Resumen Durante el proceso de microfundición de joyería con preengaste de gemas, el molde cerámico compuesto de sílice y yeso se estabiliza a temperaturas cercanas a 650 °C, mientras que la temperatura de vertido de la aleación preciosa ronda los 1000 °C. Por tal motivo, este método no es apropiado para gemas como la esmeralda, dada la gran susceptibilidad de esta al daño cuando se expone a temperaturas superiores a 350 °C. Teniendo en cuenta lo anterior, el objetivo del presente trabajo fue desarrollar un revestimiento cerámico de chamota/yeso que adquiera estabilidad dimensional a bajas temperaturas, con el fin de implementar el método de preengaste de esmeraldas para la obtención de piezas de joyería. La metodología experimental se dividió en tres etapas: i) caracterización química, mineralógica, granulométrica y térmica de las materias primas; ii) preparación y caracterización de las mezclas chamota-yeso-agua, incluyendo tiempos de vertido y fraguado de las pastas, junto a la permeabilidad y resistencia a la compresión y al choque térmico, y además, análisis microestructural, y iii) evaluación del nuevo revestimiento en el proceso de microfundición con preengaste de esmeraldas. Se encontró que las condiciones más adecuadas para obtener el revestimiento chamota/yeso son: granulometría fina del refractario (100 % con pasante malla 200), relación chamota/yeso 75/25 y adición de 50 % de agua a la mezcla seca. Los resultados de la investigación permiten concluir que es posible someter las esmeraldas al proceso de microfundición de joyería, con gemas preengastadas, empleando el revestimiento chamota/yeso desarrollado aquí, cuyo tratamiento térmico no supera los 350 °C, así como una aleación de metal precioso de bajo punto de fusión (398 °C).

Abstract During the jewelry stone-in-place micro-casting process, the ceramic mold, which is composed of silica and plaster is stabilized at temperatures close to 650 °C, whereas the casting temperature of the precious metal alloy hovers around 1000 °C. Therefore, this type of micro-casting technique is not suitable for gems such as emeralds, since they are highly susceptible to damage at temperatures over 350 °C. In the face of this fact, the main purpose of the present work is to develop a refractory mold based on chamotte and plaster, which can be stabilized at low temperatures, in order to use it for the micro-casting of pre-set emeralds. To achieve this, the experimental methodology was divided into three phases: i) raw chemical materials, mineralogical, granulometric and thermal characterization; ii) preparation and characterization of chamotte-plaster-water mixtures, including the measurement of casting times, mechanical and microstructural properties, permeability, and resistance to thermal shock; and iii) evaluation of the resulting refractory mold in the micro-casting of pre-set emeralds. The results showed that the refractory mold with composition chamotte/ plaster 75/25, an addition of 50 % of water to the dried mixture, and fine granulometry (-200 mesh) had a stabilization temperature of 350 °C. Furthermore, it is possible to subject the emeralds to the stone-in-place micro-casting process that employs the refractory mold developed here, with a thermal treatment that does not surpass the 350 °C, along with a low melting point (398 °C) for the precious metal alloy.

Resumo Durante o processo de micro fundição de joias com pré-engaste de gemas, o molde cerâmico composto de sílice e gesso, estabiliza-se a temperaturas próximas a 650 °C, enquanto que a temperatura de vertido da liga lindo ronda os 1000 °C. Por tal motivo, este método não é apropriado para gemas como a esmeralda, dada a grande suscetibilidade desta ao dano quando se expõe a temperaturas superiores a 350 °C. Tendo em conta o anterior, o objetivo do presente trabalho foi desenvolver um revestimento cerâmico de chamota/gesso que adquira estabilidade dimensional a baixas temperaturas, com o fim de implementar o método de pré-engaste de esmeraldas para a obtenção de peças de joalheria. A metodologia experimental dividiu-se em três etapas: i) caracterização química, mineralógica, granulométrica e térmica das matérias primas; ii) preparação e caracterização das misturas chamota-gesso-água, incluindo tempos de vertido e fraguado das pastas, junto à permeabilidade e resistência à compressão e ao choque térmico, e, além disso, análise microestrutural, e iii) avaliação do novo revestimento no processo de micro fundição com pré-engaste de esmeraldas. Encontrou-se que as condições mais adequadas para obter o revestimento chamota/gesso são: granulometria fina do refratário (100 % com passante malha 200), relação chamota/gesso 75/25 e adição de 50 % de água à mistura seca. Os resultados da pesquisa permitem concluir que é possível submeter às esmeraldas ao processo de micro fundição de joias, com gemas pré-engastadas, empregando o revestimento chamota/gesso desenvolvido aqui, cujo tratamento térmico não supera os 350 °C, assim como uma liga de metal lindo de baixo ponto de fusão (398 °C).

N-F codoped TiO2 films were immobilized on stainless steel sheets through a combined approach involving a dip-coating technique and a hydrothermal treatment, followed by calcination at 400°C in the presence of air. Photocatalyst characterization was conducted using XRD, Raman and UV-VIS spectroscopy as well as SEM. The films were tested in a three-electrode cell for the photoelectrocatalytic degradation of CN-containing compounds. The results showed that the increase in the degradation rate of CN-containing compounds is both influenced by a synergistic effect of the doping agents and strongly dependent on the concentration of CN-containing compounds in the solution. Nitrogen contributed to the enhanced photoactivity under visible light due to the generation of localized states within the band gap of TiO2, whereas the presence of fluoride improved the superficial properties of the film, which resulted in higher amounts of CN-containing compounds that were degraded by direct charge transfer through the photogenerated holes.

El objetivo de este trabajo fue caracterizar química, mineralógica, granulométrica, plástica y térmicamente tres arcillas, denominadas "Cascajo" (C), "Roja" (R) y "Amarilla" (A), procedentes de Barichara, Santander. Así como obtener pastas cerámicas aptas para la fabricación de ladrillos y tejas. Para lo cual se fabricaron probetas cerámicas por prensado, las cuales se sinterizaron a 1050ºC y posteriormente se caracterizaron física y mecánicamente. Los resultados demostraron que la pasta de arcilla "Roja", con contenido de agua entre 24 y 38%, es la más adecuada para la fabricación de tejas; ya que los cerámicos presentan baja absorción de agua (8.2%) y alto módulo de ruptura (9.3 MPa). Para la obtención de ladrillos se recomienda la pasta con mezcla de arcillas de 55% "Cascajo" y 45% "Amarilla", con contenido de agua entre 23 y 32%, obteniéndose cerámicos con absorción de agua menor al 15% y módulo de ruptura mayor a 5 MPa.

The aim of this work was to characterize chemical, mineralogical, granulometric, plastic and thermally three clays, named "Cascajo" (C), "Roja" (R) and "Amarilla" (A), proceeding from Barichara, Santander. As well as, to obtain ceramic pastes suitable for the manufacture of bricks and roofing tiles. For which ceramic test pieces were made by pressing, which were sintered to 1050ºC and later them were physical and mechanically characterized. The results demonstrated that the paste of clay "Roja", with water content between 24 and 38%, is the most adequate for the manufacture of roofing tiles; since the ceramics present low water absorption (8.2 %) and high rupture module (9.3 MPa). To obtain bricks it is recommended the paste with clays mixture of 55% "Cascajo" and 45% "Amarilla", with water content between 23 and 32%; so, ceramics with water absorption minor to 15 % and rupture module bigger than 5 MPa are obtained

Se benefició una arcilla caolinítica santandereana, con el fin de obtener una materia prima apta para la fabricación de refractarios sílico-aluminosos. Aplicando las operaciones metalúrgicas de tamizaje en húmedo, sedimentación y lixiviación se pretendió eliminar la mayor cantidad posible de cuarzo y hierro. El proceso de separación del cuarzo mediante tamizaje y sedimentación no fue eficiente, dado que éste presenta un tamaño de partícula muy fino, similar a la de las especies arcillosas; tan sólo se logró reducir la relación Si/Al en un 12% (de 1,52 a 1,35). Mediante lixiviación a 90°C, con solución de ácido oxálico 0,4 M, se eliminó casi la totalidad del hierro de la arcilla (cerca del 98%), lográndose el "blanqueo" de la misma. Finalmente, se determinó que es posible obtener refractarios con punto de ablandamiento de 1600°C a partir de la arcilla beneficiada.

A kaolinitic clay of Santander was beneficiated in order to obtain a suitable raw material for the manufacture of silico-aluminous refractories. With the application of the metallurgical operations of wet screening, sedimentation and leaching it was pretended to eliminate the major quantity of quartz and iron. The separation process of the quartz through screening and sedimentation was not efficient, since this one presents a particle size very small, similar to that of the clayey species; the relation Si/Al was reduced only in 12% (from 1,52 to 1,35). Almost the totality of the iron of the clay (near 98 %) was eliminated through the leaching to 90°C, with solution of oxalic acid 0,4 M, with which the "whitening" of the clay was achieved. Finally, it was determined that it is possible to obtain refractories with softening point of 1600°C from the beneficiated clay.