Size reduction equipment

Overview

• Types of size reduction equipment
• Considerations when choosing size reduction equipment
• Drum mills and Bond-Index testing

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.

Considerations when choosing size reduction equipment

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:

• Properties of the material (like breaking behavior)
• Sample feed size
• Required final fineness
• Feed quantity
• Material of the grinding tools

Properties of the sample material influence the size reduction equipment

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.

How sample feed size and required final fineness lead to the best sample reduction equipment

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.

The feed quantity plays an important role for the best size reduction equipment

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.

Which quantity is required to ensure representativeness of the original sample?

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).

Which grinding tools should be chosen for optimized sample reduction equipment?

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:

• Metal (steel, cast iron, titanium)
• Ceramics (tungsten carbide, zirconium oxide, sintered aluminium oxide, hard porcelain)
• Natural stone (agate)
• Plastics (PTFE, PP, PC)

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.