extraction of copper is Chalcopyrite (CuFeS₂) also known as Copper Pyrites and other such sulphides.

The steps involved in extraction of copper

Step1: Crushing

Step2: Concentration

Step3: Reduction

Step4: Bessemerisation

Step5: Refining

Crushing

The copper pyrite ore is crushed in a big jaw crusher. The output from crusher is not fine. Therefore, it is passed through a stamping mill. The stamping mill convert smaller lumps of ore to finely powdered ore.

Concentration

The process of removal of unwanted impurities (gangue) from the ore is called ore concentration or ore dressing or ore benefaction. The concentration can be done either by a physical method or chemical method.

Concentration by physical method (Froth floatation method)

In physical method no chemical reaction takes place during concentration. The ore is concentrated using physical property. The finely powdered ore can be concentrated by froth floatation method. This method is extensively employed for the preliminary treatment of the minerals especially sulphides. Particle sized ore particles are mixed with water. The mixture obtained is called slurry. This is added to the container filled with water and then air jets are forced into it to create bubbles. The required mineral is repelled by water and thus gets attached to the air bubbles. As these air bubbles rise up to the surface with mineral particles sticking to them, these are called froth. 

Concentration by chemical method (Roasting)

For concentration of ore by chemical method, it requires some chemical changes to ore. This can be done using roasting method. During roasting following chemical reactions occur

Copper pyrites decompose to form cuprous and ferrous sulphides

A part of these sulphides gets oxidised to corresponding oxides

Moisture is eliminated and impurities like sulphur are removed in their volatile oxides

Reduction

Reduction of copper ore is done by smelting. The roasted ore is mixed with coke and sand and is heated in blast furnace in the presence of excess air. Copper Smelting means that the concentrated ore is heated strongly with silicon dioxide (silica), calcium carbonate (CaCO₃) and air in a furnace. The major steps in the extraction of copper are

ü  Copper in Chalcopyrite is reduced to copper sulphide.

ü  Calcium carbonate is added as a flux to create the slag.

ü  Iron in Chalcopyrite is removed as iron silicate slag.

ü  Most of the sulphur in Chalcopyrite turns into Sulphur dioxide (SO₂).

The copper extracted from this process is mixed with the slag and is called Matte Copper due to its texture and appearance. Matte is a mixture of molten Cu₂S + FeS.  This molten matte so produced is then converted to blister copper by Bessemerisation.

Bessemerisation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The molten metal is then transferred to a Bessemer converter. Bessemer converter is a furnace made up of steel plates and lined with basic lining of lime or magnesia. It is mounted on pivot and can be tilted in any position. The furnace is provided with pipes known as twyers through which sand and hot air is blown into it. The tuyeres are fitted sufficiently high above the bottom so that the molten metal drops below the level of twyers and escapes the oxidising action of the air. FeS is converted to slag. Slag is poured off by tilting converter. Cu₂O is reduced to metallic copper.

After the reduction is complete, the molten copper is poured into sand moulds. As the metal solidifies, SO₂ escapes and leaves blisters on the surface. The solid metal thus obtained is called as blister copper which contains 96% to 98% copper. The remaining 2% is iron with small amounts of An, Ni which were present originally in the ore. This copper can be used in construction of pipes, boilers, etc. But to conduct electricity copper must be extremely pure. There blister copper goes to refining stage.

 

Refining

Refining of copper can be done in two stages as discussed below to obtain extremely pure copper. However, either of the stages can be used for refining of copper.

Poling

In this process, blister copper is melted in a reverberatory furnace in the presence of air. The molten mass is then stirred with a green log of wood. The green log of wood produces hydrocarbon gases which reduces cuprous oxide to metallic copper. This refining reduces copper with 99.2% to 99.6% concentration

Electrolytic refining

The copper obtained from poling can be concentrated up to 99.9% by electrolytic refining. However, blister copper can be directly refined using this method. This is a standard electrolysis setup, where the impure copper (the sample to be refined) is placed as anode and a thin strip of pure copper is placed as a cathode.

 

 

 

 

 

 

 

 

 

 

 

 

Impure or blister copper is about 99% pure when extracted from the ore. Copper metals can be refined up to 99.99% by electrolytic refining. The anode (positive electrode) is made from impure copper, and the cathode (negative electrode) from pure copper. Copper sulphate acidified with sulphuric acid is used as an electrolyte in this process. By passing electric current, the impure metal dissolves from the anode into the electrolytic solution. In this phase, copper sulphate dissociates into Cu⁺⁺ and SO₄^2- ions. The positive copper ions, also known as the cations, move towards the cathode, which is made up of pure copper metal. The metal cations absorb electrons from the cathode and get deposited on the cathode as Cu atoms. Thus, pure copper is produced at the cathode.

The following reactions take place when an electric current is passed through the electrolytic solution:

At cathode:

At anode:

The soluble impurities dissolve in the electrolytic solution, while insoluble impurities can be found at the bottom of the anode. This insoluble impurity is known as anode mud. In this method, the cathode is coated with graphite as it makes the separation of pure copper easier.

Properties of copper

Engineering properties of copper

Copper is a heavy reddish-brown metal. It gets polished easily and hence used for making utensils.

ü  It has high melting point and boiling point. Also, it is a very good conductor of electricity thus making it useful for electrical connections

ü  It is malleable and ductile and can be drawn into wires of small diameter

ü  Copper is next to silver in terms of conductivity to heat and electricity

ü  It casts very well and forms alloys

Chemical properties of copper

Action of air

Copper is not attacked by dry air at ordinary temperatures. However, if it is heated till it becomes red in the presence of air, it forms cupric oxide and on further heating it forms cuprous oxide.

Action of water

Water at ordinary temperature has no action on copper. However, red hot copper reacts with steam and form oxide

Action of acid

Action of HCl (hydrochloric acid)

Cold and dilute HCl has no action on copper. Copper reacts with warm dilute HCl in presence of oxygen to form cupric chloride

Also, copper powder dissolves slowly in hot and concentrated HCl with the evolution of hydrogen

Action of H₂SO₄ (sulphuric acid)

Cold and dilute H₂SO₄ has no action on copper. It dissolves slowly in hot and diluted H₂SO₄ in the presence of oxygen.

On heating with concentrated H₂SO₄, it produces SO₂

Action of HNO₃ (nitric acid)

Dilute HNO₃ dissolves the metal with the evolution of nitric acid

Concentrated HNO₃ dissolves the metal and gives nitrogen dioxide

Action of CH₃COOH acetic acid

In presence of air, copper reacts with acetic acid to form cupric acetate

Metallurgy of aluminium

Aluminium is a soft, silvery-white, corrosion-resistant metal. It is the most abundant metal in the earth’s crust as it makes up 8% of the crust and it is the third most abundant element after oxygen and silicon. It occurs widely as constituent of rocks and soils. Its main ores are:

Oxides: Bauxite (Al₂O₃.2H₂O), Corundum (Al₂O₃)

Fluoride: Cryolite (Na₃AlF₆)

Silicates: Feldspar (KAlSi₃O₈), Mica (KAlSi₂O₁₀(OH)₂)

Basic sulphates: Alunite (K₂SO₄.Al₂(SO₄)₃.4Al(OH)₃)

Bauxite ore is most widely found and hence it is commonly used to extract Aluminium.

Extraction of aluminium

Extraction of aluminium involves following three steps:

Step1: Purification of Bauxite

Step2: Electrolytic reduction of alumina

Step3: Refining of alumina

Purification of bauxite:

Bauxite ore generally contains ferric oxide and silica as impurities. Dressing of Bauxite ore is done by crushing and pulverising. Purification method varies according to the type of impurities present.

ü  Baeyer’s process for the removal of impurities of iron (red bauxite)

ü  Serpeck’s process for the removal of impurities of silica (white bauxite)

ü  Hall’s process for the removal of both iron and silica impurities

Baeyer’s Process

The powdered bauxite ore is roasted to convert ferrous oxide (FeO) to ferric oxide (Fe₂O₃). This roasted ore is then heated with concentrated NaOH under pressure for few hours in autoclave. This process is referred to as leaching. Aluminium oxide dissolves forming sodium meta-aluminate, while ferric oxide remains undissolved.

Undissolved Fe₂O₃ is removed by filtration. The sodium meta-aluminate is diluted with water to form a precipitate of aluminium hydroxide Al(OH)_3.

The precipitate of Al(OH)₃ is then filtered out, dried and heated at 1500℃ to get pure alumina

Serpeck’s process

Serpeck process is a method of purification of bauxite ore containing silica (SiO₂) as the main impurity. When Silica is the main impurity, it is called white bauxite. The powdered bauxite is mixed with carbon and heated up to 1800℃ and a current of nitrogen is passed, aluminium nitride is obtained.

Aluminium nitride thus obtained is hydrolysed with water to get a precipitate of Al(OH)_3.

The precipitate of Al(OH)₃ is filtered, washed and dried.

The silica present as an impurity in bauxite is reduced to silicon which is volatile at high temperature. So, it is removed easily.

Hall’s Process

This method is used for the ore having both Fe₂O₃ and SiO₃ as impurities. The powdered ore is bright red heated with sodium carbonate. Aluminium oxide dissolves to form sodium meta-aluminate while insoluble Fe₂O₃ and SiO₂ are left residue.

The fused mass is extracted with water and the insoluble Fe₂O₃ and SiO₂ are removed by filtration. The filtrate is then heated to 50^o C to 60^o C and a current of CO₂ is passed through it and Al(OH)₃ is precipitated out.

 

Al(OH)₃ is filtered off, dried and ignited to get pure alumina.

Reduction

 

 

 

 

 

 

 

 

 

 

 

 

 

Alumina cannot be reduced to metallic aluminium by carbon, because aluminium has great affinity for oxygen and thus the reduction of alumina by carbon under ordinary condition is not possible. Thus, alumina is reduced to aluminium using electrolytic reduction. However, alumina is a bad conductor of electricity and has very high melting point. If electrolysis is carried out at 2000^o C, aluminium obtained vaporises. Hall and Heroult overcome these difficulties by carrying the electrolysis of alumina in the presence of cryolite (Na3AlF₆) which acts as a proper electrolyte. In Hall-Heroults process, pure Al₂O₃ is mixed with CaF₂ or Na₃AlF₆. This results in lowering the melting point of the mixture and increases its ability to conduct electricity.  A steel vessel with a lining of carbon and graphite rods is used.

The carbon lining acts as a cathode and graphite act as an anode. When electricity is passed through the electrolytic cell which consists of carbon electrodes oxygen is formed at the anode. This oxygen formed reacts with the carbon of the anode to form carbon monoxide and carbon dioxide. In this method of production of aluminium, for every 1 kg of Al produced, approximately 0.5 Kg of carbon anode is burnt. This electrolytic bath is covered with powdered coke to avoid the oxidation of metal and to avoid the loss of heat. Heat produced by the current keeps the mass in fusible state. The temperature of bath is maintained at 900^o C – 1000^o C. On passing current, alumina decomposes to aluminium and oxygen

During the process of electrolysis, Aluminium ions that are positively charged gain electrons from the cathode and form molten aluminium. Oxide ions lose electrons at the anode and form molecules of oxygen.

The molten aluminium sinks to the bottom (cathode) from where it is tapped off from time to time. Oxygen is liberated at anode, which then combines with carbon anodes and form CO and CO₂. These gases escape through outlet.

At the cathode:

At the anode:

The concentration of alumina falls in the cell below a certain limit, the resistance of the cell increases and therefore, more current flows through lamp connected in parallel with cell and it glows. This indicates that alumina is exhausted and more alumina needs to be added to continue the process.

 

Refining of aluminium

 

 

 

 

 

 

 

 

 

 

 

 

 

Aluminium is further purified by Hoope’s process. The electrolytic cell consists of an iron box lined inside with carbon. The cell consists of three layers which differ in specific gravities.

a)   The upper layer is of pure aluminium which acts as cathode.

b)   The middle layer consists of a mixture of the fluorides of Al, Ba and Na.

c)   The lowest layer consists of impure aluminium which acts as anode.

When electrolysis occurs, current passes through the middle layer and the Al⁺³ ions migrate from the middle layer towards the cathode where they get reduced to form Al. The lowermost layer or the anode produces an equal number of Al⁺³ ions, which move to the middle layer and then to the upper layer where they form Al.

In this way all the pure aluminium gets deposited at the uppermost layer and the impurities are left behind. The aluminium obtained at the end was 99.99% pure.

Properties of aluminium

Physical properties

ü  Aluminium is silvery and lustrous white metal which takes very high polish.

ü  Aluminium is a good conductor of heat and have high melting and boiling point and thus is used to make utensils

ü  Aluminium is a good conductor of electricity and thus used to make wire

ü  It is malleable and ductile and can be rolled into sheets, foils and wires.

Chemical Properties

Action of air

Aluminium is not affected by dry air and form a thin layer of oxide in moist air. This prevents aluminium from further corrosion. On heating in air or oxygen, it burns readily which produces light with the evolution of a huge quantity of heat.

Action of water

Pure aluminium is not affected by pure water. However, impure aluminium is readily corroded by water containing salts. It decomposes boiling water with the evolution of hydrogen.

Amalgamated aluminium Al - Hg reacts most instantly with cold water liberating hydrogen. Hence it is used as reducing agent.

Action of acid

Aluminium dissolves readily in HCl to form aluminium chloride and liberates hydrogen.

Diluted H₂SO₄ reacts slowly with aluminium and liberates hydrogen

Hot and concentrated H₂SO₄ liberates SO₂ gas with aluminium

Nitric acid has no action on pure aluminium

Action of base

Aluminium readily dissolves in strong bases like NaOH, KOH forming metal aluminates and liberates H₂ gas.

It dissolves in hot concentrated solution of Na₂CO₃ forming sodium meta-aluminate

 

Alloys

The meaning of the term ‘alloy’ is a substance formed from the combination of two or more metals. Alloys can also be formed from combinations of metals and other elements. Pure metals have properties like metallic lustre, good electrical conductivity, high malleability, and ductility. These metals are very soft, highly chemically reactive. This makes metal useless for engineering purposes. To make metal useful, desired properties are incorporated by mixing it with other compounds. To impart certain properties to metals, or in order to strengthen some of their existing properties, certain other metals/elements can be added to the metals in specific ratios to form alloys. For example, pure aluminium is a relatively soft metal. Pure copper is also quite soft. However, when aluminium is alloyed with copper, the strength of the resulting alloy is far greater than that of its parent metals.

Properties of alloy

An alloy’s properties are distinct from those of the individual metals from which it is produced. Some properties of alloys are given below.

ü  An alloy is a homogeneous mixture of two or more elements one of which must be a metal.

ü  Alloys are harder than their constituent metals.

ü  Alloys are more resistant to corrosion than pure metals.

ü  Alloys are more durable than the metals they are made from.

ü  The electrical conductivity of alloys is lower than that of pure metals.

ü  Alloys have a lower melting point than the metals from which they are made.

ü  Alloys have greater ductility than their constituent metals.

Preparation of alloys

An alloy can be prepared by any of the following method

(a) Fusion

(b) Electro deposition

(c) Compression

(d) Reduction

(a) Fusion

This method uses alloying elements in a fixed proportion and fuses them together in a refractory melting pot or in a brick-lined crucible. The component metal with a higher melting point is melted first and then the other component with a lower melting point is added to the melt. Both metal components are mixed well and allowed to melt further. The molten mass is covered by powdered Carbon to avoid oxidation of the molten alloy components because they are very reactive to the surrounding atmospheric oxygen. The resulting molten mass is allowed to cool at room temperature. In the production of brass, an alloy of copper and zinc is melted first and then required quantity of zinc is added to it which melts immediately. The molten mass is stirred, covered with charcoal, and allowed to cool to avoid oxidation of copper and zinc.

(b) Electro deposition

This method involves simultaneous deposition of different component metals from the electrolytic solution containing their salts solution mixture by passing direct electricity. Brass can also be obtained by electro deposition method. The electrolysis of mixed solution of copper and zinc cyanides dissolved in potassium cyanide is carried out to obtain brass.

(c) Compression

Alloy can also be made by method of compression. In this method, two or more metal powders are mixed and compressed under a high pressure in a mould. The moulded article is then heated to a temperature just below the melting point of an alloy. Due to this, tiny particles of metal are firmly welded to one alloy. An alloy of lead and tin known as solder alloy is made by this method. Wood’s metal is an alloy of lead, tin, bismuth, and cadmium is also made by this method.

(d) Reduction

The alloy is obtained by the reduction of a suitable compound, generally oxide of one component metal in the presence of the other component metal. The component metal oxide is reduced to metal in the presence of other metal. Aluminium bronze is alloy which is prepared by reducing aluminium oxide in the presence of copper in an electric furnace.

Need of alloys

To improve the hardness of metal: Pure metals are generally soft. But when the metal is alloyed with another metal or non-metal, its hardness is increased. Pure gold and silver are soft and hence cannot be used. But to make it useful, it is hardened by addition of small amount of copper. Similarly, pure iron is very soft and cannot be used for any engineering purposes. The iron is hardened by addition of small amount of carbon. The resultant iron is nothing but steel.

To lower the melting point: When an alloying element is added to base metal, it acts as an impurity. The impurity lowers the melting point of base metal. Wood’s metal is an alloy of bismuth, lead, tin, and cadmium has melting point of 70^o C.

To increase the tensile strength: While a metal is alloyed, it increases its tensile strength. For example, when iron is alloyed with 1% of carbon, it increases its tensile strength by 10 times.

To increase the corrosion resistance: The metals in pure form are highly reactive. They get easily corroded with atmospheric air and moisture. But if metal is alloyed, it resists corrosion. For example, iron gets easily corroded in air and moisture, but its alloy steel resists corrosion.

To get good casting: Pure metals contract on solidification so it becomes difficult to cast fine impression. Hence, in order to get good castings, metals must be alloyed. Alloy expands on solidification.

To reduce malleability and ductility: Pure metals are highly malleable and ductile. Pure metals change their shape even when very small force is applied. Its malleability and ductility can be reduced by making it alloy.

To modify chemical activity: Pure metals are highly reactive and can easily react with other elements. But when it is alloyed with other element it becomes less reactive. For example, sodium is highly reactive but when alloyed with mercury (sodium-amalgam (Na-Hg)) it becomes less reactive.

Classification of alloys

Alloys are generally classified in two classes

Ferrous alloys

Non- ferrous alloys

Ferrous alloys:  Ferrous alloys are those alloys which contains iron as its main component. The term ‘ferrous’ has been derived from the Latin term ‘ferrum’ meaning iron. The most common ferrous alloys are steel.

There are Different Forms of Ferrous Metals Available in the Market. Some of the Major Types and Their Characteristics are Stated below:

Stainless Steel – Resistance to Corrosion

Cast Iron - Hard, brittle, strong, self-lubricating, economical

Mild Steel – Ductile, tough, high tensile strength. Due to low carbon content, it cannot be toughened and tempered. It should only be case hardened.

High Carbon Steel - The hardest of the carbon steels. Tough and malleable but less ductile.

Methods of steel making

Steel is manufactured by blowing hot air or oxygen through molten cast iron whereby impurities are oxidized and removed as volatile gases. Small amount of carbon is added to the pure molten iron to make steel of required strength and malleability. Steel can be made by any of the following methods: Electric process, Bessemer process, Open hearth process and duplex process. These are old methods of making steel. New methods include Kaldo process and Linz-Donawitz process.

Bessemer Process

The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is removal of impurities from the iron by oxidation with air being blown through the molten iron. In this method, a furnace called Bessemer converter is used. It is a pear or egg shaped vessel constructed by using steel plates. It is supported on side arms called trunnion to orient it in different angles. Twyers are provided at the bottom and hence it is also known as bottom-blown converter. From inside it is lined with refractory lime bricks or silica bricks. When acidic impurities are to be removed, basic lining such as dolomite (CaCO₃.MgCO₃) is used and the process is referred to as basic Bessemer process. When basic impurities need to be removed, acidic lining of silica is used and the process is referred to as acid Bessemer.

Acid Bessemer process

In this process the converter is lined with silicious refractory material. The converter starts in horizontal position and charged with about 60 tonnes of molten cast iron at 1200^o C. Then it is turned to vertical position and a hot blast of air blown from the twyers. Impurities get oxidized and temperature rises to 1900^o C. The reactions taking place in converter are as follows:

The oxides of silicon and manganese combine to form a slag of manganese silicate

A little amount of iron is also oxidized to ferric oxide, but it gets readily reduced by carbon present in the cast iron.

Ferric oxide formed above also oxidizes manganese and silicon to their respective oxides

These reactions are then observed by the colour of burning waste gases. Carbon monoxide produced burns at the mouth of the converter with a blue flame and orange-red tinge, throwing out shower of sparks. When whole carbon is oxidized, the blue flame suddenly dies down. When the blue flame dies, a calculated amount of Fe, Mn, and C is added.

Blast of air is passed; mass is mixed well. Molten iron contains dissolved gases such as O₂, N₂ and CO₂ which create blow holes or gas bubbles in castings and defects are created. This is then followed by the addition of aluminium. Aluminium reacts with these gases and removes them as slag.

Basic Bessemer

This process is used to treat cast iron containing phosphorus. In this process, the converter is lined with magnesia and lime prepared by calcinations of dolomite (CaCO₃.MgCO₃). Some limestone is added into the converter. Molten cast iron from blast furnace is then run into the converter and the blast is continued. Carbon, sulphur, and manganese are oxidized first as usual, but if the blast is continued even after the flame sinks down, then the phosphorus forms phosphorus pentoxide.

The phosphorus pentoxide thus formed, combines with lime to form a basic slag, containing calcium phosphate

After complete removal of slag, required amount of carbon is added and the product is thoroughly mixed in presence of air.

Merits of Bessemer Process

ü  This process is useful for rapid production of steel.

ü  Cost of operating process is low

ü  No extra fuel is required as molten cast iron is directly taken to converter from blast furnace.

Demerits of Bessemer Process

ü  Steel produced is of inferior quality

ü  Approx 15% of iron is lost in slag

ü  Process is not continuous and charging is tedious

Linz-Donawitz Process

LD is named after the two places of Austria namely, Linz and Donawitz where the process was first formed. It is a refining process which is done through an LD convertor or LD vessel. LD is also known as Basic Oxygen Process. The furnace is an egg-shaped steel vessel, sealed from bottom and supported on the side arms called trunnions. It is lined from inside with basic lining of magnetise or limestone. At the top, it also has an oxygen lance made up of concentric steel tubes with a tip of copper.

Charging: Five materials are required for this purpose. The converter is charged with cast iron, scrap, limestone, coolants, pure and dry oxygen.

Blowing: Once the charging is done then the converter is rotated upright in the vertical position and the lance is lowered to the position of blowing. O₂ is then turned on for around 20 minutes, at a pressure of 9 to 11 atoms that raises the temperature and impurities are burned off.

Sampling: for analysis, slag and metal samples can be taken and the temperature of the bath needs to be measured by immersion of thermocouple.

Tapping: the molten steel is tapped in the ladle if the tapping temperature is in the required range. Ladle is used to make deoxidizers and alloying additions. It has a tap-to-tap time of 40-50mins.

Slag off: after tapping steel into the ladle and putting the vessel upside down tapped the remaining slag into the slag pot.

Merits

ü  It is rapid process with high productivity.

ü  Energy needed is comparatively low.

ü  Steel produced is of superior quality.

ü  Molten cast iron and scrap can be used directly.

Composition, properties, and application of plain carbon steel

Plain carbon steel is an alloy of iron and carbon where the amount of carbon ranges from, 0.015% to 2%. Carbon steel is by far the most widely used kind of steel. The properties of carbon steel depend primarily on the amount of carbon it contains. In fact, there are 3 types of plain carbon steel and they are low carbon steel, medium carbon steel, high carbon steel, and as their names suggests all these types of plain carbon steel differs in the amount of carbon they contain. Indeed, it is good to precise that plain carbon steel is a type of steel having a maximum carbon content of 1.5% along with small percentages of silica, sulphur, phosphorus and manganese.

Plain carbon steels are further subdivided into four groups:

Low carbon steels (carbon: 0.05% <carbon< 0.30%): Often called mild steels, low-carbon steels are the most commonly used grades. They machine and weld nicely and are more ductile than higher-carbon steels.

Use: It is used for thin soft wires, ropes, chains, tubes, etc.

Medium carbon steels (0.30% < carbon < 0.60%): Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining.

Use: It is mainly used in rail roads, wheels, axles, crane hooks, etc.

High carbon steels (0.6% < carbon < 0.90%): these steels can be challenging to weld. Preheating, post heating (to control cooling rate), and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding.

Use: It is used for making tools, drills, knives, hammers, etc.

Very high carbon steels (0.9% < carbon < 1.5%): very high-carbon steels are used for hard steel products such as metal cutting tools and truck springs. Like high-carbon steels, they require heat treating before, during, and after welding to maintain their mechanical properties.

Use: It is mainly used for preparation of cutting tools, blades, saws, etc.

Limitations of plain carbon steel:

ü  There cannot be strengthening beyond about 100000 psi without significant loss in toughness (impact resistance) and ductility.

ü  Plain-carbon steels have poor impact resistance at low temperatures.

ü  Plain-carbon steels have poor corrosion resistance for engineering problems.

ü  Plain-carbon steel oxidises readily at elevated temperatures.

Effects of various alloying elements on steel

Steel is alloyed with various elements to improve physical properties and to produce special properties such as resistance to corrosion or heat. Steel is alloyed with number of elements such as Nickel, Silicon, Chromium, etc. Specific effects of the addition of such elements are outlined below:

Some important alloy steels have been listed:

Name	Composition	Properties	Uses
Molybdenum Steel	0.3% to 3% Mo	Increases strength, hardness, hardenability, and toughness and strength even at high temperatures. It improves machinability and resistance to corrosion.	Axis and cutting tools
Nickel steel platinate	46% Ni	Increases strength and hardness without sacrificing ductility and toughness. High co-efficient of expansion	Wire scales, electric bulbs, etc.
Chrome steels	2% to 4% Cr	Increases tensile strength, hardness, hardenability, toughness, resistance to wear and abrasion	Ball bearings, cutting tools, etc.
Manganese steels	6% to 15% Mn	It increases tensile strength, hardness, hardenability and resistance to wear.	Crushing Machine, helmet, railroad, etc.
Stainless steel	12% to 20% Cr, 8% Ni	Resistant to corrosion, non-magnetic, brittle.	Cutlery, utensils, surgical appliances
Nickel Steel	2.5% to 5% Ni	Hard, resistant to corrosion	Wires, cables, gears, etc.
Cobalt steel	Upto 35% Co	High magnetism, hard, resistant to corrosion	High speed tools and permanent magnets
Silicon steel	Upto 15% Si	Extremely hard, resistant to acids	Pumps, pipes carrying acids, transformers
Tungsten steel	15% to 20% W	Increases strength, wear resistance, hardness and toughness. Tungsten steels have superior hot-working and greater cutting efficiency at elevated temperatures.	High speed machine, drilling tools, cutting tools, etc.
Vanadium steel	3.5% to 5% V	Increases strength, hardness, wear resistance and resistance to shock impact. It retards grain growth, permitting higher quenching temperatures	Cutting tools

 

Non-ferrous alloys

Non-ferrous metals are alloys or metals that do not contain any appreciable amounts of iron. Non-ferrous alloys are formed by mixing atoms of transition metal other than iron with a non-transition metal. Example: Brass is an alloy of copper and zinc. It does not contain iron and hence it is non-ferrous alloy. Several non-ferrous alloys are discussed below:

Copper Alloys

ü  Alloys which contain copper as base metal are known as copper alloys.

 

Properties of copper alloy

ü  Copper alloys are generally reddish in colour.

ü  Its specific gravity is 8.93 and hence it is a heavy metal.

ü  It has high melting and boiling point.

ü  Copper alloys are generally good conductor of electricity and heat

ü  Copper alloys are malleable, tough and ductile

ü  Copper alloys are resistant to corrosion

Composition of copper alloy

Name	Composition	Property	Application
Brass	66% Cu 34% Zn	Bright gold appearance. Highly ductile, low friction and resistant to corrosion	Bells, horns, utensil, jewellery
Bronze	88% Cu 12% Tin	Hard and brittle. Low metal to metal friction. High resistance to corrosion from saltwater	Used in sculpture, musical instruments
Nickel silver or German silver	18% Ni 62% Cu 20% Zn	Silver white appearance Highly lustrous, tough and corrosion resistance	Cutlery, marine fittings, heating coils

 

Aluminium alloys

An aluminium alloy is an alloy in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, etc.

Properties of aluminium alloy

ü  Aluminium alloys are generally bluish white

ü  Its specific gravity is 2.7 and hence it is a light metal

ü  It has comparatively low melting point and high boiling point

ü  It is malleable and ductile

ü  Aluminium alloys are generally good reflector of light

Composition of aluminium alloy

Name	Composition	Property	Application
Duralumin	94% Al 4% Cu 1% Mg 0.5-1% Mn	Whitish appearance, resistant to corrosion, hard, good conductor of heat and electricity	Wire, bar, rods, wheels, aircraft fittings, etc.
Magnalium	95% Al 5% Mg	Light weight and brittle. Poor castability and good machinability	Suitable for making aircraft and automobile parts.

 

Solder alloys

Solder alloy is a metallic material that is used to connect metal workpieces. Solder alloys are generally alloys of lead and tin and has low melting point. The solder alloy becomes softer as the percentage of lead increases and harder as percentage of tin increases. Solder alloys easily adhere to the metallic surface.

Some more alloys

Name	Composition	Properties	Uses
Soft solder	37%-67% Lead 31.60% Tin 0.12% Stibium	Melt at low temperatures. Percentage of lead is more and hence it is a soft solder	Used for soldering electrical connections.
Tinman’s solder	66% Tin 34% lead	It melts at 180o C. Percentage of lead is less and hence it is a hard solder	Used for soldering tin articles
Plumber’s solder	67% lead 33% tin	Superior wetting and capillary filling characteristics	Used in making wiped joints and seams
Brazing alloys	92% tin 5.5% lead 2.5% Copper	It melts at comparatively higher temperature	Generally used for steel joints.
Rose Metal	50% Bismuth 25%-28% lead 22-25% tin	It is a fusible alloy. Very low melting point	Used for making fire alarms, dental works, automatic sprinkler system
Wood’s metal	50% Bismuth 26% lead 12.5% Tin 12.5% Cadmium	Very low melting point. It is very dense and easily fusible	Joining two metallic parts, fire alarms, electric fuse wires.