Pilot study on the reduction and decomposition of phosphogypsum using high-sulfur coal in a circulating fluidized bed

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Phosphogypsum is an industrial waste generated during the production of phosphoric acid, with CaSO4 as its main component. At present, the discharge volume of phosphogypsum in our country has reached 30 million tons per year. Phosphogypsum piles occupy large areas of land; after being soaked by rainwater, harmful components such as soluble P2O5 and fluorides spread into the surrounding environment through water bodies, causing severe pollution to soil, water, and the atmosphere. Wang Yaqin et al., through thermal decomposition experiments on phosphogypsum, proposed that thermal analysis methods indicate an optimized decomposition reaction temperature for phosphogypsum of 1100°C, with gas composition P(CO):P(CO2):P(N2) = 7:15:78, and a particle size of ≤76μm. Fan Haojie et al. conducted a mechanism study on the thermal decomposition of calcium carbonate using thermal analysis methods. Regarding the properties of phosphogypsum and its treatment methods, numerous fundamental studies have been conducted by scholars both domestically and internationally. Research has been carried out on the thermological analysis, thermodynamic behavior, and thermogravimetric analysis of phosphogypsum, as well as on the biological, physical, and chemical treatment methods for it. However, these studies remain at the laboratory stage and cannot be applied in industrial-scale production. Furthermore, due to the differences in the chemical composition of phosphogypsum in different regions, it is difficult to develop a treatment method that can be used universally across all locations. Based on the characteristics of local phosphogypsum in Yunnan, this paper conducts thermal decomposition experiments using a self-designed pilot-scale circulating fluidized bed reactor, providing experimental evidence for the comprehensive utilization of phosphogypsum in Yunnan. l Experimental Setup and Methods 1.1 Experimental Materials The experimental materials are solid particles composed of phosphogypsum and high-sulfur coal, which have been processed through grinding and screening. The density of the phosphogypsum solid particles used in the experiment is ρs = 1,935.91 kg/m3, while their bulk density is ρb = 591.09 kg/m3; their chemical composition is shown in Table 1. Table 1 Chemical composition of phosphogypsum, expressed as mass percentages/%: CaO, SO3, total SiO2, Al2O3, Fe2O3, MgO, P2O5, total F. The values are 29.36, 0.36, 0.36, 9.45, 0.05, 0.13, 3.08, 2.02, 0.01, 4.10, and 0.26, respectively. The solid particle density of the high-sulfur coal used in the experiments is ρs = 1,358.59 kg/m3, while its bulk density is ρb = 576.39 kg/m3; its chemical composition is shown in Table 2. Table 2 Chemical composition of high-sulfur coal, mass percentage/%: SiO2, Fe2O3, Al2O3, CaO, MgO, SO3, TiO2 – 45.1, 10.19, 19.56, 22.86, 3.38, 1.02, 2.16, 1.14. After drying the phosphogypsum at 95°C, its particle size distribution was determined using sieves of different sizes; the sieve analysis results indicated that the particles of phosphogypsum followed a normal distribution, with particle sizes concentrated between 80–160 μm. 1.2 Experimental Setup The experimental bench is a pilot-scale circulating fluidized bed reactor as shown in Figure 1 (omitted). The flow path of the gas-solid two-phase flow within the furnace chamber (from the air distribution plate to the furnace outlet) is 5,380 mm; together with the height of the air chamber, the total height of the entire test bench is 6,300 mm. Based on the conditions on site, this experimental circulating fluidized bed was designed as a pit-type reactor. The fuel coal was manually ground and screened, and the coal powder with a particle size of less than 8 mm was fed into the fluidized bed via a coal feeder for combustion. The high-temperature flue gas generated was used to heat the phosphogypsum; when it reached the designed temperature, the phosphogypsum began to decompose. The mixture of phosphogypsum and high-sulfur coal is fed at the head of the cyclone screw dryer, where it is pre-dried by absorbing the heat from the high-temperature flue gas passing through the outer jacket of the screw. It then enters the outer jacket of the high-temperature separator for further drying, and together with the collected high-temperature dust, it is fed by a feeder into the fluidized bed for calcination. 1.3 Experimental procedure: First, the fluidized bed reactor is heated to a certain temperature. A mixture of phosphogypsum and high-sulfur coal is quickly fed into the reactor through the feed port; the material remains in a dispersed state, allowing for uniform heating. The atmosphere and temperature inside the experimental reactor can be controlled and adjusted, which facilitates the study of the reaction process of phosphogypsum under various atmospheric conditions and temperatures. The molar ratio of CO to phosphogypsum in the reaction, as well as the reaction temperature, are important factors affecting the decomposition reaction of phosphogypsum; in other words, the reaction atmosphere and reaction temperature are key factors that influence this reaction. In the experiment, appropriate components for the reaction gas are first prepared; after being metered, they are introduced into the reactor, which has been raised to the specified temperature, through a secondary air inlet. Then, the mixture is rapidly fed into the feed port via a feeding device. In the reaction, chromatography is used for the online analysis and detection of gas components; the exhaust gas after detection is fed into an exhaust gas absorption device, while the solid residues resulting from the reaction are analyzed to determine their composition, thereby calculating the decomposition rate and desulfurization rate of phosphogypsum. The flow diagram of the phosphogypsum reduction and decomposition experimental setup is shown in Figure 2 (omitted). 1.4 Criteria for determining the degree of phosphogypsum decomposition: In this experiment, the degree of progress in the decomposition reaction of phosphogypsum is indicated by two conversion rates: the decomposition rate and the desulfurization rate of phosphogypsum. The decomposition rate of phosphogypsum refers to the mass fraction of CaSO4 in the sample that has been decomposed into CaO and CaS, relative to the original mass of CaSO4. Decomposition rate = (amount equivalent to CaSO4) / total amount of CaSO4 in the sample (1) The desulfurization rate of phosphogypsum refers to the percentage of S in CaSO4 within the phosphogypsum sample that is converted into SO2. In the utilization of phosphogypsum, the concentration of SO2 in the flue gas is one of the two key factors affecting both the utilization of phosphogypsum and its economic viability; therefore, the desulfurization rate of phosphogypsum is a crucial factor determining whether it can be utilized. Desulfurization rate = (amount of S removed) equivalent to the amount of CaSO4 / total amount of CaSO4 in the sample. (2) In phosphogypsum, once CaSO4 is converted to CaS, it can be considered that CaSO4 has decomposed; however, the sulfur contained within it does not enter the gas phase. An industrially ideal reaction outcome is achieved only when CaSO4 has decomposed and at the same time the amount of CaS approaches 0, that is, when the decomposition rate equals the desulfurization rate. Therefore, the decomposition rate and desulfurization rate can not only indicate the degree of CaSO4 decomposition but also characterize the presence and amount of CaS. In industry, the presence of residual CaSO4 and CaS is undesirable; therefore, both the decomposition rate and the desulfurization rate must be very high. 2 Experimental Results and Analysis 2.1 Effect of Temperature on the Decomposition Rate and Desulfurization Rate of Phosphogypsum By calculating, with a fixed ratio of phosphogypsum to high-sulfur coal, namely n(C):n(S)=0.7, the effect of temperature on the decomposition rate of phosphogypsum was studied in different atmospheres. Among them, the ratio of P(CO)/P(CO2) is controlled via flow meters and pressure gauges in the gas distribution system. As shown in Figure 3 (omitted), as the temperature increases, the decomposition rate of phosphogypsum keeps rising, reaching its peak at 1,200°C. Moreover, the decomposition rate of phosphogypsum varies depending on the atmosphere; when reducing gases make up a larger proportion, phosphogypsum decomposes more thoroughly at a reaction temperature of 1,100°C, with a decomposition rate of over 90%. At 1,200°C, the decomposition rate of phosphogypsum under all conditions exceeds 95%, indicating that the greater the amount of reducing CO gas and the higher the temperature, the more thorough the decomposition of phosphogypsum. By fixing the mixture ratio of phosphogypsum and high-sulfur coal at n(C):n(S)=0.7, the effect of temperature on the desulfurization rate of phosphogypsum in different atmospheres was investigated. Figure 4 (omitted) shows the curve of phosphogypsum desulfurization rate as a function of reaction temperature. As the temperature increases, the desulfurization rate of phosphogypsum also increases. At temperatures below 1,000°C, the more reducing gases there are, the higher the desulfurization rate of phosphogypsum; however, at temperatures above 1,000°C, as the amount of reducing gases decreases, the desulfurization rate actually increases. This indicates that, in terms of the desulfurization rate of phosphogypsum, temperature and atmosphere can complement each other. This is different from the results of thermodynamic analysis, in which it is shown that the more reducing gases there are, the more CaS is produced as a result of the reaction of CaSO4 in phosphogypsum; the same holds true as temperature increases. The reason for this is that the experimental reaction temperature is relatively high, and the reaction does not start at low temperatures. Therefore, high temperatures are conducive to achieving the highest decomposition rate and desulfurization efficiency of phosphogypsum. 2.2 Effect of P(CO)/P(CO2) in the reaction gas on the decomposition rate and desulfurization rate. In the experiments, with a constant ratio of phosphogypsum to high-sulfur coal, namely n(C):n(S) = 0.7, it can be seen from Figures 5 and 6 (omitted) that at high temperatures (≥1,000°C), the decomposition rate of phosphogypsum is only slightly affected by changes in P(CO)/P(CO2). However, the desulfurization rate of phosphogypsum is greatly affected by changes in P(CO)/P(CO2). As P(CO)/P(CO2) increases, the desulfurization rate of phosphogypsum decreases, whereas its decomposition rate increases. Therefore, the effect of P(CO)/P(CO2) can complement the effect of temperature, indicating that the decomposition of phosphogypsum under a weakly reducing atmosphere is favorable. 2.3 Decomposition rate and desulfurization rate of phosphogypsum under different material ratios: In this experiment, a mixture of dispersed phosphogypsum and high-sulfur coal was rapidly heated to a specified high temperature to ensure that the experimental results were closer to actual industrial conditions. Figure 7 (omitted) shows the curves of the decomposition rate of phosphogypsum as a function of reaction temperature under a constant reaction atmosphere, namely at P(CO)/P(CO2) = 0.4, for different material ratios. As can be seen from Figure 7, at different material ratios, as the reaction temperature increases, the decomposition rate of phosphogypsum increases; simultaneously, as the content of high-sulfur coal, which acts as a reducing agent in the mixture, increases, the decomposition rate of phosphogypsum also increases. This indicates that a reducing atmosphere is favorable for the decomposition of phosphogypsum. Figure 8 (omitted) shows the curves of the phosphogypsum desulfurization rate as a function of reaction temperature under a constant reaction atmosphere, namely at P(CO)/P(CO2) = 0.4, for different material ratios. As can be seen from Figure 8, under different material ratio conditions, the desulfurization rate of phosphogypsum increases as the reaction temperature rises. However, this rate decreases when more high-sulfur coal is used in the mixture; this indicates that although the decomposition rate of phosphogypsum increases with an increasing reducing atmosphere, the sulfur present does not convert into SO2 but rather into solid CaS, thereby reducing the desulfurization rate of phosphogypsum. 3 Conclusions (1) Phosphogypsum begins to decompose at around 1,000°C, which is a **lower decomposition temperature** compared to natural gypsum (the decomposition temperature of natural gypsum is 1,250°C). The reason for this is that phosphogypsum contains large amounts of other impurities that accelerate its decomposition. (2) In the experiment, as the reaction temperature increased, both the decomposition rate and the desulfurization rate of phosphogypsum increased. The reaction time required for the decomposition rate of phosphogypsum to reach 97% gradually decreased, and this reduction in reaction time was further observed when a reducing atmosphere was used. (3) The stronger the reducing atmosphere, the higher the decomposition rate of phosphogypsum; conversely, the lower the desulfurization rate of phosphogypsum. This indicates that the decomposition of phosphogypsum occurs more readily in a weakly reducing atmosphere. (4) Using high-sulfur coal as a reducing agent to reduce and decompose phosphogypsum not only helps to increase the sulfur dioxide concentration in the flue gas, but also reduces the reaction temperature as well as energy consumption.