ARR - Annual Report

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.

The focus of the recent work was on the generation of different types of structure. Random

close structured granules as presented in previous years were investigated more

detailed regarding the packing density and primary particle distance distribution inside

a granule. Additional random loose structures were generated allowing porosity as additional

phase. Porous systems were generated using two different methods: fluid bed

granulation and a sintering method. Layered systems or core/shell systems were generated

using a fluid bed; a fine particle suspension was sprayed on larger core particles.

Additionally the particle-particle distance of coarse primary particles that was presented

as suitable structure measure last year was investigated more detailed. The goal was to

develop a theoretical calculation of particle-particle distance based on the known primary

particle size distribution. Results show a good agreement between values calculated

from X-ray micro-tomography images and values from theoretical calculations. Therefore

this theoretical particle-particle distance was used as structure measure in random close

structured granules.

The dissolution measurements of different sets of experiments showed that the dissolution

speed is dependent on two factors: amount of soluble material (fraction of surface

occupied by soluble material) and pore size or phase width of soluble materials. These

two factors are strongly connected and it is difficult to investigate one without the other.

Together a high fraction of soluble material and a high phase width leads to a faster dissolution

compared to granules with smaller phase width (or pore size) and fraction.

The mechanical strength of random close structured granules showed no explicit results

and was difficult to evaluate. Random loose structured granules on the other side showed

a higher strength for lower effective porosities. These results were as expected, the development

of a network of solid bridges between primary particles leads to stable granules

if more and thicker bridges are build.

Executive Summary

The understanding and control of crystallographic polymorphism and crystal habit of organic as well as inorganic compounds is scientifically and technologically important to a number of industries. To date, however, the experimental control of polymorphs (crystalline solids with different arrangements of the same constituents) is difficult. Since a polymorph is determined at the nucleation of a crystal, methods that lead to an advanced understanding of early crystal formation pathways and mechanisms are highly desirable. Towards this aim, in this project we employ arrays of self-assembled monolayers (SAMs).

Self-assembled monolayers (SAMs) are well-defined surfaces that can be used to study the relationship between the nucleation event and the final polymorph selection. Furthermore, by tuning the substrate-crystal interface energy, potentially crystalline order of SAMs can promote the nucleation of polymorphs not accessible via solution methods. It is these two advantages, i.e. the establishment of scientific correlations between nucleation and observed polymorph and access to polymorphs not accessible via solution methods, that have led us in this project to choose heterogeneous surface nucleation via SAMs as the primary means to study polymorph selection.

In the first-year of work, we have selected three types of SAMs, two hydrophilic (carboxylic acid terminated surface and hydroxyl terminated surface) and a hydrophobic (methyl terminated) surface to investigate their ability to influence the nucleation, crystal growth, and polymorph selection of a common drug, acetaminophen (ACM). It turns out that the hydrophilic surface tends to promote the formation of the monoclinic form of ACM, while the hydrophobic surface induces the formation of the less thermodynamically stable orthorhombic form of ACM. We hypothesize that this selection is due to the energetic preference of certain crystal facets interacting with the chemically specific SAMs surface. By studying the known relationships between the structure of the crystal and the nucleating surface, we will gain insights into molecular-scale recognition events that can lead to polymorphism which is a promising step to the final goal: understanding early formation pathways of crystallographic polymorphs.

Executive Summary

The ball indentation method has been applied to a range of sizes of silanised glass beads, pea protein, maize starch, titania and alumina. The penetration depth range which provides a stable hardness measurement is determined for each material, which is identical for most materials. Ball indentation and shear cell measurements at moderate stresses allow the constraint factor to be determined. The value of constraint factor varies for different materials, but remains independent of consolidation stress in the range tested for all materials except pea protein, where a larger error is observed in the ball indentation measurements, and alumina. For silanised glass beads the constraint factor increases slightly as particle size is reduced. Comparison of the materials tested in this work and those by (Zafar, 2013) indicates that constraint factor varies between 1.7 – 4.8, and for most materials the value is below 3. All tested materials except titania, alumina and durcal 15 exhibit a notable deviation in flow behaviour at low stresses, with a rapid reduction in the yield stress inferred by ball indentation.

Future work will experimentally investigate the influences of size distribution, surface energy, density and shape on the constraint factor, with DEM also used to investigate the shape effect. DEM will investigate the variation of constraint factor in the low stress range that cannot be reached experimentally. Finally the reliable penetration depth range at higher strain rates will be determined using the freefall ball indentation method.

EXECUTIVE SUMMARY

Currently, there is no first-principles, general theory of intermediate dry granular flow that predicts its rheological response as a function of particle size, shape, and friction (even leaving aside adhesion, which is more challenging still). It is an open question what constitutive equations best describe such flows. Therefore, there is a need for experimental data which tests these theories, and thereby provides an improved understanding of how particle properties control the rheology of granular materials, independent of the flow geometry. Rather than using empirical relations fit to bulk data for a particular flow geometry and particles, we aim to connect grain-scale parameters to macroscale behaviors.

In this initial year of effort, we have focused on testing the Kamrin nonlocal theory [Kamrin and Koval 2012] in experiments. Our apparatus is a modification of a 2D annular shear apparatus capable of providing much better measurements of local boundary forces than has previously been possible, in addition to providing conventional particle-tracking. A key upgrade to the apparatus was the development of a circle of leaf springs as the outer wall. We have calibrated these to provide measurements of both normal and tangential forces at the outer boundary; torque values at the inner boundary are known from an in-line torque sensor. We have run the experiment under continuous shear, allowing us to obtain high-quality measurements of the velocity profile, the internal stress field, and the fluidity field. These experiments were done with a 60-40 mix of circular and elliptical disks at four rotation rates and two packing fractions. We find that there is a single set of parameters (similar to those found by Kamrin) which provide good agreement with the model and do not need to be adjusted in order to cover wide range of shear rates and packing fractions.

By analyzing the positions of the leaf springs, we have successfully observed fluctuations in the boundary forces which arise due to the buckling of force chains. Importantly, this new technique will allow for measuring shear forces even for non-photoelastic particles. We have also installed a polariscope within the apparatus, allowing for future work on internal forces using photoelastic particles.

Segregation model development holds promise for translation of academic research into industrial

practice. One significant hindrance to model development, however, is the inherent difficulty

in measuring segregation rates (especially in an experimental setting). In this project, we seek

to establish an “equilibrium” between segregation and flow perturbation in free surface granular

flows in order to overcome this experimental hurdle. That is, by using periodic flow inversions,

we hope to alter the steady-state distribution of particles whereby there exists a balance between

the rate of segregation and the perturbation rate. In this way, we can combine the segregation

rate expressions that we are interested in testing with our previously developed segregation control

framework such that knowing the perturbation rate, we can deduce the segregation rate (much

like knowing an equilibrium concentration, along with a reverse reaction rate, one can deduce the

rate of the forward reaction). In our first year, we examined binary segregation rate models, both

computationally and experimentally, that are appropriate for free surface flows of granular materials.

We started with well established models for both size segregation and density segregation and

compare these to new and proposed models. We tested these models, both computationally and

experimentally, using an industrially-relevant device – a tumbler-type mixer – by introducing an

axially-located baffle that periodically perturbs the flow. This flow perturbation allows us to modify

the expected segregation “equilibrium” such that varying flow properties (like rotation rate) as

well as material properties (like size or density ratio) will lead to results that collapse onto a “master

curve” when using an accurate segregation model. As the project progresses, (experimentally)

validated segregation models can be incorporated into device-level transport equations in order to

yield quantitative prediction of segregation at process scale.

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.

One of the long term barriers in understanding granule breakage is the lack of a universally accepted test method or test granules to systematically evaluate agglomerate breakage propensity and mechanism. Computer simulations are often used but are limited by the lack of identical, controlled agglomerates to test and validate simple models, let alone replicate the complex structure of real industrial agglomerates.

This report summarises progress to date on a new 3D printing production method of test agglomerates with defined and "tuneable" properties. Agglomerates were designed using Solidworks 2014 software and printed by an Objet500 Connex 3D printer. Materials with different mechanical properties were used to print the particles and the inter-particle bonds, allowing combinations of bond strength, particle strength and agglomerate structure to be tested. Compression and impact tests were performed to investigate the breakage behaviour of printed agglomerates in terms of agglomerate orientations, bond properties and strain rates.

The compression and impact results reveal different agglomerate breakage characteristics. For compression tests under low strain rate, breakage occurs at the bond between primary particles, and the compressive strength is influenced by the bond strength significantly. In future, it is worth to further relate the microscopic particle-particle and particle-bond interactions to the macroscopic compressive strength. For impact tests with high strain rate, the agglomerates with flexible bond show rebound behaviour, while the agglomerates with rigid bond fracture. Under the same impact conditions, the rubber materials have high fracture toughness and the rigid materials behave in a more brittle manner that can fracture easily. For the fractured agglomerates, clear fracture planes can be observed with low impact energy. At high impact energy, a large amount of small debris occurs, and the breakage extent increases accordingly.

Now that proof of principle for the approach has been established, the next stage of the project is to conduct systematic studies of agglomerate strengths varying structure, material properties, under various breakage forces, orientations and strain rates.

This report covers the results obtained in the second year of this project. At the end of the first year, we have developed an experimental set-up capable of dispersing a sample of about 10 g of powder in a gas stream, charging it by tribocharging, collecting the sample inside a Farday cage and measuring the charge in the collected sample. During this year, we have improved the set-up by building two tribochargers: a nylon cyclone tribocharger and a steel tube tribocharger. With this new set-up we are able to measure both the charge acquired by the particles in dispersion (but only when the steel tribocharger is used) and the ccharge remaining in the powder when collected.

We have run experiments with a set of seven materials stored at different humidities and dispersed using air and dry N2. From the results we have obtained we can conclude that in dispersion particles become electrically charged up to the maximum given by the electric field that would cause a corona discharge. However, when settled, the particle must lose most of their charge to keep the field created by the settled sample below the limit imposed by corona discharge. The remaining amount of charge on the settled powder depends on the sample mass and as a result, the specific charge of the collected sample tends to decrease when the collected mass is increased. A quantitative model for a simplified sample geometry has been developed.

We have found that the charge in the collected sample decays in a relatively short time, of the order of some minutes. We have decided to measure poured and tapped densities to evaluate the effect of charge on the packing as these experiments can be performed during the time span that powder remains charged. However, the results obtained are in conclusive. Possible explanations for the lack of a clear trend in the effect of specific charge and storage conditions on the solid fraction are given in the text.

During the Annual General Meeting, we were requested to move to smaller particle sizes (below 10 μm), storage humidites (below 10% RH) and dispersion gas humidities (dry nitrogen). The upgrades done in the tribocharging set-up to meet this condition are presented in the test, as well as some preliminary results.

Work has also continued in the other experiments planned in the project: individual particle size and charge distribution determination with particle tracking velocimetry (PTV) and scanning probe microscopy (SPM) experiments. However, no new significant results have been obtained and for these reason this report will focus on the results of the specific charge obtained with the tribocharging set-up. The state of the PTV and SPM experiments is discussed at the end of the report.