The principal objective of the present investigations was to optimize leaching parameters for uranium ex1raction from Baghalchur low-grade sandstone uranium ores (0.023% U3,O8) by bacterial. heap leaching process. Baghalchur sandstone uranium ore is alkaline in nature and was found to contain 3.5% calcite (CaCO3) and 0.1% pyrite (FeS2) as the only sulfide mineral. Hematite (Fe2O3) and magnetite (Fe3O4) were the main iron oxide minerals and constituted the major portion of the heavy minerals. Quartz (SiO2) was the main silicate mineral present in the sandstone uranium ore and represented 59% of the frame grains. Actinolite, biotite, chlorite. epidote, hornblende, muscovite, pyroxene. and tourmaline were the mica minerals present in sandstone uranium ore.
Tyuyamunite [Ca(VO4)(UO2)2.5-8 H2O] was identified as the main uranium mineral present in the sandstone ore. Clay minerals fraction of the arc contained major portion of uranium content (0.021 % U3 O8) which was 91.309t of the total uranium content present in the ore sample (0.023% U3 O8) Uranium content was much higher in the fine size are fraction as compared to coarse one. The arc contained 46.90/, (wt/wt) of ore fraction with a particle size of â‰¥595 Î¼m (â‰¥30 ASTM mesh size) which was a major fraction and almost the weight of ore fraction was found to decrease concomitantly with particle size.
The indigenous strains of Thiobacillus ferrooxidans (TFe-1 and TFe-2) and Thiobacillus thiooxidans (TTh-l) were isolated from ecosystem of uranium ore-processing unit. These strains were Gram-negative, motile, and rod-shaped bacteria. T. ferrooxidans (TFe-2) oxidized Fe2+, pyrite, sulfur and reduced sulfur compounds like thiosulfate and tetrathionate. The bacterial oxidation of pyrite, sulfur and reduced sulfur compounds produced sulfuric acid which followed a drop in initial pH value of the medium. T. thiooxidans (TTh-l) oxidized only sulfur and reduced sulfur compounds to sulfuric acid through its metabolic activity. Sulfuric acid thus produced acts as lixiviant for uranium solubilization from sandstone ore.
The release of iron from pyrite was much more pronounced (about 3-folds) by inoculated system as compared to uninoculated one. It was observed that 14.66 g/L Fe1 (total iron) was released from the bacterial oxidation of pyrite by indigenous strain of 7: ferrooxidans (TFe-2) in 20-days of incubation. The release of iron (Fe1 from pyrite mineral was relative]y higher (16 g/L Fe1) in standard strain of T ferrooxidans (A TCC 13361) as compared to indigenous strain of T. ferrooxidans (TFe-2).
Thiobacillus ferrooxidans (TFe-2) produced 10.80 and 20.80 g/L H2SO4 from the bacterial oxidation of elemental sulfur and sulfur slag respectively, in 20-days of incubation. Similarly, T. thiooxidans (TTh-l) produced 13.52 g/L and 22.0 g/L H2SO4 from the oxidation of sulfur and sulfur slag, respectively, under similar experimental conditions. However, a mixed culture of T ferrooxidans (TFe-2) and T thiooxidans (TTh-l) produced 18.6 g/L and 27.0 g/L H2SO4 from the oxidation of sulfur and sulfur slag, respectively. T thiooxidans (TTh-l) showed a compareable results of sulfuric acid production and change in initial pH-value of the medium with the axenic strain of T thiooxidans (ATCC 8085), T thiooxidans (TTh-l) produced 39.80 g/L H2SO4 with a drop in initial pH-value of 2.50 to 0.68 in 30-days incubation; whereas 42.60 g/L H2SO4 was produced from sulfur oxidation by T. thiooxidans (A TCC 8085).
In shake flask bioleaching studies, the isolated strains of T. ferrooxidans (TFe-2) and T thiooxidans (TTh-l) solubilized 91 % and 88% U3O8 respectively, from low-grade sandstone uranium ore (0.023%- U3O8) in 30-days of incubation. T ferrooxidans (TFe-2) and T. thiooxidans (TTh-l) exhibited comparable leaching data of uranium solubilization from ore with those of axenic strains of T. ferrooxidans (ATCC 13661) and T. thiooxidans (ATCC 8085). In chemical control samples, only 1.67% U3O8 solubilized from sandstone ore. A 10% (wt/vol) ore pulp density was used for these studies. During bioleaching of sandstone uranium ore, a significant amount of iron (Fe,) was also solubilized by chemical reaction between H2SO4 and iron minerals like hematite (Fe2O3), magnetite (Fe3O4) and pyrite (FeS2) etc., present in the ore matrix.
A higher uranium recovery of 92% U3O8 was obtained from sandstone ore amended with sulfur slag as compared to elemental sulfur (86.7% U3O8) and pyrite (84.3% U3O8). It was due to higher acid production during bacterial oxidation of slag and. the presence of iron (Fe2+ and Fe3+) in sulfur slag, which catalyzed the chemical reaction for uranium solubilization from ore.
In batch tests, it was observed that the elemental sulfur to uranium ore ratios [So: ore] of 4:100 to 6:100 (wt/wt) was found to be the best proportions for optimal recovery of uranium (63%-64% U3O8) from sandstone ore by a mixed culture of T. ferrooxidans (TFe-2) and T. thiooxidans (ITh-1). Similarly, a uranium recovery of 68% U3O8 was obtained from sulfur slag to ore ratio of 5:100 to 6:100 (wt/ wt). It was found during acid leaching studies of sandstone ore that 130 kg H2SO4 ton of I ore was required for maximum uranium recovery of 80% U3O8. Uranium concentration of the acid leach liquor was 434.62 mg/L U3O8 with a pH-value of 1.52. Uranium ex1raction from sandstone ore was found inversely proportional to the ore pulp density. A maximum uranium recovery of 83.5% U3O8 was obtained from ore-pulp density of 1.0% (wt/vol) during 15-days of incubation. Uranium recovery of 89.12% U3O8 was obtained from ore fraction of particle size of â‰¤ 53 Î¼m (ASTM Mesh No.â‰¤ 300). Microbial leaching of tailings residues showed that a significant amount of uranium could be leached out successfully with supply of energy substrate and nutrient (s).
Bioreactor leaching studies showed a uranium recovery of 87.3% U3O8 from low-grade sandstone uranium ore by a mixed culture of T. ferrooxidans (TFe-2) and T thiooxidans (ITh-l) during 20-days of incubation. Physical appearance of bacterial leachate was light green, which indicated the presence of soluble iron salts resembling to acid leach liquor of Uranium Ore-Processing Plant, CPC, D.G. Khan.
PVC column leaching studies revealed that when the sandstone ore was amended with elemental sulfur and mine water at pH-value of 3.5, a uranium recovery of 66% U30S was achieved during 150-days leaching experiments with indigenous microbial populations of acidophilic thiobacilli. However, when mine water was used as such (pH 7.40), the uranium solubilization was found to be upto 48% U3O8 under similar conditions. The addition of ammonium sulfate [(NH4)2SO4; 3.0 g/L] in mine water of an adjusted pH-value 3.50, was found to increase the microbial populations concomitantly enhancing the uranium leaching to 90% U3O8 from column filled with ore amended sulfur slag. Similarly, maximum uranium recovery of 84.08% U3O8 was obtained from PVC column leaching studies on mill tailings residue during 100-days of leaching time.
Uranium recovery of 4.9% U3O8 was obtained (calculated on the basis of heap effluents) from low-grade sandstone ore by microbial heap process during 150-days. But on the basis of chemical data of leached residues (core samples taken at depths of 00-100 cm), an average uranium recovery of 31.64% U3O8 was leached out during heap operation. During microbial heap leaching process on sandstone uranium ore, an off-white fluffy solid material .(sludge) was observed emerging along with heap effluents. A significant amount of uranium (0.0517% to 0.6283% U3O8) was found to be present in these samples. It seemed that a major portion of the uranium leached so far during heap leaching process had precipitated/ entrapped into these sludge samples. The formation of off-white sludge and precipitation of uranium in these sludge samples might be due to the presence of high calcium content (150.0 mg/L) in subsoil water being used for inocula preparation for microbial heap leaching process. However, no such problem was encountered during microbial column leaching studies of sandstone uranium ore earlier since mine water was employed as inoculum. The mine water sample contained less calcium content (27.6 mg/L Ca2+) as compared with NIBGE water.
Amberlite IRA-400 was capable of adsorbing 97.6% U3O8 present in the microbial leach liquor as compared to Dowex-1 and DuoliteÂ® A-147 anion-exchange resins, which absorbed 82% and 87% U3O8 respectively. A 2.0 M NH4HCO3 solution of pH-value 7.94 showed the maximum uranium elution efficiency (E) and stripped out 99.8% U3O8 from the uranium loaded resin column. Strip solutions obtained by 2.0 M NH4HCO3 solution was found to contain 15.5-25.6 g/L U3O8
Uranium concentrate (yellow cake) produced from bacterial leach liquors by separation, strippnig, precipitation and drying was found to contain 89.6% U3O8 on dry matter basis with minor impurities of others metal ions. Boron (B) was present in a concentration of 4.0 Î¼g/g of the sample. The purity of the yellow cake sample was found to meet the specifications of the Canadian UO2 powder to some extent.
These studies have indicated that uranium can be leached out, microbially from low-grade sedimentary type ores and uranium mill tailings residue. Only through the applications of biohydrometallurgical processes, the valuable U3O8 contents of mill tailings residues can be recovered on commercial scale. Certain amendments like supply of external energy source, nutrient, and following proper leaching techniques i.e. heap leaching process, under natural conditions at very low capital investment are however, required. This process is simple, economically viable and free of environmental hazards. The results of present investigations are very encouraging, and yielding a high uranium recovery from low-grade ores and waste residues. As a consequence of these studies, a heap of 5,000 tons of low-grade sandstone uranium ore has been planned to extract uranium on commercial scale at the mine site, Baghalchur, D.G. Khan.