Mineral Processing


Mining Beneficiation 101 : Grinding & Milling

Q&A about grinding and milling

1. What are the types and features of the mill?

Answer: According to the different media of the mill, it is divided into: the medium is steel balls, it is called ball mill; the medium is steel rod is called rod mill; the medium is gravel is called gravel mill; the medium that uses its own ore as the medium is called self-mill. Adding an appropriate amount of steel balls to the self-grinding machine constitutes a so-called semi-self-grinding machine.

According to the shape of the casing and the method of discharging, the ball mill is divided into short cylindrical, tubular, and cone-shaped overflow discharge and lattice discharge, and the rod mill is divided into cylindrical overflow discharge and open low-level discharge , Self-grinding or semi-self-grinding machines are mainly short cylindrical.

2. What is the representation of the mill specifications?

Answer: The expression method is: diameter × length, expressed as D × L with symbols, such as a 2700 × 2100mm ball mill, its diameter is 2700mm, and its length is 2100mm.

3. Briefly describe the crushing process of the ore by the grinding machine?

Answer: When the mill rotates at a certain speed, centrifugal force is generated, which causes the medium to generate friction with the cylinder, and rotates together with the cylinder. When it reaches a certain height and the centrifugal force of its own gravity is strong, it will drop off the cylinder. Crush the ore. At the same time, due to the sliding phenomenon inside the medium, it also has a grinding effect on the ore. Under the impact and grinding force of the medium, the ore is crushed.

4. What are the trajectories of the medium in the mill?

Answer: There are three kinds of motion trajectories: 

  1. drop-down type (when running at low speed); 
  2. throw-down type (at normal speed); 
  3. centrifugal operation (the trajectory of the medium and the cylinder is the same).

5. What is the movement law of the medium in the mill?

Answer: There are mainly four points as follows: 

① When the mill is running at a certain speed, the medium makes a regular cyclic movement under the action of centrifugal force and gravity; 

② The trajectory of the grinding medium in the cylinder is determined by the centrifugal force and gravity. It is composed of upward movement of circular arc trajectory and downward movement of parabolic trajectory; 

③, the rising height of each layer of medium is different. The height of the medium from the outermost layer to the innermost layer decreases successively; 

④, the rotation period of each layer of the medium is different, the closer to the inner layer, the shorter the rotation period.

7. What are the types and characteristics of ball mills?

Answer: There are two types of ball mills commonly used in dressing plants: grid type and overflow type. The only difference between them is that the overflow type discharge part does not have a fan-shaped grid plate device. The overflow type ball mill is automatically discharged through the discharge port. The lattice type is equipped with lattice plates, so it has the effect of forced ore discharge, thereby increasing the processing capacity and reducing the over-crushing of the ore.

8. What are the components of the ball mill?

Answer: It is mainly composed of six parts: the cylinder body, the feeding part, the discharging part, the transmission part, the bearing part and the lubrication system.

9. What is the composition of the mining part?

Answer: The feed part is composed of the end cap of the hollow journal, the feeder and the inner sleeve of the journal.

10. What are the types of miners?

Answer: There are three types of feeders used in the ball mill of the beneficiation plant: drum, snail and combined feeders.

11. What is the composition of the discharge part? (lattice type)

Answer: The discharging part is composed of the end cover of the hollow journal, the fan-shaped grid plate, the center liner and the journal inner sleeve.


12. What are the working characteristics of the rod mill?

Answer: Its work feature is: it crushes ores by the crushing and grinding action produced by the “line contact” of steel rods. Therefore, it has a selective crushing effect, reduces the over crushing of the ore, and the product size is uniform.

13. What are the technological indicators of the grinding and classification unit?

Answer: The process indicators mainly include: grinding fineness, mill production capacity and operating rate, classification efficiency, and grinding efficiency. The former is a quality indicator, and the latter is a quantitative indicator.

14. What is the expression method of grinding fineness?

Answer: The fineness of grinding products is usually sieved with a 200-mesh sieve of the “standard sieve”, and expressed as the percentage of the total product under the sieve (“mesh” is the degree of each inch of sieve The number of sieve holes, and used to name the sieve).

15. What is the definition and calculation method of mill utilization factor?

Answer: The mill utilization factor is the average amount of raw ore that can be processed per cubic meter of mill effective volume per unit time, expressed in tons/m3·hour. That is, the mill utilization factor q=Q/V (ton/m3·hour).

16.What is the definition and expression of the production capacity of the mill?

Answer: The production capacity of the mill per unit, that is, the amount of raw ore that the mill can process in a unit time (hour) under the condition of a certain feed ore and product size, expressed in tons/unit·hour.

17. Briefly describe what are the operating factors that affect the work of the grinding-classification unit?

Answer: It can be roughly summarized into the following three categories:

① In terms of the nature of the ore. Including hardness, mud content, feeding particle size and required grinding product fineness, etc.

② The structure of the mill. Including mill type, specifications, speed, etc.

③ In terms of operating conditions. There are mainly grinding media, slurry, concentration, sand return ratio and classification efficiency.

18.How to choose the shape of the grinding medium?

Answer: According to the purpose of grinding and experiments, the spherical and P2 rod-shaped media have the best grinding effect; the spherical media collide with point contact during grinding, so the grinding efficiency is high and it is suitable for fine grinding, but it is in phase with rod grinding. Compared with, the phenomenon of over-crushing is serious. Generally, when the feed size is below 30mm and the product size is not less than 1mm, the rod mill is more efficient.

19.What is the principle of choosing the material of the grinding medium?

Answer: When selecting the media material, the specific gravity, hardness, wear resistance, price, processing and manufacturing conditions of the material should be fully considered. Currently, steel or iron materials are generally used. In production, steel is commonly used for large balls, iron is commonly used for small balls, steel is suitable for rough grinding, and iron is suitable for fine grinding. In recent years, it has succeeded in replacing steel balls with rare earth manganese ductile iron balls.

20. What is the size system of the grinding media loaded?

Answer: The size is determined according to the nature of the ore. The harder the ore and the coarser the ore size, a larger size medium should be added to produce a greater impact grinding effect. On the contrary, a smaller size medium can be used to enhance the grinding effect.

21. What is the definition of the medium filling coefficient and its relationship with the speed of the mill? (One of the principles of medium filling).

Answer: It is the filling rate, which refers to the percentage of the grinding medium in the volume of the mill. The relationship between it and the speed of the mill is as follows:

Medium filling rate% Suitable mill rotation speed%

32~35             76~80

38~40             78~82

42~45             80~84

22. What is grinding efficiency? In what ways does it have to be expressed?

Answer: It refers to the amount of ore that can be processed for every 1 kilowatt·hour (one degree) of electric energy consumed. The expression methods are as follows:

①. The tonnage of raw ore processed for each kilowatt-hour of electricity consumed, namely ton/kWh.

② Calculate the tonnage of grinding products according to the designated level (usually -200 mesh) for every kilowatt-hour of electricity consumed.

③. Calculated based on surface area ton/kWh.

23. What is the technical efficiency of the grinding machine?

Answer: It refers to the ratio between the percentage content of the qualified grain fraction in the product obtained after grinding and the percentage of the original content greater than the qualified grain fraction contained in the ore.

24. What is qualified particle size?

Answer: The so-called qualified particle size means that the upper limit of the particle size should be less than the specified maximum particle size, and the lower limit of the particle size should be minus the pulverized part.

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

Answer: To improve the technical efficiency of the mill, you can consider the following aspects:

①. The closed-circuit grinding process is adopted.

② In the choice of equipment, a rod mill with peripheral discharge is suitable for rough grinding, and a grid-type ball mill is suitable for fine grinding.

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

① Appropriate cyclic load should be used during closed-circuit grinding;

② The speed of the mill should be controlled appropriately;

③ The ball loading system should be reasonable;

④ The concentration of ore grinding should be appropriate.