(70am) Incidence of the mixture composition in stability angles of granular materials in cement production

Raw granular materials intervening in cement production present a wide size distribution, ranging from microns to a couple of inches. Its storage is made in pyramidal piles and, although empirical homogenization techniques are used with good results, there are important factors like segregation and grinding conditions that produce mixtures with different size distribution components that are difficult to control. For that reason, stabilization of a pile will strongly depends on this factors and a study of the influence that different proportions of particle sizes have in the mixture is important to be explored. In this work we present experimental results showing correlations among the characteristic angles of equilibrium for the piles and the relative presence of different sizes of grains that compose the material. To this end, we perform experiments by placing the raw material inside a glass container of dimensions 0.25 m x 0.45 m x 0.30 m, this last was the height. The glass box had a mobile wall which could be used like a sliding gate. It could also be rotated in order to generate avalanches. With this experimental set up we measured two angles: the angle of repose (?ÆR) and the maximum angle of stability (?ÆM) under continuous rotation. The first one was determined under two different conditions: by quickly removing upwards the sliding gate and by rotation. In any case, we wait until an avalanche develops. The material that was dropped into the box had always a constant weight and a well known size composition. To do this, we classify the material by sieving it in three families of different size: thick, medium and thin, respectively. The size of thick grains ranges from 1.2 cm to 5 cm; the medium sized ones from 1.2cm to 0.08 cm and the thin material was consider that whose grains were smaller than 0.08cm. We performed measurements of stability angles as a function of composition, i.e., as a function of the relative presence of the three different families. The dependence of these angles with size composition indicates that stability comes from the presence of thin material, i.e., from the dust provided by family of thin grains and that there is a range of proportions where ?ÆR is not affected. The obtained results allow us to draw conclusions that might be useful for treatment and piling of raw material in cement production industry.