Step 1: Understanding the Concept:
This question asks to identify a specific type of size reduction mill based on its mechanism of action: rubbing material between two rotating circular disks.
Step 2: Detailed Explanation:
Let's describe the working principle of each type of mill:
(A) Colloid mill: This mill is used for making extremely fine dispersions or emulsions (colloids). It works by subjecting the material to high shear forces in a very narrow gap between a high-speed rotor and a stationary stator. It's used for liquids and pastes, not typically for dry grains.
(B) Ball mill: This mill consists of a rotating hollow cylinder partially filled with balls (made of steel, ceramic, etc.). As the cylinder rotates, the balls are lifted and then fall, crushing and grinding the material through impact and attrition. It does not use rotating disks.
(C) Hammer mill: This mill uses high-speed rotating hammers inside a cylindrical casing to shatter and pulverize material through repeated impacts. The particle size is controlled by a screen at the bottom. The mechanism is impact, not rubbing between disks.
(D) Attrition mill (or Disc mill): This mill operates exactly as described in the question. It has two circular disks, at least one of which rotates. The faces of the disks have grooves or teeth. The material is fed into the center and moves outwards through the gap between the disks. Size reduction occurs primarily through attrition (rubbing) and shearing forces as the material is ground between the disk faces. This type of mill is commonly used for grinding grains into flour or feed.
Step 3: Final Answer:
The mill that reduces size by rubbing grains between grooved rotating disks is an attrition mill. Therefore, option (D) is the correct answer.