explain isolation chapter
Answers
Answer: General Principles & Processes of Isolation of Metals
Types of Ores:
Ores may be divided into four groups
Native Ores: These ores contain the metal in free state eg. Silver gold etc. These are usually formed in the company of rock or alluvial impurities like clay, sand etc.
Oxidised Ores:These ores consist of oxides or oxysalts (eg. carbonates, phosphate) and silicate of metal. Important oxide ore includes, Fe2O3, Al2O3.2H2O etc. and important cabonate ores are limestone (CaCO3), Calamine (ZnCO3) etc.
Sulphurised Ores: These ores consist of sulfides of metals like iron, lead, mercury etc. Examples are iron pyrites (FeS2). galena (PbS), Cinnabar (HgS)
Halide ores: Metallic halides are very few in nature. Chlorides are most common examples include horn silver (AgCl) carnallite KCl. MgCl2.6H2O and fluorspar (CaF2) etc.
Metallurgy:
It is the process of extracting a metal from its ores. The following operations are carried out for obtaining the metal in the pure form.
Crushing of the ore
Dressing or concentration of the ore.
Reduction of metal.
Purification or refining of the metal
Concentration
Physical Method
Gravity separation: The powdered ores is agitated with water or washed with a running stream of water. The heavy ore particles of sand, clay etc. are washed away.
Froth Floatation Process: The finely divided ore is introduced into water containing small quantity of oil (e.g. Pine Oil). The mixture is agitated violently with air a froth is formed which carries away along with it the metallic particles on account of the surface tension forces. The froth is transferred to another bath where gangue-free ore settles down.
Froth Floatation Process
Electro Magnetic Separator:. A magnetic separator consists of a belt moving over two rollers, one of which is magnetic. The powdered ore is dropped on the belt at the other end. Magnetic portion of the ore is attracted by the magnetic roller and falls near to the roller while the non-magnetic impurity falls farther off
Electro Magnetic Separator
Chemical Methods
Calcination: Carbonate or hydrated oxide ores are subjected to the action of heat in order of expel water from hydrated oxide and carbon dioxide from a carbonate.
Examples:
ZnCO3 --> ZnO + CO2
CaCO3 --> CaO + CO2
Al2O3×2H2O --> Al2O3 + 2H2O
2Fe2O3×3H2O --> 2Fe2O3 + 3H2O
Roasting: Sulphide ores either are subjected to the action of heat and air at temperatures below their melting points in order to bring about chemical changes in them.
Examples:
2PbS + 3O2 --> 2PbO + 2SO2
PbS + 2O2 --> PbSO4
2ZnS + 3O2 --> 2ZnO + 2SO2
ZnS + 2O2 --> ZnSO4
CuS + 2O2 --> CuSO4
2Cu2S + 3O2 --> 2Cu2O + 2SO2
Leaching: It involves the treatment of the ore with a suitable reagent as to make it soluble while impurities remain insoluble. The ore is recovered from the solution by suitable chemical method.
Al2O3 + 2NaOH -->2 NaAlO2 + H2O
Reduction of Free Metal:
Smelting:
Reduction of a metal from its ore by a process involving melting
Several reducing agents such as sodium, magnesium and aluminium are used for reduction.
The calcinated or roasted ore is mixed with carbon (coal or coke) and heated in a reverberatory or a blast furnace.
Carbon and carbon monoxide produced by incomplete combustion of carbon reduce the oxide to the metal.
Flux:
The ores even after concentration contain some earthy matter called gangue which is heated combine with this earthy matter to form an easily fusible material. Such a substance is known as flux and the fusible material formed during reduction process is called slag.
Acidic fluxes like silica, borax etc are used when the gangue is basic such as lime or other metallic oxides like MnO, FeO, etc
Basic fluxes like CaO, lime stone (CaCO3), magnesite (MgCO3), hematite (Fe2O3) etc are used when the gangue is acidic like silica, P4O10 etc.
THESE ARE SOME EXTRA QUESTION AND ANSWER WHICH MAY HDLP YOU.
Question 6.1:
Copper can be extracted by hydrometallurgy but not zinc. Explain.
Answer
The reduction potentials of zinc and iron are lower than that of copper. In hydrometallurgy, zinc and iron can be used to displace copper from their solution.
Hence, the extraction of copper from its pyrite ore is difficult than from its oxide ore through reduction.
Question 6.4:
Explain: (i) Zone refining (ii) Column chromatography.
Answer
(i) Zone refining:
This method is based on the principle that impurities are more soluble in the molten state of metal (the melt) than in the solid state. In the process of zone refining, a circular mobile heater is fixed at one end of a rod of impure metal. As the heater moves, the molten zone of the rod also moves with it. As a result, pure metal crystallizes out of the melt and the impurities pass onto the adjacent molten zone. This process is repeated several times, which leads to the segregation of impurities at one end of the rod. Then, the end with the impurities is cut off. Silicon, boron, gallium, indium etc. can be purified by this process.
(ii) Column chromatography:
Column chromatography is a technique used to separate different components of a mixture. It is a very useful technique used for the purification of elements available in minute quantities. It is also used to remove the impurities that are not very different in chemical properties from the element to be purified. Chromatography is based on the principle that different components of a mixture are differently adsorbed on an adsorbent. In chromatography, there are two phases: mobile phase and stationary phase. The stationary phase is immobile and immiscible. Al2O3 column is usually used as the stationary phase in column chromatography. The mobile phase may be a gas, liquid, or supercritical fluid in which the sample extract is dissolved. Then, the mobile phase is forced to move through the stationary phase. The component that is more strongly adsorbed on the column takes a longer time to travel through it than the component that is weakly adsorbed. The adsorbed components are then removed (eluted) using a suitable solvent (eluant).
Question 6.6:
Name the common elements present in the anode mud in electrolytic refining of copper. Why are they so present ?
Answer
In electrolytic refining of copper, the common elements present in anode mud are selenium, tellurium, silver, gold, platinum, and antimony.
These elements are very less reactive and are not affected during the purification process. Hence, they settle down below the anode as anode mud.