Technology for processing phosphogypsum into a fluorescent dye based on calcium sulfide

Objectives. There is considerable economic demand for products obtained by processing phosphogypsum. In particular, calcium sulfide-based materials having luminescent properties are the object of intensive study due to the wide range of possibilities for their use. The alloying of the structure of calcium sulfide with cations of rare earth elements leads to the appearance of a glow having various colors. However, the high cost of such phosphorescent materials is due to the high chemical purity of the reagents required for their synthesis. The development of efficient methods for obtaining calcium sulfide-based luminescent materials from phosphogypsum is part of an integrated approach to solving the problem of synthesizing economically demanded materials from production waste.
Methods. The synthesized materials were studied using X-ray phase analysis and scanning electron microscopy. Photos of the samples were taken under illumination with an incandescent lamp or a fluorescent ultraviolet lamp.
Results. According to X-ray phase analysis, phosphogypsum is mainly comprised of calcium sulfate dihydrate and calcium sulfate hemihydrate. Heat treatment of a phosphogypsum sample at a temperature of 1073 K is accompanied by the formation of anhydrous calcium sulfate. In the presence of a reducing agent, a composite material is formed containing a phase of anhydrous calcium sulfate and calcium sulfide. Due to the calcium sulfide content, phosphogypsum has luminescent properties when heat-treated in the presence of various reducing agents, including activated carbon, wood charcoal, vegetable oil, citric acid, starch, and sucrose.
Conclusions. Optimal technological conditions for obtaining a composite material exhibiting luminescent properties are revealed. The successful synthesis of phosphor from without nonpretreated phosphogypsum is demonstrated. Optimal technological conditions for obtaining a composite material exhibiting luminescent properties are as follows: heat treatment temperature is 1073–1173 K; isothermal holding time is 60 min; reducing agent quantity is 37–50% (mol). The study results are widely applicable to processing wastes obtained from large-scale chemical production involving the production of a highly demanded inorganic product.

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