ARTICLE pubs.acs.org/est
Recovery of Uranium from Mine Waste by Leaching with Carbonate-Based Reagents Eliz^angela A. Santos and Ana C. Q. Ladeira* Department of Mineral Technology, Center for the Development of Nuclear Technology, CDTN, Brazil ABSTRACT: Waste materials, stored in inappropriate places, are one of the most significant environmental issues concerning mining activities. In Brazil, one closed uranium mine has faced such a problem. The waste, produced during the neutralization of acid drainage and containing several metals including uranium, has been disposed into the mine opening as a temporary alternative for over 20 years. The present work aimed at the recovery of the uranium present in the aforementioned waste. The effect of the following parameters on the leaching procedure was investigated: solid/liquid ratio, time, temperature, extracting agents, concentration of reagents, and the use of oxidants. The chemical characterization showed that the main constituents of the sample are Ca, S, Mn, and Al. Uranium is around 0.25%. The crystallized phases are ettringite (Ca6Al2(SO4)3 3 (OH)12 3 26H2O) as the major phase followed by gypsum (CaSO4 3 2H2O), calcite (CaCO3), and bassanite (CaSO4 3 0.5H2O). Carbonate and bicarbonate were observed to be effective extractants for the uranium. However, a combination of both reagents proved to be a better option than their individual use, and extractions around 100% were achieved. The optimum experimental condition for attaining the maximum dissolution is 0.11 solid/ liquid ratio, 0.50 mol L1 Na2CO3, 1.00 mol L1 NaHCO3, 20 h, and room temperature.
1. INTRODUCTION The Caldas mining and milling plant was the first industrial plant for uranium recovery in Brazil. The uranium mine operated from 1982 to 1995, when mining operations were ceased. As a result of the mining activities, some environmental problems have occurred including acid mine drainage (AMD). Since 1982, the mine has operated an active maintenance system, which focuses on the treatment of the acid drainage originated at the mine pit and waste rock dumps. Acid mine drainage is among the main environmental impacts of mining activities due to its dynamics and persistence. The period for acid production and pollution generation may be very long, varying from 20 to 100 years.1 The AMD generated in the Caldas uranium mine occurs from the oxidation of a large amount of sulfide minerals. Most of the sulfides are present in disposed waste rocks with low uranium content (250 mg L1).2 As a consequence, sulfuric acid is produced and reduces the pH of surface waters. Such an acid solution acts as a leaching agent for the minerals present in the waste and soil, generating a solution rich in metallic ions. The current chemical treatment of the AMD is carried out by increasing the pH with lime, from 2.7 to above 10, in order to precipitate the metals and to adjust the concentration of the contaminants to the levels set by Brazilian protection standards. The precipitation procedure has been used for over 20 years, and the alkaline precipitate has been disposed into the mine opening which lacks layers to prevent contaminant seepage into the ground or groundwater. However, this disposal procedure r 2011 American Chemical Society
should be ceased according to recent plans for the mine closure. The aforementioned alkaline sludge is a residue that can pose threats to the environment. The recovery of toxic elements may be an alternative to reduce the risks significantly. Presently, there is an ongoing effort to determine how, and to what extent, this site will be cleaned up, and also the final fate of this sludge is under investigation.3 The extraction of the uranium from mine tailings and contaminated soils can be carried out through hydrometallurgical techniques such as leaching using sulfuric acid (H2SO4), sodium carbonate (Na2CO3), and sodium bicarbonate (NaHCO3) as leaching agents for uranium.4 Although sulfuric acid leaching is the most common process for the recovery of uranium from ores, alkaline leaching is considered a more selective technique than the former one. In addition to selectivity, other advantages of the carbonate leaching process are the purity of the liquor and the relatively easy way that uranium can be precipitated from the leach liquor. Carbonate leaching is also particularly attractive in cases where excessive contents of carbonated minerals render the use of acid leaching economically impractical.410 The leaching procedures comprise the dissolution of CaU2O7 due to the formation of the soluble uranyl tricarbonate anion, [UO2(CO3)3]4, when Received: June 29, 2010 Accepted: March 11, 2011 Revised: March 3, 2011 Published: March 24, 2011 3591
dx.doi.org/10.1021/es2002056 | Environ. Sci. Technol. 2011, 45, 3591–3597
Environmental Science & Technology sodium or ammonium carbonate solutions are used as leaching agents.4,1115 Similar to the Caldas mine, the existence of areas degraded or contaminated by inadequate disposal of wastes/residues represents one of the worst environmental legacies of the mining companies. According to ref 5, there are 24 inactive U mill tailings sites in the USA. In addition, Argentina, Australia, Brazil, Canada, Germany, South Africa, and the USA are wrestling with the clean up and environmental restoration issues associated with U mining and milling. The present work is a contribution to the rehabilitation of areas degraded by mining activities through the use of available hydrometallurgical techniques to treat a nonconventional waste. The recovery of uranium is the second part of a broader project; the first part has comprised the characterization of the sludge,6 and the third part will focus on the possibility of using the final waste, free of uranium, to raise the pH of agricultural soils. For this purpose, the presence of toxic elements and also sodium in the final residue should be assessed before its use. Thus, the application of the alkaline leaching to recover the uranium from a nonconventional waste, together with the possibility of reusing the final waste, is an original contribution that makes the use of this technology economically more attractive and more environmentally friendly. This work investigated the extraction of the uranium present in the sludge generated in acid mine treatment by leaching with carbonate-based reagents. The influence of some parameters such as time of leaching, bicarbonate and carbonate concentration, pH, temperature, and solid/liquid ratio were investigated on the extraction efficiency by batch experiments. In addition, a chemical and mineralogical characterization of the sludge and the final residue is also presented.
2. MATERIALS AND METHODS 2.1. Sample Preparation. The sludge sample was collected from the mine opening, where the material is disposed currently. The preparation procedures included drying in an oven at 50 °C for 24 h, milling to disaggregate the dry material, and sieving in order to obtain homogeneous samples of 50 g. Particle size distribution was carried out by Cyclosizer apparatus. The Cyclosizer cone cut sizes were (in mm) 0.44, 0.35, 0.24, 0.16, and
