Retsch Grinding Mills

The physics behind grinding mills

The grinding of solid materials occurs through mechanical forces that break up the structure by overcoming the interior bonding forces. After the grinding, the state of the solid is changed: the grain size, the grain size disposition, and the grain shape.

There are essentially five different grinding mechanisms, and Retsch offers seven different types of mills that can be utilized. The selection of a suitable mill depends on the application:

Grinding with pressure

Grinding with pressure operates on the principle of applying mechanical force or pressure to break down materials. This method involves pressing the material against a hard surface or between two surfaces, one or both of which are moving. The pressure causes the material particles to fracture and break apart, leading to size reduction. The amount of pressure applied and the duration of its application influence the fineness of the grind.

This technique is commonly used in jaw crushers and in mortar grinders. The effectiveness of this method depends on the hardness of the material being ground, the grinding surfaces, and the specific application of force.

Grinding with impact

Grinding with impact involves the rapid application of force to break down materials into smaller particles. This method employs tools or equipment that deliver sharp, quick impacts to the material, causing it to fracture along its weakest points. The impact force can be delivered by moving hammers, balls, or discs within a grinding machine. As the material is subjected to these high-energy impacts, it breaks apart into fine particles.

Impact grinding is particularly effective for hard and brittle materials that easily brakes under force. The final particle size can be controlled by adjusting the speed of the impacting object and the design of the mill. Equipment such as hammer mills and ball mills utilize this principle for rapid and efficient size reduction.

Grinding with friction

Grinding with friction involves the use of mechanical forces to generate wear through direct contact between materials or between a material and a surface. This method relies on the relative movement of surfaces in contact with each other to produce fine particles. As the surfaces rub against each other, the generated mechanical wear break down the material into smaller pieces.

Friction grinding is especially suitable for softer materials or those that can be made more brittle by cooling (as in cryogenic grinding). The efficiency and effectiveness of friction grinding depend on the hardness of the materials, the texture and material of the grinding surfaces, and the speed and intensity of their movement. During frictional grinding, heat development should be considered because a significant part of the energy is lost as heat. Equipment such as ball mills and disc mills use this grinding principle, where the action of the surfaces moving against each other results in size reduction through frictional forces.

Grinding with shear

Grinding with shear involves the application of cutting forces that slice through materials to reduce their size. This method uses sharp edges or surfaces to apply opposing forces that effectively cut or shear the material into smaller pieces. Unlike impact or pressure, shear grinding relies on the relative movement of the cutting surfaces past each other at different speeds or in different directions, creating a shearing effect.

Shear forces are particularly effective for fibrous materials, plastics, and other substances that can stretch or deform. These materials are efficiently reduced in size when subjected to the cutting action of shear forces, which can cleanly slice through them rather than crushing or shattering.

Equipment such as rotor mills and knife mills utilize the principle of shear for grinding. The design of these machines often includes sharp blades or rotors that rotate at high speeds, applying shear forces to the material being processed. The size and shape of the resulting particles can be controlled by adjusting the size of the sieve through which the material passes after being sheared, as well as by modifying the speed and configuration of the cutting mechanism.

Grinding with cutting

Grinding with cutting involves the reduction of material size by using sharp blades or knives to slice through the material. This method employs a cutting action rather than pressure, impact, or friction to achieve size reduction. The material is fed into a machine where it encounters rotating or stationary cutting elements that slice it into smaller pieces. The size and shape of the resulting particles are determined by the nature of the cutting blades and the configuration of the cutting chamber.

Cutting grinding is particularly effective for materials that are too tough for impact or pressure grinding but can be efficiently sliced, such as certain plastics, fibrous materials, and foods. The precision of the cut can be adjusted by changing the speed of the cutting blades, their sharpness, and the spacing between them. Equipment like knife mills and shredders typically operate on this principle, offering controlled size reduction with minimal heat generation, preserving the integrity of the material being processed.

Selecting the right mill for a specific application

For sample preparation, Retsch's innovative and reliable grinding solutions can accommodate a broad spectrum of materials and applications. When selecting a grinding mill, it's essential to define the intended use, the type of material to be processed, and the desired final particle size to ensure optimal results. To choose the right product not only knowledge of the instruments is required but also profound experience in the preparation of different materials. RETSCH offers you different ways to experience our grinding mills in action. So that you can be sure to find the optimal process for your requirements.

To find the best suited grinding mill for a specific application, the following aspects should be considered:

• Quality/characteristics of sample (e.g. dry, tough, abrasive, fibrous, brittle, hard, soft, temperature-sensitive etc.)
• Feed size
• Required final fineness
• Sample volume
• Sample throughput
• Subsequent analysis (which type of contamination by abrasion of grinding tools is acceptable?)
• May the sample be dried or embrittled before grinding?
• Not to forget: Is the laboratory sample representative?

Depending on the quality of the material different size reduction principles are applied to obtain the required fineness. Large particles cannot always be ground to analytical fineness in one step. In some cases it is possible to carry out coarse and fine grinding in the same grinding mill with different settings; in other cases two grinders or crushers are required.