Flux and Slag

Flux

A chemical substance that combines with the impurities present in the roasted or calcined ore to form an easily fusible material.

Flux + Impurity $\rightarrow$ Slag

Flux is a material used to aid in purifying metals during the smelting process. When high temperatures are applied to ores, fluxes react with impurities to form slag. The kind of flux to be used is determined by the kind of metal to be extracted and the nature of the impurities. Among the most common qualities used is limestone in iron smelting. The major function of flux is to make it possible for impurities that may cause a reduction in the grade of the final product to be easily removed.

During smelting, the ore is mixed with flux and heated to high temperatures. The flux reacts with the impurities to form a separate compound, called slag, from the molten metal, and it can be easily removed. This process not only purifies the metal but also helps in lowering its melting point, therefore making the smelting process efficient.

Types of Fluxes

Fluxes can be broadly classified according to their chemical nature and the type of impurities they remove. The chief types are as follows:

1. Acidic Fluxes: These are fluxes of the nature of silica (SiO₂ which are used to remove basic impurities such as oxides of magnesium and calcium. Acidic fluxes are used in most processes, while the impurities in the ore are of a basic nature.

2. Basic Fluxes: Basic fluxes, including limestone (\(\sqrt{3}\)), are used to get rid of acidic impurities such as silica. Basic fluxes are important in making metal from the ore that has heavyweight percentages of an acidic compound.

3. Neutral Fluxes: Neutral fluxes are used when impurities are nothing but neutral bodies. Since their use is most versatile, they are, in turn, used to turbulent metalworks—welding and soldering, to name a few.

Also Read

• Ellingham Diagram
• Refining Process Against Impurities
• Extraction Of Metals: Ores And Minerals

Slag

The fusible material is formed by the reaction of flux and gangue.

$\mathrm{FeO}+\mathrm{SiO}_2 \rightarrow \mathrm{FeSiO}_3$ (slag)

Slag is formed because of the reactions between the flux and the impurities. It is a floating, molten glassy material on the surface of the molten material and is easily removed. The presence of slag protects the molten material from oxidation and aids in maintaining the temperature by insulating it. Besides protecting the molten metal from oxidation, another role of the slag is to assist in purifying the metal.

Slag plays more functions in parallel with this role of purifying metals. It retains any residual impurities that may not have reacted with the flux under capture, therefore ensuring the metal product's purity to the highest degree possible. The slag also serves the task of temperature control of the smelting process by protecting from too rapid cooling and the consequent premature hardening of the metal.

Difference Between Slag and Flux

Example

In the extraction of iron:

$\begin{aligned}
& \mathrm{CaCO}_3 \rightarrow \mathrm{CaO}+\mathrm{CO}_2 \\
& \mathrm{CaO}+\mathrm{SiO}_2 \rightarrow \mathrm{CaSiO}_3
\end{aligned}$
Here, $\mathbf{C a O}$ acts as the flux, $\mathbf{S i O}_{\mathbf{2}}$ is the gangue, and $\mathbf{C a S i O}_3$ is the slag.

Some Solved Examples

Question 1: During smelting, an additional substance is added, which combines with impurities to form a fusible product. This fusible product is known as:

1) Mud

2) Slag (correct)

3) Flux

4) Gangue

Solution

The fusible material formed by the reaction of flux and gangue is slag.

Hence, the correct answer is option (2).

Question 2: In the extraction of iron, slag is produced, which is:

1) CO

2) FeSiO₃

3) MgSiO₃

4) CaSiO₃ (correct)

Solution:

CaO reacts with SiO₂ to form CaSiO₃, which is slag.

Hence, the correct answer is option (4).

Question 3: In a metallurgical process, the flux used for removing acidic impurities is:

1) Sodium carbonate

2) bLimestone (correct)

3) Sodium chloride

4) Silica

Solution:

Limestone (CaCO₃) acts as a basic flux to remove acidic impurities.

Hence, the correct answer is option (2).

Question 4: In the metallurgical extraction of copper, the following reaction is used:

$\mathrm{FeO}+\mathrm{SiO}_2 \rightarrow \mathrm{FeSiO}_3$

FeO and FeSiO_3, respectively, are:

1) gangue and flux.

2) flux and slag.

3) slag and flux.

4) gangue and slag (correct).

Solution:

FeO is gangue, and FeSiO₃ is slag.

Hence, the correct answer is option (4).

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