Microwave-Assisted Dissolution of Pyrite in Acidic Ferric Sulfate Solutions Soner Kuslu* ,† and Mahmut Bayramoglu ‡ Department of Chemical Engineering, Ataturk University, 25240 Erzurum, Turkey, and Gebze Institute of Technology, 41400 Gebze/Kocaeli, Turkey The use of microwaves in synthetic chemistry has gained greatly in importance in the past 10 years. An observed accelerating effect of microwaves on reactions rates has been explained either by a specific microwave effect or by a superheating phenomenon. This paper considers the dissolution of pyrite mineral in acidic ferric sulfate solutions in the absence and presence of microwaves. For this purpose, a computer-controlled microwave experimental apparatus was designed in our laboratory. The experiments show that the reaction rate between pyrite and acidic ferric sulfate solution is chemically controlled and that the reaction order with respect to each Fe 3+ and H 2 SO 4 is equal to 0.5 for both classical and microwave heating; thus, the reaction mechanism is not affected by microwave irradiation. At the same time, the activation energies calculated as 33.43 and 18.72 kJ/mol in the classical and microwave systems, respectively, reveal the net effect of microwave on the activation energy. 1. Introduction Microwave energy is a non-ionizing electromagnetic energy characterized by mutually perpendicular electric and magnetic fields. It lies in the region of the electro- magnetic spectrum between millimeter and radio waves. Microwaves are defined as those waves with wave- lengths of between 1 cm and 1m, corresponding to frequencies of between 300 MHz and 300 GHz, respec- tively. 1 With microwaves, energy transfer isoccursnot by conduction or convection as in conventional heating, but by dielectric loss. Microwaves cause molecular motion by migration of ionic species and rotation of dipolar species. The dielectric loss factor, which is a measure of the ability of a sample to dissipate energy, and the dielectric constant, which is a measure of the ability of a sample to retard microwave energy as it passes through, are two important dielectric properties of a material in microwave heating. A high value of the dissipation factor, which is the ratio of the dielectric loss factor to the dielectric constant, indicates the suscep- tibility of the material to microwave energy. 2 When microwaves penetrate a material, they vibrate its polar molecules at high frequencies and produce energy in the form of heat. Materials dissipate micro- wave energy by two mechanisms: dipole rotation and ionic conduction. Dipole rotation refers to the alignment with the electric field component of the radiation of molecules that have induced dipoles. At 2450 MHz, the field oscillates 4.9 × 10 9 times per second, and sympa- thetic agitation of the molecules generates heat. Ionic conduction is the migration of dissolved ions within the oscillating electric field. Heat generation is due to frictional losses. 3 Microwave energy has many advantages over con- ventional heating method: 4 (a) Processing rates are faster than those obtained with conventional methods. (b) Energy penetrates into the materials, generating heat internally and, thus, minimizing temperature gradients. Selective heating is possible in the case of mixtures. (c) Microwave equipment can be adapted easily to automated systems, it can be quickly started and stopped, and its power levels can be adjusted electronically. Microwave energy has been used in various areas of synthetic chemistry. It is well documented that micro- waves can be employed to accelerated chemical reac- tions. 5-10 Currently, there is a debate on the mechanism of microwaves’ effect on chemical reaction kinetics. The decrease in the activation energy observed by some researchers naturally motivates the proposal of a “non- thermal” or “athermal” specific microwave effect 11-14 that accelerates a reaction to a rate higher than would be expected on the basis of the bulk reaction tempera- ture. 15,16 An increase in the probability of contact between molecules or atoms resulting from the rapid rotation of dipoles induced by a microwave field might cause a reduction of the activation energy. 17 Meanwhile, some researchers claim that microwaves provide only a convenient way to transfer energy to a given system and give rise to an apparent acceleration of the chemical reactions as a result of a “superheating” phenomenon in the context of “hot spots” theory. 18-20 The natural consequence of this theory is that the bulk tempera- ture is no longer representative of the reaction condi- tions. Some recent researchers have focused especially on this subject, and their work can be summarized as follows: Joret et al. studied the effect of microwaves on the dissolution rates of CeO 2 and Co 3 O 4 in nitric acid and explained that microwaves give rise to an apparent acceleration of the chemical reactions as a result of a superheating phenomenon. 19 Shibata at al. studied the decomposition reaction of sodium hydrogen carbonate in water, and they found that the activation energy of the reaction is reduced by microwave irradiation. 17 Chemat et al. studied the dry hydrolysis of nitriles under microwave and conventional heating, and they reported that no microwave kinetic effect was observed. * To whom correspondence should be addressed. † Ataturk University. ‡ Gebze Institute of Technology. 5145 Ind. Eng. Chem. Res. 2002, 41, 5145-5150 10.1021/ie020203r CCC: $22.00 © 2002 American Chemical Society Published on Web 09/18/2002