Screening efficiency - Optimisation of screening performance through process-oriented ­design of screen media

Summary: The efficiency of screening processes is influenced significantly by choosing suitable screen media. The media type, specification and material are core parameters with which the screening performance and wear life can be influenced. The key to optimisation is the individual, process-oriented setup of the screen media. Haver & Boecker and its subsidiary Major Wire offer this service and high-quality screen media worldwide through a large sales network.

Screening technology is a central part of the raw material preparation and takes on various functions in both mines and quarries from scalping, through protective screening to classification of sellable products. The wide range of technical requirements makes an equally wide range of solutions necessary. This means both the selection of the right machinery equipment and its operating parameters as well as the selection of a suitable screen medium.

The various types of screen media have very different properties with respect to the achievable throughput. This is determined, amongst 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 PU-bars, etc.) and is expressed as a percentage [1]. 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, which for example are 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 (Fig. 1). 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 which get permanently wedged in the mesh, or through blinding. This effect is caused by fine material sticking to the surface of the wire due to its moisture or clay content and then continuing to accumulate (Fig. 2). Both problems can be prevented through selection of a suitable screen, for example by using opening shapes which reduce the chance of material getting stuck or through the use of particularly smooth wires to which fine material doesn’t stick as easily.

In order 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 [2] which is accomplished through the vibration of the screen machine. This leads to the physical effect of stratification, or layer formation (Fig. 4). Small particles keep dropping down into cavities between larger particles and therefore sink down in the material bed [3].

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 PU 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 (Fig. 3).

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. In order to address this problem, Haver & Boecker and Major Wire use 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 %.

Through selection of suitable screen media and individual configuration additional potential can be unlocked in many screening processes. This can mean the machine capacity, the product quality or the screen wear life. But also the screen media exchange times and plant downtimes can be reduced and the 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 machine and its maintenance condition. In a recent case in southwest Germany the wear life of the screen media could be increased from an average of 4 weeks to currently 14 weeks just by optimising details regarding tensioning and installation.

Individual consulting and technical support are therefore the only way to achieve the optimum performance with each screen. With more than 35 000 applications in Europe, North and South America, Africa and Australia in a variety of industries including aggregate, sand, coal, ores and recycling, Haver & Boecker and Major Wire have the necessary experience to achieve this.

Literatur/Literature:

[1] Schubert, H.: Aufbereitung fester mineralischer Rohstoffe Band 1, VEB Deutscher Verlag für Grundstoffindustrie, 1989, S. 208
[2] Schubert, H.: Aufbereitung fester mineralischer Rohstoffe Band 1, VEB Deutscher Verlag für Grundstoffindustrie, 1989, S. 196
[3] DIN ISO 9045:1998-12, Industrial screens and screening - Vocabulary, 3.3.12