Research On The Cooling Crystallizaiton Of Strontium Chloride

Strontium chloride is an important raw material for magnesium-strontium alloy, high purity strontium salts, and high grade magnetic material. It is also widely used in TFT-LCE liquid crystal, PDP plasma television and pharmacy industry. In this paper, the batch cooling crystallization of strontium chloride hexahydrate (SrCl2·6H2O), which is the critical process of the preparation of strontium chloride was researched in details, including the studies on metastable zone and nucleation, crystallization optimization, numerical modeling and control of crystallization process, and crystal morphological modification. The results obtained may theoretically guide the industrial production.The metastable zone and nucleation were investigated by the polythermal method and the focused beam reflectance measurement (FBRM). With the increasing of supersaturation, nucleation was facilitated, thus, the width of metastable zone and induction time decreased. With the increasing of cooling rate, the nucleus formation was accrelated, the induction time decreased. However, since temperature dropped rapidly during the induction time, the width of metastable zone increased still. With the increasing of stirring rate, mass transfer was promoted, favoring nucleation, the width of metastable zone and induction time decreased. The width of metastable zone and induction time also decreased obviously in the presence of Ca2+ions. Because Ca2+ions reduced the interfacial energy, and the adsorption of Ca2+ions on the nucleus surface was effective for the integration of the solute molecules to growing nucleus. Moreover, the introduction of crystal seeds in small size was found to be helpful to eliminate burst nucleation. Small crystal seeds provided a large surface area with high adsorption capacity for solute. They could consume more supersaturation on seed growth, thus retraining nucleation.The optimal operational range of the SrCl2·6H2O batch-cooling crystallization was in an initial supersaturation of 1.150～1.250, a cooling rate of 15.0～20 ℃·h-1, a stirring rate of 400～500 rpm, and an aging time of 5-10 min. Further, the influence significances of supersaturation, cooling rate, stirring rate, aging time, and their interactions were investigated, using the response surface method. For crystal size, the most significant parameter was supersatruation, then was cooling rate, and last were stirring rate and aging time. The interactions between each pair of cooling rate, stirring rate and aging time were significant, as well. For crystal size distribution, the most significant parameter were cooling rate and aging time, then was supersaturation, and the interaction between supersaturation and aging time was significant, too. The last significant parameter was stirring rate. The best crystallization process was predicted as a supersaturation of 1.214, a cooling rate of 15.00 ℃·h-a stirring rate of 450 rpm, and an aging time of 5 min. The as-prepared SrCl2·6H2O crystals were needle-like, with an average length (Lmean) of 938.34 μm, an average width (Wmean) of 93.42 μm, and a more concentrated size distribution, with the coefficient of length variation (C. V.L) in 0.1385 and the coefficient of width variation (C. V.w) in 0.1286.The SrCl2·6H2O crystal growth was surface-reacation-limited mechanism, which could be described by the two-dimensional growth model, and the growth rate was related with crystal size. Based on the crystal growth model and two-dimensional population balance equation, the numerical modeling of crystallization process was established. The calculation results were in a good agreement with the experimental observation. The mean related error (MRE) of concentration was less than 1%, the MRE of crystal size was less than 10%, and the MRE of crystal size distributuion was less than 15%. Accordingly, in order to enlarge crystal size and concentrate size distribution as possible, the crystallization process was optimized by the temperature curve design. Operated following the curve, in the initial period, the concentration reduced slowly, weakening the secondary nucleation and crystal breakage. In the middle and later periods, the concentration reduced quickly, favoring the crystal growth. The as-prepared SrCl2·6H2O crystals were in a large size, with the Lmean of 1183.25 μm, the Wmean of 81.64 μm, and a more concentrated size distribution, with the C. V.L of 0.1518 and the C. V.w of 0.1523, proving an effective optimization.The morphological modification of SrCl2 6H2O crystals using a cationic surfactant cetyltrimethylammonium chloride (CTAC) was investigated by a combination of experiments and molecular dynamics simulation (MD). The experimental results suggested that, under a supersaturation of 1.144～1.163, a cooling rate of 15.0-20 ℃·h-1, a stirring rate of 500～600 rpm, and an aging time of 10 min, the addition of CTAC in an optimal dosage could transformed the morphology of SrCl2·6H2O crystals from needle-like to short rod-like. The Lmean of as-prepared SrCl2·6H2O crystals decreased from 1206.78 μm to 792.71 μm, the Wmean increased from 80.06 μm to 233.25 μm, and the corresponding aspect ratio dropped from 15.07 to 3.40. Therefore, the reduction of specific surface area would improve the anti-caking property, filtrability, and flowability of SrCl2·6H2O crystals. The MD simulation results indicated that, CTAC preferentially attached to the (1100) face, then to the (1 2 01) face, and to the (0001) face. The morphological modification could be attributed to the fastest growth of the (0001) face and the radial growth caused by the growth retardation of the (1 1 00)and(1 101) faces.