Question

Silica refractories are also known as

• A. Acid refractories
• B. Basic refractories
• C. Neutral refractories
• D. Magnesia refractories

Hint:

Silica refractories resist acidic slags but react with basic slags, hence classified as acidic.

Answer 1

The Correct Option is A

Silica refractories are used to line furnaces where the working atmosphere or slag is acidic, for example the roofs of acid electric-arc furnaces and coke-oven walls.

1. Acid refractories: \( \text{SiO}_2 \) does not react with acidic slag but is attacked by basic oxides, which is precisely the acid-refractory behaviour, so silica brick is grouped here.
2. Basic refractories: these are magnesia or dolomite based and would be chemically incompatible with an acidic material like silica.
3. Neutral refractories: alumina or chromite based materials are inert to both slag types, unlike silica which reacts strongly with basic slag.
4. Magnesia refractories: this is simply the basic refractory made of \( \text{MgO} \), the opposite chemical class from silica.

So silica refractories are correctly identified as acid refractories.

Answer 2

Using the Lux-Flood acid-base theory for oxides, an oxide is termed acidic if it can accept an oxide ion (\( O^{2-} \)) from a basic oxide to form a complex silicate, and basic if it donates one. Let's classify each option against this rule.

1. Acid refractories: \( \text{SiO}_2 \) readily accepts \( O^{2-} \) ions from basic oxides such as \( \text{CaO} \) or \( \text{MgO} \), forming silicates. This oxide-ion-accepting behaviour is the defining trait of an acidic oxide, exactly how silica refractories behave in service.
2. Basic refractories: Basic refractory oxides like \( \text{MgO} \) donate \( O^{2-} \) ions rather than accept them, the opposite chemical role to silica.
3. Neutral refractories: Amphoteric oxides such as \( \text{Al}_2\text{O}_3 \) can act as either donor or acceptor depending on the slag, but pure \( \text{SiO}_2 \) shows no such dual behaviour, it is consistently an acceptor.
4. Magnesia refractories: Magnesia is a strongly basic oxide, chemically the reverse of the acidic \( \text{SiO}_2 \) network that makes up silica brick.

Because \( \text{SiO}_2 \) consistently behaves as an oxide-ion acceptor in the Lux-Flood sense, silica refractories are classified as acid refractories.

Therefore, the correct answer is Acid refractories.