As the domestic economic situation improves, investment in infrastructure construction is increasing, and the cement industry has become one of the biggest beneficiaries. The production of cement can generally be divided into three processes: raw material preparation, clinker calcination and cement production. The production process of Portland cement is representative in cement production. It uses limestone and clay as the main raw materials, which are crushed, batched, and ground into raw materials. The raw materials are then fed into the cement kiln to calcine the clinker, and then the cooked materials are processed. It is made by grinding an appropriate amount of gypsum (sometimes mixed with mixed materials or additives) into the material.
The layout of cement production is to establish a cement clinker production line in the mining area and a cement grinding station near the cement sales market to reduce transportation costs. If the cement grinding station and the clinker production line are built together in a mine, since most of the mixed materials are waste residues generated in the city, these mixed materials have to be transported to the cement factory built near the mine, ground into cement, and then transported to the city. Increased transportation costs. Therefore, the clinker production line should be built near the mine, and the cement grinding station should be built near the market. Cement grinding station is an industry with green benefits. The cement grinding station is a finished cement production unit that separates the final finished product stage in cement production. At this stage, cement clinker is added to an appropriate amount of mixed materials for powdering to produce finished cement.
At present, the main types of mills include: high-pressure suspension roller mill, trapezoidal mill, high-pressure micro-powder mill, three-ring medium-speed micro-powder mill and vertical mill. Facing the new situation in the cement industry, the high-pressure suspension roller mill launched by Liming Heavy Industry is based on Mr. Bai Yinghui, chief engineer of Liming Heavy Industry Technology, based on his years of experience in mill research and development, combined with the actual situation of users using grinding equipment for many years, and is aimed at In order to meet the needs of the development of the grinding industry, the new patented products developed by ourselves have created a new era of high efficiency and low energy consumption in international industrial grinding.
The quality of cement grinding aids should meet the requirements of the building materials industry standard JC/T667-2004. The soon-to-be-promulgated national standard "Universal Portland Cement" stipulates that grinding aids are allowed to be added during cement grinding, and the amount added should not exceed 0.5% of the cement mass; the new national standard also adds a chloride ion limit. Requirements, that is: the chloride ion content in cement should not be greater than 0.06%.
First, analysis of factors affecting the quality of mill production
The function and significance of grinding
Function: to convert cement clinker (and retarder, performance Adjust the material, etc.) and grind it to the appropriate particle size (expressed in terms of fineness, specific surface area, etc.) to form a certain particle gradation, increase its hydration area, accelerate the hydration rate, and meet the cement slurry coagulation and hardening requirements.
Significance: Improve the utilization rate of grinding power, improve the quality of cement products, save energy consumption, reduce production costs, and the main factors affecting the quality of the mill
1. Grinding Material particle size, moisture, grindability, and cement outlet temperature; 2. Mill structure (diameter, length, inlet and outlet devices, partition and liner forms, mill speed, etc.); 3. Grinding body grade Preparation and loading capacity; 4. Mill ventilation; 5. Grinding system operation and management; 6. Grinding process and powder concentrator performance.
This. In the normal production process, many factors affect the output and quality of the mill, mainly the particle size, grindability, moisture, cement temperature, ventilation of the mill, gradation and loading of the grinding body, and the grinding machine. Operation and management, the following focuses on a brief analysis of these influencing factors:
(1) The particle size and moisture of the materials entering the grinding process
1. Reduce the particle size of the materials entering the grinding process to ensure that the mill eats " "Fine grain" is the fundamental way to achieve high output, high quality and low consumption of mills. The effective utilization rate of electrical energy of the crusher is about 30%, while the effective utilization rate of electrical energy of the mill is only about 0.6%, that is, the effective utilization rate of electrical energy of the crusher is 50 times that of the mill. At present, the new process of "more crushing and less grinding" is being widely adopted at home and abroad, so that the average particle size of the grinding materials is controlled below 4-10mm. Controlling the particle size of the materials entering the ball mill can make the particle size of the materials entering the ball mill more uniform, which can shorten the residence time of the materials in the mill, increase the output of the mill, and also greatly improve the specific surface area and grinding quality. Due to the increase in mill output, unit power consumption has been significantly reduced. 2. Reducing the moisture content of the materials entering the grinding mill and ensuring that the mill "eats dry food" is the prerequisite for the mill to achieve high and stable production. The different moisture content of the materials entering the grinding mill has a great impact on the output of the mill and the quality of the grinding conditions. Because the grinding materials are different, their grindability is also different.
Generally speaking, wet materials are tougher than dry materials and are not easy to grind. On the other hand, during the grinding process, due to the impact, grinding and rolling friction of the grinding body, as well as the heat brought into the mill by the material itself, the temperature inside the mill will increase. When the temperature rises, the water in the material becomes water vapor after being heated. If it cannot be discharged in time, the moisture content of the dusty gas in the mill will increase, so the fine-grained material will stick to the working surface of the grinding body and lining plate. Forming a cushion layer, the grinding efficiency will be significantly reduced. In severe cases, the compartment plate and the discharge grate will be blocked, hindering the material flow, and then the "full grinding" phenomenon will occur. Production practice has proved: when the average moisture content of the grinding material exceeds When the moisture content exceeds 2.5%, the mill output will be reduced. If the moisture content exceeds 2.5%, the mill output will be reduced by 10-25%.
It is not good to have too much water in the grinding mill, but problems may arise if the material is too dry. If the material is too dry, the flow speed of the material in the grinding mill will be accelerated, and the phenomenon of "channeling and grinding" will occur, resulting in poor grinding fineness. Hard to control. Keeping an appropriate amount of water in the material can take away part of the heat inside the mill and reduce the grinding temperature when it "vaporizes" in the mill. In addition, an appropriate amount of water vapor can also form a thin layer of water "vapor film" on the surface of the material particles, which is beneficial to the dispersion of dry powder particles. This shows that an appropriate amount of water vapor can actually improve grinding efficiency.
(2) Temperature of the cement after grinding
1. The reasons for the high temperature of the cement after grinding are as follows:
① Due to the large number of grinding bodies During the period, the impact and friction between the grinding body and the lining plate will generate a large amount of heat, which will increase the temperature of the cement; ②. The temperature of the clinker entering the grinding is high and the grindability is poor, which will increase the temperature of the cement coming out of the grinding mill; ③. Ventilation of the mill It is not good, or due to the limitation of process conditions, the ventilation volume is insufficient, the heat in the mill cannot be taken away in time, and the temperature of the cement being ground is high; ④ Due to the enlargement of the mill, the proportion of heat dissipation on the surface of the cylinder per unit of cement output has become smaller, and it cannot be effectively discharged. The heat is taken away, thereby increasing the temperature of the cement coming out of the mill; ⑤. The cement fineness requirement is too fine, the material flow rate in the mill decreases, and the heat taken away by the material decreases significantly, causing the temperature of the cement to rise; ⑥ Due to the high seasonal temperature, the material entering the grinding mill High temperature and slow heat dissipation of the system eventually lead to high temperatures in the mill and finished cement.
2. The hazards of high cement temperature after grinding:
①. Cause the dehydration of gypsum into semi-hydrated gypsum and even produce some anhydrous gypsum, causing false setting of the cement and affecting the quality of the cement. Moreover, it is easy to cause the cement entering the warehouse to agglomerate; ②, it seriously affects the storage, packaging and transportation of cement, making the packaging paper bags brittle, increasing the breakage rate, and deteriorating the working environment of workers; ③, it is also detrimental to the mill machinery itself, such as The temperature of the bearing increases, the lubrication effect decreases, and a certain thermal stress occurs on the cylinder, causing the liner screws to break. Even the mill cannot run continuously, endangering the safety of the equipment; ④. It is easy for the cement to agglomerate due to electrostatic attraction. In severe cases, it will adhere to the grinding body and lining plate, resulting in ball wrapping, which reduces the grinding efficiency and the output of the mill; ⑤. The temperature of the material in the powder separator increases, the adhesion on the inner wall of the powder separator and the fan blades increases, and the electrostatic attraction between the material particles becomes stronger, which affects the dispersion of the material after spreading and directly reduces the efficiency of the powder selection. , increase the circulation load rate of the grinding system and reduce the output of the cement grinding table; ⑥. High cement temperature will affect the construction performance of the cement, resulting in rapid setting, large concrete slump loss, and even easy to cause temperature difference stress in the cement concrete. Cause hazards such as concrete cracking.
3. Methods to reduce the temperature of cement after grinding:
First, we will introduce the practices of some old companies, but the effect is not very good.
?¨1 Reduce the temperature of the clinker entering the grinding mill: (The temperature of the material entering the grinding mill should generally be <60°C) a. Strengthen material management to avoid high-temperature clinker entering the grinding head bin, and eliminate red The materials are fed into the grinding mill; b. Water is sprayed into the cooling machine to reduce the temperature of the clinker; c. Water is sprayed onto the clinker belt entering the grinding mill.
The above two water spray measures have obvious cooling effects, but they also have side effects. Spraying water in the cooler will easily bring some water vapor into the crusher at the rear of the cooler. The water vapor will capture the clinker dust and form a bond on the inner wall of the crusher, which will affect the normal operation of the crusher over time. Spraying water on the clinker belt will reduce the cement content. intensity. The increase is conducive to reducing the cement temperature. In view of the serious over-grinding phenomenon in the mill caused by the high temperature of the grinding system and the decrease in the powder selection efficiency of the powder separator, by using grinding aids to reduce the degree of adhesion in the mill, the efficiency of the powder separator can be improved, thereby reducing the output to a certain extent. The temperature of grinding cement.
2) Strengthen the ventilation of the mill
Good ventilation of the mill is conducive to the timely discharge of heat and water, which is conducive to the timely discharge of fine powder and improves the grinding efficiency. Every time the machine is stopped, It is necessary to check and clean up the broken balls and debris stuck in the compartment grate plate and the grinding grate plate hole to ensure the ventilation area of ??the mill. Enhancing the ventilation of the mill can take away more heat. However, according to the heat balance calculation of the mill, the heat taken away by the ventilation of the mill usually only accounts for 20% of the total heat discharged by the mill. Although strengthening the ventilation in the mill can reduce the temperature of the materials, the ventilation in the mill is limited by system resistance, air lock, air leakage, etc., and is also restricted by the fineness of the product. Therefore, by increasing the ventilation in the mill, the temperature of the materials coming out of the mill can be reduced. There are certain limits to temperature. 3) Bypass the cold air when the ventilation volume in the mill cannot be changed. According to our existing process system, we can increase the natural wind volume to reduce the temperature of the material in the powder separator. By reducing the temperature of the coarse powder, we can reduce the temperature in the mill. temperature.
3. Grinding body gradation and grinding body loading capacity
Whether the grinding body gradation and loading capacity are appropriate needs to be tested through production practice. Steel ball gradation is still appropriate. When adjusting the steel ball gradation with more multi-level balls, it should be considered that the reduction in steel ball size is not consistent. For example, according to literature, it is concluded through experiments and calculations that when the 90mm steel ball wears to 80mm, the 80mm steel ball wears to 71.11mm, the 70mm steel ball wears to 63.20mm, and the 60mm steel ball wears to 56.20mm. . Obviously, if only the larger balls are replenished, the average ball diameter will inevitably tend to become larger.
1. Inspection and analysis based on the output of the mill and the fineness of the product
① When the output of the mill is low and the fineness of the product is coarse, it means that the loading capacity of the grinding body is insufficient or the grinding capacity is insufficient. The body wear is too large, and the grinding body should be added at this time.
②. When the output of the mill is low and the product fineness is coarse, it means that the material flow rate in the mill is too fast, the crushing capacity is too strong, and the grinding capacity is insufficient, which is caused by the average ball diameter being too large. At this time, the large balls should be appropriately reduced and the small balls and steel forging should be increased to improve the grinding capacity, reduce the gaps between the grinding bodies, slow down the flow rate of the material in the mill, and extend the residence time of the material in the mill, so as to obtain sufficient grinding power. Grind.
③ If a warehouse is often full of grinding and the material flow in the grinding mill is not smooth, and at the same time the output is low and the product fineness is fine, sometimes the feeding has to be stopped, indicating that the grinding capacity in the grinding mill is insufficient. You can increase the size of the big ball and take out some of the small balls. To increase the crushing capacity of a warehouse. However, attention should be paid to other factors. If the grinding phenomenon is caused by excessive moisture in the grinding material, the method of adding larger balls cannot be adopted, but the moisture content of the material should be controlled. If the mill output is high and the product fineness is fine, it means that the loading capacity and gradation of the grinding body are reasonable.
2. Judgment based on the grinding sound
Under normal feeding conditions, the impact of the steel balls in the first warehouse is strong and there is a rattling sound. If the impact sound of the steel balls in the first warehouse is particularly loud, it means that the average ball diameter of the steel balls in the first warehouse is too large or the filling rate is large; if the sound is muffled, it means that the average ball diameter of the steel balls in the first warehouse is too small or the filling rate is too high. The rate is too low. At this time, the average ball diameter and filling rate of the steel balls should be increased. When the second chamber is normal, you should be able to hear the swishing sound of the grinding body. 3. Check the condition of materials in the mill
When the mill is feeding normally, stop feeding and mill operation at the same time, open the grinding door and observe the condition of the ball materials in the mill. According to production experience, half of the steel ball in the first warehouse should be exposed on the material surface. In the second warehouse, the grinding body should be covered with a thin material layer of 10-20mm, indicating that the steel ball gradation is reasonable. If too many steel balls are exposed in a warehouse, it means that the loading capacity is too much or the average ball diameter of the steel balls is too large; on the contrary, it means that the loading capacity is too small or the average ball diameter of the steel balls is too small. The ball-to-material ratio is too small, and the material flow 3. Check the situation of materials in the mill
When the mill is feeding normally, stop feeding and running the mill at the same time, open the grinding door and observe the situation of ball materials in the mill. . According to production experience, half of the steel ball in the first warehouse should be exposed on the material surface. In the second warehouse, the grinding body should be covered with a thin material layer of 10-20mm, indicating that the steel ball gradation is reasonable. If too many steel balls are exposed in a warehouse, it means that the loading capacity is too much or the average ball diameter of the steel balls is too large; on the contrary, it means that the loading capacity is too small or the average ball diameter of the steel balls is too small. The ball-to-material ratio is too small, the material flow rate is slow, and the crushing capacity is insufficient.
Slow speed and insufficient crushing ability. The same principle applies: if the spherical surface of the second warehouse is exposed, too many balls are added; if the material layer of the second warehouse is too thick, it means there are too few balls.
4. Judgment based on the screening curve
Generally, when the mill is in normal operation, the material is stopped and grinding is stopped and the warehouse is opened for sampling at a certain distance from the grinding head to the grinding tail. (0.5 meters for large grinding, 0.3 meters for small grinding or the length of one liner), take 4-5 samples along the cross section of each section (take one sample from each side of the cylinder lining, and 2-3 from the middle). Both sides of the compartment plate and the hollow shaft of the grinding head and tail are certain sampling points.
Mix the samples taken at each sampling point evenly and take them as the average sample at that sampling point and number them to prevent mistakes. Screen and analyze them with 0 and 08mm sieves respectively, and measure each segment. The sieve analysis percentage is taken, and then the ordinate is the measured fineness, and the abscissa is the segmented distance of each sampling point along the length of the mill. Mark the sieve analysis data on the coordinate line and make a sieve analysis diagram, which is the sieve analysis curve. The OA section should drop rapidly. If the drop is not significant or there is no such section, it means that the grinding capacity of the first warehouse is insufficient. The grinding body needs to be adjusted and the average ball diameter should be appropriately increased. If a long horizontal line segment appears in each bin, it means that the fineness of this section has not changed much and the grinding body is not working well. The reason may be that the gradation, loading capacity and average ball diameter of the grinding body are not appropriate. It should be Appropriately change the grinding body gradation or clear out the broken and small balls. If the difference between the screening percentages on both sides of the partition plate is too large, it means that the capacities of the two compartments are unbalanced. At this time, you should first check whether the width of the compartment plate grate hole meets the requirements. If it is too wide and exceeds the specified value by more than 2 mm, it should be replaced or blocked; if there is a blockage, the blockage should be removed. The CD horizontal section should not be too long. If the horizontal section is too long, it means that the grinding capacity of the second warehouse is excessive. If the BD section becomes a straight line, it means that the grinding capacity of the second warehouse is insufficient. The fineness of the product is unstable. If the feeding amount increases slightly, it will become obviously thicker. The steel section should be appropriately increased. The AB section is too flat and the fineness does not change much, indicating that the crushing capacity of the first warehouse is insufficient.