R.K. Biswas

Solvent extraction of Fe3+ from chloride solution by D2EHPA in kerosene

Abstract The solvent extraction of Fe 3+ from chloride solution by di-2-ethylhexylphosphoric acid (D2EHPA, H 2 A 2 ) dissolved in kerosene has been investigated over a wide range of aqueous acidity as a function of phase contact time, Fe 3+ , HCl, H + and Cl − concentrations in aqueous phase, D2EHPA concentration in organic phase and temperature. The dependencies of the distribution coefficient on the concentrations of Fe 3+ , H + , Cl − and D2EHPA vary with the extraction conditions used which indicates that the composition of Fe 3+ species in both the aqueous and the organic phases vary with the HCl or Cl − concentration in the aqueous phase. The experimental results are best fitted by the following equation: D=K 1 x 0 [ H 2 A 2 ] 3 ( O ) [ H + ] 3 +K 2 x 1 [ H 2 A 2 ] 2 ( O ) [ H + ] 2 +K 3 x 2 [ H 2 A 2 ] 1.5 ( O ) [ H + ] +K 4 x 3 [ H 2 A 2 ] 1.5 [ HCl ] where, x 0 , x 1 , x 2 and x 3 are the fractions of Fe 3+ , FeCl 2+ , FeCl 2 + and FeCl 3 present in the aqueous phase at the particular HCl/Cl − concentration used; the values of K 1 , K 2 , K 3 and K 4 are found to be 1.315, 15, 12 and 320, respectively. The treatment of the temperature dependence data gives the apparent enthalpy changes (Δ H ) of 125, 104, 20.8 and 33.3 kJ/mol for the extraction of the species Fe 3+ , FeCl 2+ , FeCl 2 + and FeCl 3 , respectively. The loading of D2EHPA in the organic phase by Fe 3+ has also been investigated and the structures of the maximum loaded extracted species obtained at 0.05 and 1.0 M aqueous HCl concentrations have been demonstrated to be [FeCl(H 2 O)A 2 ] and [FeCl 2 ·A·(HA) 2 ·(HA·HCl) 2 ], respectively.

Solvent extraction of zirconium(IV) from chloride media by D2EHPA in kerosene

Abstract The apparent equilibrium of extraction of zirconium(IV) from chloride media by D2EHPA in kerosene is found to depend on ageing of the aqueous phase. When the aqueous phase, just after preparation, is used in the extraction study, reproducible results are not obtained. With increasing ageing time, the apparent distribution ratio ( D ) is increased for a particular set of experimental parameters. The system has been thoroughly investigated for 1- and 30-day ageing. In both cases, the apparent equilibration time is 15 min. D is decreased with increasing Zr(IV) concentration in the aqueous phase but the nature depends on ageing. In both cases of ageing, log D vs. −log[HCl] plots are peculiar in nature. For 1-day ageing, the slope is ∼−1 above 3 M HCl and also below 0.3 M HCl, and is zero within 0.3–3.0 M HCl. In contrast, for the 30-day aged system, the slope is ∼−2 above 2 M HCl, ∼−1 below 0.5 M HCl, and zero between 0.5 and 2.0 M HCl. The extractant dependencies are 2 and 4 for 1- and 30-day aged, systems respectively. Chloride ion has a large effect on D for the 1-day aged system, but no effect on D for the 30-day aged system. From the temperature dependence data, Δ H values have been determined. The possible extraction equilibrium reactions have been suggested and supported by the loading tests, as well as by the molecular weight, Zr/P ratio, Zr/Cl ratio and infrared spectral data of the extracted species.

Kinetics of backward extraction of Mn(II) from Mn-D2EHP complex in kerosene to hydrochloric acid medium using single drop technique

Abstract The extraction equilibrium constant, K ex for the extraction of Mn(II) from hydrochloric acid medium by di-2-ethylhexylphosphoric acid (D2EHPA or H 2 A 2 ) in kerosene (mostly aliphatic) has been determined to be 10 −2.16 . The rate of backward extraction of Mn(II) from Mn-D2EHP complex dissolved in kerosene by hydrochloric acid has been measured by the single rising drop technique and it is found that the flux equation at 30 ± 1°C is: J (kmol/m 2 s) = 10 −4.08 [Mn-D2EHP] (o) (1 + 0.011[H + ] −1 ) −1 (1 + [H 2 A 2 ] (o) 0.5 ) −1 . Analysis of this rate equation suggests that the process is almost chemically controlled at lower aqueous acidity and higher free D2EHPA concentration regions; whereas it is almost diffusion controlled at higher aqueous acidity and lower free D2EHPA concentration regions. In the investigated D2EHPA concentration (0.04-0.50 kmol/m 3 ) and aqueous acidity (pH = 1.0–3.0) regions, the process is intermediate controlled. The activation energy depends on the back-extraction parameters and is of the order of ∼ 20–∼ 40 kJ/mol. This low value of activation energy supports the conclusion that the process is intermediate controlled. Temperature dependence data give ΔH ± and ΔS ± values of 20–40 kJ/mol and (−180)–(−250) J/K mol, respectively. The high negative value of ΔS ± suggests that the chemical controlling step occurs via S N 2 mechanism.

Study of kinetics of forward extraction of Fe(III) from chloride medium by di-2-ethylhexylphosphoric acid in kerosene using the single drop technique

Abstract The kinetics of the title system have been investigated as functions of Fe3+, H+ and Cl− concentrations in aqueous phase, D2EHPA (di-2-ethylhexylphosphoric acid, H2A2) concentration in organic phase and temperature using the single rising drop technique. The reaction order wrt [Fe3+] is always unity. The logarithm of flux vs. −log[HCl] plots pass through a minimum at 0.5 M HCl with a limiting slope of −2.4 in the high concentration region (hcr) of HCl and +1 in the low concentration region (lcr) of HCl. This effect is due to the combined effect of H+ and Cl−. At lcr of Cl−, the rate is inversely proportional to [H+], whereas at hcr of Cl− the rate is directly proportional to [H+]. The exponent of the [Cl−] term in the rate equation is found to vary within 0–1.2 on varying [H+] from lower to higher values. When the aqueous acidity is kept low, the rate is directly proportional to [D2EHPA] and at higher acidities of aqueous phase, the rate is independent of its concentration. A rate equation involving the extractions of Fe3+, FeCl2+, FeCl2+ and FeCl3 has been derived empirically to describe the system. The treatment of the temperature dependence data on rate gives the activation energies (Ea) of 76, 63, 15 and 10 kJ/mol for the extraction of species Fe3+, FeCl2+, FeCl2+ and FeCl3 respectively. Similarly, the enthalpy (ΔH±) and entropy (ΔS±) of activation have been calculated to be 76 kJ/mol and −130 J/K mol, 55 kJ/mol and −140 J/K mol, 20 kJ/mol and −392 J/K mol, and 6 kJ/mol and −280 J/K mol, respectively, for the four species. The mechanisms of extraction particularly of the rate-determining steps have been suggested.

Kinetics of forward extraction of manganese(II) from acidic chloride medium by D2EHPA in kerosene using the single drop technique

Abstract The rate of forward extraction of Mn(II) from acid chloride medium by di-(2-ethyl hexyl) phosphoric acid (D2EHPA, H2A2) dissolved in kerosene has been measured by the single rising drop technique. The rate of mass transfer per unit area is found to be proportional to [Mn(II)] in its lower concentration region, [H2A2](0) and [H+]−1 at the interface. The forward extraction rate constant, kf, has been measured to be 10−6.35 m/s. From the temperature dependence of rate measurements, the values of Ea, ΔS±, ΔH± and ΔG300± are calculated to be 7.6 kJ/mol, −224 J/mol K, 7.6 kJ/mol and 81 kJ/mol, respectively, at temperatures > 303 K and 77 kJ/mol, −15.3 J/mol K, 77 kJ/mol and 81 kJ/mol, respectively, at lower temperatures. The rate expression can be written as; J, kmol m 2 s = 10 −6.35 [Mn 2+ ][H 2 A 2 ] (0) [H + ] −1 Analysis of the rate expression suggests that the formation of [Mn(HA2)]+ at the interface from Mn2+ and HA2− is the rate-determining step in the forward extraction of Mn(II). Negative entropy of activation suggests that the above rate-determining step occurs via the SN2 mechanism.

Some physico-chemical properties of D2EHPA: Part 2. Distribution, dimerization and acid dissociation constants in n-hexane/1 M (Na+,H+)SO42− system, interfacial adsorption and excess properties

From the distribution data for di-(2-ethylhexyl)phosphoric acid (D2EHPA, RH) between n-hexane and 1 M (Na+,H+)SO42−, the equilibrium constants of D2EHPA for dimerization, distribution and acid dissociation constants were determined as K2=3.17×104 m3/kmol, Kd=3.17×103 and Ka=4.47×10−2 kmol/m3, respectively. Using the interfacial tension data, the cross-sectional areas (A) of D2EHPA molecules adsorbed at the interfaces were measured for the 1 M (Na+,H+)SO42−–RH in different diluent systems and found to vary in the following order: cyclohexane (88.4 A2)<n-hexane (94.7 A2)<kerosene (113.7 A2)<carbon tetrachloride (137.8 A2)<o-xylene (172.4 A2)<toluene (186.3 A2)<benzene (207 A2)<chloroform (240 A2)<amyl alcohol (423 A2). This order has been correlated with the Ti(IV)-extraction data. The extraction of Ti(IV) was high when A was small. Interactions of D2EHPA with n-hexane, n-octane, cyclohexane, carbon tetrachloride, toluene and nitrobenzene were studied in terms of excess properties, i.e. excess molar volume (VmE), excess viscosity (ηE), excess Gibbs free energy of activation of flow (GE). Variation of these values and the interaction parameters (d) for D2EHPA–diluent binary mixtures showed that D2EHPA interacts with diluents in varying degrees. However, no correlation between d, GE, ηE or VmE and the metal extraction characteristics was noticed.

Kinetics of VO2+ extraction by D2EHPA

Abstract The kinetics of the forward extraction of VO2+ from sulphate–acetate medium by di-2-ethylhexylphosphoric acid (D2EHPA) in toluene has been investigated with Hahn and Lewis cell techniques using both (rate/area) or, pseudo-rate constant (q) and flux (F) methods for data treatment. It has been found for both Lewis and Hahn cell experiments that the product of the rate constant obtained from the (q) method and the volume of each phase taken in m3 almost equals the rate constant obtained from the (F) method. It is concluded that kfF values are absolute, whereas kfq is organic phase volume dependent. The kf value obtained from the Hahn cell measurements is 60 times lower than the Lewis cell kf values. The reason for this variation has been discussed. Analyses of the rate expressions suggest the following reaction step occurring in the aqueous film of the interface: VO2++A−→VOA+ as rate-determining. The activation energy in the higher temperature region, it is for a diffusion control reaction, whereas in the lower temperature region, it is for an intermediate control reaction. The entropy of activation, ΔS± suggests that the rate-determining step proceeds through SN1 mechanism.

Kinetics of solvent extraction of zirconium(IV) from chloride medium by D2EHPA in kerosene using the single drop technique

Abstract The kinetics of the title process have been investigated by the single drop technique for 1-day ageing of the aqueous phase. The flux equations have been derived for three aqueous acidities of 0.10, 1.00 and 5.00 M HCl, respectively, as: J (kmol m−2 s−1)=10−5.37 (1+0.00036 [Zr(IV)]−1)−1 [H+] [H2A2](o) (1+0.69 [Cl−]), J (kmol m−2 s−1)=10−5.77 (1+0.0056 [Zr(IV)]−1)−1 [H2A2](o) [Cl−] and J (kmol m−2 s−1)=10−6.62 (1+0.0056 [Zr(IV)]−1)−1 [H+] [H2A2](o) [Cl−]. The values of Ea (kJ mol−1), ΔH± (kJ mol−1) and ΔS± (J K−1 mol−1) for each of the above conditions have been estimated to be 8.6, 7.7 and −425; 19.2, 16.8 and −369; and 26.4, 23.4 and −330, respectively, in the higher temperature region (h.t.r) of investigation; and 89.6, 76.6 and −192; 69.5, 71.9 and −187; and 59.9, 60.1 and −205, respectively, in the lower temperature region (l.t.r). On the basis of these rate data, the mechanisms of extraction in different conditions have been suggested.

Kinetics of extraction and stripping of Ti(IV) in HCl–D2EHPA–kerosene system using the single drop technique

Abstract Kinetics of the forward extraction of Ti(IV) and stripping of Ti-D2EHP chelate in HCl–di-2-ethylhexylphosphoric acid (D2EHPA, H 2 A 2 ) kerosene system have been investigated using the single drop falling technique. In the lower concentration region of Ti(IV), the rate of forward extraction at 303 K can be represented by: J f (kmol m −2 s −1 )=10 −4.02 [Ti 4+ ][H 2 A 2 ] (0) [H + ] −1 and this suggests that the rate-determining step is: TiO (i) 2+ +HA 2(i) − →[TiO(HA 2 )] (i) + . In the higher concentration region of Ti(IV), the rate of forward extraction expression at 303 K is represented by: J f (kmol m −2 s −1 )=10 −6.06 [H 2 A 2 ] (0) [H + ] −1 ; and this suggests that the diffusions of H 2 A 2 to and of H + from the interface are rate-determining. The activation energy ( E a ) of 98±3 kJ/mol is decreased to 18±4 kJ/mol on increasing Ti(IV) concentration from 2.08 to 41.6 mM. The Δ H ± and Δ S ± values in forward extraction have been evaluated. The rate of stripping of Ti(IV) at 303 K is governed by: J b (kmol m −2 s −1 )=10 −8.67 [Ti–D2EHP] 3(0) 1/3 (1+22.4[H 2 A 2 ] (0) ) −1 [H + ]. The rate-determining steps have been assigned in both lower and higher concentration regions of H 2 A 2 . In the higher concentration regime of D2EHPA, the values of E a , Δ H ± and Δ S ± have been estimated to be 94 kJ/mol, 92 kJ/mol, and −108 J/K mol respectively. The equilibrium constant obtained from the rate study seems comparable with that obtained from the rate studies.

Solvent extraction of tetravalent titanium from chloride solution by di-2-ethylhexyl phosphoric acid in kerosene

The solvent extraction of Ti4+ from chloride solution by di-2-ethylhexyl phosphoric acid (D2EHPA, H2A2) dissolved in kerosene has been investigated over a wide range of aqueous acidity as a function of phase contact time, Ti4+, H+ and Cl− concentrations in aqueous phase, D2EHPA concentration in organic phase and temperature. The equilibrium is reached within 2 h. The distribution coefficient is found to be independent of Ti4+ concentration at least up to 1 g/l Ti4+ in the aqueous phase. The pH and log extractant concentration dependences are approximately −2 and 2, respectively. The extraction is independent of Cl− concentration. These results suggest that extraction occurs via the following reaction: TiO2++2H2A2(O)⇌[TiO(HA2)2](O)+2H+. The temperature dependence data give ΔH values of 30 kJ/mol in the higher temperature region (>30°C) and 78 kJ/mol in the lower temperature region (∼15°C) under investigation. The equilibrium constant, Kex at lower loading is 1×103.95 with a standard deviation of log Kex of 0.023. The loading capacity of D2EHPA for Ti4+ is 7.31 g Ti(IV)/100 g D2EHPA. The loading study suggests that a variation of the extraction equilibrium reaction occurs during the course of loading. At high loading Ti4+ is extracted by the reaction: TiO2++H2A2(0)⇌[TiOA2](0)+2H+. The structure of the solid complex prepared from the high loaded organic phase has been established by determining the Ti/P ratio and using infrared spectroscopy.