Grinding & Micronisation

Mill operating principle

The ball mill operates on a simple yet effective principle: a horizontal rotating tank, partially filled with the grinding components and the material to be ground.

As the vessel rotates, the balls are driven upwards by the inner wall, rise under the effect of rotation, then fall back due to gravity.

This constant movement generates two fundamental grinding mechanisms:

• Impact, caused by the falling balls, which fractures the particles;
• Attrition, resulting from repeated friction between the balls and the material, which progressively refines the particle size.

The choice of grinding balls (steel, ceramic, tungsten carbide, etc.) and the vessel lining allow the mill to be adapted to all types of materials, even the hardest or most abrasive.

Parameters influencing grinding fineness

The fineness of the ground product depends on several parameters:

• Ball size;
• Grinding duration;
• The ratio between the grinding load and the quantity of material to be processed.

Advantages of ball milling

• Robust technology;
• Compatible with a wide range of materials, regardless of their nature or initial particle size;
• High grinding efficiency, with the possibility of achieving very fine particle sizes;
• Excellent granulometric flexibility, from coarse to ultra-fine grinding;
• Good homogeneity of the ground product.

Mill operating principle

The knife mill is based on the principle of mechanical cutting.

It consists of a rotor equipped with sharp blades, which rotate against fixed counter-blades mounted on a stator.

During operation, the material to be ground is drawn into the grinding chamber, where it is trapped between the cutting elements. It is then fragmented until it reaches a size compatible with the outlet screen. This calibration screen retains oversized particles, thus regulating the particle size of the final product.

Parameters influencing grinding fineness

The fineness of the ground product depends on several parameters:

• Rotor rotation speed;
• Outlet screen mesh size;
• Mill feed rate;
• Blade and counter-blade design, in particular the air gap (distance between cutting elements).

Advantages of knife milling

• Modular technology, with the possibility of integrating different types of rotor;
• Compatibility with complex materials such as soft plastics, textiles, sheets, or fibrous compounds;
• Compatibility with materials of varying initial particle sizes;
• Wide range of fineness obtained.

Mill operating principle

The impact mill functions by creating high-speed collisions between the material to be ground and a dynamic grinding medium.

The material is fed into a chamber where a rotor fitted with grinding tools (pins, hammers, blades, etc.) operates at high speed. The material is fragmented under the impact of these tools.

In some configurations, static elements (stators) are added to complete the grinding action.

Some models, such as hammer mills, also incorporate a calibration screen at the chamber outlet. This screen retains oversized particles and allows the particle size of the final product to be adjusted as required.

Parameters influencing grinding fineness

The fineness of the ground product depends on several parameters:

• Type of rotor used;
• Rotation speed;
• Screen mesh size (where applicable);
• Material feed rate.

Advantages of impact milling

• High grinding efficiency, particularly for soft to moderately hard materials;
• Adaptable technology, with interchangeable grinding tools on some models.

Mill operating principle

The disc mill operates by shearing the material between two discs rotating against each other.

The material to be ground is fed between these discs, at least one of which is rotating.

Under the effect of relative movement between the discs, particle size is progressively reduced.

Parameters influencing grinding fineness

In addition to the nature of the material to be ground, the fineness obtained depends on:

• Disc rotation speed;
• Distance between the discs;
• Material feed rate

Operating principle of opposed air jet milling

Opposed air jet milling is a particle size reduction process based on direct impact between particles set in motion at high speed.

The powder is fed into the grinding chamber, which contains several nozzles oriented in opposing directions.

A flow of compressed air is injected at high speed through these nozzles, setting the particles in motion.

Under the effect of turbulence and high velocity, the particles collide violently in the centre of the grinding chamber.

This particle-against-particle impact phenomenon achieves very fine size reduction, without the use of mechanical tools.

An integrated classifier then separates particles according to their size, retaining oversized particles in the grinding chamber and thus ensuring that the required fineness of the final product is obtained.

Parameters influencing grinding fineness

Besides the properties of the material to be processed, the fineness of particles obtained depends mainly on:

• Compressed air pressure, which determines the impact velocity between particles;
• Classification turbine rotation speed, which selects particles according to their size.

Advantages of opposed air jet milling

• Maximised powder fineness, capable of achieving sizes of less than 5 microns;
• Autogenous milling(operating without grinding media) minimising the risk of metallic contamination;
• Heat-free process suitable for heat-sensitive products;
• Clean and controlled technology, perfectly suited to purity and granulometric precision requirements.

Operating principle of spiral air jet micronisation

Spiral air jet micronisation is based on the combination of two phenomena:

• Impacts between the particles themselves;
• Collisions between particles and the internal walls of the mill.

The powder to be processed is fed into a cylindrical grinding chamber equipped with several nozzles, but without any moving mechanical parts.

Compressed air is injected tangentially into this chamber through the nozzles, generating a spiral air flow within which particles undergo repeated collisions, both with each other and against the walls of the mill.

These impacts cause progressive fragmentation of the particles, reducing their size without the use of mechanical tools.

The finest particles are then drawn towards the centre of the chamber and discharged, while a natural classification effect retains coarser particles in the peripheral zone of the vortex.

Parameters influencing grinding fineness

In addition to the intrinsic properties of the material, the fineness achieved depends mainly on:

• Compressed air pressure;
• Powder feed rate.

Advantages of spiral air jet micronisation

• Process suitable for sensitive powders (heat-sensitive, fragile, or reactive products, as there is no significant temperature increase);
• Fine particle size;
• Design limits retention zones and thus maximises yields.

Mill operating principle

The vibratory ball mill operates on the principle of grinding by impact and friction, combined with cryogenic cooling.

The material to be ground is placed in a grinding bowl containing balls.

This bowl is subjected to rapid and powerful vibrations along a linear axis, causing repeated collisions between the balls and the particles.

Grinding is carried out at very low temperatures, via controlled injection of liquid nitrogen into the double jacket surrounding the bowl.

The vibratory mill is mainly used to generate samples (a few grams to a few dozen grams) of fine, cryogenically ground powders.

Advantages of cryogenic vibratory milling

• Thermal stability: low temperatures prevent the product from heating and preserve physicochemical properties;
• Effective grinding of difficult materials such as soft, elastic substances (rubber, plastics, polymers, etc.) or those rich in fat;
• Fine particle reduction;
• Enhanced safety: closed system limiting risks associated with liquid nitrogen and fine dust.

Mill operating principle

The cryogenic impact mill operates on the same principle as the impact mill at room temperature.

Advantages of cryogenic impact milling

It represents a highly attractive alternative:

• For deformable materials with elastic or ductile behaviour, such as plastics and polymers. Cryogenic grinding maintains the product below its glass transition temperature, rendering it more brittle and facilitating particle size reduction.
• For sensitive products (oxidisable or heat-sensitive), grinding at negative temperatures preserves product quality by preventing degradation.

Mill operating principle

The impact mill under inert gas operates on the same principle as the impact mill without inert gas.

Advantages of impact milling under inert gas

The ambient air present in the grinding chamber is replaced by an inert gas (nitrogen) to eliminate oxygen and reduce the risks of oxidation, ignition or explosion.

Operating principle of opposed air jet milling under inert gas

The opposed air jet mill under inert gas operates on the same principle as without inert gas: the material is injected into a grinding chamber in which several jets of compressed gas (air or inert gas) meet at high speed, causing particle size reduction. A dynamic selector then classifies particles according to their size.

Advantages of opposed air jet milling under inert gas

The main difference between the two processes lies in the type of gas used.

In the conventional version, milling is carried out using compressed air. This process is suitable for stable, non-flammable products that are not sensitive to oxidation.

However, if the powders are flammable, explosive or sensitive to oxygen, milling under inert gas is necessary.

In this case, the air is replaced by nitrogen, an inert gas, to eliminate any risk of explosion and oxidation, providing a secure environment compliant with ATEX safety requirements.

Mill operating principle

Wet bead milling relies on the combination of several mechanical forces: impact, attrition and shearing.

The powder to be ground is dispersed in a liquid to form a homogeneous suspension. Stabilising agents are often added to prevent particle re-agglomeration.

This suspension circulates through the grinding chamber, where beads are set in motion by a rotor. The interactions between the beads and the particles cause the material to fragment, thus reducing its size.

At the mill outlet, a separation system retains the beads in the chamber while allowing the ground suspension to pass through.

The grinding chamber has a double jacket through which a cooling fluid circulates, preventing the suspension from overheating during the process.

The mill can be used in different modes (single-pass, multi-pass or recirculation) depending on the project objectives.

Advantages of bead milling

• Ability to achieve very fine particle sizes (down to the nanometric scale);
• Preservation of physical and chemical characteristics of heat-sensitive materials (via efficient cooling provided by the double jackets of the grinding chamber and the intermediate tank);
• Versatility in the range of materials to be ground (made possible by the choice of materials for the grinding media and the grinding chamber lining);
• Dust reduction, resulting in improved safety and cleanliness.