ARR - Annual Report
The rheology of concentrated suspensions containing rigid fillers is important in many technological areas since it mirrors the dynamic behavior of the structuring units, such as disperse particle/particle aggregates as well as macromolecular components (binders) in the continuous fluid phase. For systems with high solid content, air is easily entrapped during different process steps, creating a compressible wet powder. The aim of the work is to investigate the phase transition between a 3-phase wet powder to a 2-phase system suspension (3P2S) occurring in the extrusion processing and to describe its mechanism. Extrusion processing of highly concentrated powder-binder systems always applies uni- or multiaxial shear and/or elongation flow fields, along the screw channel. The main Micro Structuring Mechanism (MSM) acting on the structuring units in such flow process are de-agglomeration, deformation, orientation and agglomeration. The increase of the pressure along the extruder channel and at the die zone, in combination with the high shear stresses present, compress and induce the plastification of the powder system. The 3P2S transition takes place in a well define layer of the final product creating concentric adjacent layer of wet powder and concentrated suspension. Modifying the system components, process steps and operating condition can achieve a different microstructure and distribution of the wet powder/- suspension zones.
The rheology of the model system has been investigated and the solid filler extensively characterized with respect to the agglomerate strength. The 3P2S transition has been investigated in the High Pressure Powder Shear reactor and in extrusion processing. The influence of the mixing quality can show a strong influence on the transition; thus different mixing process has been proposed and NIR spectroscopy has been used to evaluate the quality. Finally the product microstructure has been qualitatively analyzed with scanning electron microscopy (SEM).
Executive Summary
Uniform anatase-type TiO2 nanoparticles of different shapes have been formed by phase transformation of Ti(OH)4 gel matrix in the presence of shape controllers. For example, triethanolamine (TEOA) was found to change the morphology of TiO2 particles from cuboidal to ellipsoidal at pH above 11. The shape control can be explained in terms of the specific adsorption of TEOA to the crystal planes parallel to the c-axis of the tetragonal system in the alkaline range, as supported by the observation of preferential adsorption of TEOA to the crystal planes parallel to the c-axis at pH 11.5 and by the pH dependence of the adsorption to ellipsoidal particles. Diethylenetriamine (DETA) also modified the particle shape to ellipsoidal above pH 9.5 and the aspect ratio was much higher than with TEOA. The mechanism of the shape control could be explained in the same way as with TEOA, since analogous specific adsorption was observed with DETA as well. Similar shape control to yield ellipsoidal particles of a high aspect ratio was also achieved with other primary amines, such as ethylenediamine (ED), trimethylenediamine (TMD), and triethylenetetramine (TETA). However, secondary amines, such as diethylamine, and tertiary amines, such as trimethylamine and triethylamine, acted as a complexing agent of Ti(IV) ion to promote the growth of ellipsoidal particles of a low aspect ratio, rather than a shape controller to produce ellipsoids of a high aspect ratio. Sodium oleate and sodium stearate were found to modify the particle shape from round-cornered cubes to sharp-edged cubes. The mechanism was explained in terms of the reduction of the specific surface energies of the {001} and {100} planes of the tetragonal crystal system by the preferential adsorption of oleate or stearate ion to these planes, based on the adsorption experiment using ellipsoidal and cubic particles.
Introduction
There is enormous potential for utilisation of the solvent to control both the rate processes and product physical form characteristics in the isolation of active materials by crystallisation. This is because, while solvent crystallisation is widely used and solvent choice widely recognised to influence nucleation rates, crystal morphology and aggregation, little is known about the nature of molecular assembly processes in supersaturated solutions nor about the way in which solvents interact with and determine the structure of crystal faces.
This project aims to explore work on solvent effects in nucleation and growth processes, with the initial work exploring the nature between solution chemistry and crystallisation in a polymorphic system, tetrolic acid. This system has been chosen since, it is a simple monocarboxylic acid, CH3C=COOH, for which two crystal structures are known [1] and the difference between dimers and chains of carboxylic acids can be readily detected by a combination of Raman and infrared spectroscopy [2]. In the first year this detection strategy to differentiate between the two crystal packings of carboxylic acids was verified by a range of carboxylic acid and extended to saturated solutions, both yielding promising results. In second year, more emphasis was placed on tetrolic acid with attempts to crystallise and characterise both polymorphic forms, as well as solution spectroscopy utilising the optical probe, transmission cell and HC-32 cell accessories. In addition work has started on the use of atomic force microscopy to image the growing surfaces of molecular crystals with adipic acid chosen as the the first candidate for study.
The process of disintegration of liquid/solid suspension jets and sheets by atomization is analysed in a fundamental manner and visualized by suitable measurement methods, which allow qualitative and quantitative evaluation of the process. Supporting numerical analysis and theoretical derivations will contribute to basic understanding and control of the suspension atomization process. Model suspensions will be atomized by means of conventional and specifically designed atomizers.
The fifth year activities that are reported here include:
- Further Experimental investigations of suspension atomization in twin-fluid atomizer
- Experimental investigations of atomization suspension sheet in flat pressure nozzle
Model suspensions based on water and glycerol/water-mixtures with different kind of suspended particles (Kaolin, Polymer, Glass) have been atomized by means of twin-fluid atomizer. Model suspensions based on water and Glycerol/water-mixture with suspended particles (glass and Kaolin) have been atomized by means of a flat pressure nozzle.
SUMMARY
The fluidised dense-phase (FDP) conveying of powders and low-velocity slug-flow (LVSF) of granular bulk solids are the most common and popular modes of dense-phase used in industry. However, the accurate prediction of conveying performance still is not possible from first principles and relies heavily on empiricism.
The main aim of this project is to develop the necessary understanding, databases, guidelines and models for the purpose of predicting accurate optimal operating conditions for the two modes of dense-phase. However, as was mentioned in the original research grant application, it was unlikely that both the FDP and LVSF sections could be completed thoroughly in a single 3-year period (i.e. due to the amount of work involved). Hence, top priority was given initially to the LVSF section of the project, although some progress was also made with the FDP section of work. However, with the 3-year extension to the research grant a substantial amount of work can now be completed in the FDP section, as well as tying off some loose ends from the LVSF section.
Several difficulties have been encountered during the course of the project (e.g. unexpected results and phenomena) and have delayed progress in various areas. In some cases, it was not possible to complete certain scheduled tasks (e.g. testing aluminium and mild steel pipe and wide range of granular solids). In other cases, it was necessary to pursue new work (e.g. rotary valve air leakage, new pipe friction and stress transmission testers). However, in terms of achieving the main goals, there is no doubt that the project will be successful in terms of improved understanding and the development of new databases and models for the prediction of LVSF performance. Unfortunately, due to the various problems and delays to date, the full range of pipe wall materials and bulk solids will not be able to be tested – such work is necessary to confirm the accuracy and validity of the new models (e.g. majority of work to date has concentrated on poly pellets). Also, a significant amount of additional time will be needed for the relatively more complex FDP section of work (e.g. only one product and a few different pipelines were able to be tested by the end of the initial 3-year period). The 3-year extension is allowing a more concentrated effort in this area, as evidenced by the extensive testing completed over the past 12 months.
This Annual Report summarises the research progress and major achievements to date, as well the forward plan for the next 12 months.
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.