LEACHING OF URANIUM AND THORIUM FROM MONAZITE .1. INITIAL LEACHING

Interaction of three natural monazite specimens with a bicarbonate-carbonate solution was investigated for times up to 6.8 years. Dissolution was observed to be incongruent with respect to 238U and 232Th as well as their radiogenic daughters 234U, 230Th, and 228Th. Leaching was divided into a very rapid initial stage lasting a few hours and a slower process active for the remaining time. The initial stage was modeled as the sum of a contribution from a mechanico-chemically damaged portion of the specimen, which did not exhibit isotopic selectivity in leach properties, and a contribution from the selective removal of recoil daughter products from their recoil tracks in the surface of the otherwise undamaged bulk mineral. The latter effect is greater for short-lived 228Th compared to long-lived 234U. A correlation between the magnitude of the effect and the half-life of the radiogenic nuclide suggests an upper limit of ~ 106 years for the timescale of natural track annealing. After the initial dissolution stage, insoluble precipitates of the intermediate product in the Th chain, 228Ra, provide a supplementary source of 228Th by radioactive decay. Contributions to these precipitates come from dissolution-released and recoil-released 228Ra. This source is manifest as large apparent release rates of 228Th which begin after several weeks of leaching. Preannealing of a specimen at 800°C depresses the elemental Th leach rate but enhances the amount of 228Th 232Th fractionation. This enhancement is associated with rejection of Ra from the mineral during annealing of α-recoil damage. © 1990.