JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II MINERAL PROCESSING Previous Question Paper (August/September - 2022 - R18)

A sample is a small portion of a bulk material that is representative of the whole, ensuring that its chemical and physical properties accurately reflect the entire lot. Sampling is the process of extracting this representative fraction from a large quantity of ore or mineral product for analysis or testing.

Common sampling techniques include:

• Manual Sampling: Involves taking increments by hand using shovels or scoops. It is prone to human bias and is generally used for small-scale operations or preliminary checks.
• Mechanical Sampling: Uses automated devices like cross-stream cutters or Vezin samplers. These cut through the entire stream of material at regular intervals, minimizing bias and ensuring high accuracy.
• Coning and Quartering: The material is piled into a cone, flattened, and divided into four quarters. Two opposite quarters are kept, and the process is repeated until the desired sample size is reached.
• Riffle Sampling: A device with a series of parallel slots (riffles) is used to split the material into two equal streams, effectively reducing the sample size while maintaining representative distribution.

The ball mill operates on the principle of impact and attrition. As the cylindrical shell rotates, the grinding media (steel balls) and the ore are lifted by the friction and liner design until they reach a point where gravity overcomes the centrifugal force, causing them to cascade or cataract down onto the material.

The operation is characterized by three distinct zones:

• Cataracting Zone: Balls are carried to the top of the mill and fall freely in a parabolic path, striking the ore at the bottom. This provides high-impact energy suitable for coarse grinding.
• Cascading Zone: Balls roll over each other in a continuous stream. This creates attrition and abrasion, which is effective for fine grinding.
• Centrifuging Zone: At high speeds, balls are pinned against the shell by centrifugal force and do not fall. This is an inefficient state for grinding and is avoided in practice.

Sketch description: A circular cross-section of the mill shows the shell rotating. The charge is lifted along the rising side (rising zone), falls through the center (cataracting zone), and rolls along the bottom (cascading zone). The critical speed is the point where the centrifugal force equals the gravitational force, preventing the balls from falling.