Foam Glass from Uzhur Nephelines

A. V. Petrov, S. S. Nesterenko, Zh. V. Rodionova

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

A heat- and sound-insulating material of cellular structure, called foam glass, is finding ever wider application for the thermal insulation of refrigerators and heat-generating units and as a sound-absorbing material in construction [1].

The production of foam glass is based on foaming glass powder with gas-forming agents in the temperature range 700–900°.

Uzhur nephelines, which are one of the deposits of nepheline-bearing rocks of Krasnoyarsk Krai, have in recent years been studied at the Department of Silicate Technology as a raw material for the glass industry, replacing scarce alkali-containing and other raw materials. The possibility of using nepheline-bearing rocks in glass production, as was shown earlier [2], is due to the similarity of their compositions to those of glass batches. The content of alkalis, which are the most valuable constituent, reaches 13% in Uzhur nephelines; the high content of aluminum oxide (up to 25%) imparts to the glass high thermal and mechanical strength and high chemical resistance with respect to alkalis and water. A considerable amount of iron oxides (up to 10%) makes it possible to produce dark-green and black glasses in the form of building marblites, facing tiles, green bottles, and other products. It is known from the literature [3, 4] that a porous material from rocks can be obtained without converting them to a glassy state and from glass powder obtained as a result of melting batches containing rocks.

The objective of our investigation was to obtain a porous material directly from nepheline rock and from glass powder prepared on its basis. The studies carried out showed that nepheline rock, ground and mixed with a gas-forming agent (silicon carbide), foams at a temperature of about 1200°C (the melting temperature of the rock is 1220°C), forming a porous material of nonuniform structure with a wide range of bulk density, 0.5–1.3 g/cm³. By introducing fluorspar and calcined soda it is possible to lower the foaming temperature to 1100°C, but owing to the short sintering interval (20°C) the foaming process of the rock and of its synthetic mixtures becomes extremely sensitive to fluctuations in the thermal regime of the furnace, as a result of which the specimens rapidly deform and melt. The addition of quartz sand up to 30% contributes

leads to the formation of a melt capable of sharply reducing its viscosity even with a slight increase in temperature, as a result of which the sintering interval also hardly broadens (25–30°C); therefore, obtaining a porous material from the raw material under study by the indicated technology under industrial conditions is impossible.

Further work was based on the preliminary production of glasses from batches containing 50 percent or more of nepheline rock, with their subsequent foaming. For this purpose, one of the glass compositions developed by us was used (Table 1). The batch included nepheline rock, enriched quartz sand from the Tugansk deposit, chalk, and sodium sulfate; their calculated chemical compositions and quantities by weight are given in Table 1.

Table 1

Calculated chemical compositions and quantities by weight of raw materials

The prepared batches were placed in corundum crucibles with a capacity of 100 cm³ and melted in an electric furnace with silite heating elements at a temperature of 1350°C. The melts were not brought to complete clarification; they were rapidly cooled in cold water, and the resulting granulated powder was ground in a porcelain ball mill with a capacity of 3 l until it passed through a 0.15 mm sieve. Coke breeze, introduced in an amount of 3% of the batch weight, was selected as the gas-forming agent. From the thoroughly mixed and moistened batch, under a pressure of 400 kg/cm², cylindrical specimens 25 mm in diameter were pressed; they were then foamed in a silite furnace at experimentally established temperatures: 780, 820, 860, 880, and 900°C, with holding times of 5, 10, 15, and 20 min. Foaming of the glass powder occurs by the gaseous phase formed as a result of the thermal decomposition of the gas-forming agent and chemical reactions taking place between the gas-forming agent and certain components of the glass.

We have established that the method for obtaining a porous material from nepheline-containing glasses is based on the reaction of reduction of the sulfur contained in the glass by the carbon of the gas-forming agent to sulfide sulfur, as indicated in the literature [5], since glasses melted with calcined soda did not foam under any conditions. The foamed specimens were annealed in a muffle furnace. Annealing of the thin walls of isolated pores of foam glass can be completed in a short time, but, owing to its low thermal conductivity, the temperature gradient between the surface and the core of the product persists for a long time; therefore, in order to prevent the appearance of stresses that sharply reduce

mechanical strength, the cooling of the foam-glass specimens was carried out at a rate not exceeding 5–7°C per minute. After cooling, the specimens were subjected to determination of various types of physicomechanical properties. From glass prepared on the basis of Uzhur nephelines under laboratory conditions, foam glass was obtained with a bulk density of 0.36–0.5 g/cm³ (in a dense body), water absorption up to 30%, true porosity of about 85%, and mechanical strength up to 30 kg/cm². The temperature at which softening of the foam glass begins is 700°C; the coefficient of linear expansion is \(\alpha = 80 \cdot 10^{-7}\ \mathrm{deg.}^{-1}\). The structure of the foam glass obtained is homogeneous, finely porous, with a maximum size of individual pores of 3–5 mm.

Fig. 1. Change in the compressive strength of foam glass as a function of temperature and holding time

Petrographic and X-ray phase studies established that at a temperature of 870°C and above the walls of the foam-glass cells begin to crystallize intensively, forming a structure of the foam-crystal type. This is also indicated by the rapidly increasing mechanical strength of specimens (Fig. 1) foamed at a temperature of 900° \((\sigma_{\mathrm{comp}} > 50\ \mathrm{kg/cm^2})\). Pyroxene, plagioclase, and wollastonite form as the crystalline phase.

Thus, nepheline-containing rocks of Krasnoyarsk Krai with additions of sodium sulfate, chalk, and quartz sand can be used to obtain foam glass satisfying the requirements imposed on heat-insulating and sound-absorbing materials. In this case, the saving of alkalis may reach 65%.

REFERENCES

1. I. I. Kitaygorodskii. Glass and Glassmaking, Promstroiizdat, 1950.
2. A. V. Petrov, S. S. Nesterenko, G. P. Anpilogova. Glass from Goryachegorsk nepheline-containing rocks, Proceedings of the Mendeleev All-Union Chemical Society, Tomsk, 1969.
3. Method for obtaining foam glass, U.S. Patent No. 3174870, RZhKh, 16, 1966.
4. I. I. Kitaygorodskii, T. N. Keshishyan. Foam Glass, Promstroiizdat, 1953.
5. F. Schill. Penove sklo, Praha, 1962.