FSc Notes Part 2 Chemistry Important Chemicals Copper (Extraction), Sodium Carbonate (Ammonia-Solvay Process) and Sodium (Down’s Process)

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Copper (Extraction)

Extraction Of Copper

Copper is widely distributed in nature. It occurs in free as well as combined state. Blister copper is mainly extracted from sulphide ores (copper pyrite). The extraction of Blister Copper proceeds through following four steps.

Step I – Concentration

The sulphide ores are concentrated by any of the following methods.

a. Gravity Separation

The powdered ore is passed over a wheel so that it falls in two section due to difference in specific gravity.

b. Forth Floatation Process

The finely powdered copper pyrite ore is introduced into water containing Rine oil. The suspension is vigorously agitated with air. The stony matter settles to the bottom and the ore is carried to the top in the forth. The object of this process is to increase the concentration of copper in the ore.

Step II – Heating / Roasting

The concentrated ore is heated strongly (roasted) in a multiple heater roaster (Reverberatory Furnace) in excess of air in order to achieve the following.

1. To remove sulphur as sulphur dioxide.
2. To remove arsenic and antimony as their volatile oxides.
3. To convert the copper pyrite into a mixture of cuprous sulphide and ferrous sulphide.
4. To partially oxidize cuprous sulphide and ferrous sulphide to copper oxide and ferrous oxides respectively.
5. To remove fusible impurities and moisture.
6. To change the ore into porous form.

Step III – Smelting

The roasted ore is mixed with Silica (SiO2) and smelted in a water jacketed blast furnace about 5ft to 6ft high and 1.6 m in diameter. It is a tower like structure made of steel sheet and lined inside with firebricks. A blast of air is necessary for the combustion of ore. This ore is blown through the pipes provided at the base. Since, most of the heat is produced by the combustion of ore itself, therefore, small amount of fuel is required. The cuprous oxide formed during roasting react with unoxidized ferrous sulphide to form cuprous sulphide and ferrous oxide.

CuO + FeS —-> Cu2S + FeO

Silica act as flux, it reacts with ferrous oxide to form fusible slag (Ferrous Silicate) because iron has more affinity for oxygen than copper.

FeO + SiO2 —-> FeSiO3 (Slag)

Slag is removed from the slag hole while a molten mass containing mostly cuprous sulphide, with a very little ferrous sulphide is taken out from the bottom and is called Matte.

Step IV – Bessemerization

The molten is now transferred into Bessemer converter. It is a pear shaped furnace made of steel plates and provides with a basic lining like that of lime or Magnesium Oxide.

The main features of this process is that air is blasted through the molten matte. As a result, ferrous sulphide still present gets oxidized to ferrous oxide and is removed as slag. Cuprous sulphide is partially oxidized to oxide, which reacts with remaining cuprous sulphide to form metallic copper.

Cu2S + 2Cu2O —-> 6Cu + SO2↑

The molten copper is run into sand moulds and allowed to solidify. When it gives out absorbed SO2, it leaves blister type appearance at the surface of the metal. The metal thus obtained is known as Blister Copper. It is about 98% pure.

Refining

The crude copper obtained by the above methods contain about 2% of impurities consisting of Fe, Ni, Zn and Ag. It is refined by electrolysis. The process is carried out in a large tank lined with lead. Thick plates of crude copper served as anode and thin plates of pure copper act as cathode. The cathode is coated with oil which helps in easy scraping of pure copper. These electrodes are dipped into electrolytic copper sulphate solution. The electrolysis is then carried out by a current of 1.3 volts. The pure copper is deposited at the cathode while impurities like Fe, Ni, Zn passes into solution and other like Ag and Au fall down as anode mud. During electrolysis following reactions occur.

Cu0 —-> Cu11 + 2e- (At Anode)

Cu11 + 2e- —-> Cu0 (At Cathode)

Copper thus obtained is 99.99% pure and is known as Electrolytic Copper.

Sodium Carbonate (Ammonia-Solvay Process)

Introduction

Sodium Carbonate is an important compound. It is manufactured by Ammonia-Solvay Process.

Raw Materials

The raw materials for the manufacture of sodium carbonate are

1. Brine
2. Ammonia, which is made by Haber’s Process
3. CO2, CaOH, which are obtained from limestone.

Ammonia-Solvay Process

The Ammonia-Solvay Process consist of the following steps.

Step I – Ammonation of Brine

In first step, ammonia gas is mixed with brine. This process is carried in Ammonation Tower. The ammonation tower consist of mushroom shaped buffels at short intervals. Brine is introduced from the top and ammonia is introduced from bottom. They both flow towards each other. Buffels control the flow of brine and ensure that they are mixed to the point of saturation with ammonia.

Step II – Carbonation of Ammonated Brine

In this step, ammonated brine is mixed with carbon dioxide brine is mixed with carbon dioxide in a tower called carbonating tower or Sonvai tower. Ammonated Brine is fed from the top where as carbon dioxide ascends from the bottom. When these two substances meet, the following chemical reactions takes place.

2NH3 + CO2 + H2O —-> (NH4)2CO3

(NH4)2CO3 + CO2 + H2O —-> 2NH4HCO3

2NH4HCO3 + NaCl —-> NaHCO3 + NH4Cl

Sodium bicarbonate is relatively insoluble, which is precipitated out from the solution by cooling the lower part of the tower. Sodium bicarbonate is separated from soluble ammonium chloride by vacuum filtration.

Step III – Production of Soda Ash

Sodium bicarbonate is heated in a long iron tube to obtain anhydrous sodium carbonate or Soda Ash.

This carbon dioxide is recycled to the solvay tower. This hydrated sodium carbonate is also called washing soda.

Recovery of Ammonia

Ammonia gas is recovered from the remaining solution by treating it with Calsium Hydroxide.

2NH4Cl + Ca(OH)2 —-> CaCl2 + 2H2O + NH3

Sodium (Down’s Process)

Introduction

On large scale, sodium is manufactured by electrolysis of fused sodium chloride. The process was given by scientist Dawn, therefore, it is called Dawn’s Process.

Construction of Down’s Cell

A special electrolytic cell known as Dawn’s cell is used for the electrolysis of sodium carbonate. The cell consists of a cylindrical shape iron basin lined inside by firebricks. The iron cathode is separated from anode by iron gauze diaphragm. Anode consists of a carbon rod, which is present between two iron cathodes. The electrolytic solution consists of a mixture of sodium chloride (NaCl) and calcium chloride (CaCl2). Calsium Chloride (CaCl2) is added to decrease the melting point of sodium chloride from 801ºC to 600ºC.

Working of Down’s Cell

When electric current is passed through molten NaCl, the sodium ion migrates towards cathode. It gains and electron and converts into molten sodium metal, which floats inside the cathode compartment. This molten sodium metal is allowed to pass through pipe ‘p’ which collects it in a vessel ‘V’ outside the cell