IAEA TM on Low Grade Uranium Deposits Vienna, March 29-31, 2010

PROCESS DEVELOPMENT STUDIES FOR LOW-GRADE URANIUM DEPOSIT IN

ALKALINE HOST ROCKS OF TUMMALAPALLEAK SURI*, NPH PADMANABHAN*, T SREENIVAS*, K ANANDRAO*,

AK SINGH*, KT SHENOY, T MISHRA and SK GHOSH

*Materials and Chemical Engg.GroupsBHABHA ATOMIC RESEARCH CENTRE

MUMBAI INDIAaksuri@barc.gov.in

Structure of Presentation The Tummalapalle uranium ore deposit Location and

Resource.

Mineralogical, Liberation and Chemical Characteristics.

Process Challenges and Options.

Parametric optimization of various unit operations in both Preconcentration Chemical Leaching and Whole-ore Leaching.

Integrated Flowsheet Development and Testing.

Concluding Remarks.

Tummalapalle Deposit

Kadapah Basin

Out of 160 km,only about 9.5 km belt along the strike and 1 to 2 km along the dip in Tummalapalle -Rachakuntapalle tract is explored by drilling wherein 29000 tonnes of uranium oxide contained in about 61 million tonnes ore of 0.05% eU3O8 average grade.A.K. Raia, S. Zakaullaa, A. Chaki (URAM 2009)

Tummalapalle Exploratory Mine

Mineralogical CompositionMineral Wt.%

Carbonates (dolomicrite, dolospar & microstyolite)Quartz and feldspars (Qz, microcline, plagioclase)Collophane (Phosphates)ChloriteChertPyriteChalcopyrite & GalenaMagnetiteIlmenite including leucoxeneIron Oxides (goethite)Pitchblende (that which could be observed as a distinct phase in intimate association with pyrite)

79.812.23.901.200.821.240.060.180.230.410.01

Chemical AnalysisConstituent Wt% Remarks

U3O8Fe2O3FeONa2OK2OCaOMgOMnOP2O5Al2O3TiO2SiO2LOI

0.0481.230.650.170.5824.713.70.22.762.170.0619.431.8

No discrete minerals

Apatite, limestone and dolomite

Due to phosphate minerals

From QuartzDue to elimination of CO2 and H2O(Oxidation of FeO to Fe2O3)

62.8

17.89.2

62.3

28.9

8.8

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BRL (Sp.G 2.8) MIL (Sp.G 3.2) MIH (Sp.G >3.2)Heavy Media Liquid (Specific Gravity)

%

WtU3O8 Dist.

Mass and Uranium Distribution as functions of Density

Only 9% of U3O8 values are distributed in MIH.This is mostly discrete pitchblende in association with pyrite

Process problems of TummalapalleUranium ore

The occurrence of the uranium values in fully disseminated form in very fine* (

Process Options

Whole ore Leaching

Pre-concentration (Physical / Thermal Processing)

Bio-Leaching Chemical Leaching Chemical Leaching of Pre-concentrate

PROCESS OPTIONS FOR THE EXPLOITATION OF LOW-GRADE URANIUM ORE

This is not suitable for alkaline host-rock

The two options suitable were Reverse Beneficiation followed by Chemical Leaching and Whole

Ore Chemical Leaching.

Methods for Pre-concentration(Reverse Physical Beneficiation)

Thermal Technique: As the ore sample is predominantly with carbonate gangue calcination quenching would lead to expulsion of Carbon dioxide and create thermal stresses. Chances of exposure of locked-up uranium phase is more. Flotation of calcinedmass with suitable collector reagent will reduce the mass for subsequent chemical leaching stage.

Physical Beneficiation Technique: Presence of sulphide minerals is detrimental in leaching of ores as they consume both leachantsand oxidant leading to formation of sulphate ion. Excessive presence of sulphate ion is harmful both during leaching and precipitation of uranium values.Besides this it also affects the effluent quality. Flotation of sulfides diminishes the above-mentioned process constraints.

Calcination-Quenching-Flotation

PRODUCT Wt.% U3O8 Dist.%

FLOAT 19.6 16.3

TAILS 59.1 83.7

Wt.Loss due to Calcination 21.3 0

Calcination and quenching was carried out at 9000 C for 2h. The quenched mass was dispersed with sodium silicate and flotation of Calcium hydroxide particles were attempted with sodium oleate as collector and methyl Isobutyl carbinol (MIBC) as the frother.

Selective Flotation of Very-fine size Calcium Hydroxide from Quenched pulp was not very effective. Overall Weight reduction both thermal +

physical together is not very encouraging.

Flotation of Sulfide MineralsGrind Size --100# 100# Collector- Isopropyl Isopropyl XanthateXanthate Frother MIBC MIBC pH - insituinsitu (8.6)(8.6)

PRODUCT Wt.% U3O8 Dist%

Float 37 50

Sink 63 50

Pitchblende is present as discrete and also along with pyrite as garland, some uranium is also present as disseminations which are not completely separated during flotation process. Flotation of pyrite did not yield selective separation of sulfides. Uranium values are indiscriminately distributed in both the sinkand float fractions.

Leaching Host rock for uranium

mineralization is dolostonetype, only ALKALINE LEACHING is suitable.

Most important unit operations are comminution, leaching and solid-liquid separation

R.O.MR.O.M

COMMINUTIONCOMMINUTION

LEACHINGLEACHING

SOLIDSOLID--LIQUID LIQUID SEPARATIONSEPARATION

PURIFICATIONPURIFICATION

PRECIPITATIONPRECIPITATIONREAGENT RECOVERY

and RECYCLE

Focus in Benchscale Leaching studies Atmospheric and autoclave leaching at elevated temperature and pressure.

Solid-liquid separation of leach slurry. Determination of threshold uranium concentration for direct precipitation of uranium from the leach liquor efficiently-both qualitatively and quantitatively.

Reagents recovery. Flowsheet development and testing.

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Mesh-of -gr ind of Feed (Tyler Mesh)

Atmospheric Alkaline Leaching

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Temperat ure ( C ent ig rad e)

Grind -200#Grind -150#Grind -100#

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0 6 12 18 24 30 36 42 48 54Cont act Time (h)

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30 40 50 60 70% Solids

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Temp 80 CTime 12 hSolids 50%, Na2CO3 50kg/t, NaHCO3 50kg/t, KMnO4 10 kg/t.

Time 12 hSolids 50%Na2CO3 50kg/tNaHCO3 50kg/KMnO4 10 kg/t

Mog -100#Temp 80 C Solids 50% Na2CO3 50 kg/tNaHCO370 kg/tKMnO4 10 kg/t

MoG -200#Time 12 hSolids 50%Na2CO3 50 kg/t, NaHCO3 50 kg/t, KMnO4 10 kg/t.

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Mesh of grind of feed (Tyler Mesh) Temperature (Centigrade)

Contact time (hours) % Solids

Atmospheric Alkaline Leaching

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0 20 40 60 80 100 120Na2CO3 / NaHCO3 Dosage (Kg/ton)

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NaHCO3 20 kg/tNa2CO3 50 kg/t

0 20 40 60 80 100

NaOCl

Cu-NH3

KMnO4

Oxygen

U3O8 Leachability (%)The average leachability of U3O8 under optimum conditions was only 68%.Uranium values associated with pyrite could not be effectively leached out.The concentration of leachants are very highAll the alternate oxidants studied NaOCl, Cu-NH3, Oxygen or Air are not as effective as KMnO4. That is highly oxidizing conditions are essential for the system Diffusion of leachants is not sufficient enough to penetrate gangue phases such that uranium values locked-up as ultra-fine dissemination are leachable.

Leaching under High Temperature and Pressure

The Physical refractory nature necessitate more drastic diffusion conditions only then leachant can interact with ultra-fine disseminated uranium values. These objectives can be accomplished only under elevated temperature using an autoclave reactor. Dissolution of pyrite and release of associated uranium values. Scope of using gaseous oxidant instead of chemical oxidant. Improving the diffusion characteristics of leachantsto attack physically refractory uranium values.

Leaching under High Temperature and Pressure

Alkaline Pressure Leaching

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80 100 120 140 160 180Temperature (Centigrade)

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P=6Kg/cm2P=8.5Kg/cm2P=11Kg/cm2

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0 2 4 6 8 10partial pressure of Oxygen (kg/sq.cm)

Temp. 125 C

020406080100

0 2 4 6 8Contact Time (hours)

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U3O8

Pyrite

020406080100

50 55 60 65 70 75 80 85 90 95 100Weight % -200#

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Temperature (Centigrade) partial pressure of Oxygen (kg/sq.cm)

Leachability of Uranium enhanced to 80% at optimum operating conditions. Enhanced leachability of uranium is due to complete dissolution of pyrite. Apparently Kinetics of leaching is also faster. Expensive KMnO4 efficiently replaced with cheaper Oxygen. Stabilized leach residue.

Complete dissolution of pyrite increased sulfate concentration in the leach liquor.

Pyrite dissolution led to increased consumption of oxidant and leachant particularly Na2CO3. Increased concentration of NaHCO3 in leach liquor leads to higher consumption of precipitant NaOH.

Alkaline Pressure Leaching

Upgrading Uranium Values in Leach Liquor

The concentration of U3O8 is very-very less in comparison to other dissolved solutes - sodium carbonate, sodium bicarbonate sodium sulfate etc. This led to poor loading on ion-exchange columns, the usual purification / concentration operation in any process flowsheet Due to the failure of IX in Tummalapalle alternate strategy of concentrating the U values have to be explored.

Partial chemical composition of leach liquor in single-stage autoclave leaching of Tummalapalle ore.

U3O8Na2CO3NaHCO3SO4-2TDS

425 mg/l16.3 g/l63.9 g/l25.3 g/l95.7 g/l

Recycle scheme helps in build-up of uranium concentration. Partial bleed will give the product stream and control impurity build-up including activity build-up. Residual Na in leach liquor can be effectively utilized, thus minimizing the fresh reagent inventory.

Concentration of Uranium in Leach LiquorThe concentration of uranium in leach liquor can be enhanced by recycle of the liquor.

Simulation of uranium concentration in leach liquor at different percentage of bleed out

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Leach&Filter

Wash 1Leach

liquor bleed

Fresh Ore

Residue

Reagents (supplementary qty.)

Bleed of 26 28% volume of leach liquor implies CAKE MOISTURE restricted to 16 18%

Recycle of Leach Liquor with Partial Bleed

Recycle volume72%. U3O8 in leach liquor at steady state 1.6 g/l NaSO4 is 120 g/lSodium addition is exactly equal to the sodium quantity going

along with sodium sulfate

020406080100120140

I II III IV V VI VII VIII IX XCycle Number

020040060080010001200140016001800

U3O8 mg/l

Na2SO4 g/l

Solid-Liquid Separations Counter Current Decantation (CCD): CCD involves use of large volumes of washing with water for dissolved solute recovery. This dilutes the uranium concentration in the leach liquor henceCCD should necessarily follow with IX or SX. Filtration: Direct Filtration of Leach Slurry would not dilute the uranium concentration. Wash of leach residue in counter current mode would yield better solute recovery with minimum fresh solution for wash.

Tight solution balance is essential in the leaching filtrationoperation as the strategy of recycle of leach liquor imposes restrictions on overall water / solution input or output. So solid liquid separation of leach slurry for Tummalapalle ore needs only filtration for solution recovery without concentration dilution and for washing of leach residue with minimum fresh water / solution inventory.

0100

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R6100 R6130 R6140 R6160 R6186 R6220 R6240 R6225

Flocculant

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R6100R6130R6140R6160R6186R6220R6225R6240

Non-ionic PAM high m.w.Anionic PAM low charge high m.w.Anionic PAM low medium charge high m.w.Anionic PAM medium high charge high m.w.Anionic PAM high charge high m.w.Cationic PAM low charge high m.w.Cationic PAM low medium charge high m.w.Cationic PAM medium charge high m.w.

Heat exchanger for washing with hot solution

Solid-Liquid SeparationsLeach slurry has high sodium bearing solutes, filtration rate as such is poor, only 200 kg/h-sq.mAddition of flocculants and filtration in hot condition improved the process performance 500 kg/h-sq.m

Filtration of Leach SlurryPARAMETER VALUE

Leach Slurry Temp.Wash Solution Temp.

Wash displacement ratioCounter-current Wash

VacuumFlocculant TypeRate of FiltrationSolids in FiltrateCake ThicknessCake Moisture

Filtration EfficiencyDissolved Uranium in barren

55 C50 C1:1

4 Stages380 - 400 mm Hg

Non-ionic480 - 500 kg/h-m2

~1%5 - 6 mm18%>90%

Precipitation of Yellow Cake

Effect of uranium concentration in the leach liquor on precipitation efficiency.

Effect of temperature on precipitation efficiency/grade

Effect of reaction time on precipitation efficiency.

Precipitate characterization.

NaOH was used as precipitant to obtain Uranium as Sodium Di-Uranate

Aspects Studied

Leach Liquor Chemical Composition

U3O8Na2CO3NaHCO3Na2SO4TDSCl-SiCaMg

1.622.862.61121754.3

0.0015

0.0040.001

2

gplgplgplgplgplgpl%%%

PO4-3FeMnMoVCuNiPbZnCd

0.00220.0006

Precipitation of Sodium Di-Uranate

Threshold concentration of U for precipitation is 1.2 g/l.Precipitation Conditions: Reaction time: 6h; Temperature: 60 C NaOH concentration should be 6 times of uranium concentration, required for neutralizing the NaHCO3

5041

87.794

55

81.4 82.9

0102030405060708090100

0 200 400 600 800 1000 1200 1400 1600Leach Liq. Conc. (mg/l)

% U3O8

%Precipitation Efficiency

92 91

75

81.4 82.9

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%

Precipitation efficiency, %

Precipitate grade, % U3O8

Chemical Composition of SDUU3O8SiO2TiO2MoFeMgOMnONa2O

77.4%2.07%

Reagent Regeneration? Sodium reagents are expensive. Need for maintaining relatively high concentration of sodium carbonate and sodium bicarbonate during the leaching stage. Recycle of sodium values would make process economics reasonable. High concentration of sodium bicarbonate in the leach liquor requires high dosage of sodium hydroxide during precipitation of sodium di-uranate. Recycle mitigates the negative effect of release of effluents with high TDS in environment.

Barren Solution

Na loss from the process scheme is as sodium sulfate and this cannot be recycled. Na from Uranium leaching circuit can be recovered as marketable grade sodium sulfate by-product with radioactivity within permissible level of max. 1 Bq/g.

Constituent Conc.U3O8Na2CO3NaOHNa2SO4

0.078 gpl97.0 gpl6.2 gpl141.0 gpl

Process Options for Treating SDU Barren

SDU Barren

FREEZE CRYSTALLIZATION

FILTRATION

CAUSTICIZATION

FILTRATION

GlaubersSalt

CaCO3Sludge

Filtrate

Slurry

Dilute Caustic Lye solution

SDU Barren

FREEZE CRYSTALLIZATION

FILTRATION

CAUSTICIZATION

FILTRATION

GlaubersSalt

CaCO3SludgeFiltrate

Slurry

Dilute Caustic Lye solution

Comparison of Regeneration ProcessesProcess A Process B

Direct Freeze Crystallization of SDU Barren leads to higher contamination of sodium sulfate with sodium carbonate.(Solubility of Na2CO3 decreases dramatically as the temperature is lowered or approaches 0C)

Causticization followed by Freeze Crystallization helps in lower levels of sodium carbonate contamination in sodium sulfate.(NaOH solubility is much higher than Na2CO3 )

Causticization Freeze Crystallization sequence gives lesser radioactive contamination in Glaubers salt. This is so because some of the activity in dilute caustic lye (DCL) is precipitated as insoluble calcium salt during causticization.

Constituents Crystallization of Dilute Caustic Lye (DCL) solutionDirect crystallization of SDU

Barren solutionU3O8Na2CO3NaOHNa2SO4.10H2O

0.0002%0.066%0.49%94.6%

0.001%2.1%0.8%89.8%

Purity of sodium sulfate is 94.6%, Na2CO3 contamination is less than 0.066%Uranium oxide contamination is only 2 ppm, total radioactivity is 1 Bq/g..Process B:

Integrated Process Flowsheet

Dil. Caustic Lye liquor

Dil. Caustic Lye

SDU Barren

SDU SlurryLeach Slurry

Bled-out Leach Liquor

Recycle Leach Liquor and Wash

Feed for Leach

Crushing

Wet Grinding

Filtration

Re-pulper cum Reagent Adjustment

Autoclave Leaching Reactor

Filtration and CCWashing

Clarifier

Precipitation

Filtration

Causticization

Filtration

Crystallizer

Filtration

Splitter

Make-up water

R.O.M.

Leach liquor+ Wash

Wash-water for Filter

Sodium Diuranate

Glaubers Salt(Na2SO4.10H2O)

CaCO3Sludge

Washwaterfor Filter

Washed Leach Residue

Na2CO3, NaHCO3CO2FEEDU3O8 0.048%Total S 1.6%

Sodium diuranateU3O8 77 - 80%Recovery 77%

Sodium SulfatePurity 95%

Calcium carbonatePurity 98%

Fresh leachant dosages significantly reduced due to regeneration and recycle. Process water requirement is only 300 kg per ton of ore, minimized due to tight solution balance. Purity of Oxygen is not very critical. So O2 produced by PSA Technology is sufficient. The O2production cost by PSA is very low. Both the reagents used for regeneration of Na2CO3 and NaHCO3 i.e. CaO and CO2 are cheap.

Chemical requirements for Integrated Process Flowsheet

InventoriesSodium Carbonate 20 kg/tSodium bicarbonate 15 kg/t

Sodium hydroxide 10 kg/t

Oxygen 19 kg/t

Carbondioxide 10 kg/t

Calcium oxide 20 kg/t

Water 220 300 kg/t

Concluding Remarks A good understanding of the nature of mineralization of

uranium values in the ore, uranium process chemistry and different unit operations, has led to the development of well integrated specially engineered alkaline leaching process flowsheet to treat the low-grade uranium deposit in the alkaline host rock.

The objective of making a techno-economically viable process flowsheet could be realized primarily by reducing the number of stages of unit operations and conservation of leachants by regeneration and recycle.

Effective recycle of process solution led to minimization of fresh water inventory as well as quantum of liquid effluent discharge.

Inevitable chemical species viz. sodium sulphate and calcium carbonate, in the process were converted into useful by-products by carefully tailored sequence of chemical steps.