Geochemical hosts of solubilized radionuclides in uranium mill tailings

Abstract

The solubilization and subsequent resorption of radionuclides by ore components or by reaction products during the milling of uranium ores may have both economic and environmental consequences. Particle-size redistribution of radium during milling has been demonstrated by previous investigators; however, the identification of sorbing components in the tailings has received little experimental attention. In this study, uranium-bearing sandstone ore was milled, on a laboratory scale, with sulfuric acid. At regular intervals, filtrate from this suspension was placed in contact with mixtures of quartz sand and various potential sorbents which occur as gangue in uranium ores; the potential sorbents included clay minerals, iron and aluminum oxides, feldspar, fluorspar, barite, jarosite, coal, and volcanic glass. After equilibration, the quartz sand-sorbent mixtures were separated from the filtrate and radioassayed by gamma-spectrometry to determine the quantities of 238 U, 230 Th, 226 Ra, and 210 Pb sorbed, and the radon emanation coefficients. Sorption of 238 U was low in all cases, with maximal sorptions of 1–2% by the bentonite- and coal-bearing samples. 230 Th sorption also was generally less than 1%; maximal sorption here was observed in the fluorspar-bearing sample and appears to be associated with the formation of gypsum during milling. 226 Ra and 210 Pb generally showed higher sorption than the other nuclides - more than 60% of the 26 Ra solubilized from the ore was sorbed on the barite-bearing sample. The mechanism (s) for this sorption by a wide variety of substrates is not yet understood. Radon emanation coefficients of the samples ranged from about 5 to 30%, with the coal-bearing samples clearly demonstrating an emanating power higher than any of the other materials.