JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year MINERAL PROCESSING Previous Question Paper (February/March - 2022 - R18)

Grinding is the final stage of size reduction in mineral processing, where material is reduced to fine particles. Grinding is broadly classified based on the method of operation and the environment:

• Batch Grinding: The mill is charged with a fixed amount of material and grinding media, operated for a set duration, and then discharged completely. It is primarily used for small-scale operations or laboratory testing.
• Continuous Grinding: Material is fed into the mill at a constant rate and the product is discharged continuously. This is the standard method for industrial-scale mineral processing plants.
• Dry Grinding: Performed without the addition of water. It is used when the final product must be dry or when the material reacts with water.
• Wet Grinding: Performed with water added to the feed, creating a slurry. This is more efficient than dry grinding as it prevents dust, reduces power consumption, and facilitates easier material transport.

In continuous grinding, the mill acts as a flow-through reactor. The feed enters through a hollow trunnion at one end, travels through the rotating shell where it is subjected to impact and attrition by grinding media, and the ground product exits through the opposite end. The residence time of the particles is controlled by the feed rate and the internal design of the mill.

A common example of continuous grinding is the Ball Mill used in a closed-circuit grinding system. In this setup, the continuous discharge from the ball mill is fed into a classifier, such as a hydrocyclone. The cyclone separates the fine particles (overflow) as the final product, while the coarse particles (underflow) are returned to the mill feed for further grinding. This ensures that only particles of the desired size leave the circuit, maintaining high efficiency and consistent product quality.

Comminution laws describe the relationship between the energy input required for size reduction and the resulting change in particle size. The three primary laws are Rittinger's Law, Kick's Law, and Bond's Law.

1. Rittinger's Law: Proposed by Rittinger in 1867, this law states that the energy required for size reduction is directly proportional to the new surface area created. It is most applicable to fine grinding where the increase in surface area is significant.

Where E is energy, K_R is Rittinger's constant, and d₁ and d₂ are the initial and final particle diameters.

2. Kick's Law: Proposed by Kick in 1885, this law states that the energy required for size reduction is proportional to the reduction ratio (the ratio of the initial size to the final size). It is generally considered more applicable to coarse crushing where the deformation of the particles is the primary energy consumer.

Where K_K is Kick's constant.

3. Bond's Law: Proposed by Fred C. Bond in 1952, this is a semi-empirical law that acts as a compromise between Rittinger's and Kick's laws. It states that the energy input is proportional to the square root of the surface-to-volume ratio of the product. It is the most widely used law in industrial mineral processing for calculating the power requirements of ball mills and rod mills.

Where W_i is the Work Index, defined as the kilowatt-hours per short ton required to reduce a material from theoretically infinite size to 100 microns.