Preparation And Characterization Of High Purity Boric Acid And Borax And Zinc Borate

Boric acid and borax are the basis of boron industry as raw materials. They have a wide range of applications in both traditional industries and high-tech fields. At present, the domestic purification technologies of industrial boric acid and borax have a certain gap for preparation of nuclear grade boric acid and borax for high-borosilicate glass. Impurity content is obviously higher than standard or required. With the development of high-temperature engineering plastics, the processing temperature of some specialty plastics is close to the decomposition temperature of the commercial low hydrated zinc borate flame retardants. This paper has deeply processed boric acid and borax in the aim of increasing added values, using industrial grade boric acid and borax as raw materials to produce nuclear grade boric acid and borax meeting the requirement of high borosilicate glass industry by esterification-batch distillation process and complexation-crystallization process, respectively. And the main process parameters have been examined. On this basis, a new high thermal stability low hydrated zinc borate which raises highest decomposition temperature from currently415℃to500℃and higher has been prepared. The morphologies of this zinc borate were characterized and influencing factors on preparation process were studied. The main contents are as follows:(1) Preparation process of nuclear grade boric acid has been studied. Using solid acid as catalyst, nuclear grade boric acid was prepared by esterification-batch distillation process and process conditions have been determined. Effect of molar ratio of boric acid and methanol, temperature of collecting azeotrope of trimethyl borate ester and methanol, temperature of excessive methanol recovery and water amount used in hydrolysis to conversion ratio and yield were studied. Use inductively coupled plasma atomic emission spectrometry and ion chromatography to characterize the purity of the product. Experiments show boric acid conversion rate of no less than85%and a70～75%yield. The purity of the product meets nuclear industry requirement..(2) Preparation processes of high purity borax decahydrate and borax pentahydrate by complex-crystalization method have been studied. Based on complexing balanced theory and stability constants of complex ions, borax solubility gradient has been used on borax purification. Main cationic impurities of the product were characterized using inductively coupled plasma atomic emission spectrometry. The influences of type and amount of complexing agent, time of complexation, complex temperature and concentration on the complexation have been studied and high purity borax decahydrate has been prepared. Then the process was combined with borax pentahydrate borax high temperature crystallization process and high purity borax pentahydrate meeting the requirement of high-borosilicate glass industry was prepared by one step.(3) A novel zinc borate Zn6O(OH)(BO3)3has been prepared. With borax decahydrate and zinc sulphate as raw materials, granular zinc borate Zn6O(OH)(BO3)3was prepared in the present of surfactant. Molar ratio of borax decahydrate and zinc sulphate, reaction temperature and reaction time on the yield has been studied. The product was characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR). The average particle size of product is1.21μm. The decomposition temperature is higher than500℃. The average particle size of the product can reduce to0.73μm while polyethylene glycol was introduced in reaction system. In the mean time particles show a kind of lamellar surface structure.