The crystallisation of Alkaline-earth Metal Tungstates from solutions in Sodium Tungstate Melts
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Date
1977-03
Authors
Subramaniam, Appalasami
Journal Title
Journal ISSN
Volume Title
Publisher
Universiti Sains Malaysia
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.
Description
Keywords
Alkaline-earth Metal Tungstates