Preparation Of Calcium Metaborate And Sodium Perborate From Borax And Slaked Lime

Sodium perborate is an important boron product, whose yield ranks the world's third place next to borax and boric acid. It is one of the most versatile inorganic peroxide used as oxidant of indanthrene dye color development, stripping agent of reactive dyes staining, fabric bleaching and decoloration, disinfectant and fungicide in medicine and so on. There are some new discoveries of sodium perborate application in organic synthesis in the recent 20 years. It not only replaces hydrogen peroxide as oxidant, but also overcomes the disadvantages of the hydrogen peroxide, such as poor stability, explosiveness, high toxicity, complex operation and expensive cost. Its efficiency is better than organic peroxide acid in some oxidation reactions. Therefore, the application of sodium perborate in chemical industry will be extended in the future.In this paper, sodium perborate product is prepared from mother liquor of sodium metaborate which is the by-product from the calcium metaborate hexahydrate production process, and calcium metaborate hexahydrate was prepared from borax and slaked lime. There are two projects in this study:The first one is that borax was employed to react with slaked lime in water, and the resultant mixture was then filtrated to obtain calcium metaborate hexahydrate (CaO·B₂O₃·6H₂O). The contentration of B₂O₃ in mother liquor reaches 48-52g/L, Na₂O/B₂O₃ mol ratio is about 1, and there is no the composition of magnesium oxide and calcium oxide basically. Therefore, borate in the mother liquor exists in the form of sodium metaborate. Raw material ratio, reaction temperature and reaction time have a great effect on synthesis reaction and quality of the product. The little influence of liquid-to-solid ratio was found. Based on experiments, suitable processing parameters were obtained, i.e. m(B₂O₃)/m(CaO) of reactive raw materials, 2.2:1; liquid-to-solid ratio, 2.76:1(mL/g); reaction temperature and time, 35℃and 8-10h. The mass fractions of B₂O₃ and CaO in the product have reached 29-31%, 23-25% and that of Na₂O is less than 0.5% respectively. The calcium metaborate prepared by this process can be applied to low-alkali and alkali-free glass fiber industry as boron containing raw material.The second one is that mother liquor is evaporated, concentrated and crystallized to obtain crystal of sodium metaborate, which was employed to react with 30% of hydrogen peroxide in the presence of stabilizer in water, and the resultant mixture was then filtrated and dried to obtain sodium perborate tetrahydrate (NaBO₃·4H₂O). Sodium perborate tetrahydrate was dehydrated to obtain sodium perborate monohydrate by rotary evaporator. The effect of raw material ratio, reaction temperature, reaction time, liquid-to-solid ratio and kinds of stabilizer on synthesis reaction and quality of the product was investigated. Based on experiments, suitable processing parameters were obtained according to the principle of maximum yield, i.e. n(NaBO₃·4H₂O)/n(H₂O₂) of reactive raw materials, 1:1.1; liquid-to-solid ratio, 3.3:1(mL/g); reaction temperature and time, 25℃, 60-75min. The mass fraction of B₂O₃ in the product has reached 22-24%, that of NaBO₃·4H₂O is more than 96%, the content of active oxygen is above 10wt% and that of Fe is less than 0.004wt% respectively. The content of active oxygen of sodium perborate monohydrate is about 15wt%.