| As an important chemical product and industrial raw materials, boric acid can be widely used in lots of fields, such as glass fiber, ceramic, daily chemicals, medicine and nuclear energy. Boric acid is usually prepared by leaching boron minerals with sulfuric acid in the world. In our country, high-grade szaibelyite in Liaoning, Jilin, Qinghai and Tibet, is used to produce boric acid. However, the high-grade szaibelyite will be exhausted after half a century of mining. In recent years, some companies have started to import ulexite from abroad to produce boric acid, but there existed some technical problems in industrial production, for example, the content of sodium chloride in ulexite is too high, which erodes equipments, and the price of by-product of sodium sulfate prepared by decomposing ulexite with sulfuric acid is a bit low, which results in high production costs. In order to solve above problems, this paper has developed a new process to make comprehensive utilization of ulexite.In this thesis, ulexite from Bolivia was used as a raw material to react with nitric acid instead of sulfuric acid, in order to produce boric acid. The mother liquor obtained after separating boric acid from the leaching solution of ulexite, can be used to produce higher value-added by-products of calcium carbonate and sodium nitrate by adding sodium carbonate.First, the crude ulexite was washed by water to remove sodium chloride. The effects of liquid to solid ratio (V/w) and washing time on Cl" removal rate were investigated, respectively. On the conditions of liquid-solid ratio6ml/g and washing time20～30min, Cl-removal rate achieved92.1%, Cl-concent in ulexite concentrate was only0.56%. The ulexite after washing was leached by nitric acid. The optimal leaching conditions included:nitric acid consumption was0.9times of theoretical amount, liquid-solid ratio5:1, leaching temperature and time were60℃and40-60min, respectively. Under the optimal leaching conditions, B2O3leaching rate reached98.44%. and ulexite was nearly decomposed completely. Boric acid was crystallized from leaching ulexite solution. At the reasonable temperature of25℃and crystallization time of6-7h, the boric acid crystallization rate reached more than40%.After separating boric acid, boric acid mother liquor was obtained, which was applied to precipitate calcium carbonate by adding saturated sodium carbonate solution. The suitable calcium carbonate precipitation conditions were observed. At the reaction temperature of50 ℃, sodium carbonate addition amount of1.2times of theoretical amount, sodium carbonate addition speed of more than or equal to7.0mL/min and reaction time of10～30min, the decalcification rate of the boric acid mother liquor reached over99%. Besides, the morphology of calcium carbonate precipitated from the boric acid mother liquor was spherical calcite, the surface of calcite was smooth and the particle size of calcite was around1.0μm.The residual solution after precipitating calcium carbonate was decalcification mother liquor. The decalcification mother liquor was collected to extract sodium nitrate through evaporating, cooling and filtering operations. The influence of the final evaporating concentration and crystallization temperature of the decalcified mother liquor on the production and purity of sodium nitrate were discussed. The high purity sodium nitrate was produced at final concentrations of44～47baume degrees (°Be) after evaporating and crystallization temperatures of35～40℃. Finally, the steady circulations were carried out under all above optimal conditions of whole process, the results demonstrated the average yield of boric acid could get98.73%, and the average yield of sodium nitrate was85.16%. Boric acid, calcium carbonate and sodium nitrate produced by steady circulations could meet the requirements for industrial application standards. |
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