【正文】 ll size of crystals when solution is highly supersaturated and difficult to be controlled. The third kind of nuclei is from the existed crystal. The tiny crystals which was broken down from the existed crystal, this kind of nuclei was called as secondary nucleation. Secondary nucleation process is plex and influenced by many factors which mainly include properties of crystal and operation conditions, such as mixing intensity, suspension density and degree of supersaturation. It was found that the main factors which affect secondary rate are mixing intensity, suspension density and degree of supersaturation of the solution []. The relationship between secondary nucleation rate and the factors are mostly expressed by the following equation.B = KNNiMTj(ΔC)n Where, B is the nucleation rate No. /m3s, KN is the coefficient of nucleation rate, N is mixing intensity, MT is suspension density, Kg/m3 , ΔC is the degree of supersaturation, i, j, n are the power of the relative parameters in the nucleation equation, which can be obtained by experimental data. Crystal growth If the crystals which no matter e from the seed or secondary or primary nucleation were being in the supersaturated solution, they will grow. This kind of process is called as crystal growth. According to the diffusion theory, the crystal growth process have three steps: (1) the crystallized solute passes through the solution layer beside the crystal surface to crystal surface by diffusion。 (3) the heat of crystallization returns to the solution by conduction. Therefore the degree of supersaturation is the driving force of crystal growth, and directly influences the rate of the crystal growth. Usually the relationship of crystallization growth rate and the degree of supersaturation solution is represented as: G = KgΔCgWhere G is the crystal growth rate, m/s, Kg is the coefficient of growth rate, ΔC is the supersaturation solution , kg/m3.Because the supersaturation is the driving force for the nucleation and growth rate, the degree of supersaturation is the key factor in crystallization process. The higher the supersaturation is, the higher the nucleation and crystal growth rate are. Therefore, the degree of supersaturation has to be controlled in a proper level so that the excessive nucleation can be avoid and existing crystal growth grow. According to the report, different sizes of crystals may have different growth rates. On the other hand, the same size of crystals may also have different growth rates. This is socalled crystal size dependent crystal growth rate and crystal growth rate disperse. About the characteristics of crystal growth rate, and the influences to crystallization process as well as product size distribution are more plicated and not discuss here. Crystallization Kinetics of NaCl NaCl Crystal Growth Rate The growth rate of NaClBase on diffusion theory[1] the crystal growth rate may be controlled by diffusion or by surface reaction. At the high temperature, the surface reaction rate can be greatly enhanced but the diffusion rate increases limitedly so that the crystal growth rate of NaCl belongs to the diffusion control. On the other hands, the process could belong to surface reaction control at low temperature. The growth rate of NaCl is given in Fig. 2. When temperature is higher than 50℃, the crystal growth rate is proportional to the degree of supersaturation as a straight line. It means that the crystal growth is controlled by diffusion. When the temperature is lower than 50℃, the relationship bees a curve, it is a surface reaction control. NaCl Crystallization Kinetics Theoretically speaking, the solubility curve of NaCl is quite flat, and the degree of supersaturation is also small, transition area is narrow and system is easy to have nucleation. If the degree of supersaturation is small the crystal growth rate is small relatively. In order to get large size of crystals, the sufficient time for crystal growth should be provided, and crystal nucleation process should be controlled effectively. Zhang et al.[5] studied NaCl crystallization kinetics in the fluid bed crystallizer. The relationship between NaCl crystal growth rate and the crystal particle size, the liquid flow rate and the degree of supersaturation was experimently obtained. The dependence of crystal growth rate G on the crystal particle size were not evidence when the particle sizes are to 。s KNconstant of nucleation rate Kg – constant of crystal growth MT suspension density, Kg/m3 G Crystal growth rate, m/s ΔC supersaturation, kg/ m3 P0 power input number d Impeller diameter, m τ Residence time, sReference:[1] Ding Xuhuai, Industrial Crystallization, Beijing, Chemical Industry Publishing Company 1985[2]Cayey, N. W . Estrin, Secondary nucleation in agitated magnesium solutions (J) Ind. Eng. Chem. Fundam. 1967 (1). [3] Sung, . Secondary nucleation of magnesium sulfate by fluid shear (J). AIChE. 1963 (19): 957 [4] Su Jiaqing, Vacuum evaporation saltmaking craft [M]. Nation well rock salt industry science and technology information post, 1992, 5766.[5] Zhang Hong, Experimental study on sodium chloride crystallization dynamics in fluid bed [J]. Journal of Qinghai University (Nature Science), 2006 (5): 89,13 [6] Zhou Limin, Kinetic Study on Crystal Growth of NaCl and K2SO4 [J]. Journal of East China Institute of Technology, 2005 (3): 266269 [7] of Scaleup on Secondary Nucleation Kinetics for the Sodium Chloridewater System [J].Institution of Chemical (62):179189 [8] Wu Xiangqi. Approaches of Increasing the Granularity of Crystals in Vacuum Salt[J]. China Well and Rock Salt, 2002 (13): 12