Abstract

Three different kinds of morphology with various sizes of barium sulfate particles were produced by reactive precipitation in a Taylor-Couette flow reactor. It is found that particle morphology transition is strongly related to the hydrodynamics in the reactor, clearly indicating an interfacial interaction between feed solutions and aggregated particles. At low concentration, particle morphology transition is observed at the onset of turbulent Taylor-Couette flow. Such morphology transition also appears at the onset of turbulent Taylor vortex flow at high concentration. Based on different transition status, supersaturation is found to play an important role in nucleation and growth processes. In addition, it is revealed that the synthesized particle reduces its size as the consequence of the transition in particle morphology, indicating the effect of variation of the feeding rates. Experimental results have confirmed that controllable synthesis of barium sulfate particles with a particular morphology can be achieved through suitable selection of the controlling parameters such as the rotational speed of inner cylinder of Taylor-Couette flow reactor, reactant feeding rate and supersaturation ratio.

Keywords: barium sulfate, Taylor-Couette reactor, particle morphology, flow pattern, supersaturation, agglomeration

#### 1. Introduction

Precipitation is a traditional industrial process to produce solid particles. It has been used in many applications. Typical examples can be found in fabrication of pigments, ceramics, pharmaceuticals and bio-chemicals among others. For various purposes, the requirements of particle property are different. As particle size and morphology play important roles in determining particle property [1], many previous studies have paid attention to realization of particle size distribution control and to acquirement of a particular morphology [2–4]. As many factors can affect final particle property, such as reactant concentration, feeding modes, additives, and reactor system, their interrelationships are complicated so that many precipitation mechanisms have been proposed [5]. However, none of these proposed precipitation mechanisms can fully address the dynamic processes involved for synthesis of the particles given a particular reactor system. The present work focuses on the effects of three main parameters, rotational speed, feeding rate and supersaturation when employing a Taylor-Couette reactor for synthesis of particles on the change of particle morphology, in particular investigating barium sulfate system. Even though the synthesis of barium sulfate particles has been extensively studied [6–8], controllable synthesis of the particles is still not fully understood, which requires further investigations.

Barium sulfate is a sparingly soluble salt, whose crystallization kinetics has been widely studied. Also, its precipitation system from aqueous barium chloride and sodium sulfate is available according to a series of studies by Nielsen [9–11]. In terms of particle morphology, a number of studies have been conducted to investigate different aspects of the potential possible factors. Many morphologies were observed in connection with the synthesis processes, involving tabular particles (or flat particles), dendritic particles, round-shaped particles (or spherical particles), rhombic particles and rice-shaped particles. Barresi et al. [12] produced barium sulfate particles in a continuous Couette reactor, and they observed dendritic tabular crystals and tabular crystals with pyramidal by changing supersaturation, rotational speed and internal diameter of inner cylinder at the same time. They suggested that supersaturation was the dominant factor in determining particle morphology and size. However, they failed to reveal the effect of each individual variable. Marchisio et al. [13] found a series of morphology change from tabular particles, then dendritic particles to rounded-shaped particles with increase of sodium sulfate concentration. They indicated that the excess of species had a stronger effect on particle morphology due to the preferential absorption of the excess ion. The effect of additives, such as EDTA, phosphate, and lanthanum, has been widely investigated experimentally in their study. From the results as reported by Li et al. [14], various morphologies of barium sulfate particles were exhibited when treatment is done by using polyacrylic acid, such as ellipsoids, monodisperse spheres and rose-like aggregates. They contributed the occurrence of these morphologies to the interactions between carboxyl groups of the additive and inorganic ions. However, the mechanism of randomly coiled conformation of the additive was not clearly illustrated, which results in controllable preparation of each morphology in industrial application to be still questionable. By means of barium sulfate precipitation system, Baldyga and his co-workers [15–17] have experimentally conducted a number of studies using the mixing tank and numerically explored the mixing behavior, aiming at fundamentally reveal the mechanism involved. During this process, they found that various parameters, including feed volume ratio, stirred speed, feeding time, volume ratio, initial concentration and stoichiometric ratio have impacts on particle size distribution and morphology. Also, they proposed a mixing-precipitation model which reasonably described the interaction between the micro-mixing timescale and reaction timescale, consistent with the experimental results. Pagliolico et al. [18] have also illustrated the effect of the mixing on the production of various morphologies of barium sulfate, including dendritic particles, tabular particles, rose aggregates. They proposed shape factors for qualitative description of different morphologies. It can be seen that the abovementioned studies have focused on the effect of both solution conditions (typically excessive species, additives or supersaturation) and operating parameters (typically stirred speed, feeding point location, or addition time) on particle properties. However,

#### Interfacial Phenomena in the Synthesis Process of Barium Sulfate Particles Precipitated… DOI: http://dx.doi.org/10.5772/intechopen.85647

most of deductions in these studies only focused on the change of particle size with the parameters as mentioned while the obtained results just showed the morphology for a particular condition. Little attention has been paid on how the morphology transits with respect to the variations of operating parameters and the correlations between the morphology and these parameters, in particular flow field environment and initial solution conditions.

Taylor-Couette reactors have been employed to prepare different kinds of particles due to its advantages [3, 6, 19]. As no stirrer is involved, the breakage of particles due to the elastic collision can be avoided. Taylor-Couette flow reactor consists of two co-axial cylinders, whose internal cylinder is rotating while the external cylinder is usually kept stationary. The typical characteristics of hydrodynamics of the reactor are a narrow shear rate distribution and relatively uniform kinetic energy dissipation. An increase in the rotational speed can effectively enhance the mixing efficiency. Jung et al. [3] employed a Taylor-Couette reactor to prepare calcium carbonate particles in a gas–liquid system. After comparing the effects of species excess and shear stress, they suggested that the excessive species have a stronger effect on synthesized particle size and morphology while the effect of flow dynamics on the synthesis can be negligible. It should be pointed out, however, that their analysis focused on the aspect of mass transfer, and overlooked the effect of shear rate variations. Mayra and Kim [20] and Thai et al. [19] have systematically conducted a series of studies on the synthesis of Ni-rich hydroxide crystals, a kind of cathode material for lithium ion battery, by using the Taylor-Couette crystallizer. They have clearly demonstrated that under the condition that the Taylor-Couette reactor operates in the range of the critical Reynolds number based on different gap sizes (Rec = 128.5–219.4), the formed Taylor vortices promote the mixing and enhance the mixing residual time, which yields the uniform agglomerate particles with high tap density. This clearly indicates that the hydrodynamics of the Taylor-Couette flow reactor will have a significant impact on the synthesis of particles and the particle crystals growth.

Even though reactive precipitation processes are very fast, Barresi et al. [12] have demonstrated that mixing, especially micro-mixing, has a significant influence on the precipitation process itself. The mixing in mixing vessels that is caused by shear can be characterized by macro-mixing, meso-mixing, and micro-mixing. According to engulfment-deformation-diffusion model (EDD) as proposed by Baldyga and Bourne [21], engulfment due to the micro-mixing generates the local supersaturation and dilution of species. Such local supersaturation is the driving force for crystallization. Macro-mixing occurs on the scale of the reactor, functioning to convey the reactive solutions throughout the entire available space of the reactor [22]. Macro-mixing provides the environment for the following mixing processes, and affects the distribution of supersaturation. Meso-mixing refers to the exchange of fresh feed and its surroundings on a coarse scale, lager than Kolmogorov scale, but smaller than integral scale of turbulence. It can influence micro-mixing by changing the local environment [23]. Micro-mixing brings the fluid elements into contact, followed by molecular diffusion. It controls the generation of supersaturation.

The aim of the present study is to investigate the morphology transition process of barium sulfate precipitated in a Taylor-Couette reactor with a lobed internal cylinder [24]. The effects of three parameters, rotational speed, feeding rate, and supersaturation, on the synthesized particle morphology are assessed and the correlations are obtained. In order to examine the effect of hydrodynamics characterized by shear rate which closely associates with the rotational speed, a wide range of rotational speeds were chosen varying from 25 to 1000 rpm. The interrelationship of shear rate and final particle morphology is acquired. Then, results will be discussed from the perspective of micro-mixing, as precipitation process is

triggered by supersaturation, whereas micro-mixing controls the generation of local supersaturation. The experimental data obtained from current work can be used for the further validation of multiphase micro-mixing model.
