As far as grinding process is concerned, there are mainly two kinds of grinding process: open circuit and closed circuit. The former has the advantage of simple operation and finished products after grinding. The disadvantage is that the material flow speed is slow and the retention time is long. In order to ensure that the size of the grinding material meets the requirements, the grinding material can not be discharged in time. Often cause grinding phenomenon. Open-circuit grinding system has relatively low production capacity and high energy consumption, so it is impossible to adjust the grinding material fineness flexibly at any time. The latter is equipped with flour selection equipment, which can discharge the fine grinded powder out of the mill in time, effectively avoid the phenomenon of over-grinding, and flexibly adjust the fineness of the finished cement by adjusting the working parameters of the flour separator.
In addition, the material flow rate in closed-circuit grinding is accelerated, and the grinding bodies of each bin are properly responsible for grinding or grinding tasks, so the output is increased and the power consumption is reduced, especially when the fineness of cement is required to be higher, the advantages of high yield and low consumption are more obvious. Compared with open-circuit grinding, the output of closed-circuit grinding process is increased by more than 30%, and the power consumption is reduced by about 20%. Therefore, the use of closed-circuit grinding technology is an inevitable trend in the development of cement grinding technology.
Ball mill is ideal as grinding equipment, but as crushing or crushing equipment, it is inefficient. It is an effective way to improve the production capacity of ball mill to move the grinding process of materials with larger particle size of more than ten mm or even tens of mm from inside to outside in special grinding equipment. "More crushing and less grinding", that is to say, efforts to reduce the size of grinding materials, has been a valuable experience in cement grinding practice for many years. Therefore, the pre-grinding process came into being naturally at the historic moment.
According to the crushing theory, brittle materials are crushed from tens of millimeters to several millimeters. The essence of the crushing is the continuous generation and expansion of internal cracks, and the external factors that impel the process are the powerful force exerted on them in a certain way - crushing force.
(1) Roller press + ball mill grinding process.
This process also includes various vertical mill and ball mill cascade grinding process configuration. In this paper, the cascade grinding system of roller press and ball mill is analyzed. The crushed cement clinker is more advanced into the roller press, and the strong roll pressure crushes it from tens of millimeters to several millimeters or even finer and then into the ball mill. As the clinker particles are crushed by roll pressing, there are also many micro-cracks in the interior, so it is easy to further crush in the ball mill and quickly enter the grinding stage. In this grinding system, the main task of the ball mill is only grinding, so a smaller size grinding body can be used in the rough grinding bin. The increase of the surface area of the grinding body is obviously conducive to the improvement of grinding efficiency, thereby greatly improving the production capacity of the grinding system. According to the empirical data, the output of cement grinding system can be increased by 40% and the grinding power consumption can be reduced by 10% after the cascade grinding process of roller press and ball mill is adopted.
(2) Impact crusher + ball mill grinding process.
The development of fine grinding equipment is one of the hotspots in clinker pre-grinding. From the fine jaw clinker crusher in 1980s to various impact crushers in recent years, the reform and innovation of cement grinding process have been strongly promoted. These clinker fine crushing equipment are roughly the following: fine jaw crusher (PEX), vertical counterattack crusher (PCF), vertical hammer crusher (PCL), vertical hammer crusher (XCL), high efficiency energy-saving crusher (PGXJ), vertical impact crusher and eddy impact crusher. Their average discharge particle size can reach 3 mm to 5 mm, in which the discharge particle size of Vertical Impact Crusher & lt; 2.5 mm can reach 80% to 90%. It should be pointed out that, whether it is roller press or all kinds of fine crushers mentioned above, the prominent technical problem is the material quality of roll surface or crushing working parts. Only working parts with high hardness and high wear resistance can ensure their long-term and effective work; otherwise, shorter service life and replacement cycle will inevitably affect the operation of the system. Conversion and production capacity will also increase material consumption, which is not conducive to the improvement of economic efficiency.
The structure of the ball mill itself is also an important aspect affecting its grinding ability. It includes the setting of the length of each bin, the form of liner, the type of partition board, the through-hole ratio and the layout mode.
At present, the length of grinding bin of ball mill of various specifications is mostly designed according to the large grain size of 25 mm, so the length of grinding bin is generally relatively long. With the introduction of pre-comminution technology, the particle size of grinding materials decreases significantly, and the pressure of rough grinding in grinding decreases greatly. Therefore, if the original bin length is maintained, the unbalanced phenomena of excess capacity of rough grinding bin and tight capacity of fine grinding bin will be caused. In order to match the capacity of each warehouse, many manufacturers have adopted the method of shortening the length of coarse grinding warehouse and increasing the length of fine grinding warehouse, and achieved good results.
In recent years, the development of high-fineness grinding has promoted the improvement of partition board, which not only has the traditional functions of blocking bulk materials, preventing abrasive body from bunching, guaranteeing ventilation and forcing feeding, but also has a new grading function. For example, the double-layer separator with grading sieve can return the coarse particles with larger size to the grinding bin for further grinding besides forcing feeding. As a result of this classification, the stability of the particle size of the material entering the fine grinding bin is greatly improved, and the existence of the grinding coarse particles can be avoided to a considerable extent. At the same time, it has positive significance for stabilizing the circulating load rate of the system and stabilizing the working state of the whole system.
Mill liner has various forms and different effects. Among them, stepped liner is widely used in cement grinding bin. Its Archimedean spiral curved surface ensures that the grinding body can be raised to a certain height in a balanced way during the operation of the mill, thereby increasing its impact on crushing materials. But this kind of liner still can not overcome the shortcoming of point contact between steel ball and it. The appearance of new liners such as round square liner, angle spiral liner and groove liner makes the matching between liner and abrasive body more reasonable. There is a layer of material between steel ball and liner which is not easy to separate. The sliding friction between them is fully utilized, and the grinding efficiency is improved accordingly. Angular helical liner can improve grinding efficiency by changing the motion law of grinding body and making the detachment angle of grinding body changeable, so as to enhance the cross-penetration and full contact grinding of grinding body and material, as well as automatic grading of steel balls. On the basis of annular groove activation liner, a new type of energy-saving liner, annular groove hyperboloid liner, has been developed. The liner makes the lifting of grinding body more reasonable, the contact friction between grinding body and material more opportunities, the output can be further increased by 10%-18%, and the power consumption can be reduced by 10%-20%.
The load of grinding body in ball mill is usually determined by filling coefficient or filling rate. The filling coefficient of general ball mill is mostly 0.28-0.32. Many studies and practices have proved that the filling coefficient of the abrasive body can be increased appropriately, that is to say, the load of the abrasive body can be increased appropriately, especially in the fine grinding bin. Firstly, increasing the filling amount of the grinding body can increase the probability of grinding or grinding without changing the gradation. Secondly, with the increase of filling rate, the center of gravity of the grinding body is close to the axis of the grinding machine, and the total moment of action is not significantly increased, so it will not affect the power transmission. In fact, the power reserve of the matching motor of the mill is fully allowed to increase the load in a certain range. Practice has proved that the output of the mill can be increased by about 20% when the load of the grinding body is properly increased.
Gradation optimization of grinding body in grinder has always been one of the hot topics in cement grinding technology. In recent years, there have been many reports on the practice of steel ball gradation, but there are also many regular experience summaries, such as average ball diameter method, Steiner curve method and so on. Several important parameters to be solved in gradation determination are: large ball diameter, average ball diameter, steel ball (or segment) series and the proportion of each stage. In fact, there are many factors affecting the gradation of grinding body, and many methods are only the better choice under specific material and process conditions. Here are some opinions on the problems to be considered in determining the gradation of grinding bodies.
1) The grindability or fragility of materials should be taken into account in the calculation of large and average spherical diameters.
The brittleness of materials calcined in different kilns is different. Even clinkers calcined in the same kiln type have different grindability due to different technological formulas and firing conditions. In addition, when the mixture is slag, because its particle size is relatively small and difficult to grind, it is necessary to increase the proportion of small size grinding body in ball matching.
2) In the pre-crushing grinding process, the adjustment of the average ball diameter and its series of the grinding body in the rough grinding bin should depend on the pre-crushing equipment.
Specifically, the clinker crushed by roll pressing and extrusion has a large number of crack defects in addition to the smaller particle size, so it is easy to crush after grinding; while the clinker particles crushed by impact are more uniform, and the crushing is mainly caused by the expansion of internal cracks, the internal defects of particles are relatively small, so it is relatively difficult to grind after grinding. Break it up. Therefore, when matching balls, the two pre-comminution systems should be treated differently.
3) Fineness and properties of grinding cement.
Fineness of finished cement is one of its direct quality indexes. For open-circuit system, this index depends on the gradation of grinding body in grinding; for closed-circuit system, if the fineness of grinding cement is too coarse, it will inevitably lead to excessive cyclic load rate and increase the pressure of flour separator. another
Stable grinding process conditions largely depend on the material of liner and abrasive body. If the hardness, wear resistance and impact resistance of liner material are poor, the inner surface of liner material will change the original geometry quickly. Similarly, the gradation of abrasive body is dynamic and changing in the process of grinding machine operation, if the abrasive body. Wear resistance and mechanical strength can not meet the requirements, after a period of grinding operation, the original good gradation is obviously difficult to guarantee. Although many methods of filling balls have been summarized, the wear rules of different size grinding bodies are different in the grinding process, so filling balls can only maintain the relative balance of loading at best, and can not maintain the consistency of gradation. Therefore, improving the material quality of lining plate and abrasive body is the fundamental guarantee for the long-term stability of the gradation of abrasive body and the working conditions of the mill and the improvement of its operation efficiency and production efficiency.
For liners, the necessary material conditions for long-term work are: uniform overall hardness and structure, high impact fatigue strength, low wear rate, no deformation and continuous cracking. Obviously, it is difficult for ordinary steel liners to have these characteristics. High chromium white iron liner has high hardness and wear resistance, but it is sensitive to stress concentration. Bainite ductile iron liner has high flexural fatigue strength and excellent wear resistance. Compared with high manganese steel liner, its service life can be increased by more than one time. The service life of Multi-element Low alloy steel liner is more than four times that of high manganese steel liner.
The consumption of grinding media is the main material consumption in cement grinding process, which includes the wear caused by friction between grinding bodies and materials and the wear caused by self-fragmentation. The excessive wear of grinding body not only affects the grinding ability of the mill, but also leads to low operation rate and unstable working state due to frequent shutdown and ball filling, which directly increases the grinding cost. The wear loss of grinding body of 1 t cement steel ball is about 1000g, and the filling period is about 15 days. The wear loss of wear-resistant ball such as bearing steel ball, high chromium ball and low alloy steel ball can be reduced to 30 g/t to 140 g/t. The average wear loss is about 60 g/t. The wear loss is only 1/15-1/20 of the former, and the filling period can be more than 180 days. If the factors such as operation rate and grinding efficiency are taken into account, its economic benefits are self-evident.
It is not difficult to see that although the price of wear-resistant ball is more expensive, its excellent performance can not only greatly reduce the work intensity of clearing and filling balls, but also greatly stabilize the working state of the mill, improve the grinding capacity of the mill, but also significantly reduce the grinding cost, which can bring considerable economic benefits.
It is worth pointing out that the influence of the density of the grinding body is greatly weakened when the material is finely grinded. The important thing is its hardness and surface area. The results of slag fine grinding with steel balls and ceramic balls of the same size of mm and 5mm in the test mill show that the particle size distribution of the material after grinding under the same grinding condition is different.（ Especially 10um-30um level and & lt; 10um level) are basically the same, which has a positive significance for reducing transmission power and grinding power consumption.