Geochemistry and mineralogy of a uraniferous sub bituminous coal

Abstract

A sample of subbituminous uraniferous coal from the Red Desert, Sweetwater County, Wyo., has been studied mineralogically. The coal contains gypsum (6 percent), kaolinite (1 percent), quartz (0.3 percent), calcite (trace), and limonite (trace). This suite of minerals and the absence of pyrite show that the coal has been subjected to weathering and oxidation. No uranium minerals have been found; mechanical fractionation has indicated that the uranium is associated with the organic constituents of the coal. The minerals that have been isolated contain 0.0006 percent uranium, a content that is to be expected for nonuraniferous sedimentary rocks. The organic components of the coal contain approximately 0.002 percent uranium. On the basis of material-balance calculations, the organic components carry 98 percent of the uranium in the coal. Batch extraction of the Wyoming coal with 6 N hydrochloric acid leads to the solution of almost 90 percent of its uranium. Recovery of uranium is independent of the particle size of the coal between - 4 and - 20 mesh, and is accompanied by the solution of approximately 70 percent of the inorganic constituents (ash) of the coal. The extract contains, together with uranium, a concentration of several elements such as manganese, cerium, and vanadium that are present in the coal. Fischer assays of this weathered coal from the Red Desert indicate a yield of 16.7 gallons of tar per ton. Yields of both tar and char are about 15 percent lower if the coal is first treated with 6 N hydrochloric acid to extract the uranium. It is suggested that uranium was introduced into the coal by means of ground water carrying soluble alkaline or alkaline-earth uranyl carbonate complexes. The mineral schroeckingerite-a similar complex-is found near this subbituminous coal in the Red Desert of Wyoming. These complexes, which are unstable in acid medium, release the uranyl ion (U02++), which may then react with organic constituents of the coal to form ionic uranyl-organic compounds that are insoluble above a pH of about 2.2. Preliminary data indicate that the uranium is associated with various humic components of the coal. The distribution of uranium among the components of the subbituminous coal from Wyoming is similar to the distribution in a lignite from South Dakota. There is no indication that uranium was introduced into or retained by the two coals by appreciably different geochemical processes. © 1955 Society of Economic Geologists, Inc.