2600
Ind. Eng. Chem. Res. 2010, 49, 2600–2607
The Dissolution Kinetics of Gibbsite in Sodium Hydroxide at Ambient Pressure Henrik Gre´nman,† Tapio Salmi,† Dmitry Yu. Murzin,*,† and Jonas Addai-Mensah‡ Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi UniVersity, Biskopsgatan 8, FI-20500 Åbo/Turku, Finland, and Ian Wark Research Institute, UniVersity of South Australia, Mawson Lakes, Adelaide, 5095, Australia
The dissolution kinetics of pure gibbsite in concentrated sodium hydroxide solutions was studied. The experiments were performed under ambient pressure and at 60-85 °C. The changes in morphology of the solid phase were studied during the reaction by particle size, porosity, and surface area measurements as well as scanning electron microscopy. Reaction rates were observed to be strongly dependent on temperature and hydroxide concentration. Several kinetic models, based on different representation of the surface area of gibbsite, were compared to the experimental data and a good correlation between a model incorporating rough/porous geometry and the experimental data was obtained. 1. Introduction The solid-liquid reaction between gibbsite (eq 1) with aqueous sodium hydroxide is of interest in many fields of industry. One of the most obvious ones is the production of alumina in the traditional Bayer process, in which gibbsite, the most abundant mineral in bauxite ore, is selectively digested at high temperatures and pressures. Thereafter the reverse reaction, that is, crystallization, is applied in ambient pressure. The overall reaction is shown below: Al(OH)3 + Na+OH- T Na+Al(OH)4
(1)
Another application, where the selective leaching of gibbsite at ambient pressures is of great interest is in the radioactive waste treatment.1 Approximately 60000 t of high level waste (HLW) sludge are currently stored at the Hanford Nuclear Reservation in Richland, Washington, United States. The intention of the U.S. Department of Energy is to vitrify this sludge into final glass waste form for storage. Many of these sludges contain high concentrations, up to 70%, of aluminum compounds.2,3 The target is to remove sufficient aluminum from the sludge in a way that it is no longer the waste limiting component. To decrease the volume of the radioactive sludges, selective leaching of these compounds should be performed. This diminishes the need for further processing, which is of great technical, economic, and environmental importance. The leached low-radioactive waste can with modern technology be solidified and then deposited, for instance, in various construction industries. Moreover, the depositions of aluminum hydroxides on heat exchangers is a source of major costs in industry, and the prevention of these depositions as well as the effective removal of them is of interest in many fields. The leaching kinetics of gibbsite has often been studied at elevated temperatures and pressures due to the operational conditions of the Bayer process. Even in those conditions, plants are generally operating on an empirical basis because the overall leaching kinetics of the aluminum from the raw material, bauxite, varies as a function of a vast amount of factors, for example, the chemical composition of the ore.4 For example, silicon can influence the leaching kinetics significantly already * To whom correspondence should be addressed. E-mail: Dmitry.
[email protected]. † Åbo Akademi University. ‡ University of South Australia.
in small concentrations due to adsorption on the surface.5 The basis, however, for being able to estimate the overall reaction kinetics as well as the process, is reliable data for pure compounds. Once that is established the influence of the other factors on the kinetics can be quantified in a reliable way. Without these kinds of fundamental studies, the kinetic results remain applicable only to specific sludges of a specific composition and the tools for estimating the behavior of other sludges remain absent. One of the key factors in studying the kinetics of dissolution and factors affecting it is the available surface area for a reaction. Even though there are several indirect techniques for this purpose, nitrogen adsorption being the most common one, most of them are not very reliable for materials with low surface areas, for example,
