Chemical Engineering Journal Chemical Engineering Journal 68 (1997) 87-94 Kinetic study of the direct causticization reaction involving titanates and titanium dioxide Magnus Palm, Hans Theliander * Department of Chemical Engineering Design, Chalmers Uniwrsi~ of Technology, S-412 96 GBteborg. Sweden Received 1 December 1996; revised 17 March 1997; accepted 26 March 1997 Abstract The solid state reaction between titanium dioxide and sodium carbonate forming sodium titanates was investigated. Reactions between sodium carbonate and titanium dioxide and/or sodium tri-titanate play a key role in the direct causticization of kraft black liquor. Experiments were carried out in a microdifferential reactor made of quartz glass at varying temperatures up to a maximum of 880 “C. Kinetic data were obtained by measuring the release of carbon dioxide. The physical and chemical properties of the reactants and products were analysed in order to obtain a maximum understanding of the reaction path. Scanning electron microscopy (SEM) and the specific surface area of the reactants and products were applied for morphology determination. X-Ray diffraction (XRD) was employed to characterize the phase composition of the product. The results showed that 100% conversion can be obtained at temperatures above 830 “C. Different kinetic models were. taken into consideration, such as the Jander and Valensi-Carter models for diffusion-controlled reaction rates and the phase-boundary model for first-order reaction kinetics. One model was based on the theory that the kinetics can be described by the phase-boundary theory at the beginning of the reaction but, as the reaction proceeds. the rate becomes diffusion controlled. This mode1 gave a good fit to the experimental data collected at 840 “C. However, this model did not work as well at high temperature (880 “C) or at lower temperatures as at 840 “C. This was explained by the fact that the reaction path is different at these temperatures, i.e. other reaction products are involved. 0 1997 Elsevier Science S.A. Keywords: Kinetics; Sodium carbonate; Solid statereaction; Titanates; Titanium dioxide 1. Introduction 1.1. Direct cuusticization Virtually all the pulp mills in operation in the world today are based on the conventional process of black liquor recov- ery. The conventional recovery process gives satisfactory chemical recovery, but there are certain drawbacks, e.g. high capital cost, smelt-water explosion hazard and the corrosive nature of the smelt. The industry has been searching for new ways of recovering the chemicals used in the pulping process for more than 2.5 years. Certain processes, including direct causticization, are considered to be promising alternatives. A direct alkali recovery system (DARS) was developed and patented in the late 1970s [ 1,2], whereby the black liquor from the soda process is evaporated and burned with ferric oxide in a furnace/reactor. The product is then fed into a leacher where sodium hydroxide is regenerated. The DARS process is operated commercially at Bumie Mill in Tasmania, * Corresponding author. Fax: 00463 18 14620. 1385.8947/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved I’IIS1385-X947(97)00050-8 Australia [ 31. The most important benefits of this process, compared with the conventional one, are that it is safe and easy to operate, harmful emissions are very small and the sodium hydroxide of the white liquor can be controlled to almost any concentration up to 300 g I- ’ [ 31. The DARS process cannot be used for kraft black liquor (KBL) since the ferric oxide reacts with the sulphur-contain- ing compounds in the KBL to form ferric sulphides. There- fore, during the last 20 years, a lot of effort has been placed into finding a compound that can be used in the direct caus- ticization of KBL. The most (and probably the only) suitable direct causticization agents are titanium dioxide ( TiOz) and titanates. Studies of the direct causticization reaction with titanium dioxide have been carried out [ 4,5]. In a modem concept of KBL recovery, shown in Fig. 1, the conventional causticization step is replaced by direct causticization in the gasifier, where sodium carbonate reacts with added titanium dioxide or recycled sodium tri-titanate to form solid sodium titanate (4Na,O. 5Ti02). The temper- ature should be over 840 “C in order to achieve sufficiently high reaction rates IS]. The sodium titanates produced are