SOURCES OF MATERIALS AND THEIR EXTRACTION

INTRODUCTIONIn the context of the topic ‘Sources of materials and their Extractions’, before starting with the actual description we must clarify the definition of ‘Materials’.

We would like to converge the boundary of word ‘Materials’ to ‘Metals’ only.

We are going to discuss metals only because out of three vast categories of Engineering Materials:

1. Metals

2. Ceramics

3. Polymers

The only materials that can be extracted are the ‘Metals’.

Converging the ‘Materials’ furthermore from category of ‘Metals’, to make the topic more precise we will be discussing the Sources and Extractions of the following metals:

1. Iron

2. Aluminium

3. Copper

IMPORTANCE OF THE SELECTED METALS

GENERAL TERMINOLOGY RELATED TO METALS

Minerals:

A solid element or compound which occurs naturally in the Earth's crust is called a mineral.

Ore:

A mineral from which metals can be extracted profitably is called a metal ore. Profitable extraction means that the cost of getting the metal out of the ore is sufficiently less than the amount of money made by selling the metal. So All Ores Are Minerals But All Minerals Are Not Ores.

The most common metal ores are oxides and sulphides. Metals are obtained from their ores by reduction.

Metals seldom occur in nature in their pure form commonly found as ores (ionic compounds of the metal + a non-metal) Bauxite (aluminum) Cassiterite (tin) Galena (lead) Hematite (iron) Chalcopyrite (copper)

• Native Metals:Gold and platinum occur in the Earth as native metal, which means that they are found as the element, not the compound, and so do not need to be reduced. Silver and copper may also be found as native metal. These metals occur in nature in their pure form.

SOURCESMetals are often extracted from the Earth by means of mining.

Ore is located by prospecting techniques, followed by the exploration and examination of deposits.

Mineral sources are generally divided into surface mines, which are mined by excavation using heavy equipment, and subsurface mines.

Once the ore is mined, the metals must be extracted, usually by chemical or electrolytic reduction. The methods used depend on the metal and their contaminants.

OCCURANCE OF SELECTED METALS

World Iron (Hematite Ore) Reserves.

World Aluminum (Bauxite Ore) Reserves.

World Copper (Chalcopyrite Ore) Reserves.

RECYCLING RESOURCES

Demand for metals is closely linked to economic growth. During the 20th century, the variety of metals uses in society grew rapidly. Today, the development of major nations, such as China and India, and advances in technologies, are fuelling ever more demand.

More and more of the world's metal stocks are above ground in use, rather than below ground as unused reserves. An example is the in-use stock of copper. Between 1932 and 1999,

copper in use in the USA rose from 73g to 238g per person.

Metal ore deposits are a finite resource (there are only a certain amount of them) and non-renewable (once used, they are gone and will not be replaced). Many metals are obtained today from recycling (melting and refining) scrap metals.

Metals are inherently recyclable, so in principle, can be used over and over again, minimizing these negative environmental impacts and saving energy at the same time.

Metal stocks in society can serve as huge mines above ground.

TRADE

The World Bank reports that China was the top importer of ores and metals in 2005 followed by the United States and Japan.

METAL AND ORE IMPORTS IN 2005

EXTRACTION:

• Extraction (chemistry), the separation of a substance from a matrix.

• The method used to extract metals from the ore in which they are found depends on their reactivity. For example, reactive metals such as aluminum are extracted by electrolysis, while a less-reactive metal such as iron may be extracted by reduction with carbon or carbon monoxide.

Reactivity Series of Metals(Carbon and Hydrogen are added for comparison)

Potassium KSodium NaCalcium CaMagnesium Mg

Aluminum Al

Zinc ZnIron FeTin SnLead PbCopper Cu

Mercury HgSilver AgGold AuPlatinum Pt

Ways Of Extraction

Extracted by

electrolysis of molten

chlorides

Extraction by

electrolysis of molten

Al2O3 dissolved in

cryolite

Extraction by

reduction of oxides

using carbon

Roasting ore by

heating alone

Extraction of Iron:

Blast furnace in a modern steel works

• Iron is very reactive and is found in nature in form of its oxides,carbonates and sulphates.

• The main iron ore is Haematite (iron (III) oxide - Fe2O3).

Hematite ore

Raw material Contains Function

iron ore (hematite) iron oxide a compound that contains iron

coke carbon burns in air to produce heat, and reacts to form carbon monoxide (needed to reduce the iron oxide)

limestone calcium carbonate helps to remove acidic impurities from the iron by reacting with them to form molten slag

air oxygen allows the coke to burn, and so produces heat and carbon monoxide

The raw materials for extracting iron and their function in the process

Step1: ConcentrationThe ore is crushed in crushers and is broken to small pieces. It is concentrated with

gravity separation process in which it is washed with water to remove clay, sand, etc.

Step2: CalcinationThe ore is then heated in absence of air (calcined). This results in decomposition of

carbonates into oxides and then ferrous oxide is converted into Ferric Oxide.

322

23

24 COFeOFeO

COFeOFeCO

Step3: SmeltingThe concentrated ore is mixed with calculated quantity of coke, limestone and the

mixture is put in the Blast Furnace from top.

BLAST FURNACE

Blast Furnace:•It is a tall cylindrical furnace made of steel.

• It is lined inside with fire bricks.

• It is narrow at the top and has an arrangement for the introduction of ore and outlet for waste gases.

• Heated with help of Hot Gases.

Chemical Reactions that take place in a Blast Furnace

1. Formation of Carbon Monoxide:

COCOC

HeatCOOC

22

22

2. Reduction of Haematite to Iron:

232 323 COFeCOOFe

3. Functions of limestone:

32

23

CaSiOSiOCaO

COCaOheatCaCO

(slag)

Extraction of Aluminum:

• The current method for extracting aluminum is expensive because it involves several stages and uses large amounts of costly electrical energy.

• Aluminum is very abundant in the Earth's crust but it is always found as very stable compounds .

• Bauxite has the highest concentration of aluminum in these sources and is mined extensively around the world.

Bauxite ore

Raw materials for the electrolysis process:

• Bauxite ore of impure aluminum oxide [Al2O3 made up of Al3+ and O2– ions]

• Carbon (graphite) for the electrodes.

• Cryolite reduces the melting point of the ore and saves energy, because the ions must be free to move to carry the current and less energy is needed to melt the aluminum oxide obtained from the bauxite ore.

Electrolysis:

•When p.d is applied, Al3+ is attracted to the negative cathodeO2- is attracted to the positive anode

•At the cathode, Al3+ gains 3 electrons from the cathode to form molten aluminum, which is tapped off

Al3+(l) + 3e- Al (l)

•At the anode,O2- loses 2 electrons to the anode to form oxygen

2O2-(l) O2(g) + 4e-

Oxygen released attacks carbon anode, to form Carbon monoxide/dioxide. Carbon anode dissolved. Needs to be replaced regularly.

Anodising:•Aluminum when exposed in air forms a thin protective coat of aluminum oxide.•For better protection, a thicker coat is made•Through the process: Anodising •Make aluminum the anode in sulphuric acid bath•Oxygen produced at the anode then combines with aluminum to form a protective porous layer aluminum oxide 1000 times thicker, compared when exposed to air•Pores can be sealed by dipping into hot water or colored by using dyes which can be absorbed into it.

Extraction of Copper:

Chalcopyrite ore

• Copper is extracted usually from the sulphide ore chalcopyrite (CuFeS2) by smelting process.

•Ores containing 4% or more copper are treated by smelting process.

There are following steps for the Hydrometallurgical extraction of copper from Chalcopyrite:

1. CONCENTRATIONCrushed ore is suspended in water containing pine oil. The pine oil binds to the copper compounds, but not to the unwanted rocky material. A blast of air is passed through the suspension. The particles of ore get wetted by oil and floats as a froth which is skimmed. The gangue sinks to the bottom.

2. ROASTINGThe concentrated ore is roasted in the furnace in the presence of oxygen. Sulfur is oxidized to SO2 and impurities of arsenic and antimony are removed as volatile oxides. The following reactions take place:

2CuFeS2 + O2 Cu2S + 2FeS + SO2

S + O2 SO2

4As + 3O2 2As2O3

4Sb + 3O2 2Sb2O3

Cu2S + O2 Cu2O + SO2

FeS + O2 FeO + SO2

3. SMELTINGThe roasted ore is mixed with coke and silica sand (SiO2) and is introduced into a blast furnace. The hot air is blasted and FeO is converted into ferrous silicate (FeSiO3)

FeO + SiO2 FeSiO3

Cu2O + FeS Cu2S + FeO

FeSiO3 (slag) floats over the molten matte of copper.The resulting product of smelting is the combination of copper sulfate and iron sulfide called matte (Copper matte).

Bessemer

4. BESSEMERIZATIONCopper metal is extracted from molten matte through bessemerization . The matte is introduced in to Bessemer converter which uphold by tuyeres. The air is blown through the molten matte. Blast of air converts Cu2S partly into Cu2O which reacts with remaining Cu2S to give molten copper.

BLISTER COPPERThe product of Bessemerization is called as “Blister Copper” because, as it solidifies, SO2 hidden in it escapes out producing blister on its surface.

2Cu2O + Cu2S 6Cu + SO2

2Cu2S + 3O2 2Cu2O + 2SO2

5. REFINING OF BLISTER COPPER• Blister copper is refined by electrolysis. • The copper is cast into copper anodes and placed in an electrolytic cell. Once charged, the pure copper collects on the cathode and is removed as 99% pure. • Blocks of blister copper are used as anodes and thin sheets of pure copper act as cathodes. The cathode plates are coated with graphite in order to remove depositing copper. • The electrolyte is copper sulfate (CuSO4) mixed with a little amount of H2SO4 to increase the electrical conductivity. • Optimum potential difference is 1.3 volt for this electrolytic process.

Cu Cu+2+2e-

Cu+2+2e- Cu

Fe(s) → Fe2+(aq) + 2e–

Zn(s) → Zn2+(aq) + 2e–

EXTRACTION OF PRECIOUS METALS FROM ANODE SLUDGEImpurities, such as Ag, Au, and Pt, are less easily oxidized than Cu. These remain in metallic form and fall to the bottom of the cell, forming “anode sludge” from which they can later be recovered. The anode sludge from copper-refining cells provide one fourth of U.S. silver production and about one eighth of U.S. gold production.

Conclusion:

From the above report we learned about the importance of Metallic Materials. We selected 3 extensively used metallic elements that are iron, aluminum and copper, took notice of their existence and sources, their ores and the extraction processes the ores must go through so that we may attain pure metals to be used for Engineering purposes in industries.

END