NEET Chapter - General Principles and Processes of Isolation of Elements

This chapter covers the techniques for separating metals from their sources as well as the methods for separating metals from their sources. Metals are taken from their mineral sources on a regular basis. These ores, which are found in the earth's crust, are collected by mining. 

This chapter focuses on the extraction and purification of metals. You will learn about the wide range of methods and processes that are particular to different metals. General Principles and Processes of Isolation of Elements is an important chapter because it explains how the metals you use on a daily basis are refined.  

Ore is a natural rock or silt that contains one or more valuable minerals, generally metal-bearing minerals, that may be mined, processed, and sold for profit. As a result, these ores might be considered minerals from which metal can be recovered inexpensively and swiftly. Metals are rarely found in their pure form, and some, like potassium, sodium, calcium, and magnesium, are found in mixed conditions. 

The goal of general principles and procedures of metal isolation in chemistry is to teach students about the various techniques of metal extraction from ores. This article provides an example of the types of questions that might be asked about this subject in the JEE 2022 exam. 

Important Topics of General Principles and Processes of Isolation of Elements

• Principles of Metallurgy

• Extraction

• Various refining methods

Some Important Definitions 

Metallurgy 

• The earth's crust contains a large diversity of metals. Minerals are naturally occurring metallic compounds combined with sand, soil, and rocks that have a certain chemical makeup. Ore is a metallic compound with a reasonably high metal concentration that may be utilized to extract certain elements in a practical and cost-effective manner.

• The definition of metallurgy is "the discipline of chemistry concerned with the extraction of metals in their pure state from their ore."

Occurrence of Elements

• Atmospheric Elements: The atmosphere is mostly made up of nitrogen (78.09 percent), oxygen (20.95 percent), and other gases (about 1 per cent). 

• Sea Elements: The sea is a key source of elements such as Br, I, Ni, Cu, Zn, Sn, Au, and others. 

• Earth's Crust Elements (Lithosphere): In the earth's crust, elements exist in two states:

Free or Native State: In the free or native state, less reactive metals or noble metals with a low electropositive character, such as copper, silver, gold, and platinum, are present.

Occurrence of Metals

• Native Ores: They contain the metal in the free state. Silver, gold, and platinum occur as native ores.

• Oxidized Ores: They consist of oxides or oxysalts, such as carbonates, phosphates, sulfates, and silicates of metals.

• Sulphurised Ores: They consist of sulfides of metals like iron, lead, zinc, and mercury.

• Halide Ores: They consist of halides of metals.

Extraction of Ores

• The extraction process is used to reduce costs and to get the purest form of element possible.

• Metal extraction from ores must be made easier, better, and more cost-effective for the industry.

• This is the method an extraction process is constructed, and it is based on the principles of metallurgy used to verify the inorganic chemical characteristics of the elements of ore.

• Impurities such as pebbles, sands, and limestone are common in ore. It's called gangue. Flux is a chemical that is introduced to ore in order to eliminate the impurities that are there. The interaction of gangue and flux in ores then results in the creation of slag, a fusible substance.

Non-fusible impurity + flux → Fusible slag 

Acidic flux is used to eliminate basic impurities such as silica (SiO2), Borax, and other similar substances. 

3SiO2 + Fe2O3→Fe2(SiO3)3 (slag) 

Acidic impurities such as CaO, MgO, Fe2O3, and others are removed using basic flux. 

3CaO + P2O5 →Ca3(PO4)2 (slag) 

Steps Involved in Metallurgy

The following principles are involved in metallurgy:

(1) Crushing and Grinding: This includes crushing ores into fine powder in a crusher, which is the basic process in metal metallurgy.

(2) The Concentration of Ores: The ores mined from the earth's crust contain a huge number of undesired impurities known as gangue, which include quartz, silicates, sand, feldspar, mica, and other minerals. The dressing is the process of removing undesirable contaminants from ore and is also known as the concentration of ore because it steadily raises the amount of metal.

(3) Reduction to Free Metal: Reduction is the process of heating metal oxides to convert them to metal.

(4) Purification or Refining of the Metal: The resulting metal is processed using a variety of techniques.

Methods Involved in the Concentration of Ores

There are a number of methods for the concentration of ores and the methods are based on the properties of the ore. These methods are -

Physical Methods 

Hydraulic Washing: Ores are passed through an upward stream of water in this process, which separates the lighter gangue particles from the heavier metal ore. This is a gravity separation technique as well.

Magnetic Separation: The separation is carried out using the magnetic characteristics of either the ore or the gangue in this process. The ore is crushed into fine bits and then fed onto a conveyor belt that passes over a magnetic roller in this procedure. The magnetic ore stays on the belt, while the gangue falls off.

Froth Flotation Method: Gangue is discovered to be extracted from sulfide ores using this approach. The ore is first pulverized, after which it is suspended in water. Stabilizers are introduced to the collectors and froth. Pine oils, fatty acids, and other collectors improve the non-wettability of the metal component of the ore, allowing it to form a froth, while froth stabilizers (cresols, aniline, and other compounds) keep the froth in place.

The oil used wets the metal, and the water wets the gangue even more. Paddles and air are used to continually stir up the suspension in order to generate the foam. In order to recover the metal, the frothy metal is scraped off the top and dried.

Chemical Methods

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 that involves the roasting of ores to the temperatures below their melting points, that too mainly in the presence of air.

Leaching: When the ore is determined to be soluble in a solvent, this procedure is utilized. The powdered ore is subsequently dissolved in a chemical solution, most often a strong NaOH solution. The chemical solution dissolves the metal in the ore, and the chemical solution may be recovered and separated from the gangue. This method is used to extract aluminium from bauxite ore.

Various Reduction Processes 

Some of the methods commonly used to get free metal from the concentrated ore are given below,

Smelting: Smelting is the process of removing metal in a state of fusion. The ore is combined with carbon derived from the preceding processes and heated in a suitable furnace in this procedure. During the procedure, a sufficient flux is supplied to convert the non–fusible gangue to fusible slag. 

The metal can be obtained in molten form or as condensed vapours when the metallic oxide is reduced by carbon. This method is used to extract metals such as tin, zinc, and lead. Flux is a chemical used in the smelting process to transform infusible silicons or earthy impurities into slag, a fusible material. 

Slag is made up of impurities and flux. The slag has a low melting point and density and is incompatible with the metal. The slag floats on top of the metal, shielding it from oxidation. The slag hole is used to remove it from the furnace. If the impurities in the ore are acidic (SiO2), a basic flux, such as CaO, MgO, FeO, and so on, is added; if the impurities are basic (CaO, FeO, and so on), an acidic flux (SiO2) is utilized.

Goldschmidt AluminoThermic Process: When there are oxides that cannot be easily reduced by carbon, the reduction method is utilized. Metallic oxide ore is combined with aluminium powder, also known as thermite, and placed in a steel crucible lined inside with a refractory substance, which is then fired by a magnesium ribbon. A variety of metals, such as chromium and manganese, may be produced in a very pure condition on a commercial scale using this method.

Self-Reduction Process: This procedure is also known as the auto reduction or air reduction procedure. Sulphide ores containing less electropositive metals such as Hg, Pb, Cu, and others are heated in air to convert a portion of the ore to oxide or sulphate, which interacts with the remaining sulphide ore to produce the metal and sulphur dioxide. In this procedure, no external reducing agent is utilised.

Electrolytic Reduction Process: Alkali and alkaline earth metals, zinc, and aluminium are all extracted using this method. The substance that will be used to make a metal is heated first and then electrolyzed. To reduce the melting point of the material consumed, some additional salt is sometimes added.

Various Refining Methods 

Metals obtained as above are usually impure and need purification. The following are some of the processes used in metal refining.

By Poling: Greenwood poles are used to stir the molten metals. Wood, when exposed to the high temperatures of molten metals, produces hydrocarbons such as methane, which is formed by the reduction of any oxide present in the metal, such as copper oxide in blister copper. In the instance of tin, the impurities oxidize and float as scum on the molten metal, which is then removed.

By Cupellation: When impurities are oxidized and blown away, the impure metal is heated in a blast of air. When impure silver is heated in the air, the lead in it oxidizes to litharge (PbO) and is blown away, leaving a gleaming silver surface.

By Liquation: This method is used to refine easily flammable metals such as lead and tin. The impure metal is heated in a reverberatory furnace's sloping hearth. Impurities are left behind as the metal melts and flows down.

By Fractional Distillation: The separation of cadmium from zinc is done using this method. In zinc metallurgy, the metal is inextricably linked to cadmium. When the initial component of the condensate contains cadmium, the impure zinc is combined with powdered coke and heated, while zinc is produced in the succeeding sections.

Mond's Process: This process is used to purify nickel. The volatile chemical nickel carbonyl is generated when impure nickel is heated with carbon monoxide at 60–80°C. At 180°C, nickel carbonyl decomposes to pure nickel and carbon monoxide, which may be reused.

Van-Arkel Process: This technique is commonly used to obtain ultrapure metals. The impurities are unaffected as the impure metal is transformed into a volatile chemical. The volatile molecule is subsequently electrically decomposed to provide pure metal. This process has been used to purify Ti, Zr, Hf, Si, and other materials.

Zone Refining or Fractional Crystallization: This process refines semiconductor elements such as Si, Ge, Ga, and other similar elements. It is possible to acquire metals that are extremely pure. The approach relies on the difference in impurity solubility between the molten and solid states of metals. A moveable heater is wrapped around a metal rod. The heater is pushed across the rod carefully. The metal melts at the point of heating, and the pure metal crystallizes as the heater advances from one end of the rod to the other, while the impurities pass through the neighbouring melted zone.

Park's Distribution Methodology: This method is used to separate Ag and Au from Pb. The concept behind this procedure is that silver and gold are more soluble in molten zinc than in molten lead in a two-phase system. The resulting Zn-Ag alloy freezes out first, is extracted, and the zinc is distilled away.

Solved Examples from Chapter

Example 1: Tin stone is contaminated with Wolframite. Which one of the following methods can be employed to dress the tin stone ore? 

(a) Electromagnetic separation 

(b) Levigation 

(c) Roasting 

(d) Floatation process 

Solution: 

Electromagnetic separation method is used to dress the tin store ore because tin store contains wolframite as a magnetic impurity. When the roasted ore is reduced by carbon, the zinc sulfate is transformed back to zinc sulfide. So, the correct option is (a).

Key point to remember: Electromagnetic separation is the technique based on differences in magnetic properties of the ore or gangue.

Example 2: A mineral of iron among the following is - 

1. Malachite 

2. Cassiterite 

3. Pyrolusite 

4. Magnetite 

Solution: 

With the chemical formula Fe3O4, magnetite is a mineral and one of the most important iron ores. It is ferrimagnetic and one of the iron oxides; it is attracted to magnets and maybe magnetized to create a permanent magnet. Hence the correct option is (4).

Key point to remember: Magnetite is basically Fe3O4 and contains up to 70% of the metal.

Solved Questions from the Previous Year Question Papers

Question 1: The basic principle of froth floatation is: 

1. Impurities and metal ore have different wetting properties. 

2. Impurities and metal ore have different solubility. 

3. Impurities and metal ore have different diversity. 

4. Impurities and metal ore have different compressing power. 

Solution: 

The wettability of gangue and mineral particles differs, which is the rationale of the froth flotation process. Wetting the mineral particles with oil and the gangue particles with water separates the mineral and gangue particles. Therefore Impurities and metal ore have different wetting properties. So, the correct option is (a).

Trick: Familiarity with the froth floatation method.

Question 2: Cassiterite is concentrated by: 

(a) Hand picking 

(b) Levigation

(c) Electromagnetic separation 

(d) Wilfley table method 

Solution: 

Magnetic impurities are found to be present in cassiterite ore. So, the electromagnetic separation method is used to concentrate Cassiterite. Hence the correct option is (c).

Trick: Electromagnetic separation is the technique based on differences in the magnetic properties of the ore or gangue.

Question 3: Native silver metal forms a water-soluble complex with a dilute aqueous solution of NaCN in the presence of

(1) Nitrogen 

(2) Oxygen 

(3) Carbon Dioxide 

(4) Argon 

Solution: 

In the presence of air, Ag interacts with NaCN to generate sodium argentocyanide. This can be shown as -

4Ag + 8NaCN + 2H2O + O2 → 4Na[Ag(CN)2]+ 4NaOH

So, the correct option is (d).

Trick:  To know the reaction being taking place and the product.

Practice Questions

Question 1: Which of the following ores is concentrated by the chemical leaching method?

(a) Cinnabar

(b) Argentite

(c) Copper pyrites

(d) Galena

Answer: (b) Argentite

Question 2: Common impurities present in bauxite are

Options: 

(a) CuO

(b) ZnO

(c) CaO

(d) SiO2

Answer: (d) SiO2

Conclusion

Thus, the study of metal extraction from ores is vital because it enables the utilization of minerals found deep inside the earth's crust. A mineral is a combination of metals and non-metals that occurs naturally. Metals may be extracted using minerals. An ore is a naturally occurring solid material from which a valuable metal or mineral may be profitably mined. Ore is polluted by impurities or undesirable particles known as gangue. Minerals are all ores, but not the other way around.

To carry out the extraction of various metals from their respective ores, a range of ideas are involved, which further aid in comprehending a phenomenon in depth. As a result, this is crucial not just for competitive tests like JEE or NEET, but also for everyday life.