Transforming phosphogypsum waste into products with market value
Ron Zevenhoven1, Victor Morales-Flórez2,3, Alberto Santos4, Luis Equivias2,3
1bo Akademi University, Finland; 2University of Seville, Seville, Spain; 3Institute of Materials Science, ICMS (CSIC/US), Seville, Spain; 4University of Cádiz, Puerto Real, Spain
Vast amounts of phosphogypsum (PG) by-product from producing phosphorous acid from apatite rock using sulphuric acid have been deposited in large piles at many locations worldwide. Often, material is still added at rates of the order of megatonnes (Mt) per annum. Recognising these piles to be a problem and a threat to the environment resulted in the development of technology aiming at transforming the PG piles into useful products. The sheer amounts of PG material to be processed and the available markets for products makes it necessary to use a portfolio of approaches.
Researchers from Spain and Finland have investigated methods for converting PG into precipitated calcium carbonate (PCC) and other products. While process conditions (near ambient temperatures and pressures, in aqueous solutions) and equipment can be practically identical, the use of either ammonia and CO2, or sodium hydroxide can give ammonium sulphate and calcium carbonate, or sodium sulphate and calcium hydroxide that can still bind CO2 subsequently. Thereby, besides PG valorisation, large amounts of CO2 are fixed into carbonates, 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 aluminium industry. Important is the presence of rare earth elements and radionuclides in the PG, dictating special treatment while recovering materials with large market value. For products with high purity also removal of phosphorus from PCC product may be necessary.
This paper summarises 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, 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 process economy and commercialisation are given.