Occurrence of Metals

The main source of metals is the earth's crust. Most metals are quite reactive. Therefore, they do not occur as free elements in nature. The less reactive metals like copper, silver and gold are found in the free state (native state) as well as in the combined state (in the form of compounds). All the metals placed above copper in the reactivity series are found in nature only in the form of their compounds. Silver, gold, platinum, ruthenium, and iridium are known as noble metals because of their lack of reactivity.

They are found in the native state.

Minerals and Ores

Minerals are the natural materials in which the metals or their compounds are found in the earth. Those minerals from which metals can be extracted conveniently and profitably are called ores. Thus, all ores are minerals, but all minerals are not ores. Commercially, the most important sources of metals are oxide, sulphide and carbonate minerals. The table below shows the ores of some metals.

Ores of Some Common Metals

Extraction of Metals

The various processes involved in the extraction of metals from their ores and refining are known as Metallurgy. The process used depends on the metal. The three major steps involved in the extraction of a metal from its ore are:

1. Concentration of Ore: This step involves the removal of unwanted impurities (called gangue), like sand, limestone, mica, stones, etc. from the ore.
 

2. Conversion of Concentrated Ore into Metal: Different methods are used for extracting metals, depending on their reactivity. The highly reactive metals, like potassium, sodium, calcium, magnesium and aluminium are extracted by the electrolytic reduction of their molten chlorides or oxides. During electrolysis the metal is produced at the cathode (negative electrode) and chlorine or oxygen at the anode (positive electrode). For example,

The moderately reactive metals, such as manganese, zinc, iron, tin, lead and copper are extracted by the reduction of their oxides with carbon, aluminium, sodium or calcium. It is easier to obtain metals from their oxides (by reduction) than from carbonates of sulphides. The concentrated ores are, therefore, first converted into metal oxides. Calcination is the process in which a carbonate ore is heated strongly in the absence of air to convert it into metal oxide. For example,

Roasting is the process in which a sulphide are is strongly heated in the presence of air to convert it into metal oxide. For example,

Oxides of zinc, iron, nickel, tin, lead and copper are reduced by carbon (in the form of coke), whereas oxides of manganese and chromium are reduced by aluminium powder.

ZnO + C → Zn + CO
3MnO₂ + 4Al → 3Mn + 2Al₂O₃ + Heat

The reduction of a metal oxide by aluminium powder is a highly exothermic reaction and is called thermite reaction. This properly is used in joining the broken pieces of heavy iron objects, like railway tracks or cracked machine parts. This is done by igniting a mixture of iron (III) oxide and aluminium powder with a burning magnesium ribbon. Due to the evolution of heat, iron metal is produced in the molten state, which is poured between the broken iron pieces to weld them. This is called aluminothermy or thermite welding.

The less reactive metals, like mercury and copper, are extracted by the reduction of their oxides by heat alone. The sulphide ores of mercury and copper are roasted in air when their oxides are formed. On strong heating the oxides get reduced to metals. For example,

3. Refining of Metals: The process of purifying impure metals is called refining of metals. Different methods are used for refining different metals. The most widely used method is electrolytic refining. In this method, a thick block of impure metal is made the anode, a thin strip of pure metal is made the cathode, and a water soluble salt (of the metal to be refined) is taken  as the electrolyte. On passing electric current, impure metal dissolves from the anode and goes into the electrolyte solution. Pure metal from the electrolyte deposits on the cathode. The soluble impurities go into the solution whereas the insoluble impurities settle down at the buttom of the anode as anode mud.

If an ore gives carbon dioxide on heating, or on treatment with a dilute acid, then it is a carbonate ore. If an ore gives sulphur dioxide on heating in air or gives hydrogen sulphide gas on treatment with a dilute acid, then it is a sulphide ore.

Prevention of Rusting of Iron

Rusting of iron can be prevented if damp air is not allowed to come in contact with iron objects. This can be iron surface:

• Painting the iron surface
• Applying grease or oil
• Galvanisation, i.e., depositing a thin layer of zinc metal on iron objects. Iron sheets used for making buckets, drums, dust-bins and sheds and iron pipes used for water supply are galvanised to prevent rusting. Zinc reacts with air to form a thin coating of zinc oxide.
• Tin-plating and chromium-plating as tin and chromium are resistant to corrosion. Steel tiffin boxes are plated with nickel or tin. Chromium plating is done on taps, bicycles handles and car bumpers made of iron and steel.
• Alloying it to make stainless steel. Stainless steel is obtained when iron is alloyed with chromium and nickel. Cooking utensils, knives, scissors and surgical instruments, etc., are made of stainless steel.

Alloy

An alloy is a homogeneous mixture of two or more metals or a metal and small amount of non-metals. The properties of an alloy are different from the properties of the constituent metals. Alloys are stronger and/or harder, and have lower melting points than the constituent metals. They are  more resistant to corrosion and have lower electrical conductivity than pure metals. Some of the common alloys are:

• Duralium (or Duralumin), an alloy of aluminium with copper, magnesium and manganese, is used for making aircraft bodies and parts, space satellites and kitchenware.
• Magnalium, an alloy of aluminium with magnesium, is used to make balance beams and light instruments.
• Steel, an alloy of iron with carbon, is used for making nails, screws, bridges, railway lines, ships, vehicles, buildings, etc.
• Stainless steel, an alloy of iron with chromium and nickel, is used for making cooking utensils, knives, scissors, tools, surgical instruments etc.
• Brass, an alloy of copper (80%) and zinc (20%), is golden in colour. It is used for making utensils, nuts, bolts, screws, wires, tubes, ornaments, instruments, fancy lamps and flower vases.
• Bronze, an alloy of copper (90%) and tin (10%), is used for making coins, statues, medals, utensils and propellers of ships.
• Solder, an alloy of lead (50%) and tin (50%), has a low melting point. It is used for welding (soldering) electrical wires together.
• Amalgam is an alloy of mercury with one or more other metals. An amalgam consisting of mercury, silver, tin, and zinc is used by dentists for fillings in teeth.
• Alloys of gold with small amounts of silver and copper are used for making jewellery. Prue gold (known as 24 carat gold) is very soft due to which it is not suitable for making jewellery. Ornaments are usually made of  22 carat gold (i.e., 22 parts of pure gold alloyed with 2 parts of either silver or copper).

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