Solvent Extraction and Its Applications on Ore

Separation & Purification Reviews

ISSN: 1542-2119 (Print) 1542-2127 (Online) Journal homepage: http://www.tandfonline.com/loi/lspr20

Solvent Extraction and Its Applications on Ore Processing and Recovery of Metals: Classical Approach Y.A. El-Nadi To cite this article: Y.A. El-Nadi (2017) Solvent Extraction and Its Applications on Ore Processing and Recovery of Metals: Classical Approach, Separation & Purification Reviews, 46:3, 195-215, DOI: 10.1080/15422119.2016.1240085 To link to this article: http://dx.doi.org/10.1080/15422119.2016.1240085

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Date: 18 July 2017, At: 11:28

Separation & Purification Reviews, 46: 195–215, 2017 Copyright © Taylor & Francis Group, LLC ISSN: 1542-2119 print / 1542-2127 online DOI: 10.1080/15422119.2016.1240085

Solvent Extraction and Its Applications on Ore Processing and Recovery of Metals: Classical Approach Y.A. El-Nadi Hot Laboratories Center, Atomic Energy Authority, Abo Zaabal, Kalyobeia, Egypt

Solvent extraction is widely employed in a variety of industries for both the upgrading and purification of a range of elements and chemicals. The technology is used in applications as diverse as ore processing, pharmaceuticals, agriculture, industrial chemicals, petrochemicals, food industry, purification of base metals and refining of precious metals. This review deals with the basics of solvent extraction technique and discusses in detail its applications in several fields focusing on ore processing and recovery of important metals from economic and industrial point of view. Keywords: Solvent extraction, separation, metals, applications, ionic liquids

INTRODUCTION The term (solvent or liquid–liquid extraction) refers to the distribution of a solute between two immiscible liquid phases in contact with each other, i.e., a two-phase distribution of a solute. It can be described as a technique, resting on a strong scientific foundation (1). Solvent extraction techniques have a broad field of applications in inorganic and organic chemistry and large-scale industrial separations, in analytical chemistry, in pharmaceutical and biochemical industries, and in waste treatment. In addition, solvent extraction is a good instrument for studying fundamental understanding of equilibrium and kinetics of complex formation processes. Extraction methods have now become a routine procedure in separation technologies. In metal recovery, it is one of the favored separation techniques because of its simplicity, speed and wide scope. By utilizing relatively simple equipment and requiring less time to perform, extraction procedures offer much to chemists and engineers (2). Since the early days of the Manhattan Project, when scientists extracted uranyl nitrate into diethyl ether to purify

Received 23 January 2016, Accepted 1 September 2016. Address correspondence to Y.A. El-Nadi, Hot Laboratories Center, Atomic Energy Authority, Kalyobeia 13759, Egypt. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lspr.

the uranium used in the first reactors, solvent extraction has been an important separation technique for radiochemists. Thus, it is a technique used both in the laboratory and on the industrial scale. The schematic diagram of the solvent extraction process is shown in Figure 1. Usually the transfer of metal ions from the aqueous to the organic phase does not occur completely in one contact (3). Multiple contacts are necessary. This also holds true for scrubbing (contacting the loaded organic phase with an aqueous solution to collect back the impurities extracted by the solvent) and stripping (contacting the scrubbed organic phase with an aqueous solution to recover the main extracted species from the organic phase, back to the aqueous phase) (4). Principles of Solvent Extraction Extractants Because metals generally exist in aqueous solution as hydrated ions before the metal can be extracted into a nonpolar organic phase, the water molecules must be replaced and any ionic charge reduced or removed. This can be achieved in different ways by using three types of extractants: acidic, basic, and solvating, which extract metals according to the following equilibrium reactions (5): Acidic :Mzþ ðaqÞ þ zHAðaq or orgÞ , MAzðorgÞ þ zHþ ðaqÞ

(1)

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Y.A. EL-NADI

These extractants require the presence of stable anionic metal complexes to form the extractable ion pair complexes, so that only metals that produce such species can be extracted with these compounds. Thus, in halide solution, gold(III) > iron(III) > zinc > cobalt > copper >> nickel. An example of this kind of extractants is observed in the work performed as a comparative study of vanadium extraction by Aliquat-336 (N-methyl-NNN-trioctylammonium chloride, R3CH3N) from acidic sulfuric acid and alkaline sodium hydroxide media, respectively, giving the following equilibria (10): ½H2 V10 O28 4 ðaqÞ þ 4½R3 CH3 NClðorgÞ   , ðR3 CH3 NÞ4 H2 V10 O28 ðorgÞ þ 4Cl ðaqÞ

FIGURE 1 Typical solvent extraction flow sheet.

(5)

½VO3 OH2 ðaqÞ þ ½R3 CH3 NOHðorgÞ Basic

ðnzÞ : ðn  zÞR4 N ðaq or orgÞ þ MXn ðaqÞ , ðn  zÞR4 Nþ MXn ðnzÞ ðorgÞ

, ½R3 CH3 NVO3 ðorgÞ þ 2OH ðaqÞ

þ

Solvating : MXzðaqÞ þ mSðorgÞ , MXz SmðorgÞ þ mH2 O

(2)

(3)

Acidic Extractants. Acidic extractants include simple reagents such as carboxylic acids and organophosphorus acids, as well as chelating acids such as β-diketones, 8hydroxyquinoline and hydroxyoximes (6). For metals in the first transition series the extraction extent follows the order: V < Cr < Mn < Fe < Co < Ni < Cu > Zn, for divalent metals. Thus, unless other factors are involved copper will be extracted at lower pH values than the other elements. In addition, it is found that M4+ > M3+ > M2+ > M+, so that thorium(IV) will be extracted before iron(III) in turn before copper(II) before sodium. In this respect, Fe(III) was successfully extracted from sulfuric acid solution containing chromium using D2EHPA (HA) mixed with kerosene according to the following reaction (7):     Fe3þ ðaqÞ þ 3 ðHAÞ2 ðorgÞ , FeðHA2 Þ3 ðorgÞ þ 3Hþ ðaqÞ

(4)

Basic Extractants. Basic extractants normally consist of alkylammonium species (8). Quaternary ammonium species or uncharged alkylammonium compounds may be used as extractants. It has been observed that the magnitude of extraction follows the order R4N+ > R3NH+ > R2NH2+ > RNH3+ with the size of the alkyl group R generally between 8 and 10 carbon atoms. In commercial practice the trialkylammonium compounds are generally more commonly used in spite of their inferior extraction properties because they are cheaper than the quaternary extractants (9).

(6)

Solvating Extractants. Solvating extractants operate by replacing the solvating water molecules around the aqueous metal complex, making the resulting species more lipophilic. The types of organic compounds used include those based on carbon, i.e., alcohols, ethers, esters and ketones with compounds such as dibutylcarbitol, nonyl phenol and methyl isobutyl ketone (MIBK). Amides, RCONR2, have also been proposed for specialized applications such as the extraction of lanthanides, actinides (11–14) and precious metals (15–17). However, the most commonly used oxygen-donating solvating extractants are based on the organophosphorus compounds: alkylphosphates, (RO)3PO; alkylphosphonates, (RO)2RPO; alkylphosphinates, (RO)R2PO; and alkyl phosphine oxides, R3PO (18, 19). Some commercial solvating extractants include Cyanex 921, Cyanex 923 and Cyanex 925. However, sulfur donor extractants are less common, although dialkylsulfides have been used in precious metal extraction, and a trialkylphosphine sulfide, R3PS (Cyanex 471X), is available commercially. Such sulfur-donating extractants will need to be considered when extracting and separating soft metals, such as second- and third-row transition metals (e.g., cadmium, mercury and palladium). Actually, Cyanex reagents are considered as potential extractants for different elements of nuclear importance as U(VI), U(IV), Th(IV), etc. from different aqueous media (20). These reagents have the advantage of not extracting nitric acid, which is extracted by tri-n-butylphosphate (TBP) usually used in the extraction processes of the spent nuclear fuel. Cyanex reagents are also effective extractants for lanthanides and rare earth elements as well as platinum group metals as Pd(II). Moreover, the extraction equilibrium using these reagents is usually reached after few minutes and the separation factors between different elements in the Cyanex systems are usually higher than that of other related systems (20).

SOLVENT EXTRACTION FOR METAL RECOVERY

Synergistic Extraction On occasion, mixtures of two different extractants will enhance the extraction of a metal above that expected from the summation of the performance of the two reagents separately. This gives rise to the synergistic factor (SF), defined using the metal distribution ratios D in the different extractant media as follows: SF ¼ DAB =ðDA þ DB Þ

(7)

A large number of examples of synergism can be found in the literature (21–50), although very few of these have actually been commercialized. The main reason for the lack of industrial interest is probably the difficulty in maintaining the optimum ratio of extractants in the organic phase to provide synergism. The most common synergistic system consists of a mixture of an acidic and a solvating extractant acting on a metal ion where the preferred coordination number cannot be satisfied by just the acidic extractant. Table 1 summarizes the structure of the above referenced synergistic mixtures, the metal ions extracted, the aqueous media used and the stripping conditions as well as their applications if present. It is noteworthy that the extractants used in synergistic systems are usually composed of a mixture of an acidic extractant with a neutral one. The acid liberated during the process of solvent extraction of metals with those acidic extractants adversely affects the metal extraction. One extra example is the study of extraction behavior of the metal ions, La, Nd and Y, using a convenient mixture of neutral reagents such as trioctylphosphine oxide (TOPO) and trialkylphosphine oxide (TRPO) extractants (51). In this system, yttrium was separated in a pure form based on its trend in the extraction and stripping procedures. Besides, the method was applied on the hydrous oxide cake resulted from Egyptian monazite to separate yttrium from lanthanum and neodymium including in the ore. Extraction Techniques and Considerations The maximum degree of separation that can be achieved between two metals (A and B) in a single equilibrium of the two phases is governed by: 1. The separation factor (α), which is the ratio of the distribution ratios (D) of the two metals. 2. The phase ratio (r), which is the ratio of the volume of the organic phase to the volume of the aqueous phase. A comparison can be made of the fraction A extracted (RA) with the fraction of B extracted (RB) utilizing the following equation:  RA =RB ¼ α

DB þ 1=r DA þ 1=r

 (8)

197

Thus, the ratio, RA/RB, the recovery factor ratio, is an expression of the degree of separation (52). A single equilibrium is unlikely to provide an adequate degree of separation and further contact stages are almost necessary. Cross-current Batch Extraction In a cross-current extraction process, the feed containing the solutes is contacted with fresh solvent for each extraction stage. The extracts can be collected separately or combined for further processing, Figure 2. Counter-current Batch Extraction In counter-current batch extraction, Figure 3, the aqueous phase is extracted with successive volumes of solvent such that the fresh organic phase always extracts from the weakest aqueous phase and the most concentrated solvent extracts from the strongest aqueous phase. This method of extraction provides the lowest residual concentration of solute in the final raffinate while at the same time producing maximum solute loading of the organic phase (52, 53). In industrial practice batch processing is impracticable. It is much more effective from the chemical engineering aspect to carry out the extraction with continuous countercurrent flow of the two phases. Continuous Counter-current Extraction Contactors for counter-current extraction, Figure 3, described in detail (9), may be divided into two types: 1) stage-wise contactors and 2) truly continuous contactors. The former is typified by mixer-settler contactor, Figure 4, in which the two phases are equilibrated and then separated in the settler section before passing on counter-current to each other (54). Truly continuous, counter-current operation is obtained when the two phases pass continuously in opposite directions as in a simple packed column. A continuous counter-current contactor, Figure 5, can be designed from the number of equilibrium stages required to affect a separation (55). A graphical construction can be used to relate the feed concentration to the aqueous and organic solute concentrations for a given number of contact stages and for specific flow rate. The resulting diagram is usually termed an extraction equilibrium diagram or a McCabe–Thiele diagram, Figure 6 (51). A number of published papers in which the mixer-settler contactor has been extensively used can be found in the literature. Some of them include the extraction of copper from ammoniacal waste solution (56), extraction and recovery of zinc from simulated and real industrial waste resulting from rayon industry (57), separation of thorium from leached monazite solution using counter-current extraction (58), and recovery of U(IV) from phosphoric acid by octylphenyl acid phosphate (OPAP) extractant (59).

N/A La, Eu, Lu Zn, Cd Nd, Sm La, Nd, Sm, Gd Ce, F Ce La, Nd, Sm, Tb, Y Ni, Co In, Ga Co, Mn, Li

TPTZ+HL-10-LH in CHCl3 TPTZ+HL-10-LH in CHCl3

TBP+N235 in heptane D2EHPA+ HEH/EHP in kerosene D2EHPA+ HEH/EHP in kerosene D2EHPA+ Cyanex 923 in heptane D2EHPA+ HEH/EHP in kerosene CA100+Phen in benzene

Stripping agent

Chloride Nitric acid Chloride Acidic iodine Strong chloride

Cu, Fe U Sm Cd, Co, Ni Cu, Ni, Co, Zn Co U Zn Y Nd, Dy, Y Ni Zn Li

D2EHPA+LIX 860 in toluene D2EHPA+ TOPO in Isane IP 185 HA+TOPO or TBuP in nonane D2EHPA+ Cyanex 272 in kerosene D2EHPA+ EHEHPA in n-heptane Versatic 10 acid+LIX84-I in kerosene Mextral 54-100+ Mextral 84H in kerosene

DOP+TBP in kerosene

Separation of Ni and Co from Mn, Mg and Ca Separation of In and Ga Recycling of spent cathodic materials of Li-ion batteries

H2SO4 N/A >0.04 M HCl or >0.01 M H2SO4 N/A

N/A

6 M HCl

(50)

(43) (44) (45) (46) (47) (48) (49)

(38) (39) (40) (41) (42)

Proposal of flow sheet for copper conventional electrowinning Recovery of uranium from nitric acid media Overcome emulsification of HQ Separation of Cd from industrial wastewater containing Co and Ni Proposal of flow sheet to recover Cu, Co, Zn and Ni from strong chloride leach N/A Derivation of the uranyl, D2EHPA and TOPO speciation Recovery of zinc from ammoniacal solutions using β-diketone extractant N/A Enhancement the separation of Nd Separation of Ni from Co, Mn and Li contained in Li-ion batteries N/A 53 g/L Cu+ 180 g/L H2SO4 1 M Na2CO3 N/A 2 M NaOH 100 g/L H2SO4 N/A N/A 2 M H2SO4 N/A N/A 3 M H2SO4 2 M H2SO4

(37)

(34) (35) (36)

(28) (29) (30) (31) (32) (33)

(26) (27)

(23) (24) (25)

(21) (22)

Ref.

N/A

N/A 3 M HCl N/A N/A N/A N/A

N/A 1 M HNO3

Batch continuous extraction test Spectrochemical study of the adduct formation Increasing a selectivity among the lanthanides Proposal of the interaction complexes stoichiometries Improvement of the selectivity of Eu over Lu and Eu over La Separation of Zn from bulk Cd solutions N/A Calculation of equilibrium and stability constants Separation of Ce and F from bastnasite N/A Separation of the lanthanoids from yttrium

20 g/L H2SO4 0.1 M HClO4 N/A

Separation of Y from rare earths Improvement of Co and Ni by elevating the temperature

Application

CA100 = sec-nonylphenoxy acetic acid, CA-12 = sec-octylphenoxy acetic acid, Cyanex 272 = bis(2,4,4-trimethylpentyl)phosphinic acid, Cyanex 302 = di-2,4,4,-trimethylpentyl mono-thio-phosphinic acid, Cyanex 471x = triisobutyl phosphine sulfide, Cyanex 923 = trialkyl phosphine oxide, D2EHPA = bis-2-ethylhexyl phosphoric acid, DEO6 = dodecyl polyether, DOP = dioctyl phthalate, DNPPA = dinonyl phenyl phosphoric acid, EHEHPA = 2-ethylhexylphosphonic acid mono-2-ethyl hexyl ester, HA = 4-ethyl-1-phenyl-1,3-octadione, HEH/EHP = 2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester, Hhfa = hexafluoroacetylacetone, HL-10-LH = heterocyclic β-ketoenol, HP = 4-benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-one, HQ = 8-hydroxyquinoline, HRJ-4277 = nonylsalicylic acid, Htta = 2-thenoyltrifluoroacetone, LIX 63 = 5,8-diethyl-7-hydroxy-6-dodecanone oxime, LIX 84I = 2-hydroxy-5-nonylacetophenone oxime, LIX 860 = 5-dodecylsalicylaldoxime, Mextral 54-100 = 1-benzoyl-2-nonyl ketone, Mextral 84H = 2hydroxy-5-nonylacetophenone oxime, N/A = not applicable or not found, N235 = trialkyl amine, Neutral oxodonors = TBP, TEHP, Cyanex 923, PAN = 1-(2-pyridylazo)-2-naphthol, PC-88A = 2-ethylhexyl phosphinic acid mono-2-ethylhexyl ester, Phen = 1,10-phenanthroline, TBP = tri-butyl-phosphate, TBTZ = 2,4,6-tri(2-pyridyl)-1,3,5-triazine, TBuP = tributyl phosphine, TEHP = tris(2-ethylhexyl) phosphate, TIOA = triisooctylamine, TOPO = trioctyl phosphine oxide, Versatic 10 = neodecanoic acid.

KNO3 H3PO4 (NH4)2SO4 Chloride HCl Sulfate Ammoniacal/ (NH4)2SO4 MgCl2

Distilled water

Lanthanides

Tap water Chloride H2SO4

Distilled water Diluted sulfate Diluted sulfate Sulfuric acid Sulfuric acid Nitrate

Distilled water Distilled water Deionized water 0.1 M nitrate 0.1 M nitrate

Chloride Diluted HCl Diluted sulfate 200 g/L H2SO4

Aqueous medium

DEO6+ β-diketone in 1,2-dichloroethane Versatic 10+ LIX 63 in Shellsol D70 DNPPA+ neutral oxodonors in n-paraffin Cyanex 272+ HQ in heptane TIOA+TBP in dichloromethane LIX63+ Versatic 10+ TBP in Shellsol D70

Versatic 10+ LIX63 or 4PC in Shellsol D70 CA100+N235 in heptane Cyanex272+ PC-88A in heptane

Cd, Zn lanthanides Lanthanides

Y Co, Ni

Metal ion

CA-12+ Cyanex 272 in heptane Cyanex 272+ D2EHPA and Cyanex 302+D2EHPA in kerosene HRJ-4277+ Cyanex 471x in Shellsol 2046 Hhfa and Htta+ Co(III) chelates in toluene HTTA or HP+PAN in benzene

Extractant

TABLE 1 Extraction data obtained from synergistic systems (21–50)

198 Y.A. EL-NADI

SOLVENT EXTRACTION FOR METAL RECOVERY

199

FIGURE 2 Cross-current extraction schemes (53). © Roussel-Robatel Company. Reproduced by permission of Roussel-Robatel Company. Permission to reuse must be obtained from the rightsholder.

FIGURE 3 Counter-current extraction scheme (53). © Roussel-Robatel Company. Reproduced by permission of Roussel-Robatel Company. Permission to reuse must be obtained from the rightsholder.

FIGURE 4 Flow diagram of a mixer settler unit [courtesy of Rousselet-Robatel, Annonay, France (55)]. © Roussel-Robatel Company. Reproduced by permission of Roussel-Robatel Company. Permission to reuse must be obtained from the rightsholder.

APPLICATIONS Solvent extraction (SX) was employed mainly as an analytical tool for the separation and analysis of elements with very similar chemical properties. The discovery and isolation of the lanthanide and actinide elements provided

impetus for the further development of these technologies (60). SX is applied nowadays in various industries. This is a classic method of separation and concentration of metal ions from aqueous solutions. It is a selective, cost-effective and uncomplicated chemical process applied in hydrometallurgical separation and purification of various metals (61).

200

Y.A. EL-NADI

FIGURE 5 Eight-stage mixer-settler battery with turn-key skid-mounted accessories [Rousselet-Robatel, Annonay, France (55)]. © Roussel-Robatel Company. Reproduced by permission of Roussel-Robatel Company. Permission to reuse must be obtained from the rightsholder.

Ore Processing Uranium The process used for recovery of uranium from its ores depends on the nature of the ore. All the processes include a leaching step that solubilizes the metal. Solvent extraction is used most frequently for the recovery and purification of uranium from the leaching liquors (9). Most uranium-bearing ores are readily leached in sulfuric acid and the uranium is recovered by solvent extraction using amines or dialkylorganophosphorus acids. Phosphate ores are leached in a mixture of sulfuric and phosphoric acids or in phosphoric acid alone. Hot nitric acid has also been used as a lixiviant for uranium ores. FIGURE 6 McCabe–Thiele diagram for extraction of La(III), Nd(III) and Y (III) from HNO3 solution by a TOPO and TRPO mixture in kerosene at 25 °C and phase ratio = 1 [adapted from Ref. (51)]. Reprinted from (51) with permission from Elsevier.

The use of solvent extraction in hydrometallurgy extends to a wide range of metals from different feeds including low-grade ore, scrap, waste leachates and dilute aqueous solutions. The technology was used first in nuclear technology; then, some high-value metals like precious group metals solvent extraction technologies became commercially viable and were developed and used. After the development of selective chelating extractants, the solvent extraction was able to compete with classical separation–concentration operations in this field (2).

Sulfuric Acid Leach Liquors. Organophosphorus reagents were among the first used for the commercial recovery of uranium from solutions obtained from the leaching of low-grade ores with sulfuric acid. In the Dapex process (62), diethyl hexyl phosphoric acid (D2EHPA) is used to selectively extract uranium from vanadium and iron(III) under conditions of controlled pH and electrochemical potential (63). TBP or isodecanol is added to the organic phase to prevent the formation of a third phase. The extraction of uranium(VI) by D2EHPA (represented as the dimeric H2A2 structure) occurs via the reaction: UO2 2

þ

ðaqÞ

þ 2H2 A2ðorgÞ , UO2 ðHA2 Þ2ðorgÞ þ 2Hþ

(9)

SOLVENT EXTRACTION FOR METAL RECOVERY

201

TABLE 2 Comparison of the Amex and Dapex processes Point of comparison

Amex process

Dapex process

Coextracted ions Stripping

Mo(VI), Zr(IV) Simple, many possibilities

Extractant stability Diluent

Rather low stability Tendency to form third phases with aliphatic hydrocarbons. Longchain alcohols as phase modifiers or aromatic diluents are necessary Rapid Slow Slow (sensitive to suspended solids) Rapid

Extraction kinetics Phase disengagement

Uranium is stripped from the loaded organic phase with a solution of sodium carbonate and recovered as sodium uranyl tricarbonate, while the extractant is regenerated in the sodium form. Today, amines are more widely used for the recovery of uranium from sulfate leach liquors, and uranium recovery is one of the most important commercial uses of amines. Amine systems achieve higher uranium purity than organophosphorus systems (due to the greater selectivity of amines for uranium) and have lower extractant losses due to their lower aqueous-phase solubility. Solvent extraction is applied either directly to the weakly acidic leach liquor (AMEX process, Vaal River West, South Africa) or to the strongly acidic eluate from an ion-exchange preconcentration treatment of the leach liquor (63). A more modern variation (such as at Southern Cross Resources Uranium One, South Africa) is to treat the ore by pressure leaching followed by solvent extraction. The extraction of uranium (VI) by amines occurs in the order of tertiary > secondary > primary amines. The extraction of iron(III) occurs in the reverse order, so tertiary amines represent an obvious choice of extractant. The tertiary alkyl amine sold as Alamine 336 or Armeen 380 is widely used, usually in conjunction with an alcohol phase modifier (such as isodecanol) to prevent the formation of a third phase and inhibit the formation of emulsions. Uranium can be stripped from the loaded organic phase using a variety of reagents, including NaCl, (NH4)2SO4, Na2CO3, ammoniacal ammonium sulfate or ammonia gas (63). These stripping systems present few choices based on technical grounds, so the choice of reagent is usually determined by economic factors. The advantages and disadvantages of the Dapex and Amex processes are compared in Table 2. The Amex process is more widely used than the Dapex process because of the greater selectivity of trialkylamines than D2EHPA for uranium in H2SO4 solution. Phosphoric Acid Leach Liquors. Many phosphate rock deposits contain quantities of radioactive elements such as uranium and thorium. Selective leaching of uranium from raw phosphate ores is difficult because the U(VI) ion is

Fe(III), Th(IV), V(IV), Ti(IV), Mo(VI), Rare Earths With Na2CO3 which necessitates addition of TBP to avoid the separation of NaDEHP in a third phase More stable than amines Many diluents can be used

incorporated into the crystal structure of apatite (Ca5(PO4)3 (OH,F,Cl)), rather than adsorbtively associated with it. Uranium is, therefore, typically recovered from phosphate rocks by recovering it from phosphoric acid produced by sulfuric acid leaching of phosphate ores. Uranium in phosphoric acid solution is extracted as the UO22+ species also using D2EHPA mixed with tri-n-octylphosphine oxide (TOPO) that synergistically forms an organic-phase adduct:

UO2 ðHA2 Þ2ðorgÞ þ TOPOðorgÞ , UO2 ðHA2 Þ2  TOPOðorgÞ

(10)

TBP has also been used as a synergist, but TOPO typically gives higher synergism and higher selectivity than TBP. Di(nonylphenyl)phosphoric acid can also be used in place of D2EHPA (64). Uranium is stripped from the loaded organic phase with ammonium carbonate, and precipitated as ammonium uranyl tricarbonate. Nitric Acid Leach Liquors. High-grade uranium ores are sometimes leached in hot nitric acid, particularly when significant amounts of radioactive elements such as thorium are present in the rock (65). Uranium forms the uranyl nitrate species that can be extracted by a solvating mechanism using TBP, forming the species UO2(NO3)2 (TBP)2 (63). TBP extraction of uranium from nitric acid is also practiced for the purification of uranium and for its recovery from irradiated nuclear fuel (66). Thorium Thorium, which consists mainly of the almost stable isotope Th-232 (alpha emitter with half-life, t½ = 14.109 years), has been proposed as a secondary source of nuclear energy (67). Irradiation of Th-232 with neutrons produces Th-233 (t½ = 22.4 minutes), a beta-emitter that decays to fissionable U-233. The world reserves of Th232 are about three times larger than those of uranium. However, because the uranium nuclear fuel cycle was

202

Y.A. EL-NADI TABLE 3 Commercial processes for the recovery of Th from its main ores

Types of ores (main locations) Monazite (Brazil, India, Australia, South Africa, United States) Thorianite/Uranothorite Thorite/Uranothorite (United States, Canada)

Ore composition (La,Ce,Th)PO4 ThO2/(U,Th)O2 ThSiO4/(U,Th) SiO4

Leaching

Purification

Hot concentrated NaOH or H2SO4 HNO3 or H2SO4 Hot H2SO4

Dissolution of oxides into HNO3 and then TBP; alkylamines for Th in sulfate solutions TBP for nitrates/Alkylamines for sulfates Alkylamines

developed first and is easier to use for nuclear energy production, the thorium cycle has not received as much attention. However, thorium must be considered as a future nuclear energy source. Fission of thorium produces less hazardous waste, because the amount of long-lived transthorium isotopes is about 100 times less than that in the uranium cycle. Accelerator-driven reactors projects based on the thorium cycle have been proposed (68). Table 3 contains, in a simplified way, the composition, location and treatment of main thorium ores. The purification of thorium by TBP extraction takes place after the dissolution of thorium in nitric acid, generally from a hydroxide cake. When thorium is dissolved in sulfuric acid, purification is achieved by extraction with longchain alkylamines. Rare Earth Elements The rare earths are moderately abundant elements in the earth’s crust that occur in a large number of minerals. Rare earths typically occur as carbonates, oxides, phosphates and silicates in the forming minerals. Even though the rare earth metals are fifteen in number, about 95% of all the world rare earth resources occur in just three minerals, bastnasite, monazite and xenotime. These three therefore are the principal ore minerals for rare earth extraction. Among these, again, bastnasite occurs most frequently, monazite is second and xenotime is the distant third (4). The mineral monazite is a phosphate mainly of the cerium group rare earths and thorium. Monazite is found in many geological environments. It occurs as an accessory mineral in acidic igneous rocks, in metamorphic rocks and in certain vein deposits. Due to its chemical stability it also develops into detrital mineral in placer deposits and beach sands (69). The recovery of mixed rare earths and removal of thorium from monazite are accomplished by a variety of methods (70), after chemically attacking the mineral with sulfuric acid or sodium hydroxide. Acid Treatment. The sulfuric acid method had been used most extensively in the United States (71). With this method, depending on the acid/ore ratio, temperature and concentration, either thorium or the rare earths can be

selectively solubilized or both thorium and rare earths totally solubilized. Rare earths and thorium are subsequently recovered from the solution. The processes available are shown in Figure 7. The process of rare earth recovery based on rare earth double sulfate precipitation was largely developed by Pilkington and Wylie (72) and has found industrial application. Yttrium and the heavy rare earth double sulfates are quite soluble and go with thorium. Even in the stepwise neutralization procedure investigated at Ames (73), yttrium and the heavy rare earths are precipitated along with thorium as a basic compound. The rare earths, however, are recoverable from the thorium fraction during solvent extraction for the purification of thorium and uranium. Solvent extraction with TBP from an aqueous 8 M nitric acid solution of thorium and mixed rare earths permits the recovery of thorium, uranium, cerium and cerium-free rare earths. Other commercially significant processes essentially involve precipitation of thorium pyrophosphate or basic salts from the leach liquor and subsequent recovery of the rare earth in solution as double sulfates, fluorides, or hydroxides or even selective solubilization of thorium in the ore treatment stage itself (74). The sulfuric acid process does not yield pure products and is no longer in commercial use. Alkali Treatment. The phosphate content of the ore is recovered as a marketable by-product, trisodium phosphate, at the beginning of the flow sheet, and this has been a major attraction for the commercial use of this process. In this concern, a modified leaching and extraction process of uranium from hydrous oxide cake of Egyptian monazite was developed (75). In this process, a monazite sample is ground to mesh size 200–270, then digested by boiling with sodium hydroxide (50%) for 4 hours at 140 °C and filtered at 80 °C. The filtrate can be recycled for recovery of sodium phosphate and excess sodium hydroxide, whereas the precipitate is washed with water and dried. The dried precipitate, which consists mainly of thorium and rare earths as well as uranium hydrous oxides, was leached several times with alkaline solution composed of sodium carbonate, sodium hydroxide and hydrogen peroxide to separate uranium selectively. The solution is filtered and the filtrate containing uranium is diluted and shaken with 0.1 M Aliquat-336 in kerosene

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FIGURE 7 Monazite processing by acid treatment [adapted from Ref. (4)]. © Taylor & Francis. Reproduced by permission of Taylor & Francis. Permission to reuse must be obtained from the rightsholder.

containing octanol in one-stage solvent extraction to recover and purify uranium. The resulted uranium which is mainly tricarbonate complex is precipitated as hydroxide by decreasing pH of the solution with addition of nitric acid (75). Thorium and rare earths precipitate is dissolved in 4 M nitric acid with heating. Thorium is extracted from the above mixture by Aliquat-336 in kerosene, stripped by shaking with HCl solution and precipitated as hydroxide by addition of ammonium hydroxide giving thorium (76). On the other hand, cerium(III) is oxidized to its tetravalent state by addition of sodium bromate, extracted by Aliquat336 in kerosene containing 1-octanol and stripped by shaking the organic phase with diluted HNO3 solution and then precipitated as hydroxide by addition of ammonia (77). An integrated schematic diagram displaying the whole process is shown in Figure 8. The aqueous solution principally containing La, Nd and Y is subjected to solvent extraction using a synergistic TOPO–TRPO mixture in kerosene in 2:1 O/A phase ratio at 25 °C. The organic phase is stripped by H2SO4 in equimolar ratio to separate most of Y and then treated by HCl as stripping agent to recover La and Nd. The organic layer is finally washed with water to separate the remaining Y and then used for another cycle of extraction. Overall recovery of yttrium from the strip liquors was about 94%, while it was 79% and 81% for La and Nd, respectively. Yttrium was

finally recovered from the solution as the oxalate and calcined to oxide. The final product was found to contain 84.5% Y2O3 (51). Copper One of the most remarkable success stories in the commercial application of solvent extraction occurred in the copper industry. The use of solvent extraction for the primary processing of copper has enjoyed spectacular growth over the past 35 years. The production of high-purity copper by a combination of leaching in sulfuric acid, upgrading and purification of the copper by solvent extraction (SX), and recovery of the metal by electrowinning (EW) has increased steadily, now approaching 30% of total copper production as shown in Figure 9 (78–80). The first commercial reagents were all based on ketoxime functionality and were used exclusively for copper extraction for over a decade after the first full-scale application at Bluebird Ranchers Mine, Arizona (81). Today, ketoximes are still successfully used in niche applications for the recovery of copper from dilute leach liquors and also find applications in nickel solvent extraction from ammoniacal solutions and in precious metal refining. Particular applications of ketoximes in copper production are at El Tesoro and Lomas Bayas in the Atacama Desert of Chile, where the

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FIGURE 8 A suggested flow sheet of monazite processing by alkali treatment (51). Reprinted from (51) with permission from Elsevier.

FIGURE 9 Flow sheet for recovery of copper by leaching, SX and EW. RH = organic extractant (60). © Taylor & Francis. Reproduced by permission of Taylor & Francis. Permission to reuse must be obtained from the rightsholder.

leach liquors of circuits often contain high levels of nitrates and chlorides. Ketoximes are stable to oxidation under these conditions (82, 83). Although ketoximes do not generally allow the extraction of copper at pH values below 1.8, and have slow extraction kinetics, they exhibit copper selectivity over iron of about 300:1. Their selectivity as a function of pH for other cations commonly found in sulfate leach liquors is shown in Figure 10. The second-generation aldoxime extractants were developed to overcome the shortcomings of the ketoximes (84, 85). Aldoxime extractants exhibit very fast extraction

kinetics, high selectivity (Cu:Fe ≈ 2000:1), and high loading capacity. Because they extract at pH levels below 1—the stripping of copper is difficult. Today, modified aldoximes and aldoxime–ketoxime mixtures are the most widely used copper extractant systems. With advances in chemistry and manufacturing processes, these reagents now have high purity and many of the limitations of earlier reagents have been overcome. They have faster reaction kinetics, greater selectivity for copper over other base metals at low pH values, better extraction performance, and more rapid phase disengagement.

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FIGURE 10 pH dependence of various cations by LIX 84-I ketoxime extractant (60). © Taylor & Francis. Reproduced by permission of Taylor & Francis. Permission to reuse must be obtained from the rightsholder.

As a consequence of significant advances in reagent customization during the past 30 years, large reductions in capital costs for copper solvent extraction plants have been possible. Copper production per unit area of plant size has also increased dramatically, from about 4 ton of copper/y/m2 of settler area in the early plants to 16 t/y/m2. Some of the most innovative advances in equipment design and circuit configurations have also been achieved in copper extraction (86, 87). Earlier plants were designed using only series configurations of the mixer-settler trains. The trend in recent years leans toward the use of series parallel configurations, especially in very large plants, and from series-parallel to all-parallel (88). This increases overall throughput and reduces capital costs. Nickel Laterite Processing While high-grade nickel sulfides are processed by smelting, low-grade laterites (0.5–3% Ni), until relatively recently, could not be economically treated with available technologies. Since 1998, several new laterite flow sheets have been commissioned or are under advanced development. All of these processes use pressure acid leaching (PAL) to solubilize the metals of interest, but the downstream flow sheets have significant differences. They all involve solvent extraction as one or more of the unit operations, either for the removal of cobalt from the nickel-rich leach liquor or for the purification of nickel liquor. Bulong Process. The Bulong nickel operation near Kalgoorlie, Western Australia, was commissioned in 1998 with a design capacity of 9000 ton/y Ni and 720 ton/y Co (89). This plant closed in 2003, but later reopened as Avalon Nickel to treat nickel sulfide material. The original flow sheet remains of interest, since it has many similarities with those planned for Tati Nickel, Botswana (90), and the Nkomati project in South Africa (91). Figure 11 shows a simplified flow sheet of the Bulong downstream purification process. Following dissolution of the ore at 4500 kPa and 250 °C, the

leach liquor was then processed directly for the recovery of the valuable metals. Fe(III), Al and Cr(III) were removed to 99%, from which nickel cathode (> 99.5% purity) was produced by electrowinning. Goro Process. The Goro process, Figure 12, developed by Vale-INCO for the treatment of a nickel laterite deposit in New Caledonia (93, 94) also treats the laterite leach liquor directly but uses a completely different approach. The lateritic ore is acid leached under pressure at 270 °C. The clarified autoclave discharge liquor is partially neutralized to precipitate Al, Cr(III), Cu, Fe and Si. Trace quantities of residual copper are reduced to levels of PtCl62− ≈ IrCl62− > PdCl42− > RhCl63− ≈ IrCl63−. In most processes that use solvent extraction, gold is removed from solution first, followed by palladium, and then platinum. The remaining platinum group metals (PGMs) are recovered in a variety of ways, either up-front or at the end of the overall flow

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FIGURE 12 Goro process for recovery of nickel from laterites (60). © Taylor & Francis. Reproduced by permission of Taylor & Francis. Permission to reuse must be obtained from the rightsholder.

sheet. In each case, a pure solution of the respective metal is obtained, which is then subjected to a reduction to produce the pure metal powder or sponge. The final products are obtained by melting to obtain ingots, granules or delivery bars. Gold INCO (United Kingdom) first used solvent extraction for the refining of gold from chloride solution (100, 101). The extractant, dibutylcarbitol (DBC), is a straight-chain, high-molecularmass molecule that contains three oxygen atoms in ether positions. The extraction mechanism has been shown to involve the formation of an ion pair between the solvated auric chloride anion and the oxygen-donor reagent (102). Although several PGM refiners around the world have used DBC, this reagent suffers from several disadvantages. Under the high acid conditions of the leach, it is not particularly selective over some of the metalloid elements often found in solutions of this nature. It is also very difficult to strip effectively, so recovery of gold from the loaded organic phase is by direct reduction with oxalic acid in a batch process. The extractant is also fairly soluble in the

aqueous phase (~3 g/L) and dissolved organic losses are higher than are generally considered acceptable. Therefore, the Minataur process for the refining of gold was developed in South Africa (103). This technology uses solvent extraction as the main purification step and a variety of feed materials ranging in gold content from 50% to 99% can be treated. The identity of the extractant remains proprietary. It is known to be inexpensive and does not present the disadvantages of DBC mentioned above. Gold loadings of >100 g/L on the organic phase can be achieved and stripping efficiency is high (104, 105). The process produces gold of 99.99% purity. Advantages of the process are the reduced gold lock-ups and residence times in the circuit and the ability to produce high-purity gold with minimal recycles in the flow sheet. This process is presently operating in South Africa, Algeria and Dubai (106). Palladium Most research into the recovery of palladium by SX has centered on the use of sulfur-based extractants (107).

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FIGURE 13 Simplified flow sheet of the Skorpion zinc process (60). © Taylor & Francis. Reproduced by permission of Taylor & Francis. Permission to reuse must be obtained from the rightsholder.

TABLE 4 Precious metal oxidation states and aqueous chloro complexes Complexes formed Metal Au Ag Ru Rh Pd Os Ir Pt

Oxidation state

Coordination number

+3 +1 +3 +4 +3 +2 +4 +4 +3 +4 +2 +4

4 2 6 6 6 4 6 6 6 6 4 6

Complex geometry Square planar Linear Octahedral Octahedral Octahedral Square planar Octahedral Octahedral Octahedral Octahedral Square planar Octahedral

Organic sulfides are selective for palladium over all other precious metals except gold, so gold is removed from the HCl leach liquor ahead of palladium. The extraction reaction is:  PdCl4 2 ðaqÞ þ 2R2 SðorgÞ , PdCl2 ðR2 SÞ2ðorgÞ þ 2Cl ðaqÞ

(11)

Low [Cl−] AuCl4− AgCl2− Ru2OCl8(H2O)24− Rh(H2O)23+ PdCl42− PdCl62− Ir(H2O)xCl6-x(3−x)− IrCl62− PtCl42− PtCl62−

High [Cl−] AuCl4− AgCl2− RuCl63− RuCl62− RhCl63− PdCl42− PdCl62− OsCl62− IrCl63− IrCl62− PtCl42− PtCl62−

The extraction mechanism involves substitution of the inner sphere chloro ligands by the dialkyl sulfide. Palladium extraction with this reagent is extremely slow, taking several hours to reach equilibrium. The reaction goes to completion, however, leaving less than 1 mg/L Pd remaining in the raffinate. The reaction is carried out in batch mode. Strong ammonia solution is used for stripping. Palladium is

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FIGURE 14 Effect of HCl concentration on extraction of platinum group metals by TBP (60). © Taylor & Francis. Reproduced by permission of Taylor & Francis. Permission to reuse must be obtained from the rightsholder.

recovered by acidification with HCl to precipitate Pd(NH3)2Cl2, which is calcined to produce the metal. Various oximes, similar to those used in copper extraction, have also been widely studied as palladium extractants. Johnson Matthey (United Kingdom) and Anglo Platinum (South Africa) recover palladium with a βhydroxyoxime. Although the kinetics of this extraction is extremely slow, the extraction rate can be enhanced by the addition of amines that act as accelerators. An advantage of these reagents is that they can be stripped with strong acid, avoiding the use of ammonia (108). Stripping is typically achieved using 6 M HCl. Recovery of the metal is by salt formation, followed by reduction or calcination. Platinum Group Metals (PGM) Platinum is recovered after palladium in most flow sheets. TBP is the most widely used platinum extractant. The effect of HCl concentration on the extraction of PGMs by TBP shows the process has a very narrow operating window between about 3 and 4 M HCl. So, it is also necessary to adjust the reduction potential of the solution to ensure that iridium(IV) is not coextracted. Amines are good platinum extractants, and schemes in which primary through quaternary amines are employed have been proposed. The extraction occurs via an anion-

exchange mechanism. The reaction for tertiary amines can be written as: 2R3 NðorgÞ þ 2HClðaqÞ , 2½R3 NHþ Cl ðorgÞ

(12)

2½R3 NHþ Cl ðorgÞ þ PtCl6 2 ðaqÞ , ½R3 NHþ 2 PtCl6 2 ðorgÞ þ 2Cl ðaqÞ

(13)

In these systems, complete stripping can only be achieved by deprotonation of the extractant using sodium carbonate solution. Amines are generally employed in combination with alcohols, phenols or carboxylic acids that modify the pH dependence of extraction so that stripping is easier (109). The relative extraction efficiencies of the amines as a function of chloride concentration are shown in Figure 14. The use of trin-octylamine to recover platinum from the gold- and palladium-depleted leach liquor was reported where it is necessary to use 12M HCl for stripping with this reagent (110).

IONIC LIQUIDS AS SOLVENT EXTRACTANTS Solvent extraction employs water-immiscible organic solvents that can be toxic, flammable or volatile. Given the rising costs for their eventual disposal and the growing awareness of the environmental impact associated with

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their use, it is clear that the replacement of these solvents with less noxious alternatives is desirable (111). Recently there has been increasing emphasis on the development of environmentally benign separation processes. An important aspect of this effort to devise greener separations is, in fact, the identification, characterization and application of novel solvents exhibiting few or none of the drawbacks of their traditional organic counterparts (112). Of particular recent interest among alternative solvents have been ionic liquids (ILs). ILs are highly polar molten salts possessing large ions with delocalized charges. This leads to lower Coulomb attraction forces between ions, which results in low melting temperature of ILs (113). Ionic liquids exhibit several properties that make them attractive as a potential basis for improved extraction processes, among them are a wide liquid range, good thermal stability, the ability to solubilize a wide range of solutes, a near-absence of vapor pressure and an extraordinary degree of tunability (114). It has been observed that processes of metal ion separation from aqueous solutions using ILs as solvents, extractants and/or ion carriers have grown in significance. ILs became attractive alternatives to volatile organic solvents (VOCs) also as a result of good extractability for a variety of organic compounds and metal ions (115, 116). The hydrophobic characteristics of some ILs allow for the extraction of many heavy metal ions, such as zinc, mercury, lead, cadmium, iron, chromium, copper and nickel, from aqueous solutions (117). Despite their hydrophobic character, ILs offer relatively high solubilities for salts and can consequently be used for the extraction of salts (118). The solubility of an ILs in water strongly depends on the type of cation and anion (119). ILs possessing acid anions can be also used for the leaching of many metals from ores (120). In metal recovery and refining, extraction with ILs provides new prospects to exploit low-value metal ores and to recycle metal ions (121, 122). A number of studies have been focused on developing effective extractants for SX; however, very few reports can be found in the literature concerning the mixtures of ILs and classic extractants. Recently, numerous papers have been published on the application of ILs as extracting reagent solvents and as individual extractants of metal ions from aqueous solutions (113). ILs have also been used as passive diluents for many extractants in liquid–liquid extraction of various metal ions from aqueous phase. Imidazolium ILs are solvents with well-known physicochemical properties (123, 124). For example, butylmethylimidazolium hexafluorophosphate, [BMIM][PF6], was used as a diluent of dithizone during extraction of heavy metals ions, for example, Hg(II), Cd(II), Pb(II), Ag(I) and Cu(II) ions, from aqueous solutions. ILs have also been examined in liquid–liquid extraction of alkali and alkaline metal ions using crown ethers (125) or

calixarenes in extraction of Cs(I) (126). Dioxouranium(VI) was extracted from nitric acid solutions by TBP/IL mixtures using three types of 1,3-dialkylimidazolium (127). Sun et al. (128) reported on the use of Cyanex 923 in 1-octyl-3methylimidazolium [C8MIM][PF6] for a selective extraction–separation of Y(III) from a mixture of heavy lanthanides. The extraction efficiency of the Y(III)/Cyanex 923/ [C8MIM][PF6] system was found to decrease rapidly with increasing aqueous acidity. It was found that ILs containing disulfide or nitrile groups exhibit good extraction properties for Ag(I) and Pd (II) ions. Au(III) ions were also efficiently and selectively extracted using functionalized ILs containing functional alkenyl group and nitrile or disulfide groups (115). The extraction of Eu(III) ions by ILs showed a strong dependence on the nature of molecular solvents such as chloroform, n-dodecane and 1-octanol. The effect of diethylenetriamine pentaacetic acid on the extraction efficiency of Am(III) and Eu(III) ions from water was also described (129). ILs were also investigated as ion-selective extractants for Pd(II) ions from 0.1 M HCl solution containing many metal ions (130). It was noticed that the selectivity of the extraction of Pd(II) is dependent on the acidity of the aqueous phase and decreases with increasing concentration of HCl. Zhang et al. (131) reported the extraction of cerium(IV) ions, fluoride ions and Ce(IV)–F mixture from sulfuric acid using bifunctional IL extractants in n-heptane. The investigated systems proved strong separation capabilities, showing the following extraction order: Ce(IV) > Th(IV) > Ce (III). Rout et al. (132) used ILs to study Nd(III) extraction and found that the extraction efficiency increases with an increase in the initial pH of the aqueous solution, reaching a maximum at pH 2. Using 1-octyl-3-methylimidazolium [C8MIM] bis(trifluoromethylsulfonyl)imide as IL, Pt(IV) ion can be removed from gold (133). Extraction of Pt(SCN)62− was very high at pH 1, exhibiting a distribution coefficient of 6150. A novel ionic liquid, 1-hexyl-3-methyl imidazolium [C6MIM] dodecyl sulfonate, was synthesized and used for the construction of an aqueous two-phase system together with PEG6000, which was investigated for Au (III) extraction later (134). Under the optimum conditions, the extraction percentage was up to 97.56%. The suggested aqueous two phase system is supposed to be a promising approach for gold extraction and separation in acid solutions. In another study, a selective extraction and recovery of Au(III) from a tertiary metal solution containing Au(III), Pt(IV) and Pd(II) were investigated using ionic liquid Aliquat-336 (135). Through sequential extraction, high purity of each metal solution was separately obtained from low and high concentrations of multimetal solutions. Zhao et. al. (136) found that isohexyl-BTP in ionic liquids exhibited remarkably better extraction performance

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for lanthanides than that in octanol–dodecane system especially at lower acidity conditions (