STUDIES ON LIOUID-LIOUID EXTRACTION AND RECOVERY OF SOME INDUSTRIALLY IMPORTANT ELEMENTS

Abstract

The present thesis contains studies on liquid-liquid extraction and recovery of some selected 3d transition metals, cadmium and indium from different complex matrices by employing some alkylphosphine extractants of Cyanex series. Consequent to a large-scale consumption, over the last few decades, the richer resources of most of the 3d metals have started depleting. The scenario of good quality resources of cadmium and indium is already very grim and these two metals are generally obtained from the residues of zinc and lead processing industry. Thus there is an immediate need to minimise the wastage of 3d metals, cadmium and indium. It may be important to point out that some of the 3d metals and cadmium are serious pollutants of the biosphere. The recovery of these metals from industrial waste will not only improve economy but also partly mitigate the problem of environmental pollution. Liquid-liquid extraction is a well known technique for the recovery of metals from complex matrices. Amongst the different separation techniques, liquid-liquid extraction commands a special position because of its simplicity, versatility and relatively low operational cost. The extraction methods can be easily extended from bench scale to the macro level. The partition data also help in developing the column chromatographic separation procedures. The coupling of extraction with electrowinning using simple cells helps to yield a high purity metal. Over the years, different kinds of extractants have been used for the recovery of a variety of metal ions. However, for the last few decades the scenario of commercial extractants has been dominated mainly by carboxylic acids, chelating agents, high molecular weight amines and organophosphorus compounds. With an ever-growing demand of upgradations in the technology sector the use of many of the said extractants is on a decline because of their poor selectivity and extractant loss. The 1980's were marked with the introduction of some new alkylphosphine extractants under the trade name of 'Cyanex'. This included oxides like Cyanex 921, Cyanex 923 and Cyanex 925; a sulphide known as Cyanex 471X, an oxyacid namely Cyanex 272 and its sulphur analogues viz. Cyanex 301 and Cyanex 302. Some of the inherent advantages of Cyanex 923, such as a better extraction power, selectivity and regeneration capacity than TBP, TOPO and DEHPA and miscibility with most of the commonly used diluents, prompted the author to investigate this extractant for the recovery of 3d transition metals, cadmium and indium. The effect of some phase variables, influencing the partition of metal ions, was studied and the distribution data were utilised for achieving the separations of metals from multielement mixtures like ores and wastes. The extraction methods of Cr(III) / Co(II) / Ni(II) were coupled with electrowinning. Using Cyanex 923 it was not convenient to separate Ti(IV) from multielement mixtures and hence Cyanex 301 and Cyanex 302 were investigated. For the convenience of presentation and to maintain the clarity of the subject, the matter in the thesis is divided in to the following chapters General Introduction II. Materials and Methodology IIIA. Extraction Studies on Some 3d Transition Metal Ions Using Cyanex 923 IIIB. Electrowinning of Cr(III), Co(II) and Ni(II). IV. Extraction Studies on Cd(II) and In(III) Using Cyanex 923 VA. Extraction Studies on Ti(IV) Using Cyanex 301 and Cyanex 302 VB. Extraction Chromatographic Studies on Ti(IV) Using Cyanex 301 Chapter I embodies a general introduction to the liquid-liquid extraction technique. A classification of different kinds of extraction systems highlighting some of their important properties and the mechanism of extraction is given. A literature review on the properties and uses of Cyanex 923, Cyanex 301 and Cyanex 302 is presented. The aims and objectives of the present study are defined. Chapter II describes details of the chemicals and equipment used during the course of study. The methodology adopted for the present research has been described and the analysis of the data explained. Chapter IIIA narrates the extraction behaviour of some selected 3d metal ions namely Ti(IV), V(IV), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II) and Zn(II) from HC1, H2SO4 and HNO3 media using Cyanex 923 solution in toluene. The effect of some phase variables like the nature of diluent, type of acid and concentration of acid, metal ion, CI" ion and extractant on the distribution has been investigated. The highest extraction of most of the 3d metal ions is achieved from HCl medium. The stoichiometry of extracting species has been proposed and the loading capacity of Cyanex 923 determined. The extractant solution is evaluated for its recycling capacity and stability against long-term contact with acids. The extraction pattern of some associated metal ions namely Mo(VI), Zr(IV), Ce(IV), Al(III), Ga(III), Mg(II), Cd(II) and Pb(II) with Cyanex 923 has also been scanned. These data have been utilized for attaining some binary and multielement separations involving 3d metal ions. The developed separation methods are applied for the recovery of pure 3d metals from the spent catalysts, electroplating sludge, electroplating mud and tannery effluent. Chapter IIIB incorporates studies on the electrodeposition of Cr(III), Co(II) and Ni(II). The effect of pH, current density and concentration of metal ion / Na2S04 on the cathode efficiency of cells is shown. The deposition baths operate at reasonably good cathode efficiencies. These baths have been used for the recovery of pure metals from electroplating and tannery wastes. Chromium is obtained in the form of a grey powder while nickel and cobalt in the form of a sheet. The purity of the deposits is 99.9%. Chapter IV embodies studies on the extraction of Cd(II) and In(III) from HCl, H2SO4 and HNO3 media using Cyanex 923. The effect of nature of diluent, type of acid and concentration of acid, metal ion, CI" ion and extractant on the extraction has been observed. Cd(II) is extracted only from HCl medium while In(III) shows extraction from all the three investigated acid media. Based on the distribution data, the stoichiometry of extracting species is proposed. Experiments have been conducted to determine the loading capacity of Cyanex 923. Some aqueous solutions have been tested to back extract metal ion from the loaded organic phase. Experiments have been carried out to assess the stability and regeneration capacity of Cyanex 923. The extraction behaviour of some associated metal ions like V(IV), Ti(IV), Al(III), Cr(III), Fe(III), Ga(III), Sb(III), Tl(III), Fe(II), Cu(II), Zn(II), Pb(II) and T1(I) has been investigated. The distribution data of metal ions have been utilized for attaining some binary and multielement separations involving Cd(II) and In(III). These separations are used in devising a flowsheet for the recovery of pure cadmium and indium from zinc blend / chalcopyrite / galena / zinc electroplating mud. Chapter VA presents the data on the extraction of Ti(IV) along with those of V(IV), Zr(IV), Ce(IV), Th(IV), Al(III), Fe(III), Ga(III), Mg(II) and Mn(II) in Cyanex 301 and Cyanex 302 from HCl, H2SO4 and HNOs media. Ti(IV) shows quantitative extraction over a fairly wide range of acidity from all the three investigated acids. The composition of extracting Ti(IV) species is proposed and the loading capacity of Cyanex 301 / Cyanex 302 determined. The effect of diluent on the extraction of Ti(IV) in Cyanex 301 / Cyanex 302 is discerned. Some solutions namely 0.10 mol L1 oxalic / tartaric / citric acid and 3% H2O2 in 0.5 mol L'1 H2SO4 are scanned for their potential to back extract Ti(IV). The stability and regeneration capacity of the two extractants have been determined. The partition data of metal ions are used for the separation of Ti(IV) from binary and multielement mixtures. Finally pure titanium has been recovered from ilmenite and red mud. Chapter VB deals with the extraction chromatographic studies of Ti(IV) using Cyanex 301 impregnated silica gel (ISG). The uptake behaviour of Ti(IV) along with that of some associated metal ions has been investigated. The ISG is packed in a column for attaining some binary separations of analytical significance. The column is employed for the recovery of titanium from red mud. The thesis concludes with a discussion on the findings of the present research in the form of Conclusions. Cyanex 923 is useful for the extraction and separation of 3d metal ions, cadmium and indium. Cyanex 301 and Cyanex 302 are effective for the recovery of Ti(IV). The extractants offer a fairly wide working range of the acidity. A variety of topical separations have been achieved using simple experimental conditions. The recovery of different metals is more than 90% with purity around 99%. The extractants used are stable towards long-term contact with the acids. Also they are conveniently regenerated. The proposed chemistry has been developed on a bench level. However, it is anticipated that there may not be much difficulty in scaling up various steps as they are simple and invariably do not require rigid control of phase parameters. The author feels that the findings of the present dissertation have offered better alternatives for processing the separations involving 3d metals, cadmium and indium. Asides this, the varied potential of Cyanex 923 has been highlighted.