Use of Byproduct Gypsum

In the production of wet-process phosphoric acid, 4.5 -5.0 tonnes (dry basis) of byproduct gypsum is produced per tonne of P2O5 recovered as phosphoric acid. The global output of 40 million tonnes P2O5 per year in the form of wet-process phosphoric acid is equivalent to the production of about 150 million tonnes of phosphogypsum, of which only about 15% is reused. 

Phosphogypsum retains at least 20% of free water by weight. The disposal of this material as a waste product is discussed in Chapter 19. This chapter will briefly review ways for usir the byproduct. The prindpal me uns for using phosphogypsum are to  

Plaster consists mainly of gypsum that has been dehydrated to hemihydrate (CaS04 1/2 H2O) under carefully controlled conditions so that it will rehydrate rapidly when mixed with water, The dehydrated gypsum usually is finely ground and sometimes mixed with a filler before use. Phosphogypsum must be purified for this use the extent of purification depends on the amount and character of the impurities and how the plaster is to be used. 

For wrallboard production, rapid setHng is especially necessary, and phosphogypsum with a high P2O5 con- 

In general, the processes consist of purification of phosphogypsum including, in some cases, the addition of chemicals such as lime to neutralize acidity, thermal treatment to convert gypsum to hemihydrate or anhydrite or some combination of the two, and preparation for the market, which may include grinding or c glom-eration and, in some cases, drying. 

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Rao, D. G. 1977. Technoeconomic Evaluation of Alternate Uses of Byproduct Gypsum, FAI-lFDC Fertilizer Seminar Proceedings. 

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