International Journal of Mineral Processing 160 (2017) 58–67

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International Journal of Mineral Processing

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Uranium leaching from synthetic betafite: [(Ca,U)2(Ti,Nb,Ta)2O7]

Scott A. McMaster a,b,⁎, Rahul Ram b, Nebeal Faris b, Mark I. Pownceby c, James Tardio b, Suresh K. Bhargava b

a Chemistry - Institute of Fundamental Sciences, Massey University, Private Bag, 11 222 Palmerston North 4442, New Zealandb Centre for Advanced Materials and Industrial Chemistry, School of Applied Sciences, RMIT University, GPO Box 2476, Melbourne, Vic 3001, Australiac CSIRO Mineral Resources, Private Bag 10, Clayton South, Victoria 3169, Australia

⁎ Corresponding author at: Centre for Advanced MateSchool of Applied Sciences, RMIT University GPO Box 2476

E-mail address: s.mcmaster@massey.ac.nz (S.A. McMa

http://dx.doi.org/10.1016/j.minpro.2017.01.0110301-7516/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 August 2016Received in revised form 2 January 2017Accepted 25 January 2017Available online 26 January 2017

The leaching of uranium from of a synthetic form of the naturally occurring pyrochlore group mineral betafite,was investigated in acid sulfate media. Uranium leaching curves obtained over a range of time, temperature,[H2SO4], [FeTOT] and redox potential conditions were similar with each having three discrete segmentsrepresenting significantly different rates of uranium leaching. The first segment occurred in the initial min andinvolved extremely rapid leaching. This segment made up for the majority of the overall uranium leaching ob-tained over the test period (~2.10% U). The high rate of leaching in this segment was demonstrated to be attrib-uted to liberation of surface oxidized uranium from the betafite structure. The second segment between 1 and120minwas characterised by slowuranium leach rateswhich could bepartially influencedby altering the exper-imental conditions. Uranium leaching within this period was most likely due to oxidation of uranium by ferriciron which was progressively slowed by the competing diffusion reaction. The uranium leach rate in the thirdsegment was negligible. This negligible leach rate was demonstrated to occur when approximately 2.10% Uhad leached from the sample and was shown to be due to passivation of the sample surface. Investigationsinto leaching betafite using various iron salts showed the uranium leachingmore than doubling when iron fluo-ride was substituted for iron sulfate. The additional solubility was attributed to leaching of Nb, Ti, and Ta due tothe in-situ formation of HF, where the HF either slowed the formation of a passivating layer or the HFwas able toslowly dissolve the passivating layer.

© 2017 Elsevier B.V. All rights reserved.

Keywords:BetafiteLeachingPassivationPyrochloreRefractoryUranium

1. Introduction

There can often be vast differences in chemistry andmineralogy be-tween uranium ores sourced from different deposits. These may in-clude: differences in the types and amounts of uranium minerals, thecomposition of theuraniumminerals present and the associated ganguemineralogy. Generally, the uraniumminerals preferentially targeted forprocessing/extraction are the primary uranium minerals uraninite(UO2), pitchblende (U3O8) and coffinite (U(SiO4)1 − x(OH)4x). This isdue to their abundance in many uranium ore deposits at comparativelyhigh concentrations and the ease of extraction of uranium (Powncebyand Johnson, 2014; Bhargava et al., 2015). As depletion of high grade de-posits occurs, there is an increased focus on the processing of refractoryuranium ore minerals (Charalambous et al., 2013). One such refractorymineral receiving increased attention is the uranium pyrochlore groupmineral, betafite, A2B2O7 − x(OH)x where A = Na, K, Ca, Sr, Sn, Ba, Pb,Bi, REE, U and B = Ti, Nb, Ta, Zr, Al (Hogarth, 1961, 1977; Lumpkinand Ewing, 1996).

rials and Industrial Chemistry,Melbourne 3001 New Zealand.ster).

Multiple studies conducted on naturally occurring betafites havedemonstrated considerable stoichiometric variation. The most notablevariation within these samples was the B site cation (Nb, Ti or Ta) con-centration (Hogarth, 1961). As well, metamictisation occurs in betafitedue to radiation damage from α-decay of uranium (Lumpkin andEwing, 1988). This leads tomany natural betafites being amorphous, al-though some Ca-rich betafites have been reported as non-metamictand/or partially metamict (Cámara et al., 2004). Chemical alterationdue to weathering of metamict betafite has also been shown to occurin most natural samples (Eyal et al., 1986; Frost and Reddy, 2010;Hogarth, 1961; Lumpkin and Ewing, 1996). Primary alteration consistsof the loss of A site cations Na, Ca, and U via cationic mobilisation. Upto 20–30% of the initial uranium can be lost in the primary alterationstep (Lumpkin and Ewing, 1996). After the primary alteration stage,charge stabilisation of the betafite structure is achieved via hydrationfrom water in the environment (Lumpkin et al., 1985). Lumpkin andEwing (1996) also described a secondary alteration whereby betafitedecomposes into pyrochlore [(Ca,Na)2Nb2O7] and microlite[(Ca,Na)2Ta2O7] after A site cationicmigration of Ca andNa. The remain-ing unstable mineral reacts further to form uranpyrochlore[(U,Ca)2Nb2O7], liandratite (UNb2O10) and rutile (TiO2). More recently,McMaster et al. (2015) showed that after primary alteration, naturally

59S.A. McMaster et al. / International Journal of Mineral Processing 160 (2017) 58–67

altered amorphous betafite contains liandratite and rutile which reactforming UTiNb2O10 (McMaster et al., 2015).

Despite high concentrations of uranium and associated alterationand metamictisation, betafite is considered a refractory uranium oremineral and difficult to process (Abraham, 2009; Geisler et al., 2005;McMaster et al., 2013). The aim of the current work is to conduct a sys-tematic study of the parameters that affect uranium leaching frombetafite in acid sulfate media. The approach is to initially prepare a syn-thetic betafite sample and then to determine the uranium leaching overa range of time, temperature, [H2SO4], [FeTOT] and redox potential con-ditions. The leaching matrix chosen for the current work was selectedbased on leaching parameters commonly varied in commercial uraniumleaching circuits.

1.1. Previous work

Little previous work has been conducted on the leaching of betafitewith acid ferric sulfate solution. van Rensburg (2014) conducted smallscale leaching studies conducted on betafite-containing ore fromRössing, Namibia. Betafite accounts for ~5% of the total uranium foundin the Rössing deposit (Berning et al., 1976; vanRensburg, 2014), the re-mainder being present predominantly as uraninite and minor amountsof coffinite and uranophane. Under acidic, oxidative conditions similarto those used in the Rössing leach circuit, ~80% of the uranium wasleached. The remaining unleached uranium was present in betafite,coffinite and poorly liberated uranophane. McMaster et al. (2012) alsostudied the leaching of natural betafite under Rössinguranium leach cir-cuit conditions. They showed only ~5% of the uraniumwas leached frombetafite (McMaster et al., 2012).

In a further study, McMaster et al. (2014a, 2014b) investigated theleaching of two natural betafite samples over a range of processing con-ditions. Significant differences in leachingwere observed between sam-ples (~58% U for sample A and 5%U for sample B). The conditions underwhichmaximumuraniumwas leached from both sampleswas; 100 g/LH2SO4, 3 g/L [FeTOT], ORP (510mV vs. Ag|AgCl), 35 °C, 6 h. The degree ofchemical alteration/weathering was shown to significantly influencethe rate of leaching with the highly altered sample more readilyleached. The leach rate was shown to decrease however when the al-tered and metamict samples were annealed to restore crystallinity(McMaster et al., 2014b).McMaster et al. (2015) confirmed the increasein leach rate caused by chemical weathering in metamict naturalbetafites. In the most recent betafite leaching study, Nettleton et al.(2015) observed complete extraction of uranium from leaching of nat-ural betafite under extreme conditions in a solution containing214.5 g/L H2SO4, 3 g/L Fe3+ at 89 °C for 48 h. They also noted thatb12% uranium was extracted from the same betafite when re-crystallised in air at 1100 °C (Nettleton et al., 2015).

In the current study, a high-purity, crystalline betafite was synthe-sised for the fundamental leaching experiments. The use of a non-metamict sample should provide information for the minimum extrac-tion of uranium from betafite whereas for natural samples, the combi-nation of a metamict state in association with the presence ofuranium-rich decomposition products, leads to variable, but usually en-hanced, uranium solubilities.

2. Materials and methodology

2.1. Synthesis

Betafite was synthesised using themethod given in (McMaster et al.,2014a). Briefly, synthetic betafite was prepared via solid state reactionbetween uranyl nitrate, calcium carbonate, titanium dioxide, niobium(V) oxide and tantalum (V) oxide to achieve the following stoichiome-try: (U0.32Ca0.42)(Nb0.41Ti1.78Ta0.10)O7. The aforementioned metal saltsand oxides were dry ground for 20 min and then added to a platinum

lined alumina boat which was placed in a tube furnace for 48 h at1150 °C in a dry N2 atmosphere.

Approximately 8 g of synthetic betafite was prepared in multiplesynthesis batches. Each batch was characterised via XRD, Raman spec-troscopy and ICP-MS to insure consistent mineralogical and elementalcomposition between the multiple batches. These batches were thenpooled together resulting in approximately 8 g of material which wasthen thoroughly mixed and dry sieved through a 63 μm sieve prior tothe leaching experiments being conducted. This insured any differencesin leaching due to batch variation could be negated.

2.2. Powder X-ray diffraction (XRD)

Dry powdered samples (pre- and post-leach) were evenly scatteredonto a flat glass XRD plate within a circular poly methyl methacrylateholder and then covered by Kapton film prior to analysis. X-ray powderdiffraction patterns were collected using a Bruker D4 Endeavor highthroughput X-ray diffractometer using CuKα radiation, an incidentbeam monochromator and a scintillation detector. A 1° divergence slitwas used to analyse the 2θ range 5–90° with a step size of 0.02° 2θand counting times of 2 s/step. The instrument was calibrated usingquartz and corundum standards prior to use.

2.3. Leaching studies

For the leaching tests 250 mL of the desired sulfuric acid concentra-tion was added to a 250 mL three-neck flask. The solution was then ag-itated using an overhead mechanical stirrer and heated to the requiredtemperature using a heating mantle. Once at temperature, ferric andferrous sulfate was added to create the required oxygen redox potential(ORP). The solution was then allowed to equilibrate for 10 min before0.05 g of b63 μm synthetic betafite was added. The time upon additionof sample was determined to be 0 min and solution samples were col-lected at pre-determined intervals throughout the experiments.

Specific reaction condition used and in the leaching test matrix wasgiven in Table 1.

Analysis of the leach solutions was conducted using an Agilent HP7700 series ICP-MS. The instrument was calibrated using commercialuranium, niobium, titanium and tantalum standards. An internal stan-dard containing Sc, Ge, Rh, In, Tb, Lu, Bi, was added to all calibrationand test samples. The error in measurement on the instrumentwas cal-culated to be ±2.5%.

3. Results and discussion

3.1. Overview of leaching tests

The synthetic betafite was characterised by XRD prior to leaching toensure the sample was of high purity and there was no other uraniumcontaining minerals present in the sample which would result in inac-curate uranium leaching data. The XRD pattern is shown as Fig. 1. Rutileand a minor amount of pyrochlore (Ca2Nb2O7) were the only impurityphases identified. The leaching of synthetic betafite was investigatedover a range of conditions with parameters selected to reflect those en-countered in commercial processing of uranium ores.

3.2. Leaching of synthetic betafite using Rössing uranium leach circuitconditions

An initial leaching experiment was conducted at conditions similarto those used at the Rössing uranium mine (Test 1, Table 1). Resultsshowed ~0.5% of the total uranium present leached in the initial 1 minof leaching (Fig. 2). The rate of leaching then slowed within the next~45 min after which time a total of ~1.4%U had been leached. Theleach rate after ~45 min continued to decline and after a total leachtime of 6 h, 2.04 ± 0.03% of the U had been leached

Table 1Experimental conditions used in the betafite leaching experiments. For Tests 1–20, ironwas added in the form of the sulfate salt.

TestNo

[H2SO4](M)

[FeTOT](mM) Fe3+:Fe2+

ORP (mV vs.Ag|AgCl)

Temperature(°C) Other

1 0.051 53.6 9:1 520 352 0.51 53.6 9:1 520 353 0.51 53.6 9:1 560 504 0.51 53.6 9:1 590 655 0.51 53.6 9:1 620 806 0.51 53.6 9:1 640 957 0 53.6 9:1 530 358 0.015 53.6 9:1 520 359 0.102 53.6 9:1 520 3510 0.153 53.6 9:1 520 3511 0.51 53.6 9:1 520 3512 0.51 0 N/A N/A 5013 0.51 26.8 9:1 520 5014 0.51 107.1 9:1 520 5015 0.51 160.7 9:1 520 5016 0.51 214.3 9:1 520 5017 0.51 53.6 0:1 330 5018 0.51 53.6 2:3 430 5019 0.51 53.6 12:13 465 5020 0.51 53.6 1:0 620 5021 1.02 53.6 1:0 730 95 FeF322 1.02 53.6 1:0 630 95 FeCl323 1.02 53.6 1:0 600 95 FeBr324 1.02 53.6 1:0 840 95 FePO4

25 1.02 53.6 1:0 900 95 Fe(NO3)326 1.02 53.6 1:0 840 95 Fe2(SO4)327 1.02 0 N/A N/A 50 0.5 M

NaF28 1.02 0 N/A N/A 50 0.5 M KF29 1.02 0 N/A N/A 50 0.5 M

NH4F30 1.02 53.6 1:0 620 50 0.5 M

NaF31 1.02 53.6 1:0 630 50 0.5 M KF32 1.02 53.6 1:0 650 50 0.5 M

NH4F33 1.02 53.6 1:0 620 50 H2SO4

34 1.02a 53.6 1:0 710 50 H2C2O4

35 1.02a 53.6 1:0 760 50 HCL36 1.02a 53.6 1:0 710 50 CH3COOH37 1.02a 53.6 1:0 830 50 HNO3

38 1.02a 53.6 1:0 730 50 HBr39 1.02a 53.6 1:0 560 50 H3PO4

a Indicates equivalent normalised proton concentration used for each acid.

Fig. 1. X-ray diffraction pattern of the synthetic

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3.3. Effect of temperature, [H2SO4], [FeTOT] and redox potential

The leaching of synthetic betafite was investigated under conditions(temperature, [H2SO4], [FeTOT] and ORP), comparable to the range ofconditions used in commercial leaching of uranium-bearing ores(Tests 1–20, Table 1). The effects of varying each of these parametersare discussed individually below:

3.3.1. TemperatureThe influence of temperature on uranium leaching was investigated

at 35, 50, 65, 80 and 95 °C (Table 1, Tests 2–6). All other parameterswere kept constant excluding ORP due to the interrelationship betweenredox potential and temperature. Results are presented in Fig. 3.

The extent of uranium leaching after 6 h increased marginally withan increase in leach temperature from 1.97% U (Test 2 at 35 °C) to2.16% U (Test 6 at 95 °C). The major effect of temperature on theleaching was however only observed in the first 15–20 min, wherethe leach rate was highest in all tests; beyond this time, the rate of ura-nium leaching decreased by a similar extent in all tests. The reaction ki-netics for synthetic betafite leachingmost closely fitted a first order ratebetween the 15–75 min (Table 2, Fig. 4). The range between 15 and75 min was chosen for calculating the kinetics as the rate was signifi-cantly different either side of this interval. The change in leachingmech-anism is discussed in a later section.

3.3.2. Sulfuric acid concentrationStudies were conducted to investigate the influence of sulfuric acid

concentration at 35 °C on the leaching of uranium by varying the acidconcentration while other parameters remained constant (Tests 1, 7–11, Table 1). Results shown in Fig. 5 indicate that the total amount ofuranium leached in 6 h varied between 1.90 and 2.10% U suggestingthat for the range of sulfuric acid concentrations used, there was no sig-nificant effect on the overall amount of uranium leached.

The leaching curves had similar profiles to those observed in the in-fluence of temperature tests – a region of fast leaching (0–30min), par-ticularly in the first min, followed by a region of moderately fastleaching (30–120 min), and a final region beyond 120 min where theleach rate was significantly reduced. The rapid leach rate in the firstminwasmost likely due to the higher acid concentrations causing fasterleaching of oxidized uranium on the surface of the synthetic betafite.The leach rate between 30 and 120 min however was not significantlyimpacted by acid concentration.

betafite used in the leaching experiments.

Fig. 2. Six hour leaching of synthetic betafite at conditions similar to those used in the Rössing uranium leach circuit (Test 1, Table 1).

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3.3.3. Total iron ([FeTOT])The effect of varying total iron concentration, [FeTOT], on uranium

leaching is shown in Fig. 6. All experiments were conducted at a con-stant temperature, acid concentration and ORP (Table 1).

The experiment at 0 mM [FeTOT] (Test 12) showed all uraniumleaching occurred within the initial 1 min. of the experimentrepresenting the solubility of surface oxidized uranium. The uraniumconcentration then remained constant with no further leaching. Fortests using iron concentrations ranging from 26.8 to 214.3 mM, thedata showed that the overall amount of uranium leached was greaterthan thatmeasuredwithout any iron present however, in general, vary-ing [FeTOT] had no significant effect on uranium leaching. The only ob-servable difference was that [FeTOT] had a slight influence on the rateof leaching in the first ~120min, where the rate of leachingwas slowestin in tests using high [FeTOT]. After 120 min however, all tests with ironproduced the same level of uranium solubility reaching a maximum of~2.10% U. Beyond 120 min total leach time the leach rate was not influ-enced by [FeTOT] remaining constant at 2.10% U.

3.3.4. Redox potentialFive experiments were conducted to investigate the influence of

redox potential on U leaching. A range of Fe3+:Fe2+ ratios were used

Fig. 3. Effect of temperature on the leaching of synthetic bet

ranging from 0:1 (330 mV vs. Ag|AgCl) to 1:0 (610 mV) while the[FeTOT] was kept constant (Tests 3, 17–20, Table 1).

As the ratio of Fe3+/Fe2+ is increased, the extent of U leaching in-creased over the duration of the experiment (Fig. 7). Results of the420 and 475 mV experiments (Tests 18 & 19) showed a lesser amountof U leaching in the initial min of leaching relative to the others. Ramet al. (2013) describe lower leaching of UO2 at redox potentials of470mVdue to excess sulfate present leading to the formation of less re-active iron sulfate complexes. Although no oxidation step was noted tooccur over this period with betafite, these more stable sulfate com-plexes could be the possible cause of the slow U leaching over the initialmin at low redox potentials. At the end of the 6 h leach period both Tests18 & 19 had an increasing amount of U leaching which indicated thatthe extent of leaching had not yet reached a maximum. The two testsconducted at the highest redox potentials (Tests 3 & 20) both showedthat ~2.10% U leaching was attained in both cases although this valuewas reached in ~15 min at the higher ORP (610 mV) compared to~90min at 520mV. Test 17 conducted at 330mV(~100% Fe2+) showedno significant increase beyond the initial 1min following removal of ox-idized surface uranium. Although auto-oxidation of Fe2+ will occur, thetime required to reach equilibrium at the acid concentrations and tem-perature used is significantly longer than theduration of the experimentand hence Fe3+ oxidation of uranium is negligible (Pollard et al., 1961).

afite. See Table 1 for a list of experimental parameters.

Table 2Calculated rates for the influence of temperature on uranium leaching from syntheticbetafite.

Test no. Rate (k) Intercept R2

Test 2 (35 °C) 4.55 × 10−5 −0.358 0.96Test 3 (50 °C) 4.75 × 10−5 −0.283 0.97Test 4 (65 °C) 5.00 × 10−5 −0.217 0.92Test 5 (80 °C) 5.14 × 10−5 −0.148 0.90Test 6 (95 °C) 5.30 × 10−5 −0.145 0.94

62 S.A. McMaster et al. / International Journal of Mineral Processing 160 (2017) 58–67

3.4. Effect of Fe counter ion, fluoride ion and lixiviant

3.4.1. Fe counter ionThe influence of different Fe counter ions (SO4

2−, NO3−, PO4

3−, Br−,Cl− and F−) on betafite leaching was investigated under the followingconditions: 53.6 mM [FeTOT], 9:1 Fe3+:Fe2+ (560 mV ORP) T = 95 °C(Tests 21–26, Table 1). The different counter ionswere selected becausethey are commonly found within gangue minerals typically associatedwith betafite-containing ores (Abraham, 2009; Berning et al., 1976;van Rensburg, 2014). Results are shown in Fig. 8.

In general, similar extents of uranium leaching were obtained usingthe different Fe counter ions reaching a maximum leaching of ~2% Uafter about 45min. This observation was true except in the case of fluo-ridewhere FeF3 significantly increased the uranium leaching to ~4.5%U.A weak trend of higher leaching was observed over the initial min foriron halides than for the oxyanions, sulfate, nitrate and phosphate, butthe differenceswereminor. The redox potentialwas lowerwith iron ha-lide salts than the oxyanions indicating the higher initial rate of leachingwas not due to a higher oxidation potential in the solution andwasmostlikely due to monodentate halides forming more reactive complexes insolution compared to the bidentate or polydentate oxyanions (Laxen,1971; Levanon et al., 1969).

While the effect of iron halides had a positive effect on U leachingfrom betafite over the initial 30min, it is known that the rate of leachingfor the anions studied can be influenced by several factors. These in-clude: the reactivity of the species formed at the Fe3+:Fe2+ ratios and[FeTOT] studied; the redox potential effect of the anion ligand; thestabilised complexes formed with the uranyl ion, and; the solubility ofthe Fe salts and their stability constants (Cardarelli, 2000; Laxen,1971; Levanon et al., 1969; Nicol et al., 1975; Ram et al., 2013). Forthis reason deducing the influence of the Fe counter ion is difficult inthis limited data set. All leaching experiments conducted except for F−

showed the leach rate remained constant after approximately 45 min.

Fig. 4. Plot of Ln (U leached) vs. time (s) between 15 and 75min (900 and 4500 s) for the leachgiven in Table 2.

This was most probably due to causes explained previously for the ef-fects of acid strength, [FeTOT], temperature and ORP.

Test 24 showing the effect of iron phosphate on U leaching initiallyshowed a lower leach rate over the initial 120 min before reaching asimilar extent to all others. We also noted a significant amount ofFePO4 precipitate in the leach solution. In an electrochemical study onthe influence of PO4

3− on leaching of UO2, Nicol et al. (1975) reportedcomplexes such as FeH2PO4

2+ and FeHPO4+ were more reactive than

Fe3+, FeOH2+ and Fe2OH24+ complexes. Assuming the presence of

such compounds, the underlying insolubility of FePO4 appeared tohave negated any influence of these complexes on uranium leaching.Moreover, the slow rate of leaching over the initial 120 min was as-sumed due to the betafite grains acting as seeds for the precipitationof FePO4 and hence passivating the surface temporarily and resultingin limited active sites available for leaching.

The effect of nitrate on uranium leachingwas shown to have no sub-stantial impact (Test 25). Ram et al. (2013) examined the effect of NO3

−

on the leaching of uraninite and showed that increasing the concentra-tion of NO3

− increased the rate of UO2 leaching. This was attributed tothe increase in redox potential due to oxidation of Fe2+ from NO3

−.Moreover the excess NO3

− could form uranyl nitrate complexes withthe solubilised UO2

2+ (Ram et al., 2013). Although this had a positive in-fluence on leaching of uraninite, the influence of the aforementionedfactors were shown to have negligible effect on betafite leaching.

3.4.2. Fluoride ionThree fluoride saltswere used to investigate the influence of fluoride

on betafite leaching. The salts used were NaF, KF and NH4F which wereadded at amounts sufficient to form 0.5 M F− in each experiment. Ex-periments were conducted with either 53.6 mM Fe (as the sulfate salt– Tests 30–32, Table 1) or with no Fe added (Tests 27–29, Table 1). Re-sults of these experiments are presented in Figs. 9 and 10.

Both sets of experimentswith the addition offluoride show a greaterextent of uranium leaching in the presence of fluoride compared to F−-free systems. Addition of fluoride to the iron-containing leach solutionsshowed 2.6, 3.25 and 3.25% Uwas leached in experiments using NaF, KFand NH4F, respectively. Furthermore, when fluoride was addedwith noiron present, the amount of uranium leached was greater and the leachrate was still positive even after 360 min of leaching (Fig. 10).

Increased uranium leaching in the presence of fluoride ions is pri-marily due to dissociation of the fluoride salt into F− ions leading tothe formation of hydrofluoric acid (Eq. (1)). Over time however, theHFwill reactwith Fe3+ formingmore stable and less reactive complexes

tests conducted at different temperatures. The calculated reaction rates and R2 values are

Fig. 5. Effect of [H2SO4] on the leaching of synthetic betafite over 360 min. For all other parameters, refer to Table 1.

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(e.g. Eq. (2)) leading to a gradual decrease in the leach rate (Dodgen andRollefson, 1949).

2NaFþH2SO4↔2HFþNa2SO4 ð1Þ

3HFþ Fe3þ↔FeF3 þ 3Hþ ð2Þ

Ferric fluoride (FeF3) will preferentially form when ferric sulfate isadded to a dilute hydrofluoric acid solution (Teufer, 1964). However,octahedral Fe3+ complexes with fluoride in the presence of H2O ligandswill stabilise the ferric iron and hence direct Fe3+/U4+ oxidation is lesslikely to occur. Levanon et al. (1969) reports species [Fe(H2O)2F3] and[Fe(H2O)F4−] complexes are likely to be present at the 1.02 M sulfuricacid concentration and ~9.3:1 F/Fe ratio used in this experimental se-ries. Thismechanismexplains the lower extent of leachingwhen a com-parison between ferric iron and no iron is made (Figs. 9 and 10). Whenno iron is present in the system the formation of the ferric fluoride com-plexes does not occur.

Niobium, titanium and tantalum are known to be easily leached inHF (Gupta and Suri, 1993) and therefore it is presumed that the increasein leaching compared to sulfuric acid/iron sulfate leaching is due to in-creased Nb, Ti and Ta solubilising in the HF solution. This mechanism al-lows more uranium to be exposed to the solution thereby allowingmore U leaching overall. It is important to note however, that even

Fig. 6. Effect of [FeTOT] as iron (II,III) sulfate salts of the leaching of betafite over

though the group 4 and 5metals are dissolving, the rate of their leachingalso slows throughout the experiment. No correlation between Nb, Ti orTa and leached U concentration could be determined suggesting thatthe higher rate of uranium leaching was not due to congruent leachingof B site metals in the betafite structure. We speculate that the leachingmechanism is therefore similar to those previously discussed,where theuranium leaching is more heavily influenced by the solid/liquid inter-face reactions through a series of ligands allowing effective electrontransfers to solubilise the uranium.

As indicated above, the type offluoride salt used also had an effect onthe leach rate of uranium. In the presence of iron, over the initial 30minNaF showed a higher U solubility rate than KF and NH4F possibly due toa more rapid dissociation to form HF than for the other fluoride salts.Thereafter the solubility of uranium remained constant while in thecase of KF and NH4F salts, uranium leaching continued. The highestrate of U leaching occurred when 0.5 M ammonium fluoride wasadded. The redox potential for NH4F is ~650 mV whereas for NaF andKF it was 620 and 630 mV respectively. It was therefore assumed thatthe cause of the higher rate of leaching for addition of NH4F was a dueto a redox potential effect.

Similar trends were observed when fluoride salts were added in thepresence of iron. The biggest difference however was that in all casesthe rate of uranium leaching reached a plateau after approximately100 min (NaF) and 250 min (KF). This is most likely due to the

a 360 min period. All other parameters were kept constant as per Table 1.

Fig. 7. Effect of redox potential on the leaching of synthetic betafite over a 360 min period. All other parameters were kept constant as per Table 1.

64 S.A. McMaster et al. / International Journal of Mineral Processing 160 (2017) 58–67

aforementioned less reactive ferric fluoride complexes [Fe(H2O)2F3]and [Fe(H2O)F4−] forming preferentially and which have been shownto reduce the rate of leaching (Ram et al., 2013). We note howeverthat uranium leaching was still occurring, albeit at a slower rate, up to360 min in the experiment with NH4F.

3.4.3. LixiviantThe influence of lixiviant was studied to investigate the effect of the

conjugate base on uranium leaching (Tests 33–39, Table 1). In this ex-perimental series the proton equivalent of 0.51 N (1.02 M) H2SO4 wasused for the respective acids.

The results in Fig. 11 show the majority of the mineral acids pro-duced leaching curves similar to those observed previously. There wasan initial rapid rate of leachingwithin thefirstmin caused by surface ox-idized uranium followed by a period up to 45–50min where the rate ofuranium leaching increased until eventually reaching a plateau. Theamount of uranium leaching approximated the trend in the acid disso-ciation constants (pKa) with a lower pKa resulting in greater leaching.Sulfuric acid however caused a higher rate of leaching than the otheracids even though it had the 3rd highest pKa under the conditions ofthis study. This was due to the increased concentration of the sulfate

Fig. 8. Effect of different iron-based counter ions on the leaching of synth

oxo-anion which reacts with iron and forms the more reactive iron sul-fate complexes, FeSO4

+ and Fe(SO4)2−, both of which increase the rate ofuranium leaching.

Phosphoric acid produced the lowest rate and amount of uraniumleaching from betafite. The acid reacts with Fe3+ to form insolubleFePO4 (Haynes, 2014) which decreases the redox potential and pro-hibits the Fe3+/U4+ oxidation reaction:

2H3PO4 aqð Þ þ Fe2 SO4ð Þ3 aqð Þ↔2FePO4 sð Þ þ 3H2SO4 aqð Þ ð3Þ

The slow rate of leaching over the initial period is due to betafite par-ticles seeding the precipitation of the sparingly soluble FePO4 (con-firmed via XRD). This inhibits the redox process from occurring andhence initially the only uranium leached is the surface oxidized urani-um. Huang et al. (2004) showed the precipitation of FePO4 occurredquicker when a seeding compound was added to the phosphate con-taining effluent (Huang et al., 2004). It was assumed a similar mecha-nism occurred in this leaching experiment where betafite acted as theseed; although confirming that FePO4 had coated the surface was diffi-cult due to the vast excess of FePO4 precipitate with respect to betafitein the leach solution. Over the duration of the experiment however,

etic betafite. All other parameters were kept constant as per Table 1.

Fig. 9. Effect of fluoride ion with 3 g/L [FeTOT] as (Fe2(SO4)3) present on the leaching of betafite. All other parameters were kept constant as per Table 1.

65S.A. McMaster et al. / International Journal of Mineral Processing 160 (2017) 58–67

the FePO4 does partially dissolve and re-precipitate allowing newly ex-posed areas of betafite to undergo leaching. A maximum leaching limitof ~1.30% U was reached after approximately 120 min. This value wassignificantly lower than in the other experiments.

The amount of uranium leaching was also observed to be greatly di-minishedwhen oxalic acid was used for leaching. Again, themajority ofthe leaching occurred in the initial min of the experiment. Oxalic acid isknown to be a reducing agent and forms insoluble Fe2+ and Fe3+ saltssuch as FeC2O2 and Fe2(C2O2)3 (Haynes, 2014). Moreover, solubleferrioxalate ion [Fe(C2O4)3]3− will form when Fe3+ reacts with oxalicacid (Jeong and Yoon, 2005). Initially however, no precipitate was ob-served indicating that the iron was likely present as the soluble[Fe(C2O4)3]3− and [Fe(C2O4)2]2− complexes or [Fe2(SO4)3] which areused in the redox process to solubilise the uranium. The results showa very similar leach curve to that conducted with 53.6 mM ferrousiron (Fig. 7, Test 17) where no Fe3+ was present and hence the U/Fe3+ oxidation appears not to take place. This suggests that the reduc-tion of Fe3+ by C2O4

2– has occurred and hence no oxidation of U followswhich results in the small amount of uranium leached.

Fig. 10. Effect of fluoride ion with no Fe present on the leaching of b

3.5. Discussion of leaching

In experiments to determine the solubility of betafite under varyingconditions, similar uranium leach curveswere observed for themajorityof the tests. The leach curves were typically comprised of three relative-ly distinct segments, which are described below and shown in Fig. 12.

1. 0–1min:A period of initial rapid leachingwhich accounts for thema-jority of the leaching of uranium (80% of the total leaching). In eachexperiment ~1.80% of the uranium in the sample was leached outin the initial min (segment 1). This instantaneous leaching is pro-posed to be due to the liberation of oxidized surface uranium U(VI)from the sample.

2. 1–120 min: A very slow leach rate interval that continues to slowuntil ~120 min. Of all the segments, this was the one where changesin leach rate were most closely influenced by variations in experi-mental conditions. The second leaching segment was likely due tothe more traditional ferric oxidation/sulfate liberation reactions anddiscussed more extensively in previous sections.

etafite. All other parameters were kept constant as per Table 1.

Fig. 11. Effect of varying the lixiviant on the leaching of synthetic betafite. Experiments were conducted with different acids at equivalent proton concentrations relative to 1.02 M H2SO4

(0.51 N). Dissociation constants are also provided for each of the mineral acids. All other paramaters were kept constant as per Table 1.

66 S.A. McMaster et al. / International Journal of Mineral Processing 160 (2017) 58–67

3. 120–360min: The third segment showed very little leaching in mostcases. By this stage it is likely that the surface of the betafite has be-come passivated by leaching of the exposed uranium sites leavingbehind a uranium-depleted layer that is impermeable to the leachsolution.

Slight variations in the presence/absence and length of each ofthese segments are noted depending on variations in the experimentalset-up.

There are two possible explanations for the overall low yield of ura-nium leaching from synthetic betafite. The first explanation is that asthe oxidized uranium is removed from the mineral structure the insol-uble metals Nb, Ti and Ta remain causing the new uranium sites to be-come less exposed to the solution. The other explanation is leachingand re-precipitation of the sparingly soluble metals Nb, Ti and Tacould be forming an insoluble layer coating the betafite and hence cre-ating a passivation layer. This therefore limits the leaching by creatinga diffusion controlled mechanism between 1 and 120 min. Beyond

Fig. 12. The three segments of leaching observed for uranium leaching from betafite (using Tesegment 3: 120–360 min (purple).

120 min the leach rate approaches zero due to two possible factors.These factors include:

• The overall change in the speciation of uranium as leaching occurs.• Passivation due to re-precipitation of semi-soluble group 4 and 5metals or structural rearrangement of mineral once some uraniumhas been liberated.

In work conducted by Nettleton et al. (2015) characterisation ofannealed natural betafite showed an increased amount of tantalumwas observed on the surface layer of the annealed material. This wassuggested to have caused the reduced rate of leaching in the recrystal-lised samples (Nettleton et al., 2015). This is supportive of the previous-ly discussed hypothesis where an insoluble layer could be forming onthe surface of the synthetic betafite upon leaching which inhibits anyfurther leaching of the sample. If this is proved correct, further studiesshould examine the potential for preferentially attacking the passivat-ing layer and removing the semi-soluble metals.

st 3 as an example). Segment 1: 0–1 min (yellow), segment 2: 1–120 min (dark yellow),

67S.A. McMaster et al. / International Journal of Mineral Processing 160 (2017) 58–67

4. Summary

High purity betafite was synthesised and used for leaching studies.The parameters investigated were; temperature, [H2SO4], [FeTOT],redox potential, iron salt, fluoride salt and lixiviant. The results showedthe leaching of uranium occurring in three distinct segments. The firstsegment had a high rate of leaching and occurred in the initial min.This leaching was due to liberation of surface oxidized uranium. Thesecond leaching segment between 1 and 120 min was shown to bedue to the typical redox reaction between ferric iron and uranium.This segment was most affected by changes in leaching parameterssuch as acid concentration, total iron concentration and redox potential.As the experiments progressed the leach rate decreased due to theredox and diffusion reaction competing. By the start of the third seg-ment at approximately 120min the leach rate had slowed to a negligiblerate and remained slow for the remainder of the experiment. The extentof U leaching in the majority of the experiments was demonstrated tobe b2.00% U demonstrating the highly refractory nature of the mineral.

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