Metals are electropositive i.e. they ionize by lose of electrons to form positively charged ions. They are therefore reducing agents.
Occurrence of metals
Metals occur in the earth crust as ores.
An ore is a rock which contains metal compounds from which metals can be extracted e.g. rock salt (NaCl) for sodium; spathic iron ore (FeCO3) and iron pyrite (FeS2) for iron.
Extraction of metals
For a metal to be extracted, its concentration in a given ore must be high. There fore, it is necessary to concentrate (purify) the ore before extracting the metal.
The method of metal extraction normally depends on the position of the metal in the electrochemical/reactivity series.
Extraction of metals as a reduction process
Metals in their ores (oxides or other salts) exist in the ionized condition. During extraction, metallic ions in the ores gain the necessary number of electrons to form the corresponding metal atoms. Since the process involves taking up electrons, extraction of metals is essentially a reduction process. For example:
Principle/chief ores of sodium are:
- Rock salt (sodium chloride), NaCl. This is common in salt lakes like lake Katwe in Uganda.
- Soda ash (sodium carbonate), Na2CO3. This is common at lake Magadi in Kenya.
- Sodium nitrate (Chile salt petre), NaNO3. Common in chile.
Sodium is extracted by electrolysis of fused sodium chloride in the Down‘s cell using Downs‘s process. As the melting point of sidium chloride is high (about 800˚C), calcium chloride is added to lower the melting point to about 600˚C.
The Down’s cell
The Down‘s cell has an outer iron shell lined with firebrick. Iron gauze cylinders separate the graphite anode from the ring shaped iron cathode; this prevents the mixing of the products at the different electrodes.
A high current (about 30000 A) is used to keep the electrolyte in a molten state.
Chlorine escapes via the hood. Sodium collects in the inverted trough placed over the cathode, rises up the pipe and is tapped off through the iron vessel. Sodium metal is collected upwardly in the Down‘ cell because of its low density that makes it to float over the reacting mixture.
Reaction at the cathode
At the cathode, sodium ions gain electrons and therefore are discharged to form sodium metal which is deposited in the molten state and collected over dry nitrogen.
Reaction at the anode
At the anode chlorine ions are discharged as they lose electrons to form chlorine atoms. The chlorine atoms pair up forming chlorine molecules (gas) which is collected as a valuable product.
Uses of sodium
- It is used in the manufacture of sodium cyanide (NaCN) which is used in the extraction of gold.
- Manufacture of sodamide (NaNH2)
- Manufacture of sodium peroxide (Na2O2)
- Its alloy is used in the manufacture of anti-knock additives for petrol e.g. tetraethyllead.
- Sodium is used for the withdrawal of heat from some nuclear reactors. It is therefore used as a coolant.
- Sodium is used as a reducing agent in the laboratory.
- Sodium vapour lamps are used for street lighting.
Extraction of intermediate metals
The metals are extracted by reduction and there have four basic stages in the process.
- Concentration of the ore
- Roasting of the ore to convert the carbonates and sulphides to oxides and remove water vapour. It is easier to reduce the oxide than the carbonate.
- Reduction of the ore
- Purification/refining of the metal extracted
Concentration of the ore
This process involves removing impurities. Impurities can be separated by physical means e.g. picking by hand, washing, using a magnet or solvent extraction. In the case of copper, the ore is first crushed and then mixed with water. Air is then blown through the mixture and the clean ore separates and collects at the surface as froth. This is referred to as froth floatation.
- The concentrated (pure) ore is roasted in air at high temperatures to produce oxides which are easier to reduce than carbonates or sulphides. Sulphur is removed as sulphur dioxide and carbon dioxide driven off from the carbonates.
Reduction of the ore
- The ore is usually heated in a furnace in the presence of a suitable reducing agent, usually coke (carbon) or carbon monoxide which converts the ore into the required metal.
Purification/refining of the metal
- The metal obtained is often impure. Purification is normally done by electrolysis in the case of copper and zinc. In other cases, the impure metal is heated in a hearth open to air where the impurities oxidize and rise to the surface as a scum and can be removed.
The principle/chief ores of iron are: Haematite, Fe2O3; Magnetite, Fe3O4; Iron pyrite, FeS2; Siderite or Spathic iron ore (FeCO3); Limonite (Fe2O3.xH2O).
The iron ore is crushed and roasted in air to remove water and other non metallic impurities especially sulphur and phosphorus which are oxidized away as gaseous oxides. The roasted ore is now mainly Fe3O4. When an ore is roasted in air and Fe3O4 is the main product, it is known as sintering
The roasted ore is mixed with coke (carbon) and limestone (calcium carbonate) and introduced into the blast furnace where the reduction of the ore takes place.
The blast furnace
Very hot air is introduced from low down into the blast furnace. As the hot air passes through the mixture of roasted ore, coke and limestone, the coke burns to form carbon dioxide (is oxidized) in an exothermic process.
As the carbon dioxide produced rises through the furnace, it is reduced by the excess hot coke to produce carbon monoxide.
The carbon monoxide formed reduces the iron ore at a high temperature (about 1000˚C) to form iron metal.
The molten iron formed sinks to the bottom of the blast furnace where it is tapped and solidified into blocks of pig iron.
The role of limestone
Limestone removes Silicon(IV) oxide which is the main impurity in the iron ore. Limestone at high temperature decomposes to form calcium oxide and carbon dioxide. The calcium oxide formed combines with silicon (IV) oxide (impurity) to form molten calcium silicate (slag).
The slag being less dense than iron forms a separate layer above iron and thus are tapped separately.
The slag protects the molten iron against any further oxidation by oxygen in the hot air in the blast furnace. The carbon dioxide produced as a bi-product in the furnace is quickly reduced by hot coke to carbon monoxide which is required as a reducing agent.
By products from the blast furnace include: calcium silicate/slag (used for making roads, manufacturing cement, manufacturing glass); waste gases e.g. carbon dioxide, carbon monoxide, steam and unreacted nitrogen; calcium phosphate (used as an in organic fertilizer).
There are three types of iron which are classified based on their percentage purity. The percentage purity also determines the strength and use of the iron. The types of iron are cast iron (pig iron), wrought iron and steel.
This is an impure iron which contains relatively high proportions of carbon (4%) and small proportions of other substances such as silicon, phosphorus and sulphur. Such impurities make cast iron to be hard, brittle and to have a lower melting point than pure iron. It cannot be welded and has little tensile strength.
Cast iron can be used to make hot water pipes, Bunsen burner bases, cookers, in railings and other purposes where little strain is imposed.
This is the purest form of iron (contains about 0.3% carbon) and is obtained from cast iron by heating it with iron(III) oxide in a furnace by a process known as ―puddling‖. The oxygen of the iron oxide oxidizes carbon and sulphur to their respective gaseous oxides, phosphorus to Phosphates(V) and silicon to silicates which form slag. The semi molten mass is then hammered and rolled so that the slag is squeezed out and a mass of almost pure iron remains.
It is very tough, malleable and ductile and is there fore used to make iron nails, sheeting, ornamental work, horse shoes and agricultural implements. Wrought iron is some times referred to as low carbon steel.
Steel is an alloy of mainly iron with carbon and other elements like manganese, chromium, silicon, cobalt and some time tungsten. The quality of steel depends on the amount of carbon present and this in turn determines its intended use.
Steel is generally used in the construction of buildings, bridges, ships, car bodies, cutting and boring tools, crushing machines and stainless cutlery such as knives, forks e.t.c.
The principle/ chief ores of copper are: copper pyrites (CuFeS2); cuprite (Cu2O); copper (I) sulphide (Cu2S) and malachite (CuCO3,Cu(OH)2).
Extraction of copper from copper pyrites
The ore is first concentrated by a process of froth floatation and then it is roasted in air to produce copper(I) sulphide.
By adding silicon dioxide (sand) and heating the mixture in the absence of air, the solid iron(II) oxide impurity is converted into slag which is poured off with the reaction mixture leaving behind only copper(II) sulphide.
The copper(II) sulphide is reduced to metallic copper by heating in a regulated supply of air (oxygen).
The metallic copper produced is impure copper (blister copper) and has to be purified. Purification of the copper is done by the process of electrolysis.
Purification of blister copper
The impure copper/blister copper is made the anode and strip of pure copper serves as the cathode. The electrolyte is acidified solution of copper(II) sulphate. During electrolysis, pure copper is transferred from the impure copper anode to the pure copper cathode. There fore, the anode dissolves and decreases in size as the cathode grows bigger.
Ions above copper in the reactivity series remain in the solution.
Uses of copper
- For wiring electrical circuits since it is a good conductor of electricity and is relatively cheap. The copper used for this purpose must be very pure since impurities increase electrical resistance.
- It is used for making ornaments like ear rings and pins, bungles e.t.c. being little attacked by air.
- It is used for making alloys like bronze (copper and tin) and brass (copper and zinc),copper coinage (copper and tin), German silver (copper, zinc and nickel)
- Used for making water pipes and boilers.
- Copper is used as a roofing material because it weathers to acquire a coating of green basic copper carbonate, CuCO3.Cu(OH)2.nH2O, which lends a colourful touch to the building.
A fertilizer is a substance containing essential elements for the healthy growth of plants. When added to the soil, fertilizers normally improve soil fertility. The most important elements are: nitrogen (N), potassium (K) and phosphorus (P). Some times sulphur.
These elements are absorbed by plants as compounds especially as nitrates and phosphates. There are mainly two types of fertilizers i.e. artificial/inorganic fertilizer and organic fertilizers.
These are synthetic fertilizers that contain a high percentage of essential elements i.e. nitrogen, phosphorus and potassium; the fertilizers should be cheap and soluble in water. The examples of such fertilizers include: ammonium sulphate, ammonium phosphate, potassium nitrate, liquid ammonia and potassium chloride.
- Liquid ammonia is very soluble; it can easily be washed away by rain water and can easily evaporate off the soil.
- A composite/balanced fertilizer contains nitrogen, phosphorus and potassium in certain correct proportions and it is normally labeled NPK.
- Excessive use of ammonium fertilizers renders the soil acidic. It is there fore necessary to add quick lime (calcium oxide) to neutralize the acidity. The added lime also makes the soil more porous for proper air and water absorption.
Organic fertilizers (Farm yard manure)
These are fertilizers from decomposed animal and plant waste matter such as cow dung, chicken droppings, dead animals, dead leaves, plant branches, coffee husks e.t.c.
Advantages of organic fertilizers
- It is cheap since it is prepared by the farmer
- No elaborate instruction for application
- They provide a suitable habitat for nitrifying bacteria
- They last for a long time in the soil thus improving soil texture
- They hold water thus keeping the soil moist even in dry season
- They cannot be used selectively i.e. on soil which lacks a particular nutrient
- They harbor pests and diseases which attack the crop
- They have to be applied in bulk for effectiveness
- They act slowly and are not ready for use any time since they have to be dry first.
Advantages of artificial fertilizers
- They do not habour pests and diseases
- They are available for use whenever needed
- They are effective when used in small amounts
- Different fertilizers for different crops and soil types are usually available
- They normally give good yield
- They are expensive in terms of production costs
- They do not give desired results if the instructions for application are not strictly followed
- They may exhaust soil fertility if used for a long time
- The crop/yield is of inferior quality to one produced using organic fertilizers.
Biogas is a gas with a high methane content which is produced by microbial fermentation of organic wastes.
Biogas is formed by bacterial activities on animal and vegetable wastes. It consists of mainly methane and others gases like; ammonia, hydrogen sulphide, carbon dioxide e.t.c.
A simple biogas generator consists of a container in which animal and vegetable wastes are mixed with a correct (limited) amount of water and then covered to prevent atmospheric oxides. A temperature of about 25˚C to 30˚C is maintained. The bacteria present in the wastes break down (decompose) the waste to form biogas.
The quality of biogas produced depends on the type of waste used. For instance, a mixture of cow dung, human excreta and bean stalks produce high quality biogas.
The biogas produced is mainly used as fuel for cooking purposes and lighting.
Advantages related to biogas production
- Biogas is easy and cheap to produce
- Sewage materials can be converted to biogas
- The solid waste from a biogas plant can be used as an organic fertilizer since it contains a high percentage of nitrogen.
- Forests and wild life would be conserved if institutions and rural communities used biogas instead of wood and charcoal for their fuel needs.
However the main disadvantage related to biogas production is the release of gases such as sulphur dioxide, carbon dioxide which pollute the air.
Sugar is a food or drink sweetener.
Extraction of sugar
In Uganda, sugar is made from sugarcane plant. The sugarcane is cut into small pieces, then crushed and squeezed to force out the juice which contains mainly sucrose.
A little lime is added to prevent the sucrose from hydrolyzing into simple sugars like glucose. The mixture is filtered and the clear filtrate obtained is diluted with correct amount of water. The diluted filtrate is then concentrated by evaporation to crystal formation point. The solution is then allowed to cool as sugar crystals form. This forms brown sugar crystals.
To obtain white sugar crystals, the brown sugar is dissolved in water and the solution boiled with animal charcoal. The mixture is filtered to remove the charcoal and the filtrate evaporated as before to form white sugar crystals.
Sample questions on metal extraction, Biogas and fertilizers
- Define the term ore. Explain why the extraction of metal is regarded as a reduction process. Name one principle ore of sodium and how sodium is extracted from the ore by electrolysis method. Write equations for the reactions at the electrodes. Outline five uses of sodium.
- Name the principle ores of iron. Describe how iron is extracted from a named ore.
- Copper can be extracted from copper pyrite. Describe how copper can be extracted from copper pyrite. Mention the uses of copper. Outline how copper is refined by electrolysis.
- Describe the processes involved in the extraction of sugar from sugar cane. What is meant by biogas and how is biogas produced. Outline the advantages related to biogas production.
- What is a fertilizer? Differentiate between the artificial and organic fertilizers. Give the advantages and disadvantages of artificial and organic fertilizers.