Eficiência da moagem de pó cerâmico em moinho agitador de esferas

Abstract

In this research, wet milling of silica powder was carried out up to nanoparticles level. A work strategy has been established in which maximum efficiency is achieved. The grinding efficiency is related to the smaller particle size obtained, with the lower energy consumed, in a less time of grinding, with consequent lower cost. The adopted work method begins with the selection of the most suitable equipment for the initial and final particle size range. Among a universe of existing grinding systems, a grinding border was established, where two types of mill were selected: the ball mill and the stirred media mill. Specific energy was determined for each milling system used, as well as milling time. It has been shown how the particle size varies according to the specific energy consumed, as well as the main parameters of the process influence in this relation. The results obtained (milling curves) showed agreement with the mathematical model of Kwade; Blecher; Schwedes, 1996. A comparative study of specific energy and grinding time, between the ball mill and the stirred media mill was performed, and the stirred media mill showed the best results. The grinding time was 1 h and the specific energy was 2.500 kJ/kg, against 60 h e 200.000 kJ/kg in ball mill, for the same final particle size of 600 nm. The most important variables, in high energy grinding, in stirred media mills, were identified. In order to describe the milling process and to optimize it, the fundamental parameters "stress energy" and specific energy were used. By means of economic analysis, the minimum energy costs for different grinding speeds (and different powers) were obtained: R$ 2,00/kg and R$ 6,00/kg, at 4000 rpm, for the final particle sizes required of 300 nm and 150 nm, respectively. After 5,5 h of process, an apparent grinding limit of 133 nm was identified. The search for energy efficiency was the main focus of this research, unlike most previous studies, which focused on the search for the smallest particle. By manipulating the suspension pH, the milling efficiency was increased by ~ 18%, primarily by increasing the repulsive forces between the quartz particles and the zirconia beads. It was investigated the dependence of the specific surface area of the particles as a function of the specific energy consumed, showing that the colloidal grinding process is linearly proportional to this magnitude. After 3 hours of milling, a specific surface area of 30 m2/g was reached, corresponding to a specific area up to 3 times higher than the values available in the literature. The equivalent spherical diameter of 75 nm was obtained for that specific surface area. This equivalent diameter represents and characterizes the set of particles that have this specific surface area and serves as an indicator of milling efficiency.