Vertical mill
Vertical mills are mills for grinding brittle materials. They are usually associated with internal separators operated. The basic structure consists of a rotating grinding table onto which the grinding tools are pressed with their own weight and often with additional force from hydraulic cylinders. These grinding tools can be balls, cylindrical, conical or convex rollers. The wearing components of a vertical mill (mainly the rollers and the grinding track support of the grinding table) usually consist of replaceable parts in chilled cast alloys (example: NiHard ), the hardness of which is on the verge of machinability.
Important key figures of such mills are:
- Energy consumption (in kWh / to ground material, specifying the type and fineness of the ground material)
- Wear (grams of hard material per ton of ground material)
- Air requirement (m³ / h) and
- Pressure loss (mbar)
The material is fed using a chute from above or diagonally above, generally to the center of the grinding table.
The given material, the regrind, is moved to the edge of the plate by the acting centrifugal forces . On the way there, the grinding material is picked up by the grinding tools and rolled over one or more times. It is crushed in the process. The result is a mixture of coarsely and finely ground material. An attempt is made to adjust the milling process in such a way that the highest possible "grinding bed" is formed, an uninterrupted layer of ground rock that prevents direct contact between the hard and abrasive grinding media and the grinding track support.
An upwardly directed air stream enters the grinding chamber via a nozzle ring on the edge of the grinding bowl. With this air flow, the lighter, ground material is discharged up to the classifier (or separator). The internal sifter is used to separate coarse and fine material. Fine material then leaves the mill, while coarse material falls back down and is rolled over again. Particularly coarse material is not conveyed pneumatically. It falls out through the nozzle ring as so-called waste material and is fed back to the mill via an external circuit with a bucket elevator .
Another common name for this type of mill: vertical roller mill
Vertical mills are widely used in the cement industry and also in coal-fired power plants . Up to five roller mills can be used in a single cement line: one or two mills for grinding raw materials in front of the furnace, one or two mills for grinding clinker (cement), and another roller mill for grinding coal-fuel.
Ground materials are: raw meal (raw material for cement production), cement clinker, limestone in general, coal , clay , gypsum , blast furnace slag (slag)
Advantages of vertical mills compared to other types of mill:
- favorable energy requirements
- relatively low wear
- simultaneous drying of the material in a hot gas stream possible, d. H. very moist grist (limestone-chalk sludge with up to 15% moisture) can be fed to the mill.
Three-flap lock
A three-flap lock is a device that improves the filling of vertical mills . It reduces the amount of false air when conveying material into a room with negative or positive pressure .
The three-flap lock consists of a pipe that is filled from above. In this tube, chambers closed at the bottom are formed by three flaps.
When the flaps are opened, the product only falls further down into the next closed chamber. Since two flaps always remain closed, an upward flow of air is prevented ("false air flow"). This reduces operating costs .