Otimização da síntese de pigmentos zeolíticos ultramarinos derivados de rejeito de caulim usando um DOE 2ˆ3 completo.

Abstract

The state of Pará is recognized as an important center for kaolin production for paper coating, resulting in a significant amount of waste, mainly composed of kaolinite. Research groups at the Federal University of Pará have been exploring the use of this waste as an alternative source of silicon and aluminum in the synthesis of zeolites, particularly zeolite A, which has the potential for producing ultramarine pigments. These inorganic pigments are highly valued due to their deep blue color and good chemical and physical stability. LTA and sodalite zeolites are the main crystalline structures involved in this process, with the S3⁻ anionic radicals responsible for the blue color. The synthesis process involves calcination of the zeolite with sulfur and sodium carbonate, resulting in the diffusion of sodium polysulfides through the zeolite pores and the production of pigments with color variations depending on the synthesis conditions. The aim of the study was to understand how the crystalline structure of zeolites (LTA zeolite and sodalite) affects the pigment color and to propose an optimization for the more efficient synthesis of these pigments. To achieve these objectives, a sequential experimental design methodology (DOE) was applied. A Full Factorial Design followed by a Central Composite Design was used to identify the significant and influential factors in obtaining the ultramarine blue color. For optimization, the response surface methodology was employed in conjunction with the Excel Solver function. The response obtained was color, and the factors were temperature, time, and S/Na2CO4 ratio. The analyses that supported the crystallochemical discussion were: Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray fluorescence (XRF), differential scanning calorimetry (DSC), and thermogravimetry (TG), particle size distribution (PSD), zero point charge (ZPC), and UV-Vis diffuse reflectance spectroscopy, and Munsell Color System was used for color classification. The crystallochemical analyses of the pigments obtained from LTA zeolite revealed the insertion of S3⁻ chromophores into the zeolitic structure, which was corroborated by FTIR and DSC-TG analyses, highlighting characteristic peaks of these phases. UV-Vis spectroscopy revealed the presence of both S2⁻ (associated with green hue) and S3⁻ (associated with blue hue) chromophores, with the optimized product showing a higher concentration of S3⁻ chromophore, indicating the effectiveness of the planning. On the other hand, UV-Vis analysis of the pigment derived from sodalite did not show characteristic peaks of the chromophores of interest. Finally, this research assumes significant relevance in the global context of environmental impacts resulting from waste generation, offering a promising alternative for the management of kaolin waste and the production of synthetic ultramarine pigment. The synthesis of this pigment from LTA zeolite represents a more environmentally sustainable approach due to the almost negligible reduction in sulfur oxide emissions, positioning itself as a viable and environmentally conscious solution for obtaining this type of pigment.