JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MINERAL PROCESS ENGINEERING Previous Question Paper (November/December - 2020 - R16)

Crushing machines are primary equipment used in mineral processing to reduce the size of run-of-mine ore into smaller, manageable fragments. The main types of crushing machines used in the industry include:

• Jaw Crushers: Used primarily for primary crushing of hard and abrasive ores.
• Gyratory Crushers: High-capacity machines used for primary crushing of large volumes of ore.
• Cone Crushers: Typically used for secondary and tertiary crushing stages.
• Impact Crushers (Hammer Mills): Utilize impact forces to break softer, non-abrasive materials.
• Roll Crushers: Employ compression between two rotating rolls for size reduction.

A Jaw Crusher operates on the principle of compression between a fixed jaw and a swinging (moving) jaw. The crushing chamber is V-shaped, where the ore is fed from the top. As the moving jaw oscillates, it compresses the material against the fixed jaw, breaking it until the particles are small enough to pass through the discharge opening at the bottom.

The key components include a heavy-duty frame, a fixed jaw plate, a swinging jaw plate mounted on a pitman, an eccentric shaft that provides the motion, and a toggle plate system that acts as a safety mechanism to prevent damage from uncrushable materials. The discharge size is controlled by adjusting the toggle setting, which dictates the gap between the jaws at their closest point.

Description of the figure: The diagram would show a vertical cross-section of the crusher. At the top, the feed opening is wide. One side is a vertical fixed jaw plate. Opposite it is the swinging jaw plate, angled to create a narrowing gap. At the bottom, the discharge opening is shown. The eccentric shaft is located at the top of the pitman, and the toggle plate is positioned at the bottom of the pitman, connecting it to the frame to allow for the characteristic crushing stroke.

Grinding machines in the mineral processing industry are classified based on the mechanism of size reduction, which typically involves impact, attrition, and abrasion. Common types include:

• Tumbling Mills: These include Ball Mills, Rod Mills, Pebble Mills, and Autogenous/Semi-Autogenous (SAG) mills. These use a rotating cylindrical shell containing grinding media.
• Stirred Mills: These include Tower Mills and IsaMills, which use an internal agitator to move grinding media, typically used for fine and ultra-fine grinding.
• Roller Mills: These use heavy rollers pressing against a rotating table or ring to crush material, commonly used in cement and coal industries.
• Impact Mills: These include Hammer Mills and Impact Crushers, which use high-speed rotating elements to shatter particles.

A Ball Mill is a cylindrical device used for fine grinding. It consists of a hollow cylindrical shell rotating about its horizontal axis, partially filled with grinding media such as steel or ceramic balls. As the shell rotates, the balls are lifted along the inner wall by friction and then cascade or cataract down onto the ore, causing size reduction through impact and attrition.

Key components include the feed inlet (trunnion), the discharge end, the shell lined with wear-resistant liners, and the drive system (motor and gear). The grinding efficiency depends on the rotational speed, which is typically maintained at 65-80% of the critical speed to ensure the balls are lifted and dropped effectively rather than centrifuging against the wall.

A neat sketch of a Ball Mill would show a horizontal cylinder supported on trunnion bearings. The interior is depicted with a charge of grinding balls and ore. The feed enters through the hollow trunnion at one end, and the ground product exits through the discharge trunnion or a grate at the opposite end. The shell is shown rotating, with the grinding media shown in a cascading motion profile within the lower half of the cylinder.