A thermodynamic model for silica and aluminum in alkaline solutions with high ionic strength at elevated temperatures up to 100 degrees C: Applications to zeolites

作者：Xiong Yongliang^*

来源：American Mineralogist, 2013, 98(1): 141-153.

DOI：10.2138/am.2013.4089

• Pitzer equation
• solution chemistry
• sodium silicates
• hydrothermal synthesis

In this study, a thermodynamic model for silica and aluminum in high ionic strength solutions at elevated temperatures up to 100 degrees C is constructed. Pitzer equations are utilized for the thermodynamic model construction. This model is valid up to ionic strengths of similar to 24 molal (m) in NaOH solutions with silicate concentrations up to similar to 1.5 m. The speciation of silica (including monomers and polymers) and aluminum at elevated temperatures is taken into account. Also, the equilibrium constants for silicic acid and its polymer species (H4SiO4, H5Si2O7-, H4Si2O3-, and H5Si3O3-) at elevated temperatures up to 100 degrees C, are obtained based on theoretical calculations. Using this thermodynamic model, thermodynamic properties, including equilibrium constants, and respective reaction enthalpies are obtained for sodium silicates, zeolite A, and the amorphous form of zeolite A, based on solubility experiments at elevated temperatures. The equilibrium constants for zeolite A and amorphous precursor of zeolite A regarding the following reactions up to 100 degrees C,
NaAlSiO4 center dot 2.25H(2)O(cr) + 4H(+) = Na+ + Al3+ + H4SiO4(aq) + 2.25H(2)O(1) (1)
and
NaAlSiO4-2.25H(2)O(am) + 4H(+) = Na+ + Al3+ + H4SiO4(aq) + 2.25H(2)O(1) (2)
can be expressed as follows
log K-1 = 7963 +/- 327/T - 16.46 +/- 0.96
and
log K-2 = 12971 +/- 160/T - 30.80 +/- 0.50
where T is temperature in Kelvin.
The enthalpy of formation from elements, Gibbs free energy of formation from elements, and standard entropy derived for zeolite A and the amorphous form of zeolite A with the chemical formulas mentioned above at 25 degrees C and 1 bar are -2738 +/- 5 kJ/mol, -2541 +/- 2 kJ/mol, 373 +/- 10 J/(K.mol); and -2642 +/- 3 kJ/mol, -2527 +/- 2 kJ/mol, and 648 +/- 10 J/(K.mol), respectively. The enthalpy of formation from elements for zeolite A derived in this study based on solubility experiments in hydrothermal solutions agrees well with those obtained by calorimetric measurements and by theoretical calculations.

• 出版日期2013-1