Question

Low specific gravity of slag is desirable mainly due to the fact that, it

• A. is more fusible
• B. makes the slag more fluid
• C. permits clean and better separation from metal/matte
• D. protects the metal from overheating due to poor conductivity

Hint:

Think of a smelting furnace like a salad dressing bottle with oil and vinegar. For easy separation, you need two immiscible layers. The less dense layer (oil/slag) floats on top of the denser layer (vinegar/metal), allowing you to pour them out separately.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
In pyrometallurgical smelting operations, two primary liquid phases are typically produced: the molten metal (or a molten metal sulfide mixture called matte) and a molten oxide mixture called slag. The slag consists of the gangue (unwanted minerals) from the ore, fluxing agents added to lower the melting point, and any oxidized impurities. A successful smelting operation depends on the effective separation of these two liquid layers.

Step 2: Detailed Explanation:
For two liquids to be separated easily, two conditions must be met:

• They must be

immiscible (they do not mix, like oil and water).

• They must have different

densities (or specific gravities).
The slag is designed to have a lower specific gravity than the molten metal or matte. Because of this density difference, the lighter slag floats on top of the heavier metal/matte, forming a distinct, separate layer. This physical separation allows the two products to be tapped from the furnace at different levels, ensuring a clean separation.
Let's look at the other options:

• Fusibility (melting point) and fluidity (viscosity) are also important slag properties, but they are not a direct result of its low specific gravity.

• The slag layer does protect the metal from the furnace atmosphere, but this is a secondary benefit. The primary reason for controlling its specific gravity is for separation.

Step 3: Final Answer:
The most important reason for slag to have a low specific gravity is to allow it to float on top of the molten metal or matte, thereby permitting a clean and efficient physical separation of the two phases.