Germany‘s highest-lying quarry in the Northern Black Forest is located around 900 m above sea level near Seebach (Fig. 1). Seebach granite is a widely known building material in the region and used largely for hydraulic engineering (e.g. for backfill of the stages at the Rhine bridges at Maxau or the oil port). The material, however, is very versatile and processed on site to a range of products including ballast, chippings, mineral blends and boulders, which can be used in track, road, forestry road construction as well as gardening and landscaping.
Seebach is one of five production sites operated by VSG Schwarzwald-Granit Werke. Another of its sites is located around 30 minutes away by car in Forbach-Raumünzach. While Seebach concentrates on the production of bulk solid products, the Forbach site extracts engineered stone (Fig. 2). At Forbach, the focus is on products such as ashlar blocks, dry masonry blocks, embankment surfacing to ornamental stone and plate-like granite, giving lots of scope for individual design. From walls to natural stone stairs, stream bedding materials to attractive boulders, everything is available here on customer request.
Seebach and Forbach differ significantly in their methods of extraction. The “gentle” extraction in Forbach centres on the use of a “slow” explosive with a “pushing” effect to further process large individual raw blocks weighing 30 to 40 t. The weight classes of the finished products then often range between 500 kg and 2 t, if, for example, gravity dams for road surfacing are ordered. In Seebach, on the other hand, a “fast” explosive is used as usually required for bulk solids (Fig. 3). Individual fragments of 1.5 to 2 t are rare after blasting and used as boulders or elements for retaining walls.
The two plants supply customers located from Frankfurt to Basel and from Alsace to Munich. In the production line for armourstone in which heavy-duty screens are used, a stronger ballast screen supplied by Metso was installed (Fig. 4) in summer, as the predecessor machine was no longer able to meet the plant management’s capacity requirements. The old plant did not deliver the necessary throughput rate because the amplitude was not sufficient and the screen media wore out too quickly.
The low amplitude was a significant factor with regard to the problem of pegged particles, which in turn led to excessive downtime and reduced throughput rates. In addition the machine was fitted with pressure-welded mesh, which was worn or torn at the latest four weeks after it had been fitted in the upper deck of the screen – caused largely by the unbalance as a result of the pegged particles.
After an inspection of a reference plant relevant to the VSG, which was fitted with a Metso screen, the company decided to buy a double-deck screen from Metso (Classic series, type 2SP). This was to be mounted on a screen buck available on site precisely below a pre-installed discharge belt. Metso was faced with the task of supplying a machine to fit into the tight installation situation and provide the required pitch (Fig. 5).
The structural steel engineering company August Müller based in Rottweil, a partner of both Metso and VSG, provided considerable support in the realization of the project (Fig. 6). The project ran smoothly. Jürgen Schnurr, Managing Director at VSG, was delighted four weeks after commissioning. “The days of interruptions are finally gone. Every 200 t or every 45 minutes, we had to stop the machine to get rid of the pegged particles. The new screen, on the other hand, has not shown any wear on the screen media.”
Indeed, since the installation of the modular Panelcord® rubber screen media from Metso, the wear that usually directly impacts wire screen media in the case of granite hardly exists at all (Fig. 7). Jürgen Schnurr is confident that he can process the total annual throughput of 250 000 to 300 000 t granite planned for this part of the plant with far fewer problems than before. The following particle groups/cut-points run in regular operation through the plant: cp 45/125, cp 90/250 and LMB 10/60 (other particle size groups or cut-points are supplied by the company on request). The plant works, for instance, with a GP300 cone crusher from Metso.
As the highest German quarry operation, VSG has to be prepared for the sudden onset of winter at any time. So that production is not affected in such situations, the screen has been put under cover.