Analysis of bentonite specific surface area by kinetic model during activation process in presence of sodium carbonate L. Karimi, A. Salem ⇑ Mineral Processing Research Center, Chemical Engineering Department, Sahand University of Technology, Tabriz, Iran article info Article history: Received 7 April 2010 Received in revised form 20 June 2010 Accepted 18 October 2010 Available online 30 October 2010 Keywords: Bentonite Specific surface area Activation Kinetic model Sodium carbonate abstract In this paper activation of a typical bentonite in the presence of sodium carbonate was studied by a mod- ified kinetic model. Activation process was carried out at different concentrations of sodium carbonate, ranging 3.0–4.5 wt.%. The effects of sodium carbonate concentration, particle size distribution of natural bentonite, activation time and temperature on specific surface area were investigated. It was found that the activation rate significantly depends on sodium carbonate concentration and it was maximized in presence of 4.5 wt.% of alkali solution. Also, the specific surface of bentonite was greatly affected by par- ticle size distribution of starting raw material. The kinetic calculation results indicated that the concen- tration of alkali solution and particle size of starting bentonite negligibly change the activation energy. The mathematical calculations showed that there is an optimum activation time to achieve the maximum specific surface area in each condition. The validity of proposed model to estimate the optimum activa- tion time was substantiated by comparing computational results with experimental data. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Sodium bentonite is widely used in chemical and oil industries such as catalyst beds, drilling mud and ceramic body compositions, due to their high specific surface area [1]. This material is preferred to use in drilling mud composition because of its suitable rheolog- ical properties and maintaining ability of the cut pieces due to suf- ficient yield stress [2]. The properties of bentonite are function of montmorillonite amount which determines quality of raw material as well as the specific surface area [3]. Also, the type of montmoril- lonite and other crystals which exist in natural bentonite such as illite, quartz, albite and opal CT influence the rheological character- istics of suspension [4]. Therefore, the control of bentonite compo- sition is necessary to get reliable performance in industrial applications [5]. On the other hand, bentonite should provide suf- ficient swelling properties to support the rheological characteris- tics, high green strength after shaping process of ceramic bodies such as extrusion and high specific surface area in catalyst manu- facturing process [6]. Regardless the above properties, Na-mont- morillonite can provide intensive interactions with polymeric materials to achieve suitable mechanical properties [7]. The most commercial used process for producing Na-bentonite with high specific surface area is activation by sodium carbonate in industrial scale. The theoretical formula for montmorillonite is Na y + nH 2 O(Al 2y Mg y )Si 4 O 10 that is classified as dioctahedral clays, having two thirds of octahedral sites occupied by trivalent cations [8]. During the activation process, a considerable amount of cations were substituted by sodium cations which improve the specific surface area. This variation in montmorillonite layers leads to sig- nificant changes in cation exchange capacity, CEC. Also, the specific surface area of bentonite can be affected by alkali solution due to decomposition of smectite structure by alkali ions [9,10]. Activation of bentonite by alkali and acid solutions is almost important step in industrial scale. This process is carried out in a continuous stirred tank reactor at low temperature 50–120 °C in the presence of activation agents such as sodium carbonate, sodium hydroxide, sulfuric and chloride acids [10–12]. Recently, many investigations have been reported about the cation exchange capacity and specific surface area of bentonites [13]. Different methods have been developed for CEC and specific surface area measurements [14–16]. Bentonite usually contains different pore types. The pores in the material that are smaller than 2 nm are named micropores and those between 2 and 50 nm are called mesopores. The pores larger than 50 nm are considered as macropores. The large amount of smectite minerals contained in bentonite is the main source of porosity [17]. Yildiz and Calimli studied the pore size distribution of original and activated bento- nites by Na 2 CO 3 and CaCl 2 solutions. They showed that the pore size of activated bentonite by alkali agents is smaller than those for starting material. This change in pore size is the result of cation exchanges that occur in bentonite structure [18]. Babaki et al. developed a kinetic model based on activation temperature and time to estimate the specific surface area of 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.10.031 ⇑ Corresponding author. Tel.: +98 4123459159; fax: +98 4123444355. E-mail address: salem@sut.ac.ir (A. Salem). Microporous and Mesoporous Materials 141 (2011) 81–87 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso