Alkaline earths separation

Reduction to Gaseous Metal. Volatile metals can be reduced and easily and completely separated from the residue before being condensed to a hquid or a soHd product in a container physically separated from the reduction reactor. Reduction to gaseous metal is possible for 2inc, mercury, cadmium, and the alkah and aLkaline-earth metals, but industrial practice is significant only for 2inc, mercury, magnesium, and calcium. 

Direct attack by hot 70—80 wt % hydrofluoric acid, sometimes with nitric acid (qv), is effective for processiag columbites and tantalo-columbites. Yields are >90 wt%. This method, used in the first commercial separation of tantalum and niobium, is used commercially as a lead-in to solvent extraction procedures. The method is not suited to direct processiag of pyrochlores because of the large alkaU and alkaline-earth oxide content therein, ie, ca 30 wt %, and the corresponding high consumption of acid. 

The reaction of chlorine gas with a mixture of ore and carbon at 500—1000°C yields volatile chlorides of niobium and other metals. These can be separated by fractional condensation (21—23). This method, used on columbites, is less suited to the chlorination of pyrochlore because of the formation of nonvolatile alkaU and alkaline-earth chlorides which remain in the reaction 2one as a residue. The chlorination of ferroniobium, however, is used commercially. The product mixture of niobium pentachloride, iron chlorides, and chlorides of other impurities is passed through a heated column of sodium chloride pellets at 400°C to remove iron and aluminum by formation of a low melting eutectic compound which drains from the bottom of the column. The niobium pentachloride passes through the column and is selectively condensed the more volatile chlorides pass through the condenser in the off-gas. The niobium pentachloride then can be processed further. 

Separation and Recovery of Rare-Earth Elements. Because rare-earth oxalates have low solubihty in acidic solutions, oxaUc acid is used for the separation and recovery of rare-earth elements (65). For the decomposition of rare-earth phosphate ores, such as mona ite and xenotime, a wet process using sulfuric acid has been widely employed. There is also a calcination process using alkaLine-earth compounds as a decomposition aid (66). In either process, rare-earth elements are recovered by the precipitation of oxalates, which are then converted to the corresponding oxides. 

Lewis acids, such as the haUde salts of the alkaline-earth metals, Cu(I), Cu(II), 2inc, Fe(III), aluminum, etc, are effective catalysts for this reaction (63). The ammonolysis of polyamides obtained from post-consumer waste has been used to cleave the polymer chain as the first step in a recycle process in which mixtures of nylon-6,6 and nylon-6 can be reconverted to diamine (64). The advantage of this approach Hes in the fact that both the adipamide [628-94-4] and 6-aminohexanoamide can be converted to hexarnethylenediarnine via their respective nitriles in a conventional two-step process in the presence of the diamine formed in the original ammonolysis reaction, thus avoiding a difficult and cosdy separation process. In addition, the mixture of nylon-6,6 and nylon-6 appears to react faster than does either polyamide alone. 

The properties of hydrated titanium dioxide as an ion-exchange (qv) medium have been widely studied (51—55). Separations include those of alkaH and alkaline-earth metals, zinc, copper, cobalt, cesium, strontium, and barium. The use of hydrated titanium dioxide to separate uranium from seawater and also for the treatment of radioactive wastes from nuclear-reactor installations has been proposed (56). 

Zirconium tetrachloride forms hexachlorozirconates with alkab-metal chlorides, eg, Li ZrCl [18346-96-8] Na2ZrClg [18346-98-0] K ZrCl [18346-99-1y, Rb2ZrClg [19381 -65-8] and Cs2ZrClg, and with alkaline-earth metal chlorides SrZrCh [21210-13-9] and BaZrCl [21210-12-8]. The vapor pressure of ZrCl over these melts as a function of the respective alkah chlorides and of ZrCl concentration were studied as potential electrolytes for the electrowinning of zirconium (72). The zirconium tetrachloride vapor pressure increased in the following sequence Cs < Rb < K < Na < Li. The stabiUty of a hexachlorohafnate is greater than that of a comparable hexachlorozirconate (171), and this has been proposed as a separation method (172). 

Alkaline-earth metals are often deterruined volumetricaHy by complexometric titration at pH 10, using Eriochrome Black T as indicator. The most suitable complexing titrant for barium ion is a solution of diethylenetriaminepentaacetic acid (DTPA). Other alkaline earths, if present, are simultaneously titrated, and in the favored analytical procedure calcium and strontium are deterruined separately by atomic absorption spectrophotometry, and their values subtracted from the total to obtain the barium value. 

The proportion of hydrochloric acid in the mobile phase was not to exceed 20%, so that complex formation did not occur and zone structure was not adversely affected. An excess of accompanying alkaline earth metal ions did not interfere with the separation but alkali metal cations did. The hthium cation fluoresced blue and lay at the same height as the magnesium cation, ammonium ions interfered with the calcium zone. 

The person whose name is most closely associated with the periodic table is Dmitri Mendeleev (1836-1907), a Russian chemist. In writing a textbook of general chemistry, Mendeleev devoted separate chapters to families of elements with similar properties, including the alkali metals, the alkaline earth metals, and the halogens. Reflecting on the properties of these and other elements, he proposed in 1869 a primitive version of today s periodic table. Mendeleev shrewdly left empty spaces in his table for new elements yet to be discovered. Indeed, he predicted detailed properties for three such elements (scandium, gallium, and germanium). By 1886 all of these elements had been discovered and found to have properties very similar to those he had predicted. 

Discussion. Some of the details of this method have already been given in Section 11.11(C), This procedure separates aluminium from beryllium, the alkaline earths, magnesium, and phosphate. For the gravimetric determination a 2 per cent or 5 per cent solution of oxine in 2M acetic add may be used 1 mL of the latter solution is suffident to predpitate 3 mg of aluminium. For practice in this determination, use about 0.40 g, accurately weighed, of aluminium ammonium sulphate. Dissolve it in 100 mL of water, heat to 70-80 °C, add the appropriate volume of the oxine reagent, and (if a precipitate has not already formed) slowly introduce 2M ammonium acetate solution until a precipitate just appears, heat to boiling, and then add 25 mL of 2M ammonium acetate solution dropwise and with constant stirring (to ensure complete predpitation). 

A mercury cathode finds widespread application for separations by constant current electrolysis. The most important use is the separation of the alkali and alkaline-earth metals, Al, Be, Mg, Ta, V, Zr, W, U, and the lanthanides from such elements as Fe, Cr, Ni, Co, Zn, Mo, Cd, Cu, Sn, Bi, Ag, Ge, Pd, Pt, Au, Rh, Ir, and Tl, which can, under suitable conditions, be deposited on a mercury cathode. The method is therefore of particular value for the determination of Al, etc., in steels and alloys it is also applied in the separation of iron from such elements as titanium, vanadium, and uranium. In an uncontrolled constant-current electrolysis in an acid medium the cathode potential is limited by the potential at which hydrogen ion is reduced the overpotential of hydrogen on mercury is high (about 0.8 volt), and consequently more metals are deposited from an acid solution at a mercury cathode than with a platinum cathode.10... 

There is little evidence for 1 1 compounds between elements in this group under normal conditions. The diatomic van der Waals molecules, CaMg, SrMg and SrCa, however, have been synthesized by codepositing the atoms from separate sources with argon or krypton into solid matrices at 12 K. These low-T species are identified from their laser-induced fluorescence spectra. The ground-state spectroscopic data for these alkaline-earth dimers form a sensible series between the parent molecules Mg2, Caj and Sr2. ... 

Figure 4.20 Separation of comon anions and alkaline earth cations by ion chronatography using conductivity detection.

Nair, L. M., Saari-Nordhaus, R., and Anderson, Jr., J. M., Simultaneous separation of alkali and alkaline-earth cations on polybudaiene-maleic acid-coated stationary phase by mineral acid eluents, /. Chromatogr., 640, 41,1993. 

In many instances electrogravimetry must be preceded by a separation between metals suitably this can be an electroseparation by means of constant-current electrolysis as previously described, but more attractively an electroseparation by means of controlled-potential electrolysis at a mercury pool or sometimes at an amalgamated Pt or brass gauze electrode. In this way one can either concentrate the metal of interest on the Hg or remove other metals from the solution alternatively, it can be a rougher separation, i.e., the concentration of a group of metals such as Fe, Ni, Co, Cu, Zn and Cd on the Hg whilst other metals such as alkali and alkaline earth metals, Be, Al, Ti and Zr remain in solution151. In all these procedures specific separation effects can be... 

Starting from the corresponding hydroxymethyl-benzocrown, it has been possible to generate the immobilized system (186) by reacting the above precursor with chloromethylated polystyrene (which is available commercially as Merrifield s resin). Typically, systems of this type contain a polystyrene matrix which has been cross-linked with approximately 1-4% p-divinylbenzene. In one study involving (186), a clean resolution of the alkali metal halides was achieved by HPLC using (186) as the solid phase and methanol as eluent (Blasius etal., 1980). In other studies, the divalent alkaline earths were also separated. 

Systems of the above type incorporating larger rings form binuclear complexes with a number of alkali and alkaline earth metal ions as well as with ions such as Ag(i) and Pb(n). For example, (212) forms a symmetrical dinuclear Na+ species in which the Na+ ions are associated with the respective azacrown and crown cavities and are separated from each other by 6.40 A in the solid (Fisher, Mellinger Weiss, 1977). 

For the heavier alkaline earth elements the characterized compounds are octahedral monomers, dimers, or separated-ion species. The complexes M(SR)2 (M=Ca, Sr, Ba) have been obtained by aminolysis and proved to be soluble in solvents such as py these compounds decompose rather cleanly to their metal sulfides.86 The monomeric species include [M(EMes )2(thf)4] (M=Ca, Sr, Ba E=S, Se),87 89 [Ca(SC6F5)2(py)4],90 [Ca(SMes )2(18-crown-6)] thf,90 or [Ba(Se-Trip)2(18-crown-6)].87 Only a dimeric barium derivative, [Ba(SeTrip) (py)3(thf)]2, has been described.87 For these elements separated ion-triple species, such as [M(18-crown-6)(hmpa)2][EMes ]2 (M=Ca, Sr, Ba E=S, Se)87,88,91 or contact/separated ion-triple species as [Ba(SMes )(18-crown-6)(hmpa)][SMes ] have been reported.91... 

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