CHEMISTRY OF METALS AND ALLOYS

Metallurgy

• The entire scientific and technological process used for isolation of the metal from its ores

• Those principles shall include the thermodynamic and electrochemical aspects involved in the effective reduction of the concentrated ore to the metal.

Minerals

The compounds of metals in which metal occurs in nature

Minerals

The naturally occurring chemical substances in form of which the metals occur in the earth crust along with impurities

Minerals

a naturally occurring, inorganic solid that has a definite chemical composition and an ordered internal atomic structure

Ores

the mineral from which metal can be extracted easily & economically

Ore

metal associated with non-metallic impurities, rocky materials and siliceous matter which are collectively known as gangue.

• Pyrometallurgy

• Hydrometallurgy

• Electrometallurgy

3 Types of metallurgy

Pyrometallurgy

Extraction by using heat

Hydrometallurgy

Extraction by using aqueous solution

Electrometallurgy

Extraction by using electrolysis process

Free Form (Native Form) & Combined Form

2 types of occurences

Free Form

Less reactive type of occurence

Combined Form

more reactive type of occurence

• Oxide

• Halide

• Sulfide

• Sulfate

• Carbonate

• Phosphate

6 metals found in combined form

K - Potassium
Na - Sodium
Ca - Calcium
Mg - Magnesium
Al - Aluminum
Zn - Zinc
Fe - Iron
Pb - Lead
H - Hydrogen
Cu - Copper
Hg - Mercury

Ag - Silver

Au - Gold
Pt - Platinum

Reactivity series of metals

1. Concentration or dressing of ore

2. Conversion of ore into oxide

3. Conversion of oxide into pure metal

4. Purification of metal

4 Main processes in metallurgy of metals

Concentration or dressing of ore

The preliminary step in metallurgical process in which involves removal of impurities.

1. Gravity separation or Hydraulic wash

2. Froth Flotation

3. Leaching

4. Magnetic separation

4 types of concentration or dressing of ore

Gravity separation or hydraulic wash

• The ore having high specific gravity is separated from the gangue that has low specific gravity by simply washing with running water.

• Ore is crushed to a finely powdered form and treated with rapidly flowing current of water.

• During this process the lighter gangue particles are washed away by the running water.

• This method is generally applied to concentrate the native ore such as gold and oxide ores such as hematite (Fe2O3), tin stone(SnO2) etc

Froth flotation

• In this method, the metallic ore particles which are preferentially wetted by oil can be separated from gangue.

• The crushed ore is suspended in water and mixed with frothing agent such as pine oil, eucalyptus oil etc.

• This method is commonly used to concentrate sulfide ores such as galena (PbS), zinc blende (ZnS) etc

Leaching

• This method is based on the solubility of the ore in a suitable solvent and the reactions in aqueous solution.

• In this method, the crushed ore is allowed to dissolve in a suitable solvent, the metal present in the ore is converted to its soluble salt or complex while the gangue remains insoluble

Magnetic separation

• This method is applicable to ferromagnetic ores and it is based on the difference in the magnetic properties of the ore and the impurities.

Conversion of ore into oxides

• The concentrated ore is converted into the oxide of metal which is then reduced to metal

• This conversion is done by the process of calcination or roasting

• The ore is heated very strongly in the absence of air, keeping the temperature below its melting point so that volatile impurities are driven off.

1. Calcination

2. Roasting

2 types of conversion of ore into oxides

Calcination

• The ore is heated in the absence of air

• It is used for oxide or carbonate ores

Roasting

• The ore is heated in the presence of air

• It is used for sulphide ores

Conversion oxide into pure metal

• Reduction process is used for converting metal – oxides into metal

• The reduction reaction chosen depends on the chemical reaction or reactivity of metals

• Generally reduction by heat,chemical reduction or electrolytic reduction processes are utilized

1. Reduction by heat

2. Chemical Reduction

3. Reduction by carbon

4. Reduction by electrolysis

4 types of conversion of oxide into pure metal

Reduction by heat

Metals that are unreactive, like Hg, can be reduced from their ores by heating them

Chemical Reduction

• Various reducing agents are used for different metal – oxides to obtain free metals

• Carbon, Al, Na, Ca are some reducing agents that are put in use

Reduction by Carbon

• Oxides of Zn, Fe, Ni, Sn, Pb are reduced by heating them with carbon.

• Metal – oxide is mixed with coke, a source of carbon, and heated in a furnace

• Carbon reacts with oxygen and free metal is obtained.

Reduction by Electrolysis

• Metals such as Na, K, Mn, Ca have to be freed from their ores by electrolytic processes

• These metals are so reactive that they themselves are powerful reducing agents. Molten metal – oxides or chlorides form the electrolyte in an electrochemical cell.

• The cathode of the cell provides the electrons needed for the metal to free itself from the metal – oxide or metal – chloride bonds

• In the electrolysis, metals atoms get deposited on the cathode electrodes, which then have to carefully removed and stored

1. Electrolytic Refining

2. Zone Refining

3. Mond’s Process

3 types of purification of metal

Electrolytic Refining

• Applicable to many metals such as Cu, Ag, Pb, Au, Ni, Sn, Zn etc

• The blocks of impure metal form the anode and thin sheets of pure metal form the cathode

• A solution of a salt of the metal is taken as an electrolyte. On passing an electric current through the solution pure metal dissolves from the anode and deposits on cathode

Electrolytic Refining

• By this process, more metal ions undergo reduction and pure metal is deposited at the cathode

• The insoluble impurities either dissolve in the electrolyte or fall at the bottom and collect as anode mud

Zone Refining

• It is based on the principle that melting point of a substance is lowered by the presence of impurities.

• Consequently, when an impure molten metal is cooled, crystals of the pure metal are solidified, and the impurities remain behind the remaining metal

• The process consists in casting the impure metal in the form of a bar

Zone Refining

• A circular heater fitted around this bar is slowly moved longitudinally from one end to the other

• At the heated zone, the bar melts, and as the heater moves on, pure metal crystallizes, while the impurities pass into the adjacent molten part.

• In this way, the impurities are swept from one end of the bar to the other. By repeating the process, ultra pure metal can be obtained.

Mond’s Process

• The carbonyl method is used for the refining of metals like Ni and Fe.

• For example, in case of nickel, the impure metal is heated with CO

• The nickel carbonyl thus formed is then decomposed (after distilling off the impurities) to get pure nickel metal and CO. The process is known as Mond's process.

Alloys

a material composed of a metallic base and additional metal or non-metal elements

Alloys

Substances that are made by mixing two or more elements, at least one of which is a metal

Alloys

can be formed from combinations of metals and other elements

Alloy

is a mixture of chemical elements where the primary component is a metal

Substitutional Alloys and Interstitial Alloys

2 Types of Alloys

Interstitial Alloys

form when the mixed elements in the alloy have different atomic radii, and the smaller atoms fill in the interstitial spaces (holes) in the lattice structure of larger atoms

Substitutional Alloys

form when the mixed elements in the alloy have similar atomic radii. The atoms of one element can substitute into the crystal lattice structure of the other element

Interstitial

If the atoms of the elements being mixed have different radii, ______ alloys will form. This is the case with steel composed of iron and carbon; the much smaller carbon atoms fill in the tiny spaces between the crystal lattice of iron atoms

Substitutional

If the atoms of the metals being mixed have similar radii, ______ alloys will form. The oldest example of a _______ alloy is bronze, made up of copper and tin metal. Some of the copper atoms in the lattice get replaced by the tin atoms.

Substitutional alloys

Formation - Atom substitution

Size of Atoms - Relatively the same size atoms

Types of elements - Usually occurs between metals

Properties compared to metals - Corrosion-resistant, lower melting point, increased hardness

Interstitial Alloys

Formation - Interstitial Filling

Size of atoms - Different sized atoms, There must be at least a 15% difference in size

Types of elements - Usually, one metal has large atoms and another smaller element that fills the interstitial spaces

Properties compared to metals - Less malleable, Less ductile, increased hardness, higher melting point

1. Hard and less malleable

2. Have lower melting points than the component elements

3. Have low electrical conductivity

4. Have corrosion resistance & chemical resistance

4 Properties of alloys

1. Increases the hardness of metals.

2. Lowers the melting point of metals.

3. Increases the corrosion resistance of metals

4. Modifying the chemical activity of metals

5. Modifying the color of metals

6. improves the properties of metals

6 Importance of alloying

Ferrous Alloy and Non-ferrous alloy

2 Classification of Alloys

Ferrous Alloy

______ _______ mainly contain iron as the major constituent along with C, Al, B, Cr, Co, Cu and Mn to improve the properties of steel

Nichrome

is an alloy of Nickel and Chromium.

Nichrome

• Composition: Nickel – 60% Chromium – 12% Iron – 26% Manganese – 2%

• Properties: shows good resistance to heat and oxidation, high melting point, high electrical resistance, and can withstand heat up to 1000 - 1100˚C

• Uses: Used for making resistance coils, heating elements in stoves, making parts of boiler, gas – turbines, aero – engine valves and making electric irons and other household electrical appliances

Alnico

is a family of iron alloys which, in addition to iron, are composed primarily of aluminum (Al), nickel (Ni), and cobalt (Co),hence the acronym al-ni-co. They also include copper, and sometimes titanium. ____ alloys are ferromagnetic, and are used to make permanent magnets.

Stainless Steel

It is otherwise called corrosion resistant steel. It contains chromium, molybdenum and nickel along with iron. It does not corrode due to the formation of dense film of chromium oxide.

Heat treatable stainless steel & Non-heat treatable stainless steel

2 Types of Stainless Steel

Heat treatable stainless steel

• Composition: 1.2% carbon,12 – 16% chromium

• Properties: water and weather resistant; can be used up to 800 ˚C; magnetic, tough & can be used in cold condition.

• Uses: It is used in making blades, scissors and surgical instruments.

Non-heat treatable stainless steel

• They possess less strength at high temperatures.

• They are more resistant to corrosion

Magnetic stainless steel & Non-magnetic stainless steel

2 types of non-heat treatable stainless steel

Magnetic stainless steel

• Composition: It contains 0.35% carbon and 12 – 22% chromium

• Properties: It is more resistant to corrosion than heat treatable stainless steel. It can be forged, rolled and machined

• Uses: It is used in chemical equipment and automobile parts.

Non-magnetic stainless steel

• Composition: 0.15% carbon,18 – 26% chromium, 8 – 21% nickel

• Properties: Resistant to corrosion and is increased by adding a little quantity of Mo Resistance to oxidation

• Uses: It is used in making sinks, utensils, dental and surgical instruments

Heat Treatment

The process of heating and cooling of solid steel article under carefully controlled conditions

1. Improves magnetic and electric properties

2. Improves corrosion resistance

3. Removal of thermal stress

4. Removal of the imprisoned gases

4 Purposes of heat treatment of steel

1. Annealing

2. Hardening

3. Tempering

4. Normalizing

5. Carburizing

6. Nitriding

6 Main processes of heat treatment

Annealing

is done by heating the metal to high temperature, followed by slow cooling in the furnace

Low temperature annealing (process annealing) & High temperature annealing (full annealing)

2 Types of annealing

Low temperature annealing

Heating steel to a temperature below the lower critical temperature followed by slow cooling

High Temperature annealing

Heating steel to a temperature about 30 to 50˚C above the higher critical temperature and holding it at that temperature for sufficient time and then cooling to room temperature

Hardening (Quenching)

It is the process of heating steel beyond the critical temperature and cooling it suddenly either in oil or brine water or any fluid

Tempering

It is the process of heating the already hardened steel to a temperature lower than its own hardening temperature and then cooling it slowly

Normalizing

It is the process of heating steel to a definite temperature and allowing it to cool gradually in air

Carburizing

• Mild steel article is taken in a cast iron box containing small pieces of charcoal.

• It is heated to 900 to 950˚C and kept as such for sufficient time, so that carbon is absorbed.

• Then the article is cooled slowly within the iron box itself

Nitriding

• It is the process of heating the metal alloy in presence of ammonia at a temperature to about 550˚C.

• Nitrogen combines with the surface of alloy to form hard nitride