JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II MINERAL PROCESSING Previous Question Paper (July/August - 2021 - R16)

A jaw crusher is a primary crusher used for size reduction of hard and abrasive ores. It consists of a fixed jaw and a swinging jaw, which forms a V-shaped crushing chamber. The swinging jaw is mounted on a pitman that moves up and down via an eccentric shaft, creating a reciprocating motion that crushes the material against the fixed jaw.

Operation:

1. Material is fed into the top opening of the V-shaped chamber.
2. As the swinging jaw moves toward the fixed jaw, the material is compressed and crushed.
3. As the jaw moves away, the crushed material falls by gravity toward the discharge opening at the bottom.
4. The size of the product is controlled by adjusting the discharge gap (closed-side setting).

Advantages:

• High crushing capacity and efficiency for hard, abrasive materials.
• Simple, robust construction with low maintenance requirements.
• Capable of handling large feed sizes with high reduction ratios.

Limitations:

• Produces a relatively high percentage of fines and non-uniform, flaky particles.
• Not suitable for sticky or plastic ores as they can clog the crushing chamber.
• High vibration levels during operation requiring heavy foundations.

A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes. It consists of a hollow cylindrical shell rotating about its axis, partially filled with grinding media such as steel balls. As the shell rotates, the balls are lifted by the friction and centrifugal force along the inner wall until they reach a certain height, where they cascade or cataract down onto the material, causing size reduction through impact and attrition.

The critical speed of a ball mill is the rotational speed at which the centrifugal force acting on a ball at the inner surface of the mill shell equals the component of the gravitational force acting towards the center of the mill. At this speed, the balls remain pinned to the shell wall and do not cascade, rendering grinding ineffective.

To derive the expression for critical speed, consider a ball of mass m at the inner surface of a mill with radius R, rotating at an angular velocity ω. Let θ be the angle made by the ball with the vertical axis. The forces acting on the ball are the centrifugal force mω²R acting outwards and the component of gravity mg cos θ acting inwards. Equilibrium occurs when:

At the point of critical speed, the ball loses contact with the shell at the top of the mill, where θ = 0 and cos θ = 1. Substituting these into the equation:

Solving for the angular velocity ω_c:

Since ω = 2πN / 60, where N is the speed in revolutions per minute (rpm), we substitute to find N_c:

Converting to diameter D = 2R:

In practice, ball mills are operated at 65% to 80% of this critical speed to ensure optimal cascading action for efficient grinding.