The role of liquid-liquid extraction on separation/recovery of zirconium and hafniu-A general study

Liquid-liquid extraction (LLE) is a process of transferring a chemical compound from one liquid phase to a second liquid phase, immiscible with the first. For the separation and purification of metal ions, this method is known since 1842 [1] . In LLE, a solute distributes itself between two immiscible liquids. The distribution of a solute between two immiscible solvents is univariant at constant temperature and pressure. That is, if we choose the concentration of the solute in one phase, its concentration in the other phase is fixed. In analytical chemistry, this method enjoys a favored position among separation techniques because of its simplicity, speed and wide scope. By utilizing simple apparatus no more complicated than a separatory funnel and requiring several minutes at most to perform, extraction procedures offer much to the analytical chemist. In chemical technology, the LLE of metal chelates play an important role in the purification of chemical reagents. This method is also frequently used in nuclear chemistry and technology for the separation of various radioisotopes and for the reprocessing of nuclear fuels. ZrSiO4. Zirconium’s main minerals are zircon (ZrSiO4) and baddeleyite (ZrO2) found in the USA, Australian and Brazil and invariably containing hafnium, most commonly in quantities around 2% of the zirconium content. Only in a few minerals, such as alvite, MSiO4.X H2O (M=Hf, Th, Zr), hafnium content occasionally exceed that of zirconium. As a result of the lanthanide contraction the ionic radii of zirconium and hafnium are virtually identical and their association in nature >parallels their very close chemical similarity. Hafnium, a co-genitor of zirconium, occurs to the extent of 1–2 % in the mineral zircon, which is an important constituent of the beach sands of south– west and east–coasts of India, representing about 8 % of the Global reserves. Indian Rare Earths operates three plants of mining and beneficiation of beach sand minerals and their current production of zircon is about 20,000 turns per year. In order to achieve these objectives, the zirconium and hafnium metals industry and the need to separate zirconium and hafnium from other associated metals like Ti, Fe, Si and Al etc are indispensable. The major uses of hafnium involve the metal as an alloying additive (1-2%) in the preparation of nickel-based super alloys. These alloys are used in turbine vanes in the combustion zone of jet aircraft engines. The second major use of hafnium is as control-rod material in nuclear reactors. Hafnium tetrachloride has been used to prepare hafnium metallocene Zieglar–Natta type catalysts, which were the first catalysts to provide high yields of high molecular mass isostatic polypropylene [3] and in some heavy – metal fluoride glass cladding [4] . cyanides; the solvent itself is highly volatile and exhibits about two percent solubility in the aqueous phase. In the TBP process, the organic phase containing TBP diluted in kerosene is contacted with aqueous phase containing below 30 gpl metals (Zr + Hf) and about 3 mol.dm –3 HNO3 and NaNO3. This

Zirconium metal basically finds application in the nuclear power program due to the combination of variety of properties such as (i) low thermal neutron absorption cross section, (ii) adequate strength and ductility at reactor operating temperatures, (iii) good corrosion resistance at high temperatures in aqueous environments, (iv) reasonable dimensional stability under irradiation and (v) good compatibility with the fuel material. Zirconium and hafnium co-exist in nature and are difficult to separate because of similar chemical properties due to the lanthanide contraction. To produce high pure zirconium, the processing involves production of ZrO2 free of hafnium. Klaproth [2] had previously isolated the oxide of zirconium from a sample of zircon Nuclear power plants in India Initially, two different LLE [5] techniques were used to promote zirconium and hafnium separation: the MIBK (Methyl-isobutyl ketone) – thiocyanic- hydrochloric acid process and the TBP (Tri-butyl-phosphate)–nitric acid process. The former began to be developed at the Oak Ridge National Laboratory in 1949 and was optimized in a pilot plant at the US Bureau of Mines. The second process was developed in France 1954 by the French Nuclear Agency and was improved at Iowa State University. In 1978, a French state company, CEZUS began to operate using a new pyrometallurgical process. In the MIBK process, the thiocyanate complexes of zirconium and hafnium formed in HCl medium (2 mol.dm –3 ) exhibit different solubility in MIBK. In this process, it is hafnium, the minor component that is concentrated in the organic phase and exhibits a separation factor (ß = DHf /DZr) of seven. Nevertheless, the generated waste streams contain high concentrations of ammonium process is selective for zirconium and exhibits a separation factor (ß = DZr /DHf) of ten. Presently, this process is used in India. The drawback of this process is its inability to produce nuclear grade hafnium. In spite of the commercial metal separation processes available based on LLE, there has been a