(169b) Precipitation of Tailored Crystals in an Ultrasound Levitator

In a wide range of industries powders with particle sizes in the micron to sub-micron size range are desired especially with specific crystal morphologies. Precipitation is a common method to produce such particles. Within these processes the choice of chemical agents as inhibitors, dispersing agents and chemicals to obtain a desired morphology is decisive [1, 2]. These chemicals are often expensive and contaminate the product. An alternative possibility to influence particle size, particle size distribution and morphology of crystals is by the influence of external mechanical forces like shear stress which is a phenomenon acting on crystals in a stirred vessel [3, 4]. It is a goal of this study to show options to influence particle size and crystal morphology in a precipitation route in which gas is the continuous and liquid the disperse phase by varying physical parameters as residence time, temperature and shear stress. Therefore the precipitation system of calcium carbonate is investigated as an exemplary system.

Experiments are carried out in an ultrasound levitator (UL), where a single droplet can be positioned locally stable at an acoustic standing wave against gravity. The acoustic wave is generated by an ultrasound transducer and is standing between transducer and a concave reflector. The UL works with a frequency of 57 kHz. To evaluate mechanical forces acting in the droplet the sound pressure level is measured by a piezosensor located in the reflector. This sensor is connected to an oscilloscope. The reaction gas is delivered by a mass flow controller and enters the levitator at a small hole in the lower part of the glass cylinder. The droplet is introduced with a small syringe through another hole in the glass cylinder where the gas exits the levitator. After a predefined residence time the droplet is taken out of the levitator using an empty syringe and put on an electron microscope sample holder or a glass plate for light microscopy, where it is drying under a nitrogen purge gas at ambient temperature.

In the ultrasound levitator a droplet of calcium hydroxide solution is reacting with carbon dioxide, which is blown through the reaction room as continuous medium. With these conditions the reaction chamber of the levitator acts as three-phase-reactor. In other experiments a levitated droplet containing carbonate ions is merged with a second droplet containing calcium ions within the levitator. Through these experiments nucleation, growth and agglomeration of calcium carbonate crystals are investigated.

By using an ultrasound levitator it is possible to perform investigations of containerless precipitation and to apply specific forces at the growing crystals. The sound pressure of a standing sound wave generates an acoustic levitation force acting on the droplet. This levitation force deforms the droplet into a rotational ellipsoid. The acoustic field also causes toroidal vortices in the droplet, which produce shear forces [5]. These shear forces act on the growing crystals in the liquid suspension. By varying the power input at the ultrasound generator the intensity of the sound field and therefore levitation force, streaming and shear forces are increased. So the parameter shear stress can be investigated.

By changing physical reaction conditions as residence time, temperature and shear stress it is possible to achieve various amounts of different crystal habits. These are rhombohedral, shell-like and spherical crystals. Selected area electron diffraction is used to characterize the crystal modification.

As far as morphology is concerned high temperature and shear stress encourage the formation of spherical particles. At these conditions the rebuilding of spheres out of rhombohedral crystals can be observed with increasing residence time at constant temperature and shear stress.

Also particle size distributions are changed by varying reaction temperature and amount of shear stress. The particle size distributions of the primary particles are determined optically based on scanning electron microscopy pictures. The equivalent sphere diameters are calculated from the volumes of the rhombohedral and shell-like crystals. The volume of rhombohedrals is determined from two projected diameters of the crystals. The shell-like crystals are approximated by two spherical segments. Particle size distributions of crystals precipitated in the gas-liquid-system have been presented [6]. Figure 1 shows particle size distributions of calcium carbonate crystals which have been precipitated by mixing two solutions within the ultrasound-levitator. For both precipitation systems the same effects are shown: With increasing temperature particles become bigger, while particle size is reduced with increasing shear stress. As there is no mass transfer in this experimental method, effects issuing from a gas-liquid mass transfer rate can be excluded, which is also enhanced by a higher power input to the sound field.

Figure 1: Particle size distributions of calcium carbonate precipitated in the ultrasound levitator at various conditions.

In industrially stirred vessels besides high shear stress also micro-mixing is enhanced by an increase of the mixing intensity [7]. It is general knowledge that micro-mixing plays an important role in crystallisation processes. A higher power input of mixing leads to a smaller and steeper particle size distribution. But in the levitated droplet there is shear stress without turbulence and micro-mixing. Thus solely the influence of shear stress on the crystallisation process is investigated without interference of micro-mixing and mass transfer.

References

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