Flux - Flux, Its Types, Practice Problems and FAQs

What role does metal have in human life?

Metals are crucial to human existence. Many disorders in the human body could develop if they weren't present.
Metals have also been used to create medicines that are effective in treating conditions like cancer, arthritis, and ulcers.
A high energy society relies heavily on metals since they perform a variety of functions, including transporting electricity in the electrical grid.
They are utilized in a variety of home goods and industrial activities, as well as being crucial for the construction of buildings and highways.

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But in order to use these metals effectively, we must first purify them to the highest degree possible. To do this, we use a chemical known as flux, which purifies the metal by removing unwanted impurities from it.

Table of Content:

Flux
Types of Fluxes
Practice Problems
Frequently Asked Questions

Flux:

If the calcinated or roasted ore still contains non-fusible earthy matter impurities, then flux is also added to the ore, which reacts with impurities to generate an easily fusible product known as slag.

$F l u x + i m p u r i t i e s \to f l a m m a b l e p r o d u c t (s l a g) (p r e s e n t i n t h e o r e)$

Slag is not soluble in molten metal, but because it is lighter, it can be easily skimmed off the surface of the molten metal, making removal of gangue easier. Flux is a substance that chemically combines with gangue (earthy impurities) that may still be present in the calcinated or roasted ore to form easily fusible material called slag.

Types of Fluxes:

Fluxes are classified into two types: basic fluxes and acidic fluxes.

The type of flux used is determined by the nature of the impurities present in the "ore."

(a) If the ore contains acidic impurities like SiO₂ or P₂O₅ then basic fluxes such as lime (CaO), limestone (CaCO3), and magnesite (MgCO₃) and Haematite (Fe₂O₃) are used.

$S i O_{2} + C a O \to {C a S i O}_{3}$

Acidic Impurity Basic Flux Calcium silicate (Slag)

$S i O_{2} + {C a C O}_{3} \to {C a S i O}_{3} + {C O}_{2}$

Acidic Impurity Basic Flux Calcium silicate (Slag)

$S i O_{2} + {M g C O}_{3} \to {M g S i O}_{3} + {C O}_{2}$

Acidic Impurity Basic Flux Magnesium silicate (Slag)

(b) If basic impurities such as CaO, FeO and MgCO₃ etc. are present in the ore, acidic fluxes such as sand (SiO₂), borax (Na2B4O7.10H2O) and acids like HCl, H₃PO₄ are introduced."

$F e O + S i O_{2} \to {F e S i O}_{3}$

Basic Impurity Acidic Flux Ferrous silicate (Slag)

$C a O + S i O_{2} \to {C a S i O}_{3}$

Basic Impurity Acidic Flux Calcium silicate (Slag)

Practice Problems:

Q1. Which of the following given compounds can be considered as acidic flux?

A. CaO
B. MgO
C. CaCO₃
D. SiO₂

Answer: (D)

Solution: In option (A), (B) and (C), all contain alkaline earth metal which form basic oxides, carbonates and nitrates etc, where as in option (D), SiO₂ contains a p-block element which forms acidic oxides. Hence the only acidic flux given in these above options is SiO₂ .

Hence, the correct answer is option (D).

Q2. Which of the following given compounds can be considered as basic flux?

A. MgCO₃
B. SiO₂
C. HCl
D. H₃PO₄

Answer: (A)

Solution: In option (B), (C) and (D), all contains p-block element like Si, Cl and P which form acidic oxides, whereas in option (A), MgCO₃ contain alkaline earth metal which forms basic oxides, carbonates and nitrates etc. Hence the only basic flux given in these above options is MgCO₃ .

Hence, the correct answer is option (A).

Q3. Which of the following given reactions can be considered as the correct representation of formation of slag?

$A . C a O + M g {C O}_{3} \to {C a M g O}_{2} + {C O}_{2} B . N O + S i O_{2} \to {N S i O}_{3} C . C a O + S i O_{2} \to {C a S i O}_{3} D . F e O + {F e}_{2} O_{3} \to {F e}_{3} O_{4}$

Answer: (C)

Solution: In order to form slag, acidic flux should be added in basic impurity or basic flux should be added in acidic impurity.

In option (A) CaO & MgCO₃ Both are basic in nature. Hence they can not make slag and is not a correct representation of formation of slag.

In option (B) NO & SiO₂ Both are acidic in nature. Hence they also can not make slag and is not a correct representation of formation of slag.

In option (C) CaO being basic flux and SiO₂ being acidic impurity can make slag and is a correct representation of formation of slag.

In option (D) again FeO & Fe₂O₃ Both are acidic in nature. Hence they can not make slag and is not a correct representation of formation of slag.

Hence, the correct option is (C).

Q4. Why do we need to form flux in the process of metallurgy?

Slag formed from reaction reaction with flux and impurity is heavier in nature and hence it can be easily settle down at the surface of the molten metal
Flux is a substance that chemically combines with slag that may still be present in the calcined or roasted ore to form easily fusible material called gangue.
Slag produced by reactions involving flux and gangue is lighter and can be removed from the surface of molten metal by scraping.
Slag produced from flux and gangue can be either lighter or heavier depending on the procedure.

Answer: (C)

Solution: Flux is a substance that chemically mixes with gangue (earthly impurities) that may still be present in the calcined or roasted ore to generate an easily fusible substance called slag. Option (B) seems also incorrect as per the information.

Slag is insoluble in molten metal, but since it is lighter, it may be quickly scraped off the top of the molten metal, making gangue clearance easier.

Option (A) and option (D) are incorrect as Slag is always lighter.

Hence, the correct answer is option (C).

Frequently Asked Questions-FAQs:

Q1. What are brazing and soldering, and how do they relate to flux?
Answer: In both brazing and soldering, two metals are connected by melting a metal alloy (solder), which flows over them.

The primary distinction between the two is the temperature. Brazing is a procedure in which the filler metal (i.e. solder) has a liquidus temperature above 450o C. Soldering, on the other hand, uses filler metals with liquidus temperatures below 450o C.

Flux is used in the process of brazing and soldering. It is used to clean the metals that will be joined, removing any oxidation and preventing new oxidation from forming. Using the appropriate flux for the job necessitates ensuring that the flux is chemically compatible with the metals and solder

Q2. Why is flux required in metallurgy?
Answer: In metal alloys, flux is any substance added to the smelting of ores to promote fluidity and remove undesirable contaminants in the context of slag. Limestone is commonly used in the industrial processes of iron ores for this objective. Flux is utilized during heating to remove oxide films, promote wetting, and prevent surface reoxidation.

Q3. What exactly is rosin flux?
Answer: The base material of rosin flux is mainly rosin extracted from pine tree sap. It also contains various acids as activators, which promote burning hot solder wetting by discarding oxides from the metal.

In addition to the rosin and activators, the flux will incorporate added solvents and ingredients to aid in soldering and protect the metal against corrosion.

Q4. What exactly is metallurgy and how it is helpful in our survival?
Answer: Metallurgy is the study and understanding of the physical and chemical behavior of metallic elements, their constituents, and alloys, and is part of the Materials Sciences and Engineering discipline.

Some important applications of Metallurgy are:

Metals and mineral products are all around us: at home, on our way to and from work, and in our workplaces or factories.
They play an important role in the production of commercial airliners, means of transport, and commercial vehicles, as well as buildings, surgical implants, kitchenware and cooking utensils, coins and jewelry, firearms, and string instruments.