Inter-particle Forces in Powder Flow

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H M Pollock and R Jones, D Geldart and A Verlinden
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Research Area: 
Powder Flow
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United Kingdom

The main aim of this Lancaster University-Bradford University collaborative project is to understand the forces between a variety of dry materials at the single particle level, to relate these to the complimentary bulk powder flow measurements, and hence assess how far such single particle data are able to predict flow behaviour of real value to chemical engineers. In particular, we are interested in (a) the role of ambient conditions such as relative humidity, and properties such as particle size, roughness and surface condition, and (b) the role of both particle-particle interactions and particle-wall interactions, in flow and adhesion behaviour.

The selection of particulate material for recent study has been determined mainly by the following criteria:

(1) Availability as particles of controlled shape, size, roughness and surface chemical condition, for use as model systems for adhesion and flow studies;

(2) For single particle studies, the extent to which the bulk flow and compaction properties of the material had already been studied in testers at Bradford and in laboratories elsewhere;

(3) Suggestions to us from other IFPRI members for materials of particular industrial relevance and interest to the powder flow community.

The details of the force-curve technology, using a commercial AFM, the details of the bulk powder flow experiments, sample preparation, humidity control, and other experimental details have been extensively described in previous IFPRI annual reviews, and will not be repeated here. To achieve the objectives stated above, our work during the first half of this third year (December 1998-May 1999) has fallen into three main areas:

(1) At Lancaster, single-particle adhesion studies in well-defined model systems comprising relatively large glass spheres and flat surfaces, and related systems, using the force-distance software available with the Topometrix Explorer AFM.

(2) Adhesion studies, using force-distance curves, of more cohesive and finer powders, mainly those whose bulk cohesion properties had already been extensively studied in several testers at Bradford and elsewhere, or powders of particular industrial relevance (zeolite, hydrated alumina). In particular, we hope to relate the effects of particle size, ambient conditions, and powder-wall adhesion noted in the bulk experiments to any difference we see in the single particle or small-scale adhesion experiments.

(3) At Bradford and elsewhere, the continued improvement and standardisation of the bulk powder flow experiments in the Warren Spring-Bradford Cohesion Tester (WSBCT) using a small range of well-characterised powders, and in particular by comparative studies with several other testers in different laboratories.

Many details of the work in (1) - (3) above have already been described in our previous annual report (Year 2, ending 14 November 1998) or in the report for the 1999 AGM. Only the main conclusions, and some details not discussed in the Year 2 Report, will be given in this report. Ongoing experimental work on model systems, involving the adhesion between glass spheres and flat surfaces, and fitting the experimental results to theoretical models, is being pursued in collaboration with Dr J A S Cleaver and colleagues in the Department of Chemical and Process Engineering at the University of Surrey. The program of comparative studies of bulk powder flow and cohesion using various testers is now essentially complete and includes work at Albi in the laboratory of Professor J Dodds, using a ring-type shear cell developed by Schwedes. (Once certain software problems have been solved, we plan to include also data obtained at Delft data using the bi-axial cell being developed in Professor B Scarlett’s laboratory). The force curve studies at Lancaster on cohesive powders have been directed recently towards answering questions of more particular relevance to bulk cohesion testing, such as the role of particle-wall adhesion or particle size effects, but we have an ongoing interest in explaining some of the more unusual aspects of the force curves, e.g. humidity effects and long-range forces, in terms of the fundamental interactions involved.

The main innovation since the June 1999 AGM has been the introduction of single- particle frictional force measurements using the AFM to complement the normal adhesion measurements. Frictional forces are just as important to powder flow as these adhesion forces which we have studied exclusively in the first 2 years of the project, but we had not anticipated this major development to any extent in our “forward look” from previous reports. Cohesive materials studied so far include hydrated alumina and a limestone powder used as a standard material to calibrate cohesion testers. In particular, we are attempting to correlate single particle friction- load data with the corresponding shear stress-load plots from bulk cohesion testers. To this end, we have begun discussions with theoreticians at Surrey (Professor M Ghadiri, Dr S J Anthoni, Dr A Sharif) and elsewhere who have performed distinct element analysis to construct models for the statistics of transmission of normal and shear forces through a powder assembly. These models will be essential to provide the necessary links between single particle and bulk cohesion and flow. The current project has been granted a l-year extension, and it is expected that friction and modelling studies of this type will form the bulk of the last year of the project, to November 2000.