Size Reduction

Mills and classifiers are primarily used for particle size reduction and separation across various industries, including paper, paint, plastic, pharmaceuticals, ceramics, cosmetics, foods, and fine chemicals. They are typically used to achieve a targeted particle size distribution or specific particle shape. Numerical modeling has the potential to inform equipment design to control particle transport and predicting high-wear spots. Given the substantial volume of particles processed in industrial contexts, even marginal improvements in grinding efficiency can lead to significant economic benefits. This review summarizes advancements in understanding and modeling key processes taking place at the scale of individual particles and coarse grain approaches to simulate processes at the scale of industrial units. The review concludes with a brief perspective on future research directions, including the use of machine learning for constitutive modeling and design optimization.

It is well known that attractive particle-particle interactions become more decisive with decreasing particle size. Especially in dry fine grinding processes, where small particles are produced within a dry environment by different types of mechanical stress, these forces lead to a variety of challenges, such as a complicated control of the powder behavior, a decrease of grinding efficiencies and production rates as well as obtaining high product finenesses. In order to control these forces, chemical liquid or solid additives – so called grinding aids – are added to the process in many industrial dry fine grinding applications. Even though the benefits of grinding aids have already been shown in various experimental studies and industrial applications, their selection and application is still mostly based on empirical knowledge. As shown in this review article, the variety of applied substances, ground materials and target finenesses, but also available mill types, process designs, mill and process parameters as well as analysis methods complicate the development of a comprehensive understanding. Within this article, we present the basic mechanisms of action of various liquid, gaseous and solid grinding aids. Subsequently, it is shown how grinding aid molecules interact with the solid particle surface, leading to decisive changes of the particle and bulk behavior. Based on various scientific studies it is shown, how this may affect the micro- and macro-processes inside the mill as well as the whole grinding plan.

Note to reader

Milling is an important technology which has a wide range of potential applications in many different fields. It can be applied in different ways to any type of compound. Its effects range from obtaining nano-particles to chemical synthesis, from the increase in reactivity to the forced formation of alloys.

In all cases, changes in the physical state, in the form of polymorphic modification between crystalline varieties, amorphization etc. may occur during milling. The accidental formation of these new states can dramatically affect the stability and expected performance of a product. On the contrary, milling can be used to induce voluntarily changes of state, which makes it possible to produce new solid forms, with interesting new capacities (for example, in pharmacy, to obtain an amorphization, which allows to improve dissolution properties of poorly soluble compounds). Milling is then a "green" route of synthesis that avoids the use of a solvent or a chemically destructive heating. The amorphization induced during co-milling can be a desirable intermediate of a chemical synthesis. These physical transformations occur in conditions that are far from equilibrium. Their fundamental understanding is a challenge for materials physics. At the present time, there is no universal theoretical framework for describing and predicting transformations induced by milling. Even a simple definition of the relevant control parameters is an open problem.

In this paper, I will consider essentially the issue of changes in physical states and will only briefly mention the other aspects: chemical synthesis, forced formation of alloys or co-crystals. Similarly I will not go into the details of the multiple technical aspects associated with the performance of the mills.

After some general considerations, I will give a rapid description of the classical thermodynamic conditions for obtaining changes in physical states (polymorphisms, phase transitions, amorphization and glass transitions). This is useful for making a comparison between the thermodynamically induced and the mechanically forced changes. I will then briefly introduce the main experimental techniques that are useful to identify and quantify the changes. The numerous examples described later will give an idea of the relevance of these different techniques, according to the types of compounds studied. I will then present typical examples of transformations for different classes of compounds (elemental compounds, minerals and oxides, metal compounds, molecular and macromolecular compounds). I will dwell more on the behavior of these organic compounds, which have an important practical interest in the fields of food, pharmacy, pigments, energetic materials and so on. My area of expertise essentially concerns these compounds. They have a high sensitivity to milling and temperature changes. They present a very rich polymorphism and are good glass formers with a glass transition that is well marked and close to room temperature. It is a favorable situation to investigate in detail the effect of changing milling conditions on the nature of the end products.

I will conclude with a presentation and discussion of the many theories which compete to describe the physical transformations under milling. They are usually based on thermal equilibrium considerations. However there are many shortcomings in these approaches. I will show how more recent non equilibrium approaches could provide a more universal framework of description.

Executive Summary

of the proposed DEM-PBM multiscale method for the optimization of milling devices.

predictions of product size distribution is achieved, which indicates a promising application

experiments at different rotary speeds. A good agreement between the tests and the

model was then used to predict the milling outcomes for the other three set of milling

12000RPM using a constraint optimisation technique. The DEM-PBM coupled multiscale

i.e. particle material dependent parameters, were evaluated from the milling test at

parameters and mill operating dependent parameters. The remaining parameters of PBM,

scale. Variables in the PBM kernel were classified into particle materials dependent

operating dependent parameters in the Population Balance Model (PBM) at the process

velocity distributions obtained through DEM simulations were utilized to inform the mill

the particle dynamic and stressing conditions inside the pin mill. Furthermore, the impact

and feed rate. DEM simulations were then performed to understand the fundamentals of

The UPZ100 pin mill experiments were conducted to study the effect of rotary speed

Timoshenko beam theory considering axial, shear and bending behaviour of the bond.

model by Brown et al. (2014) was utilized in which the bond contact is based on

which is then compared to experimental results. A recently developed new bonded contact

breakage subject to impact loading was conducted to evaluate the breakage propensity

of numerical results. A Discrete Element Method (DEM) simulation of single particle

comminution characteristics of the test solids, which provides the basis for the validation

experiments were carried out using the UPZ100 impact pin mill to measure the

component to be rationalized. In particular, the contribution of tangential component velocity was incorporated in the new model using the mobilized dynamic friction. Milling

a new breakage model, which enables the contribution of the normal and tangential velocity

velocity identified from experiment, the effect of impact angle is considered in developing

in predicting breakage under oblique impact and the significance of tangential component

crack accounts for the chipping mechanism. Considering the limitation of existing models

velocity. A new particle breakage model was proposed assuming that the subsurface lateral

velocity plays an increasingly important role in particle breakage with increasing impact

impact tests. It was found from the zeolite particle impact test that tangential component

by single particle loading experiments, including indentation tests and single particle

elucidate the particle breakage mechanics. The material grindability was first investigated

A hybrid of experimental, theoretical and numerical methods have been adopted to

particle breakage model calibrated against the defined grindability.

etc. pertaining to a milling process, which in turn will provide the basis for an improved

capable of analysing particle breakage subjected to particle impact, compression, and shear

provide the fundamental scientific basis for developing appropriate grindability measure

and how they relate to the mechanical properties and the final size distribution. This will

fracture and breakage mechanisms of individual particles under different loading regimes,

stressing events. The material grindability will require a detailed study of the fundamental

prevail in a milling operation and establishing material grindability in the context of the

in mill performance optimization. This involves characterizing the stressing events that

grindability with particle dynamics in a mill in order to provide an innovative step-change

reduction. In this project, we aim to develop new ideas and methodologies to link material

Milling is commonly deployed in many industrial sectors for intended particle size

Executive Summary

Milling is commonly deployed in many industrial sectors for intended particle size reduction. In this project, we aim to develop a robust methodology to link material grindability with particle dynamics in a mill in order to provide an innovative step-change in mill fingerprinting and optimization. This involves characterizing the stressing events that prevail in a milling operation and establishing material grindability in the context of the stressing events. The material grindability will require a detailed study of the fundamental fracture and breakage mechanisms of individual particles under different loading regimes, and how they relate to the mechanical properties and the final size distribution. This will provide the fundamental scientific basis for developing appropriate grindability measure capable of analysing particle breakage subjected to particle impact, compression, and shear etc. pertaining to a milling process, which in turn will provide the basis for an improved particle breakage model calibrated against the defined grindability.

The centrifugal impact pin mill has been selected to be studied for this project, in collaboration with Hosokawa Micron Ltd. UPZ100 pin mill experiments with varying rotary speeds and feed rates were reported and analyzed in the past reports. The work performed in Year 5 of the project is to develop a coupling framework between discrete element method (DEM) and population balance model (PBM) to predict the product size distribution of milling experiments. At the particle scale, DEM simulations were performed to understand the fundamentals of the particle dynamic and stressing conditions inside the mills. Variables in the PBM kernel were classified into material dependent parameters and mill operation dependent parameters. The impact velocity distributions obtained through DEM simulations were utilized to inform the mill operation dependent parameters of PBM at the process scale. The remaining parameters of PBM, i.e. material dependent parameters, were estimated based on the milling test at 12000 RPM. The resulting DEM-PBM coupled model is then used to predict the milling results for the other three rotary speeds to validate the proposed DEM-PBM model. A good agreement between the tests and the predictions of product size distribution has been achieved, which indicates the potential application of the proposed DEM-PBM multiscale method for scale-up and optimization of milling processes. The follow-on work will focus on further improving the material dependent parameters evaluation and studying the particle breakage mechanism using the Edinburgh bond DEM model.

Executive Summary

Milling is commonly deployed in many industrial sectors for intended particle size reduction. In this project, we aim to develop a robust methodology to link material grindability with particle dynamics in a mill in order to provide an innovative step--change in mill fingerprinting and optimization. This involves characterizing the stressing events that prevail in a milling operation and establishing material grindability in the context of the stressing events. The material grindability will require a detailed study of the fundamental fracture and breakage mechanisms of individual particles under different loading regimes, and how they relate to the mechanical properties and the final size distribution. This will provide the fundamental scientific basis for developing appropriate grindability measure capable of analysing particle breakage subjected to particle impact, compression, and shear etc. pertaining to a milling process, which in turn will provide the basis for an improved particle breakage model calibrated against the defined grindability.

The centrifugal impact pin mill has been selected as the first mill to be studied for this project, in collaboration with Hosokawa Micron Ltd. The work performed in Year 4 of the project is summarized here. UPZ100 pin mill experiments were conducted with the effect of rotary speed and feed rate examined. Six parameters including particle size distribution, median product size, relative size span, bond’s grinding energy, size reduction ratio and specific surface area are chosen to characterize the milling results. In particular, the relationship between relevant parameters is investigated. The grinding energy approximately follows a linear relationship with the size reduction ratio. The alumina particle requires more grinding energy as compared to the zeolite particle. The population balance modelling (PBM) is used to predict the product size distribution in the milling impact tests. As indicated by the general form of PBM, it shows that two functions, i.e. selection and breakage functions have to be considered. The capacity of PBM is exemplified by predicting product size distribution in the impact pin mill considering two simple selection and breakage functions. The coupling framework of PBM-DEM is presented considering the deficiency of PBM, which forms the platform for the follow-on work.

Mechanical size reduction methods have dominated the scene in nearly all industries involved with comminution since their beginning. However, the various challenges and limitations of mechanical methods have maintained widely open the opportunities for non-mechanical milling methods. This review analyzes critically several different approaches, including thermal shock, microwaves, lasers, pressure variation, high voltage pulses and ultrasound. While interest in some of them, namely thermal shock and pressure variation, has nearly vanished, others are intensively studied at present and seem to be on the brink of becoming industrially-available technologies. Examples of the later are microwaves and high voltage pulses. The large variety of materials and applications that involve size reduction and the nearly universal reliance on mechanical methods suggests that non-mechanical methods have found niche applications and that type of application with continue to grow. Examples of these are lithotripsy and laser ablation, which already find applications in narrow, but important fields.

Milling is commonly deployed in many industrial sectors for intended particle size reduction. In this project, we aim to develop a robust methodology to link material grindability with particle dynamics in a mill in order to provide an innovative step--change in mill fingerprinting and optimization. This involves characterizing the stressing events that prevail in a milling operation and establishing material grindability in the context of the stressing events. The material grindability will require a detailed study of the fundamental fracture and breakage mechanisms of individual particles under different loading regimes, and how they relate to the mechanical properties and the final size distribution. This will provide the fundamental scientific basis for developing appropriate grindability tests capable of analyzing particle breakage subjected to particle impact, compression, shear and abrasion etc. pertaining to a milling process, which in turn will provide the basis for an improved particle breakage model calibrated against defined grindability.

The centrifugal impact pin mill has been selected as the first mill to be studied for this project, in collaboration with Hosokawa Micron Ltd. The work performed in Year 3 of the project is summarized as below.

• UPZ100 pin mill experiments were conducted with the effect of rotary speed and feed rate examined.
• Conceptual design of instrumented pin was trialed in laboratory and its mechanical response was explored in both static and dynamic loading.
• An inverse analysis scheme has been developed and validated for determining force and angles from strain measurement on an instrumented steel pin.
• With the finding of the significance of tangential component of impact velocity, a new breakage model was proposed based on a mechanistic approach assuming that lateral crack accounts for the chipping mechanism.
• The new model was then assessed and compared with other breakage models which demonstrates that the new model is superior in predicting both normal and oblique impact regimes for the range of velocities studied.
• DEM simulations of single particle breakage were conducted to study the damage ratio arising from the velocity regimes pertaining to an impact mill.
• A recently developed new DEM bond contact model was utilized considering axial, shear and bending behaviour of bond.

The breakage pattern of chipping and fragmentation under low and high impact velocity was successfully reproduced in DEM.

The mechanical properties of spherical alumina and zeolite particles were measured to provide an insight to their susceptibility to milling in a pin-mill. Nanoindentation was applied to determine Young’s modulus and hardness, whilst microindentation and SEM observation were carried out to measure fracture toughness. Preliminary indents determined the suitable load to apply for each material. The Young’s modulus, hardness and fracture toughness were found to be greatest for alumina, and increased with size of zeolite particles. Large scatter was present in the measurements, as is typically the case. The scatter was greater for the zeolite particles than the alumina. The mechanical and physical properties of these particles lead to the prediction that the larger zeolite is more prone to impact breakage caused by a pin mill, with the alumina particles being least susceptible.

The impact breakage of the smaller zeolite particles was assessed in a single particle impact rig at a range of impact velocities and angles. The extent of breakage was shown to correlate with normal impact velocity, regardless of impact angle. This is expected to be the case for larger zeolite particles, however alumina particles should be subjected to similar tests to assess if impact angle is influential on the extent of breakage.

Executive Summary

Milling is commonly deployed in many industrial sectors for particle size reduction. In this project, we aim to develop a robust methodology to link material grindability with particle dynamics in a mill in order to provide an innovative step--‐change in mill fingerprinting and optimization. This involves characterizing the stressing events that prevail in a milling operation and establishing material grindability in the context of the stressing events. The material grindability will require a detailed study of the fundamental fracture and breakage mechanisms of individual particles under different loading regimes, and how they relate to the mechanical properties and the final size distribution. This will provide the fundamental scientific basis for developing appropriate grindability tests capable of analyzing particle breakage subjected to particle impact, compression, shear and abrasion etc. pertaining to a milling process, which in turn will provide the basis for an improved particle breakage model calibrated against defined grindability. The IFPRI Grindability Project commenced on January 2013 and is planned for 6 years. The centrifugal impact pin mill has been selected as the first mill to be studied for this project, in collaboration with Hosokawa Micron Ltd. In the first year, the work focused on developing a computer model and conducting the discrete element modelling of pin mill to examine the particle dynamics during the milling process. This provided a better understanding of the comminution process such as the local stressing events and spatial distribution of impacts etc. under different operating conditions. This report summarizes the work performed within the second year of the project. Two materials, zeolite and alumina, have been chosen as the test particles with semi--‐brittle and brittle failure behaviour respectively. The compressive test of single particle coupled with X--‐ray microcomputed tomography (XìCT) was carried out to give a better understanding of particle breakage from the micro--‐scale standpoint. The 3D volume of particle was visualized and the 3D object was segmented to characterize the inherent crack in the particle by specifying thresholding value in Avizo Fire™. The spatial orientation of particle during projection was investigated and this problem was solved using a linear interpolation algorithm. By using 3D Digital Image Correlation (DIC), with a pair of images taken before and after an inherent crack propagates, full field displacement can be observed and the extent of crack development can be quantified to some extent. The presented results show that the combined use of XCT and DIC is an enhanced tool to study the breakage mechanism of individual particles under compression. Single impact test has been conducted to investigate the particle behaviour under dynamic impact. The breakage event was captured using a high--‐speed camera and the breakage pattern under high impact velocity was observed. The effect of impact velocity and incidence angle on particle breakage has been examined. The breakage rate increases with the increase of impact velocity, with chipping being the main breakage type at low impact velocities whereas fragmentation dominates at high impact velocities. Whilst the normal velocity component plays an important role in particle breakage, one key finding is that the tangential component of the velocity becomes increasingly important as impact velocity increases. Preliminary results of pin mill test for both zeolite and alumina were presented and compared with the predictions from the most common particle breakage models in the literature. The model fitting with the impact test data suggests that current models is deficient in predicting the breakage behavior of particle under impact comminution. IFPRI has funded a further collaboration project between Edinburgh and Dr Colin Hare of the University of Leeds to supplement the characterisation of these test particles using the well established facilities at Leeds to provide further measurements deploying the impact and nano--‐indentation techniques. The results from the collaboration are reported separately. Over the next 12 months, the project will focus on: particle breakage model development, which will consider the measured mechanical properties and loading conditions. Then breakage comparison with the newly developed model will be made with the final set of milling experiments to be conducted. Further investigation will be done in terms of in--‐situ loading single particle compression under simultaneous X--‐ray ìCT. The numerical work will continue to compute particle dynamics in real--‐geometry pin mill based on the first year’s numerical work. The new understanding will form the basis for developing the grindability test(s) capable of characterising particle breakage subjected to the dominant loading events identified within a milling operation.