Development trend and key points of cement grinding processDate: 2019-07-08 Views:
Cement is a hydraulic micron-sized powder made of cement clinker, mixed materials, gypsum and other materials (such as grinding aids) that are ground together or separately. The new view of modern cement grinding technology believes that good cement is "grinded out". Today's world cement grinding technology has shown a diversified trend, and grinding equipment is also developing in the direction of large-scale, low-cost, high-efficiency and automation. With the continuous advancement of science and technology, the mechanism of cement grinding is no longer limited to the traditional low-efficiency ball and calcination method, but gradually transitions to high-efficiency and energy-saving roller mill.
As far as the current cement grinding process is concerned, there are the following: tube mill (open or closed circuit) grinding system, vertical grinding system, barrel roller grinding system and roller final grinding system. The power consumption of the grinding process accounts for more than 70% of the total electricity consumption of the cement. The selection and application of the grinding process directly affects the production, quality and production cost of the cement, and plays a pivotal role in cement preparation.
Current Status and Development Trend of Cement Grinding Process Throughout the modern cement grinding process, most of the process still uses the tube mill as the grinding equipment. At present, the design diameter of the domestic cement pipe mill has reached about 5m, and the output is above 150t/h. Internationally designed large-scale tube mills with a diameter of 5.8m or more for grinding cement, the output is over 200t/h. The grinding mechanism of the tube mill is to transfer energy to the lining plate during the rotation process of the cylinder body, and the lining plate lifts and throws the grinding body to impact crushing and grinding the grinding material to complete the grinding operation. The shape of the grinding body used in the tube mill is mostly a conventional ball and column shape, and the spherical grinding body impact-breaks the material to be ground in a point contact manner, and the grinding efficiency is low. Especially when the size of the grinding material is large and the wearability is poor, the contradiction between the low efficiency and high power consumption of the tube mill is more prominent.
In order to improve the grinding operation conditions, increase the mill system output, and reduce the grinding power consumption, cement engineering technicians start by reducing the granularity of the grinding material, optimize the design of the working surface shape of the liner, and change the lifting of the grinding body in the grinding chamber. The throwing trajectory and the use of grinding aids and other technical means, to a certain extent, greatly improve the production efficiency of the mill.
In the cement grinding process, in addition to the tube mill process, the roller presses which were introduced in Germany in the mid-1980s were mainly used for pre-grinding of cement raw materials and cement clinker, that is, semi-final grinding. The grinding mechanism of the roller press is the crushing of the bed. At this stage, the transition from the past semi-finished grinding to the final grinding for cement preparation has been carried out. The material extruded by the two high-pressure rolls produces a large amount of cracks and fine powder, which significantly improves the wearability of the material. After the extruded cake is dispersed and classified, a closed loop is formed, the finished product is selected, and the coarse granular material is re-injected into a roller press to be pulverized. Although the power consumption of the roller press system is lower than that of the tube mill grinding system, the finished cement particles obtained by the final grinding of the roller press have a polygonal structure, and the standard consistency requires an increase in water volume during the concrete preparation process. The work performance is not as good as the cement milled by the tube mill.
Vertical mill is widely used in the raw material preparation process due to its high system output and low power consumption. At present, there have been reports of vertical grinding and grinding cement (final grinding) in the world. The grinding mechanism of the vertical mill has similarities with the roller press, and both are crushed by the bed. The difference is that the contact mode of the vertical grinding roller to the material is the cylinder surface and the plane, and the contact mode between the roller and the material of the roller press is the cylinder surface and the cylinder surface. In addition, the vertical mill itself does not need to set a separate powder sorting system, and the roller press must be set separately, and the system is more complicated than vertical grinding. At this stage, the world's largest vertical mill yield is already at 600t/h, which is unmatched by the tube mill and roller mill grinding system. At the same time, the power consumption of the vertical mill system is significantly lower than that of the roller press system.
Key points of cement grinding process
1. Transformation of large tube mill (4m or more)
In today's cement industry production, tube mills still dominate the grinding equipment. As mentioned above, the tube mill has low power utilization and the grinding power consumption is higher than that of the roller press, the vertical mill and the barrel roller mill system. In order to reduce the power consumption of grinding, most enterprises add material pretreatment process in front of the tube mill, reduce the size of the grinding mill through the pretreatment equipment, and significantly reduce the grinding output (30%~50%) while significantly reducing the grinding. System power consumption (20%~30%) and production cost, improve the physical quality of cement. Taking the roller press + dispersing grading + tube mill pretreatment grinding system (closed circuit) as an example, the new dry kiln clinker is ground, and the power consumption is 28kWh/t~32kWh/t, which is better than the tube mill alone. The power consumption when the pretreatment process is not set is as low as 8 kWh/t~12kwh/t. It can be seen that strengthening the pretreatment of the grinding material can maintain the grinding system with high and stable grinding efficiency and low for a long time. Grinding power consumption. At the same time, due to the reduced particle size of the grinding material, it is possible to optimize the design of the grinding body grading and reduce the average size of the grinding body, which is more conducive to significantly improve the grinding degree (specific surface area) and the strength of the cement.
The inside of the large tube mill should adopt lifting, grading lining, screening device, activation device, and anti-string device for grinding body. Due to the large amount of grinding of the large-scale tube mill, in order to maintain stable and high-yield system for a long time, it is required to use high-quality cemented carbide grinding bodies, such as high- and medium-chromium alloy materials (wearing<50g/t, breakage rate). <1.0%). At the same time, other parts of the wearer, such as lining boards, partition boards, etc., should also be selected from the same material as the grinding body to obtain the best anti-wear effect and good surface finish, for stable system production, Quality creates conditions.
In order to improve the roundness of the cement, some enterprises use the micro-shaped ball of 8mm~12mm in the fine grinding bin, and the effect is good. The large-scale tube mill has multiple positions, and the grinding bodies used in each bin have different specifications. The general rule is that the grinding body specifications of each bin from the feeding end to the discharging end are gradually reduced to enhance the grinding function of the grinding body. The filling rate of the abrasive body is generally<32%, and most of them are selected between 26% and 30%.
2. Transformation of small and medium-sized cement grinding process (below 4m)
For small and medium-sized tube mills, whether it is an open circuit or a closed-circuit grinding system, a pre-grinding material pretreatment process must be provided. The pre-treatment methods include pre-crushing, pre-crushing and pre-grinding. In the three pretreatment processes, pre-grinding (that is, using short-thick bar mill or barrel roller mill) has the best technical effect and low power consumption. Long-term operation is reliable, the maximum particle size of the treated materials is stable below 2mm, which still contains about 30% of finished products. The setting of the pretreatment process partially or completely replaces the function of the grinder coarse grinding bin, which is equivalent to extending the fine grinding bin of the mill, and is more conducive to the improvement of the long diameter (L/D≈3) medium or long grinding or Short-run system production (30%~50%) and reduced grinding power consumption (10%~30%). The reformation of several grinding processes after pretreatment is now discussed.
3. Pretreatment open circuit high-fine grinding system
It is well known that the proportion of particles below 30μm in the finished cement product determines the strength of the rubber sand. The more the characteristic particle size is 16μm~24μm, the better. Small and medium-sized mills generally have a short grinding body, and the time during which the material stays in the grinding mill is also short. It relies entirely on the grinding and grinding of the material in the grinding body. The material is often not easily ground, resulting in coarse particles in the finished product. More than one, seriously restricting the performance of cement hydration. The setting of the pretreatment process is of great significance for increasing the production of the open grinding system, saving electricity and increasing the degree of grinding of the cement.
The grinding material is pretreated, and the function of the grinding machine is completed by the pretreatment equipment. The average size of the grinding body in the grinding is reduced, which enhances the fine grinding ability of the material, and the proportion of particles below 30 μm in the finished cement product increases remarkably.
After the pretreatment open-circuit high-fine grinding process is formed, it is advisable to carry out corresponding transformation on the grinding machine, install the screening and classifying compartment board, and simultaneously activate the fine grinding silo liner to fully activate the grinding energy of the micro-shaped grinding body, remarkable Improve the grinding degree and gelling activity of cement. The content of a certain particle size in the cement particle grading after being ground by the open circuit process is relatively concentrated, that is, the so-called "narrow grade cement". The quilting of the inner partition plate and the discharge raft is generally ≤6mm. The open high-friction system must strengthen the ventilation and dust collection measures. The wind speed in the mill should be 0.5m/s~0.8m/s. If the surface of the abrasive body affects the grinding efficiency due to electrostatic adsorption of fine materials, it may be considered to introduce a grinding aid. The power consumption of the grinding process is generally 30 kWh/t~33 kWh/t. 4. Pretreatment closed-circuit grinding process closed circuit The grinding process is modified on the basis of open-circuit grinding by adding high-efficiency powder selection equipment. The most important technical part of the closed-circuit grinding process is that the classification accuracy of the selected classifier must be high (such as the powder selection efficiency of more than 85%), stable performance, long-term reliable operation, otherwise it is difficult to achieve the best technical results. The optimal configuration of the process is as follows: pre-preparation pre-grinding + grinding internal sieving + high-efficiency powder selection outside the mill, which can avoid the phenomenon of widening the gradation of cement particles in closed-circuit grinding, and strive to make the powder content of the characteristic particle size more. Conducive to the further development of cement hydration activity and mechanical strength. The power consumption of the closed-circuit grinding system is lower than that of the open circuit system, generally ≤28kwh/t. The researchers of Shandong Building Materials College have used the pretreatment technology of the 2.2×6.5m closed-circuit cement grinding system of a certain plant to modify the average grain size of the grinding material by 9.7. The mm is reduced to 5.3mm, and the grading of the grinding body in the grinding mill is optimized, the filling rate of the two bins is adjusted, the internal structure of the classifier is improved, and the system cyclic load rate is appropriately reduced. After the transformation, the specific surface area of the finished cement product is increased by 70%, and the 3-day compressive strength is increased by 65%. 5. The material grinding process separately grinds the material to maximize the gelation activity of the cement product, which is used for high utilization. Active industrial waste residue has created good conditions for purifying the ecological environment. The cements that have been separately ground and then "blended" have more mixed materials. At the same time, due to the reduction of the clinker content, the obtained cement not only has a low alkali content, but also has a low hydration heat, which can significantly improve the durability of the concrete product.
The cement particle grading prepared by the grinding process is more reasonable, the strength increasing rate is high, the manufacturing cost is low, and the grinding power consumption is in the middle, generally 40~50, which is one of the directions of development and transformation of the grinding process.