ARR - Annual Report
The paper shows the possibility to produce alumina nanoparticles in a stirred media mill by an appropriate adjustment of the suspension properties and the milling parameters. Besides a high electrostatic suspension stability that can be realised for metal oxides by means of pH value adjustment small grinding beads favour the production of alumina particles with a median particle size of around 10 nm. In addition to size reduction mechanochemical changes and the formation of alumina hydroxid are detected during wet grinding of alumina. This is analysed by means of X-ray diffraction analysis, (XRD), thermogravimetry (TG) and dynamic scanning calorimetry (DSC) measurement and a quantitatively good agreement between the three methods could be obtained. Further, it is proved that the hydroxide produced dissolves at pH values lower than 5 thus influencing the grinding process under these conditions.
The increasing industrial demand for nanoparticles challanges the application of stirred media mills to grinding in the sub-micron size range. It was shown recently [1] that the grinding behavior of particles in the sub-micron size range in stirred media mills and the minimum achievable particle size is strongly influenced by the suspension stability and thus the agglomeration behavior of the suspension. Therefore, an appropriate modeling of the process must include a superposition of the two opposing processes in the mill i.e. breakage and agglomeration which can be done by means of population balance models. Modeling must now include the influence of colloidal surface forces and hydrodynamic forces on particle aggregation and breakup.
In this report the modeling of the sub-micron grinding is done by a superposition of the population balance models for agglomeration and grinding with the appropriate kernels. This leads to a system of partial differential equations, which can be solved in various ways numerically. Here a modified h-p Galerkin algorithm which is implemented in the commercially available software package PARSIVAL developed by CiT (CiT GmbH, Rastede, Germany) and the moment methodology according to Diemer [2], [3] are used and compared. This includes a comparison of the derived particle size distributions, moments and its accuracy depending on the starting particle size distribution and the used agglomeration and breakage kernels. Finally, the computational effort of both methods in comparison to the prior mentioned parameters is evaluated in terms of practical application.
Furthermore the fundamental work on the agglomeration process and its mechanism described in the IFPRI report 2002 was continued and improved. Experiments are performed on a well-characterized, model system of monodisperse primary nanoparticles that are destabilized and aggregated under various milling conditions. Conditions spanning Brownian to turbulent collision aggregation in model stirred media mills are explored to study the effects of colloidal stability on the aggregation process. The agglomeration kinetics are measured using dynamic light scattering (DLS) as a function of particle and electrolyte concentrations. Further information on the agglomeration process and the structure of the agglomerates are also obtained from small angle neutron scattering (SANS) and rheo-optical light scattering experiments (ROA). Theoretical predictions based on independently measured particle and solution properties as well as mill characteristics are compared against the experimental results to demonstrate that particle aggregation kinetics in a stirred media mill can be controlled through the colloidal interactions and the milling conditions. This research provides a theoretical basis for understanding stirred media milling of nanoparticle slurries and as such, is a step towards a predictive model of sub-micron stirred media milling.
Finally the paper reports about mechanochemical changes during wet grinding of alumina in stirred media mills. The results show the formation of alumina hydroxide that can be analysed by means of X-ray diffraction analysis, thermogravimetry and dynamic scanning calorimetry. The formation of hydroxide is strongly dependent on pH value which influences the grinding mechanism in the nanometer size range.
With using AFM, measurement of force (force curve) working on a single particle holding electrostatic charge generated at contact with metal target was conducted. As sampl, PS particles with 100 micro-meter in diameter were used, and 8 kind of metal as Al, Au, Cr, Ni, Pt, Ti, Zn, and Zr were used as the target on which every surface was polished and mirror finished. Results can be summarized as follows:
- First of all, force curve was successfully measured with the method, and the measurement procedure was established.
2. By focusing to “force curve” measurement, not looking at only maximum adhesive force, electrostatic interaction was successfully observed by separating other interactions as liquid bridge and intermolecular force.
3. The force curve attributed to the electrostatic interaction was analyzed by theory with an approximation of disk-to-disk interaction based on image force method. The fact of successful agreement between the observed force curve and theory revealed that the force curve observed can be surely attributed to the electrostatic interaction, and that the amount of charge on the particle and the radius of the charged (contact) area can be estimated from the analysis.
4. The order of magnitude of the measured charge density was 10-2 C/m2, which is much greater than that obtained with “impact charging experiment,” which has been previously studied, as 10-4 C/m2.
5. From the fact, it was concluded that the force curve measurement with AFM can catch the net amount of the charge generated before charge relaxation due to gas discharge takes place.
6. The net charge generated was revealed a good compatibility to the conventional simple condenser model based on metal-to metal contact model with the term of contact potential difference in its order of magnitude.
7. No correlation was revealed for the charge density measured against work functions of metal targets, regrettably.
In the respect, subject left over could be summarized as:
- More careful experiments and data accumulation, especially in the terms of reproducibility and accuracy, will be required to establish the method in the near future studies.
2. Because the work functions used in the analysis were from literature, it might be necessary to measure their real values practically.
3. Variation of sample particles with different size and materials are interesting and necessary to be addressed.
The investigations of the last five years have shown that a production of stable suspensions of hard ceramic materials with a median particle size smaller than 10 nm is possible by wet grinding in a stirred media mill. Nevertheless, no absolute limit of the grindability was yet reached.
By means of experimental results the influence of the electrostatic stabilization on different pH-values during the comminution of fused corundum on the grinding progress and the grinding media wear is discussed.
Results for comminution of fused corundum with grinding media of different diameters between 100 μm and 1300 μm and stirrer tip speeds between 6 m/s and 15 m/s are presented and discussed with regard to the grinding progress and the grinding media wear.
The particles in stirred media mills are stressed and ground between two grinding beads, one grinding bead and a wall or between a grinding bead and an agitator element in zones of high energy density [1, 2, 3]. Only for a desagglomeration or a desintegration of microorganisms shear forces are a further grinding mechanism.
During the approaching process of two grinding beads or a grinding bead and a wall the fluid between the collision partners will flow out of the gap. The smaller the product particles are, the better they will follow the fluid out of the active volume being the volume in which particles can be captured and stressed [4, 5].
With decreasing product particle sizes two opposite effects can be observed:
- While two grinding media approach each other, the fluid between them will be displaced. Wereby particles will follow the fluid flow out of the gap more easily the smaller the particles are. This may lead to media contacts without any grinding.
- On the other hand, with decreasing product particle size the absolute number of particles increases with the reduction factor to the power of three. Thus, a capture and stressing of more than one particle or a particle bed is possible.
The loss of kinetic energy during the collision is partly used for comminution. It has to be asked, how the energy is transferred to the feed particles. For that reason it is interesting to know how many particles are captured. According to the number of captured particles three cases can be distinguished [6]:
a) Only one particle is captured, which is stressed with the total energy (single particle stressing).
b) More than one particle is captured between two beads, all particles have contact to both beads during the stress event and all particles are stressed independent of each other. In this case at first that particle is captured, which has the largest size and/or which has the smallest distance to the connection line of both bead centers. This particle is stressed with the maximum energy. The particles, which are captured between the two beads after the first particle, are stressed with a considerably reduced energy. At the end of the stress event diverse single particle stressings with different intensities occur.
c) A particle bed is captured and stressed between two grinding beads.
Up to now it is not investigated how many particles are captured and stressed.
For a better understanding of the stress events in the sub micron particle size range, the particle motion in the displacement flow between a grinding chamber wall and an approaching grinding media was observed. Due to the small sizes of grinding beads and especially of product particles the product particle motion cannot be investigated optically in existing (real sized) stirred media mills. For that reason a model was developed. Parameters as volume concentration, approaching velocity and angle, model grinding bead diameter and material were varied.
Nanotechnology applications in the pharmaceutical, materials, and chemical industries has renewed interest in the use of wet grinding in stirred media mills for the production of nanoparticles. However, challenges arise in the production of sub-micron particles that are, in part, due to colloidal surface forces influencing slurry stability and rheology. As often observed in the literature, a grinding limit in the range of 0.5 μm is reached despite high energy inputs and aggressive milling conditions. Furthermore, the product size can even increase with increased energy input, a seemingly counterintuitive result that may be attributed to aggregation of fine particles during the comminution process. In this work we postulate that colloidal stability and rheology must be considered in wet grinding to understand these results and to surmount limitations on the production of nano-sized particles. Experiments are performed on a well-characterized, model system of monodisperse primary nanoparticles that are destabilized and aggregated under various milling conditions. Conditions spanning Brownian to turbulent collision aggregation in a model stirred media mill are explored to study the effects of colloidal stability on the aggregation process.
The agglomeration kinetics are measured using dynamic light scattering (DLS) as a function of particle and electrolyte concentrations. Further information on the agglomeration process and the structure of the agglomerates are also obtained from small angle neutron scattering (SANS) experiments both at rest and under flow. Theoretical predictions based on independently measured particle and solution properties as well as mill characteristics are compared against the experimental results to demonstrate that particle aggregation kinetics in a stirred media mill can be controlled by tailoring colloidal interactions and the milling conditions. Furthermore it is shown that the concept of electrostatic stabilization during wet grinding of nanoparticles can also be applied to the system of tin oxide. It is shown that in contrast to alumina no mechanochemical changes occur for the system of tin oxide during the wet grinding process. Thus the obtained median particle sizes are the result of pure mechanical grinding. In addition the suspension rheology in a stirred media mill as function of grinding time and inter particle interactions is studied.
This research provides a theoretical basis for understanding stirred media milling of nanoparticle slurries and as such, is a step towards a predictive model of nanogrinding in stirred media milling.
Summary of 3rd year
In this part of the project work a new mixing/extrusion concept was further developed to study the microstructure changes of a model system, which undergoes a transition from a 3- phase wet powder to a 2-phase concentrated suspension system (3P2S Transition) (Arancio and Windhab, 2003).
The further developed model system used consists of:
- powder/binder: liquid fat melt/oil (e.g. cocoa butter)(i)
- powder/solid filler: hydrophobic silica particles (Sip.D17, Degussa, (D)) (ii)
The first advantage of using a fat melt, which is in the fully liquid state at moderately elevated temperatures (eg. 40°C for cocoa butter), was given by the ease of solidification close to room temperature. This made investigations of the wet powder/suspension microstructure possible without additional structural changes during sampling from the process and during sample preparation. The second advantage proved was the good homogeneous mixing ability of the hydrophobic silica powder particles within the fat binder, even at extremely low binder fraction due to the fact that the binder fat melt was cold sprayed, thus producing small solidified binder particles. These solidified binder particles were mixed with the filler particles under NIR-based homogeneity control. If this powder mixture was heated up to 40°C, a significantly better homogeneity of the mixture of powder and liquid binder was obtained compared to the conventional/alternative process of mixing by spraying the low fraction of liquid binder into the filler powder (formation of local lumps).
The performed process consisted of the following steps:
- spray cooling of the binder-fluid phase in order to obtain a solid fine binder powder
- mixing of this binder powder with the filler powder and mixing quality control by near infrared spectroscopy
- pre-compaction of the model system “powder/binder powder mixture (PBPM)” in a mechanical testing machine (Zwick)
- ram extrusion of the pre-compacted system
- micro-structural investigation of the extruded product
The powder/binder mixture in the solid powder state of the binder (A) was used to investigate the local stresses/stress distribution leading to deformation of the solid cocoa butter (binder) particles. This allows identifying the zones where the transformation from a three phase wet powder to a two phase concentrated suspension (3P2S) preferably happens. The solid liquid mixture above the melting temperature of the binder (cocoa butter) (B) reflects the powder binder system in the normally applied state, with air additionally entrapped. The acting pressure and shear stresses lead to a synergistic reduction/compression of the air included until the 3P2S transformation takes place. The impact of pressure, shear and pressure + shear on the resulting powder/binder microstructure were investigated in detail and 3P2S phase diagrams derived, which form a new basis for design calculations of extruders/extrusion processes and extrudate property optimization.
The aim of this project is to enhance our understanding of how solvents affect the polymorphic forms of compounds. Polymorphism is very important to the pharmaceutical industry as a compound’s polymorph can exhibit different physical properties, some more desirable than others. Being able to control which polymorphic form is nucleated is thus very useful.
To study the effects of solvent type on polymorphism, enantiotropic compounds will be used. These compounds have multiple polymorphs but the distinguishing feature about them is that there is a transition temperature where the solubilities of the polymorphs are equal. This transition temperature is constant regardless of the solvent type as at this temperature the free energies of the polymorphs are equal. This allows us to study the effects of solvent type by decoupling it from the effects of the supersaturation.
The work done in this first year is primarily aimed at identifying the compounds which are enantiotropic in nature, selecting a model compound, characterizing it and determining its exact transition temperature. Subsequently, we will perform experiments using different solvents at the transition temperature and measure both induction times and nucleation rate.
Project Overview
In the first year of this project, a multidimensional population balance model has been developed for a general granulation process. The model details the evolution of particles with respect to distributions in size, porosity, and binder (moisture) content the three critical properties that are related to processing as well as end product properties.
Model Utilization
The model has been utilized to determine the control-relevant properties of the granulation process. The influence of changes in the binder addition rate as well as the agitator speed were determined using sensitivity analysis of the population balance model.
Results and Insights
The results provide insight into the degree to which size, porosity, and moisture content can be adjusted in a feedback control model of operation. The present analysis is flexible enough to address both batch and continuous granulation.
Implications
The insights generated from this analysis point to more effective strategies for the improved operation of industrial granulators, notably with regard to reduced recycle streams (both ¯nes and oversized particles).
The aim of the project is to establish a relationship between the product properties and feed material and the mill functions for milling of organic solids. The specific objectives are:
- to characterise the physical, mechanical, and thermal properties of model organic (feed) materials (material function) at the single particle level, and to examine the effects of temperature and humidity on these properties
- to investigate the breakage behaviour of single organic particles under various conditions
- to characterise the properties of milled products, and to correlate the product properties to material and mill functions Models materials that are planned and approved for use in the project by the TC of IFPRI include aspirin (low hardness, relatively high fracture toughness, ductile), á-lactose monohydrate (áLM, high hardness, high fracture toughness, semi-brittle), sucrose or sorbitol (high hardness, relatively low fracture toughness, regarded as brittle in the literature), starch (ductile and mechanical properties greatly affected by the strain rate), and microcrystalline cellulose (MCC, medium hardness, high fracture toughness, semi-brittle). These materials cover a fairly wide range of physical, mechanical and thermal properties, hence ensuring generality of the results to be achieved. This report summarises the work done over the first year on five model organic materials, á-lactose monohydrate (áLM), sucrose, sorbitol, starch and microcrystalline cellulose (MCC). The work includes extensive experimental investigation into the behaviour of both the single particle impact breakage and the bulking milling under various conditions, and preliminary mathematical modelling based on the distinct element method (DEM) and the population balance method. Also included in the report are some results of the milling of MCC and áLM obtained by the investigators before the project was started. The single particle impact experiments provide data of the breakage extent of single particles as a function of impact velocity, which is then used to infer the physical and mechanical properties of the tested particles by utilising the Ghadiri-Zhang model for semi-brittle materials. The results show that the modes of failure of the five tested materials agree well with that reported in the literature. Sucrose and sorbitol, which are generally regarded as brittle materials, demonstrate the highest extent of breakage. On ii the other hand, starch and MCC, two materials regarded as more difficult to mill, show the lowest extent of breakage. Sub-ambient impact experiments are conducted on áLM, sorbitol and MCC. The results suggest that the breakage propensity, thus the mechanical properties, of áLM and MCC are little affected by the temperature, but a lower temperature is seen to reduce the breakage extent of sorbitol particles. The bulk milling experiments are conducted in a single ball mill and the results are quantified by an analogy to the first-order rate process. The results show that milling of both MCC and áLM is little affected by the temperature, in agreement with the single particle testing results.
- Attempts are made to relate the single particle impact breakage behaviour to the bulk milling behaviour. For MCC, starch and áLM, the milling behaviour expressed by the milling rate constant relates linearly to the single particle parameter containing the physical and mechanical properties of the particles. However, the relationships for sucrose and sorbitol are non-linear, indicating alternative methods should be explored for these types of materials. The relationships between the single particle breakage and the bulking milling behaviour are different for different materials. Preliminary efforts are made to unifying these relationships. This is achieved by quantifying the mill function with a term called ‘milling power’ predicted by DEM simulations of motions of both the milling media and the feed particles. The new bulk milling parameter is shown to relate to the single particle behaviour well and a unified relationship is obtained for áLM, starch and MCC. Preliminary modelling work using the population balance method has been carried out on the milling of sucrose in collaboration with Dr R Bertrum Diemer of DuPont. The results will be reported in the future.