JEE Chapter - Extractive Metallurgy

The art and science of extracting metals from their ores are known as extractive metallurgy. The majority of naturally occurring components are classified as minerals, which contain metal compounds and are found in the earth's crust. Ore is defined as a natural rock or silt containing one or more valuable minerals, primarily metal-bearing minerals, that may be extracted, processed, and sold for a profit. As a result, these ores may be thought of as minerals from which metal can be extracted cheaply and quickly. 

Metals are not usually present in their free state, and some metals, such as potassium, sodium, calcium, and magnesium, are found in a mixed state. Extractive metallurgy 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. 

Important Topics of Extractive Metallurgy

• Principles of Metallurgy

• Extraction

• Various refining methods

Some Important Definitions 

Metallurgy Definition - What is Meant by 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 utilised 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."

Extraction of Ores - Extraction Metallurgy

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

Steps Involved in Metallurgy: Process of Extraction 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’s metallurgy. Therefore, the first basic step in metallurgy is Crushing and Grinding.

(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 Ore

It is referred to as the process of the removal of gangue from ore. There are a number of methods for the concentration of ores and the methods are based on the properties of the ore.

These are physical and chemical methods. Let’s discussed these methods in detail.

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 utilised.

• Goldschmidt Aluminothermic Process: When there are oxides that cannot be easily reduced by carbon, the reduction method is utilised. 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 electrolysed. 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 oxidise and float as scum on the molten metal, which is then removed.

By Cupellation: When impurities are oxidised 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 oxidises 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 Crystallisation: 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 crystallises as the heater advances from one end of the rod to the other, while the impurities pass through the neighbouring melted zone.

Solved Examples from the Chapter

Example 1: Zone refining is a technique used for which one of the following processes?

A) Alloying

B) Tempering

C) Sintering

D) Purification

Solution: 

This process purifies 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.

Therefore option (D) is the answer.

Key point to remember:  The definition of the Zone refining technique.

Example 2: Which one of the following ores is best concentrated by the froth-flotation method?

(A) Magnetite

(B) Cassiterite

(C) Galena

(D) Malachite

Solution:

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, stabilisers 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 (C) is the answer.

Key point to remember: The froth floatation method.

Solved Questions from the Previous Years’ Question Papers

Question 1: During the process of electrolytic refining of copper, some metals present as impurity settle as ‘anode mud’. These are        

1) Sn and Ag                  2) Pb and Zn

3) Ag and Au                 4) Fe and Ni

Solution: 

The impure metal is used as an anode in electrolytic refining. As a cathode, a pure strip of the same metal is utilised. They are submerged in a suitable electrolytic bath containing a soluble metal salt. The more basic metals stay in the solution, whereas the less basic metals end up in the anode mud.

An electrolytic process is used to purify copper.

Antimony, selenium, tellurium, silver, gold, and platinum impurities from the blister copper deposit as anode mud; recovery of these elements may pay the cost of refining. Hence crude Cu contains Au and Ag as impurities. So, the correct option is (3)

Trick: Familiarity with electrolytic refining.

Question 2: A heating mixture of Cu2O and Cu2S will give  

1) Cu + SO2                   2) Cu + SO3

3) CuO + CuS                4) Cu2SO3

Solution: 

Self-reduction occurs upon heating a mixture of oxide and sulfide of copper.

2Cu2O + Cu2S → 6Cu + SO2

This procedure is also known as the auto reduction or air reduction procedure. Sulfide 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 sulfate, which interacts with the remaining sulfide ore to produce the metal and sulfur dioxide. In this procedure, no external reducing agent is utilised. Hence the correct option is (1).

Trick: Remember the process of self-reduction.

Question 3: The process that involves the removal of sulfur from the ores is :

(1) Roasting

(2) Smelting

(3) Leaching 

(4) Refining

Solution:

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 (1) is correct.

Trick:  Remember all these processes i.e., Roasting, Smelting, Leaching, and Refining.

Practice Questions

Question 1: Which process of reduction of mineral to the metal is suited for the extraction of copper from its ores with low copper content

A) Metal displacement

B) Auto reduction

C) Chemical reduction

D) Electrolytic reduction

Answer: B) Auto reduction 

Question 2: Pb and Sn are extracted from their chief ore by

A) Carbon reduction and self-reduction.

B) Self-reduction and carbon reduction.

C) Electrolysis and self-reduction.

D) Self-reduction and electrolysis.

Answer:  B) Self-reduction and carbon reduction.

Conclusion:

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.