Analysis and Countermeasures of Fracture Failure of Semi autogenous Grinding Machine Lining Plate

The semi autogenous mill has been widely used since the 1950s, and after more than half a century of development and improvement, its technology has become increasingly mature. It has now become the most important ore processing equipment in mineral processing plants, with both crushing and grinding functions, and can complete crushing and grinding operations equivalent to two-stage or three-stage crushing and screening. The semi autogenous mill has a high rotational speed, with high-frequency and high-energy steel balls continuously and strongly impacting and grinding ores. It is mainly used for grinding various hardness ores, rocks, and other suitable grinding materials in wet processes, and has the characteristics of high productivity and low cost. The function of the mill liner is to lift steel balls and ores, protect the liner from direct impact of steel balls and erosion by slurry. However, due to the continuous impact and erosion of high-frequency and high-energy steel balls inside the semi autogenous mill, the liner may experience unexpected wear and fracture, especially in the service conditions of semi autogenous mill liners with a diameter of over 8m. Early failure caused by liner fracture often occurs, seriously affecting the service life of the liner and threatening the safety of mill operation.

Analysis of the Failure Causes of the Half Self Grinding Machine Lining Plate Fracture

The usage effect of the semi autogenous mill liner is closely related to the design structure, manufacturing quality, steel ball hardness, steel ball filling amount, ore properties, service conditions, and operating habits of the liner. It is the result of the combined effect of multiple influencing factors. The abnormal failure modes of lining plates are mainly divided into unexpected wear failure and fracture failure. The fracture failure of the lining plate can seriously disrupt the production rhythm and may cause damage to the mill equipment. Therefore, it is urgent to conduct a systematic analysis of the causes of the fracture failure of the semi autogenous mill lining plate and find practical and feasible improvement measures based on the influencing factors.

1.1 Unstable operation of grinding machine

The working conditions during the debugging period of the mill are unstable, the supply of mineral materials is insufficient, and the proportion of steel balls in the mill is high. During the debugging period of the mill, due to equipment failure or inadequate debugging, frequent start-up and shutdown are required, and the slurry needs to be discharged before each shutdown. Only steel balls remain in the mill, and when restarted, the steel balls will heavily hit the unprotected lining plate.

1.2 Operating characteristics of the medium inside the mill

It is generally believed that the discharge form of the semi autogenous mill is a grid type forced discharge, where the material is continuously discharged outward through the grid plate. The material level near this position is lower, so the impact of the steel ball medium on the cylinder lining plate is more severe, and the cylinder lining plate near the discharge end is prone to damage. The discrete element method was used to analyze the motion and mass distribution of the medium at different cross-sectional positions of the semi autogenous mill cylinder with a diameter of 3.6m × 2.1m. The mass distribution of the medium at different cross-sections of the mill is shown in Figure 1, and the trajectory of the medium near the discharge end cover is shown in Figure 2.

Figure 1 Mass distribution of media with different cross-sections

Figure 2: Movement trajectory of the medium near the end cap

From Figure 1, it can be seen that in the section near the end cap inside the semi autogenous mill, the mass of the steel balls is only 90% of the average mass of the medium in the middle of the mill. Because the material level near the end cap is lower, the distribution of steel balls in the middle section of the mill fluctuates less.

From Figure 2, it can be seen that the toe of the pile is located at an angle of 33.3 ° with the vertical axis, representing the ultimate position where the steel ball slides and forms the pile. When the dropping point of the medium such as steel balls in the mill is around 33.3 °, the crushing efficiency is the highest; When the landing point of the medium is outside 33.3 °, the medium begins to directly impact the lining plate, resulting in an increased probability of lining plate fracture. The farthest throwing point of the medium can reach the angle of 68.8 ° with the vertical axis, which is an important reason for the easy cracking of the lining plate near the end cover.

1.3 Defects in lining plate structure design

The lining plate structure is an important factor affecting the trajectory and landing point of steel balls inside the mill. If the lifting angle is too small, the height of the steel ball being lifted is too high, and the throwing distance is far, some steel balls are difficult to fall on the toe of the material pile, which will directly impact the lining plate. Continuous high-energy impact causes the surface and internal stress of the lining plate to accumulate continuously, ultimately forming cracks or fractures.

1.4 Mismatch between lining material selection and working conditions

There are four main types of iron-based wear-resistant materials for semi autogenous mill liners: austenitic manganese steel, martensitic steel, pearlite steel, and wear-resistant white cast iron. With the large-scale development of semi autogenous mills, the specifications of the world's largest semi autogenous mill have reached φ 13.41m × 7.92m, and the diameter of the grinding medium can reach up to φ 150~φ 200 mm. The lining material of large semi autogenous mills must have high hardness, impact toughness, and fatigue strength to resist the huge impact force and strong scouring that the lining plate bears during use. Austenitic manganese steel is widely used in the lining plates of small and medium-sized semi autogenous mills. Practical application of large semi autogenous mills has shown that the impact of the grinding medium can cause severe deformation of austenitic manganese steel; Martensitic steel has high hardness and strength, but its martensitic structure is brittle, and large impacts can easily lead to the generation and propagation of cracks; Pearlite steel can meet the performance requirements of different specifications of grinding machines for lining plates by adjusting the content of C and alloying elements and using different heat treatment processes. The adjustable range of hardness and impact toughness is wide, and it is widely used in large semi autogenous grinding machines; Wear resistant white cast iron has high hardness and brittleness, and is rarely used in large semi autogenous mills. Under normal circumstances, the operating conditions of the grinding medium in different parts of the semi autogenous mill are different, and there are significant differences in the impact that the lining plate can withstand. The requirements for the impact toughness and strength of the lining plate are also different.

In large semi autogenous mills, the high-frequency and high-energy impact of large-diameter grinding media results in harsh operating conditions for the lining plate. Improper material selection or mismatched hardness and impact toughness of the material selection increases the probability of early fracture failure of the lining plate during use.

Countermeasures for the fracture and failure of the lining plate of the 2.5 autogenous grinder

The properties of the ore, mill speed, and efficiency have a significant impact on the fracture of the lining plate, which is an inherent property of mill operation. Although it can be adjusted appropriately, the range of adjustment is relatively small. Usually, the problem of lining plate fracture can be effectively solved by adjusting the grinding medium, optimizing the lining plate structure, selecting materials reasonably, and optimizing the manufacturing process.

2.1 Quantity of steel ball added

During the debugging period of the mill, insufficient feeding or unstable system can lead to frequent start and stop of the mill, and high steel ball filling rate inside the mill can cause early fracture and failure of the lining plate. In order to avoid early fracture and failure of the lining plate, the amount of steel ball filling should be minimized during the debugging period, and the steel ball filling rate should be adjusted after the mill runs stably. The filling rate of steel balls in a semi autogenous mill is usually between 7% and 15%, and during the debugging period, the filling rate of steel balls should be controlled below 10% as much as possible.

2.2 Axial height distribution of cylinder liner plate

According to the analysis in section 1.2, the operating characteristics of the semi autogenous mill are as follows: the material level near the end cover is low inside the semi autogenous mill, and the trajectory of the steel ball is complex due to the influence of the lining plate on the end cover. The impact of the steel ball medium on the cylinder lining plate is severe. To solve this problem, the height of the lifting strip near the end cover of the cylinder liner is reduced to control the throwing of steel balls and reduce the impact of steel balls on the liner. The high wear area of the semi autogenous mill is usually located in the middle of the cylinder, which can be optimized using auxiliary structures such as 3D scanning and G eomagic analysis, or designed using the equal life method with different heights of lifting bars in different high wear areas. The axial height distribution of the cylinder liner plate is shown in Figure 3.

Figure 3 Axial height distribution of cylinder liner plate

2.3 Optimize the lifting surface angle of the cylinder liner plate

At a constant mill speed and liner height, the smaller the angle of the liner lifting surface, the farther the steel ball will fall. When the throwing distance exceeds the toe of the material pile inside the mill, the steel ball directly impacts the lining plate, increasing the probability of lining plate fracture. The lifting surface angle of the cylinder lining plate increases with the wear of the lining plate, and the lifting capacity of the lining plate decreases accordingly, resulting in a smaller and smaller wear effect on the material. The lifting surface angle of the lining plate is simulated using the professional lining plate design software M illTraj to ensure that the steel ball is thrown onto the toe of the material pile. The simulation results of the steel ball throwing trajectory are shown in Figure 4. The surface angle design of the cylinder liner can adopt a variable surface angle scheme of "larger top and smaller bottom", and the variable lifting surface angle design is shown in Figure 5. Due to the varying wear rates in different regions, in addition to implementing measures to increase the height of high wear areas in section 2.2, a variable cross-section structure from high wear areas to non high wear areas can also be used to improve the structural strength of high wear areas.

Figure 4 Simulation of Steel Ball Dropping Trajectory

Figure 5 Variable lifting surface angle design

2.4 Bolt hole and base plate reinforcement

The bolt holes on the lifting strip are the weak parts of the lining plate, which are most susceptible to wear and tear, resulting in cracks at the bolt holes. During design, reinforcement platforms are usually installed at bolt holes to enhance the structural strength of the bolt holes. Bolt hole reinforcement usually adopts semi-circular, elliptical or trapezoidal shapes, and the intersection with the lifting bar should have a smooth transition. In order to reduce the influence of the strengthening platform on the trajectory of the steel ball throwing, the draft angles on both sides should be kept as consistent as possible with the angle of the lifting strip surface.

2.5 Reasonable Material Selection

The material selection for the lining plate of the semi autogenous mill should simultaneously consider high wear resistance and high impact toughness, while avoiding severe deformation of the lining plate under impact. At present, the mainstream material for large semi autogenous mill liners is high carbon low-alloy pearlite steel. Pearlite steel widely used in semi autogenous mills with a diameter of over 5 meters, with a C content usually between 0.8% and 1.0%, and the main alloying elements are Cr and Mo, with a total content of less than 5.0%. Its characteristics are simple heat treatment process, good matching of hardness and toughness. The development of the ultra large semi autogenous mill with a diameter of 10m has put forward higher requirements for the impact resistance of the lining material. In order to meet the requirements for the use of ultra large semi autogenous mill liners, the impact toughness of the liners can be improved by appropriately reducing the C content to 0.5% to 0.7% and increasing the Ni content to 0.3% to 0.5%. But after reducing the C content, it is necessary to improve the hardness and hardness uniformity of the lining plate by reducing the heat treatment furnace density and increasing the cooling rate during the high-temperature stage.

3 Conclusion

The main reason for the fracture and failure of the lining plate of the semi autogenous mill is the unstable working conditions during the commissioning period, and the high proportion of steel balls in the mill; The grinder starts and stops frequently, and the steel ball directly impacts the lining plate for a long time; The operating rules of the semi autogenous mill result in a lower material level near the end cover inside the mill, and the impact of steel balls on the cylinder liner is more severe; The design of the lifting surface angle of the lining plate is unreasonable, and some steel balls are difficult to fall onto the toe of the material pile, directly impacting the lining plate; Unreasonable selection of lining materials, low impact toughness, and fatigue fracture caused by bearing impact.

The main measures to solve the fracture of the lining plate of the semi autogenous mill are to strictly control the steel ball filling rate during the debugging period, usually below 10%; Reduce the height of the lifting strip near the end cover of the cylinder liner to control the throwing of steel balls; The surface angle design of the cylinder lining plate can adopt a variable surface angle scheme of "larger top and smaller bottom", which takes into account avoiding early fracture and improving later grinding efficiency; At present, the best material selection scheme for the lining plate of large semi autogenous mills is the pearlite steel scheme with high wear resistance and high impact toughness. The impact resistance of the lining plate can be improved by reducing the C content and increasing the Ni and Mo alloy content.