B.Sc. 2nd Year Chemistry — Inorganic Chemistry
Metals are naturally occurring substances that are generally characterized by lustre, high electrical and thermal conductivity, malleability and ductility. Most metals do not occur in nature in a free state. They are usually present in the form of minerals and ores combined with other elements.
The process of obtaining a metal from its naturally occurring ore and converting it into a useful form is known as metallurgy. The metal obtained after extraction is often not completely pure. It contains small amounts of unwanted substances called impurities.
Therefore, the crude or impure metal must be purified before it can be used for industrial, laboratory or technological purposes. The process of removing impurities from a crude metal is called refining of metals.
A mineral is a naturally occurring substance containing a metal or its compound in the earth's crust.
An ore is a mineral from which a metal can be extracted conveniently and economically by suitable metallurgical processes.
The unwanted earthy or rocky materials such as sand, clay and other impurities associated with an ore are collectively called gangue or matrix.
A flux is a substance added during metallurgical operations to react with gangue and form an easily fusible material called slag.
Slag is the fusible product formed by the reaction between flux and gangue during metallurgical operations.
Metallurgy is the science and technology concerned with the extraction of metals from their ores, purification of the metals and preparation of useful alloys.
Refining is the process of removing remaining impurities from crude metal to obtain a metal of high purity.
Metals occur in the earth's crust in different forms. Less reactive metals may occur in the native or free state, whereas highly reactive metals generally occur in the combined state.
The occurrence of a metal depends mainly on its chemical reactivity. Metals such as gold, platinum and sometimes copper and silver may occur in the native state. Highly reactive metals such as sodium, potassium, calcium and aluminium are generally found in the combined state.
| Metal | Common form of occurrence |
|---|---|
| Gold | Native state and minerals |
| Silver | Native state and sulphide ores |
| Iron | Oxides, carbonates and sulphides |
| Aluminium | Oxide minerals such as bauxite |
| Zinc | Sulphide, carbonate and oxide ores |
| Copper | Sulphide and oxide minerals |
Minerals containing sufficient quantities of a metal from which the metal can be extracted economically are called ores. All ores are minerals, but all minerals are not ores.
| Metal | Important ore | Formula |
|---|---|---|
| Iron | Hematite | Fe₂O₃ |
| Iron | Magnetite | Fe₃O₄ |
| Aluminium | Bauxite | Al₂O₃·2H₂O approximately |
| Zinc | Zinc blende | ZnS |
| Lead | Galena | PbS |
| Copper | Copper pyrites | CuFeS₂ |
Metallurgy involves a series of physical and chemical operations through which a metal is obtained from its ore and purified.
The exact procedure depends on the nature of the ore, the reactivity of the metal and the physical and chemical properties of the impurities.
The ore obtained from mines usually contains large pieces of rocks and minerals. It is therefore first broken into smaller pieces by crushing.
The crushed ore is then ground into fine particles to increase the surface area and facilitate the separation of the valuable mineral from the gangue.
Crushing and grinding are mainly physical operations and do not normally involve chemical changes in the ore.
The ore obtained from the mine contains unwanted materials such as sand, clay, rocks and other minerals. The process of removing these unwanted materials from the ore is called concentration of ore or ore dressing.
Concentration increases the proportion of the desired mineral and makes subsequent extraction processes more efficient.
Hydraulic washing is based on the difference in densities of the ore particles and gangue particles. The powdered ore is washed with a stream of water. Heavier ore particles settle, while lighter gangue particles are carried away with water.
Magnetic separation is based on differences in magnetic properties. A magnetic substance can be separated from non-magnetic impurities using a magnetic separator.
For example, magnetic separation may be used in the concentration of magnetite ore.
Froth flotation is commonly used for the concentration of sulphide ores. The powdered ore is mixed with water and suitable reagents and agitated with air. The sulphide mineral particles attach to the air bubbles and rise to the surface as froth, while many gangue particles remain in the aqueous phase.
Leaching is a chemical method in which the ore is treated with a suitable reagent that selectively dissolves the desired component while leaving unwanted materials behind.
Leaching is important in the extraction of metals such as aluminium and gold.
Calcination is the process of heating an ore strongly below its melting point in the absence or limited supply of air. It is mainly used for carbonate and hydrated ores.
During calcination, moisture and volatile substances are removed and carbonate ores are generally converted into oxides.
Calcination also helps in making the ore more suitable for the subsequent reduction process.
Roasting is the process of heating an ore strongly below its melting point in the presence of excess air or oxygen. It is particularly important for sulphide ores.
During roasting, sulphide ores are converted into oxides and sulphur is generally removed in the form of sulphur dioxide.
Reduction is the process by which the metal compound, generally a metal oxide, is converted into the free metal. The method of reduction depends on the reactivity of the metal.
Some metal oxides can be reduced by carbon at high temperature.
Carbon monoxide acts as a reducing agent for several metal oxides.
Certain metal oxides can be reduced by hydrogen to form the corresponding metal and water.
Highly reactive metals cannot generally be obtained by reduction with carbon because their compounds are very stable. Such metals are commonly extracted by electrolysis of suitable molten compounds.
Aluminium is an important example of a metal obtained by electrolytic reduction.
The metal obtained after extraction usually contains impurities such as other metals, non-metallic substances and unreacted materials. Such crude metal is purified by suitable refining methods.
The choice of refining method depends on the physical and chemical properties of the metal and its impurities.
Liquation is a method of refining metals based on the difference in melting points of the metal and its impurities.
The impure metal is heated to a temperature slightly above the melting point of the metal but below the melting point of the impurities. The metal melts and flows away, leaving behind higher-melting impurities.
This method is suitable for metals having relatively low melting points and impurities with higher melting points.
Distillation is based on the difference in volatility or boiling points of the metal and its impurities.
The impure metal is heated and converted into vapour. The vapour is then cooled and condensed to obtain purified metal.
Electrolytic refining is one of the most important methods for obtaining highly pure metals. It is based on electrolysis of a suitable solution containing ions of the metal to be purified.
In electrolytic refining, the impure metal is generally used as the anode, a thin sheet of pure metal is used as the cathode, and a suitable solution containing the salt of the metal is used as the electrolyte.
In the electrolytic refining of copper, impure copper is used as the anode, a thin sheet of pure copper is used as the cathode and acidified copper sulphate solution is commonly used as the electrolyte.
Copper from the anode dissolves into the electrolyte and an equivalent amount of copper is deposited on the cathode. Insoluble impurities may collect as anode mud.
Zone refining is a highly effective method for producing very pure metals and semiconducting materials. It is based on the difference in solubility of impurities in the solid and liquid phases of the metal.
An impurity is preferentially soluble in the molten state of the metal rather than in the solid state. When a narrow molten zone is moved slowly along a rod of impure metal, impurities tend to move with the molten zone.
Vapour-phase refining involves converting the impure metal into a volatile compound and then decomposing that compound to obtain pure metal.
The method is useful when the metal can form a suitable volatile compound while its impurities cannot.
Nickel can be purified by the Mond process. In this process, nickel reacts with carbon monoxide to form volatile nickel tetracarbonyl.
Nickel tetracarbonyl is then heated at a higher temperature, where it decomposes to produce pure nickel.
In oxidative refining, impurities are selectively oxidized and removed from the crude metal. The method is particularly useful when the impurities are more readily oxidized than the metal being purified.
Air or oxygen may be passed through the molten metal so that suitable impurities are converted into oxides. These oxides may either escape as gases, form a separate layer or be removed as slag.
| Method | Principle | Examples / Applications |
|---|---|---|
| Liquation | Difference in melting points | Tin, lead |
| Distillation | Difference in volatility or boiling point | Zinc, mercury |
| Electrolytic refining | Electrolysis | Copper and several other metals |
| Zone refining | Different solubility of impurities in solid and liquid phases | Silicon, germanium |
| Vapour-phase refining | Formation and decomposition of volatile compound | Nickel |
| Oxidative refining | Selective oxidation of impurities | Suitable crude metals containing oxidizable impurities |
Copper is commonly purified by electrolytic refining. Impure copper acts as the anode and pure copper is deposited at the cathode.
Nickel can be purified by vapour-phase refining using the formation and thermal decomposition of nickel tetracarbonyl.
Zone refining is used to obtain very high purity semiconductor materials such as silicon and germanium.
Distillation can be used for purification of relatively volatile metals such as zinc and mercury.
Refining is essential because metals obtained directly from ores generally contain impurities that may adversely affect their physical, chemical and electrical properties.
Metallurgy is the science and technology of extracting metals from their ores, purifying them and preparing useful metal products and alloys.
An ore is a mineral from which a metal can be extracted conveniently and economically by suitable metallurgical methods.
Gangue is the unwanted earthy or rocky material associated with an ore.
Flux is a substance added during metallurgical operations to react with gangue and form slag.
Refining is the process of removing impurities from crude metal to obtain highly pure metal.
Zone refining is a purification method based on the preferential solubility of impurities in the molten phase compared with the solid phase.
Anode mud is the insoluble residue that settles below the anode during electrolytic refining and may contain valuable metals.