Changes in the Phase Composition of a Mica Batch During Heating
Abstract Generated abstract
This study examines phase changes during heating of fluorophlogopite mica batches, first using a simplified talc and potassium silicofluoride mixture and then a stoichiometric normal fluorophlogopite charge. Differential thermal, thermogravimetric, X-ray phase, and microscopic analyses indicate early formation of potassium-magnesium fluoride, followed by cristobalite and mica formation as potassium silicofluoride decomposes actively between about 600 and 780°C. In normal fluorophlogopite batches, the corresponding reactions occur at higher temperatures, with the first mica crystals detected near 800°C and synthesis accelerating strongly with temperature. The work shows that mica formation remains incomplete after long holding at 800 to 900°C, but is nearly complete after several hours at 1100°C or within about an hour at 1200°C.
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CHANGE IN THE PHASE COMPOSITION OF MICA CHARGE DURING HEATING
P. G. Usov, E. N. Belomestnova, V. I. Vereshchagin
(Presented by the Scientific Methodological Seminar of the CTF)
In view of the complexity of the processes occurring in a multicomponent fluorophlogopite charge, their character was preliminarily studied on a two-component mixture of talc—potassium silicofluoride. The composition of the mixture corresponded to the stoichiometric ratio of the elements of tetrasilicic mica \((\mathrm{KMg}_{2.5}, \mathrm{Si}_4\mathrm{O}_{10}\mathrm{F}_2)\).
On the thermogram of this mixture at a temperature of \(300\text{—}400^\circ\mathrm{C}\) there is an exothermic effect. X-ray phase analysis of products heated to this temperature records a certain decrease in the intensity of the diffraction maxima corresponding to \(\mathrm{K}_2\mathrm{SiF}_6\) \((4.7; 2.88; 2.35)\); the appearance of any new phase is not detected. Microscopic analysis shows that the grains of \(\mathrm{K}_2\mathrm{SiF}_6\) have become somewhat smaller, and at the periphery of the talc grains extremely finely dispersed new formations with a low refractive index appear. In the literature [1] there are data indicating that at temperatures below \(500^\circ\mathrm{C}\) \(\mathrm{KMgF}_3\) is formed in a mica charge. It may be assumed that this low-temperature exothermic effect is also associated with the formation of potassium-magnesium fluoride. The weight losses of the mica charge in the temperature range from 20 to \(600^\circ\mathrm{C}\) are considerably smaller than the theoretical ones; in all probability, a large part of that released during the decomposition of potassium silicofluoride is adsorbed on the surface of the talc.
The first substantial changes in the phase composition are recorded in the charge heated to \(600^\circ\mathrm{C}\); on the X-ray pattern of this charge there appear diffraction maxima corresponding to \(\mathrm{KMgF}_3\) \((2.82; 2.29; 1.92; 1.62)\), \(\alpha\)-cristobalite \((4.03)\), and mica \((3.3)\). In the narrow temperature interval \(700\text{—}780^\circ\mathrm{C}\), substantial changes occur in the mica charge. On the DTA curve a very pronounced exothermic maximum appears; the magnitude of the loss on the DTG curve approaches the theoretical value.
Microscopic and X-ray phase analysis of the firing products at a temperature of 760°C showed that the amount of $KMgF_3$ and $\alpha$-cristobalite in the charge had increased sharply, the amount of talc had decreased considerably, and potassium hydrogen fluorosilicate was absent.
Such an energetic course of the process is due to the fact that it proceeds under conditions of active decomposition of $K_2SiF_6$. Calculations showed that, in the temperature range 600–780°C, more than half of the fluorine lost during heating from 20 to 1000°C volatilizes. At a temperature of 800–850°C, the thermogram shows a weak exothermic effect corresponding to the mica-formation process. In addition, the thermogram of the charge shows two endothermic effects: the first (at a temperature of 850–900°C) is associated with the decomposition of talc, and the second (at a temperature of 1050°C) with the melting of unreacted $KMgF_3$. To study phase changes in the charge of normal fluorophlogopite ($KMg_3AlSi_3O_{10}F_2$), briquettes and charges of stoichiometric composition were fired at temperatures of 700, 800, 900, 1000, 1100, and 1200°C with various holding times. The products were subjected to X-ray phase and microscopic analysis.
The results showed that all processes in the fluorophlogopite charge are shifted into the region of higher temperatures in comparison with the tetrasilicate charge. At a furnace heating rate of 10 degrees per minute, the first crystals of fluorophlogopite were detected in the charge fired at 800°C.
On the X-ray diffraction pattern of the mixture heated to 800°C, along with the lines of the starting components, lines corresponding to $KMgF_3$ (2.8; 2.29; 1.99) and mica (9.71; 3.31; 1.98) appear. As the firing temperature is increased, the amount of mica in the charge increases.
Experiment showed that the rate of synthesis depends strongly on temperature. Even prolonged holding (10 hours) at temperatures of 800 and 900°C does not lead to completion of the mica-formation reaction. At a temperature of 1100°C, synthesis is practically completed in 3–4 hours; at 1200°C, in 0.5–1 hour. Analysis of the firing products of mica charges based on talcs dehydrated to different degrees showed that they contain practically the same amount of impurities.
REFERENCES
- W. Eitel, R. A. Hatch, M. V. Denni. J. Amer. Cer. Soc., 36, 341—348, 1953.