A brief discussion on common problems and solutions of mill linings

1、 The principle of grinding process

Crushing and grinding are collectively referred to as pulverization. The process of transforming large pieces of material into small pieces is commonly referred to as crushing in the industry; The process of turning coarse particles into fine powder is called grinding. The grinding process in cement production is divided into two parts: raw material preparation process and cement production process, abbreviated as raw material grinding and cement grinding. Limestone, clay, iron powder, etc. are finely ground together and called raw materials; Cement is made by grinding clinker, gypsum, and mixed materials together. 1、 The purpose of crushing materials in cement production is to (1) increase the surface area (specific surface area) per unit mass of the material after crushing, thereby improving the physical effect and chemical reaction speed; (2) Several different materials can easily achieve uniform mixing in powder form. (3) Powdered materials also provide convenience for drying, transportation, and storage, and create conditions for calcining clinker and producing cement, ensuring the qualification rate of cement at the factory.

2、 The grinding body and its graded materials require both impact and grinding effects during the grinding process. The combination of grinding bodies of different specifications can also reduce the porosity between them, increase the chances of contact with the material, and improve energy utilization efficiency; When the loading capacity of the grinding body is constant, the total surface area of small steel balls is larger than that of large steel balls; To crush large pieces of material, the steel ball must have a large amount of energy, therefore, the size of the steel ball (segment) should be relatively large; If the material needs to be ground finer, smaller steel balls (segments) should be selected. Therefore, in the grinding operation, it is necessary to select the grinding material correctly and carry out reasonable grading.

3、 The basic principle of grinding material grading is that (1) when the particle size of the grinding material is large, the hardness is high, or when the product is required to be coarse, the average ball diameter of the ball matching should be selected to be larger, and vice versa. The diameter of the grinder should be small, and the average diameter of the steel ball should also be small. The average ball diameter should be balanced between the front and back positions, and the difference in ball diameter should not be too large. (2) On the premise of meeting the material fineness requirements, the average ball diameter should be smaller to increase the contact area and the number of impacts per unit time, thereby improving the grinding efficiency. 4、 The pre crushing technology and its role in grinding operations are mainly achieved by reducing the particle size of the material entering the mill, making the ball mill energy-saving and high-yielding. This technology is called pre crushing technology. It partially or completely completes the crushing work of the ball mill's * * * bin by other crushing equipment with higher energy utilization than the ball mill, reducing the particle size of the incoming material to below 5mm or smaller, which can increase the mill's hourly output by more than 30%, reduce the electricity consumption per unit by 15-20%, and make the product particle composition more reasonable. (1) After the reduction of grinding particle size, the average ball diameter of the grinding body in the * * * warehouse also needs to be reduced; ***The length of the warehouse should be shortened, and the warehouse capacity should be correspondingly reduced; (2) Strengthening ventilation inside the mill is beneficial for reducing the temperature inside the mill and improving grinding efficiency; (3) The production capacity of auxiliary equipment in the closed-circuit grinding system needs to be increased, the system cycle load rate needs to be reduced, and the powder selection efficiency needs to be improved. 5、 Strictly control the moisture content of the materials entering the mill to ensure normal operation, accurate batching, and improve the production and quality of the mill. When the moisture content of the material is high, it is easy to produce paste grinding phenomenon, and the fine powder inside the mill adheres to the grinding body and lining plate, reducing the grinding efficiency. In severe cases, it can cause blockage of the grate holes in the compartment plate, resulting in poor ventilation of the mill, making it difficult for the material to pass through, leading to a sharp decrease in production and significant fluctuations in quality. However, due to the fact that many companies are currently using desulfurization gypsum, which has a high moisture content, ventilation inside the mill will be affected, and the existing partition plate grate holes and discharge grate holes cannot be adapted to the desulfurization gypsum used.

6、 The powder selection machine of the grinding system is a grading equipment of the closed-loop grinding system. It promptly sorts the materials for grinding, with qualified fine powder as the finished product and unqualified coarse powder returned to the mill for further grinding; It can adjust the particle composition of the finished product, meet the process requirements, and ensure the quality of the ground product. The performance of the powder selector is one of the main factors affecting the production and quality of the closed-loop grinding system. But in most cases, the higher the operating efficiency of the grinding machine, the lower the load of the powder selection machine, and the lower the electricity consumption per unit weight of cement. Therefore, we usually say that cement is ground, not selected.

7、 Strengthening the ventilation of the mill is one of the main ways to improve the production capacity of the mill, which has the following advantages: (1) reducing the phenomenon of over grinding in the ball mill. The fine powder inside the grinder is promptly carried away by the airflow, eliminating the agglomeration, ball sticking, and lining plate sticking of the fine powder, as well as the buffering effect on the grinding body. (2) The water vapor inside the mill can be eliminated in a timely manner, making it less likely for the gaps in the partition plate to be blocked and reducing the occurrence of full and paste grinding. (3) It can reduce the temperature inside the mill, prevent gypsum dehydration and cement false setting, which is beneficial for the normal operation of the mill and ensures the quality of cement. (4) Beneficial for workshop environmental protection and clean production.

8、 The reason for "full grinding" (dull grinding sound, decreased reading on the ammeter, and little discharge) (1) excessive feeding amount or increased particle size of the material entering the grinding process, making it difficult to grind, without timely adjustment of the feeding amount. (2) Excessive moisture content in the grinding material, poor ventilation, and inability to timely discharge water vapor result in "paste grinding", weakening the impact of the steel ball and slowing down the material flow rate. (3) Improper grading of steel balls, too many small balls in one compartment, low average ball diameter, weak impact force, or too few steel balls added; Or the steel ball is severely worn and not replenished or emptied in a timely manner, and the grinding effect is weakened. (4) The partition plate is damaged, the grinding material is displaced, the steel balls and steel segments are mixed, and the grading is imbalanced. (5) The closed-circuit mill, due to the excessive amount of material returned by the powder selector, increases the material ball ratio inside the mill, resulting in a decrease in grinding efficiency.

9、 Factors affecting the production and quality of ball mills (1) are the particle size of the materials being ground. Due to the generally small specifications of ball mills used in shaft kiln cement plants, the particle size of the input material has a significant impact on the production and quality of the mill. If the particle size is small, the production and quality of the mill will be high and the power consumption will be low; If the particle size is large, the production and quality of the mill will be low, and the power consumption will be high. (2) The grindability of materials. The grindability of materials refers to the degree of difficulty in grinding them, and the * * * standard specifies the use of the grinding power index Wi (kwh/t) to represent it. The smaller the value, the easier it is to grind the material, and vice versa, the more difficult it is to grind. Cement plants are accustomed to using the relative grindability coefficient to indicate the difficulty of grinding materials. It uses a small experimental mill to compare the tested material with standard sand, achieve the specified fineness value, calculate the grinding time, and set the grinding time to 1 for materials with the same grinding time as standard sand. For materials with a grinding time greater than 1, it is difficult to grind; Less than 1 is easy to grind; The larger the ratio, the harder it is to grind, and the smaller the ratio, the better it is to grind. (3) Moisture content of the grinding material. For the dry grinding method, the moisture content of the feed material has a significant impact on the production and quality of the mill. The higher the moisture content of the feed material, the more likely it is to cause saturation or paste grinding, reducing the grinding efficiency and resulting in lower mill production. Therefore, it is necessary to dry materials with high moisture content before grinding. (4) The temperature of the material being ground. The high temperature of the material entering the mill, coupled with the impact friction of the grinding body, will cause excessive temperature inside the mill, resulting in ball sticking, reducing the grinding efficiency, and affecting the output of the mill. At the same time, the thermal expansion of the mill cylinder affects the long-term safe operation of the mill. Therefore, it is necessary to strictly control the temperature of the materials entering the mill. (5) Add grinding aids. Most commonly used grinding aids are organic substances with strong surface activity. During the material grinding process, they can adsorb onto the surface of the material, accelerate crack propagation during material crushing, reduce the mutual bonding between fine powders, improve grinding efficiency, and contribute to energy-saving and high-yielding of ball mills. ***Standard regulation: Grinding aids are allowed to be added in the cement production process, but the dosage should not exceed 1%.

10、 System configuration defect: (1) Length of each compartment of the grinder. Improper selection of the length of each bin leads to an imbalance in their capacity, thereby affecting the grinding efficiency. (2) Insufficient ventilation in the system. Strengthening ventilation can discharge water vapor and fine powder in the mill, prevent ball sticking and blockage, reduce over grinding phenomenon in the mill, lower the temperature inside the mill, improve grinding conditions, and enhance grinding efficiency, which is conducive to improving the production and quality of the mill. (3) The type, grading, average ball diameter, and loading capacity of the grinding material. The process of grinding materials in a ball mill is mainly achieved through the movement of the grinding body, and the reasonable selection and use of grinding bodies is an important link in energy-saving and high-yielding of ball mills. (4) Selection of powder selection machine. In a closed-loop grinding system, the powder selector is an important equipment for controlling the fineness of materials and is also a major helper for energy conservation and high yield; Its structure, performance, and system composition have a significant impact on the production process of the mill.

11、 Inspection of loading capacity and grading of grinding materials: (1) Low mill output and coarse product fineness: Open flow grinding is generally caused by insufficient loading capacity, larger ball diameter in the second bin, low total loading capacity, and difficult material grinding, while closed flow grinding reflects a large circulation volume. We should increase the loading capacity of the grinding body and adjust the ball distribution, taking into account the grinding capacity of the front and rear compartments, in order to fully utilize the capacity of the grinding system; (2) The output of the grinder is high, but the product fineness is coarse: it is due to the fast flow rate of the material inside the grinder, strong impact ability, and insufficient grinding ability. Under the condition of constant loading capacity, reduce the size of the ball, add smaller balls, and lower the average ball diameter; (3) The output of the grinder is low, and the product fineness is relatively fine: generally, there are too few large steel balls, and the filling rate is too high, which weakens the impact crushing effect. It is necessary to reduce the size of the balls, increase the size of the balls, and increase the average ball diameter while keeping the loading capacity unchanged. (4) High output and qualified fineness: Reasonable loading and grading of grinding materials are essential.

2、 Performance of ball and wear-resistant lining plate

With the rapid development of the cement industry, the diameter specifications of grinding machines are becoming larger, and the requirements for ball mill liners are also increasing. The lining structure has also evolved from a simple protective lining in the past to a highly technical and theoretical "device". The liner plate with advanced structural design can effectively increase the output of the mill, reduce power consumption, increase the specific surface area of cement, and reduce noise. Its shape is no longer just a simple plate. Especially in recent years, the material blocking rings and rings widely used in high-precision screening and grinding have deviated from the initial concept of "protection", with countless forms and shapes. So in production practice, the specific material and structure of the lining plate should be selected reasonably according to the specific working conditions and time. It is even more difficult to find two grinding machines with similar operating parameters, such as two machine tools of the same model, for cement grinding operations. Even in one workshop, two grinding machines of the same model from 10 batching lines have different output, power consumption, and specific surface area. This is the unique characteristic of grinding operations, which means that although all equipment in each subsidiary is the same, the output, power consumption, and equipment consumption of each mill are different.

3、 Differences between liner structure and compartment plate configuration

1. Grinding head

The grinding head liner is a liner installed on the inner wall of the grinding head end cover or cylinder end cover, and its main function is to protect the end cover from wear and tear. Sometimes some special shaped grinding head liners have the functions of forcibly lifting the grinding body, changing the movement trajectory of the grinding body, and assisting in grading the grinding body. When the grinder is running, the grinding body inside the grinder undergoes a "waterfall" motion, and the grinding body adjacent to the grinding head liner plate will have an impact and relative friction on it. Especially for the grinding body close to the grinding head liner, positive pressure is generated on the grinding head liner under the compression of the adjacent stacked grinding bodies. When the mill rotates, friction drives the lifting of the steel ball, as if the steel ball is "hard scraped" onto it, causing significant wear on the grinding head lining plate. This wear condition is even more severe than the conditions experienced by the cylinder lining plate. This also explains why the wear of the lining plate of the large grinding head is worse. The reason is simple: due to the large grinding diameter and the large loading capacity of the grinding body, the pressure and impact force on the lateral grinding head lining plate also increase sharply. Under the same conditions, the wear naturally becomes more severe. The wear of the grinding head liner is uneven, with a wide and severely worn area, and under normal circumstances, there are also worn cutting lines in the radius direction of the grinding machine

Below, we will focus on introducing methods and measures to solve the short service life of grinding head liners. There are generally two methods to solve the early wear of the grinding head liner: changing the shape and improving the material. Firstly, let's take a look at changing the shape, namely: (1) local thickening, which means appropriately thickening the severely worn area without affecting the grinding efficiency. (2) Setting appropriate convex edges on the grinding head liner plate controls the relative sliding between the grinding body and the grinding head liner plate, reduces wear, and also helps to lift and grade the grinding body. The effect on cement grinding is good, and the output of the grinding machine can be increased by one specification level. As long as the scrap rate does not increase, the parts of the grinding head that are prone to wear should be appropriately thickened and ribbed, which is commonly known as reinforcement. After being used by many companies, it has been proven that the reinforcement added to the grinding head liner can continue to be used for 4-6 months or more.

(3) Welding. If cost savings and prolonging the service life of the grinding head are still needed, welding repair method can also be used. Replace the old compartment plate and discharge grate plate that have been replaced with side strips on both sides, and weld them instead of reinforcing bars. However, it must be done after the * * * reinforcing bars have just been ground off and the surface of the grinding head is still flat in order to be easy to weld. If there is a depression on the surface, welding repair method cannot be used.

2. Cylinder lining plate

At present, most cement mills use a stepped type for the lining plate of the silo, only the Φ 3 * 9M mill uses a waveform step. This staircase lining board has been designed for a long time and can no longer meet the current production needs. Moreover, under the same material and process conditions, the wear is relatively fast. Some manufacturers have now switched to using groove lining plates, which have shown good performance. The benefits of groove lining plates will be explained in detail later. Normally, for cement mills with a diameter of 3.8x13m and a diameter of 4.2x14.5m, the first compartment has about five rows of stairs and the second compartment has about ten rows of graded lining plates. Under the same material, they wear out quickly and are prone to fracture. We often follow up and find that many companies have this situation. We suggest changing the first five rows of the first warehouse and the first ten rows of the second warehouse grading to groove lining plates or replacing all groove lining plates, which can benefit all aspects of operation. In this way, the steel ball cannot directly contact the lining plate, and the influence of HRC hardness on the lining plate is greatly reduced.

3. Compartment board

The partition plate is an important component of the complete set of lining plates. The performance of the partition plate not only affects the service life of the complete set of lining plates and the operation rate of the mill, but also has a significant impact on the grinding efficiency of the mill. Therefore, experts in both the field of grinding technology and wear-resistant materials have devoted considerable effort to this research. Research on compartment boards generally needs to be conducted from two aspects: material and structure. Let's start with the structure. In recent years, most of the grinding machines designed and manufactured have reasonable working principles and can basically meet the existing process conditions. But there are also two issues: one is that the partition plate and partition blind plate are too thin. The compartment plate is the heart of the grinder and a critical part of the grinder. I won't go into detail about this, as everyone will understand. When the quality is determined, increasing the thickness appropriately is beneficial for the safe operation of the mill and can reduce the number of replacements. With a fixed scrap rate, it is cost-effective for the enterprise to operate. Because many vulnerable parts originally installed only consider the contract period and the need to meet operating conditions. The thickness and width of the steel plate for the compartment rack also have this situation, sometimes only considering the contract period and operational needs. With the gradual use of desulfurization gypsum by various companies, the clothing for the compartment rack is also gradually accelerating. So when replacing the new compartment bracket, it is necessary to fully consider the service life of the new compartment bracket. The steel plates on both sides of the compartment bracket need to have sufficient strength, and the main wear parts are the steel pipes and lifting plates. When the steel pipe and lifting plate reach a certain thickness, their strength can be guaranteed.

4. Groove lining plate

Groove lining plate is a series of carefully designed and calculated circular grooves cast on the surface of the lining plate. Due to its existence, it has changed the phenomenon of material being easily squeezed out from around the steel ball in the point contact between the steel ball and the grinding body in the past. The phenomenon of a short usage time of a corrugated staircase in a warehouse has been observed for a period of time, and we believe that there are certain defects in the structural form of the lining plate (although the service life of the lining plate is related to many factors...). Because this type of grinder was designed and produced around 2000, when the technological level was far behind, the designed stepped ripple shape was the horizontal direction of the grinder, intersecting exactly with the direction in which the steel ball was running. Because one compartment is a crushing compartment, the steel ball has a greater impact on the lining plate. No matter how large the horizontal drop of the stepped waves is, the grinder first needs to intersect and cut the lining plate in the direction of rotation of the grinder during operation. This causes the waveform of the lining plate in the horizontal direction to conflict with the movement of the grinder in the direction of rotation, that is, the up and down movement, resulting in rapid wear of the steel ball and lining plate until they are worn into grooves in the direction of rotation. It has been proven that all grinding machine liners worn down to * * * have become grooves in the direction of the grinding machine's rotation. With the continuous development of building materials machinery technology, in recent years, all grinding machine compartments no longer adopt this form, but instead use standard stepped or groove shaped lining plates with high and low drops. Its characteristic is to increase the area and amount of material broken by steel balls in the grooves in the direction of rotation of the lining plate during free fall, reducing the direct impact of steel balls on the lining plate. This not only reduces the noise inside the grinding machine, but also improves the crushing capacity of one compartment, thereby increasing the output of the grinding machine. According to our production experience, after using groove lining plates (with the same weight as the original waveform step, its service life can be increased by one to two years under the same material and process conditions), the economic benefits of the enterprise are greatly improved. Such as in Nanjing Conch and Shanghai Conch. They all replaced the groove lining plate on the Φ 3 * 9M mill around the Spring Festival. In terms of process, they only adjusted the steel ball stacking ratio, and the output increased by 2-3 tons on the original basis. At the same time, due to the design and production era, the compartment system of this type of grinder can no longer meet the needs of modern cement. It is suggested that leaders boldly improve and change the current single ring double-layer compartment system to two ring double-layer.

4、 Wear principle and cause analysis of lining plate

1. Wear principle

When the grinder rotates, the grinding medium (steel balls, segments, rods, etc.) and materials inside the grinder are carried to a certain height by the lining plate and then thrown or dropped, causing impact and wear on the lining plate during the process of throwing and dropping the grinding medium and materials. Due to the requirement of fineness control in the cement industry, the length to diameter ratio of the grinder is generally greater than 2.5, and the full mill is mostly divided into 2-3 compartments (up to 5 compartments). Generally, a two bin mill is used for fine and coarse grinding, with a grinding particle size of P80 ≤ 25mm (if equipped with a fine crusher and a roller press, P80 ≤ 5mm and P80 ≤ 1.0mm respectively). The operating conditions of different enterprises may vary. Due to the large particle size of the grinding material, the diameter of the * * * ball in one warehouse is between 90-100mm, with an average diameter of 70-75mm. Large diameter steel balls crush the lining plate with a greater impact force. After the material is punched and squeezed by the steel balls, its sharp corners invade the lining plate. At this time, the wear mechanism of the lining plate is mainly caused by high stress impact chiseling and grinding, supplemented by extrusion chiseling.

2. The main forms of damage to mill liners

The main forms of damage to mill liners include fracture, wear, etc., which are closely related to unreasonable assembly, production, and other aspects.

Firstly, let's talk about the issue of fracture. Fracture is a serious problem in the production, manufacturing, and application of mill liners. In addition to being related to usage, this type of problem generally includes the following reasons in manufacturing:

Unreasonable chemical composition design or inaccurate chemical composition control;

There are serious metallurgical quality issues, such as excessive inclusions and compositional segregation;

There are serious casting defects such as shrinkage, looseness, porosity, and poor metallographic structure during casting;

Unreasonable design of heat treatment process or inaccurate control of heat treatment process;

Unreasonable stress control and elimination during the production process, such as unreasonable casting process design, unreasonable loosening and unpacking time after casting, improper grinding and welding repair methods, and failure to timely temper after heat treatment quenching.

Installation issues:

The hazards of mixed packaging:

Due to the different characteristics of the materials produced by manufacturers, mainly referring to HRC hardness and toughness.

3. The material selection of the lining plate does not match the structural design (of course, high manganese steel is not commonly used now)

This generally refers to two aspects of the problem. On the one hand, it refers to the use of high manganese steel material for lining plates in the past, as qualified high manganese steel is very brittle and has poor toughness. On the other hand, in the past, the lining plate often had two bolt holes, which were square holes, which could easily cause internal and sharp stress concentration during casting. It is recommended to split the lining plate into two sections, change the double holes to single holes, and change the square holes to elliptical holes. In this way, the problem of casting stress has been solved. At present, the grinding head and partition plate of the Φ 4.2x14.5m grinding machine are designed with square holes. It is recommended to change them all to ordinary elliptical holes. Yingde Hailuo and Yingde Longshan companies have been renovated and the results are good. Not only does it facilitate bolt procurement, but it also allows the grinding head liner to be used up to a maximum thickness of 1cm to 2cm, reducing the scrap rate.