Screening system for flexible adaption
In addition to the trend for ever higher capacities, there is a growing demand in the non-metallic minerals for screening products to well below 100 µm, so as to eliminate the need for high-investment air separators wherever possible. For fine and ultrafine sizing, for example of gypsum, silica sand, limestone and other mineral resources, the operating principle of the round tumbler screens, with up to 2 900 mm diameter, from Allgaier Process Technology boasts particular advantages with regard to throughput and screening efficiency. Latest engineering advances are drive synchronization for several tumbler screens in parallel operation and the reduction of residual forces thanks to a special electronic balancing process. Both these innovations compensate almost completely for screen vibration, thereby improving safety for screen operation in high steel structures and buildings.
1 High sizing efficiency
The operating principle of tumbler screens per se is optimally suited to sizing within narrow particle size ranges. The material is fed to the middle of the screen inserts. At the centre of the screen insert, the tumbling screen is like a flat screen, i.e. the feed material is not thrown, but has sufficient time to stratify according to particle size. Sensitive and high-value materials are protected by the extremely gentle handling. The fine material migrates downwards through the coarse material towards the screen and through the mesh. At the same time the screen feed is transported in a radial direction towards the outside edges of the inserts, i.e. into the zones of higher vertical acceleration. The increasing available area causes the bed of screen feed to be pulled apart so that the near-size particles also have the chance to reach the screening surface. As a result, screening efficiency is improv-ed by 5–15 % compared to other screening systems. Combined screen deck structures enable double screening, maximizing screening capacity and efficiency. Especially for high-value bulk solids, the additional cost of this equipment is soon repaid by the reduction in misplaced particles.
2 Limestone processing: Nine sizes in just two stages
A major quarrying company in Southern Germany is using the tumbler screens for the production of eight different grades (Fig. 1). The feed material consists of a broken stone in the size ranges 0–56 or 0–80 mm. The broken stone is dried and ground in a hammer mill and then sized in two sets of two Allgaier tumbler screens. In the first stage, two parallel operating tumbler screens, each with five screen decks, separate four useful fractions into the size ranges 4–8 mm, 2.8–4 mm, 2.0–2.8 mm and 1.5–2.0 mm, while the oversize > 8 mm is sent back to the mill. The residual fraction < 1.5 mm is sent direct to the second stage, below – also two parallel-operating Allgaier 4-deck tumbler screens. These perform fine sizing into the size ranges 1.0–1.5 mm, 0.5–1.0 mm, 0.1–0.5 mm and limestone powder in the size 0–0.1 mm.
3 Fast changeovers
More and more frequently, screens are used to separate or size a range of different products. To shorten times for screen changeovers or cleaning, Allgaier tumbler screens can be fitted with quick-action clamps that guarantee the secure fit of up to four screen decks mounted on top of each other. For smaller screen diameters, six product sizes can be produced in one step on screens with up to five screen decks. Despite the high clamping force necessary as a result, locking and unlocking does not require any great force. A spring-loaded package is used to compensate for the natural setting of the seals.
4 Drive synchronization and fine balancing
If several tumbler screening machines are operated parallel on the upper storeys of a building or in metal structures, residual dynamic forces can be generated and result in impermissible vibrations. One solution is electronic drive synchronization or special fine balancing: if necessary, the two can be combined. Residual forces that could not be previously suppressed can be reduced by up to 80 % with these solutions. These innovations can also be retrofitted, so that older installed machines can also benefit.
5 Segmented screen inserts
For screen diameters between 2000 and 2900 mm, the operators of Allgaier tumbler screens can choose to use segmented screen inserts (Fig. 2). Screens with a diameter of, for example, 2600 mm can be divided into eight outer segments and a central inner segment. Especially for screens working in tough operating conditions and subjected to high stresses, a segmented screen boasts crucial advantages. If a screen is torn or damaged, then only the damaged segment rather than the entire screening surface has to be replaced. This simplification shortens screen downtimes and reduces costs for replacement parts.