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In this paper we studied the aerated discharge of two magnesium carbonate powders dif fering in their average diameter and particle size distribution. These samples were characterized by means of f luidization experiments and rheology shear tests carried out in a rotating shear cell. In the hopper discharge experiments, besides the discharge rates and the mass of residual solids as a function of the aeration rate, the aggregative behavior was observed by means of photographic techniques. Solids aggregates were actually visible within the aerated beds of solids during the f luidization experiments and in the streams of the discharging solids. Experimental data on the powder f low properties and on f luidization were analysed in order to permit evaluation of the aggregate diameters. These values compared fairly well with the aggregate diameters observed and made it possible to evaluate the voidage values outside the aggregates. These results were used in a modified form of the De Jong and Hoelen [9] equation to predict the solids discharge rate with a reasonable degree of accuracy. * Via Ponte Don Melillo, I-84084 Fisciano (SA), Italy † Accepted: August 22, 2003 [10] aeration rates. Some terms appearing in these equations such as the pressure gradient in the proximity of the hopper outlet or the air f lux through the same orifice appear difficult to evaluate. Moreover, from experiments carried out in similar aerated discharging devices on freef lowing [11] and cohesive powders [12], it appears that a different discharge behavior is found in the two cases. Experiments on a corn starch powder [13] demonstrated that the first effect of aeration is that of breaking up the arches at the outlet and preventing their subsequent formation. At low and increasing aeration levels, this powder shows rat-holing and core f low (funnel f low). The effect of solids f low promotion by increasing aeration is to mobilize the superficial layers of solids involved in the funnel f low mechanism. Only very high aeration rates are able to f luidize the solids and to produce f low fields similar to those found with aeratable powders at high aeration. In spite of such complex phenomenology, the corn starch discharge rate under aeration is roughly predicted by the above-mentioned equations. It is also possible to extend some simplifications to these solids that make the use of the De Jong and Hoelen [9] equation possible in a predictive way. Indeed, according to their equation, the discharge rate is expressed as the following: Ws 0.55ρp(1 εb) a 2 b 0.5

Aggregative Behavior of Cohesive Magnesium Carbonate Powders during Fluidization and Aerated Discharge