Mechanical Operations

Step 1: Understanding the critical speed of a ball mill.
The critical speed of a ball mill is the speed at which the centrifugal force is equal to the gravitational force, causing the balls to stay at the top of the mill and not fall back. At speeds higher than the critical speed, the balls are carried to the top and do not effectively crush the material, thus the mill's efficiency is reduced.

Step 2: Explanation of options.
- (A) When the speed is above the critical speed, the efficiency is lowest. - (B) and (C) Efficiency does not increase above the critical speed; it actually decreases. - (D) There is no such thing as "hundred percent" efficiency at this point.

Final Answer:

Step 1: Identifying coarse crushers.
Jaw crushers are used for coarse crushing of materials. They work by compressing materials between two jaws, one fixed and the other movable. These crushers are used to reduce the size of large materials like rocks.

Step 2: Explanation of options.
- (D) Jaw crusher is the correct answer, as it is used for coarse crushing. - (A), (B), and (C) These crushers are not primarily used for coarse crushing.

Final Answer:

Step 1: Understanding the effect of tapping.
When solid particles are tapped (such as in packing or compaction), the particles move closer together, reducing the void spaces between them. This results in an increase in the bulk density of the mass, as the same amount of material occupies a smaller volume.

Step 2: Analysis of options.
- (A) Tapping does not cause a decrease in bulk density; rather, it compacts the particles, increasing the density. - (C) The bulk density does not remain unchanged during tapping; it typically increases due to compaction. - (D) More information is not needed, as the effect of tapping on density is well-established.

Final Answer:

Step 1: Understanding the process.
Sedimentation is the process of separating particles from a fluid by allowing them to settle under gravity. It is commonly used in industries like water treatment and mineral processing.

Step 2: Explanation of options.
- (D) Sedimentation is the correct process for separating particles based on their size, shape, and density. - (A) Classification is the process of sorting particles by size. - (B) Froth flotation is used for separating hydrophobic particles. - (C) Thickening is used to increase the concentration of solids in a slurry.

Final Answer: