Characteristics And Utilization Study Of CFB Ash From Zhungeer Gangue Power Plant

The burning technique of circulating fluidized bed(CFB) is a new technique, which have the advantages of wide adaptability of fuel, high rate of burning, low quantity of NOx senting out and wide load adjust. With the developing of electric power, the burning technique of CFB will become the main burning technique. At the same time, it will bring the new solid waste, circulating fluidized bed ash. At present, there are a few of reports about comprehensive utilization of circulating fluidized bed ash, especially. There is not high technique or high income research.In this paper, in allusion to the comprehensive utilization of circulating fluidized bed ash, we try to find a route of reasonable utilization.This work focuses on the analysis of characters of fly ash, especially the new characters of circulating fluidized bed (CFB) ash and finding a reasonable way to recycle it. The acid and alkali combination method is adopted according the especial characters of CFB ash. There are prominent differences between this acid and alkali combination method and the traditional way to product metallurgical grade alumina because ash from pulverized coal boiler is the raw material in the traditional way instead of CFB ash. In this view,our new acid and alkali combination method is innovative. There is a true that the content and form of alumina,which are decided by the coal used , can influence comprehensive utilization of CFB ash. So it's necessary to do reseach of phase composition, the content and activity of alumina of the coal and CFB ash from Zhun Geer plant in order to improve the reuse of it.It's tested that the 6th coal-seam in Hei daigou is rich of Boehmite, the content of which is up to14.67%. There is no other reports claiming higher content than it in the world. When Boehmite, one kind of the hydroxide minerals, is combusted, the content of alumina in the burned ash is more than 85%. Therefore the utilization of the coal minerals rich of Boehmite in or near Zhun Geer will supply the reliable recourse of high aluminum ash for industrial production. The chemical composition,mineral composition and physical and chemical properties of ash of the coal is closely related to the chemical constituents of the coal and the calcination temperature. Coal abstained from three M-1,M-2,M-3, of 6th coal-seam in Heidaigou are calcined at temperature in the range of 300℃~1300℃. Conclusions fellow :samples calcined at 300℃mainly contain kaolinite and the X-ray peak of Boehmite is significantly weaker; almost organic matter of samples burn off at 500℃for 2h. XRD patterns of samples calcined at 500℃~700℃is similar. All are amorphous. When samples are calcined at 900℃,δ-Al2O3 began to form.α-Al2O3, which is a stable crystalline form, is got by calcinations at a higher temperature. The results of lenching the calcined samples show that dissolution rate of samples of the upper and middle levels are highest, when they are calcined at 600℃. When the temperature is higher than 600℃, the dissolution rate decreases. The dissolution rate is less than 5%, when the temperature is 950℃. Dissolution rate of samples of the lower level are highest, when they are calcined at 700℃or 800℃. The dissolution rate decrease rapidly at 900℃, because of the forming ofδ-Al2O3,θ-Al2O3 and mullite.Circulating fluidized bed boiler is employed by Zhun Geer plant, which save energy and fit for low-grade fuel. Compared to pulverized coal boiler, CFB boiler is in deferent operating conditions. The furnace temperature of CFB boiler is about 850℃~ 900℃, which is lower than that of temperature of center flame of pulverized coal boiler. The CFB ash also own some special characters to ash from pulverized coal boiler (OF ash). Chemical analysis showed that the alumina content of CFB ash is 40% and the content of OF ash is as high as 50%, which can be explained by volatilization of the volatile in OF ash at high temperature. Chemical and X-ray analysis show that:free CaO exists in CFB ash adding desulfurizer instead of a small amount of gypsum in CFB ash without desulpherizer. The high temperature in pulverized coal boiler leads to the existence of mullite. By test of SEM,several ideas are caught. One is that shape of CFB ash without desulpherizer is irregular morphology and amorphous,which benefit for acid dissolving. Another one is that shape of CFB ash adding desulpherizer is flocculent and there is some rule sheet mineral, perhaps cast.The last one is that the shape of OF ash is spherical and alveolate, which is due to forming of melt minerals at high-temperature.Entire process of extracting metallurgical-grade alumina from CFB ash is divided into two mian parts:1. hydrochloric acid-soluble method to obtain raw alumina without silicon;2. Bayer method to obtain metallurgical-grade alumina without Fe. Dissolution rate is an important indicator of part of the experiment, which will affect the final product yielded.According to the results of previous studies, temperature, time, stirring method, acid concentration and ratio of acid and ash are all the main factors of rate of dissolution. Experiments results showed that dissolution rate increases with ratio of acid and ash. When the ratio is 0.7:1, the dissolution rate is largest, as high as 70.5%. However,when the ratio of acid and ash increase higher than 1.1:1, the dissolution rate of alumina did not change significantly. The dissolution rate increases to 71% until the ratio is up to 0.7:1. When the ratio is higher than 0.7:1, the dissolution rate is higher than 70%. Conserdering that excessive acid will challenge the corrosion resistance of equipments, the optimum range for the ratio is 1:1 between 0.7:1. The dissolution rate of alumina trend to upward with higher temperature. The max dissolution rate appears at 120℃. The longer reaction time is, the higher the dissolution rate is. When reaction time is shorter than 60 min, the dissolution rate increases sharply. When longer than 60 min,the increasing tend slows down. The dissolution curves of hydrochloric acid concentration shows that when the concentration is in the range of 19%~31%, the dissolution rate is gradually increased.The influence of Stirring on the dissolution rate is impotent.without stirring, the dissolution rate is no more than 62.38%. The results of pilot scale test show that the highest dissolution rate is gained when the temperature is 115℃and the ratio of acid and ash is 1.65. The existence of desulfurizer makes little effect on the dissolution rate. During the process of crystallization of AlCl3, due to impurities in AlCl3 solution causing common ion effect and heterogeneous nucleation, which reduce the solubility, it's easier for the crystallization of aluminum chloride. So that AlCl3 crystallize at a lower concentration of AlCl3 solution. The boiling point and density increase with increasing concentration of AlCl3 and the maximum boiling point is 124℃. Specific heat decreased with higher concentrations. The TG-DTA curves of crystalline aluminum chloride samples show that it decomposes completely by calcinations at one temperature of 360~600℃for 2h. Hydrogen chloride gas generating during calcinations will be recycled. Referring to Bayer method of alkali dissolved iron, the acceptable conditions to prepare raw alumina fellow: alkali concentration of NaOH content is 212 g/L, the amount of the raw alumina is 170 g/L,molecular ratio is 1.6, the dissolution temperature is 210~240℃and the dissolution time is 2h.Based on the analysis of chemical composition and phases of red mud,Fe2O3 is insoluble by alkali and exists in red mud and CaO reacting with alumina transform into insoluble hydrated calcium aluminate. The seed precipitation process include following steps:raw alumina reacting with NaOH transform into supersaturated sodium aluminate solution. The next step is adding active aluminum hydroxide and stirring. Then sodium aluminate decomposed into aluminum hydroxide precipitation. the acceptable conditions of seed precipitation fellow:the content of Al2O3 in sodium aluminate solution is 170 g/L; coefficient of seeds is 1.5; the initial temperature is 75℃,the terminal temperature is 35℃; the reactive time is 50h. Aluminum hydroxide calcined at 1200℃for 2h will change toα-Al2O3. The metallurgical-grade alumina product prepared meets the international standard of the first or secondary level.In conclusion, this study analyzed the characteristics of fly ash CFB proposed acid-alkali the base of fly ash from fluidized bed extraction of a metallurgical grade alumina production process. And the process conditions, product performance to a more in-depth study and discussion,as an important way to provide comprehensive utilization of fly ash.