Continuous manufacturing with an Oslo-type salt crystallizer was tested. For the manufacturing condition, the opening of the steam valve was changed to 8, 10, and 15 percent. The following points regarding manufacturing characteristics were clarified by this study. 1) It was possible to produce output of 81 t/day (3.38 t/h×24 h) by continuous manufacturing with 15 percent opening of the steam valve. This could thus be a useful technique for increasing production in the near future. 2) It was supposed that the large particles had a faster growth speed than the small ones in our Krystale-Oslo-type salt crystallizer, and therefore it was clarified that the cumulative residue curve on the Rosin-Rammler diagram was not a straight line, having a point at which the line deflected. 3) In comparing the Bottom Suction & Upper Feed system (BSUF) with the Upper Suction & Bottom Feed system (USBF), particle size of USBF is larger than BSUF. 4) It was clarified that production output increased more when product had smaller-size particlesthan large ones in this study. And, naturally, the larger the opening of the steam valve, the more the output increased. 5) The following was clarified by calculation. It was supposed that suspended particle size in the salt crystailizer was about 1,600μm at the under part of the crystallizer, and about 500μm at the top, and the smaller size circulated with the mother liquid, and the larger size sank and accumulated at the bottom. 6) In the above study, the frequency of the circulation pump was set to 50 Hz. If the frequency were changed to 45 and 55 Hz, perhaps there would be a change in the characteristics in cyrstallization. This subject should be studied further.
Japanese salt production companies have been developing their own technologies for various salt products as independent private companies. In particular, customers require various particle-size choices. A conventional conical classifying crystallizer was installed and various particle sizes of salt, in the range between 350 and 3,000μm, have been manufactured in our factory. Even though obtainingwider size selections mainly depends on differences in crystallizer construction, satisfactory results on the commercial production of desired particle sizes in good yields were obtained by usingan operation line diagram, after confirming factors of crystal growing rate and seed generationrate at start-up stage, in which seeds were grown up to 2,300 μm as desired, in batch and continuous commercial plant operations.
Correlation between the size of product crystals of sodium chloride and the productivity of a continuous crystallizer was tested in our plant. We used a forced circulation evaporator with a vertical external heating element as a continuous crystallizer. Feed solution used was saturated brine obtained by an ion exchange membrane electro dialyzer. Crystal sizes were tested based on production rates being controlled by the volume of heating steam, which varied from 2 to 8t/h, and circulating speed of solvent solution at the rate of 40m3/min (regulated by 50Hz of the circulating pump). The density of crystals in solution was preservedat 10%. The shape of the surface of products was observed by electron microscope. As the result, we could not find out “the first operational line” because the the circulating volume of crystals in the evaporator was supposedly too little.
Naruto Engyo Co., Ltd., has been making food salts sea water by using electrodialyzers (using ion-exchange membranes) and three effective vacuum evaporators. In recent years, crystal size control operation has become an important problem because salt users require various sizes of salt crystals. We tested crystal size control operation by using a so-called 3-B pan, which is the 2nd effective vacuum evaporator, with outside heat exchanger 400 m2 and forced reverse circulation. The 3-B pan operating condition was kept around 15% of crystal content in saturated brine and varied inlet supplysteam from 6 to 20 t/h. Output crystal size was measured and we applied Toyokura's crystallization theory. Correlation between these obtained data, crystal size, production rate, crystal growth speed, and nucleation rate were investigated. And also data of No.3 pan and No.4 pan, under commercial running, was plotted on Rosin-Rammler diagram and operation diagram. Results: (1) when we changed supply steam from 6 to 20 t/h and maintained 15% slurry, we obtained the same size of crystals; (2) when we incleased production rate, correlation between crystal growth speed and nucleation rate was a direct proportion line.
Crystallization tests of salts with various particle size were carried out in an industrial crystallizerthat was a perfect mixed bed-type with a system of reverse forced circulation. Crystallization dataobtained were applied to crystallization theory. Size distribution of crystals was plotted on a Rosin-Rammler diagram and straight lines wereobtained for 10 to 90 percent of cumulative weight of product crystals. Operational data wereplotted as a straight line on a diagram chart. This line was approximately parallel to line B, which was the correlation that (P/ρc·V')·lw became constant. Here P, ρc·P', and lw, are productionrate, density of crystal, working volume of a crystallizer, and average size of product crystaldecided at 36.8 percent on weight basis plot on Rosin-Rammler diagram, respectively. Correlationsbetween product crystal size and production rate and between suspension density of crystalbed and average crystal growth ratealso indicated that (P/ρc·V')·lw nearly equal constant wasvalid. Points logarithmic plotted between average crystal growth rateand nucleation rate gave astraight line. Application of operational data to crystallization theory of a perfectmixed crystallization modelindicated that our evaporating crystallizer had been similar to animproved fluidized bed-type crystallizerbecause of the effect of classification in salt leg. The test results by using the operational line obtained from data of crystallization tests of salts suggested that the maximum mean size of crystal obtained in this crystallizer was 1.5 to 1.6 mm.
We constructed a reverse forced circulation-type evaporator for getting middle particle size saltof approximately 800μm. The optimum operational conditions of the evaporator were taken by heated steam volume, suspensiondensity, and liquid volume in the evaporator. From our test result, we determined thatthe following points were very important: 1. Crystal growth rate in the evaporator depended on heated steam volume. This fact was themost effective factor in obtaining middle particle size salt. 2. Suspension density had the optimum operation point. 3. Increasing Iiquid volume in the evaporator militated against a good result.