Relationship between surface area coverage of flow-aids and flowability of cohesive particles

Fabio Fulchini,Umair Zafar,Colin Hare,Mojtaba Ghadiri,Hossam Tantawy,Hossein Ahmadian,Massimo Poletto

doi:10.1016/j.powtec.2017.09.013

Fabio Fulchini, Umair Zafar + Show 5 more

Open Access

https://doi.org/10.1016/j.powtec.2017.09.013

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Abstract

Poor and inconsistent flow of cohesive powders is a major issue in powder processing. A common solution is to coat the surfaces of the cohesive particles with finer particles, referred to as flow-aids. Such particles adhere to sticky surfaces and act as spacers preventing them from contacting each other and thus reducing the inter-particle forces and bulk powder cohesion. A question which naturally arises is how much flow-aid is needed to enhance the flowability to an optimum level. This work aims to establish a relationship between the degree of Surface Area Coverage (SAC) of flow-aids and the flowability, the latter as determined by a quasi-static shear cell method, as well as the angle of repose test and the FT4 powder rheometer. Glass beads of 90–150μm sieve cut are made cohesive by silanising their surfaces with a commercial chemical reagent, Sigmacote® and are used as host particles. Two types of zeolite particles are used as flow aids. The mass fraction of the flow aids required to achieve a theoretical SAC of 1, 5, 10, 20, 50 and 100% is first estimated and then the host particles are coated in a pan mixer. The SAC is measured by Scanning Electron Microscopy, coupled with image analysis, and found to correlate well with the estimated value. The optimum surface coverage is found to be when SAC is 10–20%, as this provides the greatest flowability. An increase in SAC beyond this range leads to a gradual reduction in flowability.

Highlights

Flowability and inter-particle forces are closely related
Calculation of the theoretical SAC In order to obtain a surface area coverage of 1, 5, 10, 20, 50, 100%, the required amount of flow aids is calculated according to eq 1-3, making the assumptions: (i) both host and guest particles are spherical and are sized equal to their D50; (ii) the covered surface area of the host is equal to the total projected area of the guest particles; (iii) the guest particles form a monolayer on the surface of the host particles; (iv) the host and guest particles are fully mixed and the host particles are uniformly coated
The effect of the surface area coverage (SAC) on flowability of 90-150 μm glass beads, made cohesive by silanisation, has been assessed using two types of micrometre sized particles: Zeolite A (ZA) and Zeolite A coated with nano-particles of Aerosil (ZA-Ae)

Summary

Introduction

Flowability and inter-particle forces are closely related. If particles have a weight much larger than the attractive inter-particle forces they may roll over one another, and as a consequence, they pack tightly and flow [1]. Inter-particle forces depend on the local radius of curvature at contact [5] The strength of such forces can be decreased by decreasing the local asperity radius, as irregularly-shaped fine particles flow better than round particles [1]. For this reason particles are often coated with hard very fine particles, called flow control additives, or flow aids, in order to separate energetic surfaces and to decrease the local radius of curvature at contact, reducing van der Waals interactions down to several orders of magnitude [5].

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