Notes
Frits are vitreous materials and the main raw material for the formation of enamel and ceramic coatings. They are produced with melting in melting furnaces, which is followed by rapid cooling of the melt and the formation of coarse granules. For the application to the appropriate surface frits are always milled, mostly in the large ball mills, which is time and energy consuming. On the contrary, water atomization technology, which in the world is being mostly used for the production of powders of metals and alloys, enables the powder to be directly formed upon the cooling of the melt. Ease of the process and its high flexibility lead us to explore the possibility of using this technology for the production of powdered frits. The study was based on the research of two compositionally different frits, frits A and B, which were compared with frits of same input configurations produced by conventional manufacturing process. We than compared the chemical and physical properties of the atomized and nonatomized (conventional) powder frits, as well as the formed enamels from them. The powders were investigated using modern anallytical techniques, such as X-ray fluorescence spectrometry, scanning electron microscopy, differential scanning calorimetry with thermogravimetric analysis, dilatometry etc. Next the surface of the formed enamels was evaluated with their adherance, color consistency and acid resistance. Results of the study showed that there is a great possibility for using this technology for the formation of powdered frits. The size and morphology of the powders was depended mainly on the chemical composition of the frits and the atomization parameters. The atomized powders in comparison to the non-atomized ones showed a completely different morphology, wherein the more viscous frit (frit A) formed a more irregular particles and less viscous (frit B) more spherical. The morphology of both conventional frits showed a tipical formation of particles known of the grinding powders, with cutted edges and surfaces. By developing the equation for calculation of solidification time (tstr) and by using the expression for spheroidization time (tsfr), we obtained a model that describes the morphology of particles with different sizes with the change of process parameters and properties of frits. The time ratios (R(tstr/tsfr)) have successfully confirmed the resulting morphology of the atomized frits. Addditionally, we also observed a strong interaction between the particles, as well as between the particles and the dispersing media (water), which has had a major impact on the final morphology. The particles were due to large interaction damaged, deformed and transformed, which is for frits because of their brittleness in comparison to metal systems a more decisive factor that influences the end morphology. The distribution of the particles was dependent upon the viscosity of the frits and the degree of superheating of the melt. For frits we have established, that in order to successfully atomize them, bacause of their high viscosities, a higher temperature of overheating is needed (around 300-400 °C), as for metals and alloys. Finer distributions were formed at a higher water pressures and lower orifice openings of the tundish. The atomised and the nonatomizized powders had similar characteristics, however atomised frits, due to the higher cooling rates achieved in atomization, demonstrated higher glass transition temperatures (Tg) as well as all the other temperatures of transformation (sintering temperature, softening temperature etc.). Enamels formed from the atomized frits had similar chemical, mechanical and optical properties which verifies the appropriateness of the new technology. The calculation of the economic eligibility of the process has shown that at expected costs and revenues, which certainly can change (e.g. lower purchase price of the atomization device, better optimization of used energy etc.) and significantly reduce the return on investment, the investment in water atomization to produce frit powders would be recovered in about 35,5 years.