Synthesis of Zeolites from A Low-Quality Colombian Kaolin

At present, no production of zeolites is ongoing in Colombia; thus, because of the high demand in the industrial sector, ~2500 tons is imported annually from other countries such as Cuba, Ecuador, Mexico, and the United States. In order minimize the need for these costly imports, the present study sought to evaluate the viability of producing low-silica zeolites through the hydrothermal synthesis of a Colombian kaolin, which contains quartz (40%) and iron-oxide impurities. The kaolin was subjected to a milling process to reduce the particle size to the order of 11 μm, and was heat treated to transform it to metakaolin. Optimization of the synthesis variables (Na2O/SiO2 and H2O/Al2O3 ratios, time, and temperature) was accomplished by applying an experimental design based on the ‘Response Surface Methodology’ technique. The degree of crystallinity and the cation exchange capacity (CEC) were used as response variables. The CEC was determined from the NTC 5167 standard. In addition, the mineralogical composition and the zeolite microstructure were evaluated using techniques such as scanning electron microscopy, X-ray diffraction, and solid state nuclear magnetic resonance spectroscopy. The results indicated that synthetic type A zeolites with a CEC value of 442 cmol(+)/kg can be obtained from the Colombian kaolin, with the following optimal processing conditions: Na2O/SiO2 molar ratio of 2.7, H2O/Al2O3 molar ratio of 150, temperature = 66°C, and processing time = 8 h. Note that this value (442 cmol(+)/kg) is greater than that reported for an imported commercial zeolite (408 cmol(+)/kg) of the same type, which is currently being used in industry in Colombia. The nationwide availability of the raw material and the quality of the final product present opportunities to make this material available to the Colombian market.