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Low-dust milling

Precise and efficient grinding processes with roller mills
Low-dust milling

Narrow particle size distribution, a low fines content, minimum product heating, high specific throughputs and at the same time low energy consumption are requirements that roller mills meet to a large extent. One example of the use of this mill type is low-dust crushing of coarse-grained citric acid anhydrous.

Gernot Ingenerf

In the coffee and grain industry, roller mills have been standard for decades. In the coffee grinding sector, for instance, a high yield in a defined size range with a minimum of fines is extremely important. Depending on the demands of the final consumer, the beans are ground by roller mills with corrugated or smooth rollers to approximately 1170 µm for instant coffee, 590 µm for filter coffee, 300 µm for espresso or smaller than 100 µm for Turkish coffee. Without further classification, the individually ground coffee proceeds directly to the packaging line. The ability to dispense with a downstream screening or sifting system, or at least the considerably lower costs for this process step, distinguishes the roller mill from many other mill types, where cost-intensive recirculation or disposal of undesired fines often is essential. In addition, Atex requirements are much less strict for slow-running roller mills than for faster, traditional mill types. Depending on requirements, roller mills are able to grind very hard products – even plastic – extremely homogeneously and with a minimum of fines to approximately 100 µm. Their main characteristic is a significantly narrower particle size distribution, particularly compared to impact grinding.
Mill design
Roller mills have a modular design. They can be composed of between one and four grinding stages connected in series, each consisting of two counter-rotating rollers operating at different speeds. One roller always has a fixed position, while the position of the other roller is automated and can be set in an exactly reproducible way. The selected grinding gap depends on the required grinding fineness. The first roller pair is fed directly from above by a continuously adjustable product feeding device, the precision and efficiency of which is of vital importance for the grinding result. The product then passes via other grinding stages until the specified final fineness is reached. The process parameters can be determined with a laboratory roller mill. The roller diameters are identical, which means upscaling does not cause any problems. In order to achieve a narrow particle size distribution with a minimum of fines, the roller surface, the absolute and relative speed of the rollers and the roller gap are particularly decisive factors.
Corrugated or smooth
The hardened and optionally coated rollers can have a corrugated or smooth design. A multitude of different corrugations are available to comply with the different product hardnesses and final fineness requirements. The rollers can be corrugated longitudinally or transversely, whereby the configuration is influenced by the number of flutes, the pitch, the land, the radius, the flute depth and the cutting edge and back angles. Tests are usually carried out with the original product on the laboratory roller mill to determine the optimal configuration for the application concerned. If both rollers rotate with the same speed, only compressive forces act on the product in the roller gap. In order to produce additional shearing forces, the rollers are operated with different speeds. This speed ratio is referred to as ”lead“. High leads cause high shearing forces and are required, for example, for grinding products with a low friability. The lead can therefore have an enormous influence on the particle size distribution. Very hard products are ground between smooth rollers with a hardened surface and a low lead. Corrugated rollers are recommended for soft to medium-hard products in order to reduce the wear.
The rollers are normally fed with particles which are larger than the grinding gap. The grinding process can be single or multi-grain and the gap width determines the grinding degree. As a result, the adjustment of the grinding gap plays a decisive role. Today, this gap can be adjusted automatically – and thus exactly reproducibly – on an operator panel using the very latest measurement techniques without interrupting the production process. The setting is accurate to less than 0.01 mm. In addition, exact parallelism of the rollers is necessary for homogeneous and reproducible grinding, so that the grinding gap is constant over the whole roller length. A ruggedly and precisely designed machine is an essential quality characteristic of all Neuhaus Neotec grinders. Roller mills have a much lower energy consumption than impact mill types such as hammer mills, where the hammers hit the particles repeatedly at very high speeds of up to 70 m/s. They also have much lower circumferential speeds (often limited to 10 m/s by Atex) and each particle is subjected to the compression or shear stress only once. Since – in contrast with impact mills – an aspiration system with dust removal is also not required, roller mills consume up to 40 % less power.
Practical example
One example of the use of roller mills is low-dust crushing of coarse-grained citric acid anhydrous. This salt is often used as a preservative and acidulant in the food industry as well as for pH adjustment in cosmetic products. The aim, namely crushing the up to 8 mm large input product with a high yield in the range from 200 to 600 µm, and in particular reducing the fraction <200 µm to a minimum, is fulfilled reproducibly providing the corrugation, the relative speed of the rollers and the grinding gap are carefully selected. Neuhaus Neotec recently delivered two WMC 171 roller mills designed as single-stage grinders for these applications, with a roller length of 1700 mm and roller proportioning systems with integrated metal separation. The grinders can process up to 5 t/h at a guaranteed maximum product temperature of <35 °C. Stainless steel 1.4571 was chosen as the material for the product contacting parts. The machine is equipped with bearing temperature measurement and vibration monitoring functions.
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