A mineral is a solid chemical compound that occurs in nature in pure form. A rock can have a single mineral or also have a mixture of minerals and impurities. Minerals generally occur in forms of ores.
Types of Ores
The ores can be divided into one of the following groups,
1. Halide ores
In nature, there are very few metallic halides; the most common are the chlorides. Examples include carnallite KCl. MgCl2.6H2O, horn silver (AgCl), fluorspar (CaF2), etc
2. Native ores
The metal is present in the Free State in these ores. Examples include gold, silver, etc. They are generally formed along with a rock or alluvial impurities such as sand, clay, etc.
3. Oxidized ores
These ores comprise of oxysalts or oxides. Examples include phosphates, carbonates. Few of the important oxidized ores are Al2O3.2H2O, Fe2O3, etc and important carbonate ores are Calamine (ZnCO3), limestone (CaCO3), etc
4. Sulphurised ores
These ores have sulfides of metals like lead, mercury, iron, etc. Few examples include Cinnabar (HgS), iron pyrites (FeS2), galena (PbS), etc.
The process of extracting a metal from its ores is known as metallurgy. The following steps are followed to obtain metal in its pure form.
• Crushing of the ore
• Dressing or concentration of the ore.
• Reduction of metal.
• Purification or refining of the metal
There are various methods that are used for the concentration of the ore. They are,
a. Physical Method
• Gravity separation
In this method, the powdered ores are washed with a running stream of water or are agitated with water. The heavy ore particles such as particles of sand, clay, etc are washed away.
• Froth floatation process
Finely divided ore particles are introduced into the water which has a small quantity of oil, for example, pine oil. Then the mixture is agitated roughly with air, which results in the formation of froth and this carries away metallic particles. Then this froth is transferred to a different bath in which gangue- free ore settles down.
• Electromagnetic separator
This method consists of a belt that moves over two rollers, and out of the two rollers, one of them is magnetic. The powdered ore is then dropped onto the belt from one end. Magnetic part of the ore gets attracted to the magnetic roller and thus falls near the roller, whereas, the nonmagnetic impurity falls away from the roller.
b. Chemical methods
This method is used for hydrated oxide or carbonate ores which are subjected to the action of heat so that the excess carbon dioxide can be expelled from carbonate ores and water from hydrated ores. Examples:
CaCO3 --> CaO + CO2 ZnCO3 --> ZnO + CO2 2Fe2O3×3H2O --> 2Fe2O3 + 3H2O Al2O3×2H2O --> Al2O3 + 2H2O
This method is used for sulfide ores. The sulfide ores are subjected to the action of air and heat at temperatures below their melting points to bring about chemical changes in them. Examples
This method involves treating the ore using a suitable reagent in order to make it soluble and the impurities are insoluble. Then the ore is recovered from the solution using a suitable chemical method. Example
• Al2O3 + 2NaOH -->2NaAlO2 + H2O
3. Reduction of Free Metal
There are various methods that come under this step.
This method involves the reduction of a metal from its ores. This method involves melting of the ore in order to extract the metal. Various reducing agents such as aluminum, sodium, magnesium, etc are used for reducing the metallic ore. The roasted or calcinate ore is then mixed with carbon (either coal or coke) and then heated in a blast furnace or a reverberatory. Carbon monoxide and carbon are produced due to incomplete combustion of carbon, help in reducing the oxide to the metal.
Even after concentration, some ores may contain some earthy matter, and these matters are known as gangue. Gangue, when heated, combines with the ore and become easily fusible material. And these are what are known as flux. The fusible material which is formed during the reduction process is called slag. There are two types of fluxes,
A. Acid fluxes
These include borax, silica, etc and are used for basic gangue such as metallic oxides like FeO, Mno or lime, etc
B. basic fluxes
They are used with acidic gangue like silica, P4O10. basic fluxes include CaO, limestone (CaCO3), hematite (Fe2O3), magnesite (MgCO3), etc
In general, the metals that are obtained from one of the above reduction methods are generally impure. The impure metal is then subjected to some purifying processes which are known as refining. Refining helps in the removal of undesired impurities. There are various processes that come under this,
c. Distillation process
e. Liquation process
f. Electrolytic refining
Thermodynamic Principles of Metallurgy:
ΔG =ΔH-TS or ΔG0 =-RT ln K
An element X can be only reduced by element Y if ΔG value of oxidation of Y to YO is lower than the ΔG value of oxidation of X to XO. i.e. ΔG(Y→YO) < ΔG(X→XO)
Extraction of Aluminum:
There are various important ores of aluminium,
• Corundum: Al2O3
• Bauxite : Al2O3×2H2OCryolite: Na3AlF6
• Mica: K2O×3Al2O3×6SiO2×2H2O
• Alum Stone or Alunite: K2SO4×Al2(SO4)3×4Al(OH)3
• Feldspar: K2OAl2O3×6SiO2 or KAlSi3O8
Purification of Bauxite
Hall & Herwlt Method of electrolysis of fused pure alumina
The addition of fluorspar (CaF2) and cryolite (Na3AlF6) makes alumina a good conductor of electricity. It also lowers the fusion temperature from 2323K to 1140K. Below is the reaction that takes place,
Refining of Aluminum:
The graphite rods are dipped in pure aluminum and Cu-Al alloy rod at the bottom impure aluminum works as a conductor. On electrolysis, the aluminum from the middle layer is deposited at the cathode. An equivalent amount of aluminum is then taken up by the middle layer from the bottom-most layer (aluminum layer). Thus, the aluminum is then transferred from the bottom layer to the topmost layer via the middle layer leaving the impurities behind. Almost 99.98% pure aluminum is obtained from this method.
Hydrometallurgy (solvent extraction)
Solvent extraction is one of the latest separation techniques and has become the most popular technique owing to its speed and simplicity. The basic principle of this method is preferential solubility. Liquid-liquid or solvent extraction follows a principle in which the solute has the ability to distribute itself in a certain ratio between the immiscible solvents. One among the solvents is water and the other solvent is an organic solvent which can either be carbon tetrachloride, chloroform, and benzene. In some cases, the solute can be less or more completely transferred into the organic phase. Extraction of Iron
a) Important Ores of Iron
imonite Fe2O3×3H2O (hydrated oxide of iron) Hematite Fe2O3 (red oxide of iron) Magnetite Fe3O4 (magnetic oxide of iron)
Extraction of Cast Iron:
Extraction of Copper:
Ores of Copper:
• Copper glance (chalcocite) : Cu2S
• Malachite: Cu(OH)2 ×CuCO3
• Cuprite or Ruby copper: Cu2O
• Copper pyrites (Chalcopyrite): CuFeS2
• Azurite: Cu(OH)2×2CuCO3
Refining of Metals:
1. Zone refining (fractional crystallization)
This method is generally used for preparing 100%pure metals. It is based on the principle that when a molten solution of an impure metal is allowed to cool down, then the pure metal crystallizes out while the remaining impurities melt.
This method involves converting the impure metal into a block which forms the anode. The cathode is made up of a plate or rod of pure metal. Both of these electrodes are suspended in an electrolyte. The electrolyte is a solution of a soluble salt of the metal, generally a double salt of the metal. When an electric current is allowed to pass, the metal ions from the electrolyte get deposited at the cathode in the form of pure metal and the same amount of metal dissolves from the anode and get mixed with the electrolyte solution as a metal ion. Soluble impurities in the crude metal go into the solution and the impurities settle down below the anode as anode mud.
3. Van- Arkel method
This method involves the conversion of volatile unstable compounds such as iodide leaving behind the impurities. The unstable compound formed is then decomposed to get the pure metal.
4. Cupellation and poling
These two methods are used for refining of metals. A refining process in which the alloyed metals or the ores are treated in very high temperatures and have controlled operations to split out noble metals like gold, silver, from base metals like zinc, copper, arsenic, bismuth, and lead, present in the ore is called cupellation. On the other hand, polling is used for refining of metals which has impurities that are its own metal oxides. In this process, the molten impure metal is generally stored with green wooden poles. At a high temperature of the molten metal, the wood liberates out methane which helps in reduction of metal oxide to free metal.