ARR - Annual Report
The ARR53-03 report presented some further results obtained from both laser triangulations and multiple projection techniques. It contains new developments about 3D image processing of particles with special reference to the use of a Euclidean distance function as a tool to address different shape properties (dissolution; wear; etc.). A case study concerning the 3D analysis of a Zn powder is presented. The size distribution curve is discussed and presented using different parameters to compare with laser diffraction data. The geometrical interpretation of both 2D and 3D image analysis data is very coherent and demonstrates if needed, the sensitivity of laser diffraction to shape. Because of randomized particle orientation and non spherical shapes the laser diffraction results show a wide size range and an overestimation of large sizes whereas image analysis reveals a rather well calibrated distribution.
Despite considerable efforts that have been made in the past with regards to milling, no single model has the capability to predict the milling behaviour of materials. The objective here is to further understand the role of material properties in breakage, under the influence of temperature. The methodology is to characterise the mechanical properties of materials at the single particle level using quasi-static nano-indentation and dynamic impact tests. The effect of temperature on the breakage behaviour of these materials is investigated under dynamic impact conditions and the effect of temperature on mechanical properties indirectly inferred. Furthermore, dynamic impact breakage is compared to bulk milling behaviour, under the influence of temperature. A widely used pharmaceutical active ingredient, Aspirin, and two excipients sucrose and -lactose monohydrate (-lm) are chosen as model materials. All three are regarded as semi-brittle materials with different mechanical properties.
Granulation is a complex process in which many input variables influence many product properties. As Iveson et al. describe in a review paper (2001), the understanding of the fundamental processes that control granulation behavior and product properties have increased in recent years. This knowledge can be used during process design, in choosing the right formulation and operating conditions, and it can also be used to improve process control. Although many variables are set constant during process design, variations during production in input variables occur due to the variable nature of the powder feed. Even if all granule properties, except for size, are ignored for process control, a one dimensional granule size distribution can be constructed by multiple discrete output variables, in order to represent the shape of the distribution (these can be mean sizes (with coefficients of variation), percentile sizes, moments 4 or size bins). Model Predictive Control (MPC) is an effective method to control such multiple input, multiple output processes (García, et al., 1989). More details motivating the choice of MPC for granulation processes along with examples from the literature can be found in previous reports (IFPRI# ARR51-02 and IFPRI# ARR51-03).
Partially saturated powders consist of a bulk solid whose cavities are to a certain extent lled with binder liquid. The presence of air signicantly aects the ow behavior of such systems due to interparticle frictional forces showing a strong normal force dependency. The role of the binder liquid is ambiguous as it increases normal forces between particles by capillary suction pressures and surface tensions at the contact line between liquid and solid but on the other hand reduces the coecient of internal friction between the particles by acting as a lubricant on the particle surface. Additionally interlocking eects between particles can signicantly hinder shear ow when partially saturated powders are in a densely packed state. Thus the extrusion of wet powders with minimised binder liquid fractions is often impeded by blocking of the material in the die inlet region. However, under certain process and geometrical conditions shear and pressure forces in the die inlet region can transform the 3-phase-system (3P) into a state where increased saturation as well as an orientation of the particles in shear layers reduce the resistance against ow. The reduction of the entrapped air cells ideally results in a local saturation of the system making it a 2-phase-suspension (2S) with viscous friction instead of Coulomb solid friction dominating its ow behavior. Small-scale ram extrusion combined with visual observations can be used in order to quantify critical stresses which have to be surmounted before reaching steady-state ow. Critical stresses are in uenced by particle properties (size/size distribution, shape and surface), binder liquid viscosity and particle/binder liquid interfacial properties (contact angle, interfacial tensions). An understanding of the mechanisms of wet powder ow and the interdependencies of process and material parameters during extrusion can be used for the design of product specic tailor-made processes and allow for processing of powder/binder systems with minimized binder fraction, thus reducing potential drying costs.
Project Report
We report here on work performed on the project in its third year ending October, 2009. Two previous reports have been submitted to IFPRI in 2006 and 2008, respectively. The present research is focused on the study of Powder Mechanics and the ultimate goal is to develop a quantitative description of active flows of a wide variety of powders. The study is centered on the slow, frictional and the dense, “intermediate” regimes of flow where both frictional and inertial effects are important. The novelty of the project is the study of a large range of materials and flow geometries to gain meaningful insight.
Materials and Methods
We report on a series of materials from simple (round beds) to complex (fine, odd-shaped and elastic), used in an axial-flow Couette, high shear mixer, in a centripetal geometry characteristic of a “spheronizer and two hopper flows with a funnel and a flat bottom, respectively to measure stresses and their fluctuations as a function of geometry and shear rate.
Main Novelty
The main novelty during this year is the measurement of porosity (void fraction) and porosity distribution in the flowing material using a capacitance probe in addition to stress measurements.
Executive Summary
The aim of this study is to understand the behavior of mineral particles in concentrated electrolyte solutions using surface force techniques. To this end there are two significant challenges.
- The first challenge relates to the type of surface forces that dominate at high electrolyte concentration. They are very short in range and poorly understood theoretically, but it is known that they are related to the solvation of the surface layer of a material or ions adsorbed to that layer, hence they are called salvation forces, or in aqueous solutions, hydration forces.
- The second challenge is to prepare surfaces that are suitable for investigation by surface force measurement techniques and is intimately related to the first challenge as the very short range over which hydration forces operate requires that surface roughness is controlled at a level comparable to or less than the range of the hydration forces.
At this point in the project we have successfully produced titania and alumina surfaces that are ideally smooth. The alumina surfaces have proved to be unstable in electrolyte solutions, though it was possible to measure the forces between alumina surfaces bearing adsorbed molecules that passivate the surface to dissolution. We have some evidence that stable Alumina surfaces can be formed by annealing the Ald surfaces and this is related to an increase in crystallinity. Investigations using Titania surfaces are continuing both in salt solutions and in the presence of both cationic and anionic surfactants and in the presence of a range of organic acids.
General project outline and background
The structure of granules or agglomerates can be defined as the spatial arrangement of its basic components [1]. The basic components are primary particles, binder or liquid components and intra particle porosity. The quantification of granule structure is crucial for setting up processing maps and input data for simulations such as DEM and computational modelling [2]. Spatial statistics, also called stereology, provide well-defined measures to quantify the different aspects of powder structure. For infinite systems the covariance function (CVF) defines the distance relations of the particles and pores. In agglomerates a radial distribution function (RDF) quantifies shell or boundary structures. These statistical measures are well suited as a basis for stochastical property modelling. Several image analysis procedures provide the above mentioned data, e.g. morphological sieves, point sampling techniques or chord length measurements. The characteristic curves can also be correlated with stochastical processes e.g. Poisson processes. While structure measures have been determined in a number of applications, the fundamental combination of physical properties and structure is still immature. The parameter of the above mentioned stochastic models need to be linked to the key parameter of the structure generation process e.g. primary particle size, binder content, stress and growth history. Results of other simulation techniques like VOF (Volume of fluid) or Population Balance models can serve as input to generate the correlation between statistical parameter and process. The added value of a structure model would be its statistical reliability and ease of use for parameter variations. Some key powder properties are well understood physically [3-6] but lack systematic incorporation of structure measures above phase volume and particle size. Mathematical techniques to calculate structure dependent properties include cellular automata and convolution algorithms. The additional challenge is to convert local behavioural probabilities into bulk properties.
Section I. Project results: University of Delaware
Our primary goal is to trace whether there is a connection between gelation and the attractive driven glass. If gelation is the result of an arrested phase separation, then the gel line may be an extension of the attractive driven glass line down into the two-phase region of the colloidal phase diagram. Likewise, if the local microstructure of a gel is similar to a glass, then the high density regions in a gel may have the same microrheology as a glass. Observations of the potential similarities between the microstructure of the high density regions of a gel and the attractive driven glass could potentially be missed if gels were only studied using bulk rheology techniques where measurements would be an average over the entire structure. Thus, the development of microrheology is critical for understanding the origin and properties of colloidal gels.
Our work this year focused on characterizing interaction potentials in the model colloidal suspensions used for microrheology experiments. Model suspensions are finely tuned to enable simultaneous optical micromanipulation using laser tweezers and direct confocal imaging of the suspension microstructure. At the same time, gravitational sedimentation is minimized through density matching. One of the key problems has been understanding charging and the electrostatic repulsion that arises between particles in the brominated solvents used in these suspensions. The first half of our work focused on characterizing these suspensions. This has led us to begin developing replacement model systems in order to better control the colloidal interactions in these studies.
Section II: Project results: University of Michigan
Microstructural recovery of yielded colloidal gels
Introduction and Background
Colloidal gels are dynamically arrested, space-spanning networks of particles formed when the pair potential between colloids is strongly attractive at small separations. The structure, dynamics, and rheology of gels are strongly dependent upon the colloidal volume fraction. At low volume fractions, fractal clusters have been observed. At higher volume fractions, glassy states with slow dynamics are formed when short-range attractions modify the cage structure. At intermediate volume fractions, gels form with different morphologies depending on the interparticle potential: cluster-like networks form with strong attractions and contain large voids and corresponding aggregates of particles, while string-like networks form with under stronger attractions and consist of long, conjoined branches of particles. Gelation is thought to involve the competing processes between kinetic arrest, percolation, and equilibrium phase separation.
Gels and glasses undergo yielding when a sufficiently large strain is applied, resulting in fluidization and bond rupture. This process is thought to be characterized by the formation of large voids and aggregates. The mechanism and causation of yielding has been examined extensively using light scattering and neutron scattering. Alternatively, investigation of microstructural changes using optical microscopy techniques potentially allows identification of the exact spatial location of the particles in the gel network as they undergo deformation. Unfortunately, typical deformation rates for yielding, particularly those most relevant to industrial processes, do not allow tracking of individual particle trajectories. As a result, the analysis of orientations and trajectories of colloidal particles during flow has been limited to low shear rates.
Executive Summary
The aim of this study is to understand the behavior of mineral particles in concentrated electrolyte solutions using surface force techniques. To this end there are two significant challenges.
- The first challenge relates to the type of surface forces that dominate at high electrolyte concentration. They are very short in range and poorly understood theoretically, but it is known that they are related to the solvation of the surface layer of a material or ions adsorbed to that layer, hence they are called solvation forces, or in aqueous solutions, hydration forces.
- The second challenge is to prepare surfaces that are suitable for investigation by surface force measurement techniques and is intimately related to the first challenge as the very short range over which hydration forces operate requires that surface roughness is controlled at a level comparable to or less than the range of the hydration forces.
EXECUTIVE SUMMARY
Milling is a common unit operation 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 optimisation. This involves characterising 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 analysing 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.