Advanced Materials Research Vol. 1019

Abstract: Shape memory alloys (SMAs) are a fascinating group of metals that have two remarkable properties, the shape memory effect and superelasticity. The TiPt structure with the B2 phase has been reported to undergo a reversible displacive transformation to B19 martensite at about 1200K. However, this system could serve in principle as the basis of high-temperature shape memory alloys. Molecular dynamics study of martensitic transformation in platinum titanium alloys was performed to investigate the effect of temperature dependence on B2 and B19 structures at 50 at.%Pt. The NPT ensemble was used to determine the properties of these systems and we found good comparisons with recent experimental work. The temperature dependence of TiPt shows potential martensitic change when B19 is heated to extreme high temperatures of 273K up to 1573K.

Abstract: The supercell approach method was used to investigate the effect of partial substitution of Pt with Co on the TiPt potential shape memory alloy. The first-principles calculations were carried out within the generalized gradient approximation to determine the stability of the Ti₅₀Pt_50-xCo_x for x=6.25, 18.75 and 25. We found that the calculated heats of formation and density of states predicted the 6.25 at. % Co to be the most stable structures compared. The elastic properties, thermal coefficient of linear expansion and the density of states results suggest that the partial substitution of Pt with Co decreases the Ms of TiPt with the softening of the Cʹ shear moduli.

Abstract:

Glow Discharge Optical Emission Spectroscopy (GD-OES) is an analytical technique mainly used in the analysis of solid metallic samples. The technique requires a conductive sample as the analyte serves as the cathode when generating the glow discharge plasma. GD-OES is useful for both bulk quantification and depth profiling of thin layers of conducting materials. The objective of this study was to develop a new sample support matrix for the preparation of conductive pressed pellets suitable for the analysis of non-conducting materials with GD-OES. In previous work non-conducting powders, such as uranium oxide, have been mixed with fine metal powders such as copper, silver or tantalum. Another solution has been to use a quick setting, conductive thermoplastic, such as diallyl phthalate impregnated with copper, as support. Both of these methods are, however, expensive and fairly time consuming. Graphite, a cheap, readily available conductive powder, proved not to form a strong enough pellet to withstand the conditions required during the GD analysis. This limitation was overcome by the addition of a binding agent, bakelite, to produce a relatively cheap, conductive matrix for the analysis of non-conducting powders. Spectroscopically pure zirconium oxide was used as a reference material and mixed with various quantities of graphite and bakelite powder. Two distinct regions of linearity were obtained. Samples with less than six percent zirconium yielded a gradient of 0.0011 with an R² value of 0.9949. Samples with higher zirconium content yielded a gradient of 0.0042 with an R² value of 0.9991. These results indicate the suitability of this sample matrix for analysis of zirconium materials by GD-OES.

Abstract: An important step of a new process being developed for the beneficiation of the mineral zircon (Zr (Hf)SiO₄) to produce nuclear grade zirconium (Zr) metal, is the separation of the Zr from the hafnium (Hf). Zr ores typically contain between 1 and 3% Hf , whereas the use of Zr metal in the nuclear industry requires a Hf content <100 ppm, owing to its high neutron-capture cross section. The separation step is therefore key in the preparation of nuclear grade Zr, which is considered to be very difficult due to the various similarities in their chemical properties. The preparation of hafnium free zirconium relies on the traditional wet separation systems, for example solvent extraction systems. In contrast to the traditional aqueous chloride systems, Necsa focusses on dry fluoride-based processes. Dry processes have the advantage of producing much less hazardous chemical waste. In the work reported her, separation is achieved by sublimation/de-sublimation in the tetrafluoride form. The tetrafluoride is prepared by fluorination of plasma dissociated zircon (PDZ or Zr (Hf)O₂•SiO₂) with ammonium bifluoride (ABF). The separation involves the selective sublimation of the two tetrafluorides in an inert atmosphere under controlled conditions, and subsequent similarly selective desublimation. An accurate estimation of the sublimation rates the zirconium tetrafluoride (ZrF₄) and hafnium tetrafluoride (HfF₄) as a function of temperature is required since this forms the basis of the development of a sublimation model to determine whether the concept under consideration is theoretically possible. The sublimation kinetics of ZrF₄ is reported in this paper.

Abstract:

Tantalum (Ta) and niobium (Nb) are two metals with similar chemical and physical properties and are found together in nature. One form of Ta is tantalum pentafluoride, which is stable in reducing environments, is corrosive resistant and stable under harsh conditions. Ta is currently used in nuclear reactors with a wide variety of uses and advantages. For these applications, pure Ta is needed to insure high value catalysts, contrary the higher the purity grade the more expensive the production of these high value catalysts. One way of ensuring an economic viable process for the production of high purity Ta, is to find a cost effective way to separate Ta from Nb. Ungerer et al. studied the separation of Ta and Nb, using safer chemicals and techniques for the environment in a solvent extraction (SX) process. During this study, separation was achieved in a sulphuric acid (H₂SO₄) medium with the extractants diiso-octyl phosphinic acid (PA) and di-(2-ethylhexyl) phosphoric acid (D2EHPA). The main obstacle during this study was the speciation of Ta and Nb, springing the question of why separation occurred with some extractants and not with the others. One method for determining the speciation of the compounds in a reaction mixture is by using computational techniques for molecular modelling. Several molecular modelling programs are available which uses various mathematical equations and approximations. Progress in computational chemistry over the last 20 years has made quantum mechanical calculations on large molecules, chemical systems as well as on macromolecule reactions possible. Calculations based on the density-functional theory (DFT) are now, not only used on light elements and small molecules, but also on metal complexes, heavy metals and especially on metal separation in SX. With these models at hand, SX processes were modelled within realistic margins to fit the experimental setup in a small scale laboratory. It is anticipated that the advances from this work will provide the possibility to determine, with good approximation, the outcome not only of the proposed Ta SX experiments, but also SX in general, before embarking on expensive, time consuming experiments and environmental unfriendly waste generation. In this paper molecular modelling was used to compile a partial energy profile for a proposed reaction mechanism for the reaction of tantalum- and niobium pentafluoride (TaF₅, NbF₅) with water to form tantalum- and niobium hydroxides. In the process, possible species that may form during the reaction were identified and evaluated.

Abstract: The monomeric coordination compound pentachloro(acetonitrile)niobium(V), [NbCl₅(NCCH₃)], (NCCH₃ = acetonitrile) has been prepared under aerobic conditions and characterized by single-crystal X-ray diffraction. [NbCl₅(NCCH₃)] crystallized in the monoclinic spacegroup P2₁/c with a = 5.964 (3), b = 9.888 (5), c = 15.448 (9) and β = 98.224 (2). Comparison is made to a previously published isomorphous complex that was prepared under unaerobic conditions.

Abstract: The Process Beneficiation of Minerals such as the Tantalum and Niobium Ores May Vary Significantly and the Final Sequence of Steps Normally Depends on the Physiochemical Properties. these Properties are Often Utilised in Designing a Processing Route. Removal of Iron and Titanium from the Manganotantalite and Ferrotantalite Samples in this Study was Successfully Accomplished by Magnetic Separation. Acid Leaching Using H₂SO₄ on the other Hand Removed a Large Portion of Thorium and Uranium in these Samples. Analytical Results Indicated that 66(1) and 54.7(5)% of Total Fe and Ti Respectively, and ~ 2% each of Nb₂O₅ and Ta₂O₅ were Removed from Ferrotantalite Using the Magnetic Separation Method, while only 9.0(1) and 8.61(4)% of Total Fe₂O₃ and TiO₂ Respectively, and ~2% each of Nb₂O₅ and Ta₂O₅ were Removed from Manganotantalite. Tantalite Ores Leached at 50 °C for 3 Hrs in the Presence of Concentrated H₂SO₄ was Found to be the most Appropriate Condition to Remove Radioactive Elements. Results Obtained from Sample A under these Conditions Indicated that 62(2)% U₃O₈ and 60.8(3)% ThO₂ were Leached into the Acidic Solution along with 4.45 and 0.99% of Nb₂O₅ and Ta₂O₅ Respectively.

Abstract: The formation reaction of acetylacetone to solvated tantalum pentachloride ([TaCl₅]₂) in methanol, is defined by limiting kinetic behaviour and is indicative of a two-step process. This involves the rapid formation of an intermediate, possibly trans-[TaCl₂(OMe)₃(η¹-acacH)]) with the stability constant, K₁, equal to 2.4 ± 0.3 x 10³ M^-1 at 25 °C. The second order rate constant, k₁, at 25 °C was determined as 3.1 ± 0.1 x 10² M^-1 s^-1. A fit of the data obtained from the UV-Vis study, and describing the total reaction, yielded a value of 1.7 ± 0.2 x 10³ M^-1 s^-1 for K₁, which is in agreement with the value obtained earlier. Comparison of k₁ and k₂ indicates that the first reaction is roughly six orders of magnitude (10⁶) faster than the second rate determining step.

Abstract: The suitability of Liquid-liquid extraction (LLE) for the selective extraction of chemically similar tantalum (Ta) and niobium (Nb) mixtures was investigated by determining the influence of the acid concentration, extractant to metal mole ratio (E:M) as well as the ageing of the feed on extraction using the extractants, bis (2-ethylhexyl) phosphate (D2EHPA) and di-iso-octylphosphinic acid (PA). The system consisted of a solvent in varying E:M ratio’s, diluted in cyclohexane with 3% (v\v) 1-octanol added as modifier and a feed solution containing sulphuric acid and 100 ppm of the NH₄TaF₆ and NH₄NbF₆ complexes. Depending on the acid concentration, extraction percentages (%E) of up to 100% for Ta and 10-20% for Nb were attained. An initial lack of repeatability in experimental results was shown to be caused by variations in the age of the feed solution. The change in extraction trends expressed as a normalized %E for the aged feed solutions were nearly identical for both extractants when using 3, 6, 9 and 14mol/dm³ H₂SO₄. For the 3 and 6mol/dm³ solutions, the %E decreased significantly within the first 4-5 hours of ageing. At 9mol/dm³ the %E remained stable for feed ages up to 3.5 hours before declining, while the %E remained near constant at 14mol/dm³even after ageing for 24 hours.

Abstract: Monazite is a light rare earth phosphate which is difficult to process when using conventional chemical processes. It is considered to be one of the most important commercial sources of thorium and lanthanides. Using conventional chemicals to process monazite have caused severe environmental damage as has been demonstrated in the Baotou region, China. Monazite is found in combination with other minerals in nature such as bastnaesite and xenotime. The conventional techniques and chemicals used for the processing of monazite are expensive. In order to address this issue, South Africa would like to beneficiate monazite as part of its mineral beneficiation strategy. Doing so competitively would require a new cheaper and more environmentally friendlier process. A new method for the processing of monazite is currently being investigated. It is proposed to feed the monazite is fed into a plasma reactor to crack it. Potentially the cracking will allow the monazite to be more reactive and susceptible to react with less harsh chemicals. The plasma treated monazite is reacted with a fluorinating compound such as ammonium bifluoride. Ammonium bifluoride is used rather than fluorine or hydrogen fluoride as it is less dangerous to handle. The fluorinated rare earth mixture can now be separated using various methods.