A common problem in processing high-value bulk materials is that both a large size reduction ratio and a de-fined final grain size distribution within narrow grain size limits are often required. Dust and fines must be minimized. Moreover, the materials processed are often very abrasive, for exam-ple glasses with various compositions for the ceramics industry or glass manufacture, minerals for the refractories industry, metal-lurgical products, alloys with expensive production processes or various oxides. In many cases contamination of the product must be prevented, as, for instance, any abraded iron particles can cause problems in the PPM range, and the products must be processed in further grinding stages for high-quality ceramic, dental glass or, for example, for the semi-conductor industry. A flexible use of the grinding equipment is essential as the required throughput rates are often very low. For operations with frequent product changes, simple control and adjustment to the specific grinding requirements as well as easy cleaning are needed.
For such applications, many comminution systems can be eliminated on account of their principle of operation and size reduction mechanism. High-speed (dynamic) mills, but also mills with a high shear effect incur excessive wear costs when handling abrasive products and are therefore inefficient for this type of work. Ball mills and vibrating mills generally produce an unacceptably irregular grain size distribution.
Merz Aufbereitungstechnik GmbH recommends grinding with two roller mills, which remains one of the gentlest processing methods. The parameters roller diameter, circumferential speed and friction as well as the grinding gap setting can be adjusted in order to match the size reduction ratio to the final product. Roller mills operate on the basis of single grain comminution with compression and compression-shear stresses. The advantage here is that it is possible to influence the wear rate. Especially when handling highly abrasive products, roller mills operate with minimum abrasion rates.
With the application of a Program-Controlled Grinding and Screening plant (PMS plant) from Merz, batches of abrasive products can be ground in a multi-stage process with only one roller mill. In accordance with the predefined comminution parameters for the different products, which are previously determined in dedicated tests, the plant operates semi-automatically or automatically in a sequence of comminution/milling passes. Core component of the PMS plant is a programmable control system in combination with motor-powered grinding gap adjustment. Product feed is variable depending on the plant size. For PMS plants of the type WBP 2/.. (Fig. 1), the material is fed, for instance, directly above the first comminution stage in big-bags, for larger plants of the type WBP 3/.. (Fig. 2), the material is fed from a day bin on a discharge conveyor. The batched product is fed steadily into the roller mill with the defined gap setting and then screened. The oversize is returned in an intermediate transport unit and collected in the empty chamber of the double-tank. After the next smaller grinding gap has been set in accordance with the program, the oversize is reground. The process of oversize recirculation and regrinding with reduced gap setting each time is repeated until the two chambers of the double-container signal that that they are empty. The plant is then ready for processing the next batch. The oversize can be recirculated for regrinding either on a vibrating spiral conveyor, bucket conveyor or a pneumatic conveyor. The plant can be supplied as a turnkey standalone plant, but it can also be integrated into an existing plant.
The throughput rate and the product specifications determine the design of the key plant components and therefore the size of the roller mill with regard to roller diameter and working width. Actual engineering of the plant depends on the finished product requirements. Grinding of high-value products such as, for example, dental ceramics, decorative glass, etc, generally requires zero-contamination grinding with ceramic (Al2O3) rollers. But ceramic rollers generally only enable a limited size reduction ratio, i.e. the material must be ground in several stages.
The throughput rates in a PMS plant with a WBP 2/2-K roller mill are usually low. For higher throughput rates with the same requirements, the plant can be combined with a WBP 3/3-K mill. In both cases the roller shells should be made of wear-resistant ceramic (Al2O3). Depending on the product, the rollers can be supplied with, for instance, a surfacing of zirconium, tungsten carbide or other material. Solid rollers can also be used.
Recommended in any case, however, is preliminary testing of the processing operation in practice-oriented conditions at the Merz Aufbereitungstechnik GmbH test centre.
Merz Aufbereitungstechnik GmbH, Lauchringen (D),
Tel.: +49 7741 6826-0, www.merz-aufbereitungstechnik.de