1. Principles of milling bulk solids

Milling is the action of reducing the size of particles thanks to a mechanical action. The mechanical action is submitting the particles to a stress, under the stress, some cracks will appear and subsequently the particle will be broken in different parts.

The mechanical action used to break the particles and reduce the size can be diverse and different mill exist that will use the milling principles.

Typical milling principles are the following :

• Use of 2 solid surfaces : the solid is pressed or frictionated against 2 solid surfaces
• Use of a single solid surface : the solid is impacted against a solid surface
• The solid is cut - Shear forces or pressure waves are used to break the particles

An important consideration is that milling can be either performed in dry phase or in wet phase, the equipment used are different.

Sizing of milling operations is mainly based on experience, and using reference on an existing installation, leveraging suppliers' experience and carrying out tests is strongly advised. Some models exists to try to determine the power consumption of the PSD obtained thanks to the mill but they do not appear to be very usable in an industrial environment. One point to be remembered is that milling operations are requiring a lot of energy but that only 1-2% of it is used for the size reduction, the main part is lost in friction and heating within the milling machine, thus milling operations should be a particular focus of energy efficiency in order to optimize the cost of production.

2. Common powder milling equipment

How many types of mills are there ?

The table below is listing the key milling process equipment that can be found in process industries

Table 1 : Milling technologies

Mill group	Mill types	Principles
Roll Mills	Roller mills Single stage or multi-stage High compression roller mill	The mills are equipped with 2 rolls turning in opposite direction. The product is fed between the 2 rolls. When passing in between the 2 rolls, the product is exposed to compression forces and shear forces, thus its size is reduced when passing in between the rolls. In order to make the process more efficient, several pairs of rolls can be installed in series with a gap decreasing, the rolls can also present some grooves that will make the milling more efficient by "biting" on the product To make the mill more efficient, some roll mills are equipped with a system allowing to guarantee a high pressure (10-50 bar) in between the rolls. It is also mandatory for these mills to have a spring system allowing to release the pressure if the constraint is too important (foreign body for example) and avoid mechanical damages.
Impact Mills	Hammer mills Pin mills Universal mills	The size reduction is ensured here by impacting the product against a solid surface. The solid surface is actually made by different kind of beaters rotating at very high speed. A pin mill is equipped with a static disc mounted with several pins and a mobile one also mounted with pins. When the product is forced passing through the system, it has to be impacted by the pins, and therefore be broken and its size reduced. A hammer mill is equipped with a simple rotating disc equipped with many pallets that are hitting the product and throwing it against the mill housing. A grinding sieve that will allow to enhance the milling process and reduce the Particle Size Distribution obtained can also be installed. High speed hammer mills (tip speed from 40 to 70 m/s) can allow to reach small particles sizes, below 0.1 mm A universal mill is a derivation of the hammer mill with more sophisticated beaters as well as grinding tracks and sieves. Tip speed ranges from 40 to 100 m/s.
Impact Mills	Jet mills	Jet mills are achieving a size reduction by accelerating the product to be milled and throwing it either against a static target, either against another flux of accelerated particles To achieve this purpose, speed of up to 250 m/s must be reached in order to give enough energy to the particles to break them at the impact. A single pass through the mill would create a very wide Particle Size Distribution. In order to narrow the PSD, jet mills are equipped with a classifier. Only particle below a target size will be allowed to leave the mill, the others will be recirculated.
Mills with size reduction media	Ball mills (dry)	Ball mills are basically made of a drum partially filled with a grinding media, typically beads of ceramics or steel. The mill is filled with the grinding beads as well as the product, then the mill is rotated until a speed sufficiently high so that the beads can tumble freely. It should be noted that, in some applications, a stirrer is present to agitate the beads, and the drum is therefore not rotating. When the beads tumble, roll, they actually impact and / or apply a shear stress to the product. The product, by being impacted in between 2 beads, in between a bead and the wall, or by hitting the wall has its size reduced.
Mills with size reduction media	Ball mills (wet)	Ball mills, as described above, are also a machine of choice for milling solids in wet phase. The design applied here is mainly a ball mill with an agitator, the movement of the agitator making the beads move and impact or friction the particles to be milled. Design using a loose reduction media can be found, mainly in long horizontal mills but other designs, more efficient are actually using a compact vertical bead bed, agitated. The use of beads packed and agitated allow to absorb a lot of power and mill the particles to few dozens of microns. In order to separate the beads and the product at the end of the mill, a grid is placed, with hole diameters lower than the beads one. It is important to consider that agitating beads, especially if they are densely packed in the milling chamber, release heat. These mills must therefore be cooled down and have often double jackets.
Wet mills	Colloid mills	Colloid mills are now quite widespread in the industry. They can be easily implemented on small processes requiring a milling of a slurry. Colloid mills are basically achieving a size reduction by submitting the product to intense shear forces created by a high speed rotor. The rotor is in a housing, by introducing the product in it, the rotor is creating a high that will ultimately break the particles. The housing is equipped with holes that will allow the milled product to be extracted.

Additionally, the process integration of the mill must be designed according to the performance of the mill : Open loop or closed loop milling can be implemented.

In open loop milling, the feed will go through the mill only one time, it is suitable in case the mill is performant enough to reach the desired PSD in 1 pass. It gives the simplest process.

In closed loop milling, a system to separate the particles having a too large size to the ones having the desired size must be implemented after the mill. It is typically a sieving operation where the overs are recirculated through the mill.

3. Selection of mill types

It is very important to understand which performance can achieve a type of mill, for which type of solids. It will allow the process engineer to properly select a mill type for new process, or even perform some troubleshooting on an existing process.

The 1st question to ask is the phase in which the milling must happen. If it is in wet phase, the choice will be mainly towards an agitated ball mill or, if the milling is easy and the size reduction expected not too important, a colloid mill

Table 2 : Particle size reachable for each milling technologies

Particle size distribution expected (d50)	Possible mill
Medium-coarse (10 mm)	Roll mill Hammer mill
Medium-fine (1 mm)	Roll mill Hammer mill
Fine (0.1 mm)	Roll mill (consider multi-stage) High compression roll mill High speed Hammer mill High speed Universal mill Pin Mill
Super Fine (0.025 mm)	High speed Universal mill Pin Mill Jet mills Agitated ball mills (wet)

The nature of the particles to be milled will also guide the choice of the equipment. It is especially important to understand the hardness of the product to mill.

In the literature, the hardness is sometimes referred to the Moh's hardness scale, given below :

Table 3 : Moh's hardness scale

Moh's hardness	Reference product
1	Talcum
2	Gypsum or salt
3	Calcite
4	Fluorite
5	Apatite
6	Feldspar
7	Quartz
8	Topaz
9	Corundum
10	Diamond

Having determined the hardness of the product to be milled, the following table can help to screen the type of mill that could be adapted. The result has to be coupled with the fineness desired, as per the table above.

Table 4 : Selection of mill according to hardness of material

Product hardness	Possible mill
1-3 (soft)	Roll High compression roll mill Hammer mill High speed hammer mill Universal mill Pin mill Agitated ball mill (wet Colloid mill (wet
4-6 (medium hard)	Roll mill High compression roll mill Universal mill Agitated ball mill (wet
7-10(hard)	High compression roll mill

4. Safety of milling systems

References given above on the mill types showed that most of the mills are operating at very high speed. This causing different major risks to the process safety. Main risks are listed below.

Thus, in order to prevent a fire, an explosion, or to mitigate the consequences, milling systems are often fitted with the following equipments. These are general although quite complete consideration, however, each mill operator must carry out its own risk assessment, determine which risks are specific to its installation, and take appropriate measures.

The considerations below mainly apply to dry comminution

PREVENTION of dust explosion in milling system

Preventing explosions, in the case of a mill, mainly consists in avoiding that foreign bodies enter the mill, detecting any mechanical issue and detecting if overheating of the product, due to an overfill, is happening.

Table 5 : Mill prevention of explosions

Prevention	Measure taken
Avoid entry of foreign body	The product feed must be controlled through a vibrating sieve and a magnet - in some cases, it could be a metal detector
Detect mechanical issues	A mechanical breakdown on rotating parts can lead to metal metal contact, then sparks, or overheating at a bearing for example. Some mills are equipped with vibration detection, if the vibration is above a defined threshold, the mill will be stopped. Temperatures sensors are also installed on bearings to detect overheating. Bearings need also to be flushed in order to avoid that product reach the part and be heated up to the point of fire.
Preventing overfill	The feeding of the mill must be perfectly controlled in order to avoid any overfill that could lead to heating of the product and smouldering. Control of vibration and amps on the motor can also allow to detect this incident
Inerting	Some mills will be inerted with nitrogen to avoid any explosion risks

MITIGATION of dust explosion in milling system

In case the prevention measures were not sufficient and an accident happens developping in a explosion, milling systems can be protected thanks to the following devices.

Table 6 : Mill mitigation of explosions

Mitigation	Measure taken
Explosion resistance	Some milling systems can be built in order to resist to explosions, generally up to 10 bar g. This is an efficient way to contain an explosion, however, the rest of the process must also be designed accordingly with some devices able to stop the propagation of the explosion and keep it in the area designed to withstand the pressure.
Explosion containment	Different equipments can be used to avoid the propagation of an explosion Ventex valve : it is a passive valve that is going to close under a pressure wave Explosion proof star valve : star valve with enough alveoles to stop an explosion. The explosion must be detected in order to stop the rotation of the valve. Quick acting valve : often positioned in the pipe. Those valve can be closed in few ms after the detection of an explosion. It is also needed here to have an explosion detection sensor and a safety PLC to close the valve in due time.
Venting	Explosion panels can be positioned, usually downstream the mill, in order to release the pressure of an explosion and then avoid hazardous damage. Those panels can also be coupled with a flame arrestor.
Explosion suppression	An extinguisher can be triggered at the very beginning of an explosion to avoid that the pressure wave develops to its maximum pressure.

5. Operation of mills

The main concern, in an industrial environment, is to reach a specified Particle Size Distribution (PSD) at a given throughput. The operator must therefore reacts to possible changes in the observed PSD and should understand the process levers of the process. Some of these levers are explained below.

Table 7 : Mill process driving parameters

Process lever	Change (all other constant)	Expected effect
Throughput	More	Admitting more solids in the mill is expected to increase and widen the PSD On the contrary, reducing the throughput can be a temporary way to come back to a smaller PSD
Humidity of feed	More	Having a more humid feed may cause to have a larger PSD by either making more difficult to break the particles, or by causing some powder agglomeration after breakage
PSD of feed	More	It is very important to control the PSD of the feed. Any changes will have an impact on the milled product PSD In wet milling, with agitated bead mills, having a too high PSD can ultimately lead to a blockage at the mill inlet.
Speed	More	Increasing the speed of the speed of the mill will have as an effect to reduce the PSD while absorbing more power. It must be kept in mind that the higher the speed is, the higher the wear is, the wear of the mill should therefore be managed, especially for ball mills where part of the beads are actually carried out will the product flow (acceptability of the pollution should be checked)
Fat content	More	Having a fatty feed will favor some agglomeration phenomena which will enlarge the PSD. Fatty products can also jam the mill reducing its efficiently or even causing some safety issues (overheating)
Temperature	More	A higher temperature may have negative consequences on the milling, especially with powders that can soften due to temperature. Temperature control must therefore be in place.
Bead size	Less	For agitated bead mills - smaller beads will lead to higher milling surface, thus smaller PSD

Sources

Author's industrial experience
Principles of powder technology, Martin Rhodes et al,, John Wiley and sons, 1990