| Decomposition of FGD gypsum, Ca SO4·2H2O as its main component, is one of the most promising recycle routes for reducing the stockpile of the byproduct. To obtain more valuable targeted-products and increase concentration of SO2, recent interests are focusing on lowering the decomposition temperature, improving the efficiency of reactions, finding optimum mass fraction of coal, exploring the reaction mechanism of gypsum in various atmosphere and so forth. Yet, considering the transparent nature of CaSO4 to microwaves, so far, information on the effect of microwave irradiation on the FDG gypsum decomposition is scarce. Comparison with conventional heating, there are great amounts of promising advantages such as volumetric heating, selective heating and the potential of lower processing temperature.Aimed to find an economic and feasible new way, in the present work, a number of experiments of anthracite and magnetite assisting decomposition of FGD gypsum have been investigate under microwave heating. The change trends of the samples’ microwave heating characteristics were investigated. And an optimal combination of FGD gypsum mixed with anthracite and magnetite for a high temperature as well as a high desulfurization degree was explored. The samples before and after microwave heating were characterized by X-ray diffraction(XRD) and scanning electron microscopy(SEM). Based on these testing methods, the reaction mechanism of gypsum under microwaves was discussed. Besides, compared with conventional heating method, the advantages of microwave heating during the decomposition process were also reported in this paper.The results show that the heating rate of FGD gypsum contained 20.03 mass% water rises slowly under microwave heating. Within 90 min, the temperature of FGD gypsum only rose to 299℃. The initial microwave heating rate of anthracite maintained between 25 to 30℃/min in air atmosphere. When the temperature rose to 455℃, the heating rate of anthracite increased substantially and a sharp heating peak appeared to 95℃/min as combustion of anthracite took place in air and hot spots caused by microplasma with microwave irradiation. After microwave heating for 40 min, the temperature of anthracite rose to 1200℃. As an excellent microwave absorber, magnetite can be heated up rapidly in both E and H fields during the microwave heating the heating rate firstly increase to the maximum value as 58℃/min and then decrease rapidly ascribed to the Curie temperature. Elevated temperature to 1166℃, the heating rate gradually drops to zero. The comparison of the heating rate curves between anthracite and magnetite demonstrated that the peaks of the heating rates were partially overlapped. Adding both of them to FGD gypsum, continuous increase of temperature was achieved.Adding 10 mass% of magnetite and 8 mass% of anthracite to FGD gypsum, the temperature of the mixture reached to 1040℃ within 82 min. The heating rate is intensified, in sequence, by water, magnetite, burning of anthracite and hot spots. In the process, because of the existing of gypsum, the maximum heating rate of the mixture was 30℃/min, indicating the occurring of thermal runaway had been avoided.With an increase of anthracite in the range 6-14 mass%, the desulfurization degree of the mixture at 1000℃ passes through a maximum while a threshold value for the highest desulfurization degree was obtained to be 56.82% when the mass fraction of anthracite was 8 mass%. Due to microwave selective heating, the anthracite and magnetite absorbed large amounts of microwave energy and became hot spots in the sample with much higher temperature than the surrounding particles where the adjoining FGD gypsum reacted with anthracite and magnetite to form CaxFeyOz far ahead to around 800℃ and more efficiently. Calcium ferrites( such as CaFe4O7, CaFe3O5 etc.) were also confirmed in the XRD patterns.The desulfurization degree curves with different microwave holding time show good regularity. With increasing holding time from 0 minute to 60 minute, the curves, where the temperature of the sample reached to 850℃,900℃,950℃,1000℃,respectively, can be divided into three stages: the desulfurization degree rose only slightly at 0 min≤ t ≤15 min, while the degree dramatically increased during the time range from 15 min to 45 min and once again slowed down to a certain value at 45 min≤ t ≤60 min. However, the desulfurization degree curves with different conventional holding time show no regularity. A dramatically increase of the desulfurization degree was achieved when the temperature reached to 1200℃ and the sample was held at this temperature for 45 min. After holding for 60 min at 1200℃, in this study, the residues of the mixtures sintered together and adhered to walls of crucibles which can’t be used again.With an increase of microwave holding time at 1000℃, sigmoidal shape of the desulfurization degree-time curve was showed. After microwave holding for 60 min, the maximum desulfurization degree was 93.86%. Considering the volumetric heating under microwave heating and the heat exchange between the sample layers and the atmosphere, the desulfurization degree of different layers of the residues after microwave holding times at 1000℃ was tested. The results show that from outermost layer to the inner layer, the desulfurization degrees increase significantly. According to the XRD patterns and SEM images, eutectic liquid phase has been generated, during the microwave heating process, with the decomposed productions of CaSO4 which spread from inside particles to outside particles. The SEM images also showed that there was less liquid phase in the outermost layer samples which made it possible for preventing kiln ringing under microwave heating.The residues, acted as flux slag, were added to sinter ore in sinter pot tests. No remarkable difference was found in the chemical composition between Tai-gang sinter ore and the sinter with addition of 5 % of the residues. Therefore, the decomposition residue can be used in sinter ore of shaft furnace. |
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