Thermal deformation thermodynamics of a smectite mineral
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A method has been purposed to calculate some of the thermodynamic quantities for the thermal deformation of a smectite without using any basic thermodynamic data. The Hançılı (Keskin, Ankara, Turkey) bentonite containing a smectite of 88% by volume was taken as material. Thermogravimetric (TG) and differential thermal analysis (DTA) curves of the sample were obtained. Bentonite samples were heated at various temperatures between 25–900°C for the sufficient time (2 h) until to establish the thermal deformation equilibrium.
Cation-exchange capacity (CEC) of heated samples was determined by using the methylene blue standard method. The CEC was used as a variable of the equilibrium. An arbitrary equilibrium constant (K a) was defined similar to chemical equilibrium constant and calculated for each temperature by using the corresponding CEC-value. The arbitrary changes in Gibbs energy (ΔG a 0 ) were calculated from K a-values. The real change in enthalpy (ΔH 0) and entropy (ΔS 0) was calculated from the slopes of the lnK vs. 1/T and ΔG vs. T plots, respectively. The real changes in Gibbs energy (ΔG 0) and real equilibrium constant (K) were calculated by using the ΔH 0 and ΔS 0 values. The results at the two different temperature intervals are summarized as below: ΔG 1 0 =ΔH 1 0 −ΔS 1 0 T=−RTlnK 1=47000−53t, (200–450°C), and ΔG 2 0 =ΔH 2 0 -ΔS 2 0 T=−RTlnK 2=132000−164T, (500–800°C).
Keywordsbentonite cation exchange capacity smectite thermal analysis thermal deformation thermodynamics
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