Article pubs.acs.org/jced
Phase Equilibrium Study of the AlCl3−CaCl2−H2O System for the Production of Aluminum Chloride Hexahydrate from Ca-Rich Flue Ash Junfeng Wang,*,† Camille Petit,‡ Xiangping Zhang,† and Shian Cui§ †
National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom § Department of Chemical Engineering, Lenfest Center for Sustainable Energy, Columbia University, 500 West 120th Street, New York, New York 10027, United States S Supporting Information *
ABSTRACT: The study of the solid−liquid phase equilibrium for the AlCl3−CaCl2−H2O system is of significance to separate aluminum chloride hexahydrate from the leachate obtained by the reaction of Ca-rich fly ash and a waste hydrochloride from chemical plant. The phase equilibrium data for the binary AlCl3−H2O system and the ternary AlCl3−CaCl2−H2O system over the temperature range from 278.15 K to 363.15 K were measured. A rigorous and thermodynamically consistent model representing the AlCl3−CaCl2−H2O system developed on the basis of the Pitzer’s activity coefficient model embedded in the Aspen Plus. On the basis of this, the phase behavior of the ternary AlCl3−CaCl2−H2O system at different temperatures was visualized with lucidity on an equilateral triangle. The phase-equilibrium diagram generated by modeling was illustrated to identify the course of crystallization to recover AlCl3·6H2O from the solutions containing calcium chloride. All of these will provide a thermodynamic basis for the separation of aluminum chloride from calcium chloride solutions. content limited to 15−30 wt %).3,4 The main problem with fly ash lies in the fact that (1) its disposal requires large quantities of land, water and energy and (2) its fine particles can become airborne if not be managed well and increase the risk of pollution. Therefore, the recycling of fly ash is becoming the major bottleneck for the further development of thermal power plants and represents an increasing concern due to current interest in sustainable development in China. This kind of fly ash containing 25 % to 50 % of Al2O3 is considered as a potential resource of alumina.5,6 Aluminum in Ca-rich fly ash can be extracted by acidic and alkali methods.7−12 Compared to the alkali method, one major advantage of the acidic method is the simultaneous dissolution of aluminum and other valuable elements, such as Fe and Ca, while inhibiting the silica dissolution. Different leaching agents, such as inorganic acids,13−16 organic acids,17 and inorganic salts18, have been used for the dissolution of aluminum from fly ash. Among these agents, a waste HCl solution from chemical plants was considered as an alternative leaching agent14 because it could recover aluminum from the leachate containing chloride compounds, such as AlCl3, CaCl2, and FeCl2. Therefore, the
1. INTRODUCTION Fly ash, which is also known as flue ash, is one of the industrial wastes generated usually in combustion of coal. The components of fly ash, which depends mainly on the source and makeup of the coal being burned, generally include silicon dioxide (SiO2) (amorphous and crystalline), aluminum oxide (Al2O3), iron oxide (Fe2O3) (hematite, magnetite, and maghemite), and calcium oxide (CaO). According to the American Association for Testing Materials (ASTM), fly ash is classified into two classes based on the chemical composition: Class F (Fe-rich) fly ash and Class C (Ca-rich) fly ash.1 Ca-rich fly ash will harden and gain strength over time in the presence of water due to its pozzolanic and cementitious properties. This class of fly ash is produced normally from lignite and sub-bituminous coals and, thus, contains a significant amount of the Ca-bearing minerals, such as anorthite, gehlenite, akermanite, and various calcium silicates and calcium aluminates. In China, as of 2009, approximately 37.5 million tons of fly ash was produced from coal-fired power plants.2 Except for a small proportion of fly ash used as the building materials, most of it is disposed in landfills and ash ponds, causing severe environmental problems. Especially, with the continued increase of coal consumption, lots of low-quality coals (such as lignite) have been used in power plants in China, which produced a large amount of fly ash with high-calcium content (with a higher CaO © XXXX American Chemical Society
Received: July 16, 2015 Accepted: November 2, 2015
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DOI: 10.1021/acs.jced.5b00603 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
The invariant data were obtained by the isothermal solubility technique. A total of 200 mL of salt solution with known composition was poured into a 250 mL bottle, which was equipped with a magnetic stirrer. The bottles were then immersed in a temperature-controlled water bath, allowing the solution to stir continuously for about 0.5 h to establish the temperature equilibrium. The temperature was kept constant within 0.01 °C. Then, an excess of solid was quickly added to the solutions in bottles. After the solid−liquid equilibrium was attained, the supernatant solution was then withdrawn and immediately filtered using 0.22 μm Whatman Puradisc syringe filters. The clear filtrate was added into a 25 mL volumetric flask, which was kept in the water bath and then heated to bath temperature for measuring the density of saturated solution. The contents of Al and Ca were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The solid phase was filtered and washed three times with ethanol. The washed solids were dried as 50 °C for 12 h and then analyzed by X-ray powder diffraction to determine whether the solid phase had been altered by phase transformation. All experiments were performed at least three times, and the results were averaged. The standard uncertainty of temperature was 0.01 K. The relative standard uncertainty in the measurement of phase equilibrium data was 0.025.
phase equilibrium behavior of system containing AlCl3, CaCl2, and FeCl2 is of significant importance in order to implement the utilization of fly ash with higher content of calcium. Solid−liquid phase equilibria for AlCl3−H2O and CaCl2−H2O solutions saturated with the solid phases of AlCl3·6H2O and CaCl2·nH2O (n = 2, 4, and 6), respectively, were found in the literature.19−26 The ternary system of AlCl3−FeCl2−H2O have also been systemically studied by Gao et al.27 However, to the best of our knowledge, the solid−liquid equilibrium (SLE) for the ternary AlCl3−CaCl2−H2O system, which is very important to separate aluminum chloride from the leachate obtained from Ca-rich fly ash using a waste HCl solution, has never been reported. The thermodynamic model of phase equilibria involving the aluminum chloride in water and calcium chloride solutions is also very useful for the separation of aluminum chloride from leachate. To estimate the solubility of aluminum chloride in calcium chloride solutions accurately, one of the key problems for estimating the phase equilibrium data of AlCl3− CaCl2−H2O system is to accurately calculate the mean activity coefficients of salts in aqueous electrolyte solutions using the electrolyte models.28−33Among the models used, the Pitzer model is widely used for aqueous electrolyte system in process calculations. Also, the model is conveniently embedded in the Aspen Plus software with a general process-modeling tool. Therefore, the Pitzer model is chosen for this study. In this work, the solubility of aluminum chloride in pure water and aqueous calcium chloride solutions were measured over the temperature range from 278.15 K to 363.15 K. The objective of this study is to establish a thermodynamically consistent model representing the phase equilibrium behavior of AlCl3·6H2O and CaCl2·nH2O (n = 2, 4, and 6) in the ternary AlCl3−CaCl2−H2O system over the whole temperature range. This study would provide reliable reference for the design of separating aluminum chloride hexahydrate from leachate obtained from Ca-rich fly ash using waste hydrochloride solutions.
3. THERMODYNAMIC BASIS 3.1. Speciation and Solution Chemistry. For the binary CaCl2−H2O system, the possible solubility equilibria can be described by the following dissolution reactions: CaCl 2· 6H 2O(s) = Ca 2 +(aq) + 2Cl−(aq) + 6H 2O(l) (1)
CaCl 2· 4H 2O(s) = Ca 2 +(aq) + 2Cl−(aq) + 4H 2O(l) (2) 2+
−
CaCl 2· 2H 2O(s) = Ca (aq) + 2Cl (aq) + 2H 2O(l) (3)
2. EXPERIMENTAL SECTION 2.1. Experimental Materials. Calcium chloride dihydrate (98.0 %, Xilong Chemical Group, analytical grade) and aluminum chloride hexahydrate (97.0 %, Sinopharm Chemical Reagent, analytical grade) were used without further purification in the experiments. Impurities of the two chemicals are listed in Supporting Information (Table S1). A series of alumina chloride solutions, concentrations ranging from 0 M to saturation, were prepared by dissolving alumina chloride hexahydrate in double distilled water (conductivity
