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Principles of Solvent Extraction of Organic and Mineral Acids

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Principles of Solvent Extraction of Organic and Mineral Acids
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  Chapter 2 Principles of Solvent Extractionof Organic and Mineral Acids Chapter Outline 1. Introduction 692. Extraction of Acids byCarbon-BondedOxygen-Donor Extractantsand SubstitutedHydrocarbon Solvents71 2.1. Thermodynamic (MassAction Law) ExtractionEquilibria732.1.1. Dissociation of the Acid732.1.2. Protonation 742.1.3. Dimerization 742.2. Mutual Solubilities andPhase Miscibilities752.3. Parameters of AqueousPhase802.4. Organic PhaseParameters812.5. ThermodynamicFunctions of Transfer83 3. Phosphorus-bonded OxygenDonor Extractants85 3.1. Mass Action LawEquilibria853.2. ExtractionCharacteristics86 4. Aliphatic Amine Extractants 89 4.1. Mass Action LawEquilibria904.2. ExtractionCharacteristics95 5. Extraction of Strong(Inorganic) Acids986. Summarizing Remarks 102 pH effect 103Extractant basicity 103Diluent effect 104Selectivity 104 References 108 1. INTRODUCTION Recovery and concentration of organic acids, as well as separation of acidmixtures, have attracted a great interest among researchers, especially inconnection with their recovery from fermentation broths, reaction mixtures,and waste solutions. Liquid extraction has been proposed as an alternative to Solvent Extraction. DOI: 10.1016/B978-0-444-53778-2.10002-0 Copyright    2012 Elsevier B.V. All rights reserved.  69  the classical precipitation process, and there have been several recent efforts todevelop process technologies based on extraction. 1–8 The existing informationon liquid extraction recovery of the acids is still rather limited in scope.Extraction of acetic, propanoic, lactic, tartaric, succinic, and citric acids hasreceived more attention because of industrial importance. Most of them arefreely soluble in water, some in alcohols, and some in polar solvents.Extraction chemistry is concerned solely with the state of equilibrium inmulticomponent heterogeneous systems. Experience has shown that they arebest treated by Nernst’s distribution law supplemented by sets of mass actionlaw equilibria (for details see Chapter 1). The diversity of the extractionprocesses stems from the type of reaction governing the transfer.The extraction of carboxylic acids may be presented according to threeextraction categories: 1.  Extraction by solvation with carbon-bonded oxygen-bearing extractants.The partition of the acids between their aqueous solutions and inert aliphaticand aromatic hydrocarbons and some of their substituted homologs are alsoincluded here because of the similarity of the partition chemistry involved. 2.  Acid extraction by solvation with phosphorus-bonded oxygen-bearingextractants. 3.  Extraction by proton transfer or by ion-pair formation, with extractantsbeing high-molecular-weight aliphatic amines.The first two categories involve the solvation of the acid by donor bonds of some kind which are to be distinguished from strong covalent bonds and fromionic interactions. The latter is the reaction involved in the third extractioncategory.Conventional extraction systems using water-immiscible alcohols, ketones,or ethers are relatively inefficient for acid recovery from the dilute aqueous acidsolutions found in most fermentation streams. Organophosphorus oxygen-bearing extractants and aliphatic amines are opening new rooms for therecovery of organic acids from a wide variety of aqueous solutions, includingfermentation broths, waste waters, etc.The fundamental chemistry involving all three categories of extractants isknown and defined. The basic mass action law presentations of the equilibriainvolved in the extraction process are given separately for the three categories.The distinction between the first two categories is based on the strength of the solvation bonds and the specificity of solvation. Solvation with alcohols,ethers, and ketones, the typical carbon-bonded oxygen donor extractants, is notbeing specific. The coordinate bonds between the acid hydrogen and theoxygen donor are weak for a specific number of solvating molecules per acid tobe clearly identified.On the other hand, the significantly more basic donor properties of thephosphorus-bonded oxygen move the solvation process to specific, and thenumber of solvating molecules per extracted acid is experimentally accessible. 70  PART | I  Conventional (Classical) Principles and Practice  The specific behavior of the various classes of phosphorus and amine extrac-tants, their compatibility with common diluents, and the basic chemistryinvolved in the process of acid extraction are aspects that have been extensivelyinvestigated, reviewed, presented in the chemical literature, 9–13 and discussedin this chapter. 2. EXTRACTION OF ACIDS BY CARBON-BONDEDOXYGEN-DONOR EXTRACTANTS AND SUBSTITUTEDHYDROCARBON SOLVENTS Extraction of carboxylic acids into inert noninteracting hydrocarbons andsubstituted hydrocarbons is considered with oxygen-bearing organic solvents.In spite of the substantial difference in the extractive capacity between thesetwo types of solvents, the same mass action law description of the processapplies to both.When acids do not react with other components and are not protonated,dissociated, polymerized, and hydrated, the distribution ratio of the acid  D HL  isconstant. The distribution constant  K  D,HL  equals the measured distributionratio. When HL is used for the extraction of a metal,  K  D,HL  is abbreviated  K  DE as the distribution constant of extractant.Figure 2.1a shows the effect of aqueous salt concentrations on the  D HL value of acetylacetone at constant total HL concentration and pH. The salt hastwo effects: (a) it ties up H 2 O molecules in the aqueous phase (forminghydrated ions) so that less free water is available for solvation of HL; and (b)it breaks down the hydrogen bond structure of the water, making it easier forHL to dissolve in the aqueous phase. Figure 2.1 shows that the former effectdominates for NH 4 Cl, while for NaClO 4  the latter dominates. Increase of thedistribution ratio with increasing aqueous salt concentration is described asa  salting - out   effect, and the reverse as a  salting - in  effect. Figure 2.1b shows  D HL  for the extraction of acetylacetone into CHCl 3  and C 6 H 6  for two constantaqueous NaClO 4  concentrations at pH 3, but with varying concen-trations of HL. Acetylacetone is infinitely soluble in both CHCl 3  and C 6 H 6 ; at[HL] org  ¼  9 M, about 90% of the organic phase is acetylacetone (  M  w  ¼  100),so the figure depicts a case for a changing organic phase. Polar CHCl 3 interacts with HL, making it more soluble in the organic phase; it is alsounderstandable why the distribution of HL decreases with decreasingconcentration (mole fraction) of CHCl 3 . C 6 H 6  and aromatic solvents do notbehave as do most aliphatic solvents: in some cases the aromatics seem to beinert or even antagonistic to the extracted organic species, while in othercases their pi-electrons interact in a favorable way with the solute. For ace-tylacetone, the interaction seems to be very weak. The salting-in effect is seenin both Figs. 2.1a and 1b.The distribution of an acid between water and a nonpolar hydrocarbon isnonreactive and can be regarded as physical distribution according to the 71 Chapter | 2  Principles of Solvent Extraction of Organic and Mineral Acids  (a) D (NH 4 Cl)KCl(NH 4 NO 3 )NaNO 3 NaClO 4 mol/lit (b) 2.01.51.00.502345678910CHCl 3 0.1 M NaClO 4 1.0 M0.1 MC 6 H 6 K  D 1.0 M1 2 3 4 5 6 7 8 9[HA] org M2418126 FIGURE 2.1  Distribution ratio  D HA  of undissociated acetylacetone. (a) Distribution betweenbenzene and aqueous phase containing different inorganic salts at 25  C. (b) Distributionbetween CHCl 3  (upper curves) or C 6 H 6  (lower curves) and aqueous phase 0.1 and 1.0 M NaClO 4 as a function of [HA] org . The uncertainty at the lowest  D  values is   1 for CHCl 3  and   0.2 forC 6 H 6 . 72  PART | I  Conventional (Classical) Principles and Practice  Nernst’s distribution law. But account has to be made for partial dissociation of the acid in the aqueous phase, its usual protonation, and its extensive dimer-ization (aggregation, polymerization) in the organic phase. On the other hand,acids extracted by carbon-bonded oxygen donor solvents are strongly hydratedby varying numbers of water molecules. The exact solvation number of the acidmolecules in the organic phase is usually undetermined, but it is known thata large number of solvent molecules are needed for an efficient competitionwith water molecules that hydrate the acid at the interface. 2.1. Thermodynamic (Mass Action Law) Extraction Equilibria The set of mass action equations describing the transfer of a weak (monobasic)organic acid from water into an organic phase with which the acid does notinteract chemically must be taken into account.  2.1.1. Dissociation of the Acid  One of the properties have to be taken into account is the ionization (dissoci-ation) of the acid in the aqueous solution. When the acid dissociates in theaqueous phase, the distribution ratio becomes:HL  4  H þ þ  L  ;  K  a  ¼ ½ H þ ½ L  ½ HL  AB    !  (2.1)  D HL  ¼ ½ HL  o ½ HL  w ½ L   w (2.2)where the aqueous (w) and organic (o) phases in the denominator are concen-trations of all acids––as anions and as undissociated in the aqueous phase.Equation (2.2) can be given as  D HL  ¼  K  D ; HL 1  þ  K  a ½ H þ  ð w Þ (2.3)If acid is extracted as a solute in its undissociated form HL distributionconstant is  K  D,HL . If the acid is used as extractant  K  D,HL  is identical to distri-bution constant of the extractant,  K  DE .At low proton concentration (high pH), distribution ratio becomes inverselyproportional to proton concentration and dissociated form of acid increases.The concentration of the free anion, [L  ], is an important parameter in theformation of complexes at extraction of metals by acidic extractants, referred toas the  free ligand concentration  and can be calculated bylog ½ L   ¼  log  K  a    log ½ H þ  þ  log  C  0HL    log  K  D ; HL  þ  V  w V  o  1  þ  K  a  H þ  (2.4) 73 Chapter | 2  Principles of Solvent Extraction of Organic and Mineral Acids
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