With the increase of cement mill specifications and the requirement of energy saving, high yield, and high quality of cement grinding plant, the closed-circuit grinding is an inevitable trend of cement grinding unit. The essential equipment of a closed-circuit grinding system is a cement classifier, also known as a cement separator. The function of the cement classifier is to select the granules which reach a certain particle size in time. The cement classifier itself does not produce fine powder, but because it can select fine particle materials in time, avoids the occurrence of material condensation, adhesion to the grinding body and bonding bin, which can reduce the over-pulverization of materials in the cement mill; The variety of cement production can be timely and conveniently adjusted to produce high fineness cement and improve grinding efficiency.
Function of cement classifier
The main function of the cement classifier can be summarized as: “dispersion”, “classification” and “collection”.
- Dispersion refers to the material into the classifier to be thrown out as far as possible, there must be a certain space among material particles. Therefore, the structure, speed, size of the spreader, the moisture content and the flow rate of the materials directly affect the effect of the spreader and dispersion.
- Classification refers to the dispersion of the material in the powder chamber use various forms of airflow to separate within the limited residence time, to make full use of the airflow to separate the coarse and fine particles as far as possible, and sent them to their respective outlets.
- Collection is the ability to capture coarse and fine powder, depending on the collection method and the structural of the collection component. Therefore, the gas flow rate, airflow velocity, airflow pattern, gas-solid intersection point and flow field distribution, as well as the number and structure of powder chamber have a great impact on the fractional efficiency.
All in all, the comprehensive performance of the cement classifier: one is the efficiency of powder selection; The second is the percentage of 3 ~ 30μm particles in the finished product. The efficiency of powder selection directly affects the cement mill output. Low efficiency of powder selection will choke mill output; Because of the high efficiency of powder selection, 30 ~ 80μm particles account for a large proportion in the finished product, and the specific surface area of cement decreases, which reduces the strength of cement. Therefore, too high and too low powder efficiency is not an ideal indicator. In the actual cement production line to evaluate the performance of the cement separator, the simplest detection method is to determine the percentage content of ≤30μm particles in the return material, the less ≤30μm content in the return material, the better the performance of the cement separator.
Make full use of cement classifier
During the process of a closed-circuit grinding system, the material from the grinding head into the mill is smashed in the first bin, from the gap of the ball quickly through the first bin, through the compartment board into the second bin. The grinding body in the second bin has the main effect on the material, and the crushing effect is very small. Driven by the mill barrel, the motion state of the small grinding body appears as rubbing, rolling, and sliding. The abrasive body grinds and strips the material particles during the process of dropping and rubbing. If the material cannot be discharged out of the circulation system in time after grinding, it will stay in the grinding for a long time, and it will accumulate more and more. The grinding body will continue to grind and peel these fine material particles, which will lead to the over grinding.
Therefore, to improve the ability of cement classifier is conducive to the full play of the grinding ability of the cement mill. The cement separator is an important part of the grinding system. The operation parameters of the cement separator are related to the production of the whole cement grinding unit. The higher the separation efficiency of the cement classifier has, the higher the output of the cement mill has. In the closed-loop system, the key point of cement grinding is the effect of relatively coarse particles.
Relation between cement classifier and cement mill
- The production capacity of the cement mill increases with the increase of circulating load rate, because the more coarse particles, the higher the grinding efficiency.
- The grinding efficiency increases with the increase of powder selection efficiency, but the powder selection efficiency is inversely proportional to the cyclic loading rate.
Therefore, the relationship between the grinding efficiency of the cement mill and the cyclic load rate of the cement classifier and the efficiency of powder selection needs to be balanced.
The relevant data show that: for the circular flow cement mill composed of the centrifugal classifier, the cement milling efficiency is the highest when the cyclic load rate is 200%~300% and the milling efficiency is 41%~51%. For the circular-flow mill composed of cyclone classifier, the milling efficiency is the highest when the cyclic load rate is 150%~200% and the milling efficiency is 55%-65%. For the circular flow mill composed of O-SEPa separator, the milling efficiency is the highest when the cyclic load rate is 100%~200% and the milling efficiency is 63%~79%.
With the progress of science and technology and the accumulation of production experience, the theory of the grinding process is being updated day by day. At present, some enterprises use the high-efficient cement classifier; the cycle load rate has been reduced to 100 percent below, the efficiency of powder selection increased to more than 70 percent, which promotes high production and energy saving of cement mill.
Because the closed-circuit grinding mill requires material discharged from the tail of the cement mill can be thick and fine coexistence. Therefore, the circulation amount of material in the cement mill is greatly increased. The buffering effect caused by the adhesion of fine powder to the grinding body was reduced, and the amount of material into the cement mill was increased, which greatly increased the cement mill output.
It can be seen that, under the condition that the milling capacity of the mill allows, the cyclic loading rate is in direct proportion to the cement mill output, but it is in inverse proportion to the efficiency of powder selection. Under the same condition, the cement mill output will be increased if the circulating load in the cement mill is increased. But the larger cycle load rate will make the powder efficiency decline, many qualified products cannot be selected out, back to the grinding cycle, limiting the increase in the cement mill output. This is a paradoxical state. The main measure to solve this contradiction is the improvement of the performance of the cement separator.
Matching of powder selection ability and grinding ability
To improve the capacity of the closed-loop cement grinding system, the first thing is the matching of the ability of cement classifier and cement grinding mill. According to the quality requirements of the product, the fineness of the grinding cement should be controlled within a certain range.
If the fineness of the finished product remains the same: when the fineness of feeding materials remains the same, the cyclic loading rate decreases with the thickening of feed, while the powder selection efficiency increases with the thickening of feed. When the fineness of the return material remains unchanged, the cyclic loading rate increases with the thickening of the feeding material, while the efficiency of powder selection decreases with the thickening of the feeding material. The greater the difference between the fineness of the return and the fineness of the output, the more the powder selecting ability of the cement separator will be improved, forming a lower cyclic load rate and a higher powder selecting efficiency, the cement grinding mill can increase the feeding and increase the output.
However, it is unwise to only choose the efficiency of powder as a measure of the performance of the cement classifier. Generally, the powder selection efficiency of high-efficiency cement classifier reached 65%. For any one of the classifier group, can select the efficiency of more than 90%, the simplest way is to adjust the fineness of the grinding to the fineness of the finished product, so closed circuit grinding mill will lose its significance. It must be fully realized that the improvement of grinding efficiency is a systematic project.
In a real cement manufacturing plant, it is necessary to ensure the balance between grinding capacity and powder selecting capacity. To realize the matching of powder selection ability with the grinding ability of the cement mill, under normal circumstances, the powder selection ability of the equipped cement classifier is greater than the grinding ability of the cement mill. At present, many cement plants’ grinding capacity of cement mill has not been well developed, and one of the important reasons is that they have not realized the reasonable matching of grinding capacity and selecting capacity.