JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II Year MINERAL PROCESSING Previous Question Paper (March - 2021 - R18)

Crushing is the first stage of comminution, reducing run-of-mine ore to a manageable size. It is typically divided into three stages: primary, secondary, and tertiary crushing. Primary crushing reduces large rocks (up to 1.5 meters) to 100-250 mm using jaw or gyratory crushers. Secondary crushing reduces the product to 20-50 mm using cone or roll crushers. Tertiary crushing further reduces the material to fine sizes (5-10 mm) using fine-cone crushers or hammer mills.

The mechanism of comminution involves the application of mechanical stress to overcome the internal cohesive forces of the mineral particles. This occurs through four primary modes: Impact (sudden blow), Attrition (rubbing or grinding), Compression (crushing between two surfaces), and Shear (cutting or tearing). In a crusher, material is primarily subjected to compression between a moving and a stationary plate, causing crack propagation along lines of weakness until the particle fractures.

The Reduction ratio is defined as the ratio of the size of the feed material to the size of the product material. It is mathematically expressed as:

where F is the size of the feed (usually the 80% passing size) and P is the size of the product (80% passing size). It indicates the degree of size reduction achieved in a specific crushing or grinding stage.

Its importance on processing efficiency is significant for several reasons:

• Energy Consumption: Higher reduction ratios generally require more energy per unit mass. Optimizing the ratio helps in balancing power usage with throughput.
• Equipment Selection: Different crushers have specific optimal reduction ratios; exceeding these limits can lead to mechanical failure or excessive wear.
• Downstream Performance: The reduction ratio determines the liberation of valuable minerals. If the ratio is too low, minerals remain locked in the gangue; if too high, it may lead to over-grinding, causing excessive fines and loss of recovery in subsequent separation processes.
• Capacity Control: It directly influences the throughput of the plant, as maintaining an appropriate ratio ensures a consistent feed size for subsequent grinding circuits like ball mills.