EFFECT OF HOLDING DURING GLASS MELTING ON ITS REACTIVITY AND PHASE FORMATION

P. G. USOV, Yu. I. NESTEROV

(Presented by the scientific seminar of the Department of Silicate Technology)

A steatite batch obtained in the form of glass has a higher reactivity. During firing, reactions proceed in it at lower temperatures, as a result of which the interval between sintering and the sintered state is extended. Eitel and other investigators indicate [1—5] that the structure of glasses depends strongly on the preliminary heat treatment and on the methods by which they are obtained. Therefore it was necessary to determine the influence of the duration of glass melting on its reactivity and phase formation. These studies were carried out on glass of the following composition: Onot talc—80, barium carbonate—15, and Chasov-Yar clay—5%; or, in oxides: SiO₂—56.70%; Al₂O₃—2.06%; Fe₂O₃—0.24%; MgO—28.86%; BaO—11.95%; K₂O—0.18%, corresponding to the composition of steatite batch P-5.

Table 1

Sinterability and shrinkage during firing of articles made from glass with different holding times during melting

The glass was melted in a cryptol furnace, in corundum crucibles, at a temperature of 1560—1580°C. The furnace-heating regime was maintained [[unclear: text continues on next page]].

constant, with the time required for heating to the final temperature amounting to 5 hours. After different holds at the final temperature, the melted and clarified mass was poured into water. In this way glasses were melted with holds at the high temperature of 0.5, 1.0, and 4 hours. The dry glass with the addition of a surface-active substance was ground until it passed completely through a No. 100 sieve. Test specimens were formed by pressing on a paraffin binder. Firing of the articles was carried out at temperatures of 960, 1000, 1100, 1160, 1200, 1300, 1350, and 1380°C in a silite furnace according to the standard schedule.

Studies show that the reactivity of glasses melted with different holds in the molten state is not the same; it decreases with increasing hold time. The latter is recorded by an increase in the crystallization temperature of the glasses (Fig. 1) and by the sintering data (Table 1). The temperature of complete sintering of articles made from a batch prepared in the form of glass rises as the holding time during glass melting is lengthened.

The assumption that the glasses contain structural groupings that promote crystallization at lower temperatures is confirmed also by petrographic studies.

Fig. 1. Effect of the duration of holding at the melting temperature on the DTA curves of glasses: a—hold 30 min; b—hold 1 h; c—hold 4 h.

In the glass with a 30-minute hold, under the microscope areas with barely perceptible anisotropy are observed, with an average refractive index of 1.612. The principal mass of the glass is isotropic, with a refractive index of 1.566. The longer the hold at the glass-melting temperature, the more isotropic the glass becomes; areas with barely perceptible crystallization are not observed. The melt becomes more homogeneous in its structure. Rearrangement of the ions during crystallization in the process of sintering requires greater energy expenditures, and therefore it also proceeds with more difficulty (6, 7).

According to X-ray diffraction analysis, in the crystallization products of glass melted with a 30-min hold, the predominant phase is protoenstatite. Initially, at a temperature of 960–1160°C, fixa-

its main lines with \(d = 3.15;\ 1.99\) and \(1.53\) are recorded, and from a temperature of \(1200^\circ\text{C}\) other reflections, \(d = 3.47\) and \(2.31\), also appear clearly. In insignificant amounts, the second crystalline phase of the crystallization products is quartz with \(d = 1.82\text{–}1.84\) and \(1.53\). The index 1.53 coincides with the index of protoenstatite. This reflection on the X-ray diffraction pattern is combined. The principal crystalline phase of the crystallization products of glass melted with a holding time of one hour at all temperatures is also protoenstatite, but in the region of low temperatures, along with protoenstatite, clinoenstatite is also established in the composition of the crystalline phase, with its indices \(d = 3.28\text{–}3.29\) and \(d = 2.99\text{–}3.01\). The clinoenstatite phase is established on firing at temperatures from 960 to \(1300^\circ\text{C}\). On firing at \(1350^\circ\text{C}\) it disappears completely.

The third crystalline phase of the glass crystallization products is quartz with indices \(d = 1.82\text{–}1.84\) and \(d = 1.53\). Quartz appears upon firing at a temperature of \(1000^\circ\text{C}\) and exists throughout the investigated temperature range.

A more complex phase composition is obtained in the crystallization of glasses melted with a holding time of 4 hours. Here too, the principal crystalline phase is protoenstatite. But, along with protoenstatite, clinoenstatite is established over the entire temperature range, with its indices \(d = 3.24;\ d = 2.99\text{–}3.04\) and \(d = 2.08\text{–}2.09\). Forsterite is established with indices \(d = 2.15\text{–}2.17\) and \(d = 2.23\text{–}\)

Table 2

Crystallization of glass articles upon firing
with different holding times at the melting temperature. Firing temperature \(1300^\circ\text{C}\)
(removed from the disk)

\(d\) — interplanar spacing in crystals, Å.
\(I\) — absolute intensity of the lines.

2.25. Quartz from this glass does not crystallize. The melting of glass with a four-hour holding time was carried out in a zirconium dioxide crucible.

As a result of dissolution of the latter in the glass during melting, the crystallization products contain $\mathrm{ZrO_2}$ with indices $d = 1.94$–$1.97$ and zircon with $d = 2.15$–$2.17$ and $d = 1.69$–$1.71$.

The indices of the X-ray diffraction patterns are given in Table 2.

Conclusions

1. The reactivity of glass batches depends on the duration of holding at the glass-melting temperature.

2. Glasses with the minimum holding time during melting have the greatest reactivity.

Literature

1. W. Eitel. Physical Chemistry of Silicates, p. 114, Inostrannaya Literatura Publishing House, Moscow, 1962.

2. G. V. Kukolev. Chemistry of Silicon and Physical Chemistry of Silicates, pp. 574–576, Promstroiizdat, Moscow, 1951.

3. V. N. Filippovich. On the relationship between the structures of glass and sitall melts, in: Structural Transformations in Glasses at Elevated Temperatures, Publishing House of the Academy of Sciences of the USSR, Moscow–Leningrad, 1965.

4. G. Tammann. The Glassy State, ONTI, Moscow–Leningrad, 1935.

5. N. M. Bobkova. Kinetics of the formation of the glass structure in the melting process. Collection The Glassy State, Publishing House of the Academy of Sciences of the USSR, Moscow–Leningrad, 1965.

6. V. I. Danilov. Structure and Crystallization of Liquids. Publishing House of the Academy of Sciences of the Ukrainian SSR, Kiev, 1966.

7. New Glasses and Glass Materials, edited by N. M. Bobkova, N. N. Eremenko, and L. A. Zhunina, Minsk, 1965.