Some important Terms used in Metallurgy- Pulverization, Gauge or Matrix, Flux and Slag, Poling, Practice problems and FAQs

As all languages have their own set of rules, and keywords, the same the case of science. Metallurgy is that branch of science where we study about technical and chemical processes of extraction of metals from the earth's crust or ore. There are some important terms that are used commonly in metallurgy. These terms are used at various stages of metallurgy until the pure metal is extracted.

Table of contents

• Gangue or Matrix
• Flux and slag
• Pulverization of the ore
• Poling
• Practice problems
• Frequently Asked Questions-FAQs

Gangue or Matrix

On the earth's surface, metals can be found as minerals. In most cases, gangue or matrix—earthly or undesirable materials—contaminate the metals recovered from the ores. Rocks, sand, mud, dust, and other chemical impurities including silicates, metal oxides, etc. are the main components of gangue.

Flux and Slag

The gangue particles are combined with chemical substances called flux to form light, easily fusible material. This easily fusible material formed is called slag. 

Flux             +           Impurities          -->         Slag
                                          (Gangue present in ore)          (Easily fusible product)

Depending upon the nature of the impurities present in the ore, flux can be classified into two types:

1. Acidic flux: When the ore contains basic impurities such as lime, FeO, or other basic impurities, an acidic flux such as silica, borax, or other acidic material is utilized as a flux.

Example: In the smelting of copper, impurities of basic oxides like FeO is found. So acidic flux, SiO₂ can be used to convert it into fusible slag. 

FeO            +           SiO₂              -->           FeSiO3
(Basic Gangue)          ( Acidic flux)             (Fusible slag)

2. Basic flux: When the ore contains acidic impurities such as SiO₂, P₂O₅ or other acidic impurities, a basic flux such as CaO, CaCO₃, FeO or other basic material is used as a flux.

Example: In the extraction of iron from the blast furnace, impurities of acidic oxides like is SiO₂ is found. So basic flux, CaO is added to convert it into fusible slag.

SiO₂              +           CaO                -->            CaSiO₃
(Acidic Gangue)          (Basic flux)                     (Fusible slag)

Note: During extraction of iron, limestone (CaCO3) is added which decomposes to give CaO, to act as a flux.

Pulverization of the ore

The ore extracted from the earth's crust is in the shape of large lumps. Hammers or mechanical crushers are used to break these down into smaller pieces. Stone breakers or jaw crushers are the most often used crushers. There are two plates in these crushers, one of which travels and the other of which remains stationary. Between these two plates, ores lumps are crushed. A stamp mill or a ball mill is used to grind the small bits of ore into a fine powder.

Poling

This is one of the purification methods used in metallurgy. This is used in the purification of Cu and Sn. Molten Cu containing an impurity of Cu₂O is treated with powdered anthracite and then stirred with a pole of green wood. Greenwood when burnt at high temperatures liberates hydrocarbon gasses on combustion. These gasses are converted into methane. Methane so formed combines with copper oxide to give 99.5 % pure copper known as tough pitch copper.

Green wood --> Hydrocarbon --> CH₄
4CuO + CH₄ --> 4Cu + CO₂ + 2H₂O

Practice problems

Q1. Name the machine used for grinding in the pulverization of ore?

(A) Stamp mill
(B) Ball mill
(C) Stone breakers
(D) Both (A) and (B)

Answer: (D)

Solution: In the pulverization process, big lumps of ores are first crushed using stone breakers or jaw crushers. Later the smaller pieces of ores are obtained by crushing with a stamp mill or a ball mill. 

Q2. Which of the following is an example of basic flux?

(A) MgCO₃
(B) CaCO₃
(C) CaO
(D) All of the above

Answer: (D)

Solution: Oxides and carbonates of alkali earth metals are basic in nature. Therefore MgCO3, CaCO3 and CaO are basic in nature. So, option (D) is the correct answer.

Q3. Which of the following is a refining process?

(A) Pulverization
(B) Poling
(C) Formation of slag
(D) None of the above

Answer: (B)

Solution: Pulverization is a process of breaking down lumps into small pieces. The formation of slag is the process of isolation of ore. Poling on the other hand is a process of purification of Cu and Sn.

Q4. Which of the following is an example of acidic flux?

(A) H₃PO₄
(B) HCl
(C) SiO₂
(D) All of the above

Answer: (C)

Solution: Lewis acid like H₃PO₄, HCl and silica, SiO2 are acidic in nature. So, option (D) is the correct answer.

Frequently Asked Questions-FAQs

Question 1. In poling process, which stick is used?
Answer: Bamboo stick which is rich in organic matter (hydrocarbon) acts as a reducing agent and reduces the oxide impurities.

Question 2. What is fused metal?
Answer: It is that state of the metal, in which it can be melted easily at lower temperatures.

Question 3. Why there is a need to make fusible ore? 
Answer: Metal to be extracted in their pr state are present in combined form. The process of extraction of metal from its ore often occurs at a very temperature. This adds to the high cost at the industrial level. In order to reduce the high expenditure, compounds of metals and other impurities are formed such that metal can be extracted at low temperatures.

Question 4. What is the need to form slag?

• Slag is not soluble in molten metal, thus helping in the easy separation of metal.
• It is because of the lower density of slag, it gets lighter and can be easily removed from the surface of the fused metal.
• It forms a layer on the surface of molten metal thus preventing further oxidation of the metal.