Metal fractionation

A1 Zn Zr Th Rare earth metals Fractionated rare earth metals Be Mn Ag... 

PUCI3, and MgCl2 to form a 50/50 mole % NaCl-CaCl salt phase and a molten Am-Pu-Mg-Ca alloy which is immiscible in the above salt(lO). After cooling, the metal phase is cleaved away from the salt phase and the salt phase is analyzed. Little, if any, Am or Pu remains in the salt phase and the salt residues can be discarded to waste. Metal recovery begins by evaporating magnesium and calcium from the residual metal button at about 800°C in vacuum. The americium can then be distilled away from the plutonium in a vacuum still operated at 1200°C, using yttria ceramic vessels to contain the molten metal fraction. The bottoms fraction contains the plutonium which is recycled back into the main plutonium stream. 

The individual nonferrous metal fractions thus obtained are then processed in conventional ways.32... 

Zhao K, Liu X, Zhang W, Xu J, Wang F (2011) Spatial dependence and bioavailability of metal fractions in paddy fields on metal concentrations in rice grain at a regional scale. J Soils Sediments 11(7) 1165—1177. doi 10.1007/s 11368-011-0408-6... 

Of the two models, homogeneous accretion is generally favoured. H. Wancke from the Max Planck Institute in Mainz (1986) described a variant of this model, in which the terrestrial planets were formed from two different components. Component A was highly reduced, containing elements with metallic character (such as Fe, Co, Ni, W) but poor in volatile and partially volatile elements. Component B was completely oxidized and contained elements with metallic character as their oxides, as well as a relatively high proportion of volatile elements and water. For the Earth, the ratio A B is calculated to be 85 15, while for Mars it is 60 40. According to this model, component B (and thus water) only arrived on Earth towards the end of the accretion phase, i.e., after the formation of the core. This means that only some of the water was able to react with the metallic fraction. 

If the reaction in which the metallic fraction serves as a catalyst produces water as a by-product, it may well be that the catalyst converts back to an oxide. One should always be aware that in fundamental catalytic studies, where reactions are usually carried out under differential conditions (i.e. low conversions) the catalyst may be more reduced than is the case under industrial conditions. An example is the behavior of iron in the Fischer-Tropsch reaction, where the industrial iron catalyst at work contains substantial fractions of Fe304, while fundamental studies report that iron is entirely carbidic and in the zero-valent state when the reaction is run at low conversions [6],... 

Praesodymium may be recovered from its minerals monazite and bastana-site. The didymia extract of rare earth minerals is a mixture of praesodymia and neodymia, primarily oxides of praesodymium and neodymium. Several methods are known for isolation of rare earths. These are applicable to all rare earths including praesodymium. They include solvent extractions, ion-exchange, and fractional crystallization. While the first two methods form easy and rapid separation of rare earth metals, fractional crystaUization is more tedious. Extractions and separations of rare earths have been discussed in detail earlier (see Neodymium and Cerium). 

The two variables change their role with respect to their dependent versus independent, intensive versus extensive nature. This is also true of e.g. calorimetric, conductometric and spectrophotometric titrations using UV-, IR- or NMR-spectrosco-py We additionally have to consider that in the titration of the catalytic process only the external dynamics are measured a direct comparison with the actual metal fraction of the related intermediate complexes is generally not possible We call this analysis of homogeneous catalytic systems by a metal-ligand titration the method of inverse titration and for the resulting diagrams we use the term li nd-concentration control maps ([L]-control maps) . 

Fig. 4. Separated metal fractions from different grain size classes during sample reduction (by operators metal >5 cm from mixed sample metal > 1 cm from split < 1 cm metal from 1 cm to 2 mm from BA split <2 mm metal <2 mm).

Another example is provided by the chemical fractionation of tungsten into planetary cores. Tungsten has a short-lived radioactive isotope, W, which decays into Hf. Tungsten is siderophile and hafnium is lithophile. Consequently, the daughter isotope, 182Hf, will be found either in the core or the mantle depending on how quickly metal fractionation (core formation) occurred relative to the rate of decay. The Hf- W system is used to date core formation on planetary bodies. We will discuss the details of using radioactive isotopes as chronometers in Chapters 8 and 9. 

Materials. Cyclohexene, obtained by dehydration of reagent grade cyclohexanol (3), was heated at reflux over sodium metal, fractionated on a 60-cm. Helix packed column, stored over sodium, and filtered just before use. No impurity was found by gas chromatography (column, TCP and Si-550 carrier gas, helium). Propylene (Neriki Research Grade) used showed no impurity by gas chromatography (column, active carbon and acetonylacetone). 

Treatment of waste should be considered only after source reduction and recycling options are fully addressed. Treatment includes methods for separation of the metals fraction from the wastes stream. This typically involves neutralization, precipitation, filtration and drying operations. Waste treatment, although often desirable and necessary, is not considered to be a waste minimization option by the USEPA... 

Kong, I.-C, and Bitton, G. (2003) Correlation between toxicity and metal fractions of contaminated soils in Korea, Bulletin of Environmental Contamination and Toxicology 70, 557-565. 

There are few methods which can measure well-defined metal fractions with sufficient sensitivity for direct use with environmental samples (approach B in Fig. 8.2). Nevertheless, this approach is necessary in the experimental determination of the distribution of compounds that are labile with respect to the time scales of the analytical method. Recent literature indicates that high-performance liquid (HPLC) and gas chromatographic (GC) based techniques may have such capabilities (Batley and Low, 1989 Chau and Wong, 1989 van Loon and Barefoot, 1992 Kitazume et al, 1993 Rottmann and Heumann, 1994 Baxter and Freeh, 1995 Szpunar-Lobinska et al, 1995 Ellis and Roberts, 1997 Vogl and Heumann, 1998). The ability to vary both the stationary and mobile phases, in conjunction with suitable detector selection (e.g. ICP-MS), provides considerable discriminatory power. HPLC is the superior method GC has the disadvantage that species normally need to be derivatised to volatile forms prior to analysis. Capillary electrophoresis also shows promise as a metal speciation tool its main advantage is the absence of potential equilibria perturbation, interactions... 

Development of chemical speciation schemes which can be directly related to measures of bioavailability - This would allow the determination of which active trace element species merit the most intensive research from the standpoint of environmental perturbation. Some studies have attempted to correlate metal fractions determined by a particular technique (operationally defined speciation) with those that are bioavailable (functionally defined speciation) (Larsen and Svensmark, 1991 Buckley, 1994 Deaver and Rodgers, 1996). However, any correlation is only empirical and more research is required to achieve an understanding of the mechanisms involved in bioavailability and to develop rational predictive models. 

Figura, E and McDuffie, B. (1979) Use of Chelex resin for determination of labile trace metal fractions in aqueous ligand media and comparison of the method with anodic stripping voltammetry. Anal. Client., 51, 120-128.

Scarano, G., Bramanti, E. and Zirino, A. (1992) Determination of copper complexation in sea water by a ligand competition technique with voltammetric measurement of the labile metal fraction. Anal. Chim. Acta, 264, 153-162. 

Gupta, S.K., Vollmer, M.K. and Krebs, R. (1996) The importance of mobile, mobilisable and pseudototal heavy metal fractions for 3-level risk assessment and risk management. Sci. Total Environ., 178, 11—20. 

Mester, Z., Cremisini, C., Ghiara, E. and Morabito, R. (1998) Comparison of two sequential extraction procedures for metal fractionation in sediment samples. Anal. Chim. Acta, 359, 133-142. 

Singh, S.R, Tack, F.M. and Verloo, M.G. (1998) Heavy metal fractionation and extract-ability in dredged sediment derived soils. Water Air Soil Poll., 102, 313-328. 

Tack, F.M.G., Vossius, H.A.H. and Verloo, M.G. (1996) A comparison between sediment metal fractions, obtained from sequential extraction and estimated from single extractions. Int.J. Environ. Anal. Chem., 63, 61. 

Metal speciation procedures, which have been verified under controlled laboratory conditions and evaluated by means of bioassays, will require further verification in order to determine their ecological effects. For example, how does the response of the bioassay test species to a toxic metal fraction relate to the toxicity to larger organisms such as fish in the natural environment Bioaccumulation of metals in populations has been very difficult to relate to metal speciation measurements. There is a challenge for analytical chemists to develop metal speciation procedures that are relevant to ecotoxicology (Morrison and Wei, 1991). 

An example of a method suitable for the determination of cadmium, cobalt, copper, iron, manganese, nickel, and zinc in water, using chelation and sample extraction, is as follows [113]. The sample is filtered through an acid-washed membrane filter as soon as possible after collection. It is then acidified with nitric acid for preservation until analysis. This will give the soluble metal fraction. If the total metal content is to be found, the sample is acidified and allowed to stand for 4 days with occasional shaking. Then it is filtered. 

According to the classical percolation theory, when a metal fraction tends to the critical xc the resistivity follows the power law ... 

The main restriction of Efros and Shklovskii [73] and Dykhne [74] results is the lack of consideration of quantum effects. Taking into account the quantum corrections to conductivity [57-59] leads to the following temperature dependence at metal fractions above the percolation threshold ... 

Fig. 11.15. Normalized magnetoresistance as a function of the ferromagnetic metal fraction x (a) -Co-Si02 (b) -Ni-Ag (c) -Co-Ag (d) -Ni-Sio2. The results for Ni-samples are multiplied by 10 [87],...

At temperatures above Tt, electrons pass this barrier freely while at lower temperature conductivity through the barrier is due to electron tunneling. So, raising the temperature results in a crossover from tunneling to metal-type conduction. Thus, temperature induces insulator-metal transition in the same way as a metallic fraction variation near xc. This transition was cold QSE transition. 

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