LIQUID-LIQUID EXTRACTION OF SOME METALS AND THEIR RECOVERY FROM DIFFERENT MATRICES

Abstract

In the present day society the scientific growth and the prosperity of a nation, to some extent, can be assessed by the production and consumption of metals and their salts. On a global scale there is an ever-increasing pressure on the production of metals thus resulting into depletion of richer metal deposits. In the present scenario of inflation of prices of metals the need of the hour seems to be to develop efficient extraction procedures to recover metals from low grade ores and industrial waste. The recovery of metals from the secondary sources will not only have an impact on the economy but partly help in the safe disposal of metal loaded waste. Gallium, germanium and indium fall under the category of hi-tech metals. These metals are very vital for electronics and semiconductor industries and it may be reasonable to name them collectively as e- (electronic) metals. Ironically gallium and indium have no principal ores of their own and they are recovered mainly from aluminium, zinc and lead deposits. The principal ore of germanium, germanite, has rare occurrence in nature. In a way all the three metals are scarcely available but their requirement is increasing at a very fast rate. In order to meet the supply of these metals one of the possible ways seems to recover them by recycling the waste. Many developed countries have already started programmes on these lines. As per the information available, on a global scale, about half of the gallium and one - fourth of germanium and indium consumed comes from recycling of electronic and optical waste. It may be important to point out here that the recovery from the waste is not necessarily an easy operation to accomplish. The matrix is invariably complex, the concentration of the metal is low and the purity of the metal desired may be very high. The situation for recovery from low grade ores is not very different. One particular separation technique which comes close to meet the above requirements is liquid-liquid extraction. The solvent extraction is a well established technique for the recovery of metals. It is convenient, rapid, invariably involves less operational cost and can be easily extended from bench level to plant scale. The technique can be made selective and the products of desired purity can be obtained without involving too many operations. In order to meet an ever-increasing popularity of the liquid-liquid extractions, a variety of commercial extractants keep figuring in the list from time to time and the search for more efficient and convenient extractants still continues. Various extractants of different categories have been employed for the separations involving gallium, germanium and indium. The scenario of commercial extractants in the second half of the twentieth century was mainly dominated by reagents such as carboxylic acids, high molecular weight amines, P-diketones, oximes and organophosphorus compounds. The use of these extractants is invariably limited because of extractant loss, emulsion formation, limited selectivity and other hydrometallurgical considerations. It is only in the eighties that American Cyanamid Company (now Cytec Inc.), marketed a series of alkylphosphine extractants under the trade name of CYANEX. These reagents differ from many other commercial organophosphorus extractants as the alkyl groups are bonded directly to the phosphorus atoms through P-C bonds rather than P-O-C bonding. This tends to make phosphine derivatives more resistant to hydrolysis than the other reagents of this class. This class of extractants includes alkylphosphine oxides Cyanex 921, Cyanex 923 and Cyanex 925; an oxyacid Cyanex 272 and its sulphur analogues namely Cyanex 301 and Cyanex 302 and a sulphide Cyanex 47IX. In view of some inherent advantages of alkylphosphine compounds the author has explored the potential of Cyanex 923, Cyanex 272 and Cyanex 301 for the recovery of gallium, germanium and indium. On the basis of literature survey and preliminary investigations Cyanex 923, Cyanex 272 and Cyanex 301 have been used as extractants for gallium and germanium and Cyanex 272 for indium. The effect of various phase parameters on the partition of metal ions has been studied. Based on the extraction trends optimum conditions for binary and ternary separations have been worked out. After generating the required extraction data the procedures have been developed to recover the metals from metal scrap, bottom ash and low grade ores. It may be important to point out here that the present efforts are focussed to develop a suitable separation chemistry which can be conveniently scaled up at a plant scale. For the clarity of presentation the work embodied in the thesis has been divided into the following five chapters. I General Introduction II Materials, Equipment And Methodology HI Extraction Studies on Ga(III) And Associated Metal Ions Using Cyanex 272, 301 And 923. IV Extraction Studies on Ge(IV) And Associated Metal Ions Using Cyanex 301 And 923. V Extraction Studies on In(III) And Associated Metal Ions Using Cyanex 272 ill Conclusions Chapter I embodies a general introduction to liquid-liquid extraction technique. A classification on different types of extraction systems highlighting their properties and mechanism of extraction is presented. The aims and objectives of the present study are defined and a summary of the research work executed is given. Chapter II gives details of materials and equipments used during the course of present investigations. The distribution studies were carried out by using radiotracers / Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) / Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) / Atomic Absorption Spectrometry (AAS). The procedures adopted for dissolution of different matrices are also cited. Chapter III describes the partition data on Ga(III) along with associated metal ions like V(V)/V(IV), Ge(IV), Ti(IV), Tl(III), In(III), Fe(III), Al(III), Cd(II), Zn(II), Cu(II), Ni(II) and Mn(II) from HCl / HN03 / H2S04 media using toluene solutions of Cyanex 272 / Cyanex 301 / Cyanex 923. The effect of different phase variables like the nature of acid, concentration of acid, metal ion and extractant on the distribution of the metal ions is studied. Effect of equilibration time and temperature has been investigated. The extraction data reveal that the highest extraction of Ga(III) is from HCl medium and the detailed studies have been carried out from this acid. Diluents like toluene, n-hexane and kerosene show higher extraction of Ga(III). It is also observed that kerosene (160- 200 °C) can replace toluene for commercial purposes as a cheaper diluent without affecting the results. The stoichiometric ratio of metal ion to extractant is 1:2, 1:3 and 1:3 in Cyanex 272, Cyanex 301 and Cyanex 923, respectively. Metal ion loading results indicate that the metal to extractant ratio in Cyanex 923/272 and Cyanex 301 is around IV 1:15 and 1:50, respectively. The extractants are stable towards long term contact with acid. The regeneration power of the extractants is also assessed. A number of stripping reagents have been studied for the recovery of metal ions from the organic phase. The distribution studies in Cyanex -272, -301 and -923 led to the development of conditions for a number of binary and ternary separations of gallium from associated metal ions. The data have been further extended to the recovery of pure gallium from Bayer's liquor, semi-conductor waste and bottom ash. Chapter IV incorporates studies on the extraction and separation of Ge(IV) using Cyanex 272 / Cyanex 301 / Cyanex 923 as extractants. Partition studies on Ge(IV) using 0.50 mol L"1 toluene solution of Cyanex 272 indicated that its extraction is poor(< 20%) in the entire investigated range of HCl / HN03 / H2S04 media. Further studies carried out with Cyanex 301 and Cyanex 923 revealed that in both the extractants the extraction of Ge(IV) is poor (<10%) at low HCl molarity but increases with the increase in acid molarity and attains a quantitative value at a higher acidity around 8.0 mol L' . From HN03 / H2SO4 medium the extraction of Ge(IV) is considerably low (<20%) in the entire investigated range. Therefore investigations on Ge(IV) and other metal ions namely As(V) / (III), Sn(IV), Ti(IV), Tl(III), In(III), Ga(III), Fe(III), Al(III), Hg(II), Cd(II), Cu(II) and Zn(II) were carried out employing Cyanex -301 and -923 and were confined to HCl medium. The effect of various phase parameters such as time of equilibration, temperature and concentration of extractant and metal ion on the distribution of Ge(IV) has been studied. The variation in time of equilibration suggest that the equilibrium in both the extractants is attained in about two minutes. The temperature studies reveal that the extraction of Ge(IV) increases with the rise in temperature thereby indicating the process to be endothermic. The effect of varying extractant concentration on the extraction suggests that the ratio of metal to extractant in the extracting species is 1:2 in both the extractants. In order to identify the extracting Ge(IV) species the effect of chloride ion on the extraction of Ge(IV) has also been investigated. In both the extractants log-log plot between D and [CI"] gives a straight line with a slope value around four. Based on the slope analysis data, the extracting species is proposed as GeCl4.2R (R = Cyanex 301 / 923). Metal ion loading results reveal that both the extractants can hold the metal ion up to around 1/20 of its molar concentration. In order to back extract the metal ion from the organic phase a number of stripping reagents were examined. Ge(IV) can be effectively stripped from the organic layer molar solutions of HCl and H2SO4. The stability of the extractants against acid was checked by keeping the solution of the extractants in contact with 8.0 mol L'1 HCl with intermittent shaking. Negligible change in the percent extraction of Ge(IV) was observed even after a contact period of fifty days. Regeneration power of the extractants was assessed using extraction / stripping cycles. Results show insignificant change in the extraction / stripping of Ge(IV) even after several cycles. The partition data on Ge(IV) and associated metal ions in Cyanex -301 and -923 offer conditions for several binary and ternary separations. The conditions developed for the separations are extended to develop a scheme for the recovery of Ge(IV) from the germanium diode waste. Chapter V presents extraction studies on In(III) and the associated metal ions viz., Ti(IV), Ge(IV), Tl(III), Fe(III), Al(III), Cd(II), Zn(II), Cu(II), Fe(II) and Mn(II) from HCl, HNO3 and H2S04 media in toluene solution of Cyanex 272. The extraction behaviour of In(III) is more or less similar from all the three acid media. Detailed VI investigations were carried out only from HCl medium. The effect of concentration of acid and extractant on the extraction of In(III) is studied. The effect of increasing concentration of the extractant on extraction shows that two molecules of the extractant are involved in the formation of the extracting species. The extracting species is thus proposed as In(OH)R2. The extraction of In(III) is highest in n-hexane, toluene and kerosene solution of Cyanex 272. No correlation between dielectric constant of the diluent and percent extraction is observed. For the detailed studies toluene is used as the diluent. Metal ion loading results reveal that the extractant can hold the metal ion up to one-fifteenth of its molar concentration. A number of stripping reagents were tried for the back extraction of In(III) and 0.50 mol L"1 HCl, 1.0 mol L"1 HCl, 3.0 mol L'1 H2S04 and 3.0 mol L"1 HNO3 are effective for the said purpose. Toluene solution of Cyanex 272 is found to be stable even after a contact of fifty days with 5.0 mol L'1 HCl. Recycling experiments conducted up to ten cycles reveal insignificant loss in the extraction capability of the extractant. The partition studies on In(III) and associated metal ions offer conditions for some binary and ternary separations of analytical interest involving indium. The distribution studies are further extended for the recovery of pure In(III) from zinc blend and galena. Finally based on the results the utility, of the present work, is discussed. The work embodied in the dissertation has been able to suggest simple separation procedures for Ga(III), Ge(IV) and In(III), using the three important commercial alkylphosphines. It has been possible to extend the separation chemistry for the recovery of the said metals in high purity with good yield from low grade ores and waste materials. The bench level studies indicate that in the case of all the three extractants equilibration is fast, phase VII separation is quicker and no additional modifier is required. Moreover, the extractants have good hydrolytic stability against acid contact and can be regenerated up to several cycles. These investigations suggest that Cyanex 272, Cyanex 301 and Cyanex 923 have potential to conveniently recover gallium, germanium and indium from leaner sources. The proposed chemistry is such that the procedures can be easily scaled up at the plant scale with some additional inputs.