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Chapter 6: General Principles and Processes of Isolation of Elements
A few elements like carbon, sulfur, gold, and noble gases, occur in a free state while others in combined forms in the earth’s crust. The extraction and isolation of an element from its combined form involves various principles of chemistry.
A particular element may occur in a variety of compounds. The process of metallurgy and isolation should be such that it is chemically feasible and commercially viable.
Still, some general principles are common to all the extraction processes of metals. For obtaining a particular metal, first, we look for minerals that are naturally occurring chemical substances in the earth’s crust obtainable by mining.
Out of many minerals in which a metal may be found, only a few are viable to be used as sources of that metal. Such minerals are known as ores.
Rarely, an ore contains only a desired substance. It is usually contaminated with earthly or undesired materials known as gangue. The extraction and isolation of metals from ores involve the following major steps:
• Concentration of the ore,
• Isolation of the metal from its concentrated ore, and
• Purification of the metal.
The entire scientific and technological process used for the isolation of the metal from its ores is known as metallurgy.
In the present Unit, first, we shall describe various steps for effective concentration of ores. After that, we shall discuss the principles of some of the common metallurgical processes.
Those principles shall include the thermodynamic and electrochemical aspects involved in the effective reduction of the concentrated ore to the metal.
Elements vary in abundance. Among metals, aluminum is the most abundant. It is the third most abundant element in earth’s crust (8.3% approx. by weight).
It is a major component of many igneous minerals including mica and clays. Many gemstones are impure forms of Al2O3 and the impurities range from Cr (in ‘ruby’) to Co (in ‘sapphire’).
Iron is the second most abundant metal in the earth’s crust. It forms a variety of compounds and their various uses make it a very important element.
It is one of the essential elements in biological systems as well.
For the purpose of extraction, bauxite is chosen for aluminum. For iron, usually, the oxide ores which are abundant and do not produce polluting gases (like SO2 that is produced in case iron pyrites) are taken.
For copper and zinc, any of the listed ores (Table 6.1) may be used depending upon availability and other relevant factors. Before proceeding for concentration, ores are graded and crushed to a reasonable size.
Removal of unwanted materials (e.g., sand, clays, etc.) from the ore is known as concentration, dressing, or benefaction. It involves several steps and the selection of these steps depends upon the differences in physical properties of the compound of the metal present and that of the gangue.
The type of metal, the available facilities, and the environmental factors are also taken into consideration. Some of the important procedures are described below.
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NCERT Solutions Class 12 Chemistry Chapter 6 General Principles and Processes of Isolation of Elements
Q1. Copper can be extracted by hydrometallurgy but not zinc. Explain.
Copper has a higher reduction potential than zinc or iron. Thus, in hydro-metallurgy iron and zinc can be used to displace copper from its solution.
Fe(s) + Cu2+(aq) ⇒ Fe2+ (aq) + Cu(s)
However, in order to displace zinc, metal is more reactive than it is required e.g. K, Ca, etc. The problem arising now is that all these metals react with water to liberate hydrogen gas. Thus, they cannot be used for the extraction of zinc using hydro-metallurgy.
Q2. What is the role of depressant in the froth floatation process?
The role of a depressant in the fourth floatation is to set apart two sulfide ores through selective prevention of one ore from frothing. For E.g. NaCN is used to separate two sulfide ores, PbS and ZnS. NaCN forms a complex, Na2 [Zn( CN )4], with zinc on the surface of zinc sulfide, thus selectively preventing ZnS from frothing.
4NaCN + ZnS ⇒ Na2 [Zn( CN )4] + Na2S
Q3. Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?
Hydrogen sulfite and carbon disulfide (CS2) have a larger ∆fG (Gibbs free energy of formation) than copper (I) sulfite. Thus, C and H2S will not be able to reduce Cu2S to Cu. Whereas ∆fG of Cu2O is larger than carbon monoxide’s, thus C can reduce copper (I) oxide to copper.
Therefore, it is more difficult to extract copper from pyrites than it is to extract from its oxide.
Q10. “Chromatography”, what do you understand by this term?
Chromatography is a collective term used for a range of laboratory techniques for the purification, separation, characterization, and identification of the species of mixtures whether colored or colorless. The word has been obtained from two Greek words; ‘graphy’ meaning ‘to write’ and ‘chroma’ meaning ‘color’. There are numerous chromatographic techniques like column chromatography, paper chromatography, gas chromatography, etc.
Q11. What is the criterion followed while selecting the stationary phase of chromatography?
The choice of stationary phase is done in a way that the elements of the mixture have different solubility in the phase. Thereby, different elements have different movement speeds through the phase, because of which they can be separated from each other.
Q12. Describe a method for refining nickel.
Mond’s process is a technique used to refine nickel. In this method, heat is supplied to nickel in the presence of carbon monoxide to produce nickel tetracarbonyl, which is a volatile complex.
Ni + 4CO ⇒ Ni( CO)4 [330k – 350k]
The nickel tetracarbonyl so obtained is then decomposed by heating it at a higher temperature (450 − 470 K) to form pure nickel.
Ni( CO)4 ⇒ Ni + CO [450k – 470k]
General Principles And Processes of Isolation of Elements NCERT Textbook With solution PDF Free Download