Mechanism Analysis Of Phosphogypsum Decomposition With Catalyst In Low Temperature And Ca,S Transformation

Phosphogypsum (PG) is a solid waste (by-product of the "wet process" phosphoric acid production) in phosphate fertilizer industry which is environmentally harmful. The main mineral composition of PG is CaSO4·2H2O. Nowadays, there are 0.28 billion tons per year in the world, and almost 50 million tons per year in China with low utilization and high pile-up which waste a lot of human, material, money resources and pollute our water, air, soil and the environment. With pressure of society, economy and environmental protection, the utilization of PG in large-scale has become the key sally port to make sure of the sustainable development of phosphorous chemical industry. Many scholars focus on thermal decomposition to recycle the sulfur source and produce cement with the residual solid material which could recycling the Ca and S as much as possible. However, many technological bottlenecks should be solved for the large scale utilization of PG especially the decomposing temperature which largely impact the decomposition rate of PG and the product.This paper trys to find a proper catalyst to lower the PG decomposition temperature and explore the reaction mechanism, build the catalytic reaction system and the reaction model to lower the PG decomposition temperature and provide some basic data and theoretical analysis for the utilization of PG.(1) Guided by Ellingham diagram, Fe-Ni catalyst was made to lower the PG decomposition temperature. Based on TGA analysis, a series of experiments were developed. With 0.8mol/L FeCl3, the initial PG decomposition temperature lowered to 590℃,590℃ -950℃, △TG 37.29%; With 0.4mol/L NiCl2, the initial PG decomposition temperature lowered to 644℃,644℃-900℃, △TG 38.86%; With 0.7mol/L FeCl3-0.4mol/L NiCl2, the initial PG decomposition temperature lowered to 590℃,590℃-900℃, △TG 39.93%. Meanwhile, tube furnace experiments were taken to prove the TGA experiments.(2) Based on the Factsage6.1, Born-Haber cycle, Ellingham diagram and analysis of XRD, XRF and XPS, the reaction mechanism of the simultaneous Fe-Ni catalyst during the process of PG decomposition and the major catalytic reaction path were explored by binding energy from molecular level and a major catalytic reaction system, which consists a series of ion exchange reaction and double salt reaction, surface gas-solid reaction, solid-solid reaction, crystal transition and heterogeneous reaction, was built. Fe oxide substances like FeO, Fe3O4 are the key components; they consist of the PG decomposition cycle and make sure of the proceeding continued to lower the PG decomposition temperature largely. The shrinking core model could properly imitate the process of PG decomposition with Fe catalyst in CO atmosphere.