Extraction of Metals
Introduction
The large scale processes involved in the extraction of pure metals from their respective ore is called metallurgy. There is several numbers of metallurgical processes involved in the process of extraction of different metals and other various compounds in industry and laboratory.
For the extraction of metals the following steps and processes include: Concentration of ore. The process of removal of gangue, the rocky impurities like SiO2 present in an ore is called concentration of ore or ore dressing and purified ore is called concentrate. It can be done with the following processes: Gravity Separation, Froth floatation, Electromagnetic separation, and so on. Then it will be followed by Extraction of metals from the concentrated ore. To this processes it involves the reduction process, reduction process, and refining of the metals. In order to refine the metals it should undergo distillation, liquation, and oxidation. Finally it will have to undergo Electro-refining to pure metal and compounds.
This very assignment purely contains extraction of Aluminium, Copper, Zinc, and Iron. It too has their properties- chemical and physical and applications as well. Moreover, this assignment encompasses of commercial preparation of Ammonia and Nitric Acid. The industrial preparation of Cement and preparation of Glass can also be found here.
By doing this assignment I assure that I will be able to convince and beware that any readers and learners of this very assignment would be very useful in their practical knowledge to continue and carry out their further studies hereafter.
Extraction of metal and its process
The large scale processes involved in the extraction of pure metals from their respective ore is called metallurgy. Extraction of metals from is ore take place in the following ways;
(i). Concentration of ore. The process of removal gangue, the rocky impurities like SiO2 present in an ore is called concentration of ore or ore dressing and purified ore is called concentrate. It involves the following ways:
· Gravity Separation: Where the difference in the densities of ore and gangue is the main. The ore is poured over a vibrating slopped table with grooves and jet of water is allowed to flow over it. The denser ore particles settle down in the grooves and lighter gangue washed down by water.
· Froth floatation: this process depends on preferential wettability of the ore and the gangue particles. The crushed ore is taken in a large tank containing oil and water and agitated with a current of air. The ore is wetted by the oil and separates from the gangue in the form of froth.
· Electromagnetic separation: this method is used to separate ores which are magnetic in nature. Crushed ore is placed over conveyer belt, which rotates around tow metal wheels, one of which is magnetic. The magnetic particles are attracted to the magnetic wheel and fall separately apart from the non magnetic particles.
(ii) Extraction of metals from the concentrated ore. The metal is obtained from the concentrated ore by chemical reduction or electrolytic process.
Reduction process
In the reduction process it is the oxide ore that is reduced. If the ore is not an oxide ore it is just converted to the oxide by roasting or by calcinations. Roasting is the process of heating the concentrated ore to the high temperature in excess of air. Roasting is usually done in sulphide ores like zinc blende (ZnS), and galena (PbS). A part of zinc sulphide is converted to zinc sulphate by heating to a temperature of 800- 9000C at which ZnSO4 decomposes back to zinc oxide. If the ore is carbonate or hydrated oxide it is heated in the absence of air at a temperature insufficient to melt the ore. This process is called calcinations.
Reduction process
The metallic oxide obtained is then reduced by carbon in the form of coke, carbon monoxide, or hydrogen. Oxides of potassium, calcium, sodium, magnesium, and aluminium cannot be reduced by carbon and carbon monoxides or hydrogen. These metals are placed on the top in the metal activity series, they are very reactive and have great affinity towards oxygen and so cannot be reduced by reducing agents.
(iii) Refining of the metals.
It is the separation of the above extracted metal from the residual impurity which is carried out using the following processes.
Distillation
Metals like mercury and zinc which is volatile distils over in pure form and the non volatile impurity remains behind.
Liquation
Metals like lead and tin have low boiling points, so they are heated on the sloping hearth of a furnace. The molten or fused metals flows away leaving behind the impurities.
Oxidation
Metals like iron are purified by this method. The volatile oxides of phosphorous, sulphur and other impurities rise to the surface and are removed while the molten metal remains behind.
Electro-refining
It is the widely used process. The impure slab of the metals is made the anode, while a pure thin sheet of metals is made the cathode. Electrolyte used is a salt solutions of a metal, which is to be refined. Pure metal deposits at the cathode and impurities settle down forming anode mud.
ALUMINIUMExtracting aluminium from bauxite
Introduction
Aluminium is too high in the electrochemical series (reactivity series) to extract it from its ore using carbon reduction.
Instead, it is extracted by electrolysis. The ore is first converted into pure aluminium oxide and this is then electrolyzed in solution in molten cryolite. The aluminium oxide has too high a melting point to electrolyze on its own.
Aluminium ore
The usual aluminium ore is bauxite. Bauxite is essentially an impure aluminium oxide. The major impurities include iron oxides, silicon dioxide, and titanium dioxide.
Purifying the aluminium oxide - the Bayer Process
Reaction with sodium hydroxide solution
Crushed bauxite is treated with moderately concentrated sodium hydroxide solution. The concentration, temperature, and pressure used depend on the source of the bauxite and exactly what form of aluminium oxide it contains. Temperatures are typically from 140°C to 240°C; pressures can be up to about 35 atmospheres.
High pressures are necessary to keep the water in the sodium hydroxide solution liquid at temperatures above 100°C. The higher the temperature, the higher the pressure needed.
With hot concentrated sodium hydroxide solution, aluminium oxide reacts to give a solution of sodium tetrahydroxoaluminate.
The impurities in the bauxite remain as solids. For example, the other metal oxides present tend not to react with the sodium hydroxide solution and so remain unchanged. Some of the silicon dioxide reacts, but goes on to form a sodium aluminosilicate which precipitates out.
All of these solids are separated from the sodium tetrahydroxoaluminate solution by filtration.
Precipitation of aluminium hydroxide
The sodium tetrahydroxoaluminate solution is cooled, and "seeded" with some previously produced aluminium hydroxide.
Formation of pure aluminium oxide
Aluminium oxide (sometimes known as alumina) is made by heating the aluminium hydroxide to a temperature of about 1100 - 1200°C.
Conversion of the aluminium oxide into aluminium by electrolysis
The aluminium oxide is electrolyzed in solution in molten cryolite, Na3AlF6. Cryolite is another aluminium ore, but is rare and expensive, and most is now made chemically.
The electrolytic cell. The diagram shows a very simplified version of an electrolysis cell.
Although the carbon lining of the cell is labelled as the cathode, the effective cathode is mainly the molten aluminium that forms on the bottom of the cell.
Molten aluminium is syphoned out of the cell from time to time, and new aluminium oxide added at the top.
The cell operates at a low voltage of about 5 - 6 volts, but at huge currents of 100,000 amps or more. The heating effect of these large currents keeps the cell at a temperature of about 1000°C.
The electrode reactions
Aluminium is released at the cathode. Aluminium ions are reduced by gaining 3 electrons.
Oxygen is produced initially at the anode.
However, at the temperature of the cell, the carbon anodes burn in this oxygen to give carbon dioxide and carbon monoxide.
Properties of Al
Physical properties
Al is a bluish white lustrous metal. Its density 2.7g/cc.
It is malleable and ductileIt is good conductor of heat and electricity
Its M.P= 6600C and B.P=18000C
Chemical properties
(i) Action of air
Al is not affected by air but in the moist a thin film of oxide is formed over its surface. It burns in oxygen producing brilliant light.
4Al +3O2 2Al2O3
(ii) Action of water
Pure Al is not affected by pure water. The pure Al is readily corroded by water containing salts. Al decomposes boiling water evolving hydrogen.
(iii)Action of acids
Al being strongly electropositive, very reactive and powerful reducing agent. It dissolves in HCL and dilute sulphuirc acid evolving hydrogen.
2Al +6HCl 2AlCl3 +3H2
2Al + 3H2SO4 Al2(SO4)3 + 3H2
In conc. H2SO4
2Al + 6H2SO4 Al2(SO4)3 +3SO2 +6H2O
(iv) Action of alkalies
Al is attacked by caustic alkalies with the evolution of hydrogen.
2Al + 2NaOH + 2H2O 2NaAlO2 + 3H2
Sodium meta aluminate
Copper extraction
Hydrometallurgical extraction
Oxidized copper ore bodies are with hydrometallurgical processes treat oxide ores dominated by copper carbonate minerals such as azurite and malachite, and other soluble minerals.
Such oxide ores are usually leached by sulphuric acid, usually using a heap leach or dump leach process to liberate the copper minerals into a solution of sulphuric acid laden with copper sulphate in solution. The copper sulphate solution is then stripped of copper via a solvent extraction, with the barred sulphuric acid recycled back on to the heaps. Commonly sulfuric acid is used as a leachant for copper oxide.
Froth flotation generally is not used to concentrate copper oxide ores.
Roasting
In the roaster, the copper concentrate is partially oxidized to produce calcine and sulphur dioxide gas. The reaction takes place as follows:
2CuFeS2(s) + 3O2(g) → 2FeO(s) + 2CuS(s) + 2SO2(g)
Smelting
The calcine is then mixed with silica and limestone and smelted at 1200 °C to form a liquid called copper matte.
Reactions as follows ;
For example iron oxides and sulphides are converted to slag which is floated off the matte.
FeO(s) + SiO2 (s) → FeO.SiO2
In a parallel reaction the iron sulphide is converted to slag:
2FeS (l) + 3O2 + 2SiO2 (l) → 2FeO.SiO2 (l) + 2SO2(g)
Conversion to blister
The matte, which is produced in the smelter, contains around 70% copper primarily as copper sulphide as well as iron sulphide. The sulphur is removed at high temperature as sulfur dioxide by blowing air through molten matte:
2Cu2S + 3O2 → 2Cu2O + 2SO2
Cu2S + 2Cu2O → 6Cu + SO2
In a parallel reaction the iron sulfide is converted to slag:
2FeS + 3O2 → 2FeO + 2SO2
2FeO + 2SiO2 → 2FeSiO3
The end product is (about) 98% pure copper known as blister because of the broken surface created by the escape of sulfur dioxide gas as the copper ingots are cast. By-products generated in the process are sulfur dioxide and slag.
Reduction
The blistered copper is put into an anode furnace to get rid of most of the remaining oxygen. This is done by blowing natural gas through the molten copper oxide. When this flame burns green, indicating the copper oxidation spectrum, the oxygen has mostly been burned off. This creates copper at about 99% pure.
Electrorefining
Apparatus for electrolytic refining of copper
The copper is refined by electrolysis. The anodes cast from processed blister copper are placed into an aqueous solution of 3-4% copper sulphate and 10-16% sulphuric acid. Cathodes are thin rolled sheets of highly pure copper. A potential of only 0.2-0.4 volts is required for the process to commence. At the anode, copper and less noble metals dissolve.. Copper (II) ions migrate through the electrolyte to the cathode. At the cathode, copper metal plates out but less noble constituents such as arsenic and zinc remain in solution. The reactions are:
At the anode: Cu(s) → Cu2+(aq) + 2e–
At the cathode: Cu2+(aq) + 2e– → Cu(s)
Properties
Physical properties
It has red color. It is highly malleable and ductile. It has the density of 8.93 and melting point is 1083 0C and boiling point is 2320 0C. It is a good conductor of heat and electricity.
Chemical Properties
Action of air: copper is not affected by dry air at ordinary temperature but moist air and in presence of carbon dioxide, slowly converted to green basic carbonate.
2Cu + H 2O + CO2 + O2 CuCO3.Cu (OH) 2
Action of non-metals:
(a) Oxygen. On heating(up to 11000C) copper forms first red cuprous oxide (Cu 2O) and on further heating it gives black cupric oxide(CuO)
4Cu + O2 2Cu 2O
2Cu + O2 2CuO
(b) Chlorine. A heated copper foil burn in chlorine
Cu + Cl2 CuCl2
Bromine and iodine also combine with copper similarly.
Action of acid: the action of acids on copper is important. Since copper is below hydrogen in electrochemical series, hydrogen do not evolved.
(a) Sulphuric acids. Dilute sulphuric acid reacts in presence of air or oxygen.
2Cu + 2H2SO4 + O2 CuSO4 + 2H2O
When heated in concentrated sulphuric acids it produce sulphur dioxide
2Cu + H2SO4 CuSO4 + SO 2 + 2H 2O
(b) Hydrochloric acid. Dilute hydrochloric acid reacts in presence of air.
2Cu + 4HCl + O2 2CuCl2 + 2H2O
Action of ammonia: copper has no action on nitrogen. When however ammonia gas is passed over red hot copper, hydrogen gas is liberated while nitrogen of ammonia is absorbed by copper metal.