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

Abstract

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.

Phosphogypsum Decomposition by Char under a CO2 Atmosphere

“…Thermal decomposition of PG occurs at a high temperature (>1200 °C), consuming a great deal of energy. The decomposition temperature can be lowered to below 1100 °C in the presence of reducing agents such as carbon, carbon monoxide, hydrogen sulfide, and hydrogen. − Carbon monoxide is found to be more efficient in terms of energy consumption by means of thermodynamic analyses. − Additives such as CaCl 2 and Fe 2 O 3 can further promote CaSO 4 decomposition with carbon monoxide as the reducing agent. , However, the intermediate product of CaS formed during the reductive decomposition may remain in the product due to over-reduction, leading to low CaO production . The solution to this problem is to run the process in an alternative manner between reductive and oxidative conditions, using air to further oxidize the remaining CaS into CaO .…”

Experimental study of CaS preparation from lignite‐reduced phosphogypsum in a fluidized bed

“…It was found that CO 2 could increase the CaO content by significantly inhibiting CaS. Zheng et al 14 compared the CaO preparation process by reducing PG from lignite in a fixed bed and a fluidized bed. It was found that when the molar ratio of lignite to PG was 1.2:1, the CaO purity in the fluidized bed was 93.8%, significantly higher than that of 90.6% in the fixed bed after multiple atmosphere cycles.…”

“…It was found that CO 2 could increase the CaO content by significantly inhibiting CaS. Zheng et al 14 . compared the CaO preparation process by reducing PG from lignite in a fixed bed and a fluidized bed.…”

Phosphogypsum Conversion into Calcium Fluoride and Sodium Sulfate

“…PG can be utilized as a raw material for certain chemical production processes. The best known of these processes is the PG thermal decomposition, which permit recovering SO2 gas necessary for the sulfuric acid (H2SO4) production [10].…”