Grinding Mill Terminology and Usage

A mill is a mechanical device used to grind and crush materials. It is widely used in mining, building materials, chemicals, electricity, metallurgy, and other industries. The mill crushes large pieces of material into fine particles through the interaction of rotation and grinding media to meet the requirements of the production process. Today we will introduce some mill terminology and usage.

Grinding Mill Term Explanation

Critical Speed

Critical speed is defined as the point at which the centrifugal force applied to the grinding mill charge is equal to the force of gravity. In production, the speed at which the outermost grinding medium begins to “centrifugally operate” is usually called the “critical speed” of the mill. At critical speed, the grinding mill charge clings to the mill’s inner surface and does not tumble.

How to calculate the critical speed?

Most ball mills operate at approximately 75% critical speed, as this is determined to be the optimum speed. The true optimum speed depends upon the drum diameter. Larger drum diameters operate at lower than 75% critical speed whilst smaller drum diameters operate at higher than 75% critical speed.

Grinding Efficiency　

Grinding efficiency refers to the amount of ore processed for every 1kW. h of electricity consumed. It has the following representation methods: The raw ore (tons) processed for every 1kW.

Qualified Particle Size

The qualified particle size means that the upper limit of the particle size should be smaller than the specified maximum particle size, and the lower limit of the particle size should be less than the over-crushed part.

Mill’s Throughput

What is the mill’s throughput? How to calculate it?

The mill’s throughput refers to the total amount of ore that can pass through per unit time. The calculation method is:

Grinding Fineness

The fineness of the ground product is usually screened with a 200-mesh “standard sieve” and expressed as the percentage of the amount under the sieve to the total amount of the product (“mesh” is the number of sieve holes in each inch of the sieve layer, and is used to name the sieve).

Mill Utilization Coefficient

The mill utilization coefficient is the average amount of raw ore processed per cubic meter of mill effective volume per unit time, expressed in tons/m3·hour. The mill utilization coefficient q=Q/V (tons/m3·hour).

Mill Hourly Production Capacity

The mill’s hourly production capacity refers to the amount of raw ore that the mill can process per unit of time (hour) under certain feed and product particle size conditions, expressed in tons/unit·hour.

Grinding and Classifying Unit

What are the process indicators of the grinding and classifying unit?
The main process indicators are grinding fineness, mill production capacity and operation rate, classification and grinding efficiency. The former is a quality indicator, and the latter is a quantity indicator.

Operating factors that affect the operation of the grinding-classifying unit.

The operating factors that affect the operation of the grinding-classifying can be roughly summarized into the following three categories:

① Ore properties. Including hardness, mud content, feed particle size, and required grinding product fineness.

② Mill structure. Including mill type, specifications, speed, etc.

③ Operating conditions. Mainly grinding media, slurry, concentration, return sand ratio, and classification efficiency.

Technical Efficiency of Grinding Mill

What is the technical efficiency of the grinding mill?

It refers to the ratio between the percentage of qualified particle size in the product obtained after grinding and the percentage of particle size larger than qualified particle size in the feed ore.

How to improve the technical efficiency of grinding mills? What should be paid attention to in grinding mill operation?

To improve the technical efficiency of grinding mill, the following aspects can be considered:

① Adopt a closed-circuit grinding process.

② In the selection of equipment, it is suitable to use a rod mill with peripheral discharge for coarse grinding and a grid-type ball mill for fine grinding.

The following aspects should be paid attention to in grinding mill operation.

① Appropriate circulating load should be used in closed-circuit grinding;

② The speed of the grinding mill should be properly controlled;

③ The ball loading system should be reasonable;

④ The grinding concentration should be appropriate.

Understanding these mill-related terms is of great significance for optimizing the grinding process, and improving grinding efficiency and product quality. By mastering these basic concepts, you can better apply and manage mill equipment and promote the efficient operation of industrial production.

Grinding Machine Use

Many problems may occur during the use of the grinding mill, among which “mill bloating” and “ball loading” are two common and important problems. The following is a detailed description of these two problems.

Grinding Mill Bloating

Mill bloating, also known as “mill expansion” or “mill overload”, refers to the phenomenon that the mill is filled with too much material and steel balls, causing the mill to operate abnormally.

1. What phenomena occur when the grinding mill bloats?

Generally, the following phenomena will occur:

① The main motor ammeter indicates that the current is decreasing;

② The grinding mill discharges large pieces of ore and the slurry gushes out;

③ The overflow and coarsening of the classifier are serious, and the return sand increases significantly;

④ The running sound of the grinding mill is dull, and the impact sound of the steel ball cannot be heard.

2. When the grinding mill bloats, how should it be handled in operation?

First, we need to analyze the changes in the properties of the ore, whether the feed rate, water supply, return sand amount, overflow particle size, etc. are normal. In operation, the following measures can generally be taken:

① Reduce the feed rate of the grinding mill or stop feeding for a short time;

② Adjust the water consumption, that is, strictly control the grinding concentration;

③ Reasonably add grinding media.

3. Why does the current of the main motor decrease when the mill is bloating?

The operation of the mill and the grinding of the ore require energy, and most of the energy is consumed in the grinding process of the ore. Due to the bloating, the grinding effect is greatly reduced. Therefore, the mechanical energy converted from electrical energy to grinding is greatly reduced, so the current of the main motor decreases; from the perspective of the movement of the grinding medium, due to the bloating, the material increases, the concentration increases, the height of the copper ball is reduced, and the grinding force is small, so the kinetic energy decreases and the current decreases.

Ball Mill Loading

1. Balls loaded amount & qualified filling rate
The amount of balls loaded has a certain effect on the grinding efficiency. If the balls are too few, the mill efficiency is low. If the balls are too many, the inner balls are easy to interfere during operation, which destroys the normal circulation of the balls. The grinding (crushing) efficiency is also reduced.

The practice has proved that the amount of balls loaded is related to the rotation rate of the mill. When the mill runs at a low speed, more balls can be loaded, and when the speed is high, fewer balls can be loaded.

For grid-type ball mills: filling rate φ=40-45%
For overflow ball mills: filling rate φ=35-40%

2. Steel balls size & amount
The size and dimensions of the steel balls mainly depend on the physical and mechanical properties of the ore and the particle size composition of the ore.

To process ores with high hardness and coarse particle size, a greater impact force is required, and larger steel balls need to be added; to process softer ores with smaller (less) ore particle size and require finer product particle size, grinding should be the main method and smaller steel balls can be loaded.

The ball mill at the production site is loaded with balls of various ball diameters. According to a certain proportion, materials composed of ore particles of different sizes can be processed. Theoretically, only by ensuring that there is a certain proportion of various balls can they be adapted to the particle size composition of the milled materials and achieve good grinding effects.

3. Few large or small balls 

When the particle size of the processed material is large or the hardness is high, the small proportion of large balls will cause the grinding product to run coarse and fail to meet the fineness requirements. In addition, the processing volume is also limited, affecting the grinding efficiency.

When processing materials with small ore particle sizes and low hardness, if the proportion of small balls is small, the product particle size will also be coarse. Small balls should be added appropriately, and the proportion of small balls should be increased to adapt to the nature of the ore to obtain better product fineness. Therefore, the ball loading ratio must be strictly followed to prevent imbalance.

By properly controlling the feed rate, monitoring the discharge situation, adjusting the grinding media ratio, optimizing the operating parameters, and calculating and experimentally adjusting the ball loading amount, the problem of mill bloating can be effectively solved, the reasonable ball loading amount can be determined, thereby improving the grinding efficiency and ensuring product quality.