Question

Fusion point of basic refractory material is

• A. Reduced by the addition of acid oxides
• B. Increased by addition of acid oxides
• C. Not affected by the addition of acid oxides
• D. Always less than 1000°C

Hint:

Remember the fundamental chemical principle: "Acids react with bases". In high-temperature chemistry (like metallurgy and ceramics), this reaction leads to the formation of a slag or a glassy phase, which almost always has a lower melting point than the parent refractory oxides. This phenomenon is known as fluxing.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Refractory materials are materials that are resistant to high temperatures. They are classified based on their chemical nature as acidic, basic, or neutral. The fusion point (or melting point) of a refractory is a critical property. This question explores how the fusion point of a basic refractory is affected by contamination with an acidic substance.

Step 2: Detailed Explanation:
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Basic Refractories: These are made of basic oxides, such as magnesium oxide (MgO, Magnesia) or calcium oxide (CaO, lime). They are stable at high temperatures and resistant to attack by basic slags. -

Acidic Oxides: These are oxides of non-metals, such as silicon dioxide (SiO₂, silica) or phosphorus pentoxide (P₂O₅). When a basic oxide comes into contact with an acidic oxide at high temperatures, they react chemically to form a salt, which is typically a silicate, phosphate, etc. \[ \text{Basic Oxide} + \text{Acidic Oxide} \xrightarrow{\Delta} \text{Salt (e.g., Silicate)} \] For example: \[ \text{MgO (basic)} + \text{SiO}_2 \text{ (acidic)} \rightarrow \text{MgSiO}_3 \text{ (Magnesium Silicate)} \] The product of this reaction (the silicate, in this case) generally has a much lower melting point than the original pure basic refractory oxide. For instance, pure MgO has a melting point of about 2800°C, and pure SiO₂ melts at about 1700°C, but some magnesium silicates can start to form a liquid phase (eutectic) at temperatures as low as 1543°C. This formation of a low-melting-point compound (fluxing) means that the addition of an acid oxide impurity to a basic refractory will significantly

reduce its fusion point and its overall refractoriness. Therefore, basic refractories should not be used in contact with acid slags or materials, and vice-versa, to prevent this chemical attack and premature failure.

Step 3: Final Answer:
The fusion point of a basic refractory material is reduced by the addition of acid oxides. This corresponds to option (A).