C.K Gupta

Studies on uranium extraction from phosphoric acid using di-nonyl phenyl phosphoric acid-based synergistic mixtures

Abstract During acidulation of phosphate rock with sulphuric acid to produce phosphoric acid for fertilizer application, the uranium values contained in the rock also become solubilised. The phosphoric acid produced by the ‘di-hydrate’ process, containing 26–30% P2O5, called the ‘weak phosphoric acid’ (WPA), is concentrated to >50% P2O5 grade, known as ‘merchant grade acid’ (MGA). The ‘hemi-hydrate’ process directly yields acid of high P2O5 content. The uranium concentration varies with acid concentration, provided oxidising conditions are maintained so that selective precipitation of uranium does not take place. Depending on the rock used, the acid produced can contain 0.0050–0.05 g U3O8/L. Separation of uranium by solvent extraction employing a novel synergistic combination of organo-phosphorus reagents is reported in this paper. In particular, the solvent mixture of di-nonyl phenyl phosphoric acid (DNPPA) with di-butyl butyl phosphonate (DBBP) in an aliphatic diluent has been found to be a stronger extractant than the commonly used combination of di-(2-ethyl hexyl) phosphoric acid (DEHPA) with tri-n-octyl phosphine oxide (TOPO). The DNPPA–TOPO mixture is an even stronger extractant. Results on extraction from both WPA and MGA are reported. Stripping of uranium from the organic phase was achieved with concentrated phosphoric acid in the presence of Fe+2 reducing agent at elevated temperature. Stripped uranium was subjected to a second cycle of extraction-stripping and recovered as a peroxide precipitate of high purity. Brief results on allied areas of acid pre-treatment and post-treatment are included.

Processing of xenotime concentrate by sulphuric acid digestion and selective thorium precipitation for separation of rare earths

Abstract A process is described for the recovery of rare earths from xenotime concentrate by digestion with sulphuric acid. A low grade xenotime concentrate (typically assaying 57% light rare earths, 27% Y2O3 and 15.6% heavy rare earth) and a high grade xenotime (assaying 38% light rare earths, 41.8% Y2O3 and 20.55% heavy rare earths) is routinely being produced in India. Both types of materials have been tested and >98% solubilisation of metal values has been achieved. Xenotime sulphate leach liquor has been processed for removal of thorium by selective precipitation with ammonia and sodium pyrophosphate. Although in both systems thorium removal was almost complete, the ammonia precipitation was found suitable since it facilitates the preferential precipitation of lighter rare earths along with thorium. Consequently, the heavy rare earths have been enriched in the filtrate. Other impurities such as iron, uranium, sulphate and phosphate have been effectively removed by precipitation of rare earths with oxalic acid. It has been shown possible to recover rare earths along with yttrium (∼99% recovery) by these processes.

Simultaneous purification of dysprosium and terbium from dysprosium concentrate using 2-ethyl hexyl phosphonic acid mono-2-ethyl hexyl ester as an extractant

Abstract During the solvent extraction fractionation of rare earth chloride obtained from monazite, a heavy rare earth fraction assaying 60% Y 2 O 3 is produced. This is purified further to 93% Y 2 O 3 by another solvent extraction cycle. During this step, most of the Dy and Tb are separated to yield a concentrate assaying >50% Dy 2 O 3 , 14% Tb 4 O 7 , 10% Gd 2 O 3 , 2.4% Ho 2 O 3 , and 21% Y 2 O 3 . An attempt has been made to process this Dy-rich concentrate to obtain a high grade Dy 2 O 3 by solvent extraction using 2-ethyl hexyl phosphonic acid mono-2-ethyl hexyl ester (PC 88A) in paraffinic kerosene as an extractant. The distribution ratio ( D ) of Dy was determined as a function of initial concentration of HCl for different initial concentrations of Dy. The separation factors of Dy/Gd, Dy/Tb, Y/Dy, Ho/Dy, and Er/Dy were determined experimentally. Using the experimentally determined D values of Dy and the separation factors, the distribution data for Gd, Tb, Y, Ho, and Er were derived mathematically. Based on these data, empirical mathematical models have been made to predict the concentration of these metal ions in organic and aqueous phases at various initial acidities and metal concentrations. Using these models, a computer program was developed to calculate the concentration of the metal ions in organic and aqueous phases at various stages of extraction and scrubbing in a counter-current solvent extraction cascade. Using this program, the parameters of the process, such as the initial acidity of the feed, the acidities of scrubbing and stripping solutions, the phase ratio, the number of stages in extraction, and the scrubbing sections have been optimised to obtain >97% purity of Dy. Under these optimised conditions, counter-current extraction and scrubbing tests were carried out using mixer settlers of 50-ml mixer capacity. Mixer settler runs yielded four different products from its four exit points. The raffinate coming from the first exit was essentially Gd-rich solution containing other light rare earths (LRE). The fourth and final exit point was for the stripped solution that contained 85% Y 2 O 3 with 95% recovery. The remaining two exit points, involving scrubbing stages, yielded an 83% pure Tb 4 O 7 concentrate and a 97% pure Dy 2 O 3 concentrate.

Solvent Extraction in Production and Processing of Uranium and Thorium

Abstract Solvent extraction plays a vital role in the production and processing of uranium and thorium for use as fuels in the front-end of the nuclear fuel cycle. The development of solvent extraction technology in the nuclear field in the last five decades has contributed to advances in the non-nuclear hydrometallurgy. In turn the large scale applications in the field of base metals such as copper have led to development of new equipment and techniques as well as better understanding of the process chemistry and hydrodynamics. Advances in the field of solvent extraction of relevance to the nuclear fuels, are reviewed in this paper. The significant results from the research and development work in India are also included. Various aspects discussed include chemistry of process flowsheets for uranium and thorium recovery and refining including recent improvements, diluents for use in the processes, thermal effects in extraction, process instrumentation including on-line measurements, solvent loss by entrainment, purification of feed streams prior to extraction, solvent-in-pulp processing, separation of uranium and thorium, binary ex-tractants and application of solvent extraction in uranium enrichment.

Studies on supported liquid membrane for simultaneous separation of Fe (III), Cu (II) and Ni (II) from dilute feed

Abstract Supported liquid membranes (SLMs) are used in the extraction and separation of metals from dilute feed. In this paper, we show an innovative application of SLMs in the simultaneous separation of three metal species, employing a two-membrane–three-compartment cell. Two PTFE support membranes loaded with Alamine 336 and LIX 84, respectively, were used in this study. An Alamine 336-loaded membrane was placed between the 1st and the 2nd compartment, whereas the LIX 84-loaded membrane was placed between the 2nd and the 3rd compartment of the transport cell. With this cell, we were able to separate Fe 3+ , Cu 2+ and Ni 2+ simultaneously from a feed containing these three species in 1 M NaCl and at pH of 2, into three compartments of the transport cell. Fe 3+ was separated from the feed through the Alamine 336-loaded membrane, whereas Cu 2+ was transported through the LIX 84-loaded membrane. Ni 2+ remained in the central feed compartment (2nd compartment) of the cell. The transport fluxes of Fe 3+ and Cu 2+ through the two membranes were found to be 3.6 and 5.1 μmol/m 2 s, respectively.

Extractive Metallurgy of Tantalum

This paper presents a brief review of extractive metallurgy of tantalum starting from processing of its ore to two pure intermediates K2TaF7 and Ta2O5 and their conversion to pure tantalum metal by various technically feasible processes. Though tantalum metal can be produced by several means only two processes – sodium reduction of K2TaF7 and fused salt electrolysis of K2TaF7 in the presence of oxide, have been successful on industrial scale. Besides providing salient features of these two processes, the paper presents brief accounts of studies carried out on the reduction of oxide by metallic reductants – calcium and aluminium as well as nonmetallic reductants – carbon and carbon–nitrogen. The crude metal obtained by various reduction techniques outlined are purified either by solid state pyrovacuum treatment or by melt refining in an electron beam furnace. Mechanism of refining processes taking place during these post reduction treatments are also included in the review.

Extractive Metallurgy of Beryllium

Abstract A comprehensive review of the extractive metallurgy of beryllium is presented. Due to the strategic importance and element of secrecy surrounding the metal, any open literature on beryllium is rather limited. However, this review has been made to cover all important aspects of beryllium extraction technology namely, resources of the metal; processing of ores; reduction of fluoride and oxide to get metal and alloy; refining of the metal; post preparation processes to shape metal through powder metallurgical route and preparation of foils and ceramics. Special attention has been given to the topic of toxicology and pollution control. Recyclfng of beryllium will form the concluding section. Information on the removal of impurities during the preparation of intermediate products as well as during reduction to metal and its alloys are described with process principles. Thus this review, with the help of discussion on each unit process and at the same time emphasising on the problems faced in large scale handling of this toxic metal and its compounds, shall provide substantial information presently available on the extractive metallurgy of beryllium.

Rare Earth Metals and Alloys by Electrolytic Methods

Fused salt electrolysis as applied to the production of reactive refractory metals is a useful process for rare earth metals preparation also. The relatively low melting point of some of the rare earth metals and many of their alloys is an advantage here. With great ingenuity, electrolytic cells have been designed and methods developed for the preparation of high melting rare earth metals also in consolidated forms, using oxide–fluoride electrolytes. Any process of rare earth metal preparation had to contend with the great reactivity of these metals not only with the atmospheric gases but also with a great variety of crucible materials as well. The development of electrolytic processes involving both the chloride medium as well as the oxide–fluoride compositions had to be guided by this factor. The search for solutions to circumvent the numerous challenges inherent in the electrolytic extraction of rare earth metals led to some interesting process development. One of them is the preparation of rare earth metals by a method involving a consumable cathode. In this an alloy is prepared first by electrolysis and the metal is then recovered by a simple non electrolytic route from the alloy. Using one or other of these three possibilities, all the rare earth metals, and many of their alloys, could be prepared by electrolytic methods. All these issues and possibilities are highlighted in this paper.

Thermodynamics of Interstitial Impurities Removal from Refractory Metals

The applied thermodynamic aspects of removing hydrogen, nitrogen, oxygen, carbon, and silicon from vanadium, niobium and tantalum metals by pyrovacuum treatments are considered in this paper. Two major processes operate in refining by pyrovacuum treatment. One is distillation and the other is degassing. Distillation is mainly used to remove substitutional impurities that are either already present in the metal or have been added for removing any interstitial impurity. The success is determined by the partial pressure of the impurity element as well as by the difference in partial pressures of the impurity and the metal. The maximum rate of vaporization of impurity can then be estimated using the free evaporation equation. Interstitial impurities, particularly the gases hydrogen, nitrogen and oxygen, are removed by degassing. Thermodynamics of classical degassing, which is essentially the reverse of absorption, is described usefully by the pressure-composition isotherms. This is applicable mainly for the removal of hydrogen and nitrogen. The removal of oxygen, known as deoxidation, occurs by a more complex mechanism that involves the formation and evaporation of metal suboxides. Depending on the suboxide species responsible for deoxidation, the process is known as sacrificial deoxidation, carbon deoxidation, silicon deoxidation, or aluminium deoxidation. The applicability of these different processes to a particular metal, M is determined by the thermodynamic properties of the relevant M–O, M–C–O, M–Si–O, and M–Al–O system. It is shown in this paper that all the four processes are applicable to niobium and tantalum where as only aluminium deoxidation is useful for vanadium. For the removal of carbon and silicon also, the relevant deoxidation process can be used.