ANS:B - Minimum size

The balls capable of grinding the feed in a ball mill with maximum efficiency are of maximum size. Here's why:

1. Maximum Size Balls:
   • Larger balls have more mass and surface area, which increases the impact force and grinding efficiency in the mill.
   • They also have higher kinetic energy, resulting in more effective breakage of the feed material.
2. Cast Iron Balls:
   • While cast iron balls are commonly used and effective, the size of the balls is more critical for maximizing efficiency than the material they are made of.
3. Minimum Size Balls:
   • Smaller balls have less impact and grinding efficiency compared to larger balls, as they have lower kinetic energy and may not effectively break down larger particles in the feed.
4. Elliptical Balls:
   • Elliptical balls are not standard in ball mills; spherical or nearly spherical balls are typically used for grinding operations.