Environ. Sci. Technol. 1084, 18, 658-667
Thermal Stabilization of Uranium Mill Tailings David R. Dreesen, Edward J. Cokal, Lawrence E. Wangen, and Joel M. Wllllams*
Environmental Science Group, Los Aiamos National Laboratory, Los Aiamos, New Mexico 87545 Edward F. Thode
Department of Management, New Mexico State University, Las Cruces, New Mexico 88003 The treatment of uranium mill tailings by high-temperature sintering (>lo50 OC) has been investigated as a means of controlling the release of 222Rnand leachable contaminants. Thermal stabilization in laboratory trials at 1200 "C reduced the radon emanation of various tailings by factors ranging from 37 to 1400 depending on the mineralogy of the tailings. The leachability of most contaminants (e.g., Al, Cd, Mn, Pb, U, and Zn) was substantially reduced. The weathering of thermally stabilized tailings was simulated by grinding and leaching and appears dependent on the gypsum content and particle size distribution of the original tailings as well as the amount of amorphous material produced during thermal treatment. Pilot-scale thermal stabilization tests verified the technical feasibility of this conditioning process. A conceptual engineering design of a thermal stabilization operation has been developed around the use of coal-fired rotary cement kilns; economic analysis of remedial action alternatives at several inactive uranium processing sites indicates that the cost of thermal stabilization is comparable to relocating the tailings piles. Introduction
The radiation hazards posed by uranium mill tailings have received considerable public and scientific attention during the past decade (1-3). In addition to the radiation hazards, a number of recent studies have illustrated that uranium mill tailings contain high concentrations of potentially toxic trace elements (e.g., Mo and se) which can pose significant environmental hazards if not effectively contained (4-6). The Uranium Mill Tailings Radiation Control Act (PL 95-604) directed the U.S. Environmental Protection Agency (EPA) to issue standards which would protect human health and the environment from radioactive and nonradioactive hazards posed by tailings at designated inactive processing sites (7). The initial standards proposed in Dec 1980 by the EPA required that (a) the disposal methods be effective for 1000 years, (b) zzzRnflux be reduced to 2 pCi m-2 s-l plus the contribution of the cover, and (c) groundwater not be degraded at distances greater than 1km from an existing tailings site (7). In Jan 1983, final standards were issued which were considerably relaxed regarding zzzRnflux control and imposed no numerical limits regarding groundwater contamination (8). Los Alamos National Laboratory initiated development of tailings conditioning technologies in 1980 to meet the stringent EPA proposed standards (7) as part of the overall technology development program of the U.S. Department of Energy's Uranium Mill Tailings Remedial Action Project (9). The goal of conditioning uranium mill tailings is to control contaminant mobility a t the micro or particle scale as opposed to more typical disposal scenarios which used barriers to isolated the entire tailings mass from the *Address correspondence to this author at the Los Alamos National Laboratory, Materials Science and Technology, MST-6, Los Alamos, NM 87545. 658 Environ. Sci. Technoi., Vol. 18, No. 9, 1984
environment. Our investigations of contaminant immobilization have centered on radically modifying the structure of tailings by sintering at high temperatures, i.e., thermal stabilization. In this paper we will discuss (a) laboratory thermal stabilization experiments which demonstrated the reduction in contaminant release, the mineralogical changes resulting from such thermal stabilization, and the resistance of thermally stabilized tailings to physical and chemical degradation, (b) a pilot-scale thermal stabilization study that better verified the technical feasibility of this process, and (c) a brief summary of the engineering and economic feasibility of a field-scale thermal stabilization operation. Experimental Methods
Tailings Analysis and Thermal Stabilization Procedures. Tailings samples were collected at three inactive uranium mill sites: Salt Lake City, UT (SLC); Shiprock, NM (SHIP); Durango, CO (DGO). These samples were collected from the top 1 m of tailings (2 m at SLC) and therefore represent near-surface tailings from 11 to 12 locations at each site (see ref 6 for details). Site samples that had similar physical and chemical characteristics were composited to provide a modest number of samples (i.e., two to three from each site) covering a range of elemental compositions. The designation of these composites as well as an individual tailings sample from Ambrosia Lake, NM (AML),were as follows: (a) Shiprock-SHIP Sands, SHIP Fines; (b) Salt Lake City-SLC 1AI3, SLC 4Al3, SLC 5AI3; (c) Durango-DGO LP, DGO SP Sands, DGO SP Fines; (d) Ambrosia Lake-AML Fines. These nine composites were characterized for (a) elemental composition by neutron activation analysis (NAA) and flame atomic absorption spectrophotometry (AAS), (b) particle size by sedimentation and wet-sieving techniques, and (c) total azsRa and other radionuclides by y spectroscopy. For details regarding analytical methods, see ref 6 and 10. Composite subsamples, weighing 1W200 g, were placed in either fire clay or graphite (graphite-bonded metal oxide) crucibles to about two-thirds capacity. The crucibles were loaded into a box furnace that was preheated to 800 "C. The furnace reached 1200 "C in about 2 h and was kept at that temperature for 45 min. The temperature wm set back to 800 OC, and the samples were removed about 90 min later when the temperature of the furnace had decreased to 800 "C. Sintered tailings could be separated from graphite crucibles. However, some of the tailings fused to the fire clay crucibles; in these cases, the fused tailings and crucible were pulverized together and the radon emanation of the combined materials measured. The radon contribution of a control fire clay crucible was determined to be
