THERMAL DECOMPOSITION OF ILLITE

Transcription

1 THERMAL DECOMPOSITION OF ILLITE J. H. de Araújo, N. F. da Silva, W. Acchar and U. U. Gomes Departamento de Física Teórica e Experimental-UFRN, PO Box. 1641, , Natal- RN Brazil - humberto@dfte.ufrn.br The effect of heat treatment in air at temperature from 700 o C to 1150 o C of illite has been studied using the Mössbauer effect in 57 Fe. The dependence of the Mössbauer parameters and relative percentage of the radiation absorption area has been measured as a function of the firing temperature. At 800 o C, it was observed the beginning of the thermal structural decomposition. The results have shown that the formation of hematite (Fe 2 O 3 ) increase with the increasing of the temperature to expense of the silicate mineral. INTRODUCTION Kaolinite and Illite are the most used clay minerals for structural clay products in the traditional ceramic industry. Illite is probably the most widely distributed clay mineral in the world, especially in the temperature latitudes and on the ocean floor [1]. Illites are sheet silicates with low iron content. Their composition are quite variable and strongly dependent from origin. Their structural family is 2:1 and are normally dioctahedral. i.e., an octahedral sheet is sandwiched between two tetrahedral sheets of SiO 2. The octahedral cations are surrounded by four oxigens and two hidroxyls in trans- or cis- configuration. The octahedral sites are occupied by Mg +2, Fe +2, Fe +3, Al +3, etc. and are assigned by M1 or M2 as according to be in positions trans- and cisrespectively. Although much work has been done on the thermal decomposition of sheet silicates in general, there has been little published on illite in particular. Most of the sheet silicates work having been confined to biotites and micas. The thermal decomposition of muscovites and biotites have been studied by infrared spectroscopy [2]. The Mössbauer spectroscopy is extensively used by mineralogists, geologist and physicist to analyze the clay minerals. The Mössbauer spectra of illites reported in the literature have shown to be quite variable and strongly dependent on geological and geochemical parameters [4,5,6]. The objective of this work is to study the thermal decomposition of illite as a function of the temperature by using Mössbauer spectroscopy. EXPERIMENTAL PROCEDURE Illite samples from the ceramic industry of Rio Grande do Norte state (Brazil) were collected and heat treated at different temperatures in air. The raw material were milling in a planetary ball milling biz to a fine powder (10µm). After heat treatment, the samples were mounted in an acrylic sample holder and the Mössbauer spectra recorded at room temperature. The 57 Fe Mössbauer spectra were obtained in a transmission geometry with a conventional constant acceleration spectrometer operating in a triangular wave mode with a 57 Co source in a Rhodium matrix at room temperature. The spectrometer was calibrated with a 25 µm thick α-fe foil. RESULTS Figures 1 show the differential thermal analysis curves of the sample. The clay show endothermic bands at 110 to 140 C corresponding water loss. The curve show an endothermic reaction between 470 C and 580 C due to dehydroxilation of the octahedral sheet (constitutive water). The hydroxyl groups of the tetrahedral sheet are gradually removed until a 850 C. Between 850 C and 920 C there is an exothermic peak due to crystal reformation (spinel phase). It can also be absolved that the endothermic peaks at 468

2 1110 C C are due to mulite and cristobalite phase formation. The existence of both endothermic and exothermic peaks is in agreement with the results of the reference [7]. Figure 1 - differential thermal analysis curves of untreated sample. The Mössbauer spectrum of the untreated sample is shown on the figure 2. The spectrum was fitted using a last square procedure with a sum of four Lorentzan shaped doublets corresponding to the quadrupolar splitting of Fe +2 and Fe +3 on the two sites M1 and M2 [4]. The two most intense central doublets are ascribed to Fe +3 on the M1 and M2 sites, respectively. Observe that the doublet corresponding to M1 site (positions trans) is more intense, indicating a higher population of these positions isomer. The quadupolar splitting in this site is lower, indicating that the charge distribution around of the transition metal ion is more simetric in this case. The Two right shifted doublets are attributed to Fe +2 on the same coordination sites (M1 and M2). 469

3 1,00 Rel. Transmission 0,99 0,98 0,97 0,96 0, Velocity (mm/s) Figure 2 - Mössbauer spectrum of untreated sample Table 1 shows the Mössbauer hyperfine parameters and the relative absorption area (A(%)) for Fe +2 and Fe +3 at each site position. The Absorption area is directly proportional to iron amount in the sample. Table 1 - Mössbauer parameters for untreated sample δ?mm/s)?mm/s) A(%) Fe +3 M M Fe Figure 3 shows the Mössbauer spectra for the heat treated samples at temperatures between 750 o C and 1150 o C. The spectrum for the sample treated at 750 o C show only two central doublets assigned to Fe +3 indicating that Fe +2 were completely oxidized. The high value of the quadrupolar splitting at this temperature might be due to dehydroxilation of Fe +3 hydroxide and a variety of iron sites due to O -2 and trapped water molecules bringing about distortion in the lattice [3]. The Fe 3+ components can be assigned to configurations (a) and (b) for higher and lower, respectively [3]. The (a) configuration has been assigned to the less symmetric Fe 3+ (O 5 OH) coordination and the (b) configuration correspond to the symmetric Fe 3+ (O 6 ) coordination. The samples heat treated at temperatures between 800 o C and 1150 o C also shows Mössbauer spectra with the two central doublets observed on the sample treated at 750 o C, meanwhile a sextet is observed on all spectra corresponding to a magnetic hyperfine splitting. In addition, is observed a strong change in the color of the samples, starting with orange at 800 o C up to intense red at 1150 o C. The sextet are very well fitted with the hematite Mössbauer parameters. 470

4 velocity (mm/s) Relative Transmission 1,010 1,010 1,005 0,995 0,985 0,975 0,995 0,985 T=750 0 C T=800 o C T=900 o C T=950 o C T=1100 o C T=1150 o C Velocity (mm/s) Figure 3 - Mössbauer spectra of heat treated samples We can see from above spectra that the absorption area of the hematite subspecta increase with the temperature increase to expense of the silicate subspectra absorption area, suggesting that the hematite formation is a product of the silicate thermal decomposition. Table 2 shows the Mössbauer parameters and the relative absorption areas of the silicate and hematite subspectra. 471

5 Table 2 - Mössbauer parameters for Illite as a function as a function of the firing temperature. F.T( o C) H(KOe) (mm/s) δ(mm/s) A(%) The above results for illite are very different of results of thermal decomposition of other sheet silicates existent in the literature. For example: the Mössbauer study of heat treatment in air of biotite show any evidence for formation of hematite below 900 o C [3] and other study shown hematite formation only at 980 o C [8]. CONCLUSIONS We have investigated the effect of heat treatment in air on illite. Based on results of the present study, the following conclusions can be drawn: 1- The changes in Mössbauer spectra are correlated with oxidation and dehydroxilation. 2- The dehydroxilation process begin before to the decomposition of the original silicate structure. 3- The magnetic pattern whih appear up to 800 o C are attributed to hematite. REFERENCES [1] Browenll, W. E.; Structural Clay Products. Springer-Verlag (1976). [2] Addison C. C., Addison, W.E., Neal, G.H and Sharp, J.H.; J. Chem. Soc (1962). [3] Högg, C.S. and Meads, R.E.; Min. Mag. 40, 79 (1975). [4]Raclavsky, K., Sitck, J. and Lipka, J.; Proc. 5th Int. Conf. Mössbauer Spectroscopy, 368, Bratislava (1973). [5] Malathi, N., Puri, S.P. and Sarawat, I.P.; J. Phys. Soc. Japan 26 (1969) 680. [6] Coey, J. M. D.; Atomic Energy Review 18 (1980)

6 [7] Smykatz-kloss, W., Differential Thermal Analysis: Aplication and Results in Mineralogy, Springer Verlag, New York, (1973) 974. [8]Chandra, U. and Lokanathan, S,; J. Phys. D: Appl. Phys. 15, 2331 (1982) 473