EFFECT OF ALKALI METAL OXIDE ADDITIVES ON PHASE FORMATION OF TALC DURING FIRING

P. G. USOV, N. V. SOBORA

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

The results of numerous studies on the transformation of talc during firing are often contradictory (1, 2, 3). The reason for this is partly that most investigations have been carried out on natural talc materials having different chemical compositions and crystal morphology. Obviously, minor chemical impurities can change both the decomposition temperature and the nature of the transformations of talc. Therefore, determining the influence of additives on the phase formation of talc during firing is of great importance.

For this purpose, the highest-quality talc samples from the Alguiskoye and Onotskoye deposits were taken; their chemical composition differs only slightly from the theoretical composition of the mineral talc (Table 1).

Table 1

Chemical composition of talcs

Alguiskoye powdered talc is white in color and has a scaly structure. Under the microscope, the colorless flakes show straight extinction and well-pronounced pseudoabsorption, with \(Ng = 1{,}580\) and \(Np = 1{,}539\). About 2% is present as a mechanical impurity in the form of isometric quartz grains.

Onotskoye talc is a dense, massive rock, white with a greenish tint; in ground form it is white. Under the microscope, Onotskoye talc—a monomineral talc variety—is composed of fine-grained and finely scaly aggregates. The refractive index is \(Ng \to 1{,}583\), \(Np = 1{,}541\).

Additives of \(Li_2O\), \(K_2O\), and \(Na_2O\) in an amount of 2% were introduced into the raw talcs in the form of aqueous solutions of the carbonate of the corresponding alkali metal.

Samples of talc (Onot and Alguy) without additives and with additives of alkali-metal oxides, fired at temperatures of 800–1500°C at intervals of 100°C, were subjected to optical and X-ray diffraction methods of analysis.

In samples without additives of alkali-metal oxides, a noticeable change in the talc occurs at 900°C, and in Onot talc it proceeds more strongly. The new formations produced during firing of Alguy talc acquire a fibrous structure, whereas those of Onot talc have a platy appearance, more rarely a fibrous one.

Kinter indicates [4] that when decomposition reactions occur, the deformation energy of the crystal lattice causes the emergence of parallel-platy new formations. The parallel-platy configuration of the new formations allows the crystals to grow with a minimal increase in the deformation energy of the crystal lattice.

Further heating is accompanied by an increase in fibrous formations and an increase in the refractive index, which is equal to 1.594–1.599 at 1100°C and 1.599–1.605 at 1200°C. The increase in light refraction indicates compaction of the structure of the new formations. On the X-ray diffraction patterns, along with the lines \(d = 3.16\) and \(d = 2.87\), which are common to both modifications of magnesium metasilicate, diffraction maxima \(d = 2.26\), \(d = 1.96\), and \(d = 1.49\), belonging to protoenstatite, are recorded.

In small amounts, the platy and fibrous structures are preserved after firing at 1300°C and, when the temperature is raised to 1400°C, they pass completely into a fine-grained variety of protoenstatite.

Beginning at 1100°C, in the firing products of Onot talc, and at 1200°C in the firing products of Alguy talc, crystals of clinoenstatite appear. On the X-ray diffraction pattern, they correspond to diffraction maxima with \(d = 2.10\), \(d = 1.52\), \(d = 1.61\).

X-ray diffraction records cristobalite in products fired at a temperature of 1400°C.

After firing at 1500°C, the principal crystalline phase in the firing products of both talcs is clinoenstatite.

The results of studies of talc samples (Onot and Alguy) with additions of 2% alkali-metal oxides show that the addition of \(\mathrm{Li_2O}\) has the strongest effect. After calcination at 1000°C, the structure of the firing products of Onot talc with the \(\mathrm{Li_2O}\) additive becomes homogeneous and consists predominantly of fine-grained aggregates of protoenstatite. Distinct cristobalite is also present here.

Beginning at 1200°C, clinoenstatite is recorded.

After firing at 1300°C, the amount of clinoenstatite increases and its grains grow in size. In addition to protoenstatite, clinoenstatite, and cristobalite, grains of quartz, forsterite, and isolated accumulations of glass are present.

Additives of \(\mathrm{K_2O}\) and \(\mathrm{Na_2O}\) to Onot talc somewhat accelerate the process of protoenstatite formation and the formation of cristobalite. At 1000°C, the structure of the firing products is heterogeneous. Only after firing at 1300°C is the formation of fine-grained protoenstatite completed, and the structure acquires a homogeneous texture. Cristobalite appears at 1000°C, while clinoenstatite is practically absent at all firing temperatures. Additives of \(\mathrm{Li_2O}\), \(\mathrm{K_2O}\), and \(\mathrm{Na_2O}\) to Alguy talc have approximately the same effect as on Onot talc.

References

1. A. I. Avgustinik. Changes in talc during heating. In: Talc (collection of articles). Promstroiizdat, 1952.

2. D. S. Belyankin, B. V. Ivanov, V. V. Lapin. Petrography of Technical Stone. Publishing House of the Academy of Sciences of the USSR, 1952.

3. W. Eitel. Physical Chemistry of Silicates. Publishing House of Foreign Literature, 1962, 752–754.

4. U. D. Kingery. Introduction to Ceramics. Publishing House for Literature on Construction, 1964, 226.