Sorption properties of sodium bicarbonate

Introduction From the point of view of different technological processes, the knowledge of parameters describing the structure of sorbents, such as surface area or pore size distribution play an important role for application them in chemical, cement industries, or modification of mineral raw materials. The sorption properties are determined by surface area, what binds with surface energy and reactivity [1]. As it is well known, sorbents usually characterize in surface heterogeneity. That characteristic is due to the presence of varying sizes and shapes [2]. In general, the grains of fine-grained materials are of irregular shape, exhibiting a columnar, table, needle-shaped or lamellar morphology. Less often they can be found in the form of spherical bodies [1]. There is a close relationship between the type of pores prevailing in the sorbent and its surface. In accordance with the IUPAC (International Union of Pure and Applied Chemistry) recommendation, the surface area of sorbent can be calculated from the capacity of monolayer, which covers the pores, assuming, that surface area effectively occupied by particles of adsorbent in total monolayer is known [3]. Total surface area relates to the unit mass of the adsorbate. In accordance with th same reccomendations, pores, from the point of view of their linear dimensions, are classified into micropores (with a diameter below 2 nm), mesopores (with a diameter from 2 to 50 nm) and macropores (with a diameter above 50 nm). Macroporous materials are characterized by a poorly developed surface area from a few to several m2/g, mesoporous materials – several hundred m2/g, and microporous materials – up to several thousand m2/g. Sodium bicarbonate is a sorbent that is increasingly widely used in purification processes of gaseous products from combustion of solid fuels. In the seventies of the 20th century, studies on sodium sorbents were conducted. They included the use of nahcolite (natural sodium bicarbonate) in the dry flue gas desulphurization process [4]. Nahcolite changes its microstructure as a result of decomposition at elevated temperatures, forming an inhomogeneous structure, that is very reactive in contact with acid gases. Similar properties are possessed by synthetic sodium bicarbonate. As a result of its thermal activation, the decomposition of sodium bicarbonate to sodium carbonate occurs. It influence on decreasing of molar volume of decomposed sodium bicarbonate (NaHCO3) during releasing of gasous products of decomposition: carbon dioxide and water vapor, resulting in breaking apart of compact structure and forming of pores with high surface area. In respect to different sources of informations, the temperature of decomposions varies in the range of 60 to 400 ̊C [5]. Sodium carbonate produced in that proces is characterized by a more developed surface area in comparison with crystalline sodium bicarbonate. This translates into an increase in its reactivity. The process follows the reaction below [6]: 2NaHCO3→Na2CO3+CO2+H2O (1) The reactivity of sodium bicarbonate depends chiefly on its grain size and structure [7, 8, 9]. As fine grains react more efficiently, than larger grains, the examined material was subjected to grinding and then thermal activation in order to develop its surface area. In the present study, the surface area and pore size distribution in modified sodium bicarbonate was determined, using modern methods for obtaining the surface topography and structure. The knowledge of these parameters and their correct interpretation enable the proper selection and use of sodium bicarbonate for the dry flue gas desulphurization process. The presented study on the determination of the sorption properties of modified sodium sorbent has been carried out within the development of project nr NR05000910 “Modified sodium bicarbonate in the processes of dry purification of flue gases from various types of industrial installations”.