# Type and material of grinding body:

Date:2019-05-30 13:53 writer:admin Views:

## 1.Ball mill type

Different shapes and sizes of abrasive bodies have different grinding effects in the grinding process.

The grinding bodies used by **ball mill**s in cement plants are classified as follows according to their shapes:

Steel ball steel ball is a widely used grinding machine.

According to the grinding process requirement, usually choose Φ Φ of 20 ~ 130 mm of various specifications of steel ball;

For coarse grinding ball mill warehouse Φ is generally selected 50 ~ 100 mm Φ all kinds of steel ball, fine grinding storehouse choose Φ 20 ~ 50 mm Φ all kinds of steel ball.

(2) the shape of steel forging steel forging is short cylindrical, its specifications in the diameter by the number of millimeters of length.

Steel forging is generally used to open the fine silo of the **ball mill **and also for the fine silo of the closed ball mill.

Commonly used specifications of steel forging has Φ 10 mm x 10 mm to 50 mm Φ x 60 mm.

Refined steel forging of small diameter to fine grinding storehouse Φ below 12 mm x 12 mm.

(3) steel rod steel rod mill is a kind of grinding body.

Steel bar specifications are expressed in millimeters of diameter by length.

Steel bar diameter is generally selected 40 ~ 90 mm Φ Φ, rod length should be shorter than mill bar warehouse Φ 50 ~ 100 mm Φ.

For example: Φ 2.4 m x 13 m wet grinding baseball, effective length of 2.75 m, the storehouse use steel specification for Φ 60 mm x 2650 mm, 65 mm x 2650 mm and Φ Φ 70 mm * 2650 mm.

## 2, grinding body material selection

The abrasive body should have high abrasion resistance and impact resistance.

The material should be hard, wear-resistant and not easily broken.

No obvious burrs and cracks shall be allowed on the surface of the abrasive body, and the irroundness of the steel ball shall not exceed 2% of its diameter.

In the cement industry, the consumption of grinding body and lining plate of grinding machine is quite large, the quality of grinding body material not only affects the grinding efficiency of the mill, but also relates to the operation rate of the mill.

Countries in the world have made achievements in improving wear resistance, from the 1960s to the 1970s widely used high chromium cast iron (steel) ball.

Japan's main high chromium steel ball, low chromium steel ball and alloy white cast iron ball;

Germany mainly high chromium cast iron ball and low alloy steel ball;

Alloy steel balls are commonly used in the United States and Canada.

In recent years, in China's cement industry, the abrasive materials used by the **ball mill** are as follows:

(1) high chromium cast ball high chromium cast ball is a high content of chromium (Cr10% above) alloy white cast iron ball, its characteristics are wear-resisting, heat resistant, corrosion resistance, and has a considerable toughness.

The surface hardness of high chromium cast iron ball with martensite matrix can reach 58 ~ 66 HRC.

The wear resistance of high chromium cast iron balls is 8 ~ 12 times that of ordinary carbon steel balls.

(2) low-chromium cast ball low-chromium cast ball contains a small amount of chromium elements, can maintain the white mouth of cast iron to obtain pearlitic metallographic.

The differences of toughness and wear resistance of low chromium cast balls are great.

(3) forging bearing steel ball forging bearing ball can produce a variety of diameter of steel ball, carbon content of about 1.0%, chromium content of about 0.5%, the rest of the elements for the conventional content.

The ball consumption ratio of high chromium cast iron ball is higher, but due to the low alloying element content, it still has a wide market.

Baseball mill steel rod material requirements of high hardness, wear resistance, constantly broken, not bending, commonly used 40Mn steel or 70 high carbon steel rolling.

## 3. Reasonable loading of grinding body

### (1) calculation formula of filling rate

The percentage of the filling volume of the grinding body in the mill to the effective volume of the mill is called the filling rate of the grinding body.

Phi VsVm x 100%

In the formula: phi -- filling rate of grinding body in the grinding, %;

Vs -- filling volume of grinding body in the mill, m3;

Vm -- effective capacity of mill (bin), m3.

### (2) calculate the filling rate of the measured spherical height inside the mill

When the grinding machine is loaded into the grinding body to verify whether its filling rate is consistent with the value specified in the ball-matching scheme, or when some steel balls and steel forgings are added due to the abrasion of the grinding body in the running process of the grinding machine, the filling amount is usually calculated by actual measurement and look-up table method.

The method is to measure the effective inner diameter Di of the mill with a ruler in the case that there is no material or only a few materials in the mill.

Then measure the vertical distance H from the grinding surface to the top liner through the mill center, and calculate the value of H/Di. The relationship is as follows (see figure 4).

Phi beta 360 - sine beta 2 PI

H = Di2cos beta 2

H1 = Di2 - h = Di21 - cosine beta 2

In the formula: phi -- filling rate of grinding body of the grinding machine;

Beta -- the central Angle of the filling surface of the grinding body to the center of the grinding machine;

H -- distance from the center of the mill to the filling surface of the abrasive body, m;

Di -- effective inner diameter of mill, m;

H1 -- height of filling surface of grinding body in grinding, m.

The calculated H/Di values are listed in table 7.2.

If the filling rate of abrasive is greater than 32%, the following empirical formula can be used to directly calculate the filling rate

Phi hdi 0 = 113-126

In the formula: phi 0 -- filling rate of grinding body in the grinding, %.

When measuring the filling surface height of grinding body in grinding, attention should be paid to the filling situation of materials in grinding.

If the material surface is 15 ~ 20mm higher than the grinding body, the calculated filling rate shall be subtracted by the error number of 2% ~ 4%.

Grinding filling rate has a great influence on grinding efficiency of the mill. Too high or too low filling rate of abrasive will lead to grinding efficiency reduction.

There are many factors that affect the optimal filling rate of abrasive body, such as the form, specification, internal structure characteristics of the mill and the performance of the pulverized material.

Therefore, the optimal filling rate should be obtained through experiments in production and adjusted appropriately according to the change degree of influencing factors.

The following is the filling rate of grinding body of various mills in China's cement factories:

Filling rate of grinding body of various mills in cement factory:

Medium unloading or tail unloading drying grinding filling rate phi (%)25 ~ 28;

First-level closed circuit long grinding filling rate phi (%)30 ~ 36;

Open circuit long grinding filling rate (%)25 ~ 30;

The filling rate (%) of short ball grinding powder is 35~38.

The filling rate of baseball mill is 20 ~ 25.

The filling rate of two closed circuit short ball mill (%) is 40 ~ 45.

The filling rate of grinding body of the two-stage closed-circuit **ball mill **and multi-bin mill should also be adjusted according to the balance state of grinding action of each level or bin.

For example, if a sanbin mill shows insufficient coarse crushing capacity and strong fine grinding capacity in production, the filling rate of steel balls in the first bin should be appropriately increased and the filling rate of forged (balls) in the third bin should be reduced, so that the grinding effect of each bin can reach a relative balance.

When changing product variety or quality requirements in production, the filling rate should also be adjusted appropriately.

### (3) calculation of loading quantity of abrasive body

The quality of grinding body loaded into the mill (or bin) is called the loading quantity of grinding body, which depends on the size of the filling rate of the mill.

Phi rho G = V

G = 0.00785 D2iL phi rho

Where :G -- loading amount of abrasive body in the mill, t;

V -- effective capacity of mill (bin), m3;

Di -- effective diameter of mill (bin), m;

L -- effective length of mill (bin), m;

Phi -- filling rate of grinding body within the grinding, %;

Rho -- rho rho -- rho volume density, t/m3.

The volume density of steel ball is generally 4.56 ~ 4.85t/m3.

## 4. Steel ball grading

The size and diameter of the ball and its mass of the match known as steel **ball grading**.

Steel ball gradation directly affects mill output, product quality and metal consumption.

The reasonable selection of steel ball gradation is mainly based on the physical and chemical properties of the pulverized materials, the mill structure and the required product fineness.

In the grinding process, the steel ball should have sufficient impact effect and certain grinding effect.

Under the condition that the loading amount of abrasive body remains unchanged, the total surface area of small steel balls is larger than that of large steel balls, and there are more opportunities for contact with materials. Therefore, small steel balls should be selected for the fine particle materials that are ground and finely ground.

On the other hand, large pieces of material must be crushed in order to effectively grind them, so the ball must have more energy.

The size of the selected steel ball has a certain relationship with the particle size of the grinding material, the larger the particle size of the material, the larger the diameter of the steel ball should be.

It can be seen that it is inappropriate to use steel balls with large diameter or small diameter completely in the mill, and reasonable cooperation must be carried out.

According to production experience, steel **ball grading** is generally determined according to the following factors:

(1) steel ball gradation shall be determined according to the grain size, hardness, wearability and fineness requirements of the grinding materials.

When the grinding material particle size is small, easy to wear good, product fineness requirements are fine, it is necessary to strengthen the grinding effect on the material, the selected ball diameter should be smaller.

On the contrary, if the grinding material particle size is large, easy to wear poor, it needs to strengthen the impact crushing effect on the material, at this time should choose large diameter steel ball.

(2) the mixing steel ball group with large steel ball is larger than that with small steel **ball group**.

In order to control the flow rate of materials in the mill, steel balls of different sizes are generally used.

Properly reducing the gap between steel balls can slow down the flow rate of materials in the mill, prolong the residence time of materials in the mill, and improve the grinding efficiency.

The gradation number of steel balls should not be too much, because during the movement of steel balls, due to different diameters and different linear velocities, the natural stratification of steel balls is often generated, and the inner layer with large diameters.

Too much gradation will lead to more serious stratification, which will affect the grinding efficiency.

(3) the size of steel ball diameter is also related to the amount of material passing through per unit volume in the mill.

In closed-circuit grinding, the feed back of the separator increases the amount of material passing through the mill per unit volume. In this case, the steel ball will be buffered when it is impacted.

Therefore, the amount of recycling is high, the diameter of the steel ball is slightly larger;

And vice versa.

(4) the surface shape of the lining plate of the mill is also one of the factors to be considered when matching balls.

If the shape of the lining plate surface causes insufficient ability to carry the ball, then the height of the steel ball is reduced and the impact force is reduced. The diameter of the steel ball selected at this time should be larger.

In short, as there are many factors affecting steel **ball grading**, a reasonable steel ball grading is relative, temporary, not fixed.

The most reasonable steel **ball gradation **is constantly optimized through the determination and analysis of grinding conditions in the production process and the statistics of the historical data of steel ball gradation.