| Although boron resources in Liaoning Province is rich, the dilution trend of ascharite is clearly after 50 years of substantial mining. The average grade of ascharite decreases from 18% to 12% since the early sixties of the 20th century. Moreover, the average grade of ascharite used in processing enterprises is only about 10%. In this paper, on the basis of preparation of boric acid by sulfuric acid method, we use the low-grade ascharite as the raw material to recover the magnesium sulfate in boric acid mother liquor. Since the whole process is a closed circuit, no waste is discharged. The reactivity of the reactions of diluted acid with serpentine ore and dolomite is analyzed by thermodynamic principles. The results show that the Mg can be completely leached from serpentine ore and dolomite by diluted acid. When the amount of sulfuric acid is 85% of the theoretical amount, sulfuric acid concentration is 20%, reaction time is 100min, stirring speed is about 100rmp, the boron leaching rate is up to 93.80% and the recovery rate of boric acid is 71.06%. The purity of boric acid is 99.62%, which fits the first grade industrial products (boric acid content of between 99.4%-100.8%) according to the industrial standard of boric acid (GB 538-90). In addition, when the boric acid mother liquor crystallizes at high temperature in an autoclave, the magnesium sulfate can be efficiently recycled if the concentration of magnesium sulfate is more than 25%. For example, when the concentration of magnesium sulfate in the solution is 28%, the crystallization temperature is 180℃, and the crystallization time is 4h, the magnesium sulfate can be effectively recycled. The recovery rate and purity of magnesium sulfate monohydrate are 45.03% and 96.07%, respectively.Ludwigite is the most important boron resource in China, the B2O3 reserves of which is 58% of the country total reserves. The Selective reduction of ludwigite in the electrical furnace or blast furnace will result in the Separation of iron and boron. This process will finally produce the boron-rich slag and B-containing pig iron. The grade of the boron-rich slag could up to 12%～17%. Since the obtained boron-rich slag is similar to ascharite in the chemical composition, it could replace the ascharite to be the source of Boron. The reactivity of the diluted acid with Mg2SiO4 and MgCaSiO4 in the boron-rich slag has been analyzed based on the thermodynamic principle. It was showed that Mg in the Mg2SiO4 and MgCaSiO4 can be completely leached by the dilute acid. The sulfuric acid and hydrochloric acid were used in the leaching experiments. The best acid leaching conditions for boron-rich slagⅡby sulfuric acid are:99℃of the leaching temperature,95% of the theoretical amount of the amount of sulfuric acid,6:1 of the ratio of leaching liquid to solid, and 90min of the leaching time. Under this condition, the leaching rates for B and Mg are 98.76% and 74.32%, respectively. To take advantage of sulfuric acid leaching solution, the magnesium was recovered at high temperature first, and then the boric acid was crystallized from the mother liquor. This process is different from the previous technology, in which the boric acid was crystallized at low temperature, and then the magnesium was recovered from the mother liquor. After the process of eliminating iron and aluminum, the hot solution was directly added to the autoclave. Adding the hot solution to the autoclave can use the heat of hot solution to save energy. Final filtrate could be added to acid leaching process, the boric acid and magnesium sulfate can be recovered again. The entire process is a closed circuit, no wastewater is discharged. The recovery rate of magnesium sulfate is about 50% and the recovery rate of boric acid is more than 65%. The purity of magnesium sulfate monohydrate is 95.42%。The purity of boric acid is 99.27%, which fits the qualified products under the industrial standard (GB 538-90) for boric acid (the acid content is greater than 99.0%).The results showed that the best acid leaching conditions for boron-rich slagⅡby hydrochloric acid are:95℃of the leaching temperature,95% of the theoretical amount of the amount of sulfuric acid,1:1 of the ratio of leaching liquid to solid,40 min of the leaching time. To take advantage of leaching solution, the preparation technology of boric acid and magnesium hydroxide process was developed and calcium sulfate was the by-product. The purity of the boric acid obtained by this method is 99.55%, which fits the first grade industrial products under the industrial standard (GB 538-90) for boric acid (acid content of between 99.4%-100.8%). The purity of magnesium hydroxide is greater than 80%, and the purity of the calcium sulfate is 98.81%.In the borax preparation process, the technology route of molten sodium treatment—heat treatment—water leaching—crystallization was developed. The boron-rich slag as the raw material reacts with sodium carbonate at high temperature. After the reaction, the molten slag turns to be Na2CO3-modified slag after heat treated. Na2CO3-modified slag is leached by water at atmospheric pressure or high pressure conditions. The boron could transfer to the aqueous solution and then adjusting the pH value of the aqueous solution. Subsequently, the borax is crystallized in the solution at room temperature. The results show that the chemical composition for boron-rich slag which is suitable for this technology should be met the following factors:the calcium oxide content is less than 6%; the MgO/SiO2 should be within the range of 1.4 to 2.0; alumina content is less than 9%. The optimum technology conditions are as following: the amount of sodium carbonate is 2.3 times of the theoretical amount of sodium carbonate; temperature is 650℃; heat treatment time is 4.5h; leaching temperature is 180℃. The highest leaching rate of boron is up to 87.64% in the Na2CO3-modified slag. The purity of the boric acid is up to 99.31%, significantly higher than the purity of first grade borax (95.0%) of national standards for industrial borax (GB/T 537-1997), close to the high-class borax product (99.5%). |
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