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 processes can 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 CO2 can 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 towards processing 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.