THOMAS W. CHAPMAN Department of Chemical Engineering University of Wisconsin Madison, Wisconsin 8.1 INTRODUCTION CHAPTER

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1 CHAPTER O E x t r a c t i o n M e t a l s P r o c e s s i n g THOMAS W. CHAPMAN Department of Chemical Engineering University of Wisconsin Madison, Wisconsin 8.1 INTRODUCTION Although electrolytes normally do not exhibit significant solubility in nonpolar solvents, many metal ions can react with a wide variety of organic compounds to form species that are soluble in organic solvents. Such solubility, which depends on a chemical reaction, provides a basis for separating and concentrating metals that are present as ions in aqueous solution. As with many separation processes, solvent extraction of metals was first developed as a tool of the analytical chemist. Chemical reagents and solvents are known for isolating virtually every metallic element of the Periodic Table.'^ In the 1940s and 1950s some of this basic analytical chemistry was used to develop continuous processes for separating nuclear 5 and rare-earth 6 elements. Subsequently, the availability of inexpensive or especially effective reagents led to the establishment of large-scale processes for extraction of copper, zinc, uranium, and other metals from hydrometallurgical leach liquors. 7 " 10 The flowsheet of two proposed processes, based on solvent extraction, for separating the constituents of deep-sea manganese nodules are shown in Figs and The solvent extraction of metals is similar in concept to ion exchange, which is treated in Chapter 13, and provides specific examples of separation by chemical complexation, discussed in Chapter 15. Technological implementation is based on the concepts of ordinary solute extraction, presented in Chapter 7. The choice of solvent extraction over alternative separation schemes is a complex one, but it clearly depends on the availability of an effective extraction agent. A reagent that provides a very high distribution coefficient from dilute solution usually can be found, but reagent selectivity, stability, solubility, kinetics, and cost are all important factors. The ability to reverse the extraction reaction to recover the metal and regenerate the reagent is crucial. In favor of extraction is the relative ease of handling liquid-liquid systems, compared with the solid-liquid operations required in most of the alternative processes. An extensive literature has developed that offers many choices of chemical systems, and progress is being made in refining the engineering of metal extraction processes. Relevant publications appear in the analytical chemistry literature, the Journal of Inorganic and Nuclear Chemistry, Separation Science and Technology, metallurgical journals and conferences, Hydrometallurgy, and most recently in the standard chemical engineering periodicals, such as Industrial and Engineering Chemistry. Although publications in this field have been widely dispersed, a focal point has been the Proceedings of the triennial International Solvent Extraction Conference. The Handbook of Solvent Extraction 12 provides a number of useful chapters, and Ritcey and Ashbrook 13 discuss many practical aspects of metal extraction processes. 8.2 EXTRACTION CHEMISTRY AND REAGENTS The extraction of a metal ion from an aqueous solution into an organic solvent is accomplished by the chemical formation of an uncharged species that is soluble in the organic phase. Because metal salts usually

2 Cu Extraction 4 stages 40% LIX 64N Crude leach liquor NH 3 Scrub 2 stages Ni Scrub 2 stages NH 4 HCO 3 solution Raffinate NH 3 AX) 2 solution Cu Stripping? stages Ni Extraction 2 stages Electrowinning Cathode copper Ammonium carbonate solution Ni Stripping 3 stages NH 3 Ammonia removal Solid-liquid separation Basic nickel carbonate FIGURE Solvent extraction portion of the Kennecott copper-nickel carbonate process for deep-sea manganese nodules. Adapted from U.S. Patent 3,907,966 in Ref. 11. are not soluble in organic solvents, the process requires the introduction of a reagent, or extractant, that will combine with the metal ion to form an organic-soluble species. For economic and environmental reasons, the extractant chosen to separate a metal must be practically insoluble in water. Thus, the extractant sometimes can constitute the organic-phase solvent of the process. In most cases, however, an inert organic carrier, or diluent, is used as the solvent of the organic phase. Additional organic components, called modifiers, also are employed. The proper use of diluents and modifiers can prevent formation of very viscous or solid organic phases, can modify extraction kinetics as well as equilibria, can reduce the aqueous solubility of the extractant, and allows flexibility in the choice of phase ratios in contacting equipment. The extractants that are used to form organic-soluble metal species from aqueous metal salts usually are grouped into three classes according to the type of reaction that occurs. Solvating extractants compete with water in coordination bonds around a metal ion and carry neutral salts into the organic phase. Cation

3 15(20%) secondary amine 15(20%) isodecanol kerosene Acid ph 2 ph adjust to 2 with 2 N NaOH Mn, Ni, Co, Cu chlorides (sulfates) r~ Fe Extraction 4 stages Aqueous feed Mn, Fe, Ni, Co 1 Cu chlorides Fe Stripping 3 stages FeCI 3 solution to pyrohydrolysis Cu Extraction 5 stages 2 iv NaOH added to maintain ph2 10% LIX 64N 20% isodecanol napoleum CuSO 4 solution to electrolysis Mn chloride (sulfate) 10% Kelex % isodecanol napoleum Cu Stripping 5 stages Ni-Co Extraction 5 stages ph adjust to 4.5 with 2 N NaOH 2 N NaOH added to maintain ph 4.05 Return sulfate electrolyte 3 nh + Mn, Ni, Co chlorides (sulfates) Co Stripping 4 stages Ni Stripping 3 stages 20% HCI Co Re-extraction 3 stages Return chloride or sulfate electrolyte adjusted to 3 N in H + Acidic Co chloride Ni chloride or sulfate to electrolysis 10% TIOA kerosene Co Restripping 3 stages Return chloride Co chloride to electrolyte electrolysis FIGURE Deepsea Ventures solvent extraction flowsheet for separation of metals in a manganese nodule leach liquor. From Ref. 11, with permission.

4 TABLE Examples of Some Common Metal Extractants* Extractant Trade Name Chemical Structure Molecular Weight Tributyl phosphate TPB RO \ 266 / RO R[=jCH 3 (CH 2 ) 2 CH 2 - R Trioctyl phosphine oxide TOPO R-P=O R 386 R [=] CH 3 (CH 2 ) 6 CH 2 - Di-2-ethylhexyI phosphoric acid DEHPA R-O R-O \ / P O OH 322 R [=] CH 3 (CH 2 ), CHCH 2 - C 2 H 5 R 1 CH 3 Versatic acids Versatic C R 2 COOH

5 Ri, R 2 [=] alkyl C 4 -C 5 groups R R CH-CH Naphthenic acids CH CH R CH (CH 2 X 1 COOH R Tri-iscoctylamine TIOA Adogen 38 l c Alamine 308* R \ R-N 353 R R [=]CH 3 CH-(CH 2 ) 5 - CH 3 Trioctyl methyl ammonium chloride Adogen 464 C Aliquat 36* CH 3 R-N-RCl 404 R R [=] CH 3 (CH 2 ) 7 - C 2 H 5 C 2 H 5 Hydroxyoximes LIX 63* CH,(CH 2 ) 3 C-C-CHCH(CH 2 ) 3 CH 3 Il I HON OH 257

6 TABLE (Continued) Extractant Trade Name Chemical Structure Molecular Weight C 9 H 19 Hydroxybenzohexone oximes LIX 65N* C Il I OH 339 NOH LIX 64N* LIX 65N + LIX 63 (1%) Substituted 8-hydroxyquinoline Kelex 100 N 'jxh(ch 2 ) 2 CH(CH 2 ) 3 CH H CH 3 C 2 H 5 Ritcey and Ashbrook 1 and Flett et al. give more complete lists. *Henkel Corp. c Sherex Corp.

7 exchangers include organic acids as well as specific chelating extractants. Anton exchangers extract metals that form anionic complexes. Synergistic mixtures of extractants are sometimes found to be effective in promoting a particular separation. The chemical structures of some common extractants are shown in Table Solvating Extractants Extraction of metals by solvating extractants occurs formally by the reaction (8.2-1) where MX n is an uncharged salt coming out of the aqueous phase and S is the extractant. Organic-phase species are denoted by an overline. Inorganic acid can also be extracted by a reaction of the form (8.2-2) The most common solvating extractants are alkyl phosphate esters, such as tributyl phosphate (TBP) and trioctyl phosphine oxide (TOPO), but thiophosphate esters, phosphine sulfides, ketones, alcohols, and esters also have been used. The common features of these solvating extractants is that they contain both electron-rich oxygen or sulfur atoms that can coordinate metal cations and multiple alkyl chains that make them hydrophobic. It is the electron-rich atoms that can bind protons to extract acids. Generally, those reagents possessing a carbonbound oxygen tend to extract water with the metal salt, whereas the phosphorus-based reagents do not. Because the bonds formed in the solvation-extraction reactions are not highly specific, the stoichiometric coefficients p and q y as well as the amount of coextracted water, are not definite and may vary with extraction conditions. Increased polarity of the functional group in the extractant, as is obtained in eliminating the ester linkages in trioctyl phosphate to obtain TOPO, increases the strength of the solvation, but it also tends to increase the aqueous solubility of the reagent. Solvating extractants can be used undiluted, but usually they are mixed with an inexpensive diluent such as kerosene. The solvation extraction of a metal, according to the form of reaction (8.2-1), depends on the formation in the aqueous phase of an uncharged ion pair or complex MX n to some significant level of activity. Therefore, the extent of the extraction reaction will depend strongly on the aqueous solution chemistry as complexation, association, and hydrolysis affect the distribution of metal species. Similarly, the reversal of reaction (8.2-1) to strip the metal from the organic phase usually depends on altering the aqueous-phase chemistry. This may be accomplished by stripping with an aqueous phase low in the concentration of the ligand X or by using a competing aqueous-phase complexing agent. Temperature may have some effect on the distribution equilibrium, and precipitation or redox reactions might be used to reverse the extraction reaction. Table summarizes some specific applications of solvating extractants. More extensive reviews are given in Refs Liquid Cation Exchangers Solvent extraction of metals can be accomplished by use of organic-phase extractants that function as ion exchangers. For metal cations extractants possessing an acidic functionality are appropriate. There are two types of cationic, or acidic, ion-exchange extractant available. The same functional groups that are used in cation-exchange resins, namely, carboxylic, phosphoric, and sulfonic acids, can be incorporated in a soluble organic molecule to obtain a liquid ion exchanger. Structures of moderate molecular weight (around 300) are used such that the extractant is soluble in a diluent like kerosene but exhibits a low aqueous solubility. Common examples of these acidic extractants include di-2-ethylhexyl phosphoric acid (DEHPA) and alkyl monocarboxylic acids with carbons in various branched (e.g., Versatic acids) or cyclic (e.g., naphthenic acid) structures. The other type of cation-exchange extractant available is a class of chelating extractants. Chelating extractants contain two functional groups that form bidentate complexes with metal ions. Although there are many chelating reagents used in analytical chemistry to achieve highly selective complexing of a wide variety of metals, only a few have been developed for large-scale commercial use. The notable example is the family of LIX reagents developed especially for copper extraction by General Mills Inc., now Henkel Corp., which are hydroxyoximes and possess an acidic hydrogen. Similar reagents based on the oxime group have been developed by several other chemical companies. Other acidic chelating extractants, which act as cation exchangers, include other hydroxyoximes, such as SME259 of Shell Chemical Company and the P5000 reagents of Acorga Limited, and substituted 8-hydroxyquinolines, such as the Kelex reagents of Sherex Chemical Company. With both simple and chelating acidic extractants the metal extraction reaction generally follows the form

8 TABLE Hydrometallurgical Applications of Some Common Solvating Extractants Extractant Tributyl phosphate (TBP) Methyl isobutyl ketone (MIBK) Trioctyl phosphine oxide (TOPO) Application U extraction from nitric acid Zr separation from Hf in nitric acid FeCl 3 separation from Cu and Co in HCl solution Extraction of Nb and Ta from HF solutions, followed by selective stripping Extraction of U from phosphoric acid (8.2-3) such that the reaction is strongly ph dependent and stripping occurs into strong acid. Table summarizes the applications of some acidic extractants. Ritcey and Ashbrook 6 provide a historical account of the evolution of the commercial chelating extractants Liquid Anion Exchangers In some electrolytic solutions, especially those strong in halides, many metal cations are complexed by anions to the extent that they exist primarily as neutral ion pairs or anionic species. The formation of anionic complexes may retard metal extraction by solvation or cation exchange, but it can be exploited by use of anion-exchanging extractants. The most important liquid anion exchangers for commercial use are quaternary ammonium salts and primary, secondary, and tertiary amines, which become anion exchangers by combining with mineral acids to form alkyl ammonium salts. In the former case, such a metal extraction reaction might be written (8.2-4) where m is the charge number of the metal cation M +m and X " is a univalent anion. Because aqueous complexes of various coordination numbers n coexist in an equilibrium distribution, the overall extraction reaction may be viewed as either anion exchange, with n > m, or an association reaction with n m. In either case, the stability of the organic-soluble species depends on the tendency of the metal ion to form complexes with the anion. With other amines, such as tri-isooctylamine (TIOA), an overall metal extraction reaction might be written (8.2-5) where HX and MX m both combine with the amine in an apparent addition reaction. Alternatively, this reaction could be written, and can occur, in two separate steps, TABLE Hydrometallurgical Applications of Some Common Cation-Exchanging Extractants Extractant Versatic 911 (carboxylic acid) Naphthenic acids (carboxylic acids) Di-2-ethylhexyl phosphoric acid (DEHPA) LIX 64N Kelex 100 Applications Separation of rare earths in nitrate solution Separation of Co from Ni in sulfate solution Extraction of Cu, Ni, and Co Extraction of rare earths, V, Be, Al, Co, Ni, Zn U extraction from sulfuric acid Selective extraction of Cu from acidic solutions Extraction of Cu and Ni from alkaline ammonia solution Selective extraction of Cu from acidic solutions Extraction of Cu, Ni, Co, and Zn from alkaline ammonia solution

9 (8.2-6) amine salt formation, followed by (8.2-7) an addition reaction between the ion pair and the amine salt. If attention were focused on an anionic metal complex MX "" rather than the uncharged ion pair MX m, an alternative to reaction (8.2-7) would be an anion-exchange reaction analogous to reaction (8.2-4), (8.2-8) Different amine extraction systems exhibit different stoichiometries from that shown here, and a number of different organic species may form in any one system, but the common feature of such extractions is that they depend on combining metal cations, anions, and amine into uncharged organic-soluble molecules. The extraction may be reversed by suppressing the activity of any of the product's constituents. For example, a metal extracted with a tertiary amine can usually be stripped by an aqueous phase that has either a sufficiently high ph to reverse the amine salt formation or sufficiently low coordinating-anion concentration to suppress formation of the complex. For quantitative process calculations it is important to know what chemical species form in a given extraction system. The stoichiometry of the overall reactions affects the process material balance. Also, the number of different species formed determines the number of degrees of freedom required to specify the thermodynamic state of a system. On the other hand, the detailed mechanism of a reaction, for example, whether amine extraction occurs by addition [reaction (8.2-5)] or ion exchange [reaction (8.2-7)], is of no direct concern unless slow reaction kinetics become an issue. Table summarizes applications of amines for metal extraction. The viability of amine extraction depends directly on the identity of the anions present in the aqueous phase. Amine extraction has been discussed by Schmidt Diluents Although the solvent extraction of a metal occurs by reaction with an extractant that is essentially insoluble in water, it is seldom practical to use pure extractant as the solvent phase. The pure extractants are usually very viscous and may have specific gravities near 1 so that both mixing and separating phases may be difficult. They are intrinsically surface active and tend to form emulsions with water. To avoid these problems and to provide flexibility with respect to phase ratios and the hydraulics of liquid-liquid contacting, one uses a diluent, which is often the major component in the organic phase. Inexpensive hydrocarbon solvents are chosen as diluents in large-scale processes. Table lists some examples. These solvents are chosen for their low solubilities in water and high flash points as well as some less obvious chemical properties. The diluent in a solvent extraction system does not serve simply as an inert carrier for the extractant and its metal compounds. It has been found repeatedly that the choice of diluent can affect significantly the performance of various extractants, presumably through both chemical and physical interactions with the solute species. For example, Murray and Bouboulis 8 reported the effects of systematically varying the aromatic content of the diluent on a copper extraction process, and Akiba and Freiser 9 demonstrated solvent effects on system chemistry. Ritcey and Ashbrook 10 review the various solvent properties that influence extractant performance. From the combination of a hydrocarbon solvent as a diluent with extracted metal species, which often contain some water molecules or other polar groups, there is a tendency in many systems for a third phase TABLE Hydrometallurgical Applications of Some Common Anion-Exchanging Extractants Extractant Tertiary amines (TIOA) Quaternary amines Applications Separation of Co and Cu from Ni in acidic chloride solutions Extraction of Zn and Hg from acidic chloride solutions Extraction of U, V, W, and Mo from acidic sulfate solutions Extraction of Re from acidic nitrate solution Extraction of Mo from carbonate solution Extraction of Na 2 CrO 4 Extraction of V from caustic solution

10 TABLE Some Commercial Solvents Used as Diluents in Metal Extraction Processes Composition (%) Paraffins Naphthenes Aromatics Amsco odorless mineral Spirits" Chevron ion-exchange solvent* CycloSo^' Escaid loo Escaid 110* Escaid 350 (formerly Sotvesso 150/ Kermac 470B (formerly Napoleum 470)' MSB210 c Shell 140' Shellsol R' a Union Oil Co. "Chevron Oil. 'Shell. ''Esso or Exxon. 'Kerr McGee. Source: Reference 2. to form when the extraction proceeds. This presents serious problems in fluid handling and is avoided by the addition of modifiers. The most common modifiers are alcohols, phenols, and TBP, which act as cosolvents by virtue of their polar groups. Modifier concentrations are usually 5-10% by volume of the organic phase, a level comparable to that of the extractant in many cases. Like the diluent, the modifier can also interact with the solute species and affect the extraction reaction in various ways Competing and Supporting Reactions In the outline of common extraction mechanisms given above, the phenomenon of metal extraction is viewed as a chemical reaction occurring across a liquid-liquid phase boundary. It is clear, however, that more than one reaction occurs in almost all extraction systems. In the aqueous solutions the metal ions often interact with anions or complexing agents or water (by hydrolysis) to form a variety of species. Such reactions may promote the extraction of the metal, as in the case of anionic complexes with amines, or they may hinder it and promote stripping, as with precipitation of hydroxides or sulfides or with ammonia complexing that competes with cation exchange. In the organic phase there are parallel interactions, such as association of extractant molecules into dimers and even polymers or micelles. Between phases other reactions may occur simultaneously. Water, acids, or other metals may be extracted by interaction with the extractant, and multiple species of organic-soluble metal may also form. Introduction of modifiers as well as diluents increases the number of possible interactions and the complexity of these systems. In spite of the large number of experimental studies of various extraction systems reported in the literature, the range of variables available in these highly interacting, multicomponent systems leaves many regions unexplored. Therefore, the development of a solvent-extraction process requires extensive experimentation. The general behavior of many available reagents is known, but the effects of interactions in a specific system must be determined experimentally. It must also be recognized that one is seldom working with pure chemicals in these extraction systems. Not only are the aqueous feed streams usually complex mixtures, but the commercial extractants, diluents, and modifiers are themselves mixtures and contain unknown impurities. Therefore, each system of interest must be characterized by experimental investigation. To attain a quantitative understanding of a particular metal extraction system, one must gather as much information as possible about the chemical species forming in the system. Molecular weights and states of aggregation should be determined for both extractant and extracted metal species. Maximum metal loading should be measured as an indication of the extraction-reaction stoichiometry. Degree of water coextraction should be determined, and the tendency of other species such as acids to extract should be examined. Although extensive analytical work is required, such information is needed to construct meaningful material balances as well as to formulate equilibrium and rate analyses.

11 8.3 PHASEEQUILIBRIA Often the equilibrium extent of extraction of a metal is described in terms of the phase distribution coefficient, (concentration of metal in organic phase) (concentration of metal in aqueous phase) Much of the published information about phase equilibria is presented in terms of D. Although the distribution coefficient depends on many variables, because of the multicomponent and reactive nature of these systems, many studies report D for a trace amount of metal in fixed compositions of aqueous and organic solutions, usually at ambient temperature. Such data are independent of metal concentration. Early reagent-screening experiments are summarized by Marcus and Kertes 1 in a form such as shown in Figs and Figure presents low-concentration distribution coefficient values for various metals in a 5% solution of TOPO in toluene for several concentrations of hydrochloric acid in the aqueous phase. Figure shows a similar presentation of distribution coefficients from sulfuric acid solutions into undiluted TBP. As an example of the use of such information, suppose that one wished to separate zinc and copper present in an acidic chloride solution. Figure indicates that the distribution coefficients of the two metals with 5% TOPO in toluene are on the order of 100 and 0.01, respectively, for an aqueous phase containing 1.0 M HCl. Therefore, this extractant should be effective in separating zinc from copper if the aqueous chloride level is not too high. Furthermore, the distribution coefficient of zinc falls to 0.01 in 12 M HCl so that a concentrated hydrochloric acid solution would be a candidate for accomplishing the stripping of zinc from the loaded organic phase. This information suggests that some experimental work with TOPO and the zinc-copper solution would be justified to determine the phase equilibria more completely. Trace-level distribution coefficients are often plotted as functions of acid or anion concentration for a given extractant and presented as an overlay of the Periodic Table to indicate the efficacy and selectivity of the reagent. Figures and summarize extraction of metals by some tertiary and quaternary amines from hydrochloric acid solutions. The ordinate of each graph is log D, which ranges from 10~ 2 to 10 4, and the abscissa is HCl concentration, ranging from 0 to 12 Af. The high distribution coefficients of ferric chloride with amines shown in Figs and indicate the basis for using amine extraction as the first step of the process shown in Fig Numerous charts of this form are available for various extraction systems. 2 " 4 Another way to present tracer-level metal equilibrium data is to plot the fraction of the metal extracted into the organic phase as a function of some composition variable, often ph. Implicit in such a presentation is a phase volume ratio of unity. Figure shows the percent extraction of several metal cations by naphthenic acid as a function of ph. Such data indicate the potential for separating certain metals with a particular reagent; they also show the possibility of stripping from the loaded organic by ph control. For example, Fig suggests that cupric and ferric ions can both be extracted by naphthenic acid if the ph of the aqueous phase at equilibrium is held above 4 by buffering or addition of base. On the other hand, only ferric ion should be extracted if the ph is adjusted to 2.5. With regard to stripping, addition of acid to lower the ph to 1.0 should strip all metal from the organic phase. Alternatively, the curve for copper in ammonium nitrate suggests that copper can be stripped at high ph in the presence of ammonia. Although representative values of the distribution coefficients of various metals with different extractants are useful in guiding the selection of a reagent for a particular separation process, the information provided by low-concentration D values is incomplete. For example, Fig indicates that the distribution coefficient of mercuric ion between hydrochloric acid and a solution of tri-isooctylamine in xylene(0.14 M) is on the order of 100. Figure shows the distribution coefficient of mercuric ion from 4.63 M NaCl solution at ph = 1 and indicates that the value can be considerably higher and that it varies with the level of mercury in solution. At high concentrations of metal, extractant is consumed by the extraction reaction, and the distribution coefficient falls as the finite stoichiometric loading of the organic phase is approached. A further complication in the equilibrium is shown in Fig , which shows the mercury distribution coefficient as a function of amine concentration for a fixed level of mercury. At low amine concentrations the distribution coefficient increases with amine level in the way that is expected for reaction (8.2-7) or (8.2-8), but above a certain amine level the distribution coefficient becomes constant. This behavior indicates that some separate phenomenon, such as limited solubility or association of the amine-hydrochloride salt, keeps its activity constant with increasing concentration. This is a common experimental observation in such systems. Although distribution coefficients in metal extraction systems are highly variable, they are thermodynamic quantities that can be measured and correlated as functions of the state of the system. Because of the multicomponent nature of metal extraction systems, one must take care to identify a proper set of independent variables in constructing a correlation of phase equilibrium data. Another consideration in treating equilibrium extraction data is that one desires a systematic method for treating the effects of

12 FIGURE Trace-level distribution coefficients of metal ions into a 5% solution of TOPO in toluene as functions of aqueous HCl concentration. From Ref. 1, with permission. additional species, such as other metals, modifiers, and so on, on the equilibrium distribution of a particular metal. A chemical-reaction model of extraction equilibria provides an appropriate framework for correlating data because the large, but reversible, distribution coefficients of most interesting systems are the result of reversible chemical reactions. For a cation-exchanging extractant HR, which extracts a divalent metal ion M 2+ according to reaction (8.2-3), a chemical-reaction model of the phase equilibrium would be the mass-action equilibrium expression (8.3-1) where the quantities in brackets are activities and the equilibrium constant K for a specific metal-extractant system depends on temperature and the identity of the diluent. If activities were identical to analytical

13 FIGURE Trace-level distribution coefficients of metal ions into undiluted TBP as functions of aqueous sulfuric acid concentration. From Ref. 1, with permission. concentrations, knowledge of A" in Eq. (8.3-1) would allow calculation of the equilibrium metal distribution, or D = (MR 2 V(M 2+ ), for any given experimental conditions. For example, for the case of a trace amount of metal, equal phase volumes, and an extractant concentration fixed at 0.1 Af, Eq. (8.3-1) with constant K yields the metal distribution curves shown in Fig , which may be compared with Fig A value of K = 1.0 represents an extraction equilibrium that provides complete extraction at ph > 2 and stripping at ph < 0. To account for nonidealities in solution one can use activity coefficients or simply replace activities with species concentrations in Eq. (8.3-1) to obtain an empirical mass-action equilibrium quotient that is concentration dependent. Bauer and Chapman 5 treated data for copper extraction by Kelex 100 by a chemical model, both with and without explicit activity coefficient terms, and were successful in extrapolating from results with 5% Kelex to obtain an accurate copper distribution curve for 20% Kelex. Hoh and Bautista 6 used a similar approach to correlate equilibrium data for copper with LIX 65N and with Kelex 100. The concentration-based equilibrium constant at 25 0 C was found to be on the order of 0.03 for LIX 65N in

14 FIGURE Trace-level distribution coefficients of metal ions into solutions of tertiary amines as functions of aqueous HCl concentration:, 0.14 M TIOA in xylene or kerosene;, 0.1 M Alamine 336 in diethylbenzene; <, D < 10~ 2 at M HCl. From Ref. 17, with permission.

15 FIGURE Trace-level distribution coefficients of metal ions into solutions of quaternary ammonium salts as functions of aqueous HCl concentration:, 0.1 Af Aliquat 336 in diethylbenzene + 3% tridecanol;, 0.1 M Hyamine 1622 in 1,2-dichloroethane;, 0.2 M tetra-fl-hexylammonium iodide in hexone; <, D < 10" 2 at M HCl. From Ref. 17, with permission.

16 PERCENT EXTRACTION EQUILIBRIUM pm FIGURE Fractional amount of extraction of trace levels of some metal cations from aqueous solution into organic solutions of naphthenic acid as a function of ph. From Ref. 18, with permission. toluene, 6 5 for Kelex 100 in toluene, 6 and 90 for Kelex 100 in xylene 5. From trace-level measurements Akiba and Freiser 7 found the equilibrium constant for copper extraction by LIX 65N to range from 10" 2 to in various solvents. Virtually no metal extraction system involves only one reaction. In aqueous metal salt solutions there occur homogeneous association and complexation reactions that make true species concentrations differ from the analytical concentrations that are measured and that enter into material balances. In sulfate solutions bisulfate formation occurs at low ph, and metal-sulfate ion pairing may take place. In chloride and ammonia solutions metal complexation is common. Dimerization and other association reactions often occur in the organic phases. And two-phase extraction reactions of acids or other species may take place simultaneously with metal extraction or stripping. The equilibria of these reactions must be modeled concurrently with the DISTRIBUTION COEFFICIENTS, CONCENTRATION IN THE ORGANIC PHASE (^g Hg/mL) FIGURE Distribution coefficient of mercuric ion from 4.63 M NaCl solution at 22 C and ph = 1 as a function of organic-phase mercury concentration. The organic phase is TIOA in xylene at 0.1 M and 0.5 M as indicated. From Ref. 13, with permission.

17 DISTRIBUTION COEFFICIENT, TOTAL AMINE CONCENTRATION, 5> (mol/l) FIGURE The effect of amine concentration on the distribution coefficient shown in Fig From Ref. 13, with permission. metal-extraction reaction if the true reactive species concentrations are affected by them. If such reactions are overlooked in an equilibrium model, system behavior will appear to be highly nonideal, and extreme values of empirical activity coefficients will be required. The relatively large values found for the extraction equilibrium constant of copper with Kelex 100 (5 and 90) indicate that stripping of copper from this reagent should be difficult. It is found, however, that copper does strip readily into sulfuric acid solutions because Kelex 100 reacts with sulfuric acid in preference to copper. Fitting the extraction of sulfuric acid by Kelex 100 by a chemical-reaction equilibrium constant, (8.3-2) at 50 0 C, and solving the two equilibria simultaneously describes adequately the equilibrium distribution of copper under stripping conditions. 5 This is shown in Fig The acidic copper-chelating extractants also can extract other metals from alkaline ammonia solutions, which are used in some hydrometallurgical processes to exclude iron. DeRuiter 8 modeled the extraction of copper(ii) by LIX 64N by considering the following reversible reactions: complexation of aqueous copper by four and by five ammonia ligands, the ion-exchange extraction of the free copper cation, hydrolysis of ammonia to ammonium, and extraction of ammonia by both physical solubility and by chemical association with LIX. Not only does this chemical-reaction model fit the copper-extraction equilibrium, but the equilibrium constant for extraction of free copper cations, found to be 0.02, is in good agreement with that determined in acidic sulfate solutions.

18 PERCENT EXTRACTED EQUILIBRIUM FIGURE Fractional amount of extraction of trace levels of metal cations from aqueous solution into organic solutions of an acidic extractant as a function of ph. Computed from Eq. (8.3-1) for several values of the equilibrium constant K. ph Nickel is extracted from alkaline ammonia solutions by LIX 64N, but its equilibrium constant is five orders of magnitude smaller than that of copper. 8 Also, it forms ammonia complexes more extensively than copper, which enhances the selectivity of LIX 64N for copper over nickel. Under conditions of simultaneous extraction the parameters of the chemical-reaction model for each individual metal are able to predict the two-metal equilibria provided that the equilibrium distribution of species in the aqueous phase is taken into account. It appears that solutions of the common chelating agents behave ideally in the sense that the organicphase species in these systems usually exhibit activities that are proportional to their concentrations. Unfortunately, such behavior is the exception rather than the rule. Figure illustrated nonideal behavior of TIOA in the extraction of mercuric chloride. Organic-phase nonideality is commonly encountered with solvating extractants and most cation exchangers as well as amines. As with the aqueous phase most of the nonideality can be accounted for by identifying association reactions in the organic phase and modeling the chemical equilibrium among species in solution. An example of nonideal organic-phase behavior arises with di-2-ethylhexyl phosphoric acid (DEHPA) in the extraction of metal cations. Equilibrium data of Troyer 9 for extraction of copper in the presence of nickel from sulfate solution into xylene solutions of DEHPA are shown in Fig Although the organic-phase copper concentration would be expected to rise in proportion to that in the aqueous phase, ORGANIC COPPER CONCENTRATION (kg/m 3 ) OATA 15% KELEX % NONYL PHENOL IN ESCAIO 100 H 2 SO 4 IN AQUEOUS PHASE (kg/m s ) FIGURE Comparison of chemical-equilibrium extraction models with stripping data for copper in Kelex 100. Curve A follows Eq. (8.3-1). Curve B is obtained by solving Eq. (8.3-1) simultaneously with Eq. (8.3-2). From Ref. 5, with permission.

19 FEED RATIO phos«ratio* I:I INITIAL CuSO 4 CONCENTRATION (mole/liter) FIGURE Predicted and observed organic copper concentrations in equilibrium with O.1 M DEHPA in xylene for different amounts of base added and several copper-to-nickel feed ratios. Curves are predicted by the model for the indicated initial NaOH concentrations. From Ref , with permission. the data indicate that the former is rather insensitive to the latter. Detailed data analysis indicates that the activity of free extractant is nearly independent of its analytical concentration, and this is caused by reversible polymerization of DEHPA. The equilibrium data can be modeled by accounting for the following reactions: complete dimerization of DEHPA, formation of an organic metal species of the form MR 2 2HR, metal hydrolysis to form a monohydroxy complex, formation of a metal-sulfate ion pair, formation of bisulfate, extraction of sodium to form NaR * 3HR, and association of the extractant dimers to form a hierarchy of oligomers and polymers. The equilibrium of each of these reactions is governed by a mass-action equilibrium expression. Many aqueous-phase stability constants are reported in the literature, 10 and the sodium-extraction equilibrium constant can be measured separately. Thus, metal-extraction equilibrium data can be fit with only two additional parameters, the metal-extraction-reaction equilibrium constant K and a parameter characterizing the degree of polymerization. The latter parameter must relate the concentration of the reactive extractant species to the total analytical concentration of extractant in solution. The curves in Fig have been computed from such a chemical-reaction model. 9 For an extraction reaction of the form (8.3-3) the equilibrium constant is found to be M" 1 for copper and 1.65 X 10~ 4 Af" 1 for nickel. These values indicate that copper is extracted at a lower ph than is nickel, starting at 2 or 3 compared with 3 or 4. If enough base is used, the extractant can be loaded with either metal, but when both metals are present, and the amount of DEHPA or base is limited, the extractant is quite selective for copper. The individual and simultaneous extractions of both metals as well as stripping are described adequately by the chemicalreaction model. Amine-extraction equilibria can also be modeled by chemical-reaction equilibrium constants. Figure indicates that cations such as iron(iii), zinc, cobalt(ii) and copper(ii) exhibit high distribution coefficients with chloride solutions, whereas nickel, iron(ii), and manganese are not extracted to any great extent. The basis for the differences in distribution coefficients lies mainly in the tendency for the former group of cations to form chloride complexes. Stability constants for these complexes are available in the literature, 11 and they can be used to develop quantitative phase-equilibrium models. For TIOA with hydrochloric acid the concentration-based equilibrium constant for salt formation 12 according to reaction (8.2-6) is 1.51 x 10 4 M~ 2, and the equilibrium constant for amine-hydrochloride salt dimerization 13 is 8.0 M~ x. Combination of these parameters and the ion-complex stability constants with experimental metal-distribution data allows determination of the equilibrium constants for reactions (8.2-5) or (8.2-7). This completes the description of the amine-metal extraction-phase equilibria. For cobalt(ii) in acidic sodium chloride solutions the equilibrium constant 14 for reaction (8.2-7) with TIOA is 2.0 x 10 4 M~ 2, and that for copper(ii) is 370 Af" 2. The corresponding value for zinc 15 is 7.5 x 10* Af" 2 In spite of these relative values, the order of selectivity of TIOA for extraction of the metals is Zn > Cu > Co because of the relative extent of chloride complex formation. For the same reason, zinc stripping is difficult in this system, and copper has a tendency to be reduced to cuprous, which also complexes and extracts extensively.

20 Unfortunately, few of the published studies of extraction equilibria have provided complete quantitative models that are useful for extrapolation of data or for predicting multiple metal distribution equilibria from single metal data. The chemical-reaction equilibrium formulation provides a framework for constructing such models. One of the drawbacks of purely empirical correlations of distribution coefficients is that ph has often been chosen as an independent variable. Such a choice is suggested by the form of Figs and Although ph is readily measured and controlled on a laboratory scale, it is really a dependent variable, which is determined by mass balances and simultaneous reaction equilibria. An appropriate phaseequilibrium model should be able to predict equilibrium ph, at least within a moderate activity coefficient correction, concurrently with other species concentrations. Workers at Oak Ridge National Laboratory have announced the establishment of a Separations Science Data Base, which should be useful in seeking equilibrium data for specific extraction systems EXTRACTION KINETICS Because the metal-extraction separation process involves chemical reactions, rates may be slow compared with ordinary liquid extraction. Although slow kinetics have important process design implications, more attention has been focused on mechanistic interpretations than on quantitative phenomenonological characterization. Cox and Flett 1 have reviewed some chemical aspects of extraction kinetics. One issue in the study of extraction kinetics is the question of whether the metal-extraction reaction, or its rate-determining step, occurs heterogeneously or homogeneously in the aqueous phase. 2 For practical purposes, the locale of the reaction is usually irrelevant because the kinetics of most systems can be fit by a heterogeneous model. In practical extraction systems, the aqueous solubility of the extractant is necessarily very low; organic-to-aqueous distribution coefficients of some common extractants have been determined to be 10 3 and higher. 3 A small homogeneous reaction rate constant combined with very low extractant concentrations in the aqueous phase would provide an extraction rate so slow as to make the process uninteresting. On the other hand, if the homogeneous rate constant is large enough to provide finite extraction rates, the Hatta number 4 for the reaction between metal ion and aqueous extractant will normally be large. The Hatta number is a dimensionless ratio of the homogeneous reaction rate constant to the film mass transfer coefficient. According to the theory of mass transfer with homogeneous reaction, a Hatta number greater than 3 causes the reaction to go to completion within the mass transfer boundary layer. This condition requires only that the pseudo-first-order rate constant be greater than about 10 s~\ whereas reported values of rate constants in "slow" extraction systems have generally been found to be much larger. 5 Furthermore, for a large ratio of bulk metal ion concentration to the interfacial aqueous extractant concentration, the reaction zone is located very close to the liquid-liquid interface, and the extraction rate per unit area of interface becomes independent of the aqueous-film mass transfer coefficient as well as the system volume. Under these conditions a truly heterogeneous reaction and a homogeneous process are indistinguishable in the sense that the rate of each will be a definite function of reactant concentrations in the vicinity of the interface. In either case, the functional form of the kinetics and its parameters must be determined experimentally. The critical question regarding metal-extraction (or stripping) kinetics is whether the chemical reactions that accomplish the phase transfer of the metal are fast or slow relative to the prevailing mass transfer rates. Mass transfer rates may be characterized conveniently in terms of a two-film model, with film mass transfer coefficients being typically on the order of 10" 2 cm/s in liquid-liquid systems. (This value corresponds to an effective stagnant-film thickness of 10~ 3 cm; a homogeneous reaction zone might lie within 10~ 5 cm of the interface for a bulk metal concentration 100 times the aqueous extractant solubility.) Within this context maximum, or mass-transfer-limited, extraction rates, obtained when the extraction reaction maintains equilibrium at the interface, can be computed. For a simple cation-exchange extraction, such as reaction (8.2-3), concentration profiles near the interface are represented by Fig The fluxes at the interface of all four species are related by the reaction stoichiometry. The interfacial concentrations depend on the bulk-solution concentrations, the interfacial metal flux, and the respective mass transfer resistances. Requiring the interfacial concentrations to be in equilibrium according to Eq. (8.3-1) yields the following equation 6 for the metal-extraction rate: (8.4-1) in which the interfacial metal flux N m is an implicit function of bulk concentrations, film mass transfer coefficients k h and the equilibrium constant. Figure is a plot of Eq. (8.4-1) for several combinations of bulk-phase concentrations; the ordinate is the ratio of the metal flux to its value for (M 2+ ) = 0 at the interface and the abscissa represents deviation from equilibrium of the bulk concentrations. Figure demonstrates that the extraction rate is affected by transport of ail reactive species because all transport rates must be considered in estimating interfacial concentrations.

21 ORGANIC PHASE AQUEOUS PHASE FIGURE Concentration profiles near the interface during metal extraction by an acidic extractant. If slow heterogeneous kinetics affect the metal-extraction rate, an appropriate kinetics model should be expressed in terms of interfacial species concentrations, which are obtained from bulk values by correcting for mass transfer resistances. Figure combines an empirical kinetics model 7 for copper extraction by Kelex 100 with a two-film mass transfer model to indicate the relative importance of mass transfer and kinetics in different regions of composition. For this system, the reaction kinetics become fast at higher ph and high extractant and metal concentrations. As the flux approaches the mass-transfer-limited flux, given by Eq. (8.4-1), the interfacial concentrations approach the corresponding equilibrium values. At lower ph and low extractant level, the kinetics are considerably slower. Under these conditions the interfacial concentrations maintain their bulk values. FIGURE Dimensionless extraction flux as a function of distance of bulk-phase concentrations from equilibrium as computed from Eq. (8.4-1). Curves 1 and 2 represent different bulk concentration ratios. Curve P-B represents a pseudobinary calculation where gradients in H + and HR arc neglected. From Ref. 6, with permission.

22 C C I *2 (MOLESAJTER) FIGURE The rate of extraction of copper from sulfate solution by Kelex 100 in xylene. The experimental flux is presented relative to the mass-transfer-limited metal flux for k M2 * = 10" 2 cm/s. From Ref. 6, with permission. There is considerable confusion in the literature about extraction kinetics, and little information directly applicable to process calculations is available. Part of the problem is that many rate studies have been done under conditions that were not well characterized with respect to mass transfer effects or interfacial area so that quantitative heterogeneous rate models cannot be derived. Another problem is irreproducibility caused by unknown impurity levels in extractant solutions. Kinetics are much more sensitive to impurities, additives, and changes in diluent than are equilibrium properties. Finally, it is clear that interfacial phenomena have a strong, and variable, effect on extraction kinetics. Most extractants are surface-active agents as are many modifiers and impurities. Interactions with the surface can affect extraction kinetics directly by blocking sites for heterogeneous reaction steps or providing an interfacial transport resistance, or indirectly by affecting hydrodynamic conditions. It is difficult to control these effects to obtain reproducible and meaningful data. A pragmatic approach to extraction kinetics is to measure extraction rates under mass transfer conditions that are similar to those expected in processing equipment and that are characterized well enough to allow estimation of interfacial concentrations. With such measurements one can determine whether the kinetics of a particular system are fast or slow compared to mass transfer, and if they are slow, the rate can be correlated with interfacial conditions. Several methods are used to measure extraction kinetics; these include measurements with (1) well-stirred vessels, (2) single drops or jets, and (3) the Lewis cell. Rate measurements with well-stirred vessels may be continuous or batch. An example of the latter is simply a stirred flask. The former include the AKUFVE apparatus 8 and bench-scale models of mixer-settler contactors. 9 Because the interfacial area and mass transfer conditions in these systems are not known, they generally do not yield intrinsic heterogeneous rate data for metal-extraction systems. Nevertheless, they are useful for screening relative rates and identifying cases where slow kinetics may be a problem. The advantage of single-drop and liquid-jet experiments is that the interfacial area is known. Also, the hydrodynamics may be simple enough to allow calculation of film mass transfer coefficients. With rising or falling drops, however, adsorption of surfactants can alter the hydrodynamics and possibly interfere with the extraction as well. A growing-drop experiment, 10 analogous to the dropping mercury electrode of polarography, offers the advantages of continuous generation of fresh interface as well as known mass transfer conditions. Laminar-jet measurements 11 offer similar advantages except that surfactants tend to build up on the downstream end of the interface and make the surface rigid. The Lewis cell 12 is a stirred cell with a planar interface between the two liquid phases. Although the hydrodynamics are complex and surfactants can build up on the interface, it is possible to calibrate the mass transfer coefficients empirically. Also, it appears that sufficiently intense stirring can disrupt stagnant surface layers. 13 Although not a great amount of systematic kinetics data are yet available for metal-extraction systems, a few cases have been studied in detail. It is well established that the acidic chelating extractants generally react so slowly compared to ordinary mass transfer rates that special precautions must be taken in process design to provide adequate contacting time. According to the commercial importance of the copper-lix extraction systems, much of the kinetics work to date has been aimed at this system. 14 " 16 All the methods

23 AQUEOUS PHASE ORGANIC PHASE Bulk Stagnant Stagnant Bulk Fluid Film Film Fluid FIGURE Schematic drawing of the species profiles near the interface during the extraction of copper from ammonia solution by an acidic extractant. listed above have been used, but differences in experimental conditions and mechanistic interpretations make quantitative comparison of the results difficult. With alkaline solutions containing ammonia the extraction of copper by LIX 64N becomes mass transfer limited, but extraction of nickel is controlled partly by slow kinetics. 17 From other experiments in which the effects of mass transfer have been analyzed, it appears that the following systems are mass transfer limited: uranyl nitrate extraction by TBP, 18 copper extraction by sodium-loaded DEHPA, 19 and extraction of zinc and copper(ii) chlorides by TIOA. 20 Zinc extraction by dithizone in carbon tetrachloride is mass transfer limited at high zinc concentrations but kinetically controlled at low zinc levels. 21 Ferric ion extractions are reputed to be slow because of its sluggish ligand-exchange kinetics. 22 Extraction of ferric chloride by TIOA, for example, is controlled by a slow heterogeneous reaction. 23 In mass-transfer-controlled systems in which extensive complexing or association takes place in the bulk phases, a proper mass transfer model must account for transport of all species. Otherwise, the transport model will not be consistent with a chemical model of phase equilibrium. For example, Fig indicates schematically the species concentration profiles established during the extraction of copper from ammoniaammonium sulfate solution by a chelating agent such as LIX. In most such cases the reversible homogeneous reactions, like copper complexation by ammonia, will be fast and locally equilibrated. The method of OIander 24 can be applied in this case to compute individual species profiles and concentrations at the interface for use in an equilibrium or rate equation. This has been done in the rate analyses of several of the chloride and ammonia systems cited above. 25 Stripping rates are as important for process viability as extraction rates, but less attention has been paid to their determination. In principle, stripping rates should be related through microscopic reversibility to extraction rates, but far from equilibrium the complete kinetics model is difficult to discern. An interesting aspect of kinetics is the question of how species interact when multiple extraction reactions occur. Most extractants are not perfectly selective and are capable of extracting several species from a multicomponent aqueous solution. Complete equilibrium models should indicate extraction selectivity for a given system at equilibrium, but selectivity may vary considerably in a nonequilibrium process because of rate differences. In fact, iron(iii) can be extracted by some of the copper-chelating extractants, but their effective selectivity for copper is based on the very slow kinetics of iron extraction. Even in mass-transfer-limited processes, excursions in selectivity can be observed at finite contact times. This is predicted by rate models as simple as Eq. (8.4-1) for two metals with different equilibrium constant values. 626 The phenomenon involves initially fast coextraction followed by crowding out of the less preferred metal during competition for extractant. This has been observed during simultaneous extraction of copper and zinc chlorides by TIOA in a growing-drop experiment 10 ' 25 and in extraction of uranyl nitrate and nitric acid by TBP in a Lewis cell, 27 as shown in Fig

24 Time (min) FIGURE Measured and calculated organic-phase concentrations of nitric acid (x) and uranium (O) during coextraction by TBP in a Lewis cell. Concentrations are expressed as a percentage of their equilibrium values. From Ref. 28, with permission. 8.5 CONTACTING EQUIPMENT AND DESIGN CALCULATIONS The processing equipment used to conduct solvent extraction of metals is the same as that used in conventional liquid-liquid extraction. 1 ' 2 The most common choices have been mixer-settlers, columns with agitated internals, and static mixers. Some advantages and disadvantages of several classes of equipment are summarized in Table Many of the practical aspects of equipment selection are discussed by Pratt and Hanson 3 and by Ritcey and Ashbrook. 4 Because detailed models of the chemical effects on phase equilibrium and rates in metal-extraction systems have not been available, standard chemical engineering design procedures have not been applied very extensively in equipment sizing and comparison. Although McCabe-Thiele-type calculations are often suggested for designing equilibrium-stage operations, the results are only approximately correct because the locus of the metal-distribution equilibrium curve changes from stage to stage according to changes in ph and free extractant concentration. The problem is analogous to that encountered with multicomponent distillation or nonisothermal gas absorption in the sense that the state of the system cannot be represented on a two-dimensional plot. Multiple conservation and rate equations must be solved simultaneously to simulate process paths. If the chemistry of an extraction system is well characterized and the equilibria among species and between phases are modeled adequately, equilibrium-stage calculations can be done numerically by solving simultaneously the algebraic equations that describe the equilibria and mass balances in each stage. Such an approach has been demonstrated, 5 and Table shows calculated and experimental interstage concentrations during multistage extraction of copper by Kelex. In such calculations it becomes clear that analytical reagent concentrations in feed streams must be used as independent process variables and that ph is a dependent variable along with other individual species concentrations. Although equilibrium-stage calculations are useful for preliminary design of staged contacting, characterization of equipment efficiency for a particular application requires experimental study. With mixersettlers it is common to vary experimentally the number of real stages used as well as the residence time in each stage. Scale-up then is based on these parameters as well as power input per unit volume and superficial velocity in the settlers. 3 By this approach large-scale copper-lix processes have been designed with three mixers for extraction and two for stripping. 6 * 7 The slow kinetics of the reaction are accommodated by a 2 min residence time in the mixers. Differential contactors can be characterized similarly in terms of transfer units if the equilibria and rates are sufficiently well known. 8 An interesting comparative study of six different contactors is summarized in Fig An alkaline feed stream containing copper, nickel, zinc, and ammonia was contacted with a Kelex 100 solution in each of the pilot-scale contactors. At a ph above 7 the reaction kinetics of Kelex are probably fast, but it loses some of its intrinsic selectivity for copper because of the tendency of nickel and zinc to be extracted. Although the various contactors could be made to operate at comparable levels with respect to specific throughput and copper-extraction efficiency, they exhibited quite different selectivities. Detailed liquidliquid reactor models would help to identify and predict such variations in efficiency and selectivity. With both staged equipment and differential contactors, availability of adequate phase-equilibrium models and rate expressions would allow application of existing correlations and simulation algorithms. For example, knowledge of metal-extraction kinetics in terms of interfacial species concentrations could be combined with correlations of film mass transfer coefficients in a particular type of equipment to obtain the interfacial flux as a function of bulk concentrations. Correlations or separate measurements of interfacial area and an estimate of dispersion characteristics would allow calculation of extraction performance as a

25 TABLE Some Advantages and Disadvantages of Several Classes of Liquid-Liquid Contactors Mixer-Settlers Nonagitated Differential Agitated Differential Centrifugal Advantages Disadvantages Good contacting of phases Can handle wide range offlowratios (with recycle) Low headroom High efficiency Many stages Reliable scale-up Low cost Low maintenance Large holdup High power costs High solvent inventory Large floor space Interstage pumping may be necessary Low initial cost Low operating cost Simplest construction Limited throughput with small gravity difference Cannot handle wide flow ratio High headroom Sometimes low efficiency Difficult scale-up Good dispersion Reasonable cost Many stages possible Relatively easy scale-up Limited throughput with small gravity difference Cannot handle emulsifying systems Cannot handle high flow ratio Will not always handle emulsifying systems, except perhaps pulse column Can handle low gravity difference Low holdup volume Short holdup time Small space requirement Small inventory of solvent High initial cost High operating cost High maintenance Limited number of stages in single unit, although some units have 20 stages Source: From Ref. 4, with permission.

26 TABLE Comparison of Experimental and Calculated Stream Concentrations for Copper Extraction by Kelex 100 in a Three-Stage Countercurrent Cascade of Mixer-Setters Aqueous Copper (g/l) Organic Copper (g/l) Aqueous H 2 SO 4 (g/l) Free Reagent Data Model Data Model Data Model Model Aqueous feed 1st stage 2nd stage 3rd stage Organic feed Source: From Ref. 5, with permission.

27 Total flow (gal/h-ft 2 ) Kenics mixer Pulse sieve plate column Mixco column Mixer-settlers Graesser i -3 i (a) Copper extraction (%) Mixer-settlers g j r J g S Mixco column organic cont. Mixco column aq. cont. Pulse column 60 0 C Pulse column 25 C Kenics 60 C 4 min Kenics25 C 4min Graesser Podbielniak 50 0 C Podbielniak 25 C (b) FIG URE Comparisons of the operating characteristics of several contacting devices for the separation of copper from nickel and zinc by extraction from ammonia (ph 8) solution with Kelex. (a) Comparison of specific throughput at flooding, (b) Comparison of copper extraction efficiency, (c) Comparison of Cu- Zn selectivity, (d) Comparison of Cu-Ni selectivity. From Ref. 9, with permission.

28 Cu/Ni Ratio in loaded solvent /%/»«*. ^ -i. Cu/Zn Ratio in loaded solvent Pulse 25 C column 60 0 C Pulse column Mixer-settlers 60 C-2 stages Mixer-settlers 60 C~2 stages Kenics25 C mixer 60 C 4min Kenics mixer QOQ (c) Mixco aq. cont column org. continuous 25 C Mixco org. cont. 25 C column aq. cont. Graesser Graesser 25 C Pod. 25 C 50 0 C Podbielniak 25 C 60 0 C FIGURE (d) (Continued)

29 function of equipment size. A strategy for developing such contactor models has been outlined, 10 and the approach has been demonstrated with a reaction-controlled" and mass-transfer-controlled 12 extractions in a Kenics static mixer. 8.6 PROCESSDESIGNANDENGINEERING Process flowsheets for separations of numerous metals have been published. 1 " 5 Synthesis and design of such processes for a given feed stream require consideration of the following factors: 1. Choice of Extractant. For separation of a particular metal, an effective extractant must be chosen that has the capability of providing high distribution coefficients and high organic-phase loadings. This choice depends on the chemistry of the metal and the composition of the aqueous feed solution. For example, a chelating extractant may be appropriate for extracting copper from a weakly acidic sulfate solution, but an amine may be required for a solution that has a high level of hydrochloric acid. A practical extractant must also be subject to regeneration. The extraction must be reversible such that stripping is possible with minimal consumption of reagents. Stripping should yield a purified and concentrated form of the metal product. With feed streams that contain several metals, selectivity may be a major concern. To obtain selectivity one may seek a different extractant for each metal that is highly specific by virtue of its equilibrium or kinetics properties, or one may use a single extractant for several metals and design the contacting process to achieve effective fractionation by exploiting quantitative rather than qualitative differences in metal chemistry. For example, in the process 6 represented by Fig TBP is chosen to extract iron from a chloride feed stream containing copper and cobalt. Then copper and cobalt chlorides are both extracted into an amine solution. The separation of cobalt from copper is accomplished by controlling the water-toorganic phase ratios in two separate stripping circuits. Similar strategies are represented by the flowsheets shown in Figs and Process models for multimetal fractionations with a single extractant have been presented 7 for an idealized process chemistry. 2. Choice of Diluent, Modifiers, and Extractant Concentration. The formulation and composition of the organic-phase solution play a major role in determining both the chemical and physical performance characteristics of an extraction system. The use of modifiers and mixed extractants alters both equilibrium and kinetics properties and can have a strong influence on interfacial phenomena. It is particularly important to formulate a system that minimizes organic solubility in the aqueous phase, promotes phase disengagement, and prevents emulsion formation. Problems in any of these areas can quickly render a process uneconomical or infeasible. 3. Choice of Process Configuration. The sequence of processing steps and flowsheet configuration must be selected concurrently with the choice of extraction chemistry. Some of the established heuristics of separation process synthesis 8 may be helpful here. For example, in multimetal fractionation it is probably better to try selective stripping from a single organic stream than to do multiple selective-extraction oper- Feed solution Fe Loading M/S M/S Settler Co-Cu Loading M/S M/S M/S Settler To crystallization Fe Stripping Co-Cu Stripping M/S M/S M/S M/S M/S M/S M/S M/S M/S Water Water Water Fe-eluate storage TBP storage Co-eluate storage Cu-eluate storage TIOA storage M/S = mixer-settler FIGURE Solvent extraction circuit of the Falconbridge Nikkelvert A/S matte leach plant for separation of iron, copper, and cobalt. From Ref. 6, with permission.

30 ations. The former approach, if possible, would minimize handling of the total feed stream volume and presumably reduce solvent losses and reagent consumption. 4. Choice of Contacting Equipment. Although each extraction and stripping operation in a metalsseparation process is normally operated in a countercurrent mode, either stagewise or in differential contact, the large distribution coefficients provided by the chemical-reaction basis of the separations suggest that a large number of stages, or transfer units, is not required. On the other hand, slow kinetics may restrict efficiency factors considerably so that high interfacial areas and residence times may be required. These conditions have favored the use of mixer-settlers. Where kinetics are fast, differential contactors may be preferable because of smaller power requirements and solvent inventory requirements. While efficiency is a factor in equipment selection, mechanical considerations often provide the determining criteria. One must always be sure to minimize solvent losses, and concern about entrainment and emulsion formation can dictate the mode of operation if not the choice of contactor. For example, power input for mixing may be limited or the less viscous phase chosen to be continuous to ensure good phase disengagement. Many of these and other practical aspects of process design and operation, such as appropriate materials of construction, prevention of * 1 CiUd" formation, effluent treatment methods, and typical process costs, are discussed by Ritcey and Ashbrook. 9 If any given extraction system can be characterized with respect to the dominant species appearing and reactions occurring, experimental equilibrium and rate data can be correlated in terms of the species concentrations. Such information can be combined with material balances and available engineering correlations for liquid-liquid systems to generate process models useful for design, optimization, and control. 8.7 SUMMARY Organic reagents are available that can extract metal ions into an organic solvent by virtue of liquid-liquid ion exchange or association reactions. Such reactions are reversible and provide a basis for effective metalseparation processes. Large distribution coefficients are observed, but they vary with system composition. Phase equilibria can be modeled in terms of equilibrium constants for the relevant reactions. Because of low mutual solubilities of the phases, extraction reactions appear to be heterogeneous. Some reactions exhibit slow chemical kinetics, with the reaction step constituting a resistance to extraction in series with intraphase mass transfer. Large-scale solvent extraction of metals is conducted in equipment that is similar to that used for conventional liquid extraction. Nonlinearities in the multicomponent, reactive phase equilibria and possibly slow kinetics complicate design calculations, but existing correlations and methods for treating mass transfer, interfacial phenomena, and dispersion can be used if the effects of process chemistry are property characterized. An extensive technology and a large body of experience have developed from both laboratory studies and commercial process operations. A variety of extractants, equipment, and processes is available for application to a wide range of metals-separation problems. REFERENCES Section Y. Marcus and A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes, Wiley, New York, H. Freiser, CHt. Rev. Anal. Chem., I 9 47 (1970) J. Stary, The Solvent Extraction of Metal Chelates, Pergamon, Oxford, A. K. De, S. M. Khopkar, and R. A. Chalmers, Solvent Extraction of Metals, Van Nostrand, New York, H. A. C. McKay, T. V. Healy, I. L. Jenkins, and A. Nyalor (Eds.), Solvent Extraction Chemistry of Metals, MacMillan, London, J. Korkisch, Modern Methods for the Separation of Rarer Metal Ions, Pergamon, Oxford, J. F. C. Fisher and C. W. Notebaart, Commercial Processes for Copper, in T. C. Lo, M. H. I. Baird, and C. Hanson (Eds.), Handbook of Solvent Extraction, Chap. 25.1, Wiley-Interscience, New York, G. Thorsen, Commercial Processes for Cadmium and Zinc, in T. C. Lo, M. H. I. Baird, and C. Hanson (Eds.), Handbook of Solvent Extraction, Chap. 25.5, Wiley-Interscience, New York, P. J. D. Lloyd, Commercial Processes for Uranium from Ore, in T. C. Lo, M. H. L Baird, and

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