STUDIES ON LIQUID-LIQUID EXTRACTION OF MOLYBDENUM, TUNGSTEN AND VANADIUM USING ALKYLPHOSPHINE EXTRACTANTS

Abstract

The industrial growth of a country to a larger extent can be assessed by the production and consumption of the metals and their compounds. There is an ever-increasing pressure on the production of metals thereby resulting into the depletion of richer metal deposits. This has necessitated the demand to develop efficient extraction procedures to recover metals from their low grade ores. With the increasing inflation in the prices of metals it may not possible to waste them and hence extraction procedures have to be designed to recover them from waste streams, spent catalysts, metal scraps and other similar metal bearing matrices. The recovery of metals from the secondary sector will not only improve the economy but partly mitigate the problem of metal pollution. As the science has grown in its dimensions the purity of the metals has assumed a paramount importance. In the recent years there has been unparalleled growth in certain high-tech areas and therefore some metals molybdenum, tungsten and vanadium, with a reasonable degree of purity, are in great demand. Solvent extraction technology plays a major role in the requirements of recovery of pure metals from their lower concentrations. Solvent extraction is basically a phase distribution phenomenon in which a solute distributes itself between two immiscible liquids as governed by Nernst's partition law. Solvent extraction enjoys a favoured position among the separation techniques owing to its convenience, speed, versatility and relatively low cost. Generally the technique can be safely extended from micro levels to macro concentrations. The utility of an extraction system is assessed by its efficiency to achieve mutual separations of closely associated metal ions. The study of the solvent extraction equilibria can also throw light on the metal complex formation. Liquid - liquid extraction has provided a new dimension to separation science in the form of extraction chromatography wherein the extraction data help to develop reverse phase column chromatographic procedures. The main advantage of extraction chromatography is the incorporation of multistage character of column chromatography. Because of enormous potential of the technique of solvent extraction different types of extraction systems have come up on the forefront and the search for more efficient extractants still continues. With the advent of transplutonium chemistry many organophosphorus compounds have appeared on the scene of hydrometallurgy as extractants. During the later half of the twentieth century voluminous literature has accumulated on some of the alkylphosphorus extractants namely tributylphosphate (TBP) and di(2- ethylhexyl)phosphoric acid (DEHPA). Amongst the organophosphines, tri-noctylphosphine oxide (TOPO) has been studied far more thoroughly than all the others. One of the most exciting and recent developments in solvent extraction chemistry is the introduction of some new organophosphines by American Cyanamid Company, USA, under the trade name 'CYANEX'. The list includes a phosphinic acid and phosphine oxides and their sulphur analogues. All of these reagents differ from other commercial organophosphorus extractants in that the alkyl groups are bonded directly to the phosphorus atoms through P-C bonds rather than P-O-C bonding which exists, for example, in TBP and DEHPA. This tends to make these phosphine derivatives more resistant to hydrolysis and less water soluble than other reagents. In the last few years some efforts have been made to explore Cyanex reagents as extractants. Cyanex 302 [bis(2,4,4-trimethylpentyl)monothiophosphinic acid] and Cyanex 301 [bis(2,4,4-trimethylpentyl)dithiophosphinic acid] which are mono- and disulphur analogues of Cyanex 272 [bis(2,4,4- (ii) trimethylpentyl)phosphinic acid] have lower pKa values thus enabling the extractants to work at a lower pH. Cyanex 925 is a mixture of two organophosphine oxides, bis(2,4,4-trimethylpentyl)octylphosphine oxide and tris(2,4,4-trimethylpentyl)phosphine oxide present in 85:15 ratio, respectively. The presence of highly branched alkyl chain in the major constituent of this extractant may introduce selectivity. In the light of above discussion Cyanex 301, 302, 925 and 272 have been explored as extractants for Mo(VI), W(VI) and V(IV). The effect of various variables such as the nature of diluent, type of mineral acid and the concentration of the acid, metal ion and the extractant on distribution data has been studied. The loading and recycling capacities of the extractants have been determined and the stoichiometry of the extracting species proposed. Based on the partition data some binary separations of analytical interest have been carried out. Procedures have been developed for the recovery of molybdenum, tungsten and vanadium from low grade ores and spent catalysts. The liquid-liquid partition data have provided useful guidelines to develop reverse phase column chromatographic procedure for the separations of some of the metal ions. For the sake of clarity in the presentation the work embodied in the thesis has been divided into the following six chapters. I. General Introduction. II. Materials and Equipments. III. Extraction Behaviour of Mo(VI) and Other Associated Metal Ions Using Cyanex 301 and 302 Reagents. IV. Liquid-Liquid Extraction Behaviour of W(VI) and Some Other Metal Ions Using Cyanex 925 Extractant. V. Solvent Extraction Studies on V(IV) and Some Other Metal Ions Using Cyanex 272 and 301 Extractants. VI. Reversed Phase Column Chromatographic Studies Using Cyanex 301, 302, 925 and 272 as Impregnants. (iii) Chapter I presents a brief introduction to liquid-liquid extraction of metal ions. A classification of different types of extractants is given. An overall view of the available literature on the different types of organophosphorus extractants employed for Mo, W and V is presented. The aims and objectives of the present study have been spelt out. The relevant literature on a particular type of extractant is cited in the respective chapter. Chapter II deals with the materials and equipments used during the course of the present investigations. The distribution studies were carried out by using radiotracers or Inductively Coupled Plasma Atomic Emission Spectrometry (1CP-AES). The separation studies and the analysis of the ores and the spent catalysts were done by employing ICP-AES. Chapter III present the extraction data on Mo(VI) and other commonly associated metal ions such as W(VI), U(VI), V(V), Al(III), Cr(III), Fe(III), Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Pb(II) from different mineral acids in Cyanex 301 and 302. Extraction of W(VI) from nitric acid medium could not be studied as it precipitates and for the same reasons it was not possible to investigate the behaviour of Pb(ll) from sulphuric acid. In Cyanex 301 the variations in the extraction of Mo(VI) with the change in the diluent could not be discerned because of almost quantitative extraction in all the diluents used. In the case of Cyanex 302 the highest extraction is observed in nitrobenzene and cyclohexanone diluents and lowest in n-hexane. The detailed investigations were carried out with toluene as a diluent. It was observed that kerosene fraction (160-200°C) can replace toluene without any significant change in the behaviour. The results of loading of Mo(VI) for both the extractants suggest that the extractant can hold the metal ion upto a maximum of one-tenth of its concentration. The extraction of Mo(VI) in both the extractants increases with the increasing extractant concentration. The (iv) log-log plots between the extractant concentration and the distribution ratio for both the extractants give straight lines with a slope around two suggesting the metal to extractant ratio in the extracting species to be 1:2. The extraction data have been utilized to achieve almost quantitative separation of Mo(VI) from W(VI), V(V), Al(III), Cr(III), Fe(III), Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Pb(II). The separation conditions thus developed have been utilized for the recovery of molybdenum from a low grade molybdenum ore, Cu pyrites and spent hydrodesulphurization and Co- Mo catalysts. The recovery of molybdenum from the proposed methods is around 90%. Chapter IV embodies information on the extraction behaviour of Mo(VI), W(VI), V(V), V(IV), Al(III), Fe(III),Ca(II), Mn(II), Zn(II), Cd(II) and Pb(II) in kerosene solution of Cyanex 925. The effect of various parameters like type of the mineral acid and its molarity, nature of the diluent and the concentration of the metal ion and the extractant has been studied. The highest extraction of W(VI) is observed with nitrobenzene as the diluent and the lowest with toluene. Most of other studies were conducted in kerosene (160-200°C). The results of the effect of concentration of the metal ion (1.0 xlO6 to 5.0 x 10"4M) on extraction reveal that the distribution ratio does not significantly change with the concentration of the metal ion. It has been observed that on increasing concentration of the extractant the per cent extraction of W(V1) increases. Log-log plots of extractant concentration versus distribution ratio give straight lines with a slope of two suggesting the incorporation of two molecules of the extractant in the extracting species. Based on the partition data it has been possible to achieve the separations of W(VI) from Mo(VI), Fe(III), Mn(II), Zn(II), Cd(II) and Pb(II). Encouraged by these results a scheme has been devised for the recovery of pure tungsten from samples of scheelite and wolframite ores. The recovery of tungsten from both of these ores is around 90%. (v) Chapter V includes the extraction behaviour of V(IV) alongwith Mo(VI), W(V1), U(VI), V(V), Ti(IV), Al(lII), Cr(lll), Fe(III), Mn(II), Zn(II) and Pb(Il) in Cyanex 272 and that of V(IV) and Ti(IV) in Cyanex 301 and 302. The partition data on Mo(VI), W(VI), U(VI), V(V), Al(III), Cr(III), Fe(III), Mn(II), Zn(II) and Pb(II) given in Chapter III have been utilized for the present investigations. The results of extraction of V(IV) in Cyanex 272 and 301 with the change in diluent indicate that there is no correlation between the per cent extraction and the dielectric constant. The investigations have been carried out in toluene as the diluent. The per cent extraction increases significantly with the increase in the concentration ofthe extractant. The plots between the extractant concentration and the distribution ratio of V(IV) for both the extractants give straight lines with a slope ofnearly two. The loading capacity of the Cyanex 272 and 301 for the extraction of V(IV) has been assessed. Based on the partition data it has been possible to separate V(IV) from Mo(VI), W(VI), U(VI), Ti(IV), Al(III), Cr(III), Fe(IlI), Mn(II) and Pb(II) using either ofthe two extractants. The practical utility of the extractants has been demonstrated by the recovery of pure vanadium from spent V205 catalyst. The recycling capacity of the different extractants (Cyanex 301, 302, 925 and 272) for the relevant metal ion has been found to be good. Chapter VI presents data on reversed phase column chromatographic behaviour of Mo(VI), W(VI) and V(IV) and some other metal ions using silanized silica gel columns loaded with Cyanex 301, 302, 925 and 272. The optimum conditions for the separations have been developed. Employing these columns Mo(VI), W(VI) and V(IV) have been quantitatively separated mutually and from other closely associated metal ions. These columns can be reused upto five cycles without any change in the recovery. The thesis concludes with a brief discussion highlighting some of the important features of the present study.