| Boric acid is an important inorganic chemical raw material,which plays an important role in the development of national economy.Although China is a big country of boron resources,but due to the low grade of boron ore and the limitation of processing technology,the high purity boric acid produced cannot meet the requirements of nuclear power industry.Nuclear boric acid mainly depends on imports.In order to break the monopoly of foreign technology and provide boric acid for nuclear power more conveniently and cheaply,it is very urgent to develop the preparation technology of nuclear boric acid with independent intellectual property rights in China.This paper aims to prepare high-purity boric acid of nuclear power grade.Based on the previous preparation of boric acid esters,in order to compare the effects of different boron sources on boric acid conversion rate and product purity in the preparation process,the esterification process of different boron sources was studied.In order to solve the problem that the water generated in the esterification process is difficult to remove,the transesterification technology was developed and the optimum process parameters were determined.In order to solve the problem of calcium and magnesium content fluctuation in samples obtained by transesterification,the influencing factors were studied.The main research contents are as follows:(1)Preparation of high purity boric acid by esterification.The effects of molar ratio of raw materials,reaction time and water bath temperature on the conversion rate and product purity under the optimal process parameters were studied using industrial boric acid-made pyroboric acid and methanol as raw materials.Under the same reaction time and water bath temperature,the esterification method was used with industrial boric acid,boron oxide and methanol as raw materials to study the effects of different molar ratios of raw materials on the conversion rate and product purity under the optimal conditions.The results showed that when pyroboric acid was used as raw material,the optimum process parameters were as follows: molar ratio 1:9,reaction time 1.5 h,water bath temperature 70°C,and boric acid conversion rate 92.39%.The conversion of boric acid is 90.01% when the molar ratio of boric acid is 1:9.The conversion of boric acid is 88.56% when the molar ratio of boron oxide is 1:7.The product purity test showed that all the impurities except calcium impurities were below 0.50 ppm,with no significant difference.(2)Preparation of high purity boric acid by transesterification.Using industrial boric acid,exchange agent and methanol as raw materials,the intermediate transition material was prepared from boric acid and exchange agent,and then the ester exchange reaction was carried out with methanol.The effects of molar ratio of raw materials and reaction time on ester yield and ester exchange rate were investigated.The purity of the product under the optimal process parameters was detected.The effect of hydrolysis water consumption on hydrolysis yield was also studied.The results showed that the optimum process parameters were as follows: molar ratio 1:5:10,transesterification time 3.5 h,ester yield 82.22%,transesterification rate 95.34%,water removal rate 86%,hydrolysis water consumption 1.2times the amount of azeotrope,the hydrolysis yield was 80.74%,and the purity of the product was below 0.30 ppm except calcium.(3)Study on calcium and magnesium content fluctuation during preparation of high purity boric acid.Different distillation fractions were obtained by controlling the distillation process.Different types and contents of calcium and magnesium impurities were added to the raw materials,and different common complexing agents were added to the raw materials to explore the influencing factors of calcium and magnesium content fluctuation.The results show that this process can basically completely remove magnesium impurities and reduce its content to below 0.50 ppm.For the possible reasons for the fluctuation of calcium impurities,the calcium content can be reduced to 0.52 ppm by adding EDTA,so as to meet the requirements of nuclear power grade boric acid. |
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