Several studies have been published on microwave techniques for moisture measurement of bulk materials, such as coal, soil and agricultural products. However, little research work has been carried out on the treatment of wet solids and water slurries at atmospheric pressure using microwave techniques. The microwave-assisted synthesis of inorganic materials at atmospheric pressure using hydrothermal route is still developing.

In the present study, a feasibility study on the synthesis of sodalite (Na₈(Al₆Si₆O₂₄)Cl₂) from kaolinite (Si₂Al₂O₅(OH)₄), metakaolinite (dehydroxylated kaolinite), NaOH and NaCl using a microwave process has been carried out by comparing the heating time and reaction temperature with the same factors under conventional thermal process.

Experiments have been conducted using the hydrothermal method at atmospheric pressure. Microwave power has been kept low (100W) in order to: i) maintain the same temperature as with conventional heating (80°C) and evaluate the real effect of microwave irradiation relative to conventional heating, ii) avoid the thermal runaway phenomenon [1] already observed with zeolite materials after only few minutes under microwave irradiation, iii) study the reaction evolution in time. At 80°C, when the heating time is below 30 min, the XRD analyses of conventional heating (CH) or microwaves (MW)-heated samples have revealed that all resulting products are amorphous. While a prolonged conventional heating (from 30 to 90 min) leads to an amorphous product, microwaves heating (100W) leads to the forming of crystalline products. When the heating time was increased up to 180 minutes, the XRD patterns of the resulting product mostly indicate the presence of natural sodalite together with some traces of Zeolite A. A higher reaction time of 6 hours was needed to achieve a complete transformation of kaolinite into sodalite when using conventional heating.

The efficiency of microwave irradiation over conventional thermal process was evidenced by the yield of sodalite in time. The different sodalite yields obtained at 80°C using two different heating modes (conventional and microwave irradiation) can be explained in terms of difference of activation energy for heat treatment or chemical reaction. A remarkable feature of the microwave heating process is related to the athermal-non-Arrhenius effects which are defined as a reduction of activation energy for the heat treatment or chemical reactions in the microwave heating process [2]. It is noted that some chemical reactions under microwave irradiation follow a quite different mechanism route which requires lower activation energy than that needed in the conventional method.

It is noteworthy that MW heat quickly and uniformly leading to the forming of sodalite under soft operating conditions with energy saving and short processing time.

Thermal decomposition of Na₈(Al₆Si₆O₂₄)Cl₂ sodalite occurred at high temperature (1200°C/6h heating) and led to nephelin NaAlSiO₄ formation. Both the good thermal stability and water insolubility of the synthesised sodalites were considered to be essential properties for confinement of nuclear wastes (based-halide salts).

[1] S. G. Deng and Y. S. Lin, Chem. Eng. Sci. N°10 52 (1997) 1563

[2] D. M. P. Mingos, Chem. Ind., (1994) 596

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