(206f) The Relevance of Surface Impurities on the Effect of Temperature on Powder Flow Behavior

The relevance of surface impurities on the effect of temperature on powder
flow behavior

R. Chirone¹,
D. Barletta², M. Poletto² and P. Lettieri¹

¹Department of Chemical Engineering, University
College London - Torrington Place - London WC1E 7JE - UK

²Dipartimento
di Ingegneria Industriale, Università degli Studi di Salerno - Via Giovanni
Paolo II, 132 - 84084 Fisciano (SA) - Italy

Cohesive interparticle forces may have a relevant role in several
industrial process operations involving particulate materials, such as fluidization, granulation and drying, as well as storage and solids
handling units. Several of these operations require process conditions which
involve high temperatures which, in turn, may affect the intensity of
interparticle forces such as van der Waals, capillary and electrostatic forces.
The mean by which the system temperature can affect all these forces, is the change
of particle hardness, the generation of liquid phases which determines the
formation of liquid bridges or the modification of the particle dielectric
properties. A direct measure of interparticle forces is possible but can be
affected by large fluctuations that require a great number of repetitions.
Interparticle forces, instead, play in averaged ensembles in bulk properties
such as powder cohesion. It is of interest, therefore, to have the possibility
to measure powder cohesion at the process temperature and to observe possible
changes due to temperature variations to infer possible changes at the particle
level. Powder shear testing is one of the available methods able to measure
powder cohesion and it has the great advantage of measuring well established
physical properties and of being able to produce highly repeatable results. It
has to be remarked, however that to date no established procedure exists to
relate powder cohesion measured at the bulk level to powder fluidization
properties.

In this paper a systematic study on the effect of the process
conditions on the fluidization quality of ceramic powders is presented. Tests were
carried out on powders of industrial interests, characterized by different
particle size distributions and by different amounts of surface impurities,
ranging from easy-flowing to cohesive flow behaviour. Two different
experimental facilities were used: a modified ring shear tester available at
the University of Salerno and aX-ray high temperature fluidization facility
available at University College of London. The first apparatus was used to characterize
powder cohesion at different temperatures between ambient and 500°C. Experimental
results have been interpreted in terms of possible changes in interparticle
forces as a function of temperature. The powder samples without impurities show
an increase of cohesion with temperature as a result of an increase of
interparticle van der Waals forces. A larger increase of cohesion was observed
in the case of the powder samples with chemical impurities. The behaviour can
be explained only by considering a cooperative effect of both van der Waals and
capillary forces. It is noteworthy that the amount of surface impurities that
is able to determine significant changes of powder flow properties is still so
small that no evidence of phase transition could be detected by means of sample
thermal analysis. The same powders have been characterized
in terms of fluidization quality by using the x-ray fluidization facility
available at University College of London under the same temperature range.

The changes by temperature on the flow properties of
the bulk solid evaluated with the shear cell and the behaviour of particles
under fluidization conditions are analysed and discussed. A direct quantification of the particle-particle interactions in
fluidized beds and of their changes under process conditions is very difficult.
The paper suggests a method by which powder rheology can be used to indirectly evaluate
the effects of the interparticle forces on fluidization.