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 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, plus the selection of a suitable screen medium.
The various types of 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, which, for example, are caused by the support bars for the screen, fastening points, polyurethane strips, etc. However, this is more complex to determine, since it depends upon the individual screen medium and the machine.
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 that get permanently wedged in the mesh, or through blinding. This 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, eg by using opening shapes that reduce the chance of material getting stuck, or through the use of 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 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. This process can be accelerated using suitable screen media.
Passive vs Active
There are two basic categories: passive (ie rigid) screen media, such as wire mesh, perforated plates and most rubber or polyurethane (PU) panels; and active screen media, including 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.
To address this problem, Flex-Mat uses Optimum Wire, a high tensile 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 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 screen media exchange times and plant downtimes can also be reduced and production times in difficult weather conditions can be extended.
However, achieving the best possible screening performance and wear life is ultimately dependent on correct installation and tensioning of the screens, which is a matter of the design of the screen machine and its maintenance condition.
In a recent case in south-west Germany the wear life of screen media could be increased from an average of four weeks to 14 weeks just by optimising tensioning and installation details.
Individual consulting and technical support is therefore the only way to achieve 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, and recycling, Haver & Boecker and its subsidiary Major Wire have the necessary experience to achieve this.
In Australia the distributor of Haver & Boecker and Major Wire products is Nepean Rubber & Plastics.
Lars Bräunling is the screen media product manager at Haver & Boecker and Major Wire.
1. Schubert H. Preparation of solid mineral raw materials – Volume 1. VEB German Publishing House for Basic Industry, 1989, S. 208.
2. Ibid, S. 196.
3. DIN ISO 9045:1998-12. Industrial screens and screening – Vocabulary. 3.3.12.