Classification methods for ball mills

1. Classification by length:

(1) Short mills are defined as those with a length-to-diameter ratio of less than 2; they are also referred to as ball mills. Typically, a single-compartment ball mill is used for coarse grinding or primary grinding, and two to three ball mills can also be operated in series.

(2) Medium-length mill: A mill with a length-to-diameter ratio of approximately 3 is classified as a medium-length mill.

(3) Long mills: When the length-to-diameter ratio is 4 or greater, the mill is classified as a long mill, also known as a tube mill. Both medium-long and long mills are typically divided into 2 to 4 chambers internally and are widely used in cement plants.

2. Classification by the shape of the grinding media

(1) Ball mills primarily use steel balls or steel segments as the grinding media inside the mill. This type of mill is the most widely used.

(2) Rod mill: The mill chamber is filled with steel rods 50–100 mm in diameter as the grinding media. The length-to-diameter ratio of a rod mill is typically 1.5–2.

(3) Ball-and-rod mill: This type of mill typically has 2 to 4 chambers. The first chamber is loaded with cylindrical steel rods as the grinding media, while the subsequent chambers are filled with steel balls or steel segments. The length-to-diameter ratio of a ball-and-rod mill should be around 5, with the length of the rod chamber accounting for 1.2 to 1.5 times the effective diameter of the mill. The rod length should be approximately 100 mm shorter than the rod chamber to facilitate parallel alignment of the rods and to prevent crossing and random arrangement.

(4) The grinding media loaded into the gravel ball mill include gravel, pebbles, and porcelain balls. The mill lining is made of granite or porcelain material. This type of mill is used in the production of white or colored cement as well as ceramics.

3. Classification by Inclined Material Feeding Method

(1) Tail-discharge ball mill: The material to be ground is fed in at one end of the mill and discharged at the other end; such a mill is referred to as a tail-discharge ball mill.

(2) Center-discharge ball mill: The material to be ground is fed in at both ends of the mill and discharged from the middle section of the mill shell, hence the name “center-discharge ball mill.” This type of mill is equivalent to operating two ball mills in parallel, resulting in a compact equipment layout and a simplified process flow.

(3) Discharge-type ball mills are classified, according to their discharge methods, into grate discharge, overflow discharge, peripheral discharge, and pneumatic discharge, among others.

4. Classification by Transmission Method

(1) Center-driven ball mill: The electric motor drives the mill’s rotation by transmitting power through a reducer to the hollow shaft at the discharge end of the mill. The output shaft of the reducer is aligned on the same straight line as the mill’s centerline.

(2) Edge-driven ball mill: The electric motor drives the mill shell to rotate by means of a reducer that engages with a large gear mounted on the discharge-end shell.

5. Other Categories

Based on the process operation, ball mills can be further classified into dry-ball mills, wet-ball mills, batch ball mills, and continuous ball mills. Compared with batch ball mills, continuous ball mills offer higher production capacity, lower specific energy consumption per unit of product, a higher degree of mechanization, and fewer operators required. However, they entail substantial capital investment and more complex operation and maintenance. Nowadays, batch ball mills are rarely used and are typically employed as laboratory test mills.