ARR - Annual Report
EXECUTIVE SUMMARY
Our basic concept in the project was as follows: for a fundamental discussion on the phenomena of electrostatic charging of powder, it is essential to study the charge generation due to a single impact/contact on a single particle. To realize the concept we proposed two directions:
Approach 1: Impact Charging Experiment
To extend the original (previous version of the) impact charging experiments to the actual powder particle size region, a new equipment was designed and built in the previous project years. The sensitivity of the charge measurement was extremely improved, and this allowed particles from 50 to 500 μm to be applied to the experiments. Although the actual data were dispersed, almost all of them fell in a certain region explained by the charge relaxation model, which determines the amount of impact charge non-empirically, with an assumption of localized initial charge. Particles which were plated with nickel to make its surface electro conductive lost their all the initial charge at the impact onto a metal target as the same to the cases of 3 mm metal particles. Experiments with different kind of metal targets did not show any significant difference due to the difference of work function of the metals whilst it is expected that the impact charging behavior may depend on it if the process of charge relaxation due to gas discharge is not considered. These facts encouraged us to say that the charge relaxation model works in this particle size region of hundred micro-meter as well as the case of bigger particle with 1-3 mm in diameter in the previous work. In the future study, more improvement of the sensitivity of the charge measurement will be tried to allow particles under 10 μm to be applied. As well, an experimental study chasing process of successive impacts will be interesting.
Approach 2: Development of the Method of Electrostatic Adhesive Force Measurement
This is the completely new approach to determine the amount of charge transferred onto a particle due to a contact from a measurement of adhesive force curve at approaching and separating particle against a metal target. We have actually launched AFM study: force curves of particles due to electrostatic attraction were successfully observed with PS particles of 30 and 100 μm, and with glass bead with 40μm. In the cases of smaller particle, the force curves on approach and separation were almost same after enough repeated contact, whilst they were not in the case of bigger particle: the 100 μm particle case showed that the force in the case of separation was significantly bigger than the case of approach even after enough repeated contacts. This proved that some kind of charge relaxation takes place: this must be investigated in more detail in the future study. To evaluate the force curve quantitatively, a disk charge interaction gave a good account, and it was indicated that the amount of charge and the contact area can be estimated by the method.
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.
Based on the Fokker-Planck-Equation a new model was developed to calculate the reduction of periodic concentration fluctuations entering a continuous mixer. The calculations and the experimental results show:
- The average residence time normalized with the period length of the entering mass flow fluctuations is the main influencing parameter of the mixing quality.
- The mixing process can be described very well with a system of Fokker-Planck-Equations.
By the development of an air-cleanable cowl for the Near Infrared (NIR) Spectrometer VECTOR 22/N it is possible to determine in-line the mixing quality at the end of the continuous mixer every 1.7 sec. The NIR Spectrometer can be used for the analysis of any powder mixture, the only requirement is that the tracer component contains C-H-, O-H- or N-H-groups which absorb the NIR radiation. Because the analysis results can only be as good as the calibration results the calibration was optimised and an apparatus for dust free, easier and faster calibration of the NIR Spectrometer was developed.
For calcium carbonate with an average diameter of 2 μm it could have been shown that the pulsation of a volumetric feeder with a single proportioning device can be reduced tremendously by the use of different dosing tube attachments. The best results were obtained with a self-developed rotating star attachment. The pulsations were reduced to a tenth of the pulsations of a standard dosing tube.
The increasing inter-particle interactions with increasing fineness are a serious problem during wet grinding in stirred media mills. Because these inter-particle interactions have a big influence on the stability against agglomeration and on the rheology of the suspension. As shown in the literature [1] and in this report, a lower boundary of possible particle size is typically reached at around 1 μm. Furthermore the agglomerate size can even increase with further grinding, building larger particles than the original ones, that are strong enough to withstand the comminution process.
In this work we postulate that colloidal stability must be considered in wet grind- ing to understand there results and surmount there limitations on the production of nano- particles.
To study the grinding limits of particle sizes below 1 μm a detailed understanding of the agglomeration process and its mechanism is needed. Therefore the agglomeration process of fine particles is discussed in this report first under Brownian and later on under turbulent shear motion similar to the motion in stirred media mills. The agglomeration kinetics as function of added salt was measured using dynamic light scattering (DLS). Further information on the agglomeration process and the structure of the agglomerates give small angle neutron scattering (SANS) experiments.
Theoretical predictions on the stability ratio and the critical coagulation con- centration based on the DLVO-theory were calculated using measured values of the ζ-potential.
Furthermore an initial investigation to describe the equilibrium between break- age, agglomeration and deagglomeration in stirred media mills has been conducted. The problem is described by the population balance model, which will be solved using the moment method.
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.