Typical phosphate fertilizers have specifications that allow particle sizes between 2 mm and 4 or 5 mm. In addition, these products are often expected to have a quality of more than 90 percent, which means that only 10 percent of the particles may be outside the specialized range. On the other hand, phosphate fertiliser production plants have a production rate of several hundred tonnes per hour. Phosphate granulate tends to clog the screen meshes. If this occurs, production must be stopped to allow the meshes to be cleaned or replaced. The biggest challenge of a phosphate screening machine is therefore to obtain a high quality material with a high throughput while ensuring a low downtime for maintenance.
Our solution - the WA(U) screening machine
The concept of the Rhewum wa/WA(U) screening machine consists of directly excited screening meshes which are positioned at an angle to transport the material over the screen. The whole machine housing does not move while the screen wires are tensioned by rocker shafts. The rocker arm shafts hit the screen wires repeatedly with relatively low amplitude but high forces and high frequency. This results in a high number of contacts between the mesh openings and the particles. Direct excitation of the screen makes it possible to achieve vibration of the screen at a frequency of 50 Hz with accelerations of up to 15 g. These values cannot be achieved on screening machines where the entire machine body moves.
Further advantages of the machine speak for themselves:
- Individual adjustment of the angle of each screen deck
- Individual control of the inlet and outlet vibration of each screen deck
- High acceleration forces prevent clogging of the meshes and overcome agglomeration
- Maintenance and control of the drives during production
Drive types
The impact movement is generated by two different available drive types (WA or WAU). Each of these two drive types is suitable for different tasks. The simpler and more solid WAU drive, which is based on an unbalance motor, is suitable for easy flowing materials that are dry and do not tend to clog the meshes. As standard, it is used for coarser screen cuts larger than 2 mm. The amplitude and the associated vibration forces can be adjusted by mounting more or less unbalance discs on the motor shaft. The motor is directly connected to the oscillating axle and requires no leverage and no additional housing. A cleaning cycle can be realized by a short motor stop. The WA drive is a technology specially developed by RHEWUM. It is based on an electromagnetically excited hammer which is connected to the pendulum shaft by levers. This drive is especially suitable for screen cuts between 50 microns and 4 mm. The very high, inharmonic acceleration provides advantages of deagglomeration (prevention of clogging) and especially high screening efficiency. The drive is electrically adjustable via potentiometer, low maintenance and easy to install.
Mesh tensioning system
Each screen fabric can be changed individually without removing the other. Overflows are collected from each deck and discharged via an overflow chute. The tensioning system is optimized for fast tensioning and easy mesh change, which can be done by two skilled workers within five minutes per screen.
Flexible screening process
Each individual drive can be individually adapted, making the screening process more flexible. It is therefore possible to set the inlet drives stronger than the outlet drives to mix the material first and overcome agglomerates. For longer operating times, all settings of the drives can be made during production. The steep angle of the screen decks allows material transport even when the individual drives are switched off. This ensures the availability of the screen in case of failure of individual drives. The cleaning cycle is also flexibly programmable and can be changed during production.
Plug-and-play implementation
Screening machines are mainly installed in the upper part of the building, where the height is limited and connections and floor space have to be taken into account. In order to keep the effort as low as possible and to reduce downtime during replacement as much as possible, a tailor-made design is the only solution. The economical modular design of the directly excited screening machines WA and WAU allows the given structure, hoppers and feeders to be used. As no conversion or design work is required, a short replacement is possible within 48 hours until the structure is adapted and ready for production.
Example of use
The most recent experiments with diammonium phosphate are described as an example. The task was to screen 200 t/h with screen cuts of 2.0 mm and 5.0 mm. A machine with a length of 3.6 m and a width of 3.9 m was used to achieve a high degree of purity for the desired product. The achieved purity of 96.5 percent coupled with a yield of 95 percent speaks for itself.
