Article pubs.acs.org/IECR
Solubility and Self-Consistent Modeling of Aniline Hydrochloride in H−Mg−Na−Ca−Al−Cl−H2O System at the Temperature Range of 288−348 K Shunping Sun, Junfeng Wang, and Zhibao Li* Key Laboratory of Green Process and Engineering, Institute of Process Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: Previous work has proved that the preparation method of anhydrous magnesium chloride by using the thermal decomposition of the complex [HAE]Cl·MgCl2·6H2O is a potential process for commercial application. Normally, the complex [HAE]Cl·MgCl2·6H2O is synthesized by reaction crystallization of aniline hydrochloride (C6H5NH2·HCl, [HAE]Cl) and bischofite (MgCl2·6H2O). The study on the solubility of [HAE]Cl in hydrochloric acid and various chloride salt solutions plays a significant role in the development, analysis, and engineering design for this new process. This work is a continuation of our systematic study of the solubility of [HAE]Cl in different chloride media. The solubility of [HAE]Cl in different concentrations of HCl (1.19−6.98 mol·kg−1), NaCl (0.5−3.8 mol·kg−1), CaCl2 (0.5−4.5 mol·kg−1), AlCl3 (0.5−2.8 mol·kg−1), and their mixed solutions was determined using a dynamic method in the temperature range from 288 to 348 K. With the purpose of improving AspenPlus’s prediction capability, in regard to [HAE]Cl solubility data in the H−Mg−Na−Ca−Al−Cl−H2O systems at various temperatures, new model parameters were obtained via the regression of the experimental solubility of [HAE]Cl in single electrolyte solutions, such as HCl, NaCl, CaCl2, and AlCl3, under atmospheric pressure. With the newly obtained electrolyte NRTL (ENRTL) interaction parameters for [HAE]Cl−NaCl, [HAE]Cl−CaCl2, [HAE]Cl−AlCl3, and AlCl3−H2O, and default parameters for NaCl−H2O and CaCl2−H2O in AspenPlus, a self-consistent model for the system [HAE]−H−Mg−Na−Ca−Al− Cl−H2O was established with the maximum relative deviation of 4.3% between experimental and predicted solubility values.
1. INTRODUCTION Magnesium as the lightest structural metal is used in several high-volume-part manufacturing applications, including automotive and truck components. Because of good mechanical and electrical properties, magnesium is widely used for manufacturing of mobile phones, laptop computers, and other electronic devices. Magnesium production technologies can be divided into two main types: the Pidgeon process and electrochemical methods. China has been the largest producer of magnesium in the world since 2005. Nearly 95% of the primary magnesium output of China is produced using the Pidgeon process.1 Compared to the electrochemical methods, the Pidgeon process is resource (dolomite) and energy-intensive and leads to relatively severe environmental pollution generated from the energy consumption.2,3 The environmental problems of the primary magnesium production with the Pidgeon process have already attracted much attention from the local government and enterprises in China. In addition, China is rich in brine resources containing considerable amounts of mainly magnesium, potassium, and sodium. There are thousands of saline lakes only in northwest China in Qinghai province. Among all saline lakes, Qarham salt lake is the most famous for its huge reserve of potassium occupying more than 90% of the potassium deposit in China. Simultaneously, the brines in Qarham salt lake contain significant Mg content.4 However, magnesium chloride is usually generated as a byproduct or a waste of the potassium fertilizer industry. The valuable magnesium resources cannot be utilized effectively and are discarded back into the saline lakes, also © 2012 American Chemical Society
causing a serious environmental problem. Therefore, interest in the production of magnesium metal using MgCl2 brine with electrochemical method is growing from an ecological and technological point of view. In order to produce magnesium metal from MgCl2 brine, it is necessary to first obtain the principal precursor, high quality anhydrous magnesium chloride. With the technology development in the past few decades, many methods, e.g., HCl gas protective heating,5−7 organic solvent distillation or molecular sieve absorption,8,9 and decomposition of the complex,10−14 have been tested or even commercialized to prepare anhydrous magnesium chloride from magnesium chloride hydrate. Nevertheless, further improvement is still being researched to reduce the energy cost and satisfy stricter environmental requirements. Thermal decomposition of the complex [HAE]Cl·MgCl2·6H2O ([HAE]Cl, aniline hydrochloride) was considered to be an effective way for producing high-purity anhydrous magnesium chloride.12,15 In previous work, the solid−liquid equilibria for the binary [HAE]Cl−H2O system and the ternary [HAE]Cl− MgCl2−H2O system have been investigated and modeled with success.16 The equilibrium constants of [HAE]Cl, which were obtained by its solubility in pure water using an equilibrium equation for describing the activity coefficients, were used to correlate the electrolyte nonrandom two liquid (ENRTL) Received: Revised: Accepted: Published: 3783
June 30, 2011 February 6, 2012 February 7, 2012 February 7, 2012 dx.doi.org/10.1021/ie2013949 | Ind. Eng. Chem. Res. 2012, 51, 3783−3790
Industrial & Engineering Chemistry Research
Article
All the chemical reagents were prepared by weighing the pure components with an uncertainty of 0.001 g.
interaction parameters for the first time. The obtained ENRTL parameters were used to predict the solubility of [HAE]Cl for the ternary system [HAE]Cl−MgCl2−H2O over the temperature range from 277 to 370 K. There are some impurities such as NaCl, CaCl2, and AlCl3 existing in bischofite (MgCl2·6H2O) obtained from salt lakes, which will affect the purity of anhydrous magnesium chloride. Simultaneously, hydrochloric acid plays an important role in the recycling of aniline hydrochloride. Hence, the solubility of [HAE]Cl in these chloride salt solutions and hydrochloric acid should be further investigated by experiment and modeling. Previous work16 shows that ENRTL was suitable for describing the phase equilibrium of the [HAE]Cl−MgCl2−H2O system with good result. In this work, we decided to expend the methodology to a more complicated system containing [HAE]−H− Mg−Na−Ca−Al−Cl−H2O. This work is a continuation of the systematic study of the solubility of [HAE]Cl in salt lake brines.16 To be concrete, the solubility of [HAE]Cl in hydrochloric acid, sodium chloride, calcium chloride, and aluminum chloride solutions over the temperature range from 288 to 348 K was measured by the dynamic method. New ENRTL model parameters for ionic pairs [HAE] + :Cl − −H 3 O + :Cl − , H 3 O + :Cl − −[HAE] + :Cl − , [HAE]+:Cl−−Na+:Cl−, Na+:Cl−−[HAE]+:Cl−, [HAE]+:Cl−− Ca2+:Cl−, Ca2+:Cl−−[HAE]+:Cl−, [HAE]+:Cl−−Al3+:Cl−, and Al3+:Cl−−[HAE]+:Cl− were determined by regressing the experimental data of [HAE]Cl in single electrolyte solutions with the acquired parameters of the solubility product constant. The model parameters newly obtained were tested against the solubility of [HAE]Cl in mixed NaCl−AlCl3−H2O and NaCl− CaCl2−H2O systems not used in model parametrization. Finally, the solubility of [HAE]Cl in mixed chloride salt solutions was predicted by this self-consistent model.
3. THERMODYNAMIC MODELING FRAMEWORK 3.1. Solution Chemistry. The main dissolution reactions involved in the system [HAE]−H−Mg−Na−Ca−Al−Cl−H2O investigated in the present work are as follows. [HAE]Cl(s) = [HAE]+ + Cl−
(1)
NaCl(s) = Na+ + Cl−
(2)
MgCl 2· 6H2O(s) = Mg 2 + + 2Cl− + 6H2O
(3)
CaCl2·6H2O(s) = Ca 2 + + 2Cl− + 6H2O
(4)
AlCl3·6H2O(s) = Al3 + + 3Cl− + 6H2O
(5)
The equilibrium constants are designated as K1 for [HAE]Cl, K2 for NaCl, K3 for MgCl2·6H2O, K4 for CaCl2·6H2O, and K5 for AlCl3·6H2O. The basis of calculations in this work will be molality-based solubility products for the salts of interest, which are defined as follows: K1 = γ[HAE]+m[HAE]+ γCl−mCl− = γ±2m[HAE]+mCl− (6)
K2 = γ Na+m Na+ γCl−mCl− = γ±2m Na+mCl−
(7)
K3 = γCl−2mCl−2 γ Mg 2 +m Mg 2 +a H2O6 = γ±3mCl−2m Mg 2 +a H2O6
2. EXPERIMENTAL SECTION 2.1. Experimental Materials. The chemicals used in the experiments include aniline hydrochloride (99.0%, Beijing Chemical Plant), hydrochloric acid (99.7%, Beijing Chemical Plant), sodium chloride (99.8%, Beijing Chemical Plant), calcium chloride anhydrous (96.0%, Xilong Chemical Plant), and aluminum chloride hexahydrate (97.0%, Sinopharm Chemical Reagent Co. Ltd.). All were of analytical grade without further purification. A series of solutions, in concentrations ranging from 0.5 to 4.5 mol·kg−1 with an interval of 0.5 mol·kg−1, were prepared by dissolving solute in double distilled water (conductivity < 0.1 μS·cm−1). 2.2. Apparatus and Procedure. The solubilities of [HAE] Cl in chloride solutions were determined using a dynamic method.17 First, a series of chloride solutions with different concentrations were prepared and their concentrations were accurately determined by titration method. Simultaneously, the density of the solution was measured. Then a fixed volume of solution was added into a jacketed glass vessel with a volume of 250 mL. The system was maintained at a certain temperature T using a water bath. A known mass of [HAE]Cl was added into the solvent with a magnetic stirrer providing vigorous agitation. Some time later, more weighted [HAE]Cl was added if the last trace of salts was observed to disappear. The mixture of solute and solvent was heated very slowly (
