Sequential determination of U and Th isotopes and ~(226)Ra by alpha spectrometry in phosphate fertilizer samples and phosphogypsum

摘要

The Brazilian phosphate fertilizer is obtained by wet reaction of igneous phosphate rock with concentrated sulphuric acid, giving as final product phosphoric acid and calcium sulphate (phosphogypsum) as by-product. Phosphoric acid is the starting material for the production of the majors’ phosphate fertilizers: triple superphosphate (TSP), single superphosphate (SSP), monoammonium phosphate (MAP) and diammonium phosphate (DAP). The phosphate rock used as raw material is enriched in radionuclides of the U and Th natural series. During the chemical attack these radionuclides are distributed among products and by-products. A sequential procedure was implemented to determine the content of radionuclides alpha emitters (~(234)U, ~(238)U, ~(230)Th, ~(232)Th, ~(228)Th and ~(226)Ra) in samples of phosphate rock, phosphogypsum and phosphate fertilizers produced by the Brazilian industries. The experimental procedure consisted of a chemical attack with strong acids, followed by sequential chromatography extractions in column with ionic resins. Depending upon the conditions of percolation U isotopes are separated, followed by Th isotopes and ~(226)Ra. The solutions containing U and Th are electroplated on stainless steel disks and counted on a surface barrier detector for alpha spectrometry. To the Ra final solution, a BaSO4 seeding suspension was added to allow the formation of a micro precipitated of BaRaSO4. This precipitate is filtered in a polypropylene membrane and counted on the same detector. The precision and accuracy of the method were determined by analyzing reference materials: soil IAEA-326 and sediment IAEA-300. SSP and TSP, which are obtained by reacting phosphoric acid with phosphate rock in different proportions, presented higher concentrations of all studied radionuclides. Brazilian phosphate fertilizers, which are produced directly from phosphoric acid, MAP and DAP, present in their composition low concentrations of radionuclide ~(226)Ra. As for the radionuclides U and Th, the concentrations observed are higher.