Ultrasonic Precipitation Process

Particles, e.g. nanoparticles can be generated bottom-up in liquids by means of precipitation. In this process, a supersaturated mixture starts forming solid particles out of the highly concentrated material that will grow and finally precipitate. In order to control the particle/crystal size and morphology, control over the precipitation influencing factors is essential.

Precipitation Process Background

Within the recent years, nanoparticles gained importance in many fields, such as coatings, polymers, inks, pharmaceuticals or electronics. One important factor influencing the use of nanomaterials is the nanomaterial cost. Therefore, cost-efficient ways to manufacture nanomaterials in bulk quantities are required. While processes, like emulsification and comminution processing are top-down processes, precipitation is a bottom-up process for the synthesis of nano-size particles from liquids. The precipitation involves:

• Mixing of at least two liquids
• Supersaturation
• Nucleation
• Particle growth
• Agglomeration (Typically avoided by low solid concentration or by stabilizing agents)

Precipitation Mixing

The mixing is an essential step in the precipitation, as for most precipitation processes, the speed of the chemical reaction is very high. Commonly, stirred tank reactors (batch or continuous), static or rotor-stator mixers are being used for precipitation reactions. The inhomogeneous distribution of the mixing power and energy within the process volume limits the quality of the synthesized nanoparticles. This disadvantage increases as the reactor volume increases. Advanced mixing technology and good control over the influencing parameters result in smaller particles and better particle homogeneity.

The application of impinging jets, micro-channel mixers, or the use of a Taylor-Couette reactor improve the mixing intensity and homogeneity. This leads to shorter mixing times. Yet these methods are limited it the potential to be scaled up.

Ultrasonication is an advanced mixing technology providing higher shear and stirring energy without scale-up limitations. It does also allows to control the governing parameters, such as power input, reactor design, residence time, particle, or reactant concentration independently. The ultrasonic cavitation induces intense micro mixing and dissipates high power locally.

Magnetite Nanoparticle Precipitation

The application of ultrasonication to precipitation was demonstrated at the ICVT (TU Clausthal) by Banert et al. (2006) for magnetite nanoparticles. Banert used an optimized sono-chemical reactor (right picture, feed 1: iron solution, feed 2: precipitation agent, Click for larger view!) to produce the magnetite nanoparticles “by co-precipitation of an aqueous solution of iron(III)chloride hexahydrate and iron(II)sulfate heptahydrate with a molar ratio of Fe³⁺/Fe²⁺ = 2:1. As hydrodynamic pre-mixing and macro mixing are important and contribute to the ultrasonic micro mixing, the reactor geometry and the position of the feeding pipes are important factors governing the process result. In their work, Banert et al. compared different reactor designs. An improved design of the reactor chamber can reduce the required specific energy by the factor of five.

The iron solution is precipitated with concentrated ammonium hydroxide and sodium hydroxide respectively. In order to avoid any pH gradient, the precipitant has to be pumped in excess. The particle size distribution of magnetite has been measured using photon correlation spectroscopy (PCS, Malvern NanoSizer ZS, Malvern Inc.).”

Without ultrasonication, particles of a mean particle size of 45nm were produced by the hydrodynamic mixing alone. Ultrasonic mixing reduced the resulting particle size to 10nm and less. The graphic below shows the particle size distribution of Fe₃O₄ particles generated in a continuous ultrasonic precipitation reaction (Banert et al., 2004).

The next graphic (Banert et al., 2006) shows the particle size as a function of the specific energy input.

“The diagram can be divided into three main regimes. Below about 1000 kJ/kg_Fe3O4 the mixing is controlled by the hydrodynamic effect. The particle size amounts to about 40-50 nm. Above 1000 kJ/kg the effect of the ultrasonic mixing becomes visible. The particle size decreases below 10 nm. With further increase of the specific power input the particle size remains in the same order of magnitude. The precipitation mixing process is fast enough to allow homogenous nucleation.”

Literature

Banert, T., Horst, C., Kunz, U., Peuker, U. A. (2004), Kontinuierliche Fällung im Ultraschalldurchflußreaktor am Beispiel von Eisen-(II,III) Oxid, ICVT, TU-Clausthal, Poster presented at GVC Annual Meeting 2004.

Banert, T., Brenner, G., Peuker, U. A. (2006), Operating parameters of a continuous sono-chemical precipitation reactor, Proc. 5. WCPT, Orlando Fl., 23.-27. April 2006.