Article pubs.acs.org/IECR
Study of Crystallization Kinetics of Ammonium Carnallite and Ammonium Chloride in the NH4Cl-MgCl2-H2O System Daoguang Wang and Zhibao Li* Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China ABSTRACT: Crystallization kinetics of ammonium carnallite (NH4MgCl3·6H2O), a double salt formed in NH4Cl-MgCl2 solution, and ammonium chloride in the NH4Cl-MgCl2-H2O system were experimentally investigated with the mixed-suspension mixed-product removal (MSMPR) crystallizer. The metastable zone widths of ammonium carnallite and ammonium chloride were explored. The effects of temperature and MgCl2 concentration on the nucleation and growth behaviors of ammonium carnallite and ammonium chloride crystals were investigated in both batch and continuous mode. Results showed that the crystal growth rates of the two salts are size-independent. The higher temperature enhances their nucleation and growth rates. The higher MgCl2 concentration results in a greater nucleation rate and lower growth rate of NH4Cl. The nucleation and growth kinetic parameters were determined with the experimental values at various temperatures and MgCl2 concentrations. A kinetic operation equation which agreed well with experimental data was then proposed for design purposes. The operation model capable of analyzing the effect of suspension density and residence time on the crystal size of ammonium chloride, and ammonium carnallite was finally employed to the preliminary design of crystallizers in the ammonium chloride recovery process.
1. INTRODUCTION Recently, a new approach has been proposed on the basis of thermodynamic studies of the NH4Cl-MgCl2-H2O system to recover crystalline NH4Cl from NH4Cl-rich solutions generated in the MgO production process.1 In the new recovery process, the saleable crystalline NH4Cl is obtained by three consecutive crystallization operations involving ammonium chloride and ammonium carnallite (NH4MgCl3·6H2O), a double salt formed in NH4Cl-MgCl2 solution with high MgCl2 content more than 3.5 mol kg−1 (H2O). Thus, the crystallization kinetics including nucleation and crystal growth of NH4Cl and NH4MgCl3·6H2O in the NH4Cl-MgCl2-H2O system are of great importance for the practical design and operation of large-scale crystallizers and for predictive scale-up of the recovery process. Several crystallization studies of NH4Cl have been reported in the literature. Kahlweit2 has investigated the effect of supersaturation and temperature on the stationary growth of dendritic NH4Cl crystal from aqueous solutions. He described a discontinuity in the growth velocity as a function of supersaturation in NH4Cl-H2O solution, i.e., the growth velocity of NH4Cl exhibited a sudden increase above a certain higher supersaturation. Blackmore et al.3 found that the growth rate of NH4Cl in pure solution was also disconnected as a function of undercooling. Söhnel et al.4 have studied the influence of admixtures, i.e., Al3+, Fe2+, and Mn2+, and operating conditions on the crystallization of ammonium chloride. They concluded that the best result was obtained at lower temperature, 303.15 K, and in the presence of Mn2+ (150 ppm). Cournil and his co-workers5 investigated the crystallization in a mixed vessel by creating the initial supersaturation with the addition of KCl crystals into the nearly supersaturated NH4Cl solutions. They discussed the respective roles of primary and secondary nucleation, growth, agglomeration and fragmentation in the framework of a model. However, these studies were carried out in pure NH4Cl solutions © 2012 American Chemical Society
(or with admixtures) and seem to be less helpful in understanding the crystallization process of NH4Cl in bulk NH4Cl-MgCl2 solutions in a stirred vessel. In order to appreciate the influence of temperature and solution properties, i.e., particularly MgCl2 concentration and supersaturation, on the crystallization process of NH4Cl in the NH4Cl-MgCl2-H2O system, it is desirable to investigate the nucleation kinetics and crystal growth kinetics of NH4Cl. Moreover, no study is available on the crystallization kinetics of NH4MgCl3·6H2O. Therefore, more kinetic work on the crystallization of NH4Cl and NH4MgCl3·6H2O should be conducted to meet the crystallizer operation and design requirements and to obtain high-quality crystalline products. The objective of this work is to determine the nucleation and growth kinetics of NH4Cl and NH4MgCl3·6H2O in the NH4ClMgCl2-H2O system and further to use the resulting kinetics in the primary design of crystallization process. The metastable zone width (MZW) of aqueous NH4Cl-MgCl2 solutions was measured. The influences of MgCl2 concentration, temperature, and suspension density on the crystallization kinetics of NH4Cl and NH4MgCl3·6H2O were systematically investigated by either the batch or continuous mode. The mixed-suspension mixedproduct removal (MSMPR) crystallization technique, due to its capacity to simultaneously investigate nucleation and growth kinetics while generating data that are directly applicable to the scaling-up process,6,7 was employed to determine the crystallization rates. The kinetic parameters were then extracted from the experimental data. Finally, a kinetic model that agreed well with experimental values in predicting crystal size was proposed and employed for the preliminary design of crystallizers. Received: Revised: Accepted: Published: 2397
July 18, 2011 January 4, 2012 January 4, 2012 January 4, 2012 dx.doi.org/10.1021/ie201554j | Ind. Eng.Chem. Res. 2012, 51, 2397−2406
Industrial & Engineering Chemistry Research
Article
Figure 1. Flow diagram of the experimental equipment: (a) water bath, (b) feed tank, (c) thermometer, (d) peristaltic pump, (e) thermometer, (f) MSMPR crystallizer, (g) water bath, (h) filter, (i) graduated flask, and (j) vacuum pump.
2. EXPERIMENTAL AND METHODS 2.1. Experimental Materials. Ammonium chloride (99.5%, Beijing Chemical Plant) and magnesium chloride hexahydrate (98%, Beijing Chemical Plant) of analytical grade were used without further purification in the experiments. A series of magnesium chloride solutions, concentrations ranging from 1 to 4 mol kg−1 with an interval of 0.5/1 mol kg−1, were prepared by dissolving magnesium chloride hexahydrate in double distilled water (conductivity
