【正文】 The dependent of crystal growth rate G on crystal particle size L can be expressed as G=, when the particle size is in the range of to .Zhou et al[6] studied the influences of different additive on NaCl crystal growth in the laboratory fluid bed crystallize. The results indicated that PbCl2 and K3[Fe(CN)6] could suppress NaCl crystal growth effectively even concentration of additive was very low. According to the studies of Grootsholten[7], the secondary nucleation rate in the MSMPR crystallizer (V=5591L) can be expressed as: B = *1020（P0N3d5）2/3G2MTThe secondary nucleation rate in the crystallizer (V=10001800L) can be express as: B = , KN=5*10153. THE FACTORS AFFECTING THE CRYSTAL SIZE AND CONTROLLING STRATEGY At present[4], vacuum salt production processes were mostly carried out with four effect force evaporator. The factors which may affect crystal size are analyzed as follows.. supersaturation and supersaturation controlThe most important factor which influences crystallization is the degree of supersaturation. In a crystallization process the growth rate will increase with the increase of degree of supersaturation. High supersaturation can enhance the growth of crystal, it is a advantage for getting large size of crystal. But if the degree of supersaturation is too high and over the limit of metastable zone, the primary nucleation will occur and nucleus will be too much and the product size will be small. The key of controlling crystal size is to control the degree of supersaturation in crystallizer effectively. The degree of supersaturation in the crystallizer is decided by the balance of the rates of supersaturation creation and consumption. In the evaporation crystallization process, the rate of supersaturation creation is decided by the evaporation rate, namely the evaporation intensity of equipment. If the evaporation intensity is high, the creating rate of supersaturation will be quick and the high degree of supersaturation will be easily formed. The consumption rate of supersaturation mainly relies on the processes of spontaneous nucleation and crystal growth. If there is enough crystal surfaces and having high growth rate in the crystallizer, the supersaturation which produces by evaporation can be consumed only by crystal growth. The supersaturation will not be higher than the maximum of supersaturation, and there is no primary nucleation, large crystal is possibly obtained. If the crystal surface in the suspension is insufficient and the crystal growth is insufficient to eliminate the supersaturation which produces by evaporation, the supersaturation is high, the new nuclei will form from solution, which will consume the supersaturation. Therefore the control of consumption and creation rate of the supersaturation will be the key technique for controlling product size in the vacuum salt production process. How to realize this balance process will be one of our important researches. But too high evaporation intensity is one important factor of too small particle size. Influence of temperature As shown in Figure 2, if the temperature of the crystallization is high, the crystal growth rate will be large. The temperature in each effect evaporator is over 50℃in vacuum salt production process. Therefore the crystal growth rate is under the control of diffusion. The higher the temperature is, the faster the crystal growth rate is, the bigger the product size possibly is. Influence of suspension density The surface area of crystals will be increased with the increase of suspension density in the crystallizer. The consumption rate of supersaturation will, therefore, be increased. The level of supersaturation will be lower in the crystallizer. The primary nucleation can be avoided effectively with high suspension density in the crystallizer. At same time, the high suspension density will increase the resident time of crystal in the crystallizer and have more time to grow and possibly can produce large size of crystals. However, too high suspension density will cause high secondary nucleation rate by collision of crystal and circulating pump, preheated pipe and circulation pipe wall, opportunity of collision between the crystal are also large. The large secondary nucleation rate can also reduce the size of product. The large crystal size can not be produced with too low or too high suspension density. The optimal value of the suspension density depends on the design of the crystallizer structure and other operation mode and operation condition. This is also a parameter to be studied for the vacuum evaporation salt production system. The new structure of the crystallizer, control the secondary nucleation with sufficient amount of suspension is also one research topic in consideration of the crystallizer design. Influence of circulating rate In the evaporation operation, the suitable circulating rate is very important to the control of crystal size. If the circulating rate is low, the degree of superheat of circulation fluid will increase, thus the degree of supersaturation of solution will be increased so that the nucleation on the surface of the evaporator will have more possibility to occur. It is not advantageous for the growth of crystal. On the other hand, too high circulating rate will increase the collision strength and probability of collision between crystal and crystal, crystal and wall, crystal and impeller. Therefore the secondary nucleation rate is increased. Circulating rate is very important to the salt crystal size. The suitable circulating rate which can produce the large salt crystal also depends on the structure of evaporator and operation condition of the c