Processing technology made in Germany
A common subdivision in processing technology is the division into pre and post crushing or into coarse, fine and very fine crushing. Process-related and product-specific requirements normally determine this optimum mode of action of crushing equipment. Crushing systems are used as beating, pressing, shearing, cutting or as combined systems depending on the requirements of the process or of the products.
In the same way the selection of possible material combinations for the crushing/grinding tools and the parts of the structure that come into contact with the product are selected specifically for the product in question. For example, this means that contamination-free processing is usually associated with the use of ceramic tools and housings with internal coatings. The use of structural parts or rollers made from tungsten carbide is common practice in metallurgical products (silicon, metallurgical silicon semiconductor technology). H2-forming products such as CaSi alloys are prepared under protective gas (inert gas); the overpressure atmosphere leads to the use of gas-tight constructions of up to 400 mm water column. Salts are particular moisture-dependent in their reaction with the environment and the product usually demands the use of high quality VA steels. Wet crushing using a mounted spray system necessitates corrosion protection and therefore use of high quality roller materials. The housing is cooled and sometimes the rollers are even provided with internal cooling to protect against high temperatures of > 80°C to approx. 800°C brought about by the product or the environment. Trace heating and additional insulation is installed to prevent the formation of condensation on walls. A uniform distribution across the working width of the rollers can be of importance depending on the necessity for coarse or fine crushing. Ripple or roller spreaders can be used in grain compositions from approximately 5.0 mm to a maximum of 10 mm. Chutes or conveyors can be used for coarser products.
Single-roller crushersusually assume the task of crushing predominantly coarse-lumpy, dry, medium-hard to hard products. In this process the fixed roller works against a sprung plate arranged at an angle. The size and design (profile) of the crushing plate and the roller is matched to the requirements and practical experimentation. The roller diameter can be adjusted between 350 mm and 800 mm depending on the product sizes. The rollers can be prepared as full rollers usually with welded crushing lobes or in-cast segments. The nip can be adjusted using the segmented crushing plates. As customary in the crushing process the throughput capacity and the maximum particle size determine the working width of the rollers. The different profiles are mostly developed through practical experimentation to suit the desired particle composition and requirements of the end product. The crushing performance can be significantly influenced through the interaction of the profiled crushing roller with the profiled crushing plate.
Twin-roller crushers are used either for coarse, preliminary crushing or crushing with a constant crushing nip. For a final grain size range of approximately 1.0 mm - 5.0 mm the smooth rollers are provided with either crushing lobes or flutes of different shapes and sizes. When using the system for preliminary crushing the rollers are usually designed with different interlocked lobe configurations. This significantly increases the crushing ratio. Similarly to the single-roller crusher the rollers can be of the full roller design or roller sections (body as a polygon). Roller crushers are generally equipped with an adjustable mechanical cushioning system that enables the rolls to back-off temporarily when encountering foreign substances or elastic products. The rollers immediately return to the set condition. The restoring forces are adjustable. In this way the cushioning system protects the expensive rollers from damage or even breakage due to overloading caused by foreign objects of any kind or overfeeds. The throughput capacity, maximum particle size and particle composition generally determines the working width and design of systems.
The drive for both rollers is a toothed belt drive. If product-related adjustments to the peripheral speeds and frictions are necessary in production this is easily achieved at any time by exchanging the motor pulleys. The dynamic loads that occur when crushing very hard products can be significantly reduced by the frictional mode of operation. Vibration-damping metal rails are also fitted between the transfer frames and substructures. The uniform distribution of the product is achieved by an upstream conveyor trough. This is designed to match the working width of the roller crusher. Working widths up to 1600 mm are possible depending on the design size and diameter.
Twin-roller mills are used for fine crushing of brittle, medium-hard to hard products with a feed material of uniform grain size range. The pressure crushing forces usually combined with frictional forces (different roll speeds) enable the product to be produced to specific crushing properties within narrow grain boundaries. The crushing results are mainly defined by variable parameters such as:
- Mean circumferential speed / throughput
- Friction / speed difference
- Nip adjustment / crushing ratio
Fixed factors, such as the diameter of the rollers (up to 1000 mm), and possible surface profiles are determined by practical experimental procedures and set in comparison. The throughput volume, the composition of the final product and the design layout determine the diameter of the rolling mill in the range of 200 mm - 1000 mm.
The average crushing ratio when grinding with roller mills is approximately 1:4 to 1:8, (reference D-350mm, D-500mm) and, taking the roller diameters into account (up to 1000 mm) selected for the design/process technology, may be increased to approximately 1:12 - 1:15. Depending on the throughput capacity a lower limit final grain size range of > 0.3 to approximately 1.0 mm can be achieved. Hard, brittle products such as minerals, refractory clay, quartz, carbon, glass, silicate products, etc., are crushed at constant velocity or the smallest possible friction to avoid dust and increase the service life of the rollers. However, depending on the specific grinding properties, the smaller the selected crushing nip the greater the friction must be to avoid briquetting (pressings). This means that in the production of table salts, preparation of fluidized bed granules, fertilizers, ureas, chemical products without noticeable wear properties, crushing must be performed at adapted frictions.
The requirements define the materials to be used. Wall thicknesses up to 60 mm enable repeated reworking of rollers in the case of hollow barrel (erosion) for roller widths of > 400 mm or for surface damage (e.g. scoring) by foreign substances. Not infrequently the service life period between reworking is up to 1-2 years depending on the product. The often higher procurement cost compared to high-speed mill systems and the significant benefits of lower maintenance costs resulting in high availability, as well as the requirements of customized targeted product grinding within tight grain boundaries therefore justifies the use of a rolling mill.
Rolling mills are available with working widths up to 1.6 m. Parallelism should therefore be ensured – reworking on stationary working machines is a concept of these designs. Maintenance, for example the installation and removal of rollers, can be simplified by designing in mobility and providing a manual lifting-lowering device.
Various modifications also allow adaptation of the process, production and maintenance. Wipers made from special materials are used to avoid build-up on the roller caused by moisture/dampness in the product. These are easily adjustable and access for reworking is facilitated by overhead installation. The service life can be extended by 15 - 20% by adding the ability to run in reverse when fine grinding hard, very wearing substances such as silicon alloys. The adjustment of the grinding nip to an accuracy of 0.1 mm is manual using a hand wheel. This is also available as a digital, motor driven adjustment which allows the release of the upstream processes.
Ceramic or glass products can usually only be processed using ceramic roller shells (Al2O3 or ZrO) with a low, limited crushing ratio. As a rule this is not higher than approx. 1:1.5. The intelligent control in combination with the digital, motorized nip adjustment mentioned above enables the multi-stage grinding of single batches to the desired final grain size via a grinding screen cycle using a roller mill. The PMS system principle (Pprogram-controlled milling screen system Mahl-S)) enables rapid customization through program changes for frequent product changes.