M.A. Hughes

The kinetics of vanadium (IV) extraction in the acidic sulphate-D2EHPA-n-hexane system using the rotating diffusion cell technique

Abstract The kinetics of the extraction of vanadium (IV) from acidic sulphate solutions by di-2-ethylhexyl-phosphoric acid (D2EHPA or H 2 R 2 ) dissolved in n-hexane have been investigated over a range of vanadium concentrations, extractant concentrations, pH, sulphate concentrations and temperature conditions. The rotating diffusion cell technique was employed. The experimental data have been analysed in terms of a mechanism of mass transfer with chemical reaction (MTWCR) using either the monomer or dimer models for D2EHPA. The data have been fitted to the generalised MTWCR model of Hughes and Rod using the kinetic and equilibrium parameters. The data are best fitted using the model for the monomer of D2EHPA reacting in a thin aqueous film at the interface and the value of the kinetic parameter, Θ 1 , (10 4 μ m 2 s −2 ) indicates that the extraction is under mixed control. The activation energy for the system, in the temperature range 297–309 K depends slightly on the pH of the aqueous solution and varies within 49-34 kJ mol −1 .

Synthesis and characterisation of diamide extractants for the extraction of neodymium

Abstract The aim of this study was to prepare a series of amide extractants and to assess their properties as potential metal extractants for the rare earth metal neodymium. A number of diamides and a monoamide were synthesized with different substituting groups, including alkyl chains, cyclohexyl chains and aryl chains. The general method for synthesizing these extractants was to react relevant acyl chlorides with secondary amines in a basic medium. The yields for diamides were in the range 32–50% and the yield was 90% for monoamide. A systematic investigation was carried out on the extraction properties of the extractants synthesized using the neodymium ion Nd 3+ to monitor the extraction of a typical trivalent rare earth species. The effects of the variables: diluent type, structure of diamides, aqueous acidity and salting-out agents, on the extraction of Nd 3+ are discussed. The effect of aqueous acidity on the distribution ratios proved that there was competition between the metal ion and HNO 3 for the diamide coordination sites. When the nitric acid concentration in the aqueous phase was varied, maximum extraction was observed in the region of 3 M HNO 3 for all eight diamides investigated. From the results it can be concluded that the diamides are effective extractants for neodymium in the region of 3 M HNO 3 . The neodymium can be stripped by very dilute HNO 3 .

The mechanism of vanadium (IV) extraction in a chemical kinetic controlled regime

Abstract The rate of vanadium (IV) extraction from sulphuric acid medium by di(2-ethylhexy) phosphoric acid in n-hexane diluent was investigated. The liquid mixture was stirred at a speed at which the influence of diffusion on kinetics was believed to be a minimum. The overall rate of mass transfer was found to be first order in aqueous vanadium concentration, inverse first order in aqueous hydrogen ion concentration and a half order with respect to the concentration of the extractant in the organic phase. The rate constant at 28°C is 2.8×10 −3 mol 1 2 s −1 and the activation energy is 64 kJ mol−1. A mechanism is proposed for the extraction kinetics.

Degradation in hydroxyoxime solvent extraction systems

Abstract The factors that influence the loss of oxime extractants by degradation mechanisms have been studied under conditions chosen so as to magnify these effects but far from those pertaining to present commercial practice. No decisions vis-a-vis choice of a commercial extractant should therefore be based upon these results. The reaction of mineral acid with oximes is shown to be a major factor in the degradation of oximes. LIX 63 degrades in the organic phase, probably by the hydrolysis of an acidic species such as HLIX63+HSO4− · nH2O. The process is enhanced by aqueous phase additives which increase the proton activity, reduced by copper complexation of the oxime, and is largely unaffected by diluents and other organic phase additives (except nonylphenol). There is also evidence of degradation occurring even in the absence of added acid when LIX 63 is complexed with copper at pH values around 2–3. SME529 also decomposes in the bulk organic phase; the degradation is faster than for LIX63. In contrast, LIX65N and P5100 degrade by an interfacial process, which can be accelerated by factors (such as alteration of the nature of the diluent or the addition of a second oxime) which increase water or acid uptake into the organic phase. Degradation of P5100 is faster than that of LIX65N. The degradation of kerosene (Escaid 100) is also shown to affect kinetic performance in solvent extraction adversely.

The modelling of equilibrium data for the liquid-liquid extraction of metals part III. An improved chemical model for the copper/LIX 64N system

Abstract New data are presented for the copper sulphate—sulphuric acid/LIX 64N—Escaid 100 solvent extraction system, and it is shown that these data do not exhibit the anomalous features of Robinsons (1971) results. The data are modelled using equations based on the thermodynamics of the Cu2+, CuSO4, H+, HSO4−, SO42− aqueous system and the HR, (HR)n, CuR2 organic sytem (where HR represents the active β-hydroxyoxime in LIX 64N). Although the values of the two aqueous and the two organic equilibrium constants cannot be held to have thermodynamic significance, the best models are capable of predicting the organic copper concentrations of the 86 data points at four different concentrations of LIX 64N (5–50 vol.%) from the aqueous phase analyses with average errors as low as 7%. Isotherms for desired aqueous feedstocks and desired concentrations of our batch of LIX 64N in Escaid 100 at 25–26°C can be calculated.

Rates of extraction of cobalt from an aqueous solution to D2EHPA in a growing drop cell

Abstract A technique has been developed to study the rate of extraction of cobalt from an aqueous phase into a growing drop of heptane containing di(2-ethylhexyl) phosphoric acid (D2EHPA). The hydrodynamics of the growing drop are accounted for. Experiments have been carried out at the single temperature of 25 ± 0.1°C Sphere the pH, aqueous cobalt concentration and the organic D2EHPA concentration have been varied. A mathematical model is given which is based upon the control of rate by mass transfer with chemical reaction. The model fits the experimental data well considering the experimental errors and the complexity of the transfer of solute where fresh surfaces are continually produced by the growing drop.

The modelling of equilibrium data for the liquid-liquid extraction of metals: Part II. Models for the copper/LIX 64N and chromate/Aliquat 336 systems

Abstract The equilibrium data for two examples of metal extraction are considered. The copper/LIX 64N system is an important one in that category involving the solvent extraction of acid leach liquors. In the case of alkali leach liquors the chromate/Aliquat 336 system provides the example. The data for each of these systems have been tested against various models. For the copper/LIX 64N system it has been shown that a polynomial gives the best fit, a modified chemical model is however only slightly worse. In the case of the chromate/Aliquat 336 system the chemical model is better than the polynomial. Parameters are reported for the best fits to each of the models tested and a computer display of a copper/LIX 64N surface is given.

The modelling of equilibrium data for the liquid-liquid extraction of metals Part I. A survey of existing models

Abstract The various approaches which have been made to model equilibrium data for the extraction of metals in liquid-liquid systems have been reviewed. The limitations of any one model are highlighted. Those models based on the known chemistry of the process can be used with confidence to extrapolate data outside an experimentally measured region of the equilibria but models with as good an, if not better, accuracy can be based on a generalised polynomial and this method does not require a detailed chemical study to be made.

The separation of zinc from copper by di-2-ethylhexyl phosphoric acid - an equilibrium study

Abstract The extraction of copper and zinc from sulphate media by di-2-ethylhexylphosphoric acid (D2EHPA) is reported. The equilibrium data for the extraction of (1) zinc alone (2–40 g/l), and (2) copper alone (2–60 g/l) by both 10 v/v % and 20 v/v % D2EHPA are given. The general case of extraction of two metals which compete for the same extractant is considered, and the data for the binary metal system zinc/copper extracted by 20 v/v % D2EHPA are reported. Specific chemical models and empirical models have been developed to correlate the data. Because the extractant has a high affinity for zinc in the presence of copper, the binary data can be modelled on the assumption that the interaction of copper with zinc can be neglected. However, the effect of zinc in reducing the extraction of copper is significant. We show that zinc can be decontaminated from copper and test the predictions with a laboratory scale counter current apparatus. Typical predictions show that in five stages, a 20 v/v % D2EHPA feed and an aqueous feed of 1.98 g/l Zn, 36.78 g/l Cu, zero g/l H2SO4 would give a 89.9% recovery of zinc with purity of 90%; with lower copper to zinc ratios in the feed the purity may be improved.

The mechanism of extraction of HNO3 and neodymium with diamides

Abstract A series of N , N , N ′, N ′-tetraalkylmalonamide (MA) compounds were prepared and suitable structures for Nd 3+ complexes extracted from concentrated HNO 3 aqueous solution were selected. Extraction of HNO 3 and Nd 3+ by N , N , N ′, N ′-tetraalkylmalonamides has been investigated to provide the mechanisms which were supported by infrared spectroscopy study. The extraction mechanisms vary with the change in the aqueous acidity. The HNO 3 was extracted as HNO 3 ·(MA) 2 , HMA + NO 3 − or HNO 3 ·MA and HMA + NO 3 − ·HNO 3 . The Nd 3+ was extracted as Nd(NO 3 ) 3 ·(MA) 2 , Nd(NO 3 ) 3 ·(MA) 3 and Nd(NO 3 ) 3 ·(MA) 3 MA. From the above results six possible structures for MA complexes are proposed. The infrared spectra of the above extractants were examined. The carbonyl and CN bond stretching regions were monitored for changes as nitric acid or Nd 3+ was extracted. It was found that the carbonyl groups are the primary sites for complexing the metal, as predicted from the equilibrium investigation. During the loading of the Nd 3+ into the organic phase the extractants behave as a bidentate ligand. The difference in the spectra in neutral or acidic media suggests that the extraction mechanism differs under different aqueous conditions.