Grinding machines are indispensable equipment in cement plants. In the process of cement production, the cement plant needs to complete the grinding of pulverized coal, raw meal powder, and cement products. The purpose of grinding is to increase the specific surface area of the material and reduce the particle size of the powder, so as to achieve the activity required by the next process.
Tube mill has been used as the main grinding equipment in the cement industry for a long time, and it is still widely used because of its simple structure, easy operation, and low price. However, the energy utilization rate of tube mills is only 2% ~ 20%, which is very low. With the continuous development of grinding technology, there are two kinds of cement grinding equipment with lower energy consumption, one is vertical roller mill, and the other is roller press.
The two equipments are different in grinding mechanism, structure design, and product performance, but each has its own advantages.
1. Grinding Principle
Both the vertical roller mill and roller press apply the working principle of ‘material bed milling’. During the grinding process, only a small part of material particles directly contact with the grinding parts of the mill; but under the action of grinding parts, material particles form a material bed, and through the stress transmission and interaction between the particles, they are cracked, broken, and crushed.
For vertical roller mill, the grinding parts are rollers and grinding disc. The pressure required for grinding mainly comes from the rollers. At present, the loading force is mainly formed by hydraulic pressure. The material is fed into the grinding disc from the center or side of the upper housing, and the material layer is formed in the process of moving on the grinding disc. When the material passes between the roller and the grinding disc, the grinding is completed under the extrusion and shear force between them.
For roller press, the grinding parts are two rollers of the same size. The materials fall vertically from the top to the two rollers to form the material bed. Under the extrusion force of the two rollers, materials are ground to fine particles. There are two transmission modes of roller press, namely double drive, and single drive. The double drive roller press is the more popular type. The transmission device of a single drive roller press has only one set to drive one roller, and the other roller is driven by connecting with the former one with a gear train.
2. Design Structure
There are essential differences between the design structures of vertical mill and roller press.
In a vertical roller mill, the material on the grinding disc is restricted by the air duct ring, but the side of the material between roller and disc is not restricted. In roller press, in addition to the restriction between the two rollers, side baffles will also be set on the sides of both ends of the roller, which will limit the side of the material bed of the roller press.
Because the grinding area of the roller press is restricted from all sides, the increase of applied energy will also increase the fine powder and specific surface area of material. But for vertical roller mills, the growth rate of that of material will gradually decrease. From this point of view, roller press is superior to vertical mill.
In roller press, the contact area between the two rollers and the material is small; the material is ground by only passing through the roller press once due to the large grinding ratio.
In a vertical mill, there generally are two or more grinding rollers. The contact area between the grinding roller and the grinding disc is large, and the energy utilization rate is higher than that of the roller press. Moreover, the materials are generally milled on the grinding disc by several grinding rolls repeatedly. From this point of view, the vertical mill is better than the roller press.
3. Product Performance
When grinding cement products, a grinding system composed of a roller press or an external circulation vertical mill + a powder concentrator + a tube mill is generally selected. Taking grinding P · o42.5 cement as an example, the pre grinding closed-loop system composed of roller press or external circulation vertical mill and powder concentrator has a circulation load rate of 200% – 300%, and the specific surface area of the selected materials is 180-280m2 / kg. The following table lists the main machine preparation and the product particle size of several cement grinding systems.
|Roller Press/Vertical Mill||Tube Mill||Product Particle Size Distribution (%)|
|Roller Press 120-45||Φ3.2×13||8-10||12-16||20-28|
|Roller Press 140-80||Φ3.2×13||10-14||17-21||26-32|
|Roller Press 150-90||Φ3.2×13||12-15||19-23||27-33|
|Roller Press 170-120||Φ3.8×14||12-16||18-24||28-35|
|Roller Press 180-120||Φ4.2×13||11-17||20-27||32-40|
|Vertical Mill KVM18.3||Φ3.2×13||15-19||20-26||35-41|
|Vertical Mill KVM22.3||Φ3.2×13||14-18||18-25||32-38|
|Vertical Mill KVM46.4||–||21-26||30-35||43-55|
The particle shape of the materials from the roller press and the vertical roller mill is also different. The vertical mill grinds the material by two forces, the extrusion force and the shearing force between the roller and the grinding disc; the roller press grinds the material by only one force, the force between the two grinding rollers. When the material is ground to a certain fineness, most of the fine powders from the vertical mill are irregular and prismatic, and the fine powders from the roller press are mainly irregular flake and strip particles. From the particle shape, the fine powder from vertical mill is closer to that from tube mill, and the fine powder from vertical mill is more suitable for making cement products than that from roller press.