Solvent Extraction and Ion Exchange Applications in ...

This article was downloaded by: [Kathryn C. Sole] On: 03 October 2011, At: 15:03 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Solvent Extraction and Ion Exchange Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lsei20

Solvent Extraction and Ion Exchange Applications in Africa's Resurging Uranium Industry: A Review a

b

Kathryn C. Sole , Peter M. Cole , Angus M. c

Feather & Marthie H. Kotze

d

a

Independent Consultant, Johannesburg, South Africa b

TWP Projects, Johannesburg, South Africa

c

BASF, Johannesburg, South Africa

d

Mintek, Randburg, South Africa

Available online: 03 Oct 2011

To cite this article: Kathryn C. Sole, Peter M. Cole, Angus M. Feather & Marthie H. Kotze (2011): Solvent Extraction and Ion Exchange Applications in Africa's Resurging Uranium Industry: A Review, Solvent Extraction and Ion Exchange, 29:5-6, 868-899 To link to this article: http://dx.doi.org/10.1080/07366299.2011.581101

PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/termsand-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan,

sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.

Downloaded by [Kathryn C. Sole] at 15:03 03 October 2011

The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Solvent Extraction and Ion Exchange, 29: 868–899, 2011 Copyright © Taylor & Francis Group, LLC ISSN 0736-6299 print / 1532-2262 online DOI: 10.1080/07366299.2011.581101

Solvent Extraction and Ion Exchange Applications in Africa’s Resurging Uranium Industry: A Review


Kathryn C. Sole1 , Peter M. Cole2 , Angus M. Feather3 , and Marthie H. Kotze4 1

Independent Consultant, Johannesburg, South Africa 2 TWP Projects, Johannesburg, South Africa 3 BASF, Johannesburg, South Africa 4 Mintek, Randburg, South Africa

Abstract: During the past five years, there has been a global resurgence in the processing of uranium. This is particularly evident on the African continent where exploration for uranium is booming, several plants are being commissioned, and many more projects are in the pipeline, mainly in Malawi, Namibia, Niger, South Africa, and Tanzania. This article reviews recent African uranium developments with a focus on solvent-extraction, ion-exchange, and resin-in-pulp processes. The essential chemistry is outlined, followed by discussions of process choices and selected flowsheets. Keywords: solvent extraction, ion exchange, resin-in-pulp, uranium, Africa, Rössing, Langer Heinrich, Trekkopje, Somaïr, Cominak, Kayelekera, Vaal River

INTRODUCTION The processing of uranium from primary sources first gained momentum in the late 1950s as the technology used in the Manhattan Project was introduced to the wider scientific community.[1,2] At this time, two main approaches were employed—carbonate leaching of uranium ores followed by direct precipitation of a relatively impure uranium product or sulfuric acid leaching of the ores to yield either cationic or anionic species in solution which could be respectively upgraded and purified using di(2-ethylhexyl)phosphoric acid (D2EHPA) or a tertiary amine extractant, ahead of precipitation.[3,4] Address correspondence to Kathryn C. Sole, 213 Wyoming Ave., Berario, Johannesburg 2195, South Africa. E-mail: [email protected]

869

Despite a robust industry for two decades, the processing of uranium declined in the 1980s as the market became dominated by the liquidation of commercial and military inventories. The consequently depressed price led to cuts in production and exploration. Furthermore, the nuclear associations of uranium garnered many negative perceptions amongst the general public following the Three Mile Island and Chernobyl incidents. This resulted in a stay of new nuclear projects and resulting drop in demand for uranium. Recently, however, new demand is emerging from China, India, and Russia, as these countries seek to increase their nuclear power capabilities. There is renewed global interest in this commodity and the price of uranium has escalated dramatically (Fig. 1).[5,6] Although the heydays of 2007 were short-lived, the price still remains significantly higher than five years ago. Supply is currently uncertain and is far below demand. As mining companies look seriously at entering this market again, there has been a massive resurgence in exploration and projects, including in countries that traditionally have not been uranium producers. This is particularly evident on the African continent, with new plants being commissioned and several more projects in the pipeline, mainly in Malawi, Namibia, Niger, and South Africa (Table 1). A significant number of international companies are also actively prospecting for uranium in Botswana, Gabon, Malawi, Morocco, Mozambique, Namibia, South Africa, Tanzania, Zambia, and Zimbabwe.[5,15] Figure 2 compares the uranium reserves, resources, and current production of major African countries with those of other significant global players.[7] Considerable potential exists on the African continent for the

140 120

Price (USD/lb U3O8)


SX and IX in Africa’s Uranium Resurgence

100 80 60 40 20 0 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 08 09 10 Year

Figure 1. Historical spot price of uranium (data from [5] ).

∗

Lethlakane Bakouma Kayelekera Etanga Husab Langer Heinrich Rössing Uranium Trekkopje Valencia Azelik Cominak Imourarin Somaïr Dominion Reefs Ergo Uranium Ezulwini Uranium Harmony TPM Mine Waste Solutions Ryst Kuil Vaal River South Mkuju River Chirundu Mutanga

Operation A-Cap Resources Areva Paladin Energy Bannerman Resources Extract Resources Paladin Energy Rio Tinto Areva Forys Metals CNNC International Areva Areva Areva Shiva DRDGold First Uranium Harmony Gold Simmer and Jack Areva AngloGold Ashanti Mantra Resources African Energy Resources Denison Mines

Majority owner 2011 2014 2009 2013 2014 2007 1976 2011 2011 2010 1978 2013 1971 2008 2011 2009 – 2011 2010 1978 2012 2011 –

Production start

CM: Care and maintenance; PP: Preproduction; F: Feasibility; Comm: Commissioning.

Tanzania Zambia

South Africa

Niger

Botswana Central African Rep Malawi Namibia

Country 998 2363 1493 2272 6800 2363 4500 3545 1590 700 2364 5000 3545 1360 300 404 – 420 1745 681 1680 591 804

Capacity (t/a U3 O8 ) F F 104 F F 1227 4158 PP F PP 1695 PP 1811 CM F 11 F Comm. PP 636 PP F F

Production 2009 (t/a U3 O8 )∗

Table 1. Summary of African uranium production, pre-production, and feasibility projects (as of August 2010)


[7]

[7]

[13,14]

[9]

[7]

[12]

[11]

[7,9]

[7]

[7,9]

[7,9]

[7]

[7]

[7]

[7]

[7,9]

[7]

[7,9]

[7]

[7]

[7,8]

[7]

[7]

Ref.

870 K. C. Sole et al.


871

300,000 Reserves Resources Production

250,000

U3O8(t)

200,000

150,000

50,000

-

Australia

Canada

Kazakstan

Namibia

Niger

S.Africa

Figure 2. Comparison of uranium reserves, resources, and annual production (2009) for the main uranium-producing countries in Africa and the world (data from [7] ). 20,000 Gabon Namibia Niger South Africa Canada Kazakstan Australia

18,000 16,000 Production (t/a U3O8)

14,000 12,000 10,000 8,000 6,000 4,000

2009

2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

1997

1996

-

1995

2,000 1994


100,000

Figure 3. Production trends in major uranium-producing African countries compared with the top three world producers (data from [5] ).

development of a strong uranium industry. Namibia has more than doubled its production in the last six years (to over 5000 t/a U3 O8 ) (Fig. 3) to become the world’s fourth-largest producer and there are more than 30 companies active in exploration in that country alone.[9] Husab (formerly Rössing South), a deposit only discovered in 2008, is now believed


872

K. C. Sole et al.

to be the largest uranium-only deposit in the world, as well as being the richest deposit in Namibia.[16] When this project comes on line in 2014, it will have an output greater than the combined current total Namibian production and be the second largest uranium mine in the world (after Olympic Dam, Australia) with an annual production of 6803 t U3 O8 .[17] Namibia will then become the world’s third largest uranium producer after Kazakhstan (which produced 15.0 kt U3 O8 in 2009) and Canada (12.0 kt U3 O8 ).[7] The Imouraren project in Niger, coming online in 2013, will be the third largest uranium open pit in the world and is expected to produce 5000 t/a U3 O8 for a lifetime of 35 years.[18] South Africa, estimated to possess 8% of the world’s recoverable resources,[7] produces U3 O8 as a byproduct from some Witwatersrand gold mines.[19] This article reviews recent African developments in the primary uranium processing industry with a focus on solvent-extraction (SX), ionexchange (IX), and resin-in-pulp (RIP) operations. The basic chemistry is outlined, followed by a discussion of process choices and selected flowsheets. URANIUM PROCESS CHEMISTRY Process Overview The technologies for the primary processing of uranium have not changed significantly during the past five decades. Sulfuric acid processing is typical in African operations, although carbonate leaching can be employed where acid consumption makes the project economically unattractive. Carbonate leaching is expensive and efficient reagent recovery is required to ensure that a project is economically robust. Following crushing and grinding, uranium flowsheets all involve an upfront leaching step to solubilize the metal and subsequent uranium recovery as ammonium (or, less often, sodium) diuranate (ADU, SDU), commonly known as “yellow cake,” which is calcined to give U3 O8 as a final product (Table 2).[20] Precipitation of UO2 using H2 O2 is also often considered today, as the uranium peroxide precipitate is the most crystalline and responds well to dewatering (i.e., thickening and filtration), requires a lower temperature for calcination, and the product can be directly marketed. In between the leaching and precipitation steps, one of three processing routes is typically followed for concentration and purification: (i) IX for concentration and purification of the weakly acidic leach liquor; (ii) IX for concentration of uranium, followed by SX for purification (the so-called Eluex or Bufflex process); (iii) Direct SX of the leach liquor (the Purlex or Amex process).


873

Table 2. Typical specification for calcined uranium product, U3 O8 [20]


Species Uranium (U) Arsenic (As) Barium (Ba) Boron (B) Cadmium (Ca) Calcium (Ca) Carbonate (CO3 ) Chromium (Cr) Fluorine (F) Iron (Fe) Lead (Pb) Magnesium (Mg) Mercury (Hg) Moisture (H2 O) Molybdenum (Mo) Phosphorus (PO4 ) Potassium (K) Selenium (Se) Silicon (SiO2 ) Silver (Ag) Sodium (Na) Sulfur (S) Thorium (Th) Titanium (Ti) Vanadium (V) Zirconium (Zr)

Typical value (%)

Maximum impurity limit (%)

75 0.01 0.01 0.005 0.01 0.05 0.2 0.01 0.01 0.15 0.01 0.02 0.01 1.0 0.10 0.10 0.20 0.01 0.50 0.01 0.5 1.00 0.1 0.01 0.10 0.01

>65 0.04 0.04 0.10 0.04 1.00 0.5 0.04 0.10 0.50 0.04 0.50 0.04 2.00 0.30 1.00 1.00 0.04 2.00 0.04 3.00 4.00 0.5 0.05 0.75 0.50

The choice of leaching technology depends strongly on the mineralogy of the ore.[21–23] More than 185 uranium minerals are known and the mineralogy can be complex.[9] Ores with tetravalent uranium minerals (such as uraninite (UO2 ), coffinite (U(SiO4 )1−x (OH)4x ), and uranothorite ((U,Th)SiO4 )) require addition of an oxidant to convert the uranium to the more soluble hexavalent species. Less common ores that contain hexavalent uranium minerals (carnotite (K2 (UO2 )2 (VO4 )2 ·3H2 O), autunite (Ca(UO2 )2 (PO4 )2 ·10−12H2 O), and uranophane (Ca(UO2 )2 (HSiO4 )2 ·5H2 O) are examples) do not require the use of an oxidant. In African deposits, typical impurities include Mo, Ti, and V.[9] Traditional atmospheric sulfuric-acid leaching of uraniumbearing ores often employs pyrolusite (MnO2 ) or Caro’s acid (H2 SO5 )[24] as an oxidant for controlling the redox potential to ensure economic recovery of uranium. More recently, air/SO2 is also being considered as an environmentally more attractive oxidant for uranium leaching than

874

K. C. Sole et al.


pyrolusite.[25,26] These reactions for the pyrolusite as the oxidant show the role that the ferric/ferrous couple plays in the oxidation of tetravalent uranium: 2+ UO2 + 2Fe3+ → UO2+ 2 + 2Fe

(1)

2Fe2+ + MnO2 + 4H+ → 2Fe3+ + Mn2+ + 2H2 O.

(2)

Design aspects for dispersion of especially air into a pulp mixture to ensure efficient oxygen mass transfer have to be considered carefully, as a sub-optimal design could lead to the excessive formation of thiosulfate species that, being anionic, would have a detrimental impact on resin or SX performance.[27] Other modern innovations include the use of autoclave leaching of iron sulfide concentrate at elevated temperature and pressure to generate acid and oxidant (ferric) for atmospheric leaching of uranium minerals such as uraninite,[28] while heap leaching (practiced in the copper industry for many years) has opened up opportunities for treating low-grade materials, especially when pyrite is present in the ore, and can enable smaller projects in remote locations to be viable.[29,30] As indicated in the Pourbaix diagram of Fig. 4,[30] uranium dissolves as the U(VI) species, UO2 2+ , but combines with sulfate present in the leach liquor to form the uranyl sulfate anions, UO2 (SO4 )2 2− and UO2 (SO4 )3 4− . When sufficient sulfate is present, the latter species dominates under the aggressive leaching conditions (high temperatures and high redox potentials of 600–700 mV wrt SHE) so that many other impurities are also leached, including the anions of Si, Mo, Bi, W, Sb, As, Mn, V, Zr, and P, as well as Cl− , NO3 − , SCN− , and S2 O3 − .[32] The SO4 2− , HSO4 − , and Fe(SO4 )− anions are also generated during leaching. Where uranium is hosted in minerals that are high acid consumers (>75–100 kg/t ore), such as limestone (CaCO3 ), dolomite (CaMg(CO3 )2 ), or calcrete (CaCO3 ), alkaline leaching is preferred. This medium is more selective for uranium than acid, with impurities such as Fe, V, Al, and Ti remaining largely unreacted.[33] The UO2 (CO3 )3 4− anion usually predominates, although the UO2 (CO3 )2 2− species may also exist at low carbonate concentration. A mixture of Na2 CO3 (typically 0.3 M) and NaHCO3 (0.1 M) is used. The leaching reaction, shown for the hexavalent uranium mineral, carnotite, is given in Eq. (3): UO3 + Na2 CO3 + 2NaHCO3 → Na4 UO2 (CO3 )3 + H2 O.

(3)


O2

UO22+ UO2(SO4)

Potential Eh (V)

1.0

875

U(SO4)2+

0.5

H2O UO2(SO4)22–

U4+

U3O8

U(SO4)2

U4O9

0.0 H2O


UO2(OH2)H2O

UO2

H2

–0.5

–3

–2

–1

0

1

2 3 4 5 Equilibrium pH

6

7

8

9

Figure 4. Eh -pH diagram for the U-S-H2 O system at 25◦ C; [U] = 10−2 M; [S] = 10−1 M (adapted from [31] ).

The bicarbonate acts as a pH buffer and is necessary to avoid reprecipitation of the dissolved uranium by reaction with the hydroxyl ion: UO3 + 3Na2 CO3 + H2 O → Na4 UO2 (CO3 )3 + 2NaOH 2Na4 UO2 (CO3 )3 + 6NaOH → Na2 U2 O7 + 6Na2 CO3 + 3H2 O.

(4) (5)

Purification of Uranium Liquors by Solvent Extraction Amines are used for the recovery of uranium from sulfate leach liquors and uranium recovery is one of the most important commercial uses of amines.[34] The extraction of uranium(VI) occurs in the order: tertiary > secondary > primary amine. The extraction of iron(III) occurs in the reverse order, so tertiary amines are the obvious choice of extractant, since iron is often present in the minerals or is added in the leach as an oxidant catalyst to improve the extraction efficiency of the leach.[35] The tertiary amine is a weak-base reagent so it is capable of treating feed solutions with a wide range of acidities: acidities can vary from pH 2 for pregnant leach solutions (PLS) to >100 g/L H2 SO4 when treating IX eluate in an Eluex process. Tricaprylyl amine, the tertiary alkyl amine sold as Alamine® 336 (Cognis, now BASF) or Armeen® 380 (Akzo Nobel), is widely used, usually in conjunction with an alcohol phase modifier (typically isodecanol) to prevent third-phase formation and inhibit the emulsion formation which

876

K. C. Sole et al.

can occur as a consequence of the high molecular mass of the organicphase complexes. (The use of an alcohol can be problematic from time to time; it also acts as a nutrient for microorganisms, and cases are known of bacterial and fungal outbreaks in uranium SX circuits using alcohols.[36] ) The structure of the tertiary amine can be represented as R3 N. The extractant needs to be protonated for extraction to occur. This is achieved in a single step during extraction due to the high acid content of the leach liquor. In sulfuric acid, some bisulfate extraction may also occur:[37]


2R3 N(org) + H2 SO4(aq) → (R3 NH+ )2 SO2− 4 (org)

pKa = 9.02

− + R3 N(org) + H+ (aq) + HSO− 4 (aq) → R3 NH HSO4 (org) .

(6) (7)

Once the extractant is protonated, the uranium complex is extracted by an anion-exchange process: 4− 4− + 2(R3 NH+ )2 SO2− 4 (org) + UO2 (SO4 )3 (aq) → (R3 NH )4 UO2 (SO4 )3 (org)

+ 2SO2− 4 (aq)

(8)

2− 2− + (R3 NH+ )2 SO2− 4 (org) + UO2 (SO4 )2 (aq) → (R3 NH )2 UO2 (SO4 )2 (org)

+ SO2− 4 (aq) .

(9)

The formation of adducts, such as [(R3 NH)2 SO4 ]2 ·UO2 SO4 (H2 O)3 , has also been postulated.[34] The mechanism of uranium extraction from sulfate media by amines has been comprehensively reviewed.[38,39] The equilibrium loading capacity of an amine reagent depends on the other anions in solution and the acidity of the feed liquor. The theoretical maximum loading is 1.21 g U(VI) per vol.% Alamine 336: extraction from high acid eluates will decrease this value significantly due to co-extraction of bisulfate anions. The loaded organic phase is washed with water or dilute H2 SO4 to remove any physically entrained aqueous phase since this will contribute to the carryover of impurities from the feed liquor to the product. Coextracted anionic contaminants (such as Fe(III) as an anionic sulfate complex or zinc chloride from a relatively high chloride background) are removed by scrubbing with dilute H2 SO4 (∼10 g/L) or (NH4 )2 SO4 solution at pH 2–2.5.[32] Uranium is stripped from the loaded organic phase by either pHswing or anion-exchange mechanisms using a variety of reagents, including NaCl, (NH4 )2 SO4 , Na2 CO3 , ammoniacal ammonium sulfate, or ammonia


877

gas.[32,34,40] Stripping with (NH4 )2 SO4 is most commonly practiced in Africa, although NaCl and Na2 CO3 are employed in Niger operations: (R3 NH+ )4 UO2 (SO4 )4− 3 + 4NH4 OH → 4R3 N + (NH4 )2 SO4 + (NH4 )2 UO2 (SO4 )2 + 4H2 O

(10)

(R3 NH+ )4 UO2 (SO4 )4− 3 + 7Na2 CO3 → 4R3 N + Na4 UO2 (CO3 )3


+ 4NaHCO3 + 3Na2 SO4

(11)

(R3 NH+ )4 UO2 (SO4 )4− 3 + 4NaCl → 4R3 N.HCl + Na4 UO2 (SO4 )3 . (12) Where the mechanism of stripping involves deprotonation of the amine by increasing the pH, the reaction should be controlled to avoid the precipitation of solid ADU in the strip circuit: 2(NH4 )2 UO2 (SO4 )2 + 6NH4 OH → (NH4 )2 U2 O7 + 4(NH4 )2 SO4 + 3H2 O. (13) The loaded strip liquor, known as “OK liquor,” typically has a uranium concentration of 7–10 g/L U3 O8 . This is treated with ammonia to produce an ADU slurry, which is further processed at a nuclear facility or calcined on site to U3 O8 . The stripped organic phase is treated in a regeneration circuit using a Na2 CO3 /NaOH mixture to ensure the removal of a range of other impurities (Mo, Bi, chlorides, nitrates, phosphates, silica, cyanide complexes, and organics, such as humic acids and tars) from the organic phase to avoid their build up in the circuit. Extraction by amines is very rapid (< 30 s) and stripping occurs readily under mild conditions; however, these systems can co-extract other anions present in the leach liquors, tend to be sensitive to the presence of suspended solids, flotation reagents, oils, etc., and phase disengagement can be slow. Some specific problems associated with the presence of impurity elements in the PLS in SX circuits include the following:[28,32,35] ●

●

●

Fe, As, Mo, V, and Zr can introduce product purity issues if not adequately scrubbed; Mo can form precipitates or third phases, depending on the oxidation state, pH, and presence of heteroatoms, such as phosphate, silicate, or arsenates; Nitrate (often originating from explosives used in the mining process) significantly depresses uranium extraction, can lead to severe organic integrity problems, through oxidative degradation and consequent loss of the extractant, and can reduce stripping efficiency;

878 ●

●

●


● ●

K. C. Sole et al.

Chloride depresses uranium extraction by co-extraction[41] (to a lesser degree than nitrate) and is reported to reduce the efficiency of stripping by ammonium sulfate;[40] Organics can hinder phase disengagement, reduce uranium loading capacity for the amine, and co-extract other metals, leading to product purity problems; Soluble and colloidal silica at levels above 500 mg/L can cause severe phase disengagement problems and major crud formation, and reduce the effectiveness of the amine:[42] it is recommended to operate under organic-continuous mixing conditions when high silica levels are present; Zr can contribute to crud problems in the strip circuit; Some Th may co-extract with U and attracts penalties if present in yellow cake, so needs to be separately recovered if present in appreciable quantities in the PLS.

Particularly troublesome impurities can be better managed today using modern SX reagents. For example, the use of Alamine 304-1 (trilaurylamine) is beneficial for treating Mo-containing liquors as it exhibits better solubility of the amine-Mo complex, retaining it in the organic phase, and permits selective stripping of U3 O8 and Mo.[42] Good discussions of the behavior and mitigation of impurity species present in tertiary amine circuits are available.[38,43] PURIFICATION OF URANIUM LIQUORS BY ION EXCHANGE IX can be used instead of SX, especially for lower grade ores or in systems with difficult liquid-solid (L/S) separation characteristics, either by operating with unclarified solution or as a RIP system. In RIP, the resin is added into the slurry after leaching to recover solubilized uranium, while in resin-in-leach (RIL), the resin is added directly into the leach process to scavenge uranium as it is leached and thereby drive the equilibrium of the leach reaction. RIP can be applied to both acid and carbonate leach pulps, where particle sizes up to 80% passing 75 µm and pulp densities up to 50–55% solids can be tolerated. The reactions for the loading of the uranyl sulfate or carbonate anions by strong-base resins are very similar to those for SX: − UO2 (SO4 )4− 3 + 4RX → R4 UO2 (SO4 )3 + 4X

(14)

− UO2 (CO3 )4− 3 + 4RX → R4 UO2 (CO3 )3 + 4X ,

(15)

where R represents the resin functional group which is grafted to the resin backbone (typically a polystyrene matrix) and X is the counter-ion.



879

Quaternary amine resins, which are not particularly selective, are most widely employed. Typical modern examples include PFA 600/4740 (Purolite)[44] and Amberjet 4400 (Dow).[45] Strong-base resins are less selective for uranium than tertiary-amine extractants and can be poisoned by irreversible loading of strongly complexed anionic species, such as Mo, Si, polythionates, Ti, Zr, Th, and organics, which reduce reactivity and loading capacity.[38] Sulfates and bisulfates (and carbonates and bicarbonates) compete for resin sites and reduce the resin capacity for uranium. These resins also typically lose effectiveness at chloride levels above 3 g/L. Vanadium, present as VO3 − or VO4 3− , is adsorbed more strongly than uranium and is difficult to elute. It is then common to reduce the redox potential of the solution by the addition of metallic iron. This reduces Fe(III) to Fe(II), which in turn reduces V(V) to V(IV) species which are not loaded. Co-loading of Fe(III), some of which could be present as Fe(SO4 )n (3–2n) or Fe(OH)(SO4 )2 2− in sulfate media, also depresses uranium extraction. In South Africa, where most uranium is recovered as a byproduct of gold processing, cobalti-cyanide and polythionate species have proved particularly troublesome in the past. A technical solution to the problem of cobalti-cyanide poisoning has not been found and common practice is to acid leach uranium first before sending the residue for cyanidation to recover gold. This “reverse leach” technique, used in South Africa in the 1970s and 1980s, often improved the efficiency of the subsequent gold leach with cyanide; however, any recycling of water from the gold circuit that still contains cobalt cyanide to the uranium circuit would cause a similar fouling problem. In contrast to SX using tertiary amines, where phase contact times of less than 30 s are required for equilibrium to be reached, IX kinetics are slow due to diffusion limitation into the porous resin matrix and therefore require a high resin inventory, adversely affecting the capital cost of a plant (although this can be minimized by use of a counter-current or carousel configuration). Elution of uranium loaded from acidic media is usually carried out using sulfuric acid (100 g/L) as the bisulfate anion readily displaces the uranium complex. Upgrading of a leach solution (0.1–0.8 g/L U3 O8 ) to as high as 120 g/L U3 O8 on the resin and 35 g/L U3 O8 in the eluate can be achieved by judicious selection of the operating conditions,[46] although strong equilibrium constraints exist. It is more common to employ a much lower upgrading factor and then further process the eluate by SX (see Process Choice). In alkaline leaching flowsheets, resins can be eluted using NaNO3 or NaCl, often in the presence of Na2 CO3 or NaHCO3 , with reactions similar to those occurring in the SX stripping of amines. A significant limitation to the use of resins is the occurrence of silica fouling where the PLS has a high silica content.[9,42] The mechanism and avoidance of this phenomenon remain poorly understood, but silica


880

K. C. Sole et al.

is known to slow down the kinetics of loading and stripping, to increase reagent consumption due to the higher resin flowrates required, can cause crud formation in a subsequent SX circuit, and can compromise the yellow cake quality by silica contamination.[9,47] Silica-fouled resin can be regenerated by treatment with caustic, but this has to be minimized to limit the costs and the osmotic shock on the resin. Recent resin developments in this area, such as the introduction of Ambersep 920 SO4 (Dow), are encouraging.[45] The typical (wet) sizes of resin beads used for uranium applications are 95% passing 300–850 µm for fixed-bed ion exchange (FBIX) and 90% passing 850–2000 µm for RIP slurries.[35] A typical theoretical capacity is 75 g U per liter of strong-base gel-type resin, although design values are specific for individual applications and largely depend on the presence of other anionic species in the feed. Operating capacities are generally significantly lower than the theoretical capacities, especially where resins are operated in the presence of silica. Today, weak-base resins (based on secondary or tertiary amine functionality) are also employed for acidic circuits.[48] These are only suitable for treatment of low-silica liquors as the macroporous beads absorb silica rapidly, causing loss in resin activity and yellow cake contamination. They are more selective and more resistant to abrasion than strong-base resins, but the kinetics of loading is slower and they are more expensive. These resins now allow eluate purities that approach those of SX.

PROCESS CHOICE: ELUEX vs. DIRECT SX OR RIP SX and IX both have advantages and limitations in the processing of uranium. The combination of IX and SX systems in sequential operating mode (the Eluex process) offers greater selectivity, and therefore higher product purity, than either of the individual processes. In addition to the purity limitations that may arise if SX is not employed, it is costly if all of the acid in the eluate has to be neutralized prior to precipitation. The target is to generate a higher grade eluate, which would make the acid consumption concomitantly lower for direct precipitation. The flow rate of the PLS—one of the major factors affecting process selection—has a significant impact on both the capital and operating costs, particularly in the case of SX and FBIX, and to a lesser degree for counter-current IX (CCIX). The use of an Eluex circuit, incorporating strong-base CCIX followed by SX using mixer-settlers, is often considered for flowsheets where the uranium tenor in the PLS is low, which would require a very large stand-alone SX plant: IX enables upgrading of the uranium tenor at a lower cost.[21,22] The latter arrangement has economic advantage in treating high solution throughputs. Of the 15 uranium plants operating in southern Africa



881

in the 1980s, it is interesting that the only plants to survive to 2011 all use Eluex flowsheets.[21] With the advent of large-scale SX in column contactors (such as Bateman pulsed columns, which have become the industry standard[49,50] ), the cross-over point in terms of plant throughput may in future move in favor of direct SX instead of Eluex.[22] The advantage of CCIX as a low-cost uranium pre-concentration step is not realized unless significant reduction in the size (or complete elimination) of the downstream SX circuit can be achieved.[45] The preconcentration ability of CCIX is limited by the maximum concentration that can be achieved in the CCIX eluate (4–7 g/L U3 O8 ) and the upper limit of the loading capacity of amine extractants (beyond an organic loading of 8–10 g/L U3 O8 at an extractant concentration of 10 vol. %, phase separation problems may occur). In the treatment of high concentration PLS, the direct SX route becomes more attractive and gives superior economic returns at tenors greater than 0.9 g/L U3 O8 ,[21,22] although SX has been used to treat feed tenors ranging from 0.07–14 g/L U3 O8 .[35] Flowsheets incorporating SX using tertiary amines can be configured to be selective over many impurities, including Fe, V, and Mo. Although the tertiary amine in SX is poisoned as a resin would be, tertiary amines are weak bases (in contrast to the strong-base chemistry of most resins) so regeneration is relatively easy and valuable secondary products can be produced by making appropriate adjustments in the flowsheets. An SX flowsheet is generally preferred if the ore contains contaminants that could report to the final product. Clarification of the leach solution is very important ahead of both SX and IX, particularly if silica is present, as this can precipitate at the aqueous-organic interface or cause blockages in FBIX columns by precipitating in the void volume between the resin beads. Extensive clarification, including counter-current decantation (CCD) and belt filters, is typically employed to avoid such issues (although silica cannot be effectively removed in L/S separation equipment, as the conditions may cause it to precipitate subsequent to clarification). Where it is not possible to clarify the feed liquor, fluidized bed IX may be suitable for the treatment of slimes with a maximum particle size of 45–75 µm. Pulp density is limited to 500 mg/L solids: above this value, solids are more likely to settle out and clog the bed. Studies have indicated that L/S separation equipment can comprise up to 25% of the cost of a uranium plant.[51] IX is more tolerant of high silica levels than SX (requiring