Crushing principles

The mill used determines not only the comminution principle but also the achievable particle shape, final fineness, and throughput. Poor mill selection can lead to excessive wear, heat generation, or insufficient homogenization. Vibratory mills combine impact and frictional forces and are suitable for small samples as well as for dry, wet, or cryogenic milling. In a cryogenic mill, the material is continuously cooled with liquid nitrogen, which makes it brittle and preserves volatile components. Cutting mills are ideal for fibrous or elastic materials and deliver defined particle sizes, while jaw crushers or centrifugal crushers are suitable for the pre-crushing of large, hard lumps. A well-considered combination of several mill types ensures that the material is processed efficiently and gently, and that subsequent analysis or production steps deliver reliable results.

Compressive stress is one of the oldest crushing principles. The material is compressed between two fixed or moving surfaces until it overcomes its internal strength and breaks. Typical machines such as jaw crushers or roller crushers operate on this principle: The samples are fed into a narrow gap and crushed by mechanical pressure. This method is particularly effective for hard and brittle samples such as ores or rocks, which break relatively spontaneously under pressure. In primary crushers such as gyratory crushers, an eccentrically mounted crushing cone ensures uniform stress and high throughput. The compressive principle is less suitable for tough-elastic materials, as they tend to be deformed rather than crushed. When using this method, it is important to ensure that the material is fed in uniformly to avoid bridging and uneven particle size distribution.

In impact milling, the sample is brought into contact with a solid surface at high speed. The accelerated particles strike impact surfaces or grinding tools and shatter due to the resulting impact forces. Ball mills, hammer mills, and jet mills utilize this principle by generating numerous impacts through rapid rotations or airflows. It is particularly suitable for hard, brittle, and crystalline materials, which break down into finer particles upon impact. In vibratory mills, this principle is combined with friction to efficiently homogenize small samples; they are even suitable for dry, wet, and cryogenic milling. The final fineness depends on the impact velocity, the geometry of the grinding tools, and the milling process. Impact milling can generate heat; therefore, adequate cooling is advisable for temperature-sensitive samples or materials containing volatile components.

Frictional grinding relies on the movement of the grinding tool's surface relative to the sample, generating frictional forces between the two. The solid particles are essentially abraded; compressive and shear forces act simultaneously. Disc mills and grinding plates utilize this sliding friction to grind or homogenize soft to medium-hard materials. Heat generation is typically higher than with compressive or cutting grinding because energy is continuously converted into heat during the sliding motion. Therefore, samples with low melting points or heat-sensitive components should either be ground slowly or pre-cooled. Frictional grinding is well-suited when a uniform particle size distribution and a very fine final particle size are required, for example, in the production of powders for analytical determinations. In many mills, friction is used in conjunction with impact or shear forces to achieve a more effective comminution result.

Shearing occurs when two surfaces are displaced relative to each other, and the material between them is cut or ground by a shearing motion. This principle is particularly suitable for fibrous, tough, and elastic materials such as plastics, vegetables, wood, or paper, which are difficult to grind using only compressive forces. Rotor impact mills and cross-impact mills have counter-rotating tools that shear the sample; the resulting particle size can be defined by sieves and cutting speed. A key advantage is the low heat generation, which protects heat-sensitive samples. Shearing produces relatively clean cut edges and a narrow particle size distribution. Pre-grinding may be beneficial for larger samples or fibrous materials. For extremely elastic products, a combination of shearing and cutting is often used.

In the cutting principle, sharp cutting edges separate the sample material by shearing or chopping. Cutting mills, shredders, and rotary cutters have blades or knives that cut the sample into defined particles through a rotating motion. This method is suitable for soft, elastic, fibrous, and tough materials, such as plants, textiles, plastics, or films. The sharp cut generates only minimal friction and therefore little heat, preventing discoloration or thermal alteration of the sample. Modern cutting mills, such as the SM series, allow for variable cutting speeds and screen inserts, enabling the reproducible setting of desired particle sizes. In contrast to pure compression or impact processes, the particle shape here often remains elongated or scale-like. Cutting is unsuitable for very hard and brittle materials; compression or impact-based mills are recommended for these materials.