| High purity boric acid can be used as raw material to produce high pure chemical reagent or borate crystals, and it has widespread application prospects in the fields of high-tech. Currently, sulfuric acid acidification method is common used demestically in the producing process of boric acid. But its purity can only up to reagent grade standard with this method, which can not meet the requirement of boric acid in nuclear industry. The aim of this thesis is to develop a preparation method of high purity boric acid based on the improvement and development of traditional preparation method. The high purity boric acid (99.99%) was prepared by the combination method of borax acidification, ion resin exchange processes and recrystallization, which filled blank of the high purity boric acid production in China. The environment pollution was reduced by recycling mother solution.HCl gas was adopted to acidizing borax instead of strong liquid acid for the first time. Some effects on the crystallization of boric acid, such as acid varieties, acidity of solution, neutralization temperature, crystallization temperature, solid-liquid ratio of reaction, temperature reducing way, stirring speed and so on, were studied. Comparing with the two different methods leaching experiments, the method of HCl gas acidification is better than the sulfuric acid acidification. In acidification process, the reaction temperature was raised and the solution pH value was increased. As a result, purity of boric acid was 99.55% by sulfuric acid acidification, and purity of boric acid was 99.70% by HCl gas acidification.Mixed bed ion exchange was selected, in which 732# strong-acid cation exchange resin and 717# strong basicity anion exchange resin were used. The traditional anion and cation resin volume ratio of 1:1 was changed to 3:1, which was in favor of the anion adsorption. The best absorption effect was gained by optimizing the ion exchange process parameters. The purity could be up to 99.90% and the content of Fe<0.195X 10-3%, Cl-<0.112X 10-3%, SO42-<0.514X 10-3%.Some influencing factors on the recrystallization of H3BO3,’such as the solution concentration, crystallization temperature, crystallization time and stirring speed in the solution of H3BO3-H2O system were discussed emphatically. The experiment condition for boric acid crystallization was optimized by the orthogonal experiment based on single factor experiments. "Two steps-cooling" crystallization was used, that was fast cooling step from 95℃to 60℃at 1.3℃/min, and slow cooling step form 60℃to 10℃at 0.5℃/min.The study showed that if a slow stirring in the process of rapid cooling step, most of boric acid crystals became symmetrical grain size. The purity could be 99.99% by the second recrystallization. The main impurity contents reached the requirement of importing high purity boric acid, those were Fe<0.0002%, Cl-<0.0001%, SO42-<0.0003%.Crystallization kinetics of H3BO3-NaCl-H2O system and H3BO3-Na2SO4-H2O system were studied. Some influencing factors in crystallization process of system, such as supersaturation, crystallization temperature, crystallization time, stirring velocity, and the impurity in solution were studied. Based on the experimental findings, the curve of crystallization kinetics was gained and kinetics equation was established, and the reaction order was determined.At present, there is not production of high-purity (99.99%) of boric acid in our country, and therefore standard method of analysis has not established. Comparing the purity and the main content of impurities (Fe, Cl-, SO42-) of high purity (99.99%) boric acid imported from U.S., analysis methods of potentiometric titration with NaOH standard solution and spectrophotometric method, and photoelectric turbidimetry were established, which were used to determine the pruity and imprtuity contents of high purity boric acid prepared in our laboratory. The analysis result showed that the purity and main impurity contents attented the level of the high purity boric acid imported from U.S. |
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