Most of the naturally occurring components are called minerals in which metal compounds are present and these are found in the earth's crust. Now, what is ore? Ore is said to be the natural rock or sediment that consists of one or more precious minerals, basically, those containing metals, which can be mined, treated, and sold at a profit. So, these ores can be said to be the minerals from which metal can be economically and easily extracted.
The metals are not always found in a free state while Potassium, Sodium, Calcium, Magnesium, etc are the metals that are found in a combined state. From these Metal Compounds, a pure form of metals is desired to be extracted. One should always keep in mind that all ores are minerals but all minerals are not ores. This is because some of the minerals have unwanted substances in them.
Isolation of elements in Chemistry aims to teach the students about the various processes of extraction of metals from ores. This article provides an example of the types of questions that might be asked about this subject in the JEE exam.
Concentration of Ore
Types of Ores
Ores of Different Metals
Some Important Definitions
Extraction of Ores
Extraction process is used to decrease the cost and is used to gather the purest form of the metal according to the possibility.
It needs to be simpler, better, and more cost-effective for the industries to extract metals from their ores.
It depends upon the principles of metallurgy used to check the inorganic chemical properties of the constituents of ore and this is the way an extraction process is designed.
The ore generally contains impurities like rocks, sands, and limestone known as gangue. So, a substance is added to ore in order to remove the impurities present in it is called flux. Then the combination of gangue with flux occurs in ores leading to the formation of a fusible material called slag.
Concentration of Ores
These processes include Hydraulic Washing, Magnetic Separation, Froth Flotation Method and Leaching. Now, let us discuss all these processes one by one.
This method uses the magnetic properties of either the ore or the gangue to carry out the separation. In this method, the ore is first ground into fine pieces and then is made to pass onto a conveyor belt passing over a magnetic roller. The magnetic ore remains on the belt and the gangue falls off the belt.
Froth Flotation Method:
In this type of method, gangue is found to be removed from sulfide ores. The ore is first powdered and its suspension is created in the water. To this collectors and froth, stabilizers are added. Collectors such as pine oils, fatty acids, etc increase the non-wettability of the metal part of the ore and allow it to form a froth while the froth stabilizers (cresols, aniline, etc) sustain the froth. The oil being used wets the metal and the water further wet the gangue. In order to create the froth paddles and air constantly stir up the suspension. This frothy metal is found to be skimmed off the top and dried in order to recover the metal.
This method is used when the ore is found to be soluble in a solvent. The powdered ore is then dissolved in a chemical, mainly a strong solution of NaOH. The chemical solution dissolves the metal in the ore and can be extracted and separated from the gangue by obtaining the chemical solution. Extraction of the aluminium from bauxite ore is made using this process.
Extraction of Ore from highly Reactive Metals
Calcination: It is a process that involves the heating of ore in the absence of air in order to remove water from hydrated oxide at temperatures below the melting point.
Roasting: It is a process which involves the roasting of ores to the temperatures below their melting points, that too mainly in the presence of air.
Smelting: It is the process of reducing oxide ore with carbon in the presence of high temperatures.
Difference between Minerals and Ores
Types of Ores
The types of ores are-
In this type of ore, the metals are found to be present in the sulfide form. It is difficult to obtain metals from this type of ore, so they are usually converted to oxides through the process of metallurgy.
For example, Zinc Blende and Galena.
In this type of ore, carbonate ions are found to be present. It is difficult to obtain metals from these ores, they also get converted to oxide ores first. For example, Calamine.
Some Important Ores
Classification of Ore Deposits
An ore deposit can be defined as the economically significant accumulation of the minerals in a host rock. They can be classified as:
Hydrothermal epigenetic deposits
Granite related hydrothermal
Metamorphically reworked deposits
Carbonatite-alkaline igneous related
Hydrothermal deposits formed largely from basinal brines etc.
Solved Examples from the Chapter
Example 1: The metal extracted by leaching with cyanide is
Leaching: This method is used when the ore is found to be soluble in a solvent. The powdered ore is dissolved in a chemical. The chemical solution dissolves the metal in the ore and can be extracted and separated from the gangue by obtaining the chemical solution. Extraction of the Silver from argentite ore is made using this process.
As it is extracted by leaching with cyanide. The reactions involved are given as-
Ag2S + 2NaCN → Na[Ag(CN)2]
2Na[Ag(CN)2] + Zn → Na2[Zn(CN)4] + 2Ag
Therefore option (2) is the answer.
Key point to remember: The chemical reactions involved to give free silver metal follow the process of leaching.
Example 2: Which one of the following ores is best concentrated by the froth-flotation method?
Froth-floatation method: In this method, gangue is removed from sulfide ores. The ore is first powdered and its suspension is created in the water. To this collectors and froth, stabilizers are added. The oil wets the metal and the water wets the gangue. Paddles and air constantly stir up the suspension to create the froth. This frothy metal is skimmed off the top and dried, to recover the metal.
Since this method is used for the concentration of sulfide ores and here the only sulfide ore present is galena (PbS). Therefore option (3) is the answer.
Key point to remember: The froth floatation method.
Solved Questions from the Previous Year’s Question Papers
Question 1: Sulphide ion is a soft base and its ores are common for metals.
(a) Pb (b) Al (c) Ag (d) Mg
(A) (a) and (c) only
(B) (a) and (d) only
(C) (a) and (b) only
(D) (c) and (d) only
Out of the options being given only Pb and Ag are commonly found to exist in the form of sulfide ores like PbS (Galena - a lead ore) and Ag2S (Argentite - a silver ore) while 'Aluminium' is mainly found in the form of oxide ore whereas 'Magnesium' is mainly found in the form of halide ore. Hence the correct option is (A).
Trick: Know the chemical formulas of ores containing respective metals.
Question 2: In the leaching of alumina from bauxite, the ore expected to leach out in the process by reacting with NaOH is :
Leaching- This method is used when the ore is found to be soluble in a solvent. The powdered ore is dissolved in a chemical, a mainly strong solution of sodium hydroxide. The chemical solution dissolves the metal in the ore and can be extracted and separated from the gangue by obtaining the chemical solution.
As it is extracted by leaching with cyanide. In bauxite ore the impurities like Fe2O3, TiO2, and SiO2 are found to be present, in which Fe2O3 and TiO2 are the basic oxides as they do not react with or dissolve in Sodium hydroxide while SiO2 is an acidic oxide as it gets dissolved in sodium hydroxide, hence it leaches out. Hence option (4) is correct.
Trick: To know the process of leaching.
Question 3: The process that involves the removal of sulfur from the ores is :
Roasting is a process that involves heating sulfide ore to a high temperature in the presence of air.
Smelting is the process that involves reducing oxide ore with carbon in the presence of high temperatures.
Leaching is the process that is successfully used to extract gold, silver, copper, nickel, and uranium.
Refining is the process that is used for removing the last small amounts of impurities left after the major extraction steps.
Hence option (a) is correct.
Trick: Remember all these processes i.e., Roasting, Smelting, Leaching, and Refining.
Question 1: The refining method used when the metal and the impurities have low and high melting temperatures, respectively, is :
(b) Vapour phase refining
(d) Zone refining
Question 2: The ore that contains both iron and copper is :
(a) Copper pyrites
Answer: Copper pyrites
Thus, the study of the extraction of metals from ores is important as it allows one to use the minerals present deep inside the earth's crust.
A variety of concepts are involved to carry out the extraction of various metals from their respective ores and further help in understanding a phenomenon in detail. Hence, this is important not only for competitive exams like JEE or NEET but also for a better understanding of the extraction process of metals.