The thermodynamic activity of ZnO in silicate melts
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I.
INTRODUCTION
Z I N C is unusual in that, in its oxide and crystalline silicates, Zn 2+ is tetrahedrally-coordinated with oxygen. The only oxycompounds of zinc known to contain Zn 2+ in octahedral coordination are ZnCO3 and the high-pressure modification of ZnO. ~ In this respect zinc contravenes the normal rules of crystal chemistry, and hence it is of interest to determine whether or not the factors which determine the peculiar crystal chemistry of ZnO and ZneSiO4 cause any unusual properties in silicate melts containing ZnO. Experimental studies of the activity of ZnO in silicate melts have been confined to Fe-saturated FeO-CaOA1203-SIO2 melts containing 1 to 2 wt pct ZnO at 1200 ~ 2 and up to 10 wt pct ZnO at 1250 ~ 3 These studies were specifically concerned with conditions in zinc smelting operations. In addition, calculations of the activity of ZnO in lead blast furnace slags containing up to 25 wt pct ZnO have been made from analyses of operating plant data# The only published data for binary zinc silicates are estimations of the activities of the component oxides 5 and the free energies of formation 6 of the melts at 1600 ~ The present study is concerned with the experimental measurement of the activity of ZnO in zinc silicate and calcium zinc silicate melts.
II.
EXPERIMENTAL DETAILS
The equilibrium ZnO~ + CO = Zn(v) +
CO2
[1]
was measured using a transpiration technique in which a CO-CO2 gas mixture was passed over the melt containing ZnO at 1560 ~ The transpiration cell, which is shown in Figure 1, was fabricated from an alumina crucible of 1.8 cm diameter and 6 cm height with a 0.953 cm hole drilled in the center of its base, into which was cemented a 76 cm length of 0.953 cm O.D. • 0.635 cm I.D. alumina tube. The open end of the crucible was provided with a cap fabricated
from a 3.8 cm length of 2.54 cm O.D. x 1.91 cm I.D. alumina tube which was closed at one end by a 2.54 cm diameter alumina disk containing a 0.08 cm diameter hole. The melts were contained in boats, 5 cm in length and 1.2 cm deep, fabricated by cold-pressing 0.045 cm thick platinum sheet, and the experimental runs were conducted in a horizontally-mounted molybdenum wire resistance furnace. Reagent grade CaO, SIO2, A1203, and ZnO were used and master c o m p o s i t i o n s 2 Z n O . S i O 2 , 3 Z n O - 2 S i O 2 , ZnO- SIO2, 3CaO" 2SIO2, CaO" SIO2, 2CaO" ZnO- 2SIO2, 24 mole pct ZNO-33 mole pct SiOz-43 mole pct CaO, 42 wt pct SIO2-38 wt pct C a O - 2 0 wt pct A1203, and 62 wt pct SIO2-23.3 wt pct CAO-14.7 wt pct A1203 were prepared by pelletizing and sintering. Weighed mixtures of the predried reagent oxides were isostatically pressed into rods which were fired overnight in air at 1000 ~ The master compositions were then ground to - 2 0 0 mesh and stored in desiccators. In an experimental run 8 to 9 g of the appropriate mixture of master compositions were placed in the Pt boat and held in the experimental furnace for 30 to 35 minutes to allow formation of a homogeneous melt. The boat was then removed from the furnace, cooled
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