Flux and Slag
Master the chemistry of removing infusible impurities during Smelting.
Even after physical concentration (beneficiation) of an ore, some impurities (gangue) remain intimately mixed with it. During the high-temperature extraction process (Smelting), these remaining impurities often have very high melting points and do not melt. To remove them, chemists introduce a foreign substance known as a Flux.
1. The Fundamentals: Flux & Slag
What is a Flux?
A Flux is a chemical substance added to the ore during smelting to react with the infusible (high-melting-point) gangue and convert it into a fusible (easily meltable) liquid mass.
What is Slag?
The liquid mass formed by the chemical reaction between the flux and the gangue is called Slag.
Key Properties of Slag:
- Fusible: It has a lower melting point than the metal and the gangue, so it remains liquid at furnace temperatures.
- Low Density: Slag is lighter than the molten metal. Therefore, it floats on top of the molten metal.
- Protective Layer: Because it floats on top, the layer of slag prevents the hot molten metal from coming into contact with air and getting re-oxidized.
- Immiscible: It does not mix (dissolve) with the molten metal, allowing for easy separation (tapping off).
Figure 1: Cross-section of a furnace hearth. Slag floats, protecting the metal and allowing separate extraction.
2. Types of Flux
The choice of flux depends entirely on the chemical nature of the gangue present in the ore. The general rule is: Acidic gangue requires a Basic flux, and Basic gangue requires an Acidic flux. This is essentially a high-temperature acid-base neutralization reaction.
A. Acidic Flux
Used to remove Basic Impurities (like metallic oxides such as $FeO$, $CaO$, $MgO$).
Common Examples:
- Silica / Sand ($SiO_2$)
- Borax ($Na_2B_4O_7 \cdot 10H_2O$)
The iron impurity ($FeO$, which is basic) is removed by adding Silica ($SiO_2$, acidic flux) to form Iron Silicate slag.
$FeO + SiO_2 \xrightarrow{\Delta} FeSiO_3$
(Basic Gangue) + (Acidic Flux) ➔ (Slag)
B. Basic Flux
Used to remove Acidic Impurities (like non-metallic oxides such as $SiO_2$, $P_2O_5$).
Common Examples:
- Limestone ($CaCO_3$)
- Quicklime ($CaO$)
- Magnesite ($MgCO_3$)
- Hematite ($Fe_2O_3$)
In the blast furnace, Limestone decomposes into $CaO$. This $CaO$ (basic flux) removes Silica ($SiO_2$, acidic gangue).
$CaO + SiO_2 \xrightarrow{\Delta} CaSiO_3$
(Basic Flux) + (Acidic Gangue) ➔ (Slag)
Mastery Check: Flux & Slag
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