The crystallisation of Alkaline-earth Metal Tungstates from solutions in Sodium Tungstate Melts
| dc.contributor.author | Subramaniam, Appalasami | |
| dc.date.accessioned | 2017-07-04T08:15:51Z | |
| dc.date.available | 2017-07-04T08:15:51Z | |
| dc.date.issued | 1977-03 | |
| dc.description.abstract | This work presents a study of the crystallisation of Alkaline-earth Metal Tungstates from solutions in Sodium Tungstate Melts. The solubilities of the Alkaline-earth Metal Salts in Sodium Tungstate at various temperatures from 600° to 1200° C were obtained from the Iiterature. Preliminary quantitative studies were made on the crystallisation of Alkaline-earth Metal Titanates by slow continuous cooling of saturated solutions in Alkaline-earth Metal Chloride Melts from initial crystallisation temperatures, T0 = 1000° to 1200° C, down to ambient temperature. The effects of initial crystallisation temperature and solute solubility at the initial crystallisation temperature (solute-solvent interaction) and the rate of cooling on the final crystal form and size were investigated. These results were compared with the results of the preliminary studies on the crystallisation of Alkaline-earth Metal Tungstates by slow continuous cooling of saturated solutions in Sodium Tungstate Melts from initial crystallisation temperatures, T0 = 800°to 1000° C, down to ambient temperature. Sodium Tungstate was a suitable solvent for crystallisation of the Alkaline-earth Metal Tungstates within the temperature range T0 =8000 to I000°C and rates of cooling RT = 40° to 2000C/hr. The crystallisation of Calcium, Strontium and Barium Tungstates from Sodium Tungstate Melts was studied in detail by two methods, viz. the crystallisation in alumina crucibles by slow continuous cooling, and the crystallisation in platinum crucibles by slow continuous cooling. For crystallisation in alumina crucible, crystal growth generally occurred almost after the onset of cooling. Although there was no induction period but Strontium and Barium tungstates showed slow growth at the early stages for crystallisation at T 0 = 800°C and RT = 40°C/hr. Perhaps during this period of slow growth some supersaturation developed and probably both n ucl eati on and some growth onto very small early n uc I ei occurred si m ul tan eo usly • At low-values (0< ~ 0.1), crystal length ( l) varied linearly with time (t). These results suggest that the growth of these systems was probably diffusion rate -controlled. In all the above cases, crystal anisometry did not change. Some theoretical relations have been proposed for the determination of rate constants. The experimental rate constants increased in the order: CaW04 <. SrW04 ( BaW04 • For crystallisation in platinum crucible, crystal growth generally occurred after a short induction period. During such induction periods supenahration developed and probably both nucleation and some growth onto very small early nuclei occurred simultaneous. The variation of nucleus number (N) and t- values with initial crystallisation temperature (T ) and rate of cooling ('i') was studied in some detail. t- values ii1Creased with decrease in rate of cooling. The rate constants for crystal growth of CaWO 4, SrWO 4 and BaWO 4 were much smaller than the estimated values. | en_US |
| dc.identifier.uri | http://hdl.handle.net/123456789/4094 | |
| dc.language.iso | en | en_US |
| dc.publisher | Universiti Sains Malaysia | en_US |
| dc.subject | Alkaline-earth Metal Tungstates | en_US |
| dc.title | The crystallisation of Alkaline-earth Metal Tungstates from solutions in Sodium Tungstate Melts | en_US |
| dc.type | Thesis | en_US |
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