Synthesis of Barium Sulfate Nanoparticles Using a Spinning Disk Reactor: Effects of Supersaturation, Disk Rotation Speed, Free Ion Ratio, and Disk Diameter Asghar Molaei Dehkordi* and Alireza Vafaeimanesh Department of Chemical and Petroleum Engineering, Sharif UniVersity of Technology, P.O. Box 11155-9465, Tehran, Iran The aim of this research was to synthesize barium sulfate nanoparticles using a spinning disk reactor. Barium sulfate was produced by continuously pumping two aqueous solutions of BaCl 2 and Na 2 SO 4 , respectively, into the chamber of spinning disk reactor, where a liquid-liquid reaction took place to form BaSO 4 . The influences of various operating and design parameters such as the initial supersaturation, disk rotation speed, free ion ratio, and the disk diameter on the size of barium sulfate nanoparticles were carefully investigated. By varying the supersaturation and disk rotation speed, a broad range of particle size ranging from micrometer sizes down to nanoparticles smaller than 100 nm can be produced. Using high disk rotation speed and high initial supersaturation, crystals ∼38 nm in size were produced. It was found that the variation of free ion ratio has a significant influence on the obtained particle size. Moreover, at a constant supersaturation and disk rotation speed, precipitation experiments with the excess amount of barium or sulfate ions lead to smaller mean particle size compared to those prepared under stoichiometric conditions. 1. Introduction Barium sulfate, so-called Barite, used in the paint and paper industries as well as in oil production, can be extracted from the ground or synthesized by a precipitation method. In addition, it has been largely used as a model system to investigate the effects of various process conditions on the precipitation reactions and to develop precipitation models. Nanoparticles, one of advanced materials, have tremendous potential and applications in many industries, such as electron- ics, 1,2 opticals, 3,4 chemicals, 5,6 ceramics, 7,8 metallurgy, 1 pulp and paper, 9 environment, 10 pharmaceutics, 11-13 and so forth. In the past decade, significant international research efforts have been directed toward the synthesis, characterization, and properties of nanoparticles. 14 More recently, the concerns of mass production of nanoparticles have encouraged research activities in the development of a synthesis method with low- cost and high-volume production. On the other hand, many methods for preparing nanoparticles were developed and reported in the open literature. These methods could be classified as physical vapor deposition, 15 chemical vapor deposition, 15,16 reactive precipitation, 17,18 sol-gel, 19 microemulsion, 20 sonochem- ical processing, 21 supercritical chemical processing, 22 etc. Among these methods, reactive precipitation is of high industrial interest because of its convenience in processing, low cost, and massive production. The precipitation process is fast, operable at ambient tem- perature, and involved nucleation, growth, and agglomeration phenomena. In general, the nucleation and growth processes take place concurrently to secondary phenomena, such as agglomeration, attrition, and breakage. It is therefore possible to identify primary particles, which are formed by crystal nucleation and growth, and secondary particles, which derive mainly from agglomeration phenomena. 23 Supersaturation is known to play the main role in controlling the mechanism and the kinetics of nucleation and growth processes. In particular, heterogeneous nucleation can take place at the low level of supersaturation. In this case, the generation of nuclei is catalyzed by foreign particles, typically dust. On the contrary, very high levels of supersaturation are required for homogeneous nucleation, since in this case the critical nucleus can be only generated by the collisions of a high number of solute clusters randomly moving in solution. Depending on the level of supersaturation, nucleation phenomena are generally very fast in precipitation processes. Since the driving force of the nucleation process is local supersaturation, the intensity of mixing plays a fundamental role in determining the precipitation mechanism and, hence, particle properties and crystal size distribution. Thus, very high levels of supersaturation and intense mixing are required to ensure that homogeneous nucleation is the dominant nucleation mechanism. 24 Conventional precipitation processes are often carried out in stirred tank or column reactors, therefore, the control of product quality is difficult and the morphology and particle size distribution changes during the production time. The high-gravity (Higee) technique proposed by Ramshaw 25 is an example of process intensification and is defined to minimize the equipment scale, to save space, resources, and energy, and thus to make the chemical industry cleaner and safer. During the past two decades, Higee systems have been extensively applied in chemical processes because of the great enhancement in the mass-transfer rate in many unit operations such as distillation, absorption, stripping, extraction, and adsorp- tion, and it has recently been adopted in the field of precipita- tion. 26 There are two types of devices, that is, the high-gravity rotating packed-bed reactor (RPBR) and the spinning disk reactor (SDR). Chen et al. 14 have successfully produced submicrometer particles and even nanoparticles such as CaCO 3 , SrCO 3 , and Al(OH) 3 by the RPBR without adding surfactants to the reacting solution. Cafiero et al. 27 have prepared BaSO 4 particles with a mean size of around 0.7 μm by using a SDR in which the uniform distribution of supersaturation arisen from high mixing efficiency was essential for producing particles with a narrow size distribution. Note that Cafiero et al. 27 have only examined the effects of disk rotation speed and the supersatu- ration on the crystal size of BaSO 4 . The main objectives of the present investigation were (1) to synthesis barium sulfate nanoparticles in an SDR and (2) to * To whom correspondence should be addressed. E-mail: amolaeid@ sharif.edu. Tel.: +98-21- 66165412. Fax: +98-21- 66022853. Ind. Eng. Chem. Res. 2009, 48, 7574–7580 7574 10.1021/ie801799v CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009