Kathryn C. Sole a, Peter M. Cole b, Angus M. Feather c & Marthie H. Kotze d a Independent Consultant, Johannesburg, South

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1 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: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Solvent Extraction and Ion Exchange Publication details, including instructions for authors and subscription information: Solvent Extraction and Ion Exchange Applications in Africa's Resurging Uranium Industry: A Review Kathryn C. Sole a, Peter M. Cole b, Angus M. Feather c & 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, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan,

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3 Solvent Extraction and Ion Exchange, 29: , 2011 Copyright Taylor & Francis Group, LLC ISSN print / online DOI: / Solvent Extraction and Ion Exchange Applications in Africa s Resurging Uranium Industry: A Review Kathryn C. Sole 1, Peter M. Cole 2, Angus M. Feather 3, and Marthie H. Kotze 4 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. kathysole09@gmail.com

4 SX and IX in Africa s Uranium Resurgence 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 Price (USD/lb U 3 O 8 ) Year Figure 1. Historical spot price of uranium (data from [5] ).

5 870 K. C. Sole et al. Table 1. Summary of African uranium production, pre-production, and feasibility projects (as of August 2010) Production Capacity Production Country Operation Majority owner start (t/au3o8) 2009 (t/au3o8) Ref. Botswana Lethlakane A-Cap Resources F [7] Central African Rep Bakouma Areva F [7] Malawi Kayelekera Paladin Energy [7,8] Namibia Etanga Bannerman Resources F [7] Husab Extract Resources F [7] Langer Heinrich Paladin Energy [7,9] Rössing Uranium Rio Tinto [7] Trekkopje Areva PP [7,9] Valencia Forys Metals F [7] Niger Azelik CNNC International PP [7] Cominak Areva [7] Imourarin Areva PP [7] Somaïr Areva [7,9] South Africa Dominion Reefs Shiva CM [7,9] Ergo Uranium DRDGold F [7] Ezulwini Uranium First Uranium [7,9] Harmony TPM Harmony Gold F [11] Mine Waste Solutions Simmer and Jack Comm. [12] Ryst Kuil Areva PP [7] Vaal River South AngloGold Ashanti [9] Tanzania Mkuju River Mantra Resources PP [13,14] Zambia Chirundu African Energy Resources F [7] Mutanga Denison Mines 804 F [7] CM: Care and maintenance; PP: Preproduction; F: Feasibility; Comm: Commissioning.

6 SX and IX in Africa s Uranium Resurgence , ,000 Reserves Resources Production 200,000 U 3 O 8 (t) 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] ). Production (t/a U 3 O 8 ) 20,000 18,000 16,000 14,000 12,000 10,000 8,000 6,000 4,000 2,000 Gabon Namibia Niger South Africa Canada Kazakstan Australia 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 U 3 O 8 ) (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

7 872 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 U 3 O 8. [17] Namibia will then become the world s third largest uranium producer after Kazakhstan (which produced 15.0 kt U 3 O 8 in 2009) and Canada (12.0 kt U 3 O 8 ). [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 U 3 O 8 for a lifetime of 35 years. [18] South Africa, estimated to possess 8% of the world s recoverable resources, [7] produces U 3 O 8 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 U 3 O 8 as a final product (Table 2). [20] Precipitation of UO 2 using H 2 O 2 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).

8 SX and IX in Africa s Uranium Resurgence 873 Table 2. Typical specification for calcined uranium product, U 3 O 8 [20] Species Typical value (%) Maximum impurity limit (%) Uranium (U) 75 >65 Arsenic (As) Barium (Ba) Boron (B) Cadmium (Ca) Calcium (Ca) Carbonate (CO 3 ) Chromium (Cr) Fluorine (F) Iron (Fe) Lead (Pb) Magnesium (Mg) Mercury (Hg) Moisture (H 2 O) Molybdenum (Mo) Phosphorus (PO 4 ) Potassium (K) Selenium (Se) Silicon (SiO 2 ) Silver (Ag) Sodium (Na) Sulfur (S) Thorium (Th) Titanium (Ti) Vanadium (V) Zirconium (Zr) 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 (UO 2 ), coffinite (U(SiO 4 ) 1 x (OH) 4x ), and uranothorite ((U,Th)SiO 4 )) require addition of an oxidant to convert the uranium to the more soluble hexavalent species. Less common ores that contain hexavalent uranium minerals (carnotite (K 2 (UO 2 ) 2 (VO 4 ) 2 3H 2 O), autunite (Ca(UO 2 ) 2 (PO 4 ) H 2 O), and uranophane (Ca(UO 2 ) 2 (HSiO 4 ) 2 5H 2 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 (MnO 2 ) or Caro s acid (H 2 SO 5 ) [24] as an oxidant for controlling the redox potential to ensure economic recovery of uranium. More recently, air/so 2 is also being considered as an environmentally more attractive oxidant for uranium leaching than

9 874 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: UO 2 + 2Fe 3+ UO Fe 2+ (1) 2Fe 2+ + MnO 2 + 4H + 2Fe 3+ + Mn H 2 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, UO 2 2+, but combines with sulfate present in the leach liquor to form the uranyl sulfate anions, UO 2 (SO 4 ) 2 2 and UO 2 (SO 4 ) 3 4. When sufficient sulfate is present, the latter species dominates under the aggressive leaching conditions (high temperatures and high redox potentials of 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,NO 3,SCN, and S 2 O 3. [32] The SO 4 2, HSO 4,and Fe(SO 4 ) anions are also generated during leaching. Where uranium is hosted in minerals that are high acid consumers (> kg/t ore), such as limestone (CaCO 3 ), dolomite (CaMg(CO 3 ) 2 ), or calcrete (CaCO 3 ), 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 UO 2 (CO 3 ) 3 4 anion usually predominates, although the UO 2 (CO 3 ) 2 2 species may also exist at low carbonate concentration. A mixture of Na 2 CO 3 (typically 0.3 M) and NaHCO 3 (0.1 M) is used. The leaching reaction, shown for the hexavalent uranium mineral, carnotite, is given in Eq. (3): UO 3 + Na 2 CO 3 + 2NaHCO 3 Na 4 UO 2 (CO 3 ) 3 + H 2 O. (3)

10 SX and IX in Africa s Uranium Resurgence UO 2 2+ O 2 H 2 O Potential Eh (V) UO 2 (SO 4 ) U(SO 4 ) 2+ UO 2 (SO 4 ) 2 2 U4 + U(SO 4 ) 2 H 2 O UO 2 (OH 2 )H 2 O U 3 O 8 U 4 O 9 UO H 2 Equilibrium ph Figure 4. E h -ph diagram for the U-S-H 2 O system at 25 C; [U] = 10 2 M; [S] = 10 1 M(adaptedfrom [31] ). The bicarbonate acts as a ph buffer and is necessary to avoid reprecipitation of the dissolved uranium by reaction with the hydroxyl ion: UO 3 + 3Na 2 CO 3 + H 2 O Na 4 UO 2 (CO 3 ) 3 + 2NaOH (4) 2Na 4 UO 2 (CO 3 ) 3 + 6NaOH Na 2 U 2 O 7 + 6Na 2 CO 3 + 3H 2 O. (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 H 2 SO 4 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

11 876 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 R 3 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] 2R 3 N (org) + H 2 SO 4(aq) (R 3 NH + ) 2 SO 2 4 (org) pk a = 9.02 (6) R 3 N (org) + H + (aq) + HSO 4 (aq) R 3 NH + HSO 4 (org). (7) Once the extractant is protonated, the uranium complex is extracted by an anion-exchange process: 2(R 3 NH + ) 2 SO 2 4 (org) + UO 2 (SO 4 ) 4 3 (aq) (R 3 NH + ) 4 UO 2 (SO 4 ) 4 3 (org) + 2SO 2 4 (aq) (8) (R 3 NH + ) 2 SO 2 4 (org) + UO 2 (SO 4 ) 2 2 (aq) (R 3 NH + ) 2 UO 2 (SO 4 ) 2 2 (org) + SO 2 4 (aq). (9) The formation of adducts, such as [(R 3 NH) 2 SO 4 ] 2 UO 2 SO 4 (H 2 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 H 2 SO 4 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 H 2 SO 4 ( 10 g/l) or (NH 4 ) 2 SO 4 solution at ph [32] Uranium is stripped from the loaded organic phase by either phswing or anion-exchange mechanisms using a variety of reagents, including NaCl, (NH 4 ) 2 SO 4,Na 2 CO 3, ammoniacal ammonium sulfate, or ammonia

12 SX and IX in Africa s Uranium Resurgence 877 gas. [32,34,40] Stripping with (NH 4 ) 2 SO 4 is most commonly practiced in Africa, although NaCl and Na 2 CO 3 are employed in Niger operations: (R 3 NH + ) 4 UO 2 (SO 4 ) NH 4 OH 4R 3 N + (NH 4 ) 2 SO 4 + (NH 4 ) 2 UO 2 (SO 4 ) 2 + 4H 2 O (10) (R 3 NH + ) 4 UO 2 (SO 4 ) Na 2 CO 3 4R 3 N + Na 4 UO 2 (CO 3 ) 3 + 4NaHCO 3 + 3Na 2 SO 4 (11) (R 3 NH + ) 4 UO 2 (SO 4 ) NaCl 4R 3 N.HCl + Na 4 UO 2 (SO 4 ) 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(NH 4 ) 2 UO 2 (SO 4 ) 2 + 6NH 4 OH (NH 4 ) 2 U 2 O 7 + 4(NH 4 ) 2 SO 4 + 3H 2 O. (13) The loaded strip liquor, known as OK liquor, typically has a uranium concentration of 7 10 g/lu 3 O 8. This is treated with ammonia to produce an ADU slurry, which is further processed at a nuclear facility or calcined on site to U 3 O 8. The stripped organic phase is treated in a regeneration circuit using a Na 2 CO 3 /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;

13 878 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 (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 U 3 O 8 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: UO 2 (SO 4 ) RX R 4 UO 2 (SO 4 ) 3 + 4X (14) UO 2 (CO 3 ) RX R 4 UO 2 (CO 3 ) 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.

14 SX and IX in Africa s Uranium Resurgence 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 VO 3 or VO 4 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(SO 4 ) n (3 2n) or Fe(OH)(SO 4 ) 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 ( g/l U 3 O 8 )to as high as 120 g/l U 3 O 8 on the resin and 35 g/l U 3 O 8 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 NaNO 3 or NaCl, often in the presence of Na 2 CO 3 or NaHCO 3, 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

15 880 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 µm for fixed-bed ion exchange (FBIX) and 90% passing 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

16 SX and IX in Africa s Uranium Resurgence 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 U 3 O 8 ) and the upper limit of the loading capacity of amine extractants (beyond an organic loading of 8 10 g/l U 3 O 8 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 U 3 O 8, [21,22] although SX has been used to treat feed tenors ranging from g/l U 3 O 8. [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 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 <20 ppm and preferably <10 ppm total suspended solids (TSS)). The potential ability of CCIX to handle unclarified leach solutions offers a substantial advantage to this process, particularly for ores that contain clays or other slimes that are difficult to settle. Several authors have studied this topic in some detail. [52 57] Flocculants selected for CCD and clarification should be tested for compatibility with the SX organic phase, as some can be detrimental to phase disengagement.

17 882 K. C. Sole et al. RIP and RIL processes have been widely used in the Central Asian Republics, mainly due to the low ore grade, fragmented industry, and in preference to using a CCD or filter circuit. [55,58] Uranium losses can be minimized because of lower adsorption losses ( preg-robbing ) on shales, clays, and zeolites. Water use and bleed liquor volumes are minimized since RIP/RIL can typically recover almost all of the soluble losses that would occur if CCD were used due to the virtual elimination of soluble loss in the thickener tails. [35] A case in point is the Mkuju River project (Tanzania), where the application of RIL overcomes a problem associated with soluble uranium in the leach residue. Test work conducted in the feasibility stage of the project identified that smectite, an ion-exchange clay present in the ore, caused some adsorption of soluble uranium which then registered as a decrease in leaching efficiency: resin, a superior adsorbent, introduced into the leached slurry, enables recovery of all the dissolved uranium. RIP is generally considered more cost-effective for low-grade ores and high throughputs. The main drivers for flowsheets using RIP followed by direct precipitation from the eluate are reduced capital and operating costs compared to CCD-filtration circuits and SX circuits (particularly considering the need to mitigate fire risks in all new SX plants). No L/S separation is required and no clarification of the leach liquor (as for SX and FBIX) is necessary. The use of a batch process allows control of resin inventory, but introduces other metallurgical constraints, such as the resin residence time in the adsorption circuit. One of the main limitations of this flowsheet choice is the high acid consumption when neutralization of the eluate is required prior to uranium precipitation: strategies such as increasing the uranium content in the eluate or recovering the acid prior to uranium precipitation (using techniques such as nanofiltration [41] )can circumvent this. Using IX without the inclusion of a subsequent SX step is usually only possible if the leach solution is relatively free of impurities that may be deleterious to product purity: one alternative is to precipitate impurities ahead of uranium precipitation, although this approach is not widely implemented. Silica fouling of the resin also remains a hurdle to widespread acceptance of this technology. [9] Resin durability is the primary challenge in the use of RIP or RIL, with conservative estimates of resin make-up due to attrition as high as 40% per annum. [57] However, over the last three years, significant advances have been and are continuing to be made by resin manufacturers in improving the size distribution and durability of strong-base resins. In fact, as illustrated in Fig. 5, resin technologies have progressed to the point that the costs of resin losses in an RIP process are low compared to solvent losses for an equivalent SX system. [42] Resin losses are estimated to contribute USD /lb uranium produced (compared to an annual operating cost of USD 20 30/lb for a typical South African plant). [42]

18 SX and IX in Africa s Uranium Resurgence USD million RIP CCD/SX Annual resin inventory replacement (%) Figure 5. Comparison of the annual cost of resin replacement at various breakage rates in an RIP process with the cost of organic loss and flocculant cost from an equivalent CCD/SX flowsheet, calculated for a typical South African plant (exchange rate: ZAR 7.00 = USD 1.00). The resin replacement costs for modern resins only exceed those of the conventional plant at breakage rates much higher than measured in practice (adapted from [42] ). In FBIX applications, the uranium concentration can be upgraded by a factor of 10 30: the technique of split elution with recirculation of a second portion of eluate can be used to increase the eluate concentration. Continuous elution also offers the possibility of achieving a higher grade eluate. [56,57,59,60] Counter-current continuous elution processes claim more efficient elution, achieving a higher uranium concentration, which, in turn, allows for direct precipitation from the eluate and avoiding the additional capital and operating costs of a downstream SX circuit. [61] A recent economic modelling comparison of RIP and direct precipitation flowsheets for an African project [62] indicated that the most significant variables affecting the choice of flowsheet are resin loading capacity, resin loss, the CCD underflow density that could be achieved, and the costs of key reagents. This analysis concluded that RIP is the preferred recovery technology for all uranium feed tenors, except where species that significantly depress resin loading, such as chloride, are present. For PLS tenors above 400 mg/l U 3 O 8 and chloride contents above 6 g/l, direct precipitation should be considered. Washing of the ore to remove chloride and beneficiation to improve the grade can improve capital and operating cost estimates. In alkaline leach circuits, high reagent concentrations make recirculation of barren liquor a mandatory requirement and therefore often preclude the use of RIP technology because L/S separation of the slurry is still required. However, the need to minimize uranium loss could still drive the selection of an RIP process from an economic or environmental point of view.

19 884 K. C. Sole et al. With increasing environmental and safety concerns within the mining industry, IX systems are, in general, inherently safer due to the fire risk associated with the use of organic solvents, potential for organic contamination of the environment, and potential attack by organics on materials of construction. The loss of entrained organic from raffinate and OK liquor (upto200 mg/l) can also increase reagent usage if not properly engineered and operated. On the other hand, IX suffers from physical breakdown of the resin beads due to mechanical attrition and osmotic shock. Long-term integrity of the resins, particularly under conditions where the acidity of the aqueous phase is cycled by variations of up to 100 g/lh 2 SO 4, can still benefit from improvements to resin manufacturing processes. Other important factors influencing the engineering decisions related to flowsheet selection include the project size and location, as well as availability and cost of the associated reagents. An excellent overview of process options, design criteria, and equipment selection for the hydrometallurgical processing of uranium is presented by Ivanova et al. [28] SELECTED PROCESS DESCRIPTIONS Rössing Uranium, Namibia Rössing Uranium, belonging to the Rio Tinto Group, is the third largest open-cast uranium mine in the world and produces 8% of the world s uranium. [10] It is situated near Swakopmund in the Namib Desert. In 2009, 54.5 Mt ore were mined and 4150 t U 3 O 8 produced. With current reserves, mining is expected to continue until at least [63] The uranium-bearing ore body is hosted in the mineral alaskite (quartz-orthoclase-leucogranite) with the following uranium mineral distribution: 55% uraninite, 40% uranophane, and 5% betafite (Ca,Na,U) 2 (Nb,Ta) 2 O 6 (O,OH). The ore is mined by blasting, loaded onto 180 t haul trucks with electric shovels, and delivered to the primary crushers that reduce the rock to an average size of 16 cm. It is reduced further in size in three additional crushing and milling stages. Uranium is extracted by atmospheric leaching in dilute sulfuric acid with an average leach efficiency of 88.7%. Following L/S separation by a combination of rotoscoops and CCD, uranium is recovered from solution (0.18 g/lu 3 O 8 ) by IX using Dow RPU resin. Multiple-tank fluidized bed IX is carried out in four trains of Porter contactors, each with six units. The Porter contactor [64] is suitable for treating slimes with several percent solids although it has a higher resin loss than fixed-bed designs (due to the number and nature of resin movements) and also has a high footprint. The PLS and resin are moved counter-currently on a continuous basis; resin is transferred upstream between stages by airlift. The loaded resin is transferred to one

20 SX and IX in Africa s Uranium Resurgence 885 of three elution columns, where uranium is eluted with 100 g/l H 2 SO 4 derived from the SX raffinate. Delkor linear screens installed on the barren streams reduce resin losses. The eluate, containing 3 5 g/l U 3 O 8, is pumped to an SX plant where it is further concentrated and the remaining impurities removed. The SX circuit [65] uses fiber-reinforced plastic mixer settlers designed by Davy Powergas. The SX plant has two trains of five extraction stages, two scrub, and four strip stages. The organic phase comprises 7 vol.% Alamine 336 and 3 vol.% isodecanol in Shellsol Stripping is carried out with ammonium sulfate returned from the ADU precipitation circuit with ph control by addition of aqueous ammonia, giving an OK liquor containing g/l U 3 O 8. A portion of the stripped organic phase is regenerated in a tank using a NaOH/Na 2 CO 3 mixture. Precipitation of ADU occurs on ammonia addition to the OK liquor. This is dried and calcined to produce Rössing s final product, uranium oxide (U 3 O 8 ), in a grey-black powder form. Since the plant started in the mid-1970s, there have been a few episodes where the tertiary amine in the SX circuit has been catastrophically degraded when the plant deviated from certain operating parameters. This is linked to the presence of nitrate in the feed to the SX circuit (originating from water-soluble nitrogen compounds used in mining explosives and coextracted in the upstream IX circuit). Nitrate in the presence of high acid converts to the nitronium ion. This then reacts with the amine extractant to generate an imine which is rapidly hydrolyzed in the presence of water and acid to the corresponding aldehyde and the secondary amine. The nitronium ion is simultaneously converted to nitrous acid, which, under these conditions, forms a nitrosyl cation which then reacts with the secondary amine to form the nitrosoamine. [66,67] Rössing has implemented steps to minimize and control the adverse effect of nitrate in the SX circuit, the key to which is to bleed both raffinate and strip solutions to control the nitrate concentration. Following successful column piloting studies during 2009, a demonstration heap leach process will be brought online in the near future. [63] The liquor generated will be combined with the current PLS for processing in the main circuit. Langer Heinrich, Namibia When Paladin Energy s Langer Heinrich operation started production in 2007, it was the first new uranium mine in 20 years. [68] It has subsequently gone through two phases of expansion, with Phase III (being commissioned in Q4 2011), increasing production to 2360 t/a U 3 O 8 and Phase IV feasibility study expected to be completed by the end of [69] In

21 886 K. C. Sole et al. 2009, this mine accounted for 2% of the world s uranium output. [14] The carnotite (K 2 (UO 2 ) 2 (VO 4 ) 2 ) ore (0.06% U 3 O 8 ) is a calcite conglomerate. Acid leaching is not viable due to excessive consumption by gangue so an alkaline leaching flowsheet was selected. Following beneficiation, the ore is leached in 40 g/l Na 2 CO 3,10g/L NaHCO 3 at 90 C and ph 10.5, with a residence time of 36 h required due to the slow leach kinetics. With uranium mineralized in the hexavalent state, an oxidant is not necessary for leaching. This is followed by L/S separation in a six-stage CCD circuit. The PLS (1.5 g/l U 3 O 8 ) is currently purified by IX using a fixed-bed configuration, although NIMCIX columns [64,70] (developed by Mintek, South Africa) are used for the expansion. An advantage of this equipment is that it can treat unclarified solutions (up to 300 ppm TSS), capital and operating costs are low, but recoveries are high and resin losses are low. With a significant degree of automation, the availability and reliability are excellent, and operation is efficient and stable over a wide range of feed rates and concentrations. [71] The UO 2 (CO 3 ) 4 3 anion in solution is loaded onto a strong-base anion exchanger, M500 (Lanxess). Fixed-bed elution takes place using 1 M NaHCO 3 and the resin is then regenerated using 1 M NaOH before returning to the loading cycle: Na 4 UO 2 (CO 3 ) 3 + 2R 2 CO 3 R 4 UO 2 (CO 3 ) 3 + Na 2 CO 3 (16) R 4 UO 2 (CO 3 ) 3 + 4NaHCO 3 4RHCO 3 + Na 4 UO 2 (CO 3 ) 3. (17) The subsequent precipitation is a two-step process: NaOH is firstly added to precipitate SDU and produce Na 2 CO 3 by reaction with excess NaHCO 3 : NaHCO 3 + NaOH Na 2 CO 3 + H 2 O. (18) This allows the barren solution to be recycled to the leach as a reagent source and represents a significant operational cost saving since little additional Na 2 CO 3 make-up to the circuit is required. In the second precipitation step, the SDU is acidified with H 2 SO 4 and H 2 O 2 is added to precipitate a hydrated uranium peroxide product that is then calcined. Although vanadium is present in the ore (330 g/t V 2 O 5 ), it has been found to exhibit limited solubility in the leach. The original process design made provision for its removal from the barren IX solution by precipitation with ferrous sulfate, but vanadium has not yet posed any operational problems and has not needed to be removed.

22 SX and IX in Africa s Uranium Resurgence 887 Trekkopje, Namibia The Areva-owned Trekkopje deposit is a low-grade, calcrete-hosted carnotite deposit containing 0.14% U 3 O 8. This project is the world s first commercial alkaline heap leach for uranium. [72,73] Open-pit mining is followed by crushing, screening, agglomeration, and heap leaching using a carbonate/bicarbonate medium. Although no oxidizing agent is necessary, some vanadium is also leached. Uranium recovery from solution by IX is followed by precipitation of UO 4. [74] The choice of heap leach for this project was based on the low upgrade factor, the porous, permeable, and competent nature of the ore, a water requirement with some 30% of that required for tank leaching, and the lower capital and operating costs. [75] The plant is located in a national park so there is a strong emphasis on environmental issues and rehabilitation. An on-off heap leach configuration was selected, since this significantly reduces the footprint and assists in the rehabilitation process. The leach pad of 3 km length and 810 m width will be one of the largest heap leach operations in the world. The ore also contains soluble chlorides that adversely affect resin absorption so the heaps are subjected to a two-stage wash before using a Na 2 CO 3 /NaHCO 3 mixture for leaching. The leach time is 160 days. Uranium recovery is 75%, while vanadium dissolution is very low (<5%). The spent ore will be used to refill the open pit mine. NIMCIX columns are used for both loading and elution steps. The eluate is 1 M NaHCO 3, while NaOH is used for regeneration of the resin and scrubbing of the small amounts of vanadium ( 15 mg/l) present in the PLS. Under typical alkaline leaching conditions (ph 10), vanadium is more strongly complexed than uranium by quaternary amine resins; however, when the ph is increased to 10.8, the selectivity is reversed, allowing purification of the leach liquor during IX. [33] The desert location requires water to be supplied by a desalination plant (with a capacity of 20 million m 3 /a potable water) which is the first to be built in Namibia. Following a successful demonstration phase of the project in 2009, construction of the full-scale plant has started and full production of 3200 t/au 3 O 8 is expected by [74,75] Somaïr, Niger The Somaïr (Société des Mines de l Air) operation in Niger, owned by Areva, has operated since 1968, with 2009 production of 1700 t/a U 3 O 8 from an ore grading 0.18% U 3 O 8. Mineralogy of the Niger deposits is not well characterized, but uranium is known to be associated with Mo, V, Zn, and Zr. [76] Ore is crushed to <250 mm, milled to <600 µm, screened, and then impregnated and cured with strong sulfuric acid in a process known as pugging. The pugging (agglomeration) process takes place in a rotating

23 888 K. C. Sole et al. drum in which concentrated H 2 SO 4 and NaNO 3 (as an oxidizing agent) are added to the ore with a small amount of water to achieve about 10% moisture. The off-gas is collected and scrubbed to recover and recycle nitric acid to supplement the addition of oxidizing agent. The ore is cured on a conveyor belts for several hours at 90 C, then mixed with solution to dissolve the uranium. The resulting PLS is recovered on a belt filter. Pugging allows increased recovery of uranium from refractory ores, improves L/S separation, and less silica reports to the PLS. [35] Heap leaching was practiced at this site between 1971 and 1988, but recovery was only about 50%. More recently, knowledge of heap leaching gained in the copper and gold industries has been implemented and a new heap leach operation brought on line in 2009 is expected to increase production to 3000 t/a U 3 O 8 by [18] Pilot trials indicated uranium recoveries of 68 70% after 50 days, with an average acid consumption of kg/t. [77] The PLS is treated by SX and yellow cake is precipitated from the OK liquor. The PLS from the main plant (220 m 3 /h at 1.5 g/l U 3 O 8 )isfed at 110 m 3 /h to each of two SX circuits. The heap leach circuit produces 250 m 3 /h PLS with a grade of 0.4 g/l U 3 O 8. This has required modification of the third original SX circuit to allow processing of the increased total flow by operating in a series-parallel configuration. [18] Uranium is extracted in four stages with 0.14 M ( 6.5 vol.%) tertiary amine at an advance organic-to-aqueous (O:A) ratio of 1:4. In contrast to the southern African plants, stripping is carried out in three stages using 180 g/l Na 2 CO 3 at an advance O:A ratio of 7.3. [18] Selection of this stripping agent minimizes Mo contamination of the yellow cake. The final product has a uranium content of 68 to 71%. Cominak, Niger The neighboring Cominak (Compagnie Miniere d Akouta) operation (majority owned by Areva) started in Current production is 1500 t/a U 3 O 8 from an ore grading % U 3 O 8. The main impurities are Mo, Zr, and V. A similar flowsheet to that of Somaïr is employed. The main differences are the uses of NaClO 3 as the oxidant in leaching, sodium chloride as the SX strip reagent, and magnesia as the agent for yellow cake precipitation. [18] The SX circuit comprises four stages of extraction, followed by three stages of stripping with NaCl, to yield OK liquor from which uranium product is recovered. The stripped organic is advanced to two further stages of stripping where Mo is removed using Na 2 CO 3. The fully stripped organic phase is regenerated by contact with sulfuric acid before returning to the extraction circuit. In 2010, the mine started processing a new ore with a higher V content. This brings new challenges to the SX operation in terms of controlling the