Edouard Plasari

New technologies for the precipitation of solid particles with controlled properties

Abstract Precipitation is a decisive unit operation for the production of fine particles with controlled properties. New technologies are developed in order to better control the production process. For example, sliding surface mixing device, vortex reactor or impinging jets may be cited. These reactors are studied in large intervals of operating conditions, showing therefore their possibilities and applications in precipitation. These new technologies are considered as “multifunctional”, because they allow in some ways the separation of the different mechanisms of precipitation (nucleation, growth, agglomeration, ripening). Thus, their contribution may be clearly identified in the control of the precipitation process and, particularly, their advantages in front of standard reactors.

CFD simulation of precipitation in the sliding-surface mixing device

Steady-state precipitation of pseudo-boehmite is studied in the sliding-surface mixing device, both experimentally and using a commercially available CFD package (FLUENT® 4.5). A 3-D description of the reactor geometry is employed. Solutions of the Navier-Stokes equations along with the standard two-equation k-e turbulence model provide both the velocity and energy dissipation fields, while generation and development of the solid crystal phase are described by the moment equations of the population balance. These equations and the precipitation kinetics have been implemented directly in the CFD code using Fortran User-Defined Subroutines. A size-dependent growth rate model is used to describe the crystal growth of pseudo-boehmite crystallites. Local quantities are calculated, such as species concentrations, supersaturation and mean particle size. A good agreement is obtained between experimental and predicted crystal properties. The influence of feed point location, rotating speed of the spinning disk and reagent concentrations on the quality of the precipitate is also investigated.

Determination of nucleation and crystal growth kinetics of barium carbonate

Abstract A compact apparatus of specific construction is used for nucleation measurements in accordance with Nielsens method. Experiments are realized by varying the supersaturation ratio from 35 to 280 and temperature from 10 to 50 °C. Barium carbonate is precipitated by mixing equal volumes of sodium carbonate and barium hydroxide solutions. The experimental data have shown that the nucleation rate of barium carbonate in the supersaturation range cited above is characterized by the primary heterogeneous mechanism and can be expressed by the relation r N = A 0 exp(− E / RT )exp[− B /((ln S ) 2 )], where A 0 =6.4×10 24 m −3 s −1 , E =22,065 J mol −1 , and B =58.7. An original method, using a high seed charge in a batch crystallizer, is developed for the determination of crystal growth kinetics in a large range of supersaturation variation. At high supersaturation ratio, the crystal growth rate satisfies a first-order kinetic expression, while at supersaturation ratio values lower than 2, the kinetic expression is second order. These results show that the crystal growth of barium carbonate follows the Burton, Cabrera, and Franck mechanism characterized by the kinetic expression G = α s 2 tanh( β / s ). The fitting of this kinetic expression with all experimental data at 25 °C from low to high supersaturation ratio values using a least-squares technique gives α =9.49×10 −7 m 7 mol −2 s −1 and β =1.35×10 −2 mol m −3 .

Study of silica particle aggregation in a batch agitated vessel

Abstract The aggregation kinetics of precipitated silica particles is studied in a laboratory batch reactor. A method using destabilisation of a silica sol is developed to observe the aggregation process in absence of any other precipitation phenomena. The evolution of the moments of the particle size distribution (PSD) against time shows that the silica aggregation proceeds in two stages — a perikinetic aggregation stage, for particles smaller than 250 nm, whose kinetic rate depends on physico-chemical parameters, and a rapid orthokinetic aggregation stage, for larger particles. Once they have reached their maximal size, particles undergo breakage, whose rate depends on hydrodynamic conditions. Expressions for the evolution of the first moments (up to sixth order) against operating conditions are derived.

Influence of the internal crystallizer geometry and the operational conditions on the solid product quality

From an industrial point of view, control of precipitated particles quality turns out to be crucial. Experiments carried out in a pilot-scale baffled stirred tank with and without draft tube of two different diameters under different process conditions show a great influence of internal geometry and feed point locations on crystal size distribution and morphology of precipitated particles. Several results in contradiction with those expected by intuition show that much more work is needed to elucidate the interaction between mixing and precipitation phenomena in stirred tanks. In this context, the paper provides a frame of reference and some rules of thumb useful to industrial manufacturers for controlling the product quality.

Development of new methods to accelerate and improve the agglomeration of submicron particles by binding liquids

Abstract Often, the agglomeration of submicron particles by adding binding liquids in a classical way can be very slow and uncontrolled. Here, two methods are developed for accelerating and controlling these kinds of processes. In the case of the first method, the immiscible binding liquid is added in the form of a very fine emulsion preliminarily prepared by a mechanical dispersion using a rotor–stator mixer. The second method is totally innovative; the binding liquid is directly generated within the suspension by a chemical reaction. Both methods are applied to the agglomeration of an organic pigment radically improving the process.

Rheological behaviour and heat effects during the formation and phase transformation of silver oxide gels obtained by precipitation

Abstract Freshly precipitated silver oxide gel obtained from the addition of alkali in silver nitrate solution of high concentrations is suspected to have quite non-Newtonian properties. The knowledge of its rheological behaviour can be helpful when choosing the mixing device for precipitation reactors. In this paper, the rheological behaviour of the concentrated suspension is investigated and the choice of appropriate mixer is discussed. During the precipitation, the temperature evolution exhibits two steps: the exothermic reaction is supposedly responsible for the first step, the second one seems to occur exactly when the gel is breaking down. Hence, a calorimetric study is also carried out in order to investigate this phenomenon linked to the rheological behaviour.

Characterisation and modelling of a two impinging jet mixer for precipitation processes using laser induced fluorescence

Publisher Summary This chapter focuses on a two impinging jet mixer that is used for precipitation processes highlighting its components and process mechanism by the means of the laser induced fluorescent technique. Neutralization reactions with a fluorescent pH-indicator are used to reveal the mixing process up to the molecular level. On the basis of the experimental results, a phenomenological mixing model is derived. The mixing model involves a macrodilution process and a coalescence-redispersion process through which the fluids are contacted at the molecular level. The mixing model is applied to the precipitation of barium sulfate, and it is found that its predictions are very close to experimental results. The mathematical description of the mixing process relies on the coalescence-redispersion model, which predicts very realistic distributions of concentration. The mixing model is easy to perform. Besides, it provides a global dynamical description of the process, from which the influence of the operating parameters becomes transparent. This phenomenological approach is an alternative to computational fluid dynamics (CFD) simulation, as, due to instabilities of the flow, this latter approach can provide less accurate results. The mixing model can be advantageously used to investigate the capabilities of the TIJ mixer when applied to precipitation processes, and it can serves as a basis for the design.