Size reduction equipment refers to machinery designed to break solid materials into smaller pieces or finer particles. This process, known as grinding, is a crucial operation in many industries, including mining, food processing, pharmaceuticals, recycling, and chemical manufacturing. The primary purpose of size reduction is to increase the surface area of the material, improve efficiency in subsequent processing steps like extraction, enhance product quality by homogenization, facilitate storage and transportation, and enable the separation of valuable components from waste.
Size reduction equipment varies widely in design and functionality, tailored to specific materials and reduction requirements. The main types include:
These are heavy-duty machines used for breaking large chunks of materials into smaller particles. They are often the first step in the size reduction process. Types of crushers include jaw crushers, gyratory crushers, and cone crushers. Retsch offers a whole series of Jaw Crushers.
Grinders reduce materials to a finer size than crushers. They are used for intermediate and fine grinding. Types include ball mills, rod mills, cross beater and hammer mills.
Retsch offers a wide range of grinding mills designed to accommodate a variety of applications.
Pulverizers reduce materials to a very fine particle size, which is essential in the pharmaceutical and food processing industries for creating powders. They include disc mills and vibratory disc mills.
Discover RETSCH Laboratory Pulverizers for material analysis, quality control and research.
This category includes devices that use mechanical movement to cut soft to medium hard solid materials, often used in the food industry.
The choice of size reduction equipment depends on the material properties (hardness, abrasiveness, moisture content), desired particle size, and specific application requirements. Adequate selection and operation of size reduction machinery is critical for achieving efficient and cost-effective production processes.
Important considerations when choosing the optimized size reduction equipment for special tasks include:
Laboratory mills operate on various principles of size reduction, depending on the sample material's characteristics. The choice of mill for a specific size reduction task depends on the sample's breaking properties. Materials that are hard and brittle are optimally pulverized through impact, pressure, and friction. In contrast, substances that are soft and elastic necessitate cutting and shearing effects for effective comminution. Fibrous samples also require cutting, along with friction and shearing. It is common for mills to comprise multiple size reduction principles, such as impact and friction in planetary ball mills, or shearing and impact in rotor mills. Processes like drying or embrittling serve as methods to modify the breaking properties of samples, rendering them more brittle and thus, more amenable to pulverization.
Retsch offers a wide range of grinding mills designed to accommodate a variety of applications.
For size reduction of large particles over 40 mm, the equipment is classified as crushers or shredders, while mills are used for smaller particle sizes. The initial feed size and desired final fineness dictate whether pre-crushing is necessary before fine grinding. Typically, two different mills/crushers are needed to achieve very fine particles, as mills suitable for fine grinding to the nanoscale generally only accept particle sizes up to 10 mm. The required final fineness is determined by the analytical task – ensuring good homogeneity is crucial. Particles must be sufficiently small for efficient and quick microwave digestion steps. For XRF analytics, particles smaller than 100 µm are needed, while for other analytical techniques, usually 0.5 mm is acceptable if homogeneity is maintained. For R&D tasks, particle sizes down to the nanometer scale (< 100 nm) are necessary, achievable only with ball mills and wet grinding processes.
To ensure reproducibility in sample preparation, it is essential to extract a sub-sample that truly represents the bulk material. This means the sub-sample must share the same properties as the bulk. Given that most samples are inhomogeneous mixtures, care must be taken to avoid segregation due to varying particle sizes, especially during transport. If the entire sample isn't processed, a representative portion must be taken. The quantity of the sample is crucial; it must be sufficient for analysis and proportionate to the total sample size and grain size. These factors dictate the minimum quantity needed for the sub-sample to accurately reflect the bulk.
Some industry-specific standards provide guidelines and directions on the correct sampling process, for example DIN 51701-2:1985 in the coal industry. It contains, for example, the formula
G [kg] = 0.07 [kg/mm] x z [mm]
which indicates how much sample “G” must be extracted from a bulk sample with maximum particle size “z” to obtain a representative quantity. Taking a coal sample with a maximum particle size of 5 cm as an example, the following calculation applies:
G [kg] = 0.07 kg/mm x 50 mm
G = 3.5 kg
Consequently, the size reduction equipment chosen should be capable of taking 3.5 kg of the sample (and the initial feed size).
Sample preparation refers to the series of actions taken to transform a sample from its original state to a form that is suitable for analysis in a laboratory setting.
Each RETSCH mill comes with grinding tools that are optimized with regards to their functionality and handling. However, due to the wide range of applications, the requirements may differ greatly. Therefore, RETSCH offers a great variety of accessories. For ball and disc mills, for example, the grinding sets are available in various sizes. By using distance sieves and rotors it is possible to process temperature-sensitive materials in rotor mills. All grinding tools are available in different materials.
The materials used for RETSCH grinding tools can be grouped as follows:
The suitability of a material for a specific type of mill is determined by its chemical and physical properties. All mills can utilize grinding tools made of steel. As a general guideline, the grinding tool material should be harder than the sample to minimize wear. Additionally, good wear resistance is important for abrasive samples. Cryogenic grinding typically requires steel tools, except for the CryoMill and MM 500 control, which also offer equipment made of zirconium or tungsten carbide for this purpose. Wet grinding necessitates very small grinding balls and materials with good wear resistance, commonly using zirconium oxide. Tungsten carbide, known for its wear resistance and hardness, is used for grinding very hard samples.
In the process of mechanical size reduction, abrasion is inevitable. Therefore, when selecting a material for milling, it's important to consider whether potential contamination from abrasion could negatively impact the product or subsequent analysis, such as the influence of chrome or nickel abrasion on heavy metal analysis. Materials that are neutral-to-analysis are preferred. Another key aspect of ball mills and vibratory disc mills is the energy input from different materials. For instance, grinding balls made of tungsten carbide generate a significantly higher energy input, leading to a more effective size reduction, due to their higher density compared to balls of the same size made from other materials.
The drum mill is a type of ball mill suitable for the pulverization of large feed sizes and large sample volumes up to 35 l. The grinding process is performed either in dry or wet conditions. The achievable final fineness depends on the grinding media used.
Another application area of the TM 300 is Bond Index Testing. The Bond Work Index is used to assess the grinding efficiency and to calculate the necessary grinding power when choosing size reduction equipment in the design phase of, for example, a mining plant. Precise determination of BWI is crucial for the accurate design and estimation of costs linked to the comminution process in industries like cement, mining or steel.
Both the Ball Mill and the Rod Mill module can be used for the process. At least 15 to 20 kg sample material is required to simulate a closed grinding circuit.
The Ball Mill Work Index (BWI) is used for the range from 2.1 mm down to 100 µm. The sample needs to be pre-crushed to particle sizes as defined below.
The optimum number of grinding balls is 285. With the ball diameters varying due to wear, the overall number should be adjusted from time to time to ensure a total mass of 20.125 grams.
The grinding jar of the Bond Index Ball Mill measures 12'' x 12'' and has well-rounded corners.
The Rod Mill Work Index (RWI) is used for particle size determination in a size range from 25 mm down to 2.1 mm. The sample needs to be pre-crushed to particle sizes as defined below.
The grinding jar for the Bond Index Rod Mill is 12″ x 24″ in size and has a wave-shaped design.
Přístroje RETSCH jsou používány ve vědě a průmyslu pro přípravu vzorků pro širokou škálu různých analytických metod, taktéž pro měření velikosti částic při výrobním procesu a kontrole kvality. Součástí výkonného výzkumu a vývoje je vždy snaha o nalezení řešení vhodných pro zákazníky i pro příslušný segment trhu a systematická implementace nových řešení do výrobků a služeb založena na citátu Aristotela:
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