Biohydrometallurgical Processing of Indian Uranium Ores

Abstract

The depletion of high grade ores of metals opens up the possibilities for utilization of low grade deposits which are often not suitable for cost-effective extraction by conventional processes because of high-energy consumption and environmental problems. In respect of the uranium in India, most of the ores that are exploited are of low-grade compared to the world standard and generally contain 0.03 to 0.06% U3O8 . Mining would become unattractive when the tenor still lowers. To cope up for the ongoing and forthcoming nuclear power projects in India, the short fall in availability of uranium is increasingly realised which would be a major challenge in future. It is therefore, imperative to consider the exploitation of the lean grade ores to meet the demand in the country using an alternate processing technology viz., bioleaching. The present work has thus been envisaged to examine the extraction of uranium in presence of microorganisms from the following ores: (i) Turamdih uranium ore (ii) Narwapahar uranium ore Biomineral processing is generally an environmentally benign and energy efficient approach to treat lean grade/complex ores, by - products, and wastes/residues, and has emerged as an alternative to the traditional processes for the extraction of valuable metals. The scope of using bioleaching has widened recently due to the elimination of expensive and polluting steps involved not only in base and precious metal extraction, but also in hydrometallurgical processing of uranium using oxidants. Bioleaching of copper from low grade sulphide ores, biobeneficiation of refractory gold ores and bioleaching of uranium in commercial operations are testimony to this technology which provides impetus for further research in this area. In view of this, studies on the bioleaching of uranium from the above two ores available in India are presented and discussed in this thesis. The thesis has been divided in to FIVE CHAPTERS including conclusions based on the work carried out with effect from October 2006. Chapter - 1 gives introduction to the work, particularly the resources and importance of bioleaching in uranium extraction, its applications for the treatment of low grade ores, microbes relevant to dissolution, literature review on processes and mechanism of bioleaching, influence of underlying parameters, international and Indian scenario, kinetic models for biodissolution, scope of work, etc. Subsequent to the isolation and culturing of Acidithiobacillus ferrooxidans (A.ferrooxidans) which was systematically established by 1947, bacterial leaching of uranium was first published in 1957 applying an iron oxidising autotrophic soil bacteria. The 9K media proposed for isolation and culturing of A.ferrooxidans, is even now being used for mass production of the cells. Although several important microorganisms such as autotrophs and heterotrophs are able to catalyse the leaching of metals, in the present investigation most common autotroph/chemolithotroph viz. Acidithiobacillus ferrooxidans is mostly used with a limited study with Leptospirillum ferrooxidans (L.ferrooxidans). A.ferrooxidans is an iron and sulphur oxidising bacteria and so is L.ferrooxidans. They obtain energy for the metabolism through the oxidation of ferrous sulphate, thiosulphate, elemental sulphur and other reduced valence inorganic sulphur compounds in presence of oxygen and derive carbon for the growth of cell structure from atmospheric carbon dioxide. A.ferrooxidans thrives at a pH between1.5 – 3.5 and below 40 o C temperature whereas L.ferrooxidans is active at 40 o C and above. In the bioleaching of metals, two mechanisms are generally reported to operate. These are direct and indirect mechanisms. In the direct mechanism, attachment of bacteria (A.ferrooxidans) to the mineral surface with enzymatic action causes solubilisation of the minerals as metal sulphates and sulphur is oxidised to sulphuric acid. Indirect mechanism incorporates reaction of ferric iron generated biogenically with the iron containing minerals to dissolve metals as sulphates and this mechanism is prevalent in the leaching of uranium through oxidation of U(IV) to dissolve U(VI). The objectives of the present research are to study the bio-dissolution of uranium from the two uranium ores with A.ferrooxidans besides using L.ferrooxidans in brief for Narwapahar ore while establishing the kinetics and mechanism of leaching, characterising the raw materials and leach residues, and correlating uranium recovery with redox potential/iron species. Materials and method are described in Chapter - 2. Prior to bioleaching experiments, the characterisation of Turamdih and Narwapahar ore from the mines in Jharkhand, India has been carried out. Chemical analysis, phase identification by XRD, optical microscopy and surface morphology by SEM and EPMA as far as possible are reported. The uranium ore from Turamdih mines has ~0.024% U 3 O 8 . The phase identification by XRD shows that quartz, alumina and magnetite are the major phases while kyanite (aluminium silicate), apatite, ferrosilite (ferro-silicate), pyrite and hematite being the minor phases. Uranium is present as uraninite in the ore. The uraninite ore of Narwapahar mines contains 0.047% U 3 O 8 . The mineralogical analysis of ore shows the presence of quartz, apatite, alumina and magnetite as the major phases, while kyanite, ferrosilite, pyrite and hematite as the minor phases. The uranium dissolution behaviour is expected to differ as the two ores have difference with respect to the apatite contents and grinding index. The Narwapahar ore is rich in apatite as compared to that of Turamdih. The grinding index is 7.4 kWh/T and 3.8 kWh/T for Narwapahar and Turamdih ores respectively. The bioleaching of uranium was investigated in the following modes: a) Shake flask experiments to optimise the parameters b) Column leaching to simulate the conditions for possible applications in percolation/heap leaching. c) Bioreactor leaching to examine the dissolution in controlled agitated tank reactor. For the bioleaching experiments, A.ferrooxidans was isolated as wild culture in 9K media from mine water of respective mines (Turamdih and Narwapahar). In leaching, a weighed amount of sample of known size was placed in the conical flask (500 mL) with addition of active culture grown on the ore except in control experiment. Bactericide (HgCl 2 ) at 0.02g/L concentration was added in the sterile/ control experiments. The experiments were carried out in a temperature controlled incubator shaker with orbital shaking mechanism. The pH was maintained to the desired level and maintained from time to time by (10N) sulphuric acid. Slurry sample was collected at an interval of 5 days for chemical analysis and also for redox potential measurement against SCE. Volume of the leach slurry was maintained by distilled water to account for losses. The leaching of uranium was computed from the slurry after filtration while analysing the filtrate and residue in some cases to check the material balance. The Turamdih ore was subjected to column leaching in a glass column with 2.5 kg material [1mm-150µm (200 g), 5mm-1mm (2.0 kg) and 5mm-4mm (300 g)]. Another set up with a load of 6.0 kg sized ore with only two fractions viz., 5mm-1mm (4.0 kg) and 15mm-10mm (2.0 kg) was also used. The 2cm thick plexiglass column had 75cm height and 6.5 cm internal diameter. The column was sprayed with the bacterial solution using a peristaltic pump at the rate of 3L/h for 2.5 kg and 4L/h for 6.0 kg ore. In 2.5 kg column, the lixiviant volume of 10L had a total bacterial population of 6.4x10 7 cells/mL, whereas, it was 25 L medium solution containing 10% (v/v) inoculum with the bacterial density of 6.5x10 6 cells/mL for the 6.0 kg column. Similar column leaching set up was designed for Narwapahar ore at a load of 2.0 kg with the distribution as <150µm (100g) and 5mm- 4mm (1.9 kg). The ore was subjected to a continuous recirculation mode process at bacterial solution flow rate of 3L/h. All the column leaching experiments were conducted at room temperature by spraying the solution for 8h/day while maintaining the pH of the feed to the desired level for the entire run. In order to investigate the bioleaching in stirred tank reactor at controlled temperature, a PLC based software controlled 2L bioreactor- BIOSTAT-B ® (Make-SARTORIUS) was used at 10-40% (w/v) pulp density with enriched culture of A.ferrooxidans/L.ferrooxidans. Leaching was carried out by using bacterial culture and also the inoculum containing biogenically prepared Fe(III) ions. The enriched culture of A.ferrooxidans (6x10 6 cells/mL) was used in initial experiment. Biogenic ferric sulphate generated from 10g/L ferrous sulfate at pH 2.0 and 35 o C contained Fe(III) ions with A.ferrooxidans/L.ferrooxidans. The 10% inoculum was used to make the slurry with the ore at the desired pulp density for optimization of parameters to understand the role of bacteria and Fe(III) ions. Studies on bio-processing of Turamdih ore are presented in Chapter - 3. The parameters such as effect of pH, particle size of the ore, pulp density, and temperature in shake flask have been optimised. Kinetics and mechanism of bioleaching have also been examined. The enriched culture of A.ferrooxidans was adapted on the uranium ore at pH 2.0 and was inoculated in the bioleaching experiment at pH 1.7, 20% PD and 35°C with the mixed particles of <76μm size resulting in 98% uranium recovery in 40 days time. Effect of particle size variation showed higher uranium dissolution (91%) with the medium size particles in the range 53–45μm at pH 1.7 in 35 days as compared to the coarser (76–53 μm) and finer size (<45 μm) fractions. High metal recovery with the mixed size particles (<76 μm) may be attributed to the better permeability of ferric ions in iron silicate matrix to dissolve uranium. Recovery of uranium may be correlated with the values of redox potential acquired during the experiments. Under the optimum conditions (pH 1.7, 20% pulp density, 35°C temperature with <76μm mixed size particles), rise in redox potential was recorded to be 595–715 mV in 40 days. Bioleaching of uranium appeared to follow the indirect mechanism with the involvement of Fe(III) biogenically generated by bacteria. The kinetic data on uranium bioleaching show best fit to the chemical controlled shrinking core model [1-(1-x) 1/3 =k c t, where “x” is the fraction leached in time “t” and “k c ” is the specific rate constant] for the reaction proceeding on the surface of the ore with the lixiviant, namely, Fe(III) and acid. Activation energy of 31 kJ mol -1 is acquired in the temperature range 25–35 o C (298-308K). The chemical controlled leaching mechanism of uranium is corroborated by the XRD phase identification of the ore and the leach residues, and also by the change in surface morphological features through SEM studies. A. ferrooxidans isolated and cultured from the mine water was also used for bioleaching of uranium from the Turamdih ore in laboratory-scale columns. The experiment conducted in a 6.0 kg column shows uranium biorecovery of 53.6% with a rise in redox potential from 512mV to 719mV in 30 days and bacterial population from 6.5x10 6 to 2.1x10 8 cells/mL which may be correlated with the oxidation of Fe(II) to Fe(III), improving the recovery. For 2.5 kg laboratory column, 58.9% uranium biorecovery is recorded in 40 days at pH 1.7 as against 56.8% leaching at pH 1.9 with maximum E (redox potential) values of 665mV and 623mV, respectively. The control experiment yields a lower uranium recovery (47.9%) in 40 days at pH 1.7. The dissolution behaviour of uranium from Turamdih ore in bioreactor using 10% (v/v) inoculum (A.ferrooxidans) at pH 2.0, 20% pulp density, 35 o C and 150rpm with ore particles of <76μm (mixed) size showed nearly 30% leaching in just 24h; a significant rise in uranium bio-recovery (98.3%) was obtained in 14 days at the maximum redox potential of 754mV. In the second set of experiments with the inoculum containing biogenic ferric solution, 84.7% uranium leaching was recorded at pH 2.0 and 20% pulp density with rise in redox potential from 536–674 mV in 10h only as against 14 days time with the culture of A.ferrooxidans. In the chemical leaching, uranium recovery was however, low (38.3%) besides, low redox potential value (448mV) in 10h. The uranium bioleaching rose to 87.6% in 10h at 30% (w/v) pulp density and pH 2.0 with the ore particles of <76µm size as compared to 86% and 76% uranium biodissolution for 53-45µm and <45µm size materials; the leaching decreased to 67.6% and 71.8% at 25 o C and 30 o C respectively with <76μm size particles. Low metal leaching (76%) was observed at still higher temperature (40 o C). Mode of uranium bioleaching from the ore was reflected by XRD phase analysis and SEM of the leach residue. In Chapter - 4, the parameters such as effect of pH, particle size of the ore, pulp density, and temperature on bioleaching of Narwapahar ore in shake flask have been optimised and kinetics and mechanism of bioleaching examined. The enriched culture of A.ferrooxidans isolated from Narwapahar mine water was found effective. The optimum uranium bio-recovery was found to be 96% at 10% pulp density, pH 1.7 and 35 o C in 40 days with the fine particles of <45µm size as against 98% leaching from Turamdih ore with <76μm particles at 20% pulp density. Under the optimum condition at pH 1.7, rise in redox potential is recorded to be 594-708 in 40days. Bioleaching of uranium seems to follow the indirect mechanism in this case also with the involvement of Fe (III) biogenically obtained by A.ferrooxidans. Uranium recovery was found to be 98% at 40 o C in 40 days when another isolate of L.ferrooxidans was inoculated. This shows the possibility of improving the kinetics of the process and requires further studies by applying thermophiles. The kinetic data for dissolution of uranium from the Narwapahar ore with A.ferrooxidans show best fit to the chemical controlled model with activation energy value of 28 kJ/mol, in the temperature range 298-308K. In case of 2.0 kg laboratory column leaching, 57% bio-recovery in 40 days at pH 1.7 was achieved with the enriched culture of A.ferrooxidans (4x10 7 cells/mL). The dissolution of uranium in column under chemical (control experiment) leaching conditions recorded a lower value of 39%. To test the efficacy of L. ferrooxidans, column was operated under similar conditions which resulted in high uranium biorecovery (66%). Under this condition, bacterial population was found to be 9x10 8 cells/mL with a redox potential of 663mV in 40 days. The acid (sulphuric) consumption using A.ferrooxidans and L.ferrooxidans was estimated to be 3.2 and 2.8 kg/T ore as against 5.4 kg/T in control experiments. Bioprocessing of Narwapahar ore was also investigated in the bioreactor while using the enriched culture of A.ferrooxidans and L.ferrooxidans separately. The experiments were carried out at pH 2.0, 10% (w/v) pulp density and 150rpm agitation speed with <45μm size ore and 10% (v/v) inoculum of respective bacterium at 35 o C (A.ferrooxidans) and 40 o C (L.ferrooxidans). The uranium leaching was recorded to be 57% and 63% by A.ferrooxidans and L.ferrooxidans respectively in 5 days with the corresponding redox potential values of 561 and 588mV. When 10% inoculum of A.ferrooxidans containing biogenic iron (III) solution was used for the particles of <45µm, 83% uranium was leached out in just 10h at pH 2.0, 10% (w/v) pulp density and 35 o C as compared to 77%, 74%, 56% and 52% dissolution for <76µm, 53-45µm, 76-53µm and 100-76 µm size materials, respectively. At 20% pulp density, uranium recovery rose to 87% in 10h as compared to low recovery of 75% and 72% at 30% and 40% pulp density respectively. Effect of temperature (25-40 o C) on bio-dissolution of uranium and its role on the effectiveness of A.ferrooxidans and L.ferrooxidans, were studied in bioreactor at pH 2.0 and 20% (w/v) pulp density with the finest size particles (<45µm). Uranium biorecovery was found to be 90.3% in 10h at 40 o C with L.ferrooxidans as against 77% leaching at same temperature with A.ferrooxidans. The uranium recovery was low of 66% and 72% at 25 and 30 o C with A.ferrooxidans respectively. The residues obtained from bioreactor leaching with both the microbial species have been characterised by SEM and XRD to understand the leaching behaviour of uranium. Distinct change in surface morphology of the residue and the phases identified after bioleaching are testimony to the microbial leaching of uranium. The conclusions drawn from the research presented in the thesis are summarised in Chapter-5. The highlights of each part are also vividly described while emphasising the leaching behaviour of uranium from both ores under the three modes of experiments viz., bioleaching in shake flasks, laboratory columns and bioreactor. Scope for further work in this area is separately enumerated. All the references cited in the thesis are given in the end. The thesis is appended with the list of publications out of the research conducted on the subject.