Transforming phosphogypsum waste into products with market value

Vast amounts of phosphogypsum (PG) which is a by-product of phosphorous acid production from apatite rock using sulphuric acid, are deposited in large piles at many locations worldwide. PG  materials  are  added  at  rates  of  the  order  of  megatonnes  (Mt)  per  annum.  Recognising these PG piles to be a problem and a threat to the environment resulted in the development of technologies aiming at transforming these piles into useful products. The sheer amounts of PG material to be processed and the available markets for products make it necessary to use a portfolio of approaches.

Researchers  from  Spain  and  Finland  have  investigated  methods  for  converting  PG  into  a precipitated  calcium  carbonate  (PCC)  and  other  products.  While  process  conditions  and equipment  can  be  practically  identical  (similar  near  ambient  temperatures  and  pressures,  in aqueous solutions), the use of either ammonia and CO2, or sodium hydroxide in the conversion processescan result in ammonium sulphate and calcium carbonate, or sodium sulphate and calcium hydroxide, which can bind CO2, subsequently. Thereby, besides PG valorisation, large amounts  of  CO2can  be  also  fixed  into  carbonates,  thus,  contributing  to  carbon  emissions control strategies. Other waste streams have been successfully tested as possible reactants for converting PG such as soda-rich liquid waste from the aluminium industry. Important is the presence of rare earth elements and radionuclides in the PG, requiring special treatment while recovering materials with large market value. Aiming at products with high purity, the removal of phosphorus from PCC product may be necessary.

This paper summarises the work and findings that led to a joint approach towardsprocessing 120  Mt  of  PG  deposited  at  the  salt  marshes  on  the  Tinto –Odiel  estuary  at  Huelva,  Spain, which was declared a UNESCO Biosphere in 1983. The various process routes are outlined and   the   quality/purity   and   amounts   of   products   obtained   are   quantified.   Finally,   some projections  on  development  towards  large-scale  implementation  and  commercialisation  are given.