Dry granulation through roll compaction followed by milling is a widely used pharmaceutical process. The material properties of powders and the roll compaction process conditions affect the strength of ribbons, and subsequently the granule size distribution (GSD). Accurate prediction of the granule size distribution from milling of ribbons with different properties is essential for ensuring tablet quality in the final compaction stage. In this study, MCC, PH-102 ribbons with precisely controlled porosities were produced and milled in a cutting mill and granule size distribution was analysed using QicPic. A population balance model with a new breakage function based on the Weibull function was developed to model the ribbon milling process. Eight model parameters were initially obtained for each ribbon porosity and very good agreement between the model and experimental results was obtained. Sensitivity analysis was then performed and thus reduced the number of model parameters that changed with ribbon porosity to two in the breakage function. The refined model was able to predict the granule size distribution both within and outside the experimental boundaries. It was shown that the model developed in this study has a great potential for predicting granule properties and therefore the optimisation of the dry granulation process.
This work aims to build a generic dynamic model structure, which can accommodate interchangeable sub-models of each sub-process, making it amenable to continuous upgrade without the need for redevelopment, for multicomponent tumbling grinding mills. The Generic Tumbling Mill Model Structure (GTMMS) is based on a population mass balance framework which incorporates breakage characteristics, transport, classification along the mill, a discharge function, and energy consumption incorporated in a dynamic mill model structure. Version III builds on two earlier versions by incorporating energy distributions derived from discrete element modelling, an updated version of the 4D breakage appearance function which applies to a broader size range, and addresses multi-component ore breakage via the probability distribution of energy split based on material stiffness. The model has been tested against multi-component plant survey data. GTMMS III suggests a mechanistic insight into mixture prediction through component analysis and is a step forward towards the unified comminution model (UCM) with its mechanistic, generic, and dynamic prediction capability.
Pursuing efficient and low-cost catalysts for the sluggish oxygen evolution reaction (OER) is imperative for the large-scale deployment of promising electrochemical technologies such as water splitting and CO₂ electrochemical reduction. The earth-abundant perovskite catalysts based on LaNiO₃-δ show promise in OER catalysis because of their relatively low cost and their optimal electronic structure but suffer from low electrode-area normalized activity. In this work, we partially substituted La with Sr and Ni with Fe to enable a remarkably high OER activity with an ultra-low overpotential of 374 ± 3 mV vs RHE at a current density of 10 mA cm−2 normalized by electrode geometric area. This performance even surpasses the performance of benchmark RuO2. Our results show that Sr could promote OER-active sites including Ni(III), O2−₂/O−, and optimal Ni/Fe ratios, which significantly improve the surface intrinsic activity at the perovskite surface. Therefore, this work not only developed a highly efficient earth-abundant catalyst towards OER, but also demonstrated the effective modulation of catalyst surface interactions through A-site doping for perovskite oxides for key applications such as water splitting, CO₂ electrochemical reduction and N₂ electrochemical fixations.
Possessing the advantages of both polymeric membranes and the specific inorganic nanoparticles or nanotubes, mixed matrix membranes (MMMs) have captured the imagination of researchers for a possible technological breakthrough for efficient gas separation. However, it is still very challenging to achieve defect-free interface between fillers and polymer matrix. In this study, the naturally abundant and low cost halloysite nanotubes (HNTs) were applied as fillers for MMMs synthesis. To improve the filler dispersion and filler-matrix interface affinity, the raw HNTs were modified by either alkali etching or (3-Aminopropyl) triethoxysilane grafting. After surface etching, the defect holes were formed on the surfaces of etched-HNTs, resulting in the rougher HNT walls and significant increment of surface area and CO2 adsorption capacity. The filler/polymer interfacial voids and filler dispersion were quantitatively assessed by tomographic focused ion beam scanning electron microscopy. HNTs surface etching significantly improved the HNTs/polymer interfacial affinity (void% = 0.06% for Raw-HNTs MMM, 0.02% for Etched HNTs MMMs) and filler dispersion, while grafted-HNTs mainly contribute to the filler dispersion. Compared to the pure polymer membrane and MMMs with untreated HNTs, MMMs containing 10 wt.% etched HNTs filler exhibited both increased CO2 permeability (807.7 Barrer) and higher CO2 selectivity (CO2/CH4 selectivity of 27.8) on the well-known limit of Robeson upper bound. In contrast, grafting HNTs only increased the membrane permeability without enhancing CO2 selectivity. The results suggest that surface etching can be an effective route in filler modification to improve interfacial morphology and membrane separation performance.
Dry granulation through roll compaction is a technology commonly used in the pharmaceutical industry for producing roll compacted ribbons. The significance of the feed screw speed and roll speed during ribbon production was highlighted in recent publications. However, previous studies focused primarily on the individual effects of either the feed screw speed or roll speed on ribbon porosity, and the synergetic effect of these parameters was rarely examined. The aim of this study therefore was to investigate the effects of the screw-to-roll speed ratio on the porosity of roll compacted ribbons, produced at different roll compaction conditions using the microcrystalline cellulose MCC, Avicel PH-102 feed material. It was observed that ribbon porosity decreased linearly with increasing screw-to-roll speed ratio. Furthermore, an increase in the speed ratio led to an increase in the roll gap and mass throughput while a decrease in the screw constant was observed. Thus, this study demonstrates that the screw-to-roll speed ratio can be treated as one of the critical process parameters for controlling ribbon porosity and can also be used to determine the optimum operating regimes during roll compaction.
In this paper, ibuprofen from a commercial source and its fractioned samples with narrower size distribution were characterized to assess the effect of particle properties on compaction characteristics. The compaction behavior of binary mixtures of ibuprofen with spray-dried lactose was also studied. The tablet in-die densification rate and tablet out-of-die porosity and tensile strength were measured for all samples. It was found that the particle size of ibuprofen does not affect the yield stress as derived from a Heckel plot, however the yield stress increases with the increase of ibuprofen particle size in the binary mixtures. Particle size also affects the tablet out-of-die properties, with tablet porosity and tensile strength increasing with the decrease in particle size. The effect of adding a weak compacting powder such as lactose on the tablet tensile strength is very much dependent on the ibuprofen particle size and mass ratio of the binary mixtures. Mixing lactose with ibuprofen of similar size in equal mass has no effect on the tensile strength of the tablet whereas mixing it with ibuprofen of larger size reduces the strength compared to ibuprofen alone. Adding a smaller amount of lactose can lead to an increase in tablet strength, even though the particle size of ibuprofen and lactose is quite different. Theoretical analysis on the tablet strength based on particle–particle bond strength was also carried out to explain the experimental results.
The latest state of the art on Discrete Element Method (DEM) and the increased computational power are capable of incorporating and resolving complex physics in comminution devices such as tumbling mills. A full 3D simulation providing a comprehensive prediction of bulk particle dynamics in a grinding mill is now possible using the latest DEM software tools.This paper explores the breakage environment in mills using DEM techniques, and how these techniques may be expanded to provide more useful data for mill and comminution device modelling. A campaign of DEM simulations were performed by varying the mill size and charge particle size distribution to explore and understand the breakage environment in mills using DEM techniques. Analysis of each mill was conducted through consideration of the total energy dissipation and the nature of the collision environment that leads to comminution.The DEM simulations show that the mill charge particle size distribution has a strong influence on the mill input power and on the way the energy is distributed across the charge. The smaller particles experience higher energies while the larger experience less, but this variation is strongly dependent on the mill size. The results also showed that the average particle collision energy increases with increasing mill size, whereas its distribution over particle size is strongly influenced by the mill content particle size distribution. The simulations also captured the energy distribution within different regions of the tumbling charge, with the toe impact region having higher impact energies and the bulk shear region having higher tangential energies. Regardless of the mill size most of the energy is consumed by the particles in the mid-size range, which has the highest percentage mass of the total charge distribution.
In order to develop a common mathematical and simulation platform for tumbling mills, a review of existing mechanistic models was made identifying the key aspects of grinding and slurry transport which are common to all tumbling mills. A mill model structure has been developed for all types of tumbling mills based on the population balance framework by incorporating breakage characteristics, slurry and solids transport, product classification and discharge, and energy consumption. A size-dependant breakage model developed by the JKMRC is applied. Transport is separated from breakage events and treated as a main sub-model in the new model structure. The model structure is based on dynamic time-stepping technique to enable dynamic simulation capability for non-steady-state simulation and control modelling. It is envisaged that the new model will cover a full range of milling conditions. It should enable a smooth transition between different mill types, such as from AG (Autogenous Grinding) to SAG to ball mill. The dynamic mill model structure developed here is the first step towards mechanistic modelling of grinding mills and provides great potential for the optimization of the comminution process
Powell MS, Hilden M, Ballantyne G, Liu LX, Tavares LM (2014)The appropriate, and inappropriate, application of the JKMRC t10 relationship, In: Yianatos J, Doll A, Gomez C, Kuyvenhoven R, (eds.), Proceedings of the XXVII International Mineral Processing Congress – IMPC 2014pp. 133-144
The t10 relationship developed by Narayanan and Whiten has underpinned a number of the JKMRC models and the characterisation of ore competence through impact testing with the drop weight tester. This usefulness is based on a consistent relationship between the t10 value and the overall size distribution that was noted for brittle rocks. However, like all characterisation tests this should be used only when the controlling conditions apply, which in this case is single impact breakage between two metal platens and within the range of sizes in which fragment sizes are normalizable. For any other mode of breakage the relationship should be confirmed before being applied. Soft ores or bimodal ores with widely different competence between dominant components have non-typical breakage signatures. Breakage via single point impact abrasion, compressed-bed breakage, low-energy surface breakage and incremental breakage, to name some of the other breakage modes, do not obey the general t10 relationship, so their outcomes cannot be used to provide a direct comparison. Inappropriate use of the t10 parameter can result in misleading conclusions about efficiency of energy use. These issues and alternative comparison techniques are presented and discussed in terms of choosing appropriate measures of efficiency of breakage, ore competence and energy required for comminution.
The stability of agglomerated/pelletized ores is one of the key properties for successful heap leaching of complex, low-grade nickel laterite minerals. In this paper, single pellets of saprolitic and goethitic nickel laterite with controlled binder type (tap water and 44 wt.% H2SO4), binder content and pre-set porosity were made by a pellet press and subjected to mechanical strength and rewetting stability tests. The effect of fine/coarse particles ratio on the mechanical strength was also investigated using siliceous goethitic ore. The failure strength of the pellets under different drying conditions was measured and the time taken for the pellets to disintegrate under saturated (soaking) and leaching conditions was recorded. The results showed that, with the same type of nickel laterite, the time taken to disintegration during leaching test is proportional to the pellets tensile strength. Pellets with water as binder are more stable under soak conditions. Furthermore, failure strength for oven dried pellets is greater than that of air dried. With saprolitic nickel laterite (SAP) pellets, their mechanical strength and re-wetting stability can be enhanced by drying the wet pellets or by increasing the binder content in the pellets. The pellets mechanical strength was found to be a good indication of their stability under leaching conditions as well. However, no relationship between the two was observed for goethitic nickel laterite pellets.
A mill model structure based on the population mass balance framework incorporating breakage characteristics, slurry and solids transport, product classification and discharge and energy consumption was reported in XXVII IMPC and named the Generic Tumbling Mill Model Structure Version I (GTMMS I). In this work, a new 4D (four dimensional) appearance function sub-model based on the experimental results of the JK Rotary Breakage Tester (JK RBT) was developed to describe the breakage characteristics and was applied to this new model structure. The newly developed 4D appearance function model has fewer fitting parameters and is more generic in nature. Most importantly, it is applicable to both high and low energy impact breakage. It is therefore much more versatile in comparison to the existing JKMRC t10-tn appearance function and the size-dependant JK M-p-q t10-tn appearance function. In addition, the Discrete Element Method (DEM) energy distribution model was integrated into the new model structure. With DEM results providing energy distribution information inside the mill directly, the selection function and the back-calculation method used in the existing JKSimMet modelling method are not needed in this model structure. With the above two revolutionary innovations, the previous model structure was upgraded to the Generic Tumbling Mill Model Structure Version II (GTMMS II). Furthermore, the new model structure is dynamic in nature with time-stepping technique for non-steady-state simulation. A case of the dynamic grinding of a SAG mill was studied to validate the GTMMS II. The model simulation results agreed well with the plant data. With the newly developed 4D appearance function model, the incorporation of DEM energy distribution and transport function, the Generic Tumbling Mill Model Structure Version II (GTMMS II) is an important step forward towards mechanistic modelling of tumbling mills.
In this work, a nucleation model that includes nuclei breakage/fragmentation is proposed. The model is based on the nucleation model of Hapgood and the Stokes deformation number calculated from the granule dynamic yield strength from the previously reported granule breakage work. It is proposed that breakage or fragmentation of primary nuclei from binder spray will occur if the Stokes deformation number exceeds a certain critical number. In the case where breakage occurs the model for secondary nuclei size distribution is proposed. To validate the model, the characteristics of the primary nuclei formed from nickel laterite ores with diluted sulphuric solutions as a binder were investigated. The nuclei were produced by dropping the binder solution onto a stationary powder bed. The mechanical integrity of the primary nuclei formed, the relationship between the nuclei diameter and binder drop diameter were studied. The Stokes deformation numbers for nickel laterite powders with different particle size in a lab scale drum granulator were calculated and the nuclei size distributions with different nickel laterite feed powders are predicted.
Heap leaching is a widely used extraction method for low-grade minerals including nickel and cobalt. Agglomeration of fine mineral particles as a precursor to heap leaching is an important means of enhancing leaching rates and metal recoveries. Single pellet leaching behaviour of three nickel laterite ores, namely siliceous goethitic (SG), goethitic (G) and saprolitic (SAP) was investigated to assess the effect of pellet properties (binder type, binder content, porosity and dryness) on its stability, initial leaching rate and maximum Ni recovery. The column leaching performance of agglomerates of the same ores was also investigated. Both single pellet and column leaching tests showed that the ore mineralogy played a major role in the Ni extraction rate, with G-type of ore the lowest. The Ni extraction rate was also found to be directly related to the pellet/agglomerate dryness and the highest rate was obtained at an intermediate degree of dryness due to the better wetting and diffusion of acidic lixiviant into the pores in between the particles. However, no significant effect of drying on the stability of the agglomerates (measured by agglomerate slump in the column) was found in column leaching. For G type of ore, mixing it with high clay ores during agglomeration is recommended to enhance its robustness during leaching process.
Single particle breakage characterisation at fine sizes for use in mill modelling has been addressed by only a few researchers and is not utilised in engineering design. This is mainly due to the challenge of accurately imparting a range of impact energies to sub-millimetre particles and then measuring the progeny size distribution for the tiny resultant mass. In order to fill this gap, a dispersed monolayer multi-particle breakage method was applied with a mini JK Drop weight tester in this work to extend the single particle breakage test from 16 mm down to 425 μm, covering a specific energy (Ecs) range of 0.1 - 2.5 kWh/t to provide a wide range of test conditions. A challenge that had to be addressed was switching from single particle to dispersed mono-layer due to the physical constraints of drop-height and drop mass in maintaining accuracy in input energy over the orders of magnitude required to apply the required specific range of energy input. As only a limited size range could be subjected to both single particle and mono-layer bed breakage, it was necessary to establish if the two testing techniques provide the same breakage results. A novel application of the Fréchet distance was successfully applied to quantitatively evaluate the discrepancy of progeny size distribution between single particle breakage and monolayer multiple particle breakage. Extrapolation of an empirical Fréchet distance model indicated that the application of dispersed mono-layer breakage below 2 mm provides an acceptable comparison with the single particle breakage applied to coarser sizes, thus facilitating the fitting of a single appearance function across this wide range of sizes and applied breakage energies.
A two-dimensional, along-the-channel, two-phase flow, non-isothermal model is developed which represents a low temperature proton exchange membrane (PEM) fuel cell. The model describes the liquid water profiles and heat distributions inside the membrane electrode assembly (MEA) and gas flow channels as well as effectiveness factors of the catalyst layers. All the major transport and electrochemical processes are taken into account except for reactant species crossover through the membrane. The catalyst layers are treated as spherical agglomerates with inter-void spaces, which are in turn covered by ionomer and liquid water films. Liquid water formation and transport at the anode is included while water phase-transfer between vapour, dissolved water and liquid water associated with membrane/ionomer water uptake, desorption and condensation/evaporation are considered. The model is validated by experimental data and used to numerically study the effects of electrode properties (contact angel, porosity, thickness and platinum loading) and channel geometries (length and depth) on liquid water profiles and cell performance. Results reveal low liquid water saturation with large contact angle, low electrode porosity and platinum loading, and short and deep channel. An optimal channel length of 1 cm was found to maximise the current densities at low cell voltages. A novel channel design featured with multi-outlets and inlets along the channel was proposed to mitigate the effect of water flooding and improve the cell performance.
We conduct Observing System Simulation Experiments (OSSEs) with Lagrangian data assimilation (LaDA) in two-layer point-vortex systems, where the trajectories of passive tracers (drifters or floats) are observed on one layer that is coupled to another layer with different dynamics. Depending on the initial position of the observed tracers, the model studied here can exhibit nonlinear features that cause the standard Kalman filter and its variants to fail. For this reason, we adopt a Monte Carlo approach known as particle filtering, which takes the nonlinear dynamics into account. The main objective of this paper is to understand the effects of drifter placement and layer coupling on the precision skill of assimilating Lagrangian data into multi-layered models. Therefore, we analyze the quality of the assimilated vortex estimates by assimilating path data from passive tracers launched at different locations, on different layers and in systems with various coupling strengths between layers. We consider two cases: vortices placed on different layers (heton) and on the same layer (non-heton). In both cases we find that launch location, launch layer and coupling strength all play a significant role in assimilation precision skill. However, the specifics of the interplay of these three factors are quite different for the heton case versus the non-heton case. © 2014.
Dry granulation is commonly used in the pharmaceutical industry for compressing heat and moisture sensitive feed materials into compacts, subsequently followed by milling. Population balance models (PBMs) are often used to explore the effects of milling conditions on the granule size distribution (GSD) but limited studies have investigated the effects of the feed material and ribbon properties on the resulting GSD. In this work, a variety of feed materials and ribbons with different mechanical properties were used to validate a mass-based bi-modal breakage function developed in a previous study (Olaleye et al., 2019). Ribbon like tablets (referred to as ribblets) with a range of precisely controlled porosities were produced using an Instron machine and pharmaceutical excipients including the microcrystalline cellulose MCC 101, MCC DG and a DCPA/MCC mixture. Roll compacted ribbons were also produced using MCC 102 and MCC DG excipients. The ribblets and ribbons were milled in an impact-dominated cutting mill and PBM parameters were obtained from the ribblet milling data. Mechanistic models related to the feed ribbon property were then developed. It was found that the PBM with the mass-based bi-modal breakage function can accurately predict the GSDs of both the milled ribblets and roll compacted ribbons. The model developed was successfully linked to ribbon properties such as porosity for the first time and the model parameter a that reflects the fines mode in the bi-modal breakage function increased linearly with ribblet porosity. This work demonstrates the versatility of the developed PBM and provides a systematic approach for describing the ribbon milling process.
The breakage characteristics of a two-component ore in a confined bed was studied in this work in order to predict the breakage behaviour of a multi-component ore from the properties of individual components. Bed compression tests with each single component and blended components at different bulk volumetric ratios were carried out at different volume based specific energies and the bed particle size distributions of each component in the mixtures were measured (through magnetic separation). The experimental results show that the breakage product size measured by t10 – the cumulative mass percentage of particles less than 1/10th of the feed size, is linearly proportional to the relative bed porosity reduction, for both single component and multi-component ore. There is a minimum bed porosity reduction to be reached before any breakage occurs and the less competent the ore, the lower the minimum porosity reduction value. Theoretical analysis on bed compression breakage shows that the relationship between product size measured in t10 and the specific comminution energy (Ecs) is not unique and is dependent on the testing conditions. The analysis shows the importance of testing bed breakage at conditions that are independent of bed configurations. Furthermore, models for predicting the product t10 and specific comminution energy of multi-component ore from single component compression data are developed. By compressing the particle bed at the set porosity reduction for each component and few multi-component tests, one can predict the mixture product and mixture energy consumption at any mixture ratios without the needs to physically separate the products in the mixture test.
Optimisation of grinding circuits is invariably dependent on sound process models together with process simulators that can solve the process models accurately. Most of the process models are solved numerically because analytical solutions are not available, which can lead to errors in the results due to the numerical approximation of mathematical equations. Whiten , and Valery Jnr & Morrell [2, 3] have developed a dynamic model with numerical simulation for autogenous and semi-autogenous mills, and validated the model with dynamic response of mills in terms of power draw, grinding charge level, slurry level and product size distribution to changes in feed rate, feed size, feed hardness and water addition [2, 3]. In this work, an analytical solution for their dynamic model of tumbling mills has been developed based on the knowledge of solutions to the first-order nonhomogeneous linear differential equations. Two algorithms, Direct Single Time method (DST) and Direct Multiple Time method (DMT), were applied to obtain the analytical solutions respectively. It was found that analytical solutions are more accurate than the traditional finite difference numerical methods. However, the DST analytical method has a drawback of numerical instability due to the accumulation of round-off errors which are amplified by exponential functions, whilst the DMT method can provide stable solutions. To test the DMT analytical method, two cases of SAG mill dynamic operation were studied with both the traditional numerical method and the newly developed analytical method, further proving the robustness and feasibility of the analytical solutions.
The Appearance function, also known as breakage distribution function, is used to describe the breakage characteristics of an ore impacted with a certain energy. It is the bedrock of comminution modelling. The range of applicability of the majority of existing appearance functions is limited to coarser sizes above a few millimetres. In the previous work, a 4D (four dimensional) appearance function model was developed based on JKRBT test data, but its applicable range was not sufficiently broad at −24.4 + 7.3 mm. In order to develop a more versatile appearance function model that can be used for a wide range of energy levels and feed particle sizes, drop weight tests for smaller particles with sizes ranging from 425 μm to 16 mm were carried out with the Mini JK drop weight tester. Combined with data up to 63 mm from Standard JK Drop Weight Tests, the outcomes were fitted to two types of 4D appearance functions - the P80-m based 4D model and the P80-m-q based 4D model. The proposed 4D models are more accurate and scalable than existing models. Most importantly, they can be used for a wide range of conditions, with feed particle size ranging from 425 μm to 63 mm and input specific energy from 0.1 to 2.5 kW h/t in the initial test data.
The latest state of the art on Discrete Element Method (DEM) and the increased computational power are capable of incorporating and resolving complex physics in comminution devices such as tumbling mills. A full 3D simulation providing a comprehensive prediction of bulk particle dynamics in a grinding mill is now possible using the latest DEM software tools. This paper explores the breakage environment in mills using DEM techniques, and how these techniques may be expanded to provide more useful data for mill and comminution device modelling. A campaign of DEM simulations were performed by varying the mill size and charge particle size distribution to explore and understand the breakage environment in mills using DEM techniques. Analysis of each mill was conducted through consideration of the total energy dissipation and the nature of the collision environment that leads to comminution. The DEM simulations show that the mill charge particle size distribution has a strong influence on the mill input power and on the way the energy is distributed across the charge. The smaller particles experience higher energies while the larger experience less, but this variation is strongly dependent on the mill size. The results also showed that the average particle collision energy increases with increasing mill size, whereas its distribution over particle size is strongly influenced by the mill content particle size distribution. The simulations also captured the energy distribution within different regions of the tumbling charge, with the toe impact region having higher impact energies and the bulk shear region having higher tangential energies. Regardless of the mill size most of the energy is consumed by the particles in the mid-size range, which has the highest percentage mass of the total charge distribution.
Solid pharmaceutical tablets can be manufactured via three processing technologies: direct compression, dry granulation and wet granulation before the final compaction process. Dry granulation using roll compaction generally involves production of ribbons, followed by milling to produce granules and the production of tablets. The mechanical properties of ribbons produced during roll compaction can influence breakage behaviour in a mill and hence the performance of a drug formulation. Therefore it is important to explore critical factors that determine the quality of ribbons. In this study, MCC Avicel PH-102 was used as the model powder for roll compaction and the critical operating factors affecting ribbon quality were studied. MCC 102 ribbons were manufactured at a range of process conditions (roll speed, feed screw speed and compaction pressure) using the TF mini roll compactor with a serrated die & punch (DPS), grooved roll surface. The key ribbon properties measured are the powder feed ratio (defined as the ratio of feed screw speed to that of roll speed) and mass throughput ratio (defined as the mass of ribbons produced in a given time to that of fines). The porosity of ribbons was observed to depend significantly on powder feed ratio into the compaction zone. Porosity remained constant at a fixed feed ratio irrespective of the absolute roll speed and the feed screw speed conditions used. Mass throughput ratio is another factor that was introduced to describe process efficiency i.e. to draw comparison between the amount of ribbons and fines. The amount of ribbons and fines generated at a given process condition depends significantly on the feed screw speed and the compaction pressure. In addition, ribbon porosity decreased with increasing pressure but there was negligible effect on ribbon porosity when excessively high pressure (> 60 bar) was used. Ribbon porosity was also observed to decrease with increasing feed screw speed but the converse is true with increasing roll speed. In conclusion, the impact of various roll compaction process parameters were critically investigated and a detailed insight into the main factors (powder feed and mass throughput ratios) that govern the behaviour of roll compacted ribbons was provided. In future, the focus is to relate these process properties (and ribbon porosity) to granule size distribution from a milling process.
Milling of roll compacted ribbons is a commonly used unit operation in the pharmaceutical industry to improve the manufacturability of fine powders. In this thesis, two computational techniques, the Discrete Element Method (DEM) and the Population Balance Modelling (PBM), are combined into a coupled DEM-PBM framework, to investigate the micro-mechanics of ribbon breakage in milling. The effects of interfacial energy and porosity on the mechanical properties of ribbons and the effects of interfacial energy, impact velocity and abrasion velocity on ribbon fragmentation during impact and abrasion tests are explored. On the effects of interfacial energy and porosity on the mechanical properties of ribbons, it is found that the tensile strength of the ribbon is a linear function of the interfacial energy and an exponential function of the porosity. An equation to evaluate the ribbon’s tensile strength, from both the porosity and the interfacial energy is then derived. The proposed equation, which is similar to the Ryshkewitch-Duckworth (RD) formula that considers only the porosity, can be considered as an extension of the RD equation to consider the interfacial energy effect. On the impact and abrasion tests, mathematical models are derived to describe the dependency of the number of large fragments and the fraction of fines (small fragments), resulting in DEM ribbon impact and abrasion tests on the interfacial energy, the impact velocity and the abrasion velocity. It is found that these models, when used as input for PBM, in a multiscale DEM-PBM modelling framework, reasonably well predict the experimental data for the impact tests and are in good agreement with the experimental data for the abrasion tests.