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| ORIGINAL ARTICLE |
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| Year : 2022 | Volume
: 45
| Issue : 3 | Page : 158-169 |
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The behavior of mill tailings produced from a uranium extraction plant adapted an indigenously developed alkaline leaching-assisted process
Barendra Kumar Rana1, Samim Molla2, MR Dhumale2, SK Jha1, MS Kulkarni1
1 Homi Bhabha National Institute; Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
| Date of Submission | 09-Apr-2023 |
| Date of Decision | 10-Apr-2023 |
| Date of Acceptance | 10-Apr-2023 |
| Date of Web Publication | 18-May-2023 |
Correspondence Address:
Barendra Kumar Rana
Homi Bhabha National Institute, Mumbai - 400 094, Maharashtra
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/rpe.rpe_17_23
The potential impact of tailings with respect to uranium and its associated toxic metals in the surrounding environment is assessed by identifying their concentrations and physicochemical behavior in the tailings. Tessier's sequential extraction method was used to study the speciation of U, Co, Zn, Mn, Fe, Pb, Cr, and Cd in different geochemical fractions of the alkaline mill tailings of the Tummalapalle process plant. The study indicated that uranium and other toxic metals present in the tailings have different chemical forms. Most of the toxic elements are not readily available in an exchangeable phase; rather, they are present in the tailings in stable complex form. However, a few elements, such as Co, Pb, and Cd, showed significantly higher concentrations in the oxidizing phase. The elements present in the oxidizing phases can be labile after prolonged exposure to atmospheric conditions. U (nat.) and Fe associated with the tailings are indicated to be mostly distributed in the residual phase. Radioactive disequilibrium of 238U and 232Th series radionuclides in the ore, waste rock, and tailings were studied. Disequilibrium was mostly noticed in the 238U series radionuclides in these matrices; however, the 232Th series radionuclides showed a radioactive equilibrium between their parents and daughters.
Keywords: Sequential extraction, speciation, uranium, uranium tailings
How to cite this article:
Rana BK, Molla S, Dhumale M R, Jha S K, Kulkarni M S. The behavior of mill tailings produced from a uranium extraction plant adapted an indigenously developed alkaline leaching-assisted process. Radiat Prot Environ 2022;45:158-69 |
How to cite this URL:
Rana BK, Molla S, Dhumale M R, Jha S K, Kulkarni M S. The behavior of mill tailings produced from a uranium extraction plant adapted an indigenously developed alkaline leaching-assisted process. Radiat Prot Environ [serial online] 2022 [cited 2023 May 28];45:158-69. Available from: https://www.rpe.org.in/text.asp?2022/45/3/158/377229 |
| Introduction | |  |
The higher demand for uranium has spurred the exploration, mining, and processing of uranium-bearing ores in the country. Recently, uranium mining and processing of uranium ore have been started at the Tummalapalle region in the Proterozoic South Cuddapah Basin in Andhra Pradesh, which is known to be the major uranium province by the Atomic Mineral Directorate for Exploration and Research,[1] beyond the Singhbhum thrust belt of Jharkhand. The uranium mineralization occurs within the Vempalle carbonate host rock, which is known to be a strata-bound formation. It is basically dolomitic limestone (“dolostone”) along with stromatolite limestone. The Vempalle dolostone is mainly composed of carbonate (83.2%), quartz and feldspar (11.3%), apatite (4.3%), and pyrite (0.47%).[2] Uraninite ([U+41−x, U+61−x] O2+x) is the primary uranium mineral abundant on the earth. Besides this, secondary minerals such as gummite (Na[UO2][O, OH]), autunite (CaO.2UO3.P2O5.8H2O), saleeite (Mg[UO2]2 [PO4]2·10 [H2O]), torbernite (Cu[UO2]2 [PO4]2.12H2O), and coffinite (U[SiO4]1-x[OH] 4x) are also found in nature. The radioactive minerals identified in the Tummalapalle zone are known to be pitchblende, coffinite, and U-Ti complex. For low-grade uranium ore, typically in India (<0.05% U3O8), where uranium is the only by-product, the tonnes of ore processed generate an equivalent quantity of solid waste, known as tailings, which contain the entire daughter products of the uranium decay chain with unrecovered uranium. In nuclear fuel cycle facilities, the large volume of waste generated mainly represents uranium mill tailings, where more than 900×106m3 of tailings have been stored, consisting of about 180 piles operating in more than 30 countries in the world.[3] In India, the coarse fraction of the tailings is basically used as backfill material in the exhausted mine working, and the rest fraction is dumped in an engineering-designed tailings pond for long-term impoundment. Several environmental problems that can arise from mill tailings are radon emanation, dispersal of airborne radioactive dust, and leaching of contaminants, including radionuclides and toxic metals, into the surface and groundwater.[3] Tailings can differ from the ore as they are produced after numerous physical and chemical treatments such as crushing and grinding to fine particles, leaching with acids (sulfuric acids or hydrochloric acids) or alkaline solutions (carbonate or bicarbonate), etc. Mill tailings generally contain unrecovered uranium with the entire daughter products of the uranium decay series and several toxic metals, such as iron, manganese, lead, and cadmium.[4]
Uranium and the associated toxic metals in tailings may represent a potential radiological and toxic hazard to the environment. These metals can be migrated to the natural environment by a series of transformations through physical and chemical processes, such as dissolution-precipitation, redox, adsorption–desorption, and complexation reactions, and may remain in the environment for a longer time.[5] As tailings are stored in the environment for a longer period, the different environmental conditions may influence the physicochemical properties of uranium and toxic elements that may have a significant impact on the surrounding environment. The physicochemical properties of soil and diverse microbial activity can profoundly influence the environmental behavior of radionuclides associated with the mill tailings. In addition, the presence of radionuclides showing a profound influence on soil properties and microbial diversity was verified in uranium mill tailings.[6]
The potential impact on the environment of radionuclides and toxic metals present in the tailings is assessed by estimating their total quantities available in a solid matrix. However, the assessment of the total elements may not provide a precise estimate of the potential environmental effects. Many studies indicated that the bioavailability, mobility, and toxicity of the metals to the biota depended on the physical and chemical forms of uranium and toxic metals present in tailings. The remobilization of the toxic metals and the identification of the chemical forms of toxic elements are required to understand the geochemical processes. Tessier's sequential extraction procedure is a classical method allowing the chemical separation of major mineral ingredients.[7] The present study focused on the distributions of U, Co, Zn, Mn, Fe, Pb, Cr, Cd, and Cu in different geochemical fractions of Tummalapalle mill tailing samples to understand the fate and transport of toxic metals in the environment. The distribution of other radionuclides, including the uranium and thorium decay series, was also studied in ore, waste rocks, and tailings to evaluate the potential radiological risk to the environment.
Alkali leaching process at Tummalapalle site
The extraction of uranium at Tummalapalle Mill was carried out by an indigenously developed alkali leaching route from the dolomitic ore, using oxygen and sodium carbonate as main leaching agents at high temperatures and pressure.[8] The higher-sized mined ore is reduced to 74 μm in combination with the crushing and grinding units before leaching in the autoclave. The ore with a grind size of about 80% of –74 μm and a slurry density of about 60%–65% (w/w) is fed to the autoclave for the leaching of U at an optimized O2 pressure of 7.5 kg/cm2, a Na2CO3 concentration of about 40–50 g/l, along with an optimized concentration of NaHCO3, and a temperature of about 130°C–140°C. The optimized leaching is achieved by maintaining the residence time of the slurry at about 7 h in the autoclave. The uranium existing as U (+4) in the ore is oxidized to U (+6) which easily leaches into the aqueous phase. The detailed reactions that take place in the autoclave during the leaching are described as follows:[8]
Further, the detailed reaction of the gangue material is also presented as follows:
After the leaching of the uranium, the slurry is subjected to solid–liquid separation by passing through the horizontal leached belt filter. The generated barren solid waste was discharged in the form of slurry to an engineered-designed tailing pond for impoundment. The generated liquor is clarified by the clarifier and passed through a precoat drum filter to get clarified pregnant liquor. The liquor produced has a low uranium concentration, and later, with the help of the RDS (re-dissolution system), the uranium content in the liquor is increased to an optimized level by the feeding technique before precipitation. Finally, the U in leach liquor is precipitated with NaOH solution at atmospheric pressure and 50°C–60°C temperature as sodium diuranate (SDU).[9] By recovering sodium sulfate from the effluent produced by the SDU precipitation unit, carbonation is then performed to convert sodium hydroxide into sodium carbonate and sodium bicarbonate. The carbonated effluent is further reused in the process. The flow sheet of the alkali leaching process is presented in [Figure 1].
| Materials and Methods | | |
Site description
The study was conducted around the uranium mining and ore processing facility at the Tummalapalle site in the district of Kadapa in Andhra Pradesh. The area lies in the southwestern portion of the mid-to-late Proterozoic crescent-shaped Cuddapah basin in Andhra Pradesh, India. It is overlain by a significant Eparchaean unconformity and has a thick, unmetamorphosed to mildly metamorphosed arenaceous, and argillaceous sedimentary succession. On the southwest boundary of the basin, the sediments are generally undisturbed beneath a thick sedimentary layer, which is overlain by severely metamorphosed and deformed late Achaean to early Proterozoic granite gneisses and Dharwarian schists, whereas on the eastern margin, Dharwarian schists and gneisses surged over them. Carbonate rock formations host the geological formations of this area. Lithologically, this area comprises purple shale, shale, massive limestone, cherty limestone intraformational conglomerate, and dolostone (uranium-rich).[2] Uranium mineralization is hosted by phosphatic dolostone, which is confined between purple shale and dolostone.[10] The extraction of uranium at the Tummalapalle processing plant was commissioned in 2017 using the indigenously developed alkali leaching route. The barren solids generated in the processing plant are disposed of in a tailings impoundment facility through a pipeline. The liquid effluent associated with tailings is collected in the decant water pond through the spillway and recycled to the effluent treatment plant for treatment before reuse in the processing plant. The tailings generally contain some unrecovered uranium, the entire daughter product of the uranium decay chain, and other chemical contaminants, including several toxic metals. The climate of the region has been observed to be tropical and semi-arid, with more sunny days. The winter season is short, while the summer season is long and hot. The temperature in the region varies from 15.7°C to 44.2°C in the winter and summer, respectively. The average annual (2012–2020) rainfall in this area was 377–1379 mm.[11] Least rainfall is observed during January–May, whereas maximum rainfall is observed during August–November. The locations of the tailings pond and the Tummalapalle site are shown in [Figure 2].
Sample collection and processing
Grinded ore samples of size 74 μm were collected from the horizontal neutral belt filter of the mill. Waste rock samples were collected from the waste yard, and finally, their size was reduced to 74 μm in combination with a laboratory jaw crusher (model IJC-3, made by Insmart, Hyderabad), and a ball mill. Tailings samples were collected from the tailings discharge points at the tailings pond. Ore, waste, and tailings samples were dried in a hot air oven at 105°C for 24 h, and about 400 g of samples were packed airtight in a 250 mL PET bottle for a period of 1 month before gamma spectrometric analysis.
Experimental
Tailings pH
Mobility and sorption of radionuclides and other contaminants strongly depend on pH. 30 g of tailings was placed in a 250 ml conical flask, to which 75 ml of distilled water was added, and the suspension was stirred for a few seconds. The suspension was then allowed to settle for 1 h with occasional stirring.[12] pH was recorded by immersing the pH electrode into the solution. The pH meter (make: Eutech Instruments, model: Cyberscan PCD 650) was calibrated using buffer solution standards traceable to NIST, having pH values of 4.01 ± 0.01, 7.00 ± 0.01, and 10.1 ± 0.01.
Sequential extraction
Tessier's sequential chemical extraction procedure was followed to differentiate metal fractions into five categories.
Exchangeable fraction (Exc.)
8 mL of a 1 M MgCl2 solution (pH = 7.0) was mixed with 1 g of sample (TMPL-1). The sample mixture was shaken for 1 h and centrifuged for 20 min. The residue was labeled as TMPL-2 for the next steps.
Carbonate fraction (Car.)
8 mL of a 1 M sodium acetate solution (pH = 5.0) was utilized to leach the residue (TMPL-2). The mixture was shaken for 5 h at room temperature and subsequently centrifuged for solid-liquid separation. The residue was labeled as TMPL-3.
Fe/Mn oxide fraction (Red.)
After being treated with 20 mL of a 0.04 M hydroxylamine hydrochloride solution mixed with a 25% acetic acid solution at 96°C for 6 h, the mixture sample was centrifuged for 20 min at 8000 rpm. The residue (TMPL-4) was retained for subsequent use.
Oxidizable-bound to organic matter (Ox.)
TMPL-4 was allowed to react with 3 mL of 0.04M HNO3 and 5 mL of a 30% H2O2 solution for 2 h at room temperature. The mixture was subsequently heated in a water bath at 85°C for 3 h. Finally, 5 mL of a 3.2M ammonium acetate solution (pH 2 adjusted by nitric acid) was added, and the mixture was shaken for 2 h, after which the mixture sample was centrifuged at 8000 rpm for 20 min. The residue was labeled as TMPL-5.
Residue fraction (Res.)
The last residue (TMPL-5) was digested with 2 mL of HClO4 and 10 mL of hydrofluoric acid until complete dissolution. Finally, the residue was dissolved in 12N HCl and diluted to 25 mL.
Analysis of uranium and other elements after sequential extraction
Uranium content in the filtrate was estimated using a UV fluorometer (Model: FL6224A, Make: ECIL, India). The details of the method for uranium analysis have been described elsewhere.[13] The sample solution (5 mL) was taken into a beaker and evaporated to near dryness on a hotplate. 1 mL of concentrated H2SO4 was added and brought to near dryness by evaporating. Subsequently, 10 mL of H2SO4 (pH = 2) was added to the beaker and transferred into a separating funnel. 10 ml of a 2% alamine in benzene solution was added into the separating funnel and mixed thoroughly by shaking for 5 min, and then, the solution was left for some time for the complete separation of the aqueous and organic layers. 0.1 mL of solution was transferred into a platinum dish, and 250 mg of the fusion mixture (85% Na2CO3 and 15% NaF) was added to it. The mixture was fused at 800°C in a muffle furnace for 3 min and then cooled before exposing the platinum disc to a UV excitation source (360 nm), and the emitted fluorescence reading at 554.6 nm was recorded.[13] The amount of uranium present in the sample is directly proportional to the fluorescence intensity. A known uranium standard solution and reagent blank were also processed for the calibration of the instrument. Uranium in the sample solution was estimated using Eq. (1).
Cs is the sample count, Cb is the blank count, Cstd is the standard count, Sstd is the concentration of U in the standard solution used for calibration (μg/L), Vstd is the volume of the standard solution used for calibration (mL), and Vs is the volume of the sample (mL). Cationic concentrations of Co, Zn, K, Mn, Fe, Pb, Cr, Cd, and Cu were analyzed using an atomic absorption spectrometer (AAS; make: GBC, Australia; model: SavantAA Sigma). NIST-traceable certified reference standards (Make: Inorganic Ventures, USA) were used to calibrate the AAS during the analysis. The instrument's performance was checked with a blank and standard analysis after the completion of ten consecutive samples.
Assessment of radioactivity in ore, waste rock, and tailings
A high-purity germanium (HPGe) detector-based gamma-ray spectrometer consisting of a carbon fiber (thickness: 0.6 mm) coaxial p-type HPGe detector (energy range: 3–10,000 KeV) with a relative efficiency of 50% with respect to a 3” × 3” NaI (Tl) detector was used for the assessment of 238U, 226Ra, 210Pb, 235U, 228Th, 228Ra, and 40K activities in the ore, waste rock, and uranium mill tailings. The HPGe detector was placed within a 10-cm-thick lead disc with an inner lining of 1 mm of tin (Sn), 1.5 mm of aluminum (Al), and 1.5 mm of copper (Cu) to reduce background counts. The energy calibration was performed using standard 60Co and 137Cs sources. The efficiency calibration for each of the radionuclides was performed using certified reference materials (RGU-1, RGTh-1, and RGK-1) obtained from the IAEA, Vienna. The energy resolution was calculated in terms of full-width half maxima and estimated to be 2.1 keV at 1332 keV gamma energy of a standard 60Co source. Each of the samples and the standard reference materials was packed in a 250-mL PET bottle of the same geometry to avoid any uncertainty arising due to measurement in a variable geometry. Using the two main gamma energies of 234Th, 63.3 keV (4.8%), and 92.59 keV (5.58%), the activity of 238U was estimated by gamma spectroscopy. The gamma energy line of 143.8 keV (10.9%) was used to estimate the activity of 235U. The prominent gamma radiation of 1764.5 keV (15.4%) energy was used to estimate 226Ra because its self-attenuation is small compared to other energies. The 46.53 keV line (4.25%) was used to estimate 210Pb in the samples. 228Ra concentration was measured as 228Ac using 911 (25.8%), 969 (15.8%), and 338 (11.3%) keV γ-rays. 228Th concentration was measured as decay products in equilibrium (i.e., 212Pb, using 238 and 300 keV γ-rays, 212Bi, using 727 keV γ-ray and 208Tl, using 2614, 583, and 860 keV γ-rays). 1460.8 keV (10.66%) gamma energy was used to estimate 40K activity. The activity of the radionuclide in the 60,000-second-count sample was estimated by the following equation:
Where η is the efficiency at a particular energy level, γ is the gamma emission probability (%), and W is the weight of the sample.
| Results and Discussions | | |
pH of tailings
Chemical speciation and affinity toward the solid phase for radionuclides and other contaminants are significantly dependent on pH. Acid drainage can be a prime environmental concern. At low pH, water may dissolve minerals containing radioactive elements and heavy metals. Acidity combined with deposited salts and heavy metals can prevent plant growth. In addition, downstream surface water bodies may become contaminated and affect the health of the ecosystem. The pH in the tailings samples was in the range of 10.79–11.23, which indicates an alkaline nature. The alkaline pH will reduce the leachability of toxic metals into the surrounding environment. The bioavailability of metals is reduced at higher pH due to the lower solubility of metal hydroxide and oxides.[14]
Uranium
The total uranium concentration in the mill tailings varied from 61 to 91.2 mg/kg. The fractions of uranium present in different phases are graphically presented in [Figure 3]. The order of distribution for uranium in different phases is residual > exchangeable > reducible > carbonate bound > oxidizable. About half of the uranium (51.1% of the total) was mainly distributed in the residual phase, indicating it is strongly bound to the mineral lattice. A considerable amount of uranium is present in the exchangeable (19.8%). Thus, the proper management of uranium mill tailings is required to be carried out from environmental influences, such as by covering with plastic sheets and soil or clay so that contact with rainwater and air can be minimized. Further, standard practices may be implemented during the decommissioning process of the tailings pond to minimize the impact on the surrounding environment. | Figure 3: Distribution of uranium in the different phases of the tailings
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Cobalt
The Co was mainly found in the oxidizable phase. The order of distribution Co in different phases is reducible > residual > carbonate-bound > exchangeable > oxidizable [Figure 4]. About 38% and 34% of the cobalt are bound to the reducible phase and residual phase, respectively, which are thermodynamically unstable under an anoxic and mild oxidizing environment. | Figure 4: Distribution of cobalt in the different phases of the tailings
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Zinc
The order of distribution of Zn in different phases was carbonate-bound > reducible > residual > oxidizable > exchangeable [Figure 5]. The Zn was mainly found in the carbonate-bound phase. The uranium ore is mostly composed of carbonate minerals, so the presence of smithsonite (ZnCO3), hydrozincite (Zn5 [CO3]2 [OH] 6), and aurichalcite ([Zn, Cu] 5 [CO3]2 [OH] 6) may be the reason for the high abundance (56%) of carbonate-bound zinc.[15] About 25% and 12% zinc were present in the Fe/Mn-oxide bound and residual phases, respectively, which may be attributed to the presence of calamine [a mixture of smithsonite, willemite (Zn2SiO4), and hemimorphite (Zn4 [Si2O7][OH]2.H2O) bound with Fe-oxides and clay minerals.[16]
Manganese
The total manganese content in the mill tailings varied from 370 to 1475 mg/kg. The order of distribution in different phases is reducible > carbonate-bound > oxidizable > residual > exchangeable, except in sample 1, where the carbonate-bound phase was found to be dominant (65%) [Figure 6]. This may be attributed to the differences in the mineralogical composition and association of manganese with carbonates. The exchangeable fractions of manganese in the mill tailings are < 1% except in sample 1. The Mn was mainly found in the reducible phase (79%) due to its existence as oxides and oxide-hydroxides. | Figure 6: Distribution of manganese in the different phases of the tailings
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Iron
The order of distribution of Fe in tailings in different phases was residual > reducible > carbonate-bound > oxidizable > exchangeable [Figure 7]. About 91% of the iron (4284–6920 mg/kg) was present in the residual form, and about 8% (501–602 mg/kg) was in the reducible fraction. Iron was mainly found in the residual phase, which indicates its low bioavailability. The solubility of iron in the reducible fraction may be attributed to the reduction of the Fe (III) to Fe (II) in the hydroxylamine hydrochloride-acetic acid medium by the following reaction:
Many researchers have reported that a minor amount of Fe (III) compounds such as amorphous iron hydroxides can be dissolved, whereas the crystalline Fe (III) oxyhydroxides such as goethite, lepidocrocite, and hematite persist in the residual fraction.[17]
Lead
The order of distribution of lead in different phases is reducible > residual > carbonate-bound > exchangeable > oxidizable [Figure 8]. Fifty-seven percentage of the Pb were found in the Fe/Mn oxide-bound phases, which indicates that lead cannot be mobilized in an oxidizing environment. About 12.5% (9.8 mg/kg) of the lead was leached from the carbonate-bound phase in the second step due to the formation of stable Pb-acetate complexes, which are highly water-soluble (44.3 g/L in water at 20°C).[18] Lead is well known for its toxicity, and its primary routes of potential human exposure are ingestion, inhalation, and dermal contact. Lead acetate is absorbed approximately 1.5 times faster than other lead compounds.[19] About 6.3% of the lead is also present in the exchangeable phase. The lead isotopes 206Pb, 207Pb, and 208Pb continuously originated from their parents, 238U, 235U, and 232Th, respectively, by radioactive decay, except 204Pb, which is primordial.
Chromium
The Cr was mainly found in the residual phase of the tailings. The order of distribution in different phases is residual > reducible > carbonate-bound > exchangeable > oxidizable [Figure 9]. The Cr in the uranium tailings was mobilized only after treatment with a very strong oxidizing agent in the fifth step. About 76% of the chromium was present in the residual phase, and only about 3% was in the exchangeable phase. Therefore, it may be assumed that the chromium complexes are quite stable under general environmental conditions. | Figure 9: Distribution of chromium in the different phases of the tailings
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Cadmium
The total Cd content in the uranium tailings varied from 2.2–5.3 mg/kg with a mean of 3.2 mg/kg. The order of distribution of Cd in different phases of tailings was Fe/Mn-oxide bound (37.3%–42.7%) > carbonate bound (21.5%–38.3%) > exchangeable (13.2%–21.0%) > residual (6.5%–15.0%) > oxidizable (0%–11.2%) [Figure 10]. Cd was associated with Fe/Mn-oxides and could be susceptible to leaching in a reducing environment. A significant amount of Cd was observed in the carbonate-bound phase, which may be attributed to the replacement of Ca from the calcareous mineral by Cd as both the elements have similar ionic radii (Cd2+: 97 pm, Ca2+: 99 pm).[19],[20]
Copper
Copper in the mill tailings samples was primarily portioned with Fe/Mn-oxide bound (24.5%–36.6%), residual (15.8%–37.9%), and carbonate bound (11.8%–37.8%), oxidizable (8.9%–28.8%), and exchangeable (0.2%–12.3%) [Figure 11]. The presence of high amounts of copper with the Fe/Mn-oxide bound phase may be attributed to the coprecipitation of copper ions during the formation of the ore deposit. The hydrous ferrous oxides are also well known for their strong absorption properties; thus, the adsorbed copper ions are not readily mobilizable. The high occurrences of Cu in the carbonate and oxidizable phases may be attributed to the presence of malachite (Cu2CO3 [OH] 2) and chalcopyrite (CuFeS2) in the tailings.[21] | Figure 11: Distribution of copper in the different phases of the tailings
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Nickel
The order of distribution of Ni in different phases of tailings is residual > reducible > carbonate-
bound > oxidizable > exchangeable [Figure 12]. Nickel was found to be mainly associated with residual and Fe-Mn oxide phases, which may be attributed to the decomposition of crystalline forms of Fe-minerals and the silicate minerals into less crystalline or amorphous forms under experimental conditions.[22] The tendency of nickel to be associated with limonite and Fe-silicate minerals is reported by other researchers.[23] | Figure 12: Distribution of nickel in the different phases of the tailings
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Major elements present in the mill tailings
The mill tailings had undergone various mechanical (e.g., crushing and grinding) treatments and leaching with different physical and chemical environments (e.g., Na2CO3, NaHCO3, O2, high temperature, and pressure) during uranium recovery from the ore and thus bear minimum or no similarity with the mineral or rock that was fed to the process plant. Thus, the major composition of mill tailings samples is presented in [Table 1]. The major components of mill tailings were Ca, Mg, Fe, K, and Na, Mn, Cu, Pb, Co, Ni, and Zn were also estimated to be present in trace quantities. About 120 mg/kg of unrecovered uranium (as U3O8) was present in the mill tailings, where 20% were in the exchangeable phase. Therefore, feasibility studies may be carried out to extract U from the mill tailings in the future.
Radionuclides in the ore, waste rock, and mill tailings
The statistical summary of specific activities for 238U, 226Ra, 210Pb, 235U, 232Th, 228Ra, and 40K estimated in ore, waste rock, and mill tailings is presented in [Table 2], [Table 3], [Table 4], respectively. 238U, 226Ra, 210Pb, 235U, 228Ra, 228Th, and 40K in the ore samples varied from 2207 ± 45 to 16,643 ± 33, 2128 ± 3 to 10,535 ± 7, 2854 ± 21 to 12,123 ± 41, 99 ± 2 to 652 ± 2, 4 ± 1 to 35 ± 1, 5 ± 1 to 26 ± 1, and 337 ± 5 to 665 ± 9 Bq/kg, respectively, with their respective means of 4504 ± 3169, 3953 ± 2020, 4895 ± 2560, 198 ± 122, 14 ± 8, 16 ± 4, and 486 ± 92 Bq/kg [Table 1]. Further, the respective activities in waste rocks were in the range of 631 ± 4–2052 ± 9, 618 ± 2–1977 ± 9, 895 ± 11–2490 ± 17, 33 ± 1–94 ± 1, 6 ± 1–28 ± 1, 7 ± 1–25 ± 1, and 382 ± 5–1084 ± 10 Bq/kg [Table 2]. In waste rock, comparatively, the activities of 238U series radionuclides are less than in ore. However, activities of 232Th series radionuclides and 40K are in a similar range in both ore and waste rocks. In the tailings samples, 238U, 226Ra, 210Pb, 235U, 228Ra, 228Th, and 40K activities varied from 974 ± 4 to 1824 ± 8, 2457 ± 16 to 4863 ± 3, 2425 ± 8 to 5077 ± 16, 45 ± 1 to 98 ± 1, 9 ± 1 to 19 ± 1, 10 ± 1 to 20 ± 1, and 392 ± 5–555 ± 7 Bq/kg, respectively. The primordial radionuclide 40K is abundant in tailings, with a mean of 446 ± 42 Bq/kg. The mill tailings contain almost all the uranium-series radionuclides, including unrecovered uranium. | Table 2: Activities of 238U, 226Ra, 210Pb, 235U, 232Th, 228Ra, and 40K in the ore from Tummalapalle uranium mine
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 | Table 3: Activities of 238U, 226Ra, 210Pb, 235U, 232Th, 228Ra, and 40K in the waste rock from Tummalapalle uranium mine
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 | Table 4: Activities of 238U, 226Ra, 210Pb, 235U, 232Th, 228Ra, and 40K in the Tailings samples
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In the case of ores, the activity ratios of 226Ra/238U, 210Pb/238U, and 210Pb/226Ra varied from 0.63 to 1.00, 0.73 to 1.41, and 1.02 to 1.43, respectively [Table 5], and their respective means are 0.92 ± 0.08, 1.13 ± 0.17, and 1.24 ± 0.15. In the case of waste rock, the activity ratios of 226Ra/238U, 210Pb/238U, and 210Pb/226Ra varied from 0.67 to 1.24, 0.78 to 1.58, and 0.88 to 1.60, respectively, and their respective means are 0.94 ± 0.14, 1.13 ± 0.22, and 1.21 ± 0.21 [Table 5]. The activity ratios of 226Ra/238U, 210Pb/238U, and 210Pb/226Ra in the ore and waste were found to be in a similar range [Table 5]. | Table 5: Activity ratio (average) of radionuclides in ore, waste rocks, and tailings
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Less value of 226Ra/238U than unity indicated that the 238U content in the ore and waste rock was higher than the activity of 226Ra. However, 210Pb activity was found to be higher than 226Ra as well as 238U. 238U decay chain radionuclides showed disequilibrium between their parents and daughters in the ore as well as in the waste rock. The reasons for radioactive disequilibrium are generally related to geochemical mechanisms that cause the mobilization of the radionuclide during a period comparable to the half-life period of the daughter radionuclide.[24],[25] Several geochemical processes control the dissolution and migration of more geochemically soluble radionuclides of a radioactive series, the diffusion of nuclides of radon, and the effects of α-particle recoil.[24] In general, U is more geochemically mobile than Ra due to the oxidation of U+4 to U+6 in oxidizing environments. Ra, however, is more mobile than U in the presence of SO42− and Cl− anions, suggesting an increased solubility of Ra in salt water.[25],[26] The hydrogeochemical study conducted in the Cuddapah region has indicated a higher salt content in groundwater in this region that may be a cause of disequilibrium between 238U and radium.[2] The activity ratios of 226Ra/238U, 210Pb/238U, and 210Pb/226Ra in tailings samples varied from 1.39 to 3.69, 1.42 to 3.82, and 0.79 to 1.54, respectively, and their respective means are 2.37 ± 0.60, 2.53 ± 0.66, and 1.10 ± 0.27 [Table 5]. The activity ratios of 226Ra/238U and 210Pb/238U [Table 5] have shown higher ratios in the tailings as compared to the ore and waste rock samples. The higher 226Ra/238U and 210Pb/238U ratios in tailings were attributed to the extraction of uranium during the milling process, where about 80% of the uranium is recovered from the ores. The secular equilibrium of uranium with its daughter products has been influenced by technological enhancement. The activities of 228Ra and 228Th of the 232Th series were insignificant in comparison to the uranium series radionuclides. The mean activity ratio of 228Th/228Ra [Table 5] was almost constant (0.96–1.08) in the ore, waste, and tailings, indicating that the process chemistry had little impact on the leaching characteristics of 232Th series radionuclides present in the ore. The 235U and 238U mass percentages in the ore and waste were almost constant. However, in the tailings, the % mass of 235U was higher than that of the in situ rocks, and in the case of 238U, the % mass of 238U was lower in the tailings than in the ore and waste rocks, which are assumed to be influenced by the leaching process [Table 5]. The leaching of excess 238U from the ore as compared to 235U is assumed to be the result of damage to the 238U mineral lattice point due to irradiation by more alpha and beta decay and alpha recoil effects.
| Conclusions | | |
According to the findings of the speciation analysis of tailings, uranium, and other toxic elements are found to be distributed in different proportions in various geological fractions of the tailings and thus have varying degrees of environmental mobility. More than 50% of U, 90% of Fe, and 91% of Cr were found to be distributed in the residual phase, indicating they are strongly bound to the mineral lattice. Most of the elements showed an insignificant distribution in the exchangeable phase, suggesting that the elements are present in the tailings in their stable complex form. However, toxic elements such as Co, Pb, and Cd are significantly associated with the oxidizing phase, and these elements can only be made labile under prolonged exposure to atmospheric conditions. The major elements such as Ca and Mg were estimated to be the principal components of the tailings. The 226Ra/238U ratio in ore, waste rock, and tailings indicated disequilibrium among 238U series radionuclides. The mean ratios of 226Ra/238U in ore, waste, and tailings were 0.92, 0.94, and 2.37, respectively. The elevated ratio of 226Ra/238U in tailings is attributed to the maximum recovery of uranium from the ore. The mean mass abundance of 238U in the ore, waste rock, and tailings was evaluated to be 99.30, 99.28, and 99.14%, respectively, whereas the respective values for 235U were 0.70, 0.72, and 0.86%. Less abundances of % mass of 238U and a higher abundance of 235U than the natural mass % are attributed to be influenced by the leaching process.
Acknowledgments
The authors would like to wish the most profound gratitude to Dr. D. K. Aswal, Director, HS&EG, BARC, for his kind support during the study. The authors would like to thank Dr. C. K. Asnani, C&MD, UCIL, for his encouragement during the study. The authors would also like to acknowledge the cooperation and assistance provided by the colleagues of the HPU, Turamdih, during the experiments.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]
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