Specific activity of uranium and thorium in marketable rock phosphate as a function of particle size

Marketable rock phosphate fertilizer from Florida was classified into seven particle size fractions ranging from 149 μm to less than 0.5 μm using a Bahco Microparticle Classifier and air elutriation. The resulting size fractions were assayed for U and 230Th by solvent extraction and liquid scintillation α-spectroscopy. Results indicated that the specific activity of U and 230Th increased with decreasing particle size. Maximum activities of 110 pCi/g U and 50 pCi/g 230Th were found in particles less than 1.0 μm in aerodynamic diameter. Qualitative emission spectrographic analysis of the fractions revealed that the concentrations of Al, Cu, Mg, Na, Ti and Zn also increased with decreasing particle size. (author)