STUDIES ON THE REMOVAL AND RECOVERY OF MERCURY(II) AND NICKEL(II) USING ALKYLPHOSPHINE EXTRACTANTS AND THEIR APPLICATIONS TO INDUSTRIAL WASTE

Abstract

The industrial growth of a country to a large extent can be assessed by the production and consumption of the metals and their compounds. Acertain fraction of these metals ultimately find a place in the open environment causing detrimental effects. Therefore, on a global scale, there is an ever increasing pressure on the production of metals thereby resulting into the depletion of richer metal deposits. All this has necessitated the demand to develop efficient processes to remove and recover metals from waste streams, spent catalysts, metal scraps and other metal bearing matrices. In the third world countries, like India, the discharge of industrial effluents poses agrave threat to the ecosystem because of casual implementation of regulatory measures. Avariety of organics and inorganics are being discharged indiscriminately. The removal and recovery of metals from secondary sector will not only improve the economy but partly mitigate the problems of metal pollution. Heavy metals are known contaminants of the ecosystem. Amongst the earliest metals to cause concern to the environmentalists was mercury. The main source of this is soda- lye industry using mercury cell process. Although the use of mercury cell has been banned it is still being employed in countries like India thereby contaminating some of the industrial wastes. Nickel is a strategic metal which finds applications in electroplating industry, catalysts and production of different grades of steels. However, India is not bestowed with nickel reserves except the laterite deposits of the Sukhinda in Orissa which are of inferior grade compared to other sources elsewhere. Therefore, efforts should be made to remove and recover these metals from wastes. Moreover, from the point of view of analytical chemists the separations of mercury and nickel from associated metals are of equal importance. Liquid-liquid extraction popularly known as solvent extraction (SX) offers a convenient solution to the said problems. Solvent extraction is one of the premier separation techniques and commands a special place in separation science and technology owing to its convenience, speed, versatility and relatively low cost. It can be conveniently extended from micro levels to macro concentrations. 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 keeping the inherent qualities of the extractant intact. Because of ever increasing popularity of solvent extraction different types of extraction systems have come up on the forefront and the search for more efficient extractants still continues. During the last three decades voluminous literature has accumulated on some of the alkylphosphorus extractants namely tributylphosphate (TBP) and di(2-ethylhexyl)phosphoric acid (DEHPA). Amongst the organophosphine extractants tri-n-octylphosphine oxide (TOPO) has been studied far more thoroughly than all the others. In the eighties, American Cyanamid Company, USA, marketed a series of organophosphines under the trade name CYANEX. The list mainly includes phosphinic acids and phosphine oxides and their sulfur analogues. These reagents differ from 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 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. Cyanex 301( bis(2,4,4-trimethylpentyl)dithiophosphinic acid ) has a lower pKa value than its oxyacid counterpart Cyanex 272 ( bis(2,4,4- trimethylpentyl)phosphinic acid ), thus providing a wider working range of acidity. Cyanex 47IX (triisobutylphosphine sulfide) is a soft lewis base and therefore will readily complex with soft acids like Hg(II). On the basis of literature information and preliminary investigations, Cyanex (ii) 47IX and Cyanex 301 have been explored as extractants for Hg(II) and Ni(Il), respectively. 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 extractants have been determined and the stoichiometry of the extracting species is proposed. Based on the partition data some binary separations of analytical interest have been carried out. Procedures have been developed for the removal of mercury and recovery of nickel from wastes. Solvent extraction data have provided useful guidelines to develop reverse phase column chromatographic procedure for the separation of mercury from some metal ions. The columns have been used to decontaminate paper and pulp industry waste from mercury. For the sake of clarity in the presentation the work embodied in the thesis has been divided into the following five chapters. I General Introduction. II Materials and Equipments. III Solvent Extraction Studies on Hg(II) and Some Other Metal Ions Using Cyanex 47IX as Extractant. IV Reverse Phase Column Chromatographic Studies Using Cyanex 47IX as Impregnant. V(a) Extraction Behaviour of Ni(II) and Other Associated Metal Ions Using Cyanex 301 Extractant. (b) Electrowinning of Nickel. Chapter I embodies a general introduction to liquid-liquid extraction and extraction chromatography. Aclassification of different types of extractants and an overall view of the available literature on the different types of recently marketed alkylphosphine extractants is presented. The aims and objectives of the present study are defined. Chapter II deals with the materials and equipments used during the course (iii) of the present investigations. The distribution studies were carried out by using radiotracers or Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) / Atomic Absorption Spectrometry (AAS). Chapter III presents the extraction data on Hg(II) and other commonly associated metal ions such as Cr(III), Fe(III), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Pb(II), and Ag(I) from nitric acid in Cyanex 47IX. Hg(II) shows a quantitative extraction when toluene, cyclohexanone and chloroform are used as diluents. Detailed investigations were carried out using toluene as a diluent. It was observed that kerosene fraction (160-200°C) can replace toluene without any significant change in the extraction behaviour. The log-log plot between the extractant concentration and the distribution ratio gives a straight line with a slope of around one thus suggesting Hg(N03)2.S ( where, S = Cyanex 47IX ) as the extracting species. The results of loading of Hg(II) indicate a 1:1 stoichiometric ratio of metal ion to extractant thus supporting the results of slope analysis. The extraction data have been utilised to achieve almost quantitative binary separations of Hg(II) from Cr(III), Fe(III), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II). Since the extraction of Hg(II) is more or less quantitative over the entire range of acidity it does not require a rigid control of aqueous phase conditions for separation. Moreover, Hg(II) can be recovered quantitatively by a variety of reagents depending upon the need for its further processing. Due to similar behaviour and response to stripping reagents it was not possible to separate Hg(II) from Ag(I). The regeneration and recycling experiments conducted upto ten cycles reveal practically insignificant change in the efficiency of the extractant. The hydrolytic stability of the extractant has also been assessed. The selectivity of the extractant for Hg(II) has been utilised for the decontamination of paper and pulp industry waste from mercury at the bench level. The results of batchwise uptake of Hg(II) and some other metal ions by Cyanex 47IX sorbed on chromosorb 102 and silica gel are incorporated (iv) in Chapter IV. Batch experiments have been conducted to evaluate the effect of various parameters like equilibration time, the concentration of the metal ion and the extractanton the uptake of mercury(II). Optimum conditions for the separation of mercury(II) from other metal ions on chromosorb 102 and silanized silica gel columns loaded with Cyanex 47IX have been developed. It has been possible to separate mercury(II) quantitatively from other closely associated metal ions. The column can be reused upto ten cycles without any significant change in recovery. These columns on a bench scale have also been utilised for removing mercury from paper and pulp industry waste. Chapter V (a) includes the extraction behaviour of Ni(II) alongwith other metal ions such as Cr(III), Fe(III), Mn(II), Co(II), Cu(II) and Zn(II) from sulfuric acid in Cyanex 301. The investigations have been carried out using toluene as a diluent. The plot between the extractant concentration and the distribution ratio of Ni(II) gives a straight line with a slope of around two suggesting the formation of NiR2 (where, HR =bis(2,4,4-trimethylpentyl)dithiophosphinic acid) species. The loading capacity of Cyanex 301 for Ni(II) has been assessed. Based on the partition data in Cyanex 301 it has been possible to separate Ni(II) from Cr(III), Fe(III), Mn(II), Co(II) and Zn(II). The practical utility of the extractant has been demonstrated by recovering nickel from spent hydrogenation catalyst and electroplating bath residue. The recycling capacity of the extractant has been found to be good even upto ten cycles without any significant change in the efficiency of extraction and stripping. Chapter V (b) presents studies on the electrodeposition of nickel. Nickel(II) recovered from the spent catalyst and electroplating bath residue after solvent extraction step is fed to the electrolytic bath to maintain the concentration of nickel. The purity of these deposits is reported. The solvent extraction-electrowinning (SX-EW) has been successful to obtain nickel metal from the wastes. The thesis concludes with a brief discussion on the findings of the present investigation.The present project has been successful in suggesting useful (v) commercial extractants for mercury(II) and nickel(II). Cyanex 47IX is selective for Hg(II) over many metal ions commonly encountered with it and several separations of analytical interest have been achieved. The columns of coated materials have the potential of being used for decontaminating mercury(II) containing effluents. The selectivity for mercury(II) will avoid unnecessary loading of the column with other metal ions. It has been possible to use Cyanex 301 for topical separations of nickel(II) from commonly associated metal ions. The proposed conditions of separation have resulted in the development of methods for the recovery of pure nickel from spent catalyst and electroplating bath residue.