Screening technology is a central part of raw material preparation, and takes on various functions in both mines and quarries, from scalping through protective screening to classification of saleable products.
The wide range of technical requirements makes an equally wide range of solutions necessary. This means selection of the right equipment and its operating parameters, plus selection of a suitable screen medium.
The various screen media have very different properties with respect to achievable throughput.
This is determined, among other factors, by the available open area, for which the following distinction is necessary: the often used calculated open area states the area of all holes compared to the combined area of holes and wires (or polyurethane bars, etc) and is expressed as a percentage.
However, in practice, more relevant for the capacity of the screen is the true open area, which provides the actual proportion of open and total area on a specific screen deck, including all blind areas.
The latter, for example, is caused by the support bars for the screen, fastening points, polyurethane strips, etc. This is more complex to determine, since it depends on the individual screen medium and the machine.
However, an intelligent screen panel design can increase the area available for screening and therefore the capacity of the machine.
In actual operation, this open surface is often reduced even further since, in many processes, the screened product gradually closes the openings of the screen. This occurs either through pegging of material, meaning particles get permanently wedged in the mesh, or through blinding (see Figure 1). This effect is caused by fine material sticking to the surface of the wire due to its moisture or clay content, then continuing to accumulate.
Both problems can be prevented through selection of a suitable screen, for example, using opening shapes that reduce the chance of material getting stuck, or using particularly smooth wires to which fine material doesn’t stick as easily.
To achieve the highest possible capacity on a given screen area, the undersized particles that are to be screened off have to come into contact with the screen surface as quickly as possible. Therefore, the material needs to be shaken up and turned over, which is accomplished through the vibration of the screen (Figure 2).
This leads to the physical effect of stratification, or layer formation (Figure 3). Small particles keep dropping down into cavities between larger particles and therefore sink in the material bed.
This process can be accelerated using suitable screen media. There are two basic categories: passive – that is, rigid – screen media such as wire mesh, perforated plates and most rubber or polyurethane panels; and active screen media. These include Flex-Mat, which has single wires lying next to each other with minimal fixation.
Once material runs over this surface the impact induces vibration in the wires, which accelerates the stratification process. This has led to immense increases in throughput in numerous applications, while maintaining a precise separation (Figure 4).
The goals of increasing screen capacity and maximising wear life are diametrically opposed, since an increase in open area always means a reduction in material usage, and vice versa. To address this problem, Flex-Mat uses Optimum Wire, a spring steel wire with its own formulation.
It contains the ideal amount of carbon and manganese and it is drawn in the patented process, both of which guarantee the best possible wear resistance. In a direct comparison with other spring steels, this wire can achieve wear life advantages of up to 40 per cent.
Through the selection of suitable screen media and individual configuration, additional potential can be unlocked in many screening processes. This can mean machine capacity, product quality or screen wear life. But screen media exchange times and plant downtimes can also be reduced and production times in difficult weather conditions can be extended.
Achieving the best possible screening performance and wear life is, however, ultimately dependent on correct installation and tensioning of the screens, which is, in turn, a matter of the design of the screen and its working condition.
Individual consulting and technical support is therefore the only way to achieve optimum performance with each screen.
Source: Nepean Rubber & Plastics