Development of Economically Stabilized Phosphogypsum Composites for Saltwater Application
Phosphogypsum (PG, CaSO4.2H2O) is a waste by-product produced during the wet-process manufacturing of phosphoric acid. The USEPA mandates that PG be disposed of in stacks due to the potential release of Rn222(from Ra226 decay), which may pose a potential threat to public health and the environment.
This research focused on stabilizing PG with Portland Type II cement and Class C fly ash for use in marine environments. The 73%:25%:02%, 67%:30%:03%, 63%:35%:02% and 62%:35%:03% PG:Class C fly ash:Portland Type II cement composites demonstrated promising results with no signs of degradation after 12 months of natural saltwater submergence. The Ra226 concentrations in the TCLP leachate were well below the current EPA regulatory value for drinking water (5 pCi.L–1). The metal concentrations in the leachate were also well below the USEPA toxicity characteristics limits. A one-dimensional diffusion model based on Fick’s second law with a non-zero surface concentration at the solid-solution interface was developed to calculate effective calcium and sulfate diffusion coefficients of composites placed in saltwater. This model determined that the range of effective calcium, sulfate and Ra226 diffusion coefficients were 1.36-8.04 x 10–13 m2.s–1, 2.96-7.20 x 10–13m2.s–1 and 1.46-2.90 x 10–17 m2.s–1, respectively. This model also predicted that calcium, sulfate and Ra226 will stop leaching at a critical time (tc) of 64-78, 122-137, and 150-470 days, respectively, when the leaching processes are balanced by precipitation reactions. The effective diffusion coefficients of Cu, Cr, and Zn ranged from 6.46 x 10–12 to 1.21 x 10-11 m2.s-1, 5.83 x 10-13 to 1.69 x 10-12 m2.s-1, and 3.97 x 10-14to 9.10 x 10-14m2.s-1, respectively. The tc for Cu, Cr, and Zn to stop leaching were 78-254 days, 61-168 days and 89-145 days, respectively.
The engineering properties test results indicated that the composite material could be classified as well-graded gravel or well-graded sand with little or no fines. The direct shear test determined the angle of internal friction as 49-50°. The USCS classification would also qualify the PG briquettes as a potential fill material in embankment construction projects because of the excellent workability characteristics. The economic analysis indicated that the commercial cost for the selected four stabilized PG briquettes ranged from $11.94 to $15.45 per ton–1(2003 value) in Tampa, Florida. This result indicates that the briquettes (42.5mm x 23.5mm x 14.5mm) can potentially replace the larger (15-30.5 cm) and more expensive ($35 per ton–1; average delivered cost for 2003) granite backfill currently used in the state of Florida.
It is recognized that granite would still be needed as an armoring to protect the stabilized PG from wave-derived energy.