Mechanism Analysis of Ca, S Transformation in Phosphogypsum Decomposition with Fe Catalyst

Low temperature and highly-efficient one-step decomposition of phosphogypsum via biochar by Fe3+/Co2+/Ni2+ unitary/ternary catalyst

Phosphogypsum (PG), which has great environmental harm and restricts the sustainable development of phosphorus chemical industry, is a solid waste produced in phosphoric acid production. Thermal decomposition of PG is an extensive way to reutilization of resource, and the key point is to establish an appropriate decomposition path and catalyst system of PG. In the work, the strategy for low-temperature and highly-efficient decomposition of PG is established based on the thermodynamic analysis and the experimental research by metal ions to reduce the decomposition temperature. Meanwhile, SEM(Scanning Electron Microscope) is used to characterize the composition and morphology of PG in the various conditions, also the decomposition temperature is analyzed by TGA(Thermogravimetric Analysis). Then, the decomposition ratio via Fe3+/Co2+/Ni2+ unitary/ternary catalyst is obtained by precipitation method. Through kinetic analysis combined with XRF(X-ray Fluorescence Spectrometer) and EDX(Energy Dispersive X-Ray Spectroscopy) results, it is found that there is a reaction competition in the decomposition process by Fe3+/Co2+/Ni2+ ternary catalyst. Further the mechanism of catalytic system on PG is derived. The present work can be concluded that Fe3+/Co2+/Ni2+ can effectively reduce the decomposition temperature of PG, and the effect of ternary metal is more obvious than that of unitary metal. Finally, pomelo peel is used instead of coke to successfully decompose PG at low temperature by one step method. The establishment of low temperature decomposition system of PG has potential application in phosphorus chemical industry and is in line with sustainable development.

Advances on resource utilization of semi-dry desulfurization ash by thermal decomposition: a high-efficiency and low-temperature method for large-scale processing

The semi-dry flue gas desulfurization ash (SFGDA) is an industrial waste generated by the semi-dry desulfurization process, and its resources have been continuously attracted attention. Through the method of heat decomposition, the SFGDA decomposed into CaO and SO2 has emerged as a prominent research topic. This paper summarizes various of research workers, who revealed that the decomposition temperature of CaSO4 in SFGDA is greater than 1678 K and 1603 K in the air atmosphere and N2 atmosphere, respectively, presenting challenges such as high energy consumption and limited economic feasibility. On the one hand, the effects of CO and C regulating the pyrolysis atmosphere on reducing the pyrolysis temperature were reviewed. On the other hand, the impact of additives such as Fe2O3 and FeS2 was considered. Ultimately, the joint effects of regulating atmosphere and additives were discussed, and an efficient and low-temperature decomposition route was obtained; adding solid C source and Fe2O3 for pyrolysis reaction, the decomposition temperature of CaSO4 can be reduced by at least 230 K and desulfurization efficiency exceeds 95% under the condition of micro-oxidizing atmosphere. Moreover, the CaO resulting from SFGDA decomposition can be further synthesized into calcium ferrite, while the enriched SO2 can be utilized for the production of industrial sulfuric acid, which holds promising prospects for large-scale industrial applications.

Experimental Study on Optimization of Phosphogypsum Suspension Decomposition Conditions under Double Catalysis

Phosphogypsum (PG) is not only a solid waste discharged from the phosphate fertilizer industry, but also a valuable resource. After high-temperature heat treatment, it can be decomposed into SO2 and CaO; the former can be used to produce sulfuric acid, and the latter can be used as building materials. In this paper, the catalytic thermal decomposition conditions of phosphogypsum were optimized, and the effects of the reaction temperature, reaction atmosphere, reaction time and carbon powder content on the decomposition of phosphogypsum were studied. The research shows that the synergistic effect of carbon powder and CO reducing atmosphere can effectively reduce the decomposition temperature of phosphogypsum. According to the results of the orthogonal test under simulated suspended laboratory conditions, the factors affecting the decomposition rate of phosphogypsum are temperature, time, atmosphere and carbon powder content in turn, and the factors affecting the desulfurization rate are time, temperature, atmosphere and carbon powder content in turn. Under laboratory conditions, the highest decomposition rate and desulfurization rate of phosphogypsum are 97.73% and 97.2%, and the corresponding reaction conditions are as follows: calcination temperature is 1180 °C, calcination time is 15 min, carbon powder content is 4%, and CO concentration is 6%. The results of thermal analysis of phosphogypsum at different temperature rising rates show that the higher the temperature rising rate, the higher the initial temperature of decomposition reaction and the temperature of maximum thermal decomposition rate, but the increase in the temperature rising rate will not reduce the decomposition rate of phosphogypsum.

Crystal regulation of gypsum via hydrothermal treatment with hydrogen ion for Cr(VI) extraction

Heavy metal-containing gypsum is a widespread hazardous waste. In this work, H⁺ was found to be the most essential factor of the mineralizers in hydrothermal treatment to completely (≥99.8%) extract Cr(VI) from gypsum waste to the supernatant, where the significant growth (from several μm to several hundreds of μm) and perfection of the gypsum crystals were observed. Moreover, with increasing concentration of H⁺, the crystal growth (undergoing Ostwald ripening process) was accelerated and the phase transformation temperature of gypsum was decreased from 110℃ (at 0.2 mol/L of HCl) to 100℃ (at 0.3 mol/L of HCl), which are favorable to enhance Cr(VI) extraction efficiency. Pilot experiments further certified this method to be practicable even in ton-scale. This work proposes a practicable and universal method to completely extract Cr(VI) from gypsum waste, and would also inspire the recycle of gypsum waste containing other heavy metals, such as As, Pb, Cd, and Hg.

Decomposing properties of phosphogypsum with iron addition under two-step cycle multi-atmosphere control in fluidised bed

Phosphogypsum is a solid industry by-product generated when sulphuric acid is used to process phosphate ore into fertiliser. Phosphogypsum stacks without pretreatment are often piled on the land surface or dumped in the sea, causing significant environmental damage. This study examined the reaction characteristics of phosphogypsum, when decomposed in a multi-atmosphere fluidised bed. Phosphogypsum was first dried, sieved and mixed proportionally with lignite at the mass ratio of 10:1, it was then immersed in 0.8 [Formula: see text] with a solid-liquid ratio of 8:25. The study included a two-step cycle of multi-atmosphere control. First, a reducing atmosphere was provided to allow phosphogypsum decomposition through partial lignite combustion. After the reduction stage reaction was completed, the reducing atmosphere was changed into an air-support oxidising atmosphere at the constant temperature. Each atmosphere cycle had a conversion time of 30 min to ensure a sufficient reaction. The decomposing properties of phosphogypsum were obtained in different atmosphere cycles, at different reaction temperatures, different heating rates and different fluidised gas velocities, using experimental results combined with a theoretical analysis using FactSage 7.0 Reaction module. The study revealed that the optimum reaction condition was to circulate the atmosphere twice at a temperature of 1100 °C. The heating rate above 800 °C was 5 [Formula: see text], and the fluidised gas velocity was 0.40 [Formula: see text]. The procedure proposed in this article can serve as a phosphogypsum decomposition solution, and can support the future management of this by-product, resulting in more sustainable production.