SUBJECTS 329 tungstate

All commercial materials are based on calcium hydroxide and liquid alkyl salicylates (Prosser, Grolfman Wilson, 1982) and are supplied as a two-paste pack. Zinc oxide is sometimes added to the calcium hydroxide, as are neutral fillers. A paste is formed from this powder by the addition of a plasticizer examples include A-ethyl toluenesulphonamide (o- orp-) and paraffin oil, with sometimes minor additions of polypropylene glycol. The other paste is based on an alkyl salicylate as the active constituent containing an inorganic filler such as titanium dioxide, calcium sulphate, calcium tungstate or barium sulphate. Alkyl salicylates used include methyl salicylate, isobutyl salicylate, and 1-methyl trimethylene disalicylate. An example of one commercial material, Dycal, is given in Table 9.7, but its composition has been subjected to change over the years. 

If tungsten is recovered from the wolframite group mineral, the wolframite concentrate is boiled or pressure-digested with 50% caustic soda solution. Alternatively, they may be fused or sintered with caustic soda, caustic potash or sodium carbonate and the fused mass then leached with water. The solution is filtered to separate sodium tungstate solution. The fdtrate is subjected to various treatments to remove molybdenum, phosphorus, and arsenic impurities. The filtrate at this point is essentially a solution of sodium tungstate and is treated in the same way as that obtained from the scheehte concentrate discussed above. 

The theory of electronic excitation, which occurs during plastic deformation and fragmentation of crystals, is discussed by Molotskii [99]. The mechanochemistry of inorganic solids has been reviewed by Boldyrev et al. [100] with particular reference to the work done in The Institute of Solid State Chemistry at Novosibirsk. The consequences of mechanical activation on the structures and properties of selected spinels have been examined and the rate of the reaction forming barium tungstate investigated. Prospects for the future development of this subject are assessed. 

The sodium tungstate is the process solution from which the tungsten is recovered by further treatment. The insoluble hydroxides are filtered off and constitute the tungsten ore tailings, the subject of this study. The tailings are usually stored in a self-contained site prior to ultimate disposal. 

D. P. Smith and F. A. Hartgen studied this subject. For the transition temp., vide supra. H. le Chatelier, E. GrosohuS, and M. Amadori studied the m.p. of mixtures of potassium sulphate and chromate—vide Fig. 43—a continuous series of solid soln. is formed. M. Amadori obtained continuous series of solid soln. with potassium chromate and potassium molybdate or tungstate. S. F. Sohemt-sohuschny studied the m.p. of the binary system K2Cr04-KCl, and found a typical... 

The reactor was assembled with a plasma treated microporous PVDF membrane containing sodium tungstate as catalytically active species. The PVDF membranes were subjected to plasma treatments in NHj fed radio frequency glow discharges in order to generate amine functions on the surface of PVDF membranes prepared by a phase-inversion procedure, which are suitable for the immobilization of Na2W04.The modes of reactor operation as forced-flow or contactor were compared, being the best results achieved with a transmembrane pressure AP = 0 (contactor mode), since this mode of operation is the one that better favours the contact between the reactants and the catalytic active sites. 

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