Tube mill
Polysius supplies tube mill systems for drying and grinding a very broad range of different materials.
Selection of the appropriate system for the particular application depends on different parameters, such as feed size, grindability and moisture content and drying properties of the raw material.
The customary system configurations are:
- AG/SAG mill + single-compartment mill or two-compartment separator mill
- Crusher + single-compartment mill or two-compartment separator mill
- Crusher + air-swept mill
- Crusher + DOUBLE ROTATOR®
Tube mills can be operated in open or closed circuit. In an open circuit there are no auxiliary units, such as separator and bucket elevator. This minimises the capital expenditure and maintenance requirement. The closed circuit offers process technological and energy-effectiveness advantages. Tube mills are operated with grinding media charges (balls, rods, cylpebs) of around 30 %.
Comprehensive process and plant know-how (Polysius has been successfully designing and building grinding plants since 1880), as well as our strong back-up-testing capability, such as application-specific test grinding in our own Research and Development Centre, help to ensure optimum plant design.
Flowsheet of a grinding plant with air-swept mill
Air-swept mill
Air-swept mills with one grinding compartment are used for the combined grinding and drying of very moist materials because large quantities of low-temperature gas can be passed through its drying chamber.
This mill can accept ores up to a lump size of 15 mm (in special cases up to 30 mm) and a moisture content of up to 15 %.
The ground material is carried out of the mill in the stream of gas and collected in the high-efficiency separator. Separator tailings are returned to the mill for regrinding.
The grinding plant has no dust problems as it works under negative pressure conditions, due to the pneumatic material conveyance. As there are no dust deposits (in contrast to closed circuit mills with bucket elevators), the air-swept mill is especially suitable for grinding coal.
A simple plant concept and low capital expenditure typify the air-swept mill
Flowsheet of a grinding plant with single-compartment mill and upstream POLYCOM®Single-compartment mill
Single-compartment mills are used for comminuting feed materials with a lump size of up to 15 mm (in special cases up to 30 mm) and moisture contents of up to 6 %.
For drying the feed material, the single-compartment mill can be equipped with a pre-drying compartment.
Unlike the air-swept mill, the ground material is mechanically discharged via an end discharge diaphragm and then fed to a dynamic separator. Separator tailings are returned to the mill for regrinding.
As the material is discharged mechanically from the mill, the power requirement for material conveying is lower than that of the air-swept mill.
Flowsheet of a grinding plant with two-compartment separator millTwo-compartment separator mill
Two-compartment separator mills can accept ores with up to 30 mm lump size and 6 % moisture content. The grinding circuit has the same configuration as that of a single-compartment mill.
The first grinding compartment (with lifter plate lining and large grinding media) is for coarse grinding. The second grinding compartment has a charge of smaller grinding media and a classifying plate lining for fine grinding the material. The two compartments are separated by a partition diaphragm.
The specific power requirement of a two-compartment separator mill is about 5 % lower than that of a single-compartment mill.
DOUBLE ROTATOR®
The DOUBLE ROTATOR® (a central discharge mill) is used for grinding even the hardest and most abrasive materials. Large DOUBLE ROTATOR® plants are used, for instance, for grinding extremely abrasive gold ore. The feed size can range up to 50 mm - in special cases even more. Furthermore, the DOUBLE ROTATOR® can be used for drying and grinding very moist materials (up to 12 % water content) due to the very large gas volumes that can be passed through the mill.
The typical steps in a DOUBLE ROTATOR® grinding process are drying, primary grinding, intermediate separation, fine grinding and renewed separation.
The material is pneumatically and mechanically discharged through the central outlet between the coarse and fine grinding compartments and passes to the intermediate separator, which removes the fine material. The major portion of the separator tailings is fed into the fine grinding compartment for further grinding. The remainder is returned to the coarse grinding compartment to improve the flow properties of the material being ground.
The material ground in the fine grinding compartment is also discharged through the central outlet and separated.
The hot gas required for drying the material and transporting it out of the mill is mainly sucked in through the flange-mounted pre-drying chamber and the coarse grinding compartment and extracted via the central discharge. In contrast to a two-compartment separator mill, the fine grinding compartment of a DOUBLE ROTATOR® therefore has a lower gas flow rate and thus offers optimum conditions for the grinding process. Furthermore, its pressure drop is lower.
The mill's division into two grinding compartments and the intermediate separation of the material allow optimum plant adaptation to the coarse and fine grinding requirements of the respective feed material. The DOUBLE ROTATOR® therefore has the lowest specific energy requirement of all tube mills and also has a lower wear rate than conventional two-compartment mills.
