Natural radioactivity in phosphate rock, phosphogypsum and phosphate fertilizers in Brazil

Abstract

Phosphate deposits are generally characterized by enhanced radionuclide concentrations compared to natural levels. The mining and processing of this phosphate ore redistribute radionuclides throughout the environment and introduce them into phosphoric acid and phosphogypsum. Phosphoric acid is the starting material for triple superphosphate (TSP), single superphosphate (SSP), monoammonium phosphate (MAP), diammonium phosphate (DAP), NPK fertilizers and di-calcium phosphate (DCP). Contents of natural radionuclides from thorium and uranium series, 226 Ra, 210 Pb and 228 Ra, were measured in Brazilian igneous phosphate rock, phosphoric acid, phosphogypsum and phosphate fertilizer samples, using high-resolution gamma-spectrometry. Neutron activation analysis was used for the determination of U and Th in the same samples. The fertilizers samples which are derived directly from phosphoric acid, MAP and DAP, presented activity concentrations around the detection limits of the counting system for 226 Ra (<5.0 Bq . kg –1 ), for 228 Ra (<3.0 Bq . kg –1 ) and for 210 Pb (<19 Bq . kg –1 ). As for U and Th, the concentrations found in MAP and DAP are more significant, up to 374 and 250 Bq . kg –1 , respectively. SSP, TSP and NPK, which are obtained by mixing phosphoric acid with different amounts of phosphate rock and NH3, presented higher concentrations of radionuclides, up to 871 Bq . kg –1 for 226 Ra, 283 Bq . kg –1 for 228 Ra, 1255 Bq . kg –1 for 210 Pb, 413 Bq . kg –1 for U and 538 Bq . kg –1 for Th.